Photoexcitation Dynamics of Fullerenes

Encyclopedia of Nanoscience and Nanotechnology

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Photoexcitation Dynamics of Fullerenes Mamoru Fujitsuka, Osamu Ito Tohoku University, Sendai, Japan

CONTENTS 1. Introduction 2. Photoexcitation and Relaxation Processes of Fullerenes 3. Photoinduced Reactions of Fullerenes 4. Photoinduced Processes of Fine Particles of Fullerenes 5. Fullerene Oligomers, Higher Fullerenes, and Metallofullerenes 6. Charge Separation and Recombination Processes of Donor-Fullerene Linked Molecules 7. Concluding Remarks Glossary References

1. INTRODUCTION Fullerene C60 (Fig. 1) was found in the laser vaporization of graphite in 1985 by Kroto et al. [1]. Since the first demonstration of large scale synthesis of fullerenes in 1990 [2], quite a large number of studies on fullerenes have been carried out for clarification of their basic properties and for their applications. Up to date, in addition to C60 , various kinds of fullerenes such as higher fullerenes and endohedral metallofullerenes have been isolated. Furthermore, various kinds of derivatives of fullerenes have been synthesized. Thus, many kinds of compounds are included in the fullerene group. Fullerenes show interesting properties in the field of materials science: Superconductivity, photoconductivity, ferromagnetism, and nonlinear optics are examples of characteristic properties of fullerenes [3–6]. Furthermore, it should be noted that fullerenes are also attractive materials in the field of biochemistry, since the excited fullerenes are effective in cleavage of DNA in the presence of molecular oxygen and electron donors [7, 8]. Furthermore, computer simulation made clear that C60 would be active to HIV protease [9]. ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

Photoexcitation dynamics of fullerenes have been also widely investigated. Nowadays, the excitation–relaxation processes of C60 and C70 have been well established [10–15]. Singlet and triplet properties have been investigated by using pico- and nanosecond laser flash photolysis techniques. One of the important photophysical properties of C60 and C70 (Fig. 1) is almost quantitative triplet generation, which results in effective photochemical bimolecular reactions [10]. From the viewpoint of photochemistry, fullerenes are good electron acceptors and many photoinduced reactions have been reported by using these fullerenes as acceptors [16–20]. The excellent acceptor ability of fullerene is a key feature of photoconductivity for fullerene-doped polymer films such as poly(N -vinylcarbazole) and poly(p-phenylene vinylene) [4, 21]. Furthermore, many derivatives of the fullerenes have been synthesized due to high reactivities of fullerenes [22]. Fullerene oligomers and polymers are interesting materials as well as pristine fullerenes [23, 24]. Utilization of fullerenes to mimic photosynthesis systems has been investigated, resulting in enhanced efficiencies of the charge separations, which relates to application of the highly efficient photovoltaic cells [25–27]. In this chapter, we review the photoexcitation dynamics of fullerenes including C60 , C70 , higher fullerenes, endohedral metallofullerenes, and fullerene oligomers. Furthermore, photoinduced processes in the fullerene-donor linked molecules have been also reviewed, since they will serve as important molecular devices.

2. PHOTOEXCITATION AND RELAXATION PROCESSES OF FULLERENES 2.1. Excited Singlet State Properties of C60 and C70 C60 and C70 show weak fluorescence at 700 and 660 nm (Fig. 2) [28, 29]. The quantum yield of the fluorescence of C60 is as low as 3.2 × 10−4 [30]. Small quantum yields of the fluorescence processes can be attributed to the forbidden transition due to closed shells with high symmetry (Ih -symmetry). When the symmetry of C60 is decreased by the introduction of a functional group, about a threefold Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (593–615)

594

Photoexcitation Dynamics of Fullerenes

Sn internal conversion

S1

intersystem crossing

hν

C 60

C 70

Figure 1. C60 and C70 .

S0

(b)

400

500

600

700

2.2. Excited Triplet States of C60 and C70 Photoexcitation of C60 and C70 generates the triplet excited states almost quantitatively from the singlet excited states; that is, the quantum yields for the intersystem crossing yields (ISC ) are >0.95 for C60 and C70 [10, 36]. The quantitative triplet generations of the fullerenes are important in the various photochemical processes. For example, the triplet excited fullerenes are important reagents for the quantitative generation of singlet oxygen [10], which is one of the important species in the fields of photobiology. C60 and C70 in the glassy matrix of the brominated hydrocarbon at 77 K show phosphorescence at 794 and 810 nm [37], respectively, which correspond to 1.56 and 1.53 eV of the triplet energies. Upon nanosecond laser irradiation, C60 shows a clear absorption band due to the triplet excited state at 750 nm (Fig. 4) [11–15], which is a good spectral probe for the photochemists who trace the photoinduced process via the triplet excited state of C60 , since the extinction coefficient of the transient absorption band is as large as 16,100 M−1 cm−1 at 750 nm [15]. The decay rate of the triplet excited state of C60 in solution is governed by the triplet–triplet annihilation and the self-quenching processes, 3 3

kTT

∗ ∗ ∗ C60 + 3 C60 + C60 −→ 1 C60

(1)

ksq

∗ C60 + C60 −→2C60

(2)

0.40

800

Wavelength / nm

0.60

0.30

0.40

∆Abs

x20

Figure 3. Schematic energy diagram of the excitation and relaxation processes of fullerenes.

∆Absorbance

(a)

Fluores. int. (arb. unit)

Absorbance (arb. unit)

increase of the fluorescence quantum yield is attained [31]. From the low symmetry, C70 shows a slightly larger fluorescence quantum yield (5.2 × 10−4 ) than C60 [30]. The fluorescence lifetimes of pristine C60 and C70 are reported to be 1.2 ns and 660 ps, respectively [30]. Introduction of multiaddends to C60 changes fluorescence properties to a great extent. Schick et al. reported that the hexa-adduct of C60 (Th -symmetry) shows a fluorescence peak at 550 nm, with high fluorescence quantum yield (0.024), which is 75 times larger than pristine C60 [32]. They also reported that the hexa-adduct shows apparent phosphorescence. These findings indicate that the optical devices are possible by using fullerene compounds with many addends. By using ultrashort laser pulses, transient absorption bands due to the singlet excited states of C60 and C70 can be observed. In the case of C60 , the transient absorption bands due to the singlet excited state appeared around 900 nm upon ultrashort laser pulse irradiation [33]. As for the singlet excited state of C70 , the transient absorption spectrum shows a peak around 700 nm [34]. It has been reported that the decay rate constants of the absorption bands of the singlet excited states agreed well with the corresponding fluorescence decay rate constants. When the excitation wavelength is short enough to pump fullerenes to the higher excited states (Sn ), the lowest excited state (S1 ) should be generated after the internal conversion process (Fig. 3). In the case of C60 , it has been reported that the S1 state is converted with a time constant of 250 fs from the higher excited state [35]. As for C70 , it was confirmed that the internal conversion process proceeds within 1 ps after the excitation with the laser pulse.

300

T1

fluorescence

0.20

0.20 0.00

0.10 0.00 600

700

800

900

0

200

time/ µs

400

1000 1100 1200

Wavelength / nm Figure 2. Absorption and fluorescence spectra of (a) C60 and (b) C70 in benzene. Reprinted with permission from [29], A. Masuhara et al., Bull. Chem. Soc. Jpn. 73, 2199 (2000). © 2000, Japan Chemical Society.

Figure 4. Transient absorption spectrum of C60 in toluene at 100 ns after laser light irradiation. Inset: Absorption-time profile at 750 nm.

595

Photoexcitation Dynamics of Fullerenes

Both processes were investigated by dependence of the decay rate of the triplet excited C60 on the excitation power or concentration (Fig. 5) [29, 38]. It has been reported that the bimolecular rate constants of both processes are on the order of 109 M−1 s−1 , indicating that both processes are effective quenching processes of the triplet excited states. Ausman and Weisman reported that the intrinsic lifetime of the triplet excited state of C60 is as long as 143 s in toluene at room temperature [38]. In the case of C70 , the triplet absorption band shows peak at 980 nm, of which the extinction coefficient is 6500 M−1 cm−1 [14]. In the case of C70 , the intrinsic triplet lifetime is reported by Ausman and Weisman to be as long as 11.8 ms in toluene [38]. These long triplet lifetimes of C60 and C70 indicate that the wide varieties of the photoinduced reactions are expected via the triplet excited states of both fullerenes. Triplet properties of the derivatives of C60 have been also reported [31]. In the case of 1,2-adducts of C60 , the triplet absorption band appeared around 700 nm. It has been reported that the ISC values are 0.8–0.9 for these 1,2-adducts [31]. Thus, the triplet excited states of the derivatives are also good precursors for the photoinduced reactions.

3. PHOTOINDUCED REACTIONS OF FULLERENES 3.1. Electron Acceptor Abilities of Fullerenes The first reduction potential of C60 is reported to be −0 42 V vs saturated calomel electrode (SCE) in benzonitrile [39], which is similar to the reduction potential of p-benzoquinone (−0 51 V vs SCE) [40], a typical acceptor in the photosynthesis systems. The cyclic voltammogram of C60 shows multiple reduction waves up to sixth reduction steps [41]. From these results, C60 is a good electron acceptor. Its electron acceptor ability is enhanced in its excited state, since free-energy 1.5 40

∆ k1st / 104 s-1

∆Absorbance

20mJ

1.0

30 20

0 0.0 0.5 1.0 1.5 2.0

0.5

∆A0 1.3mJ

0.0 -20

0

20

40

60

80

time / µs Figure 5. Laser power dependence of absorption-time profiles of triplet excited C60 at 750 nm. Inset: Analysis of the triplet–triplet annihila0 + tion process according to the relation −dln A0 /dt = k1st = k1st 0  k2nd , and T are T-T absorbance at 2k2nd /T A0 , where A0  k1st t = 0, an intrinsic first-order decay rate, rate constant of the T-T annihilation, and an extinction coefficient of the T-T absorption band. Reprinted with permission from [29], A. Masuhara et al., Bull. Chem. Soc. Jpn. 73, 2199 (2000). © 2000, Japan Chemical Society.

change for the electron transfer (Get ) in the excited state [Eq. (3)] is more negative than that in the ground state [Eq. (4)] [42], Get (excited state) = Eox − Ered − E00 − e2 /r 2

Get (ground state) = Eox − Ered − e /r

(3) (4)

where Eox , Ered , E00 , and e2 /r are oxidation potential of donor, reduction potential of acceptor, excitation energy, and Coulombic term, respectively. As for the oxidation, five oxidation waves were confirmed in the cyclic voltammogram, although reports on the oxidation processes of C60 are scarce compared to the reduction processes.

3.2. Electron Transfer via the Singlet Excited States of Fullerenes Since the decay rate constants of the singlet excited states are 8 8 × 108 and 1 5 × 109 s−1 for C60 and C70 , respectively, electron transfer processes via the singlet excited states become evident when a charge-transfer complex is formed in the ground state or when the concentration of the donor is as high as ∼100 mM. When C60 or C70 was excited in the presence of highly concentrated amines such as dimethyl aniline and triphenyl amines, C60 or C70 showed exciplex emission. For example, the C70 -diemethylaniline system showed exciplex emission in the 700–800 nm region with decrease of fluorescence intensity of C70 in the 600–700 nm region [43]. The exciplex formation is apparent in nonpolar solvents rather than polar solvents, since the polar solvents solvate the generated radical ions as free radical ions. When the concentration of the donor is high, one-to-one charge transfer complex formation can be observed as broadening of the ground state absorption band of C60 . Sension et al. observed electron transfer from the singlet excited state of C60 within 1–2 ps by observing the absorption band around 1000 nm upon excitation of the C60 -dimethylaniline complex with a femtosecond laser [44]. The recombination occurs on a time scale in the range from 20 to 55 ps. Electron transfer via the singlet excited states of the fullerenes also became evident in the solid state materials such as conjugated polymer films. Sariciftci et al. reported that the femtosecond laser irradiation on the polythiophene– C60 composite film, in which the charge–transfer complex was formed, results in the radical ion pair formation within 1 ps [45, 46]. Thus, laser excitation on the charge transfer complex results in fast electron transfer. This kind of electron-transfer process is important in the photoconductive materials in which the substantial carriers are generated by photoirradiation. It has been reported that the photoconductivity of these composite films of fullerenes and conjugated polymer persisted about 10 ns after a short laser pulse, indicating that the charge-migration processes take place in these films. A fast charge-separation process of the charge-transfer complex is also reported for the C60 -doped poly(N vinylcarbazole) (PVCz) film, for which enhanced photoconductivity was also reported [4]. In the C60 -doped PVCz films, the electron transfer occurred immediately after the

596

Photoexcitation Dynamics of Fullerenes

picosecond laser pulse [47]. The decay of the initial chargeseparated state with a time constant of 1.2 ns comprises three channels: the charge–recombination, the hole migration to the neighboring carbazolyl chromophores, and the formation of the local triplet excited state of C60 (Fig. 6). It should be noted that under low concentrations of these photoconductive polymers (<10 mM) in solution, the electron-transfer process via the triplet excited states of fullerenes or triplet excited polymers is a major process as shown below.

