4876
J. Phys. Chem. A 2000, 104, 4876-4881
Solvent Polarity Dependence of Photoinduced Charge Separation in a Tetrathiophene-C60 Dyad Studied by Pico- and Nanosecond Laser Flash Photolysis in the Near-IR Region Mamoru Fujitsuka,† Osamu Ito,*,† Takashi Yamashiro,‡ Yoshio Aso,‡ and Tetsuo Otsubo*,‡ Institute for Chemical Reaction Science, Tohoku UniVersity, Katahira, Aoba-ku, Sendai, 980-8577, Japan and Department of Applied Chemistry, Faculty of Engineering, Hiroshima UniVersity, Higashi-hiroshima, 739-8527, Japan ReceiVed: January 11, 2000; In Final Form: March 9, 2000
Photoinduced charge separation and recombination processes in a tetrathiophene-C60 dyad molecule (4TC60) in various solvents were investigated by observing transient absorption spectra in the near-IR region. In polar solvents such as tetrahydrofuran and benzonitrile, charge separation occurred at a rate on the order of 1010 s-1, which decreased down to the order of 109 s-1 in moderately polar solvents. In nonpolar toluene, charge separation was not observed. The quantum yields for the charge separations were nearly unity in polar solvents. Charge recombination occurred in 100 ps - 10 ns; the shorter lifetimes occurred in the highly polar solvents. After recombination, the triplet excited state of C60 was generated predominantly. In polar solvents such as benzonitrile, the charge-separated state was also observed in the microsecond time region. To explain the unprecedented long lifetime of the second charge-separation step, an equilibrium between the chargeseparated state and the triplet excited state was taken into consideration.
Introduction Recently, fullerene (C60) has been adopted in several donoracceptor-linked molecules in order to realize an efficient photoinduced charge-separation process.1-6 In such dyad molecules, fullerene acts as an electron acceptor. As for the donor of the dyad molecules, aniline,1 carotenoid,2 porphyrin,2b,3,4 pyrazine,5 and so forth6 have been employed. In these molecules, the quantum yields of the charge-separation processes were close to unity. The lifetimes of the charge-separated states were on the order of subnanoseconds. Imahori et al. reported that the reorganization energy of the dyad molecule including the fullerene acceptor is small compared with other reported electron acceptors.4b This feature seems to be one of the advantages of the fullerene-containing dyad molecules, the use of which is aimed at attaining a long-lived charge-separated state with high quantum yield for application to energy-storage systems or other sensitized reactions. As for the donor moiety of the dyad molecule, several candidates are expected in addition to those listed above because many examples of photoinduced electrontransfer reactions between fullerene and donors have been reported.7-12 Studies on various donor-fullerene dyad molecules are expected to give important information in optimizing the charge-separation yield and lifetime. In the present study, we examined the photoinduced chargeseparation and recombination processes in a novel tetrathiopheneC60 linked dyad molecule (Figure 1) by time-resolved absorption and fluorescence measurements. Oligothiophenes have been confirmed to donate an electron to the triplet-excited fullerenes in solution in previous studies.13,14 In the polythiophene film doped with C60, ultrafast photoinduced charge generation was reported.15 The present dyad molecule has a rather strict structure due to a stiff oligothiophene structure, although the alkyl chain * To whom correspondence should be addressed. † Tohoku University. § Hiroshima University.
Figure 1. Molecular structures of 4T-C60, NMPC60, and 4T.
was introduced to realize higher solubility of the dyad molecule in various organic solvents. Furthermore, oligothiophenes are stable compared with other olefins such as carotenoid because of stabilization of the π-conjugated chain by the sulfur atom. By applying pico- and nanosecond laser flash photolysis techniques to the present dyad molecule, absorption bands due to the charge-separation species were successfully observed in the near-IR region. The solvent dependence of charge-separation and recombination rates are disclosed. Furthermore, unprecedentedly long charge separation in the microsecond time region is reported. Experimental Section Materials. Syntheses of the dihexyltetrathiophene-C60 dyad (4T-C60), N-methylpyrrolidino-C60 (NMPC60), and dihexyltetrathiophene (4T) were described in a previous paper (Figure 1).16 Other chemicals were of the best commercial grade available.
