11632
J. Phys. Chem. B 2000, 104, 11632-11638
Photoinduced Electron Transfer from Oligothiophenes/Polythiophene to Fullerenes (C60/C70) in Solution: Comprehensive Study by Nanosecond Laser Flash Photolysis Method Keisuke Matsumoto,† Mamoru Fujitsuka,*,† Tadatake Sato,‡ Shinji Onodera,† and Osamu Ito*,† Institute for Chemical Reaction Science, Tohoku UniVersity, Katahira, Aoba-ku, Sendai, 980-8577 Japan, and National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan ReceiVed: June 20, 2000; In Final Form: August 22, 2000
Photoinduced electron-transfer processes from oligothiophenes (nT)/polythiophene (poly-T) to fullerenes (C60/ C70) have been studied by the nanosecond laser flash photolysis method, observing the transient absorption spectra in the visible and near-IR regions. When fullerene was selectively photoexcited in polar solvents, electron transfer from nT to the excited triplet state of fullerene was confirmed. The electron-transfer rate constants increased with the number of repeating unit (n) of nT. On the other hand, the efficiency of electron transfer showed a maximal value at n ) 4; for n > 4, electron-transfer efficiency of nT decreased, indicating contribution of other processes such as energy transfer. By the photoexcitation of nT in polar solvent, both electron and energy transfer processes were observed for 4T and 6T. In the case of 3T, energy transfer occurred predominantly even in polar solvent. In nonpolar solvent, energy transfer was a predominant deactivation process. Electron-transfer efficiencies among these oligothiophenes and polythiophene were explained on the basis of free-energy changes for the electron transfers and triplet energy levels of nT.
Introduction Fullerenes (C60 and C70) are good electron acceptors both in their ground and excited states. By doping of fullerenes to the conjugated polymer films such as polythiophene, enhancement of photoconductive properties has been reported.1 Similar doping effects have been reported for poly(N-vinylcarbazole) (PVCz),2 poly(p-phenylene vinylene),3 and poly(2,5-dialkoxy-p-phenylene vinylene).4 The generation of photoinduced carrier in these polymers can be attributed to photoinduced electron transfer between fullerenes and conjugated polymers. As for polythiophene/oligothiophene films doped with fullerene, direct observations of the photoinduced electron transfer were reported by Sariciftci et al. using several experimental techniques including photoinduced absorption spectroscopy, photoinduced electron spin resonance, and transient photocurrent measurements.5 In series of their studies, they observed fast electron transfer due to excitation of charge-transfer complexes. In the composite films of polythiophene/oligothiophene with fullerene, most of fullerenes are expected to form charge-transfer complex due to high concentration of thiophene units. Furthermore, they confirmed photoinduced energy transfer from the triplet excited poly(3-alkylthiophene) to C60 in solution. Since polythiophene/ oligothiophenes have more intense and wide absorption in the visible region than C60 due to a degenerated π-conjugation system of thiophene backbone, main photoinduced reactions should proceed from the excited polythiophene/oligothiophene in the mixture system with C60. It is well-known that electronic structures of oligothiophenes largely depend on their number of polymer repeating unit.6 The HOMO-LUMO gap of the oligothiophenes becomes small as increase of the thiophene unit, which also changes oxidation potentials, singlet, and triplet energies.7 These are important * Authors to whom correspondence should be addressed. † Institute for Chemical Reaction Science, Tohoku University. ‡ National Institute of Materials and Chemical Research.