3.3. Electron Transfer via the Triplet Excited States of Fullerenes One of the important characteristics of C60 and C70 is the high quantum yield of the intersystem crossing from 1 C∗60 (1 C∗70 ) to 3 C∗60 (3 C∗70 ). Thus, the reaction systems including donors of lower concentrations (<10 mM) attain highly efficient electron transfer via 3 C∗60 and 3 C∗70 . For observations of the electron-transfer processes via the triplet excited states of fullerenes, a nanosecond laser flash photolysis method observing transient spectra in the nearinfrared (IR) region is advantageous, since the transient species of the fullerenes appeared in the near-IR regions. For example, absorption bands of the triplet excited state and radical anion of C60 appeared at 750 and 1080 nm, respectively. As for C70 , they appeared at 980 and 1360 nm, respectively. The first paper on the electron-transfer process via the triplet excited C60 was reported by Arbogast et al. [48]. Their studies using transient absorption spectroscopy showed the generation of the radical anion in the near-IR region. Furthermore, they showed the dependence of the rate constants for the electron-transfer processes on the free-energy changes according to the tendency described by the Rehm–Weller equation, which are calculated by the oxidation potentials of aromatic amines used as donors. In addition to the aromatic amines, many kinds of organic compounds are known to show sufficient donor abilities. For example, tetrathiafulvalene (TTF) or bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) is a donor which shows metallic conduction when it forms a charge-transfer complex with appropriate acceptors [49]. C60 is one of the acceptors which forms a charge-transfer complex with TTF [50]. In benzonitrile solution which contains C60 and TTF, 1

(C60, PVCz)* Charge-separation Hole migration

(C60•-, PVCz•+)

hν

C60* + PVCz

CR

N

CR

(C60, PVCz)

C60 + n

the radical ions are generated upon nanosecond laser irradiation (Fig. 7) [51]. The electron transfer was confirmed to proceed via the triplet excited state of C60 . The bimolecular quenching rate constant (kq ) of the triplet excited C60 is 5.0 × 109 M−1 s−1 , indicating that the quenching process is a diffusion-limiting process (kdiff in benzonitrile is 5 3 × 109 M−1 s−1 at 298 K) because of sufficiently negative free-energy change for the electron-transfer process. On the other hand, the quantum yield for the electron transfer (ET ) via the triplet excited state was evaluated to be 0.75, which was estimated as a ratio of the generated radical ions to the initial concentration of the triplet excited C60 ; these values can be estimated from the transient absorption spectra by using extinction coefficients of the radical ions and the triplet excited states. Since the electron-transfer rate constant (kET ) value can be estimated by using the relation kET = ET × kq , the kET value for the C60 and TTF system was evaluated to be 3 8 × 109 M−1 s−1 . The ET value smaller than unity indicates that other bimolecular deactivation processes of the triplet excited C60 , such as an energy transfer and collisional quenching, are present (Scheme 1). In the present case, the latter quenching process seems to be plausible due to the higher triplet energy of TTF compared to that of C60 .

C60•-••• PVCz•+

3

ISC

Figure 7. Transient absorption spectra of C60 (0.1 mM) in the presence of TTF (1.0 mM) in benzonitrile. Inset: Absorption-time profiles at 750 and 1070 nm. Reprinted with permission from [51], M. M. Alam et al., J. Photochem. Photobiol. A: Chem. 104, 59 (1997). © 1997, Elsevier Science.

Charge-transfer complex

PVCz

Figure 6. Charge-separation and charge-recombination processes of C60 -doped PVCz film. CR and ISC denote charge recombination and intersystem crossing, respectively.

3

C60* + Donor

dissociation C60•- + Donor•+ (C60•-,Donor•+) geminate ion pair back electron transfer collisional in ion pair deactivation

(3C60* ••• Donor) encounter complex energy transfer C60 + 3Donor*

C60 + Donor

Scheme 1. Reaction scheme for 3 C∗60 in the presence of electron donor.

As for BEDT-TTF, the kET value was somewhat smaller than that of the C60 -TTF system due to the smaller freeenergy change for the electron-transfer system of C60 and

597

Photoexcitation Dynamics of Fullerenes

BEDT-TTF. When the polarity of the solvent decreases, the ET values decrease while the kq values are still close to the corresponding kdiff values. This observation can be attributed to the difficulty of the radical ion pair dissociation in the nonpolar solvents because of poor salvation ability of the radical ion pair in these nonpolar solvents. The electron-transfer process competitive with the energy-transfer process has been confirmed in the mixture systems of C60 with -carotene or retinol, which is reported to be a good triplet energy acceptor [52, 53]. In nonpolar solvents, only the energy-transfer process was observed. On the other hand, the electron-transfer process also becomes evident with increasing solvent polarity. In the case of the photoinduced reaction of C60 with -carotene in benzonitrile, the quantum yields for the electron transfer and energy transfer were estimated to be 0.84 and 0.16, respectively. As for the reaction system of C60 and all-trans retinol, the values were 0.45 and 0.55, respectively. The lower quantum yield for the electron transfer of C60 -retinol system can be attributed to less negative free-energy change for the electron transfer (−9.9 and −14.1 kcal mol−1 for C60 -retinol and C60 -carotene systems, respectively). Porphyrins and phthalocyanines also act as donors for fullerenes under photoexcitation [54, 55]. It was confirmed that the electron transfer proceeded via both 3 C∗60 and triplet excited porphyrins. Zinc-including porphyrin compounds such as tetraphenylporphyrin and octaethylporphyrin showed higher donor ability to fullerenes in comparison with copper and H incorporated porphyrins. A similar tendency was also confirmed with phthalocyanine derivatives [55].

the 355-nm laser in polar benzonitrile [Eq. (5)]. This mechanism was supported by the fact that the generation rate of the C60 radical anion depends on the concentration of C60 and that PVCz radical cation is generated by the 355-nm laser irradiation even in the absence of the C60 in benzonitrile. The rate constant indicates that the electron-capture process by C60 [Eq. (5)] is the diffusion-limiting process: h

+

C60

+

−

· PVCz −→ PVCz· + e− −→ PVCz· + C60

(5)

The electron-transfer rate constant of the C60 -PVCz system is larger than the system of C60 and N -ethylcarbazole (EtCz) when C60 is excited by the 532-nm laser, although EtCz corresponds to the unit structure of PVCz. The larger rate constant of PVCz can be attributed to the stable radical cation of PVCz due to the stacking of the carbazole moieties in the polymer chain as observed by the charge-resonance band in the near-IR region of the transient absorption spectra. The effect of the stacking of the carbazole moieties on the electron-transfer rate constants was discussed on the basis of the data on the electron-transfer processes between C60 and various carbazole dimer models (Fig. 8) [57]. It has been confirmed that electron transfer proceeds effectively with the fully stacked carbazole dimers which stabilize the radical cations effectively. In the case of the conjugated polymers such as polythiophene in polar solution, photoinduced electron transfer between fullerenes and the polymers proceeds via the triplet excited states of fullerenes or polymers rather than the charge-transfer complex. It has been well known that the oxidation potentials and triplet energies of conjugated

3.4. Electron Transfer with Polymers Photoconductive polymers such as PVCz and polythiophene are reported to enhance their photoconductivities upon doping of the fullerenes [4, 45]. As mentioned above, electron transfer within the charge-transfer complex in the films is responsible for the photocarrier generation upon light illumination. When the concentrations of these polymers are sufficiently lower in solution, formation of the chargetransfer complex with fullerenes in the ground state can be ignored. Under these conditions, electron-transfer processes via the triplet excited states of the fullerenes or the conjugated polymers are important. When C60 is photoexcited in the presence of PVCz, the electron-transfer process proceeds via two mechanisms depending on the excitation wavelength [56]. It was confirmed that electron transfer proceeds from the triplet excited state of C60 , when the reaction mixture is irradiated by visible light, that is, a 532-nm laser, which excites C60 only. The electron-transfer rate constant was estimated to be 5 1 × 106 M−1 s−1 , which is ca. 1/1000 of the kdiff value of benzonitrile solvent. The quite slow electron-transfer rate constant can be attributed to the somewhat positive freeenergy change for electron transfer. On the other hand, electron transfer proceeds more rapidly when the reaction mixture is excited with the laser pulse in the ultraviolet region, that is, a 355-nm laser, which excites both C60 and PVCz. The radical anion of C60 is generated by capturing the electron which is ejected from PVCz by excitation with

N N

N

n PVCz ket = 5.1 x 106 M-1s-1

m-DCzPe ket = 8.1 x 107 M-1s-1

N N

N

DCzPr ket = 2.2 x 105 M-1s-1

N

DCzMe ket = ~2 x 104 M-1s-1

N

N

DCzCBu ket = 2.5 x 104 M-1s-1

N

N

DCzBu ket = 2.6 x 104 M-1s-1

Figure 8. Carbazole dimers and the rate constants for the electrontransfer process with 3 C∗60 in benzonitrile upon laser excitation at 532 nm.

598

Photoexcitation Dynamics of Fullerenes

polymers depend on their conjugation length to a great extent. The studies on the electron-transfer processes with the structure-defined oligomers give important information on improvement of efficiency of the electron-transfer processes. In the electron-transfer system of fullerenes and oligothiophene, it was confirmed that the electron-transfer rate constants become large with an increase in the number of repeating units until tetramer (Fig. 9) [58]. On the other hand, a decrease in the kET values due to the smaller ET values was observed for longer oligomers. This finding can be attributed to the large contribution of the energy-transfer process because of the lower triplet energies of the longer oligothiophene compared to those of fullerenes. Electron-transfer processes with conjugated oligomers were also investigated in detail for the oligomers of poly(pphenylenevinylenes) including the triphenyl amine moiety [59]. From the comparison with the monomer models of oligomers, it was revealed that the rate constants of the electron transfer with oligomers are larger due to the larger collision probability of the oligomers compared to the corresponding monomer models (Fig. 10).

N N n

TPA-PV monomer ket = 8.4 x 108 M-1s-1 oligomer (n=3.8) ket = 4.0 x 109 M-1s-1

N N n

TPA-BPV monomer ket = 5.3 x 108 M-1s-1 oligomer (n=6.6) ket = 5.5 x 109 M-1s-1 excited C

3.5. Back Electron Transfer When the radical cation and radical anion decay completely after laser pulse, the generated radical ion pair returns to the corresponding neutral ground state by the back electrontransfer process [60]. When the solvent is highly polar, the generated radical ions are solvated as free radical ions; thus, the back electron transfer obeys second-order kinetics [Eq. (6)]. On the other hand, in the less-polar solvents, the radical ions are present as geminate ion pairs; thus, the back electron transfer obeys first-order kinetics [Eq. (7)]: kBET 2nd

+

−

· C60 solv + Donor· solv −

−→ C60 + Donor

kBET 1st

+

· C60  Donor· solv

−→ C60 + Donor

(6)

S 7

3T ket = 1.4 x 10 M-1s-1

S

S S R

R = C6H13

4T ket = 9.6 x 108 M-1s-1 R S

S

S S

S

S R

n

TPA-AV monomer ket = 3.2 x 108 M-1s-1 oligomer (n=3.8) ket = 5.7 x 108 M-1s-1 Figure 10. Monomers and oligomers of poly(p-phenylenevinylene) including triphenylamine and the rate constants for the electrontransfer process with C60 in benzonitrile via the triplet excited C60 .

In the electron-transfer system in the solvent mixtures with moderate polarities, the back electron-transfer process by the mixed-order kinetics has been observed, suggesting selective solvation of the radical ions, because of large fullerene molecules.

3.6. Formation of Fullerene Adducts via Electron Transfer

R

S

N

(7)

S S

N

R = C6H13

8

6T ket = 8.6 x 10 M-1s-1 Figure 9. Oligothiophenes and the rate constants for the electrontransfer process with C60 in benzonitrile via the triplet excited C60 .

Fullerenes are such active compounds that they form derivatives easily. Radical anions and cations are also intermediates for the formation reaction of the fullerene derivatives. Some examples are shown below. Akasaka et al. reported that the photoexcited C60 generates [2 + 3] cycloadducts with the disiliranes, such as 1,1,2,2tetramesityl-1,2-disilirane via the electron-transfer process (Scheme 2) [61]. It is interesting to note that the adduct formation was not observed by the thermal reactions in the dark. The photoadduct formation occurs in the nonpolar toluene, suggesting that the exciplex is a plausible intermediate. When the reaction proceeds in benzonitrile, the generated radical cation of the disilirane forms 1:1 or 1:2 adducts with benzonitrile; in this case, C60 acts as a photocatalyst

599

Photoexcitation Dynamics of Fullerenes

of bissilylation of benzonitrile (Scheme 2) [62]. Photocatalytic reactions of C60 in bisslylation were also confirmed with other carbonyl compounds used as solvents.

0.40

Mes 2 Si SiMes 2

1,2-Mes

0.30 0.20

C60*

C60

R2Si N Ph

0.10

R2Si SiR2 R = mesityl

R2 Si

C60•- R Si SiR 2 + 2 in toluene •

SiR2 R2Si SiR2 N N Ph Ph

Si R2

in benzonitrile

C60•- R2Si

+

•

SiR2

0.00 ∆Absorbance

hν

0.04

Dep2 Si O

1,4-DepO

SiDep 2

0.03 0.02 0.01 0.00 0.08 1,16-t Bu

N CPh

Sit BuPh2 Sit BuPh2

Scheme 2. Reaction scheme for C60 in the presence of disiliranes.