10.1021/jp000136j CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000
Charge Separation in a Tetrathiophene-C60 Dyad
J. Phys. Chem. A, Vol. 104, No. 21, 2000 4877
Figure 2. Absorption spectra of 4T-C60, NMPC60, and 4T in toluene. Superposition of NMPC60 and 4T is indicated by dash-dot line.
Apparatus. The time-resolved fluorescence spectra were measured by a single-photon counting method using the second harmonic generation (SHG, 410 nm) of a Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S, 1.5 ps fwhm) as an excitation source and a streakscope (Hamamatsu Photonics, C4334-01) equipped with a polychromator (Acton Research, SpectraPro 150) as a detector. The picosecond laser flash photolysis was carried out using a SHG (532 nm) of an active/passive mode-locked Nd:YAG laser (Continuum, PY61C-10, fwhm 35 ps) as the excitation light. A white continuum pulse generated by focusing the fundamental of the Nd:YAG laser on a D2O/H2O (1:1 volume) cell was used as the monitoring light. The visible monitoring light transmitted through the sample was detected with a Dual MOS detector (Hamamatsu Photonics, C6140) equipped with a polychromator (Acton Research, SpectraPro 150). For detection of the near-IR light, an InGaAs linear image sensor (Hamamatsu Photonics, C5890-128) was employed as the detector. The spectra were obtained by averaging 80 events on a microcomputer. The time resolution of the present system was ca. 35 ps. Nanosecond transient absorption measurements were carried out using a SHG (532 nm) of a Nd:YAG laser (Spectra-Physics, Quanta-Ray GCR-130, fwhm 6 ns) as an excitation source. For transient absorption spectra in the near-IR region (600-1600 nm), monitoring light from a pulsed Xe lamp was detected with a Ge-avalanche photodiode (Hamamatsu Photonics, B2834). Photoinduced events in micro- and millisecond time regions were estimated by using a continuous Xe lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu Photonics, G5125-10) as a probe light and detector, respectively. Details of the transient absorption measurements were described in our previous papers.12 All the samples in a quartz cell (1 × 1 cm) were deaerated by bubbling Ar through the solution for 15 min. Steady-state absorption spectra in the visible and near-IR regions were measured on a Jasco V570 DS spectrophotometer. Results and Discussion Steady-State Absorption Spectra. Figure 2 shows an absorption spectrum of 4T-C60 in toluene as well as those of 4T and NMPC60, which correspond to components of 4T-C60. The absorption peaks and shoulders of 4T-C60 were interpreted from the superposition of the components as indicated in Figure 2. A slight broadening of the absorption band of 4T-C60 was observed in the region of 380-470 nm compared with the superposition of the components, which indicates very weak interaction among the components due to a close distance. In the present laser flash photolysis study, excitation was carried
Figure 3. Fluorescence decay profiles around 710 nm of 4T-C60 in (a) toluene and (b) benzonitrile. Excitation is at 410 nm. Solid lines show fitted curves; hatched parts indicate laser profile.
out by the 532-nm laser light, which predominantly excites the C60 moiety of the dyad. Very weak interaction between C 60 and 4T moieties is in accord with the results of electrochemical analysis: Oxidation and reduction potentials of 4T-C60 (1.04 and -0.63 V vs Ag/AgCl in benzonitrile, respectively) are almost the same as the oxidation potential of 4T (1.07 V) and the reduction potential of NMPC60 (-0.63 V).16 The rigid tetrathienyl group may exclude structural variation, which brings about a weak interaction in the ground state, although in a film mixture composed of C60 and polythiophene or oligothiophene, substantial interaction among them was confirmed in the absorption spectra.15 Time-Resolved Fluorescence Measurement. In a previous paper, it was reported that the fluorescence peaks of unbonded 4T at 460 and 484 nm disappeared because of the connection with C60 in toluene.16 Thus, the fluorescence bands due to the C60 moiety were observed at 712 and 790 nm, which have the same lifetime as that of NMPC60 (Figure 3a). These findings indicate that the fast energy transfer from the singlet excited state of 4T to C60 occurs (eq 1) to generate the singlet-excited C60 moiety but not electron transfer in toluene: hν(