parameters governing electron and energy transfer processes.8 Furthermore, in the presence of these oligothiophenes, photoexcited fullerenes are also expected to participate in electron transfer,9 although photoinduced processes from excited fullerenes have not been well understood in polythiophene/oligothiophene-fullerene mixture systems. In the present paper, we carried out a comprehensive laser flash photolysis study on photoinduced electron transfer both via excited fullerenes and excited oligothiophene/polythiophene (Scheme 1) in solution by selective excitation using laser light at various wavelengths. To optimize photoinduced charge separation in the thiophene-fullerene system, detailed reaction parameters of electron-transfer processes were estimated. Furthermore, effect of the conjugation length of oligothiophene/ polythiophene on the electron- and energy-transfer processes was discussed on the basis of quantitative kinetic analyses. Experimental Section Materials. C60 (purity 99.5%) and C70 (99%) were purchased from Terms Co. and used as received. Poly(3-octylthiophene) (poly-T) and terthiophene (3T) were purchased from Aldrich. 3T was recrystallized. Dihexyltetrathiophene (4T) and dihexylsexithiophene (6T) were prepared by the method described in the literature.10 Concentrations of nT were calculated on the basis of oligomer unit unless otherwise stated. Other chemicals were of the best grade commercially available. Nano- and Microsecond Transient Absorption Measurements. The solutions containing C60 and oligothiophenes were excited by second-harmonic generation (532 nm) or thirdharmonic generation (355 nm) of a Nd:YAG laser (Quanta Ray GCR-130, fwhm 6 ns) or an optical parametric oscillation (Continuum Surelite OPO, fwhm 4 ns) pumped by a Nd:YAG laser (Continuum, Surelite II-10). For the measurements of the transient absorption spectra in the visible region, a Si-PIN photodiode (Hamamatsu Photonics, S1722-02) was used as a
10.1021/jp002228e CCC: $19.00 © 2000 American Chemical Society Published on Web 11/16/2000
Photoinduced Electron-Transfer from nT/(poly-T) to Fullerenes
J. Phys. Chem. B, Vol. 104, No. 49, 2000 11633
SCHEME 1: Molecular Structures of C60, C70, and nT (n ) 2, 3, 4, 6, and poly-) Investigated in the Present Study; Reduction and Oxidation Potentials and Triplet Energies (ET) Are Also Shown
detector for monitoring light from the pulsed xenon flash lamp. In the near-IR region, a Ge avalanche photodiode (APD) (Hamamatsu Photonics, B2834) was employed. Sub-millisecond phenomena were observed by using an InGaAs-PIN photodiode (Hamamatsu Photonics, G5125-10) and a Si-APD (Hamamatsu Photonics, S5343) as the detectors for the near-IR and visible light, respectively, from a continuous xenon lamp (150 W). Details of nanosecond laser flash photolysis system are described in the previous papers.11 Laser photolysis was performed for deaerated solutions obtained by Ar bubbling in a rectangular quartz cell with a 10 mm optical path at room temperature. Electrochemical Measurements. Cyclic voltammetry was carried out by using a potentiostat (Hokuto Denko, HAB-151) in a conventional three-electrode-cell equipped with Pt working and counter electrodes and an Ag/AgCl reference electrode at room temperature. Scan rate was 100 mV s-1. Sample solution was deaerated by Ar bubbling. Results and Discussion Steady-State Absorption Spectra. Figure 1 shows the absorption spectra of C60/C70 and thiophenes in o-dichlorobenzene (DCB). With increase of number of repeating unit (n) of oligothiophene, the absorption maximum showed shift to the longer wavelength close to that of poly-T, indicating extension of the delocalization of π-electrons. In the present study, to realize substantial solubility in organic solvents, alkyl groups were introduced to oligothiophenes. Perturbation to the π-conjugation-system by the alkyl groups seems to be small, since the absorption peaks and molar extinction coefficients () values were similar to those of nonsubstituted oligothiophenes.12 The values also increased with n of nT. In the case of poly-T, the value was calculated on the basis of the polymer unit, i.e., the value of poly-T in Figure 1 is a product of the value on the monomer unit and the averaged degree of the polymerization (∼700). The red-shift of the absorption maximum indicates that HOMO-LUMO gap becomes small, which agrees with the
Figure 1. Steady-state absorption spectra of (a) C60 and C70 and (b) 3T, 4T, 6T, and poly-T in DCB. Concentration of poly-T was estimated on the basis of polymer unit.