The different reactivities depending on the solvents can be attributed to the solvation of the reaction intermediate: In toluene, adduct formation is considered to proceed in the triplet exciplex; on the other hand, in benzonitrile, the solvated radical ion pair can react with solvent molecules. In the case of the reaction with the La@C82 , Akasaka et al. observed adduct formation in both the photo- and thermalreaction conditions [63]. Higher reactivity of La@C82 can be attributed to the high electron acceptor ability of La@C82 . It has been reported in the electrochemical studies on metal encapsulated fullerenes that the reduction potential of La@C82 shifts to the anodic direction by 0.7 V in comparison with C60 . With the photoirradiation on the pristine C60 and disilane in benzene, 1,16-addition product was obtained via the silyl radical formation (Scheme 3) [64]. These bissilylated fullerenes show lower oxidation potential due to the electron-donating nature of silicon addends. Actually, laser flash photolysis study on the photophysical properties of these bissilylated fullerenes shows a substantial change of the photoexcited state properties, such as triplet absorption and fluorescence spectra (Fig. 11) [65]. Thus, bissilylation changes properties of the fullerenes to a great extent.

0.06 0.04 0.02 0.00 400

600

800

1000

1200

Wavelength / nm Figure 11. Transient absorption spectra of bissilylated C60 derivatives at 250 ns after the laser light irradiation. Inset: Molecular structures. Reprinted with permission from [65], M. Fujitsuka et al., Phys. Chem. Chem. Phys. 1, 3527 (1999). © 1999, The Royal Society of Chemistry on behalf of the PCCP Owner Societies.

[67]. In the case of this reaction, the radical anion of C60 was confirmed by the laser flash photolysis experiments. Fukuzumi et al. reported that 4-t-butylated 1-benzyl1,4-dihydronicotineamide (BNAH) generated the anion of t-butylated C60 upon photoirradiation via the electron transfer [68]. Generated t-butylated C60 anion was quenched in the presence of acid or benzylbromide to generate 1,2-tBuC60 or 1,4-t-Bu(PhCH2 )C60 , respectively. In the present

C60 hν

3

C60*

R1

OSiMe3

R2

OR3

C60•

- R

+

1

O SiMe3

•

R2

OR3

SitBuPh2

(tBuPh2Si)2

hν

2 tBuPh2Si•

+ C60

SitBuPh2 + SiMe 3 - O

OR3

Scheme 3. Reaction scheme for C60 with siliy radical.

As for the C–C bond formation with fullerene, the photoinduced electron transfer with ketene silyl acetal (KSA) was reported. For example, KSA derived from ethyl acetates generated fullerene-acetates quantitatively upon photoirradiation in benzene (Scheme 4) [66]. Their reactivities to form C60 -adducts are studied on the basis of the oxidation potentials of the KSA, indicating adduct formation via electron transfer with triplet excited C60 . Furthermore, Danishefsky’s diene, which is a vinylogue of KSA, formed [4 + 2] cycloadducts of C60 by the Diels–Alder reaction (Scheme 5)

1

R R

2

proton donor

H

O OR3

R1 R2

Scheme 4. Reaction scheme for C60 with ketene silyl acetals. Reprinted with permission from [66], K. Mikami et al., J. Am. Chem. Soc. 117, 11134 (1995). © 1995, American Chemical Society.

600

Photoexcitation Dynamics of Fullerenes MeO

generation of 1,2-dihydrofullerene was reported to be attained by a similar electron-transfer mechanism. The radical cation of fullerene is also an active intermediate which generates fullerene adducts. Siedschlag et al. reported that the C60 -adduct generated via the radical cation of C60 which was formed by the cosensitizer under photoirradiation [69]. The C60 radical cation abstracts -proton from the alcohol to generate C60 H cation and alkyl radical, which forms alkylated C60 (Scheme 7).

R R

OSiMe3 OMe

C60 hν

3

+

C60•-

C60*

OSiMe3

•

OMe

OMe -+

•

OSiMe3

•

OSiMe3

DCA•- + BP•+ DCA hν + BP •+

BP + C60•+

BP•+ + C60

H

H-C60+ + •CH2OH

C60•+ + MeOH

CH2OH

OMe DCA: 1,9-dicyanoanthracene BP: biphenyl

OSiMe3

+ DCA•-

H

OMe CH2OH

O +

Scheme 7. Reaction scheme for C60 with methanol via C 60 . Scheme 5. Reaction scheme for C60 with Danishefsky’s diene. Reprinted with permission from [67], K. Mikami et al., J. Am. Chem. Soc. 122, 2236 (2000). © 2000, American Chemical Society.

case, electron transfer via the triplet excited state of C60 is responsible for the adduct formation (Scheme 6). After electron transfer, the radical cation of t-butylated BNAH generates the t-Bu radical which reacts with C60 to generate t-butylated C60 . Using 10-methyl-9,10-dihydroacridine,

3

C60*

t-Bu CONH2 N Bz

H C60•

-

t-Bu CONH2 2.00

•+

N Bz

[TDAE] 0Abs -1 /10-3 M

C60 hν

Tetrakis(dimethylamino)ethylene (TDAE) has a strong donor ability to form strong charge-transfer complexes with electron acceptors. It has been reported that charge-transfer complex salts of fullerenes with TDAE show ferromagnetism at low temperature [5]. It was revealed that C60 in polar solvents can be reduced in the presence of TDAE without photoirradiation (Fig. 12) [70]. From the relation between the

CONH2 +

N (C60• t-Bu•) Bz

Me2N

NMe2

Me2N

NMe2

TDAE 8.0 6.0 4.0 11 2.0 7 0.0 3 -2.0 1 0.0 0.1 0.2 0.3 0.4 0.5 0

1.60 Absorbance

H

3.7. Electron Transfer in the Ground State

1.20 0.80

[C60]0[TDAE] 0Abs -2 / 10-6 M2

0.40 C60•-

0.00 400

-

600

800

1000

1200

Wavelength / nm

t-Bu

Scheme 6. Reaction scheme for C60 with 4-t-butylated 1-benzyl1,4-dihydronicotineamide. Reprinted with permission from [68], S. Fukuzumi et al., J. Am. Chem. Soc. 120, 8060 (1998). © 1998, American Chemical Society.

Figure 12. Steady-state absorption spectra of C60 and various concentrations of TDAE in deaerated o-dichlorobenzene at room temperature. Numbers indicate the concentration of TDAE in mM. Inset: Plot according to the relation: [TDAE]0 /Abs = [C60 ]0 [TDAE]0 /Abs2 − 1/K. Reprinted with permission from [70], M. Fujitsuka et al., J. Phys. Chem. B 103, 445 (1999). © 1999, American Chemical Society.

601

Photoexcitation Dynamics of Fullerenes

absorption intensity of the C60 radical anion and the amount of TDAE, it becomes clear that C60 and TDAE are in equilibrium with the corresponding radical ions [Eq. (8)]. From the analysis of thermal equilibria, the equilibrium constant was estimated to be 240 for C60 and TDAE in benzonitrile. On the other hand, in less-polar solvents (dielectric constant <7), the generated radical ions are solvated as radical ion pairs [Eq. (9)]: −

+

solv + TDAE solv C60 + TDAE $ # C60 −

+

C60 + TDAE $ # C60  TDAE· solv

(8)

3.8. Electron Mediating from C60 Radical Anion When a donor decomposes to nonactive compounds after donating an electron to C60 , the radical anion of C60 shows a considerably long lifetime. For example, the generation − rate of the superoxide anion (O 2 ) via the radical anion of C60 can be estimated by applying this method [72]. The radical anion of C60 generated using tetrabutylammonium tetraphenylborate [− BPh4 (+ NBu4 )] as electron donor was persistent, because BPh4 dissociates into less electronaccepting compounds such as BPh3 and biphenyl promptly as described by −

−→ C60 + BPh3 + 1/2 Ph-Ph

(10)

The radical anion of C60 also acts as an electron mediator. In the presence of molecular oxygen, the persistent radical anion of C60 begins to decay according to the electronmediating process (Fig. 14) −

+ O2 C60

−

−→ C60 + O 2

C60R triplet state

C60R•-+ TDAE •+

ketT C60R ground state

(9)

By laser irradiation on C60 in the solution in which C60 and TDAE are in equilibrium with the corresponding radical ions, other amounts of radical ions are generated photochemically, although the reaction system returns to thermal equilibrium by the back electron-transfer process. On combination of the back electron-transfer rate and the thermal equilibrium constant, the electron-transfer rate at the ground state can be estimated. For example, the rate constant of the electron-transfer process between C60 and TDAE in the ground state was estimated to be 6 4 × 108 M−1 s−1 in o-dichlorobenzene, which is about 1/3 of the rate constant between the triplet excited C60 and TDAE. The smaller rate constant of the electron transfer in the ground state can be attributed to the smaller free-energy change for the electron transfer in the ground state. Similar electron transfer in the ground state was also confirmed with C70 and adducts of C60 [71]. In the case of the adducts of C60 , it was revealed that electron-transfer rate constants in the ground and the triplet states are affected by the substituents. The substituent effects of the reaction rate constants were successfully analyzed on the basis of the linear free-energy relationship. It was revealed that electron transfer in the ground state needs considerable activation energy compared to that for the electron transfer via the excited triplet state in the reaction system with TDAE (Fig. 13).

C60 + − BPh4

(a)

(11)

∆GT

ketG kbet ∆GG

(b) C60R triplet state C60R•-+ TDAE •+

ketT C60R ground state ∆GT

ketG

kbet ∆GG

Figure 13. Energy diagram of electron transfer processes of 1,2-adducts of C60 (C60 R) and TDAE in (a) polar solvents and (b) nonpolar solvents. ketT , ketG , and kbet denote the electron transfer rate constants from the triplet excited state, and from the ground state, and back-electrontransfer rate constant, respectively. GT and GG indicate the freeenergy changes for the electron transfer from the triplet excited state and the ground state, respectively. Reprinted with permission from [71], C. Luo et al., Phys. Chem. Chem. Phys. 1, 2923 (1999). © 1999, The Royal Society of Chemistry on behalf of the PCCP Owner Societies. −

From the dependence of the decay rate constant of C 60 on the concentration of O2 , the bimolecular rate constant for the generation of the superoxide anion was estimated to be 3.7 × 102 M−1 s−1 in benzonitrile. The small reaction rate constant for this electron-mediating process can be attributed to an endothermic process as estimated from the reduction potentials of C60 and O2 (−0.92 and −1.2 V vs ferrocene/ferrocenium (Fc/Fc+ ), respectively). It should be noted that the generated superoxide anion can act further as an electron mediator: In the presence of viologen dication, the superoxide anion donates an electron to generate the radical cation of viologen, although the quantum yield of the electron transfer from superoxide ion to viologen dication was somewhat small, 0.1–0.2 [73]. It becomes clear that triethylamine and triethanolamine also act as donors which generate the persistent radical anion of C60 , because the corresponding radical cations

602

Photoexcitation Dynamics of Fullerenes

0.04

PTA-PV monomer

a

0.03 with OV 2+

∆Absorbance

0.02

without OV 2+

0.01 0.00 0.04

PTA-PV oligomer

b

0.03

with OV 2+

0.02 without OV 2+

0.01 0.00 0

200

400

600

800

Time / µs

−

Figure 14. Absorption-time profiles of C 60 in the presence of the molecular oxygen. Inset: Pseudo-first-order plot. Reprinted with permission from [72], T. Konishi et al., Chem. Lett. 202, (2000). © 2000, The Japan Chemical Society.

easily decompose into less electron-accepting compounds, which are not identified yet. It should be noted that these donors are soluble in water. Thus, they can be employed as sacrificial donors also in aqueous systems. In the electron-transfer systems of the C60 and monomer/ oligomer of poly(p-phenylenevinylene), C60 acts as a photosensitizer and electron mediator, since photoexcited C60 accepts an electron from the donor and mediates the electron to octylviologen dication (OV2+ ), another electron acceptor, efficiently [59]. In the electron-transfer system of C60 and monomer of TPA-PV (Fig. 10) in the presence of OV2+ , the generation of the radical anion of C60 was confirmed in the transient absorption spectra. With decrease of − + C 60 , generation of OV was confirmed, indicating that elec+ − tron transfer from C60 to OV occurs (Scheme 8). It should be noted that the radical cation of TPA-PV of the electronmediating systems showed slow decays (Fig. 15), indicating that the back electron transfer between OV + and the radical cation of the TPA-PV monomer/oligomer is retarded due to the repulsion of the positive charges of both radical cations. C60•-

PPV•+ ket OV

kmd

Figure 15. Time profiles at 1600 nm of the electron-transfer systems of (a) C60 and PTA-PV monomer and (b) PTA-PV oligomer in the presence and absence of OV2+ . Reprinted with permission from [59], H. Onodera et al., J. Phys. Chem. A 105, 7341 (2001). © 2001, American Chemical Society.

water-soluble compounds and introduction of water-soluble addends to fullerenes. The water-soluble complexes with fullerenes have been achieved by using %-cyclodextrin (%-CD), water-soluble calixarene, miscelle, and water-soluble polymers. Yoshida et al. reported that C60 forms the 1:2-complex with %-CD (Fig. 16), which shows good solubility in water [74]. Andersson et al. reported that C70 also forms the complex with %-CD [75]. Since the spectral shapes of the ground state and triplet excited state are not changed by the inclusion in %-CD, the interaction between fullerenes and %-CD is quite weak. It was revealed that the bimolecular excitation–relaxation and electron-transfer processes of the inclusion complex of fullerene in %-CD are changed in comparison to the pristine fullerenes [29]. For example, the rate constants for the triplet–triplet annihilation processes of C60 and C70 in %-CD are much smaller than

OV 2+

•+

PPV

kfbet

kfbet OV 2+

PPV

3

C60*

hν C 60

PPV•+

OV •+

Scheme 8. Electron-mediating system of C60 .

