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)

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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 ( 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