more cathodic oxidation potentials of the longer oligothiophenes as summarized in Scheme 1. Within the concentration ranges employed for the present laser photolysis experiments (fullerenes < 0.2 mM, and nT < 10 mM), appreciable spectral changes were not observed in absorption spectra of mixtures, suggesting that the ground-state interactions such as charge-transfer interactions are weak in the present sample solutions. In the case of 3T, 4T, and 6T, the 532-nm laser light excites C60/C70 only. In the case of poly-T, the 610-nm laser light was used for selective excitation of C60/C70. On the other hand, the 440-nm laser light excites mainly nT in the nT-C60 systems. For C70, it was impossible to perform predominate thiophenemoiety excitation. Triplet-Triplet Excitation. Triplet-triplet absorption spectra of the components were measured in less polar solvents, preventing the photoejection processes. For nT and poly-T, the triplet-triplet absorption spectra are shown in Figure 2, which appeared immediately after nanosecond laser excitation. The absorption maximum shows a red-shift as the n of nT increases. With increase of the repeating unit, the value of T-T absorption is also reported to increase.7 In the case of poly-T, the value of the T-T absorption band was not determined, since the number of the excited site on a single polymer chain cannot be estimated. Furthermore, in the present study laser photolysis of poly-T was carried out under the condition (laser power < 5 mJ/pulse) where the linearity between the laser power and absorption intensities of T-T absorption band is retained, to avoid effect of the intramolecular T-T annihilation13 (see Supporting Information). The lowest triplet energies (ET) of nT and poly-T were also evaluated by the energy-transfer method employing the Sandrofy
11634 J. Phys. Chem. B, Vol. 104, No. 49, 2000
Figure 2. Triplet absorption spectra of 3T, 4T, 6T, and poly-T observed by laser flash photolysis in DCB. For poly-T, concentration of poly-T was 0.5 mM (monomer unit) and laser power was 4 mJ pulse-1.
Matsumoto et al.
Figure 4. Transient absorption spectra observed by 532-nm laser irradiation of C60 (0.1 mM) in the presence of 4T (3 mM) in BN:DCB (1:1); 0.1 µs (filled circle) and 1 µs (open circle) after laser pulse. Inset: Time profiles.
SCHEME 2
Figure 3. Absorption spectra of radical cations of 3T, 4T, 6T, and poly-T in dichloromethane.
equation.14 The estimated triplet energies are listed in Scheme 1. Lower triplet energies were confirmed for longer oligothiophenes, indicating similar tendency to the singlet energies of oligothiophenes. Absorption Spectra of Radical Cations of Thiophenes. Absorption spectra of oxidized nT in dichloromethane were measured by stepwise addition of FeCl3. With decrease of absorbance of neutral molecules, new absorption bands appeared as shown in Figure 3. Complete replacement of the neutral thiophenes by the radical cations was accomplished after addition of two equivalents of FeCl3 as reported in the literature.15 In each nT•+, two absorption maxima are observed in the visible and near-IR regions. With an increase in n, both absorption peaks shift to longer wavelength. For poly-T•+, the near-IR absorption maximum may locate at longer wavelength than 1600 nm, which is a limit of our instrument. Another absorption maximum at 800 nm is similar position to that of 6T•+. The values of 3T•+ and 4T•+ are almost same;7 the value of 6T•+ is larger than those of 3T and 4T by a factor of 3. In the case of poly-T, the value of the radical cation was not determined from the same reason for the value of theT-T absorption band of poly-T. Fullerene Excitation in Mixture System. By the excitation of C60 with the 532-nm laser light in the presence of 4T in a mixture solvent of benzonitrile (BN) and DCB, a transient absorption band at 740 nm appeared immediately after the nanosecond laser pulse as shown in Figure 4. The 740-nm band was assigned to the triplet state of C60 (3C60*).16 With the decay of 3C60*, new absorption bands of 4T•+ and C60•- appeared at
640 and 1080 nm, respectively. In the time profiles in Figure 4, it is apparent that the decay of 3C60* is almost mirror image with the rises of C60•- and 4T•+, indicating that electron transfer takes place via 3C60*.9 The initial quick rise within laser duration at 640 nm was attributed to the tail of 3C60*, but not to electron transfer via 1C60*, because such a quick rise was not observed for C60•- at 1080 nm. Since the intersystem crossing rate from 1C * to 3C * is as fast as 8.2 × 108 s-1, electron transfer via 60 60 1C * is competitive with the intersystem crossing process only 60 when 4T concentration is higher than 200 mM on assuming that the rate of electron transfer via 1C60* is close to a diffusioncontrolled limit (kdiff ) 5.2 × 109 M-1 s-1 in BN:DCB). For 3T and 6T, the electron-transfer processes via 3C60* were also confirmed by the decay of 3C60* and the rises of C60•- and nT•+.17 As for reaction systems of C70, similar electron-transfer processes were confirmed by observing the decay of 3C70* (980 nm) and the rises of C70•- (1380 nm) and nT•+. In the case of poly-T, the 610-nm laser light excites predominantly C60. With the decay of 3C60* at 740 nm over 30 µs, the absorption bands at 880 and 1580 nm increased reaching the maxima around 30 µs. The latter 1580-nm band was assigned to poly-T•+, while the 880-nm band was attributed to superposition of the bands of 3poly-T* and poly-T•+. A characteristic band of C60•- was also observed at 1080 nm, which rises till ∼30 µs. These observations clearly indicate that electron transfer occurs via 3C60* from poly-T, although energy transfer also takes place concomitantly. In the case of C70/poly-T, the decay of 3C70* and the rises of the ion radicals were observed. Thus, electron transfer takes place via 3C60* or 3C70* as represented in Scheme 2, in which the possible deactivation processes of 3C60* or 3C70* are indicatedsenergy transfer and collisional quenching without electron transfer. The contribution of energy transfer would be anticipated to increase for nT with larger n, because of lower ET as discussed later sections.