3.9. Water-Soluble Fullerenes Pristine fullerenes are not soluble in water, while advantageous properties are expected in the fields of biology. Thus, much effort has been devoted to prepare water-soluble fullerenes. Sufficient solubility in water was achieved by several manners: inclusion complex formation of fullerenes with

Figure 16. C60 encapsulated in %-cyclodextrin.

603

Photoexcitation Dynamics of Fullerenes

the diffusion-limiting rate of the solvent. Furthermore, the rate constant for electron transfer of C60 in %-CD with 1,4diaza[2.2.2]bicyclooctane (DABCO) is 1/10–1/100 of the corresponding rate constants of the pristine fullerenes. Thus, the %-CD is a retarder for the photoinduced processes of the fullerenes. Since %-CD acts as retarder for the back electron-transfer process, the radical anions of fullerenes in %-CD show long lifetimes compared to those of the pristine fullerenes. This aspect is favorable in the utilization of the radical anion of fullerenes as an electron mediator. Furthermore, the persistent radical anion formation was observed in the presence of triethanolamine as mentioned in Section 3.8. It was revealed that both the radical anion of C60 in %-CD and the radical cation of methyl viologen are present at the same time as the equilibrium in water media. Fullerenes are also known to form the inclusion complexes with calixarenes. In the case of the water-soluble inclusion complex of C60 and calixarene (cationic homooxacalix[3]arene), substantial interaction between fullerenes and the calixarene was observed in the ground state absorption spectrum [76]. Increase in the absorption intensity around 400–500 nm of C60 in calixarene can be attributed to the charge-transfer complex formation due to the &-electron system of calixarene. Strong interaction between the calixarene and fullerenes was also observed in the excited states. The triplet absorption peak of C60 in the calixarene appeared at 545 nm, which is largely blueshifted compared to that of pristine C60 . The triplet lifetime is as short as 50 ns. The substantial interaction between calixarene and included C60 was also observed in the singlet excited state as a large blueshift of the fluorescence peak. Pristine fullerenes become soluble in water when they are incorporated in a micellar system. It was confirmed that a micellar system of poly(vinylpyrrolidone) and C60 showed bioactivity, such as mutagenicity, lipid peroxidation, and DNA-strand scission, upon visible-light irradiation in the presence of O2 , via generation of the active oxygen species [7]. By introduction of the hydrophilic groups such as COO− to C60 , the C60 adducts are dissolved in water as clusters [77]. The cluster formation was excluded by using surfactants. Guldi reported that the excited singlet state properties of the clusters are not changed from the monomer state [77]. On the other hand, they showed acceleration of the decay of the triplet state of the adduct in the cluster due to triplet–triplet annihilation. Water-soluble clusters of fullerenes were also achieved by introduction of addend with positive charge [78]. It is interesting to note that the positively charged fullerene clusters showed fast reduction of the fullerene core in a cluster due to electron-attracting force. Furthermore, it has been reported that the fullerene with cationic addend showed cleavage of double-strand DNA via generation of the active oxygen species [79].

Introduction of dendritic branches to the fullerene core can improve solubility and processability. Furthermore, it also provides an additional functionality of the branches themselves [85]. For the fullerodendrimers, in which fullerene moieties are employed as focal points of dendrimers (Fig. 17), it has been reported that bimolecular processes of the dendrimers are varied to a great extent depending on the generation of the dendrimer moiety [86]. For example, photoinduced electron-transfer rate constants of fullerodendrimers with benzyl-ether type dendrons depend on dendrimer generation: the electron-transfer rate constants of the fourth generation were ca. 1/2–1/4 of the second generation. For the electron-transfer processes of the dendrimers, long-range electron-transfer processes were suggested from the comparison with the expectation by the semiempirical Rehm–Weller relation [42]. The findings indicate the shielding effect of the dendron groups. It should be pointed out that these shielding effects are also observed for other bimolecular processes such as triplet–triplet annihilation and energy-transfer processes and are effective for dendron groups larger than the third generation as reported for Fréchet-type dendrimers. The shielding effect of the dendrimer is important from the viewpoint of drag delivery, since it retains chemically active species for long time. As for the monomolecular processes of the fullerene moiety of the dendrimers, such as singlet and triplet lifetimes,

O

O

O

O

H

O

O

Second Generation

O

O O

O

O

O

O

H

O

O

O

O

O O

O

Third Generation

O

O

O

O

O

O

O O

O

O O

O

H

O O

O O

O

O

O O

O

O

O O

O

3.10. Fullerodendrimers

O

Dendrimers have attracted wide attention because of their artificial three-dimensional nanostructures [80]. Fullerenes have been employed as a core, in branches, or as terminals of dendrimer molecules, that is, fullerodendrimers [81–87].

Fourth Generation

O

O

O

O

Figure 17. Molecular structures of fullerodendrimers.

604

Photoexcitation Dynamics of Fullerenes

substantial dependence on the dendrimer generation was not observed, indicating that the effects of the dendron groups on the excited state properties of the fullerene moiety are small [86]. Introduction of the dendrimer moiety to the fullerene is also beneficial to attain water-soluble fullerenes. Takaguchi et al. reported that fullerodendrimer bearing dendritic poly(amidoamine) substitute shows substantial solubility in water [87]. It was confirmed that the singlet oxygen was generated by the energy transfer from triplet excited fullerene moiety of the dendrimers. Furthermore, since their dendrimer generates pristine C60 by reversible Diels–Alder reaction, their dendrimers are interesting from the biological point of view. By introduction of functionality on the dendron groups, electronic interaction between the dendron groups and fullerene moiety becomes evident. Guldi et al. reported the photoinduced charge separation and energy-transfer processes in the fullerodendrimers having phenylene vinylene dendron groups [85]. The fast and highly efficiently intramolecule processes are interesting as summarized in the next section. Further interesting properties are expected for these fullerodendrimers by adding functions both to core and branch moieties.

4. PHOTOINDUCED PROCESSES OF FINE PARTICLES OF FULLERENES 4.1. Photochemistry of Fullerene Fine Particles Fullerenes do not show sufficient solubility in the conventional organic solvents. Thus, it has been reported that fullerenes generate aggregates in poor solvents [88]. By utilizing this aspect of fullerenes, one can obtain fullerene fine particles easily. The fine particles of the fullerenes are prepared by injection of fullerene in o-dichlorobenzene solution to continuously stirring ethanol [89, 90]. The fine particles of C60 have granular-type shape and the average diameter was estimated to be 270 nm (Fig. 18). The crystalline structure of the C60 fine particles was confirmed by X-ray scattering and

transmission electron microscope (TEM) observation. In the TEM picture, it was confirmed that the crystalline structure is retained throughout the fine particle. In the case of C70 , the generation of the fine particles was also confirmed. The average diameter of the C70 fine particles is estimated to be 140 nm. It was revealed that the fine fullerene particles several tens of nanometers in diameter can be obtained by using supercritical conditions. The ground state absorption spectrum of the C60 fine particles is quite broad compared to the isolated C60 in toluene. The spectral features of the C60 fine particles are similar to those of the C60 bulk crystals. These findings indicate that substantial interactions are present between the C60 molecules in the fine particles. It is interesting to note that the absorption peak of the C60 fine particles shifts to the shorter wavelength side compared to that of the C60 bulk crystal. The shift of the absorption peak will indicate the size effect of the fine particles as observed with the fine crystals of perylene [91]. The fine particles of C60 showed weak fluorescence around 700–750 nm upon excitation with a picosecond laser. The fluorescence decay profile of the C60 fine particles shows the two-component-decay with 0.8 and 2.1 ns of the fluorescence lifetimes (Fig. 19). The two-component-decay can be attributed to the deactivation processes of the free exciton and self-trapped exciton levels as supposed for the organic crystalline materials. The transient absorption bands of the triplet excited state of the C60 fine particles can be observed by the nanosecond laser irradiation (355 nm). Immediately after the laser irradiation, the C60 fine particles show a quite broad absorption band around 700 nm (Fig. 20). The quite broad transient absorption band decays within 50 ns, and a sharp absorption band remains at 500 ns after the laser irradiation. From the comparison with the transient absorption spectrum of C60 in toluene, the sharp absorption band at 750 nm can be attributed to the localized triplet excited state of C60 in the fine particles. The fast-decaying component can be attributed to the triplet–triplet annihilation process in the fine particles, since the fast-decaying component can

Size distribution / %

40 30 20 10 0 70

178

502

1415

3990

Diameter / nm Figure 18. Size distribution of the fine particles of C60 . Inset: scanning electron microscope (SEM) picture. Reprinted with permission from [90], M. Fujitsuka et al., J. Photochem. Photobiol. A: Chem. 133, 45 (2000). © 2000, Elsevier Science.

Figure 19. Fluorescence decay profile of the fine particles of C60 . Inset: Schematic diagram of free exciton and self-trapped exciton. Reprinted with permission from [90], M. Fujitsuka et al., J. Photochem. Photobiol. A: Chem. 133, 45 (2000). © 2000, Elsevier Science.

605

Photoexcitation Dynamics of Fullerenes

C60*

0.30

0.30 3 3

Absorbance

C60*

0.20

Abs

C60*

0.20 0.10 0.00

0.00 400

0.0 0.2 0.4 0.6 0.8

time/ µs

0.10

500

600

700

800

900

1000

Wavelength / nm Figure 20. Transient absorption spectra of C60 fine particles at 50 ns (solid circle) and 500 ns (open circle) after 355-nm laser light irradiation. Inset: absorption-time profile at 740 nm. Scheme for the T-T annihilation in the C60 fine particles. Reprinted with permission from [90], M. Fujitsuka et al., J. Photochem. Photobiol. A: Chem. 133, 45 (2000). © 2000, Elsevier Science.

be attributed to the triplet excited state from the picosecond laser flash photolysis measurements, in which generation of the broad absorption band via the intersystem crossing process was observed. From the laser power dependence of the absorptiontime profiles of the C60 fine particles, it was confirmed that the fast-decaying part appears when the excitation laser power becomes high; only the slow-decaying part appears with lower laser power excitation. This finding indicates that the triplet–triplet annihilation process in the fine particles becomes apparent only when the density of the triplet excited states becomes sufficiently high by the higher laser power. On the other hand, the localized triplet excited state was not confirmed with the C70 fine particles. Furthermore, the fine particles of C60 with diameter smaller than 100 nm also did not show such localized triplet states. These findings indicate that the triplet–triplet annihilation process in the fine particles largely depends on the crystalline structure and diameter of the fine particles. When the dispersion of the C60 fine particles was saturated by oxygen, the absorption band of the localized triplet excited state was quenched. The quenching of the localized triplet excited state can be attributed to the energytransfer reaction from the triplet excited state to the oxygen generating the singlet oxygen. This finding indicates that the localized triplet excited state of the C60 fine particles is reactive to the substrate in the dispersion. The energytransfer process can be observed when the dispersion solution includes -carotene, a triplet energy acceptor. The energy-transfer process can be confirmed by the generation of the triplet excited state of -carotene with the concomitant decay of the localized triplet excited state of the C60 fine particles. The bimolecular quenching rate of the energy transfer between -carotene and the C60 fine particles was estimated to be 1.5 × 109 M−1 s−1 , indicating that the efficient energy transfer process takes place on the surface of the C60 fine particles.

Photoinduced electron-transfer processes of the C60 fine particles were also confirmed in the transient absorption spectra. When the nanosecond laser light was irradiated to the solution containing the C60 fine particles and DABCO, an electron donor, generation of the radical anion of the C60 fine particles was confirmed with a decrease of the triplet excited state of C60 localized in the fine particles (Fig. 21). This finding indicates that the localized triplet excited state is also active to the photoinduced electron-transfer processes. The bimolecular rate constant for the present electron transfer was estimated to be 2 × 108 M−1 s−1 , which is smaller than the rate constant between C60 and DABCO in solution (4 × 109 M−1 s−1 ). The smaller rate constant of the C60 fine particles can be attributed to the reaction process on the surface of the fine particles. The generated radical ions are finally recombined to form their neutral states. The photoinduced oxidation process of the C60 fine particles was also confirmed using the cosensitization reaction with the nanosecond laser flash photolysis. These observations indicate that the localized triplet excited states of the fine particles are reactive species to the molecules in the solution both for the electron-transfer and for the energy-transfer processes, although the rate constants are somewhat smaller than those for the reaction systems, in which both donor and acceptor are isolated molecules in the solution.

5. FULLERENE OLIGOMERS, HIGHER FULLERENES, AND METALLOFULLERENES 5.1. Fullerene Dimers and Trimers Fullerenes are also reactive to fullerene itself. Fullerene polymers have been synthesized by several methods [23, 92–94]. Investigations on properties of fullerene dimers C60 fine particle DABCO N 3

(a)

N

C60*

N

C

•+

•-

0.0660

N

∆Abs

3

∆Absorbance

C60 fine particle

0.04

0.02

0.00 600

0.15 720 nm 0.10 0.05 0.00 0.04 1080 nm 0.02 0.00 -0.4 0.0 0.4 0.8 1.2 1.6

time/ µs

700

800

900

1000 1100 1200

Wavelength / nm Figure 21. Transient absorption spectra of C60 fine particles in the presence of DABCO at 100 ns (solid circle) and 500 ns (open circle) after 355-nm laser light irradiation. Inset: absorption-time profiles. Scheme for the electron transfer process of the fine particles of C60 . Reprinted with permission from [90], M. Fujitsuka et al., J. Photochem. Photobiol. A: Chem. 133, 45 (2000). © 2000, Elsevier Science.

