J. Phys. Chem. 1996, 100, 15309-15313
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ARTICLES Excited States of Poly(3-octylthiophene) Studied by Picosecond Time-Resolved Absorption Spectroscopy Using a Streak Camera Takuo Kodaira, Akira Watanabe,* and Osamu Ito* Institute for Chemical Reaction Science, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-77, Japan
Motoyuki Watanabe, Haruhisa Saito, and Musubu Koishi Hamamatsu Photonics K.K., System DiVision, 812 Joko-cho, Hamamatsu 431-31, Japan ReceiVed: March 6, 1996; In Final Form: June 28, 1996X
The excited states of π-conjugated poly(3-octylthiophene) were studied by laser flash photolysis technique. The nanosecond time-resolved absorption spectra for poly(3-octylthiophene) showed the triplet-triplet (TT) absorption band (λmax 800 nm) in the near-IR region. The intersystem crossing process was investigated by picosecond time-resolved absorption spectroscopy. The absorption-time profile obtained by picosecond laser flash photolysis of poly(3-octylthiophene) using a streak camera showed an initial rising component just after the photolysis and a slow growing component within a few nanoseconds. They were assigned to excited singlet and triplet states, respectively. The absorption-time profiles were analyzed by curve-fitting considering the decay of the excited singlet state and the growth of the excited triplet state. The intersystem crossing rate constant from the excited singlet state to the excited triplet state was determined to be 1.54 × 109 s-1 by the curve-fitting. The S1-Sn and T-T absorption bands were depicted separately using the fitting parameters.
Introduction Polythiophene is a typical π-conjugated polymer which shows electrical conductivity caused by the conjugation along the main chain. There are many studies of the conduction mechanism for polythiophene. The polaron and bipolaron theory have been applied suitably to polythiophene having a one-dimensional π-conjugated main chain.1-3 On the basis of a chemical concept, the positive polaron and bipolaron states can be recognized as radical cation and dication states, respectively.4,5 The doping process can be also regarded as an electron transfer process between dopants (electron donor or acceptor) and conjugated polymers. The ionization process and the absorption band have been investigated using thiophene oligomers as a model of polythiophene. The transition energy of the radical cation band of thiophene oligomers decreased with increasing the sequence length of thiophene ring, which corresponds to the increase of the conjugation. The laser flash photolysis technique makes it possible to investigate the dynamics of the electron transfer process. Such studies have been carried out for thiophene oligomers in solution mainly. The electron transfer takes place between photoexcited states of thiophene oligomers and acceptors (donors). Scaiano, Evans, and coworkers have reported the photochemical generation of radical cations from R-terthienyl and related thiophenes.6-8 The radical cation formation via excited triplet states was discussed by quenching experiments using acceptors. Garnier et al. studied the photochemical behavior of various thiophene oligomers using laser flash photolysis.9,10 The photochemical polymerization of oligothiophene and dithienothiophene has been reported by Shimidzu et al.11 The photochemical behavior of X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)00703-4 CCC: $12.00
Figure 1. Chemical structure of poly(3-octylthiophene).
excited singlet and triplet states is important in the first step of the photochemical generation of radical ion site on the conjugated chain. In a solid-state material like a conjugated polymer film, the annihilation and trapping of the excited states following fast inter- and intramolecular migration processes disturb the observation of chemical features of conjugated polymers. The isolated polymer chain in solution shows us essential photochemical properties of conjugated polymers. However, usual conducting polymers are insoluble in common organic solvents. By introducing a large organic side chain to the main chain, some π-conjugated polymers become soluble. Poly(3-octylthiophene) is a typical π-conjugated polymer soluble to various solvents due to the long alkyl chain (Figure 1). The electrical and optical properties of poly(3-octylthiophene) films have been studied on the basis of the polaron and bipolaron theory.12,13 The photophysical and chemical properties of soluble π-conjugated polymers are attracting interest to the photophysical behavior of the excited state in relation to electroluminescent devices.14-16 The excited-state dynamics of singlet and triplet states is closely connected with the luminescence efficiency. © 1996 American Chemical Society
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Recently, several photophysical and chemical studies on the soluble π-conjugated polymers have been reported by using femto- and picosecond time-resolved spectroscopy.17-20 In this paper we report the photochemical properties of poly(3octylthiophene) in a dilute solution, where a one dimensional π-conjugated chain is isolated. The excited singlet and triplet states and the intersystem crossing process are studied by nanosecond laser flash photolysis and picosecond time-resolved spectroscopy using a streak camera. Experimental Section Materials and Sample Preparation. 3-Octylthiophene was obtained from Tokyo Chemical Industry Co., Ltd. Poly(3octylthiphene) was prepared by the polymerization of 3-octylthiophene using FeCl3 as a catalyst in CHCl3 at 30 °C for 2 h.12,13 The dark red polymer was purified by reprecipitation using MeOH as a precipitant. The molecular weight and the polydispersity were determined to be 2.0 × 104 and 2.54, respectively, by GPC using mono-disperse polystyrenes as standards. Poly(3-octylthiophene) was dissolved in spectroscopic grade tetrahydrofuran (THF), and the sample solution in a square quartz cell of 1 cm path length was deaerated with argon bubbling before measurements. All measurements were made at 22 °C. Nanosecond Laser Flash Photolysis in the Near-IR Region. Nanosecond laser flash photolysis experiments were carried out with a Nd:YAG laser (Quanta-Ray, GCR-130, 6 ns fwhm). The poly(3-octylthiophene) solution was excited at 355 nm, and the probe light from a xenon flash lamp through the sample cell was detected using a Ge-APD (germanium avalanche photodiode, Hamamatsu, B2834) module (Hamamatsu, C5331-SPL). The absorption-time profiles were recorded with a digitizing oscilloscope (HP 54510B, 300 MHz) and analyzed by a personal computer. The details of the experimental setup are described elsewhere.21 Picosecond Time-Resolved Spectroscopy Using a Streak Camera. Picosecond time-resolved absorption spectra were observed using a picosecond laser flash photolysis system which consists of an active/passive mode-locked Nd:YAG laser (Continuum, PY61C-10, 30 ps fwhm), optical delay lines, a polychromator (Acton Research Corp., SpectraPro-150), and a streak scope (Hamamatsu, C2830) with a high-speed streak unit (Hamamatsu, M2547) and a CCD (charge-coupled device) camera (Hamamatsu, C4880). A continuum probe light with relatively long duration (50 ns) is generated by the breakdown of Xe gas focusing the 1064 nm laser beam onto the Xe tube.22,23 Three-dimensional absorption and emission images can be obtained by using the streak scope system. The details of the experimental setup are described elsewhere.24 The time-resolved emission spectra were measured using an argon ion laser (Spectra-Physics, BeamLok 2060-10-SA) pumped Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S) with a pulse selector (Spectra-Physics, Model 3980), a harmonic generator (GWU-23PS), and a streak scope (Hamamatsu, C4334-01, sweep repetition rate 2 MHz). In the measurements, by setting a threshold level for an A/D-converted CCD camera signal, the photoelectron image can be clearly separated from the noise. The system enables photon counting measurements at simultaneous multiple wavelengths. Typical instrument response functions for this apparatus are 20 ps (fwhm), and the time-resolution of the detection within 5 ps can be obtained by using the deconvolution technique. The emission quantum yield was determined by using quinine bisulfate in a 0.1 N H2SO4 aqueous solution.
Figure 2. Nanosecond time-resolved