Photoinduced Electron-Transfer from nT/(poly-T) to Fullerenes
J. Phys. Chem. B, Vol. 104, No. 49, 2000 11635
TABLE 1: Quenching Rate Constant (kq), Quantum Yield (Φet), ket () kq × Φet), and ∆Get for Electron Transfer via 3C60* and 3C * in BN and BN:DCB (1:1) 70 solvent
a
acceptor
BN
3
BN:DCB
3C
60*
BN
3C
70*
BN:DCB
3
C60*
C70*
donor
kq/109 M-1 s-1 a
Φet
ket/109 M-1 s-1 a
∆Get/kcal mol-1
3T 4T 6T 3T 4T 6T poly-T 3T 4T 6T 3T 4T 6T poly-T
0.025 1.6 3.2 0.0037 0.42 3.9 10b 0.2 2.0 2.8 0.021 0.96 1.9 13b
0.56 0.58 0.27 0.21 0.47 0.32 0.15 0.45 0.32 0.25 0.32 0.31 0.27 0.19
0.014 0.96 0.86 0.00078 0.2 1.2 1.5b 0.09 0.63 0.70 0.0067 0.30 0.51 2.5b
-3.28 -6.05 -9.74 -2.47 -5.23 -8.92 -8.92 -3.51 -6.28 -9.97 -2.70 -5.47 -9.15 -9.15
Estimation error: (5%. b Estimated on the basis of polymer unit.
Figure 5. Transient absorption spectra observed by 610-nm laser irradiation of C60 (0.1 mM) in the presence of poly-T (0.5 mM, monomer unit) in BN:DCB (1:1); 2.5 µs (filled circle) and 25 µs (open circle) after laser pulse. Inset: Time profiles.
Figure 6. Plots of [C60•-]/[3C60*] against [nT] for electron transfer via 3C60*. Concentration of poly-T was monomer unit.
Quantum Yields and Electron-Transfer Rates. The efficiency of electron transfer can be calculated from the ratio of the maximal concentration of the generated radical ions to the initial concentration of the triplet states of fullerenes.18 For example, the ratios of [C60•-]max/[3C60*]max are plotted against the concentration of nT as shown in Figure 6. The [C60•-]max/ [3C60*]max value is the function of concentration of nT as eq 1:
[C60•-]max/[3C60*]max ) ket [nT]/{kT0 + kq [nT]}
(1)
where kT0, kq, and ket are decay rate of 3C60* in the absence of
Figure 7. Transient absorption spectra observed by 355-nm laser irradiation of 3T (0.5 mM) in the presence of C60 (0.3 mM) in BN:DCB (1:1); 0.25 µs (filled circle) and 2.5 µs (open circle) after laser pulse. Inset: Time profiles.