606

Photoexcitation Dynamics of Fullerenes

and trimers seem to be important to elucidate properties of one-dimensional and two-dimensional fullerene polymeric materials. From this viewpoint, several experimental and theoretical investigations have been reported on fullerene dimers and trimers. Fullerene dimer (C120 ) and trimer (C180 ) can be obtained by the high-speed vibration milling developed by Komatsu et al. (Fig. 22) [23, 95]. Ground state absorption spectra of C120 and C180 showed an absorption band around 700 nm which is characteristic of 1,2-adducts of C60 [96–98]. Furthermore, fluorescence spectra showed fluorescence bands similar to those of 1,2-adducts of C60 . fluorescence quantum yields were in the order: 1,2-adducts of C60 > dimer (C120 ) > trimer (C180 ) > C60 polymer (Table 1). In the triplet absorption spectrum of C180 , substantial broadening of the transient absorption band was observed (Fig. 23). This finding indicates that the interactions between the fullerene moieties are present in the excited triplet state. It was revealed that the extinction coefficient of 3 C180∗ is about 1/5 that of C60 . Ma et al. [96] reported that the C60 polymer did not show a transient absorption band upon excitation. Thus, properties of C180 can be regarded as intermediate between C60 and C60 polymers. In the case of C120 O, spectral features of ground state absorption and fluorescence are quite similar to those of 1,2-adducts of C60 [99]. Thus, the interactions between the fullerene moieties are also small in the ground and excited states. As for the triplet absorption band, on the other hand, transient absorption bands appear at 630 and 480 nm, which are blueshifted compared with those of C120 and 1,2-adducts. Furthermore, the triplet lifetime was estimated to be 160 ns, which is quite shorter than those of C120 and 1,2-adducts. These findings indicate a substantial interaction between the two C60 -moieties. Therefore, interaction between two C60 moieties depends largely on distance and orientation of two C60 -moieties. When C118 N2 , a dimer of azafullerene C59 HN, is excited with the nanosecond laser, transient absorption bands appeared at 1280, 1000, 880, and 680 nm which are quite

C120

C120O

different from azafullerene C59 HN: C59 HN shows a transient absorption peak at 750 nm along with a shoulder at 1050 nm (Fig. 24) [100]. It has been reported that the laser irradiation of C118 N2 generates C59 N radical [101]. However, it becomes clear that these absorption bands include the triplet excited state of C118 N2 , because these transient absorption bands are quenched in the presence of oxygen due to the triplet energy transfer generating singlet oxygen, which was observed in the near-IR emission spectra. Thus the different spectral features of the triplet excited C118 N2 and C59 HN indicate the interactions between two azafullerenyl cages. The decay lifetime of the triplet excited C118 N2 is 10 s, while that of C59 HN was 5 s. The ISC values were estimated to be 0.48 for both C118 N2 and C59 HN. It has been reported that the triplet excited C118 N2 and C59 HN work as sensitizers in the oxidation reactions of olefins; 2-methyl-2-butene and -terpinene undergo ene and Diels–Alder photooxygenation reactions, respectively, to produce the corresponding peroxides in the presence of a minute amount of C118 N2 or C59 HN (Scheme 9) [102]. Azafullerene C59HN or C118N2

C180

Figure 22. Molecular structures of C120 , C120 O, C118 N2 , and C180 .

3

(Azafullerene)* 3

Azafullerene oxide 1

O O

O2

O2

+ OOH

HOO

Scheme 9. Reaction scheme for oxidation reactions of olefins by C118 N2 or C59 HN sensitizer. Reprinted with permission from [102], N. Tagmatarchis and H. Shinohara, Org. Lett. 2, 3351 (2000). © 2000, American Chemical Society.

When the C120 is excited in the presence of N  N  N  N tetramethyl-1 4-phenylenediamine (TMPD), absorption bands of the C120 radical anion appear with the decay of the triplet state of C120 , indicating the electron transfer via the triplet excited state [97]. Spectral features of the C120 radical anion are similar to those of the 1,2-adducts of C60 , indicating that interaction among fullerene cages is also negligibly small in the radical ion state. This finding indicates that a minus charge of the C120 radical anion is localized on one fullerene cage of the dimer molecule as in the case of the excited states. It should be noted that the radical anion of C120 decayed by the back electron transfer to the ground state. On the other hand, the ground state reduction of C120 by TDAE resulted in decomposition of the C120 radical anion into C60 and C60 radical anion. Thus, the decomposition of the C120 radical anion is a slower reaction than the back electron-transfer process between the C120 radical anion and the TMPD radical cation. The rate for the decomposition should be slower than the order of 105 s−1 : C120

C118 N2

hν

✲ C −

reduction

120

+

✲ C + TMPD 120

+ TMPD

❅

❅ decomposition ❘ − ❅ C60 + C60

(12)

607

Photoexcitation Dynamics of Fullerenes Table 1. Photophysical properties of C60 , 1,2-adduct of C60 (C60 R), C120 , C120 O, and C180 . C60

C60 Ra

C120

C120 O

C180

Singlet Es (eV) (F (ns) F

1.7 1.2 3.2 × 10−4

1.7∼1.8 1.2∼1.3 (1.0–1.2) × 10−3

1.7 1.6 7.9 × 10−4

1.8 1.7 8.7 × 10−4

1.7 0.9 5.5 × 10−4

Triplet (TT (nm) T (M−1 cm−1 ) (T (s) ISC

750 1.6 × 104 55 1.0

680–700 (1.4–1.6) × 104 24–29 0.88–0.95

700 1.4 × 104 23 0.7 ± 0.1

680 7.7 × 103 0.16 0.48

700 2.7 × 103 24 0.74 ± 0.1

a

C60 (C3 H6 N)p-C6 H4 CHO [30]. Source: Reprinted with permission from [98], M. Fujitsuka et al., Chem. Lett. 384 (2001); © 2001, Chemical Society of Japan and [99], M. Fujitsuka et al., J. Phys. Chem. A 105, 675 (2001), © 2001, American Chemical Society.

In the case of C120 O, electron transfer was confirmed by the appearance of a new absorption band at 1000 nm in the presence of DABCO [99]. It became clear that the generated radical ions decayed predominantly by the back electron transfer at the diffusion-limiting rate. In Table 1, estimated properties of fullerene dimers are summarized as well as those of C60 , 1,2-adducts of C60 , and C180 . It becomes clear that the interaction between the fullerene moieties in the fullerene oligomers largely depends on C60 –C60 distance and orientation.

5.2. Higher Fullerenes Recently, photophysical and photochemical processes of C76 , C78 , C82 , and C84 (Fig. 25) have been investigated [103–107]. Compared with C60 and C70 , ground state absorption spectra of higher fullerenes are ranging to the near-IR region. Although absorption spectra of higher fullerenes depend on their size and symmetry, roughly saying, higher fullerenes are expected to have absorption edges at the longer wavelength side, suggesting the smaller highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gaps. These small HOMO–LUMO gaps of the higher fullerenes also accord with the small differences between the first oxidation and reduction potentials of higher fullerenes (Table 2)

[108]. Therefore, facile oxidations of fullerenes are expected as well as easy reductions. C76 showed transient absorption bands at >900, 625, and 550 nm upon subpicosecond laser light irradiation at 388 nm (150 fs fwhm) as shown in Figure 26. These absorption bands decayed quickly within 100 ps, and a broad absorption band remained around 550 nm. Since the 388-nm laser light pumps C76 into the higher singlet excited state (Sn ), the fast-decaying component can be attributed to an internal conversion process generating the lowest singlet excited state (S1 ) from the higher singlet excited state (Fig. 3). The slow decaying component corresponds to the deactivation process of the lowest singlet excited state to the ground and the triplet excited states. The rates for the fast- and slowdecaying components correspond to 83 ps and 2.6 ns of the lifetimes of Sn and S1 , respectively. A similar two-step decay process was also observed with C78 . In the triplet excited state, higher fullerenes show the absorption bands in the visible and near-IR regions (Fig. 27). It should be noted that the intensities of the transient absorption bands of higher fullerenes are quite low compared with those of C60 and C70 . These low signal intensities 0.30 C59HN

0.20 0.08

∆Abs

∆Absorbance

0.08

0.04

0.06

0.00

0.04 0.02

0

40 Time / µ s

80

∆Absorbance

0.10

0.10 0.00 C118N2

0.15 0.10 0.05

0.00 400

600

800

1000 1200 1400 1600

Wavelength / nm

0.00 400

600

800

1000 1200 1400 1600

Wavelength / mn Figure 23. Transient absorption spectra of C60 (triangle), C120 (open circle), and C180 (closed circle) at 100 ns after the laser light irradiation. Inset: Absorption-time profile of C180 at 700 nm. Reprinted with permission from [98], M. Fujitsuka et al., Chem. Lett. 384 (2001). © 2001, Chemical Society of Japan.

Figure 24. Transient absorption spectra of C59 HN and C118 N2 in toluene at 100 ns after the laser light irradiation. Reprinted with permission from [100], N. Tagmatarchis et al., J. Org. Chem. 66, 8028 (2001). © 2001, American Chemical Society.

608

Photoexcitation Dynamics of Fullerenes (a) ∆Abs(0.05/div.)

0 ps

400 ps

400 C76(D2)

C78(C2v ' )

500

600 700 800 Wavelength / nm

(b) ∆Abs(0.05/div.)

620 nm

900 nm

0

500

1000 1500 2000 2500 3000 Time / ps

Figure 26. (a) Transient absorption spectra of C76 in toluene upon femtosecond laser light irradiation [388 nm, full width at half maximum (FWHM) 150 fs). (b) Absorption-time profiles.

C84(D2)

Figure 25. Molecular structures of C76 (D2 ), C78 (C2 ), C82 (C2 ), and C84 (D2 ).

can be explained on the basis of low ISC values. Quite low ISC values seem to be a common feature of the higher fullerenes, in which the nonradiative deactivation process from the singlet excited states to the ground states may be an efficient pathway. The transient absorption bands of higher fullerenes are governed by the self-quenching process rather than the triplet–triplet annihilation [Eqs. (1) and (2)]. The estimated intrinsic triplet lifetimes of the higher fullerenes (Table 3) are shorter than those of C60 and C70 . Therefore, shorter triplet lifetimes seem to be a common feature of higher fullerenes. When the higher fullerenes are treated with TDAE, the radical anions of the higher fullerenes are generated: The generation of the radical anions of higher fullerenes was confirmed by the electron paramagnetic resonance (EPR) measurements. It should be noted that the radical anion

of C82 is also generated by TMPD or DABCO without photoirradiation. The generation of the radical anion of C82 by TMPD or DABCO can be attributed to the lower reduction potential of C82 compared with other fullerenes such as C60 and C70 , which do not generate the radical anions in the dark.

C 76(D2)

C 82(C 2 )

∆Absorbance

C82(C2)

900

C 84(D 2d )

Table 2. Half-wave potentials and (Eox − Ered ) for the fullerenes in 1,1,2,2-tetracholoroethane. C 84(D 2 )

E1/2 vs Fc/Fc + in volts

C60 C70 C76 C78 a C78 b C84 a

+2/+1

+1/0

0/−1

−1/−2

−2/−3

Eox − Ered

— 1 75 1 30 1 43 1 27 —

1 26 1 20 0 81 0 95 0 70 0 93

−1 06 −1 02 −0 83 −0 77 −0 77 −0 67

— — −1 12 −1 08 −1 08 −0 96

— — — — — −0 96

2 32 2 22 1 64 1 72 1 47 1 60

Major isomer. Minor isomer. Source: Reprinted with permission from [108], Y. Yang et al., J. Am. Chem. Soc. 117, 7801 (1995). © 1995, American Chemical Society. b

600

800

1000

1200

1400

1600

Wavelentgth / nm Figure 27. Transient absorption spectra of higher fullerenes, C76 (D2 ), C82 (C2 ), C84 (D2d ), and C84 (D2 ) in toluene at 100 ns after the laser light irradiation. Reprinted with permission from [103], M. Fujitsuka et al., J. Phys. Chem. A 101, 4840 (1997) and [106], J. Phys. Chem. B 103, 9519 (1999). © 1997, 1999, American Chemical Society.

609

Photoexcitation Dynamics of Fullerenes Table 3. Photophysical and photochemical properties of C60 , C70 , C76 , C78 , C82 , and C84 . Propertiesa

C60

C70

C76 (D2 )

C78 C2 

C82 (C2 )

C84 (D2d )

Singlet properties ES (eV) (Sn (ps) (S1 (ns) F

1.7 0.25c 1.2 0.00032d

1.8 <1 0.66 0.00052d

1.33 83 2.6 −e

0.9 28 1.0 −e

1.0

1.1

1.7 −e

−e

Triplet properties ET (eV) (T (s) ISC

1.5 143 f 1.0 g

1.5 118,000 f 0.97h

1.0–1.1 9.6 0.05i

<0.9 10 0.12i

0.9–1.0 56 <0.01

0.9–1.0 20 <0.01

1.6 × 109 2.0 × 109

9.6 × 108 1.2 × 109

3.5 × 109 2.6 × 109

n.r.j 1.9 × 109

n.r.j 1.5 × 109

1.8 × 109 1.6 × 109

Energy transfer rates Oxygenb -carotene

a ES  (Sn , (S1 , F , and ET refer to singlet energy, lifetime of Sn , lifetime of S1 , fluorescence quantum yield, and triplet energy, respectively. b In M−1 s−1 . c Data from [29]. d Data from [28]. e Difficult to estimate due to weak fluorescence bands. As for relative yield, see text. f Data from [35]. g Data from [10]. h Data from [38]. i Data from [104]. j No reaction.

The photoinduced electron-transfer processes with TMPD were also confirmed with higher fullerenes. The reaction rate constants are C60 ≈ C70 > C82 > C76 (Table 3). Based on the Rehm–Weller equation, free-energy changes for electron transfer from TMPD to the triplet excited fullerenes to TMPD are calculated to be −22, −22, −15, and −18 kcal mol−1 for C60 , C70 , C76 , and C82 , respectively. Therefore, the order of reaction rate constants reflects the free-energy changes for the electron-transfer processes. Oxidations of the higher fullerenes have been attempted by the two photoinduced processes: oxidation of excited higher fullerenes by the strong electron acceptor directly and oxidation by the sensitized reaction indirectly. For fullerenes, tetracyanoethylene (TCNE) can be utilized as an electron acceptor for direct photoinduced electron transfer. The generation of the radical cations of higher fullerenes was confirmed by their transient absorption spectra. In the case of C82 , oxidation of C82 in the ground state takes place with TCNE because of the lower oxidation potentials of C82 . Using N -methylacridinium hexafluorophosphate (NMA+ ) and biphenyl (BP) as sensitizer and cosensitizer, respectively, the radical cations of fullerenes are generated by the following reaction schemes [109]: NMA + +

h

−→ 1 NMA + ∗

BP + Cn

−→ BP +

+BP

−→ NMA + BP

+

5.3. Metallofullerenes Several kinds of endohedral metallofullerenes have been reported. Especially lanthanum-containing C82 ’s have been widely investigated. In the case of La@C82 , it was confirmed by the EPR study that three electrons of La transfer to the C82 cage, that is, La3+ and C3− 82 . Furthermore, fullerenes can also include two or three metal ions in one fullerene cage, such as La2 @C80 . In this section, photoexcitation and relaxation processes of La@C82 and La2 @C80 (Fig. 28) are summarized [110]. The steady-state absorption spectra of La@C82 show the absorption bands at 1412, 1002, and 636 nm, which are characteristic of the lanthanide metal-encapsulated C82 fullerenes. Since it has been reported that the change of central metal does not affect the positions of absorption maxima, the electronic transitions take place within the C82 3− . The absorption spectrum of La2 @C80 shows a broad band around 900 nm and a relatively sharp shoulder at 400– 450 nm, which are characteristic bands of endohedral metallofullerenes.

(13)

+ Cn

n = 60 70 76 and 82

(14)

The reaction rate constants for Eq. (14) are estimated to be ∼1010 M−1 s−1 for higher fullerenes, indicating that the radical cations of higher fullerenes are formed effectively as observed with other fullerenes (C60 and C70 ).

La@C82

La2@C80

Figure 28. Molecular structures of La@C82 and La2 @C80 .

610 Immediately after the laser light irradiation, the transient absorption band appeared at 780 nm with broad bands around 1500 and 840 nm. The transient absorption band decays according to two components. The decay rate constants of the fast- and slow-decaying components were 1 2 × 107 and 3 4 × 105 s−1 , respectively. The fast- and slowdecaying components may be due to the doublet and quartet states, respectively. In the case of La2 @C80 , the decay with two steps was also observed. The fast- and slow-decaying components can be attributed to the excited singlet and triplet states, respectively. On cooling the solution as low as −30 C, the intensity and decay rate were not increased. Thus, the movement of internal La ions does not affect the excited state properties much.

6. CHARGE SEPARATION AND RECOMBINATION PROCESSES OF DONOR-FULLERENE LINKED MOLECULES

Photoexcitation Dynamics of Fullerenes (a)

(b)

(c)

Me H S

Me2N

Ar

S

R

Ar N N Zn N N

S

N

S m

HN CO

Ar

(d)

(e)

t-Bu

Me N N

H

t-Bu

(f)

OCHN

NH N N HN

6.1. Donor-Fullerene Dyad Molecules Photoinduced processes of the donor-fullerene dyad molecules have been investigated widely [25–27]. In 1995, Williams et al. reported the intramolecular photoinduced charge-separation (CS) and charge-recombination (CR) processes of N N -dimethylaniline-C60 dyad molecules, in which the length between C60 and dimethylaniline was 3 or 11 --bonds (Fig. 29a) [111, 112]. They confirmed the CS process by observing a transient absorption band due to the radical cation of dimethylaniline. The CS proceeds from the singlet excited state of the C60 moiety, since the fluorescence lifetime due to C60 moiety becomes short. The CS rate constants were as fast as >1.6 × 1010 and 5.5 × 109 s−1 for the 3- and 11-bond system, respectively, indicating that the CS process of these dyads proceeds almost quantitatively. On the other hand, the lifetime of the CS state of the 11-bond system is 0.25 s. Williams et al. attributed the fast CS and long lifetime of the CS state to very strong electronic coupling with the bridge by the special symmetry properties of the fullerene &-system. As for the donor molecules, varieties of donors have been employed for the donor-fullerene linked molecules. Carotenoid, porphyrin, ferrocene, tetrathiafulvalene, oligothiophenes, aromatic amines, etc. have been employed for the dyad molecules [25–27, 113–124]. In these molecules, the quantum yields of the CS processes are close to unity. The lifetimes of the CS states are on the order of subnanosecond to microseconds. Imahori et al. reported that the reorganization energy of the dyad molecule including fullerene acceptor is small compared with that of reported electron acceptors such as benzoquinones due to the large &-electron system of fullerene. The small reorganization energy of the dyad molecules including fullerene is confirmed by analyses of the charge-transfer absorption and emission spectra of porphyrin-fullerene dyad molecules (Fig. 29b) in benzene [125]. The reorganization energy was estimated to be as small as 0.23 eV, which is comparable to the smallest values (0.22 eV) in the photosynthetic reaction

Me

R

N

N CH3

(g) Ar HNOC

Fe

N N Zn N

N

CONH

N CH3

Ar

(h) Me N N Me

O

Figure 29. Molecular structures of dyads and triads including C60 .

center. This feature seems to be one of the advantages of the fullerene-containing dyad molecules which are aiming at a long-lived CS state with high quantum yield for application to energy-storage systems or other sensitized reactions. Usually, the CS process proceeds from the singlet excited state of fullerene or donor moiety. The triplet excited state of fullerene moiety is also a precursor of the long-lived CS state of the porphyrin-C60 dyad [126], in which the CS rate from the triplet state was estimated to be 5.5 × 108 s−1 (Fig. 30). In the case of the oligothiophene-fullerene dyad molecules, it has been reported that the CS state of tetrathiophene-C60 dyad (Fig. 29c) shows lifetimes as long as 6.3 s in benzonitrile due to an equilibrium between the CS state and the excited triplet state of C60 (Fig. 31a) [120]. The long-lived CS states were also observed for the aromatic amine-C60 and retinyl-C60 dyads (Fig. 29d, e) [120–122, 124]. In the latter case the lifetime of the CS state is 20 s. The equilibrium between the CS state and the triplet state is supported by energetical consideration and some experimental facts, such as oxygen sensitivity and solvent polarity dependence of the CS state. The equilibrium is achieved when

611

Photoexcitation Dynamics of Fullerenes

6.2. Fullerene-Donors Triad and Tetrad Systems

1

ZnP*-C60

3

ZnP*-C60

CS

ISC

CS CS

hν

ZnP•+-C60•-

ZnP-1C60* ZnP-3C60* ISC

CS

ZnP-C60

Figure 30. Schematic energy diagram for CS processes of ZnP-C60 in benzonitrile.

both states have close energy levels. When the energy level of the CS state is shifted by changing solvent polarity, the equilibrium is not obtained and the lifetime of the CS state becomes quite short, <1 ns (Fig. 31b). It should be noted that the equilibrium processes and the CR to the ground state are competitive. Thus, the long lifetime components are one part of the generated CS state. For example, in the case of octathiophene-C60 dyad in benzonitrile, 85% of the CS state has lifetimes of 63 ps, while the long lifetime was 15% of the generated CS state. (a)

nT- 1C60* CS nT- 3C60*

nT•+-C60•-

hν

CR

nT-C 60 (b)

nT- 1C60* CS nT•+-C60•-

CR

nT- 3C60*

hν CR

nT-C 60 Figure 31. Schematic energy diagrams for CS and CR processes of oligothiophene(nT)-C60 in (a) polar and (b) moderatory polar solvents.

The long lifetime of the CS state was also achieved by the introduction of the multistep-electron-transfer system. Liddell et al. reported that carotenoid-porphyrinfullerene (C-H2 P-C60 ) triad molecule (Fig. 29f) generates the final charge-separated state (C + -H2 P-C60 − ) via the C-P + -C60 − upon excitation of porphyrine moiety [127]. In 2-methyltetrahydrofuran, the lifetime and the quantum yield for the generation of the final CS state were 170 ns and 0.14, respectively. The quantum yield and the lifetime become long and high in the ferrocene-zinc porphyrin-fullerene (Fc-ZnP-C60 , Fig. 29g). The lifetime and the quantum yield for the genera+ tion of the CS state (Fc -ZnP-C 60− ) were 7.5 s and 0.65, respectively [128]. A further long lifetime is expected for the tetramer including fullerene. The Fc-ZnP-ZnP-C60 tetrad shows lifetimes as long as 19 s in benzonitrile [129].

6.3. Supramolecular Dyad and Triad Systems By means of metal–ligand coordinate bonds, hydrogen bonds, etc., noncovalently linked donor–acceptor molecules have been reported [130]. To date, several noncovalently linked dyads are reported for fullerene derivatives [131– 135]. For example, zinc porphyrin was connected via a coordinate bond with pyridine moiety of fullerene derivatives. Intrasupramolecular charge separation processes have been confirmed by observing radical ion species in laser flash photolysis experiments. By utilization of metal coordinate bonding of zinc porphyrin, further functionalities can be donated to dyad molecules. For example, D’Souza et al. reported that supramolecular triad molecules could be obtained by forming a coordinate bond between zinc porphyrin and C60 porphyrin dyad with pyridyl group (Fig. 32) [135]. Furthermore, control of distance between the chromophores of zinc porphyrin-fullerene dyad by an axial coordination is also an interesting example of supramolecular systems [134]. Supramolecular dyad molecules are also available by utilizing host–guest chemistry. Konishi et al. reported that improvement of electron-transfer efficiencies could be attained in the electron-transfer systems composed of donors and fullerenes connected with calixarene, in which calixarene could capture the electron donor [136]. It is interesting to note that back electron transfer in these systems obeyed second order kinetics, indicating that the generated radical ions are solvated separately after electron transfer by deforming the supramolecular.

6.4. Light-Energy Conversion Using Donor-Fullerene Linked Molecules The long lifetime and high quantum yield of the CS state of these donor-fullerene linked molecules seem to be efficient charge-generation species in the photoactive devices. A photoelectrochemical cell has been developed using a gold electrode which is covered by self-assembled monolayers of porphyrin-C60 dyad with S–Au interaction [137].

612

Photoexcitation Dynamics of Fullerenes

N N O N N

N

N

H H N

N

Zn

+

N

N

Size distribution / %

40

30

20

10

0 10.0

708.2

2930.0

Figure 33. Size distribution of the fine particles of octathiophene(8T)C60 . Inset: SEM picture. Reprinted with permission from [141], M. Fujitsuka et al., J. Phys. Chem. B 105, 9930 (2001). © 2001, American Chemical Society.

N

N

N O N

N H H

N

N

3: 4'-pyridyl 4: 3'-pyridyl Figure 32. Supramolecular triad of zinc porphyrin and fullereneporphyrin dyad. Reprinted with permission from [135], F. D’Souza et al., J. Phys. Chem. B 106, 4952 (2002). © 2002, American Chemical Society.

Using methylviologen as a carrier in the electrochemical cell, 5 mW cm−2 of photocurrent was obtained. This kind of photocurrent generation was also reported for the cell using selfassembled monolayers of oligothiophene-C60 dyads [138]. Furthermore, the generated radical ion pair can be utilized for the electron-mediating process. For example, C 60− moiety of ZnP+ -C 60− dyad or ZnP+ -H2 P-C 60− triad donates an electron to hexyl viologen to generate a radical cation. On the other hand, ZnP+ moiety of the dyad or triad oxidizes NADH analogs. Thus, both ZnP-C60 and ZnP-H2 P-C60 act as efficient photocatalysts for the uphill oxidation of NADH by hexylviologen [139].