nT, the bimolecular quenching rate constant of 3C60*, and electron-transfer rate constant, respectively. The kq value can be evaluated from the dependence of first-order decay rate of 3C * on the concentration of nT as summarized in Table 1. 60 When kT0 , kq [nT], the [C60•-]max/[3C60*]max value takes a constant value as shown in Figure 6. Therefore, the constant value refers to the quantum yield of electron transfer (Φet) via 3C *.18 Furthermore, the k value can be obtained from the 60 et following equation, ket ) kq × Φet. The evaluated Φet and ket values are summarized in Table 1, in which the Φet values for the reactions via 3C70* were also evaluated similarly. In the case of poly-T, the kq and ket values based on polymer concentration are listed. The Φet values are usually less than unity, which indicates that some deactivation processes of the triplet excited states are competitive to the electron-transfer process as shown in Scheme 2. In Table 1, the kq and ket values increase with an increase in n of nT, while the Φet values show a maximal value at n ) 4. The reason will be discussed in the latter section in relation to the lowest triplet energy of nT. Thiophene Excitation in Polar Solvent. By the excitation of 3T with the 355-nm laser in the presence of C60 in BN:DCB, the transient absorption band at 480 nm appeared immediately after nanosecond laser, which was assigned to 33T*.7 With the decay of 33T*, new absorption bands appeared at 740, 840, and 1080 nm, which were ascribed to 3C60*, 3T•+, and C60•-, respectively. From the time profiles in Figure 7, it is apparent that both energy and electron transfers take place via 33T*. The molar
11636 J. Phys. Chem. B, Vol. 104, No. 49, 2000
Matsumoto et al. TABLE 2: Quenching Rate Constant (kq), Quantum Yield (Φet), ket () kq × Φet), and ∆Get for Electron Transfer via 3nT* in BN:DCB (1:1). k via 3nT* in Bz en acceptor C60
donor 33T* 3
C70
4T* 36T* 3poly-T* 33T* 3 4T* 36T* 3 poly-T*
ket/109 ∆Get/ ken/109 kq/109 M-1 s-1 a Φet M-1 s-1 a kcal mol-1 M-1 s-1 a 8.5 2.9 1.7 6.4 3.4 2.6 1.5 6.3
0.0 0.24 0.30 c 0.0 0.18 0.33 c
0.70 0.51 c 0.52 0.50 c
-10.8 -11.5 -11.0 -8.23 -11.7 -12.4 -11.9 -9.15
14 16 nrb 4.9 6.6 nrb
a Estimation error: ( 5%. b No reaction. c The Φet and ket values for the electron transfer via 3poly-T* were not determined because the values of 3poly-T* and poly-T•+ were not available.
Figure 8. Transient absorption spectra observed by 440-nm laser irradiation of 6T (1 mM) in the presence of C60 (0.3 mM) in BN:DCB (1:1); 0.3 µs (filled circle) and 3 µs (open circle) after laser pulse. Inset: Time profiles.
SCHEME 3
ratio of the generated 3C60* and C60•- was calculated to be 93:7, indicating that the energy transfer is the predominant deactivation pathway of 33T*. The initial quick rise of the 740nm band can be attributed to 3C60* generated by the direct excitation with the 355-nm laser light, while direct excitation is minor. By the excitation of 6T with the 440-nm laser light in the presence of C60 in BN:DCB, a transient absorption band at 710 nm due to 36T* appeared immediately after the laser pulse as shown in Figure 8. Electron transfer via 36T* was confirmed by the decay of 36T* and the rises of C60•- and 6T•+. In Figure 8, evidence for energy transfer was not found. The time profiles in Figure 8 support that electron transfer predominantly takes place via 36T*. The initial quick rise at 780 nm of 6T•+ was attributed to the tail of 36T*, because the time profile at 780 nm is a superposition of the decay of 36T* with the rise of 6T•+. These processes are summarized in Scheme 3, in which relative contribution would be expected to change with nT. For nT with high ET, the contribution of energytransfer may increase. In the case of electron transfer via 3nT*, the Φet values were evaluated from the plots of [nT•+]max/[3nT*]max against the concentration of C60 or C70. The Φet values are summarized in Table 2 with the kq and ket values via 3nT*. The Φet and ket values for the electron transfer via 3poly-T* were not determined because the values of 3poly-T* and poly-T•+ were not available while electron-transfer processes were observed. The kq values tend to decrease with an increase in n of 3nT*. This trend is an opposite to the kq values for the electron-transfer processes via 3C * or 3C * (Table 1). For the electron-transfer processes 60 70 via 3nT* (n > 4), negatively larger free-energy changes are expected for shorter oligomers due to their larger triplet energies, which compensate anodic oxidation potentials of shorter oligomers. As for 33T*, larger triplet energy than 3C60*/3C70* is considered to be the driving force for the reaction at the diffusion limiting rate.