6.5. Photoinduced Charge Separation and Recombination Processes of Fine Particles of Donor-Fullerene Linked Molecules Photoinduced processes of the donor–acceptor linked molecules in condensed condition are important, since these photoactive molecules are expected to be used in a condensed form such as fine particles and assembled membranes. Thus, the investigations on the fine particles of the donor–acceptor dyad molecules give fruitful discussion on the light-energy conversion systems. Thomas et al. reported the photoinduced CS and CR processes of aniline-C60 dyad molecule (Fig. 29h) [140]. They

found that the CS process occurs within a few nanoseconds and that CS state has a lifetime, as long as several microseconds, due to the hopping of the radical ions among the molecules in each fine particle. In the case of fine particles of octathiophene-C60 (8T-C60 ) dyad (Fig. 33), the CS occurs within 1 ps upon subpicosecond laser light irradiation, indicating that the rate constant for the charge separation is quite fast (>1 × 1012 s−1 ), while CS in benzonitrile occurs at 1 1 × 1011 s−1 (Fig. 34) [141]. The fast CS process can be attributed to the intermolecular charge separation within each fine particle. On the other hand, the CR process occurs by a two-step process: Fast CR within a few picoseconds can be attributed to CR between the geminate radical ion pair, while the slow CR in the nanosecond regions can be attributed to the recombination after the hopping of the radical ions. The quite fast CS and CR processes of the oligothiophene-fullerene dyads can be attributed to the molecular arrangement in the fine particles. (a) 20 ps

∆Abs (0.02/div.)

N

600

8T radical cation

400 ps

700

800 900 Wavelength / nm

1000

1100

(b) ∆Abs

N Zn

171.2

Diameter / nm

1: 4'-pyridyl 2: 3'-pyridyl

N

41.4

850 nm

in benzonitrile fine particles

0

10 Time / ps

20

30

Figure 34. (a) Transient absorption spectra of fine particles of octathiophene(8T)-C60 upon subpicosecond laser light irradiation (388 nm, FWHM 150 fs). (b) Absorption-time profiles of fine particles of 8T- C60 (solid line) and 8T-C60 in benzonitrile (dot line). Reprinted with permission from [141], M. Fujitsuka et al., J. Phys. Chem. B 105, 9930 (2001). © 2001, American Chemical Society.

Photoexcitation Dynamics of Fullerenes

In the case of oligothiophene-fullerene dyads, oligothiophene and fullerene moieties seem to be located at the closest position since both oligothiophene and fullerene moieties are hydrophobic. On the other hand, aniline-fullerene dyad is considered to form a spontaneous self-assembly in such a way that the hydrophobic fullerene moieties come together, leaving the polar aniline moiety outside, in which the distance between aniline in a dyad and C60 in another dyad will be similar to that of the dyad molecule in solution. These findings indicate that the self-assembly of donor– acceptor linked molecules in the fine particles is an important factor governing the rates of the CS and CR processes.

7. CONCLUDING REMARKS In the present chapter, we summarized photoexcitation dynamics of the fullerenes and related materials. In the case of the conventional fullerenes, C60 and C70 , their photophysical and photochemical properties were well established: A lot of photochemical reactions have been cleared and they have been employed for many applications such as photobiology and photoactive molecular devices. The fine particles of C60 show interesting photophysical and photochemical properties due to the migration of the singlet and triplet excited states in each fine particle. As for the higher fullerenes and metallofullerenes, although reports on their photophysical and photochemical properties were scarce, further interesting properties are expected after accumulation of the experimental data. The dyads and triads including fullerene showed a long-lived CS state with higher quantum yield than ever reported for dyads and triads. Thus, application of the donor–acceptor linked molecules to devices such as the light-energy conversion system seems to be interesting.

GLOSSARY Back electron transfer Reverse reaction process of electron transfer. Original ground state is formed. Energy transfer Process donating singlet or triplet excitation energy from an energy donor to an energy acceptor. Fullerenes Molecules composed of carbon with footballlike shapes. Carbon allotrope of diamond and graphite. Most famous C60 was found in 1985. Laser flash photolysis Spectroscopic measurement of absorption spectral change in shorter time scale upon excitation with pulsed laser. Nowadays, time resolution from femto- to millisecond region is available. Metallofullerenes Fullerenes including metal ions in fullerene cage. Photoinduced electron transfer Process donating an electron from an electron donor to an electron acceptor upon photoexcitation of the donor or acceptor.

REFERENCES 1. H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, and R. E. Smalley, Nature 318, 162 (1985). 2. W. Krätschmer, L. D. Lamb, K. Fostiropoulos, and D. R. Huffman, Nature 347, 354 (1990)

613 3. A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glarum, T. T. M. Palstra, A. P. Ramirez, and A. R. Kortan, Nature 350, 600 (1991). 4. Y. Wang, Nature 356, 585 (1992). 5. P. M. Allemand, K. C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Konovan, G. Grüner, and J. D. Thompson, Science 253, 301 (1991). 6. W. J. Blau, D. J. Cardin, T. J. Dennis, J. P. Hare, H. W. Kroto, R. Taylor, and D. R. M. Walton, Phys. Rev. Lett. 67, 1423 (1991). 7. N. Miyata and Y. Yamakoshi, in “Fullerenes Vol. 5” (K. M. Kadish and R. S. Ruoff, Eds.), Vol. 97-42, p. 345. The Electrochemical Society, Pennington, NJ, 1997. 8. S. Takenaka, K. Yasmashita, N. Takagi, T. Hatta, and O. Tsuge, Chem. Lett. 321 (1999). 9. S. H. Friedman, D. L. DeCamp, R. P. Sijbesma, G. Srdanov, F. Wudl, and G. L. Keyton, J. Am. Chem. Soc. 115, 6506 (1993). 10. J. W. Arbogast, A. P. Darmanyan, C. S. Foote, Y. Rubin, F. N. Diederich, M. M. Alvarez, S. J. Anz, and R. L. Whetten, J. Phys. Chem. 95, 11 (1991). 11. R. J. Senssion, C. M. Phillips, A. Z. Szarka, W. J. Romanow, A. R. McGhie, J. P. McCauley, A. B. Smith, and R. M. Hochstrasser, J. Phys. Chem. 95, 6075 (1991). 12. T. W. Ebbesen, K. Tanigaki, and S. Kuroshima, Chem. Phys. Lett. 181, 501 (1991). 13. D. K. Palit, A. V. Sapre, J. P. Mittal, and C. N. R. Rao, Chem. Phys. Lett. 195, 1 (1992). 14. N. N. Dimitrijevic and P. V. Kamat, J. Phys. Chem. 96, 4811 (1992). 15. L. Biczok, H. Linschitz, and R. I. Walter, Chem. Phys. Lett. 195, 339 (1992). 16. R. J. Sension, A. Z. Szarka, G. R. Smith, and R. M. Hochstrasser, Chem. Phys. Lett. 185, 179 (1991). 17. T. Osaki, Y. Tai, M. Yazawa, S. Tanemura, K. Inukai, K. Ishiguro, Y. Sawaki, Y. Saito, H. Shinohara, and H. Nagashima, Chem. Lett. 789 (1993). 18. S. Nonell, J. W. Arbogast, and C. S. Foote, J. Phys. Chem. 96, 4169 (1992). 19. H. N. Ghosh, H. Pal, A. V. Sapre, and J. P. Mittal, J. Am. Chem. Soc. 115, 11722 (1993). 20. A. Watanabe and O. Ito, J. Phys. Chem. 98, 7736 (1994). 21. L. Smilowitz, N. S. Sariciftci, R. Wu, C. Gettinger, A. J. Heeger, and F. Wudl, Phys. Rev. B 47, 13835 (1993). 22. S. R. Wilson, D. I. Shuster, B. Nuber, M. S. Meier, M. Maggini, M. Prato, and R. Taylor, in “Fullerenes” (K. M. Kadish and R. S. Ruoff, Eds.), p. 91. Wiley, New York, 2000. 23. G. W. Wang, K. Komatsu, Y. Murata, and M. Shiro, Nature 387, 583 (1997). 24. A. M. Rao, P. Zhou, K. Wang, G. T. Hager, J. M. Holde, Y. Wang, W.-T. Lee, X. Bi, P. C. Eklund, D. S. Cornett, M. A. Duncan, and I. J. Amster, Science 259, 955 (1993). 25. N. Martin, L. Sánchez, B. Illescas, and I. Pérez, Chem. Rev. 98, 2527 (1998). 26. D. M. Guldi and P. V. Kamat, in “Fullerenes” (K. M. Kadish and R. S. Ruoff, Eds.), p. 225. Wiley, New York, 2000. 27. H. Imahori and Y. Sakata, Eur. J. Org. Chem. 2445 (1999). 28. D. Kim, M. Lee, Y. D. Suh, and S. K. Kim, J. Am. Chem. Soc. 114, 4429 (1992). 29. A. Masuhara, M. Fujitsuka, and O. Ito, Bull. Chem. Soc. Jpn. 73, 2199 (2000). 30. B. Ma and Y. P. Sun, J. Chem. Soc. Perkin Trans. 2, 2157 (1996). 31. C. Luo, M. Fujitsuka, A. Watanabe, O. Ito, L. Gan, Y. Huang, and C.-H. Huang, J. Chem. Soc. Faraday Trans. 94, 527 (1998). 32. G. Schick, M. Levitus, L. Kvetko, B. A. Johnson, I. Lamparth, R. Lunkwitz, B. Ma, S. I. Khan, M. A. Garcia-Garibay, and Y. Rubin, J. Am. Chem. Soc. 121, 3246 (1999). 33. M. Gevaert and P. V. Kamat, J. Phys. Chem. 96, 9883 (1992). 34. K. Tanigaki, T. W. Ebbesen, and S. Kuroshima, Chem. Phys. Lett. 185, 189 (1991).

614 35. D. McBranch, V. Klimov, L. Smilowitz, M. Grigorova, and B. R. Mattes, in “Fullerenes Vol. 3” (K. M. Kadish and R. S. Ruoff, Eds.), Vol. 96-10, p. 384. The Electrochemical Society, Pennington, NJ, 1996. 36. R. R. Hung and J. J. Grabowski, Chem. Phys. Lett. 192, 249 (1992). 37. R. R. Hung and J. J. Grabowski, J. Phys. Chem. 95, 6073 (1991). 38. K. D. Ausman and R. B. Weisman, Res. Chem. Intermed. 23, 4311 (1997). 39. D. Dubois, K. M. Kadish, S. Flanagan, R. E. Haufler, L. P. F. Chibante, and L. J. Wilson, J. Am. Chem. Soc. 113, 4364 (1991). 40. G. J. Kavarnos and N. J. Turro, Chem. Rev. 86, 401 (1986). 41. Q. Xie, E. Pélez-Cordero, and L. Echegoyen, J. Am. Chem. Soc. 114, 3978 (1992). 42. D. Rehm and A. Weller, Isr. J. Chem. 8, 259 (1970). 43. Y. P. Sun, C. E. Bunker, and B. Ma, J. Am. Chem. Soc. 116, 9692 (1994). 44. R. S. Sension, A. Z. Szarka, G. R. Simith, and R. M. Hochstrasser, Chem. Phys. Lett. 185, 179 (1991). 45. N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Science 258, 1474 (1992). 46. N. S. Sariciftci and A. J. Heeger, Int. J. Mod. Phys. B 8, 237 (1994). 47. A. Itaya, I. Suzuki, Y. Tsuboi, and H. Miyasaka, J. Phys. Chem. B 101, 5118 (1997). 48. J. W. Arbogast, C. S. Foote, and M. Kao, J. Am. Chem. Soc. 114, 2277 (1992). 49. H. Schwenk, S. S. P. Parkin,V. Y. Lee, and R. L. Greene, Phys. Rev. B 34, 3156 (1986). 50. P. Bowmer, M. Kurmoo, M. A. Green, F. P. Pratt, W. Hayes, P. Day, and K. Kikuchi, J. Phys. Condensed Mater. 5, 2739 (1993). 51. M. M. Alam, A. Watanabe, and O. Ito, J. Photochem. Photobiol. A: Chem. 104, 59 (1997). 52. Y. Sasaki, M. Fujitsuka, A. Watanabe, and O. Ito, J. Chem. Soc. Faraday Trans. 93, 4275 (1997). 53. Y. Sasaki, T. Konishi, M. Yamazaki, M. Fujitsuka, and O. Ito, Phys. Chem. Chem. Phys. 1, 4555 (1999). 54. T. Nojiri, A. Watanabe, and O. Ito, J. Phys. Chem. A 102, 5215 (1998). 55. T. Nojiri, M. M. Alam, H. Konami, A. Watanabe, and O. Ito, J. Phys. Chem. A 101, 7943 (1997). 56. M. Fujitsuka, Y. Yahata, A. Watanabe, and O. Ito, Polymer 41, 2807 (2000). 57. S. Komamine, M. Fujitsuka, O. Ito, and A. Itaya, J. Photochem. Photobiol. A: Chem. 135, 111 (2000). 58. K. Matsumoto, M. Fujitsuka, T. Sato, S. Onodera, and O. Ito, J. Phys. Chem. B 104, 11632 (2000). 59. H. Onodera, Y. Araki, M. Fujitsuka, S. Onodera, O. Ito, F. Bai, M. Zheng, and J.-L. Yang, J. Phys. Chem. A 105, 7341 (2001). 60. O. Ito, Y. Sasaki, Y. Yoshikawa, and A. Watanabe, J. Phys. Chem. 99, 9838 (1995). 61. T. Akasaka, W. Ando, K. Kobayashi, and S. Nagase, J. Am. Chem. Soc. 115, 10366 (1993). 62. T. Akasaka, Y. Maeda, T. Wakahara, M. Okamura, M. Fujitsuka, O. Ito, K. Kobayashi, S. Nagase, M. Kako, Y. Nakadaira, and E. Horn, Org. Lett. 1, 1509 (1999). 63. T. Akasaka, T. Kato, K. Kobayashi, S. Nagase, K. Yamamoto, H. Funasaka, and T. Takahashi, Nature 374, 600 (1995). 64. T. Akasaka, T. Suzuki, Y. Maeda, M. Ara, T. Wakahara, K. Kobayashi, S. Nagase, M. Kako, Y. Nakadaira, M. Fujitsuka, and O. Ito, J. Org. Chem. 64, 566 (1999). 65. M. Fujitsuka, O. Ito, Y. Maeda, M. Kako, T. Wakahara, and T. Akasaka, Phys. Chem. Chem. Phys. 1, 3527 (1999). 66. K. Mikami, S. Matsumoto, A. Ishida, S. Takamuku, T. Suenobu, and S. Fukuzumi, J. Am. Chem. Soc. 117, 11134 (1995). 67. K. Mikami, S. Matsumoto, Y. Okubo, M. Fujitsuka, O. Ito, T. Suenobu, and S. Fukuzumi, J. Am. Chem. Soc. 122, 2236 (2000). 68. S. Fukuzumi, T. Suenobu, M. Patz, T. Hirasaka, S. Itoh, M. Fujitsuka, and O. Ito, J. Am. Chem. Soc. 120, 8060 (1998).