In Nonpolar Solvent. By the excitation of nT with the 440nm laser light in the presence of C60 in a nonpolar benzene (Bz), the transient absorption band due to 3nT* appeared immediately after the laser pulse. After several hundred nanoseconds, the absorption band of 3C60* appeared at 740 nm with the decay of 3nT* in the manner similar to Figure 7, indicating that energy transfer predominantly takes place from 3nT*. The rate constants for energy transfer (k ) via 33T* and en 34T* are listed in Table 2. The k values from 33T* or 34T* to en C60 are close to kdiff in Bz; however, the corresponding ken values to 3C70* are smaller than kdiff by factors of 3, which may be related to slightly higher ET of 3C70* than that of 3C60*.14 For poly-T, energy transfer via 3poly-T* was not observed; on the other hand, energy transfer from 3C60* to poly-T was possible as shown in Figure 5, because the ET level of 3poly-T* is slightly lower than that of 3C60*/3C70*. Free-Energy Change. The free-energy changes of electron transfer (∆Get) can be calculated by the Rehm-Weller equation (eq 2):8,19
∆Get (kcal mol-1) ) 23.06 [E(D/D•+) - E(A•-/A) e02/sa] - ET (2) where E(D/D•+), E(A•-/A), and e02/sa are oxidation potential of donor, reduction potential of acceptor, and Coulombic term, respectively. The estimated ∆Get values are summarized in Tables 1 and 2 for the processes via 3C60*/3C70* and 3nT*, respectively. For the electron-transfer processes via 3C60* or 3C *, ∆G becomes more negative with increase in n of nT as 70 et shown in Table 1. The observed log ket values are plotted against ∆Get as shown in Figure 9. The ket values increase with a decrease in ∆Get along the curve calculated by the semiempirical Rehm-Weller equation.8 The Φet values show a parabola-type curve on ∆Get values (dotted line in Figure 9). Usually, the Φet value is expected to increase for a negatively larger ∆Get value. In the electrontransfer system of excited fullerene and oligothiophene, negatively larger ∆Get value is obtained for longer oligothiophene because of its lower oxidation potential (see Scheme 1 and eq 2). On the other hand, the triplet energy of oligothiophene also becomes small with the number of the repeating unit (Scheme 1). Triplet energies of fullerenes are comparable to those of 6T and poly-T. Therefore, energy transfer should contribute to the quenching of the triplet excited fullerenes, indicating smaller Φet values of longer oligothiophenes. From contribution of these two factors, the Φet values are concluded to show a maximal value as indicated in Figure 9; the maximal value corresponds to electron transfer between 4T and triplet excited fullerenes.
Photoinduced Electron-Transfer from nT/(poly-T) to Fullerenes
J. Phys. Chem. B, Vol. 104, No. 49, 2000 11637 TABLE 3: Rate Constant (kbet) and ∆Gbet for Back Electron Transfer in BN and BN:DCB (1:1) radical anion
radical cation
kbet/109 M-1 s-1 a
∆Gbet/kcal mol-1
BN
C60
BN:DCB
C60
BN
C70
BN:DCB
C70
3T 4T 6T 3T 4T 6T poly-T 3T 4T 6T 3T 4T 6T poly-T
6.0 2.6 5.6 6.9 2.9 (12)b 8.9 8.6 5.6 1.2 2.3 7.6 1.4 5.1
-33.9 -29.0 -25.1 -33.5 -30.7 -27.0 -27.0 -38.3 -27.4 -23.5 -32.6 -29.8 -26.1 -26.1
solvent
a Estimation error: ( 5%. b Accurate measurement was difficult because of low intensity.
Figure 9. Plots of ket (open marks) and Φet (filled marks) against ∆Get for electron transfer via 3C60* or 3C70*. The solid line was calculated from the Rehm-Weller equation. The dot line was a supposed curve for Φet vs ∆Get.
Figure 11. Plots of ket against n for electron transfers via 3C60* and 3 C70*. The ket values of poly-T were estimated on the basis of polymer unit.
Figure 10. Second-order plot for the decay of C60•- at 1080 nm in the presence of 3T•+ in deaerated BN:DCB (1:1). Inset: Time profile.