Photoexcitation Dynamics of Fullerenes 69. C. Siedschlag, G. Torres-Garcia, C. Wolff, J. Mattay, M. Fujitsuka, A. Watanabe, O. Ito, L. Dunsch, F. Ziegs, and H. Luftman, in “Fullerenes Vol. 5” (K. M. Kadish and R. S. Ruoff, Eds.), Vol. 97-42, p. 296. The Electrochemical Society, Pennington, NJ, 1997. 70. M. Fujitsuka, C. Luo, and O. Ito, J. Phys. Chem. B 103, 445 (1999). 71. C. Luo, M. Fujitsuka, C.-H. Huang, and O. Ito, Phys. Chem. Chem. Phys. 1, 2923 (1999). 72. T. Konishi, M. Fujitsuka, and O. Ito, Chem. Lett. 202 (2000). 73. T. Konishi, M. Fujitsuka, O. Ito, Y. Toba, and Y. Usui, Bull. Chem. Soc. Jpn. 74, 39 (2001). 74. Z. Yoshida, H. Takekuma, S. Takekuma, and Y. Matsubara, Angew. Chem. Int. Ed. Engl. 33, 1597 (1994). 75. T. Andersson, M. Sundahl, G. Westman, and O. Wennerström, Tetrahedron Lett. 38, 7103 (1994). 76. S. D. M. Islam, M. Fujitsuka, O. Ito, A. Ikeda, T. Hatano, and S. Shinkai, Chem. Lett. 78 (2000). 77. D. M. Guldi, J. Phys. Chem. A 101, 3895 (1997). 78. D. M. Guldi, H. Hungerbühler, and K.-D. Asmus, J. Phys. Chem. A 101, 1783 (1997). 79. S. Takenaka, K. Yamashita, M. Takagi, T. Hatta, and O. Tsuge, Chem. Lett. 321 (1999). 80. S. Hecht and J. M. J. Fréchet, Angew. Chem. Int. Ed. 40, 74 (2001). 81. J.-F. Nierengarten, Chem. Eur. J. 6, 3667 (2000). 82. K. L. Wooley, C. J. Hawker, M. J. Fréchet, F. Wudl, G. Srdanov, S. Shi, C. Li, and M. Kao, J. Am. Chem. Soc. 114, 9836 (1993). 83. C. J. Hawker, K. L. Wooley, and M. J. Fréchet, J. Chem. Soc., Chem. Commun. 925, (1994). 84. E. Cloutet, Y. Gnanou, J.-L. Fillaut, and D. Astruc, Chem. Commun. 1565 (1996). 85. D. M. Guldi, A. Swartz, C. Luo, R. Gómez, J. L. Segura, and M. Martín, J. Am. Chem. Soc. 124, 10875 (2002). 86. R. Kunieda, M. Fujitsuka, O. Ito, M. Ito, Y. Murata, and K. Komatsu, J. Phys. Chem. B 106, 7193 (2002). 87. Y. Takaguchi, T. Tajima, K. Ohta, J. Motoyoshiya, H. Aoyama, T. Wakahara, T. Akasaka, M. Fujitsuka, and O. Ito, Angew. Chem. Int. Ed. 41, 827 (2002). 88. Y. P. Sun and C. E. Bunker, Nature 365, 398 (1993). 89. M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, A. Watanabe, and O. Ito, Chem. Lett. 1211 (1997). 90. M. Fujitsuka, H. Kasai, A. Masuhara, S. Okada, H. Oikawa, H. Nakanishi, O. Ito, and K. Yase, J. Photochem. Photobiol. A: Chem. 133, 45 (2000). 91. K. Kasai, H. Kamatani, Y. Yoshikawa, S. Okada, H. Oikawa, A. Watanabe, O. Ito, and H. Nakanishi, Chem. Lett. 1181 (1997). 92. N. Takahashi, H. Dock, N. Matsuzawa, and M. Ata, J. Appl. Phys. 74, 5790 (1993). 93. Y. Iwasa, T. Arima, R. M. Fleming, T. Siegrist, O. Zhou, R. C. Haddon, L. J. Rothverg, K. B. Lyons, H. L. Carter Jr. A. F. Hebard, R. Tycko, G. Daggagh, J. J. Krajewski, G. A. Thomas, and T. Yagi, Science 264, 1570 (1994). 94. S. Pekker, A. Jánossy, L. Mihaly, O. Chauvet, M. Carrard, and L. Forró, Science 265, 1077 (1994). 95. K. Komatsu, K. Fujiwara, and Y. Murata, Chem. Lett. 1016 (2000). 96. B. Ma, J. E. Riggs, and Y.-P. Sun, J. Phys. Chem. B 102, 5999 (1998). 97. M. Fujitsuka, C. Luo, O. Ito, Y. Murata, and K. Komatsu, J. Phys. Chem. A 103, 7155 (1999). 98. M. Fujitsuka, K. Fujiwara, Y. Murata, S. Uemura, M. Kunitake, O. Ito, and K. Komatsu, Chem. Lett. 384 (2001). 99. M. Fujitsuka, H. Takahashi, T. Kudo, K. Tohji, A. Kasuya, and O. Ito, J. Phys. Chem. A 105, 675 (2001). 100. N. Tagmatarchis, H. Shinohara, M. Fujitsuka, and O. Ito, J. Org. Chem. 66, 8026 (2001). 101. J. C. Hummelen, B. Knight, J. Pavlovich, R. Gonzalez, and F. Wudl, Chem. Commun. 1554 (1996). 102. N. Tagmatarchis and H. Shinohara, Org. Lett. 2, 3551 (2000).

Photoexcitation Dynamics of Fullerenes 103. M. Fujitsuka, A. Watanabe, O. Ito, K. Yamamoto, and H. Funasaka, J. Phys. Chem. A 101, 4840 (1997). 104. D. M. Guldi, D. Liu, and P. V. Kamat, J. Phys. Chem. A 101, 6195 (1997). 105. M. Fujitsuka, A. Watanabe, O. Ito, K. Yamamoto, and H. Funasaka, J. Phys. Chem. A 101, 7960 (1997). 106. M. Fujitsuka, A. Watanabe, O. Ito, K. Yamamoto, H. Funasaka, and T. Akasaka, J. Phys. Chem. B 103, 9519 (1999). 107. M. Fujitsuka, O. Ito, K. Yamamoto, and T. Akasaka, Recent Res. Devel. Phys. Chem. 4, 135 (2000). 108. Y. Yang, F. Arias, L. Echegoyen, L. F. P. Chibante, S. Flanagan, A. Robertson, and L. J. Wilson, J. Am. Chem. Soc. 117, 7801 (1995). 109. S. Nonell, J. W. Arbogast, and C. S. Foote, J. Phys. Chem. 96, 4169 (1992). 110. M. Fujitsuka, O. Ito, K. Kobayashi, S. Nagase, K. Yamamoto, T. Kato, T. Wakahara, and T. Akasaka, Chem. Lett. 902 (2000). 111. R. M. Williams, J. M. Zwier, and J. W. Verhoeven, J. Am. Chem. Soc. 117, 4093 (1995). 112. R. M. Williams, M. Koeberg, J. M. Lawson, Y.-Z. An, Y. Rubin, M. N. Paddon-Row, and J. W. Verhoeven, J. Org. Chem. 61, 5055 (1996). 113. H. Imahori, S. Cardoso, D. Tatman, S. Lin, L. Noss, G. R. Seely, L. Sereno, C. Silber, T. A. Moore, A. L. Moore, and D. Gust, Photochem. Photobiol. 62, 1009 (1995). 114. D. Kuciauskas, S. Lin, G. R. Seely, A. L. Moore, T. A. Moore, D. Gust, T. Drovetskaya, C. A. Reed, and P. D. W. Boyd, J. Phys. Chem. 100, 15926 (1996). 115. H. Imahori, K. Hagiwara, M. Aoki, T. Akiyama, S. Taniguchi, T. Okada, M. Shirakawa, and Y. Sakata, J. Am. Chem. Soc. 118, 11771 (1996). 116. N. V. Tkachenko, L. Rantala, A. Y. Tauber, J. Helaja, P. V. Hynninen, and H. Lemmetyinen, J. Am. Chem. Soc. 121, 9378 (1999). 117. J. Llacay, J. Veciana, J. Vidal-Gancedo, J. L. Bourdelnde, R. González-Moreno, and C. Rovira, J. Org. Chem. 63, 5201 (1998). 118. N. Martin, L. Sánchez, M. A. Herranz, and D. M. Guldi, J. Phys. Chem. A 104, 4648 (2000). 119. T. Yamashiro, Y. Aso, T. Otsubo, H. Tang, T. Harima, and K. Yamashita, Chem. Lett. 443 (1999). 120. M. Fujitsuka, O. Ito, T. Yamashiro, T. Aso, and T. Otsubo, J. Phys. Chem. A 104, 4876 (2000). 121. M. Fujitsuka, K. Matsumoto, O. Ito, T. Yamashiro, T. Aso, and T. Otsubo, Res. Chem. Intermed. 27, 73 (2001).

615 122. S. Komamine, M. Fujisuka, O. Ito, K. Moriwaki, T. Miyata, and T. Ohno, J. Phys. Chem. A 104, 11497 (2000). 123. T. Ohno, K. Moriwaka, and T. Miyata, J. Org. Chem. 66, 3397 (2001). 124. M. Yamazaki, Y. Araki, M. Fujitsuka, and O. Ito, J. Phys. Chem. A 105, 8615 (2001). 125. H. Imahori, N. V. Tkachenko, V. Vehmanen, K. Tamaki, H. Lemmetyinen, Y. Sakata, and S. Fukuzumi, J. Phys. Chem. A 105, 1750 (2001). 126. H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito, Y. Sakata, and S. Fukuzumi, J. Am. Chem. Soc. 123, 2607 (2001). 127. P. A. Liddell, D. Kuciauskas, J. P. Sumida, B. Nash, D. Nguyen, A. L. Moore, T. A. Moore and D. Gust, J. Am. Chem. Soc. 119, 1400 (1997). 128. M. Fujitsuka, O. Ito, H. Imahori, K. Yamada, H. Yamada, and Y. Sakata, Chem. Lett. 721 (1999). 129. H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata, and S. Fukuzumi, J. Am. Chem. Soc. 124, 5165 (2002). 130. M. W. Ward, Chem. Soc. Rev. 26, 365 (1997). 131. F. D’Souza, G. R. Deviprasad, M. S. Rahman, and J.-P. Choi, Inorg. Chem. 38, 2155 (1999). 132. N. Armaroli, F. Diederich, L. Echegoyen, T. Habicher, L. Marconi, and J.-F. Nierengarten, New J. Chem. 77 (1999). 133. T. Da Ros, M. Prato, D. M. Guldi, E. Alessio, M. Ruzzi, and L. Pasimeni, Chem. Commun. 635 (1999). 134. F. D’Souza, G. R. Deviprasad, M. E. El-Khouly, M. Fujituska, and O. Ito, J. Am. Chem. Soc. 123, 5277 (2001). 135. F. D’Souza, G. R. Deviprasad, M. E. Zandler, M. E. El-Khouly, M. Fujituska, and O. Ito, J. Phys. Chem. B 106, 4952 (2002). 136. T. Konishi, A. Ikeda, T. Kishida, B. S. Rasmussen, M. Fujitsuka, and O. Ito, J. Phys. Chem. A 106, 10254 (2002). 137. H. Imahori, S. Ozawa, K. Ushida, M. Takahashi, T. Azuma, A. Ajavakom, T. Akiyama, M. Hasegawa, S. Taniguchi, T Okada, and Y. Sakata, Bull. Chem. Soc. Jpn. 72, 485 (1999). 138. D. Hirayama, T. Yamashiro, K. Takimiya, Y. Aso, T. Otsubo, H. Norieda, H. Imahori, and Y. Sakata, Chem. Lett. 570 (2000). 139. S. Fukuzumi, H. Imahori, K. Okamoto, H. Yamada, M. Fujitsuka, O. Ito, and D. M. Guldi, J. Phys. Chem. A 106, 1903 (2002). 140. K. G. Thomas, V. Biju, D. M. Guldi, P. V. Kamat, and M. V. George, J. Phys. Chem. B 103, 8864 (1999). 141. M. Fujitsuka, A. Masuhara, H. Kasai, H. Oikawa, H. Nakanishi, O. Ito, T. Yamashiro, Y. Aso, and T. Otsubo, J. Phys. Chem. B 105, 9930 (2001).