The ∆Get values via 3nT* are more negative than those via 3C60* or 3C70*; however, no clear tendency was found from the limited data set. Back Electron Transfer. The absorbance of the ion radical begins to decrease after reaching a maximum as shown in the inset of Figure 10. The decay of the radical ion can be attributed to the back electron transfer, because any evidence for other reaction pathway was not confirmed in the steady-state absorption of the solution after the laser experiments. The decay-time profile obeys second-order kinetics (Figure 10), which indicates that each ion radical is solvated as free ion radical. The slope of the linear line corresponds to 2kbet/ir, where ir is the molar extinction coefficient of the ion radicals. On substituting the reported ir values,20 the kbet values are evaluated as listed in Table 3. The kbet values are all close to the diffusion-controlled limit, which supports that the back electron-transfer process is endothermic as was anticipated from free-energy change for back electron-transfer process (∆Gbet) calculated from the difference of oxidation and reduction potentials (Table 3). Effect of Degree of Oligomerization. Among the rate constants and quantum yields in Tables 1, 2, and 3, the ket values seem to vary regularly with n of nT. In Figure 11, the ket values via 3C60* and 3C70* are plotted against n of nT. With increase of n, the ket values increase monotonically till n ) 6. The ket
values 6T are close to poly-T, which can be rationalized from the fact that 6T and poly-T have similar oxidation potentials and triplet energies (Scheme 1), resulting in similar ∆Get values. Spectroscopic data shown in Figures 1-3 indicate that the π-conjugation of poly-T extends more than 6T. If 6-8 thiophene rings act as a unit in poly-T as an electron donor, there are about 100 units in a polymer chain, which lead the decrease in the calculated ket values down to (1.5-2.5) × 107 M-1 s-1. In the case of poly-T, each thiophene moiety is substituted by a long alkyl group; thus, steric hindrance in the bimolecular electron-transfer reaction must be taken into consideration. In the present case, poor mobility of polymer in solution did not decelerate the electron-transfer rate extensively, because C60 and C70 move freely in solution. Solvent Effects. In the present study, we measured the rate parameters in three solvents; highly polar BN, moderately polar BN:DCB(1:1), and nonpolar Bz. Energy transfer predominantly takes place in Bz; the ken values from 3nT* to C60 and C70 are all close to kdiff (Table 2), which is rationalized from the fact that the triplet energy of shorter oligomers is higher than that of C60/C70 as indicated in Scheme 1. The inferior electron transfer can be attributed to smaller stabilization energy of radical ion pair in nonpolar solvents resulting in positive ∆Get values. As for electron transfer in polar solvents, the Φet values in BN are slightly larger than those in BN:DCB (1:1) to BN, indicating stabilization of radical ions in polar BN. Energy Diagram. An energy diagram for electron and energy transfers is schematically illustrated in Figure 12 for C60, in
11638 J. Phys. Chem. B, Vol. 104, No. 49, 2000
Matsumoto et al. Supporting Information Available: Laser power dependence of T-T absorption band of poly-T in DCB. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 12. Schematic energy diagram for electron transfer processes via 3C60* and 3nT* in BN:DCB (1:1).
which the energy values were calculated relative to the ground state. From this energy diagram, it is presumed that for shorter nT electron transfer via 3nT* is more effective than that via 3C *. Indeed, slightly larger k values via 3nT* were observed 60 et for 4T. As for larger oligomers, the ∆Get values via 3C60*/3C70* and via 3nT* are comparable due to similar triplet energies. In Figure 9, parabola-type tendency was found for Φet via 3C60* and 3C70*, which indicates that the competitive energy transfer from 3C60*/3C70* to 6T/poly-T has large contribution, because of comparable triplet energy levels. Conclusion In the present study, it becomes clear that both triplet excited fullerenes and triplet excited polythiophene/oligothiophenes contribute to photoinduced electron-transfer processes. In the case of the electron-transfer processes via the triplet excited fullerenes, yields for electron transfer showed maximum around n ) 4, while kq values increased with an increase in n value of nT. Smaller electron-transfer yields of longer oligomers indicate the contribution of other deactivation processes such as energy transfers. The ket values of poly-T were similar to those of 6T, which can be rationalized from the fact that the triplet energies and oxidation potentials of poly-T and 6T are quite similar, resulting in similar ∆Get. In the case of the electron transfer via excited thiophenes, contribution of energy transfer became small for longer oligomers due to their lower triplet energies. These findings indicate that the triplet energy is also an important parameter as well as ∆Get in optimization of electrontransfer yield between fullerenes and oligothiophenes. Acknowledgment. The present work was partly supported by a Grant-in-Aid on Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (Nos. 10207202, 11740380, and 12875163). The authors are also grateful for financial support by Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Corporation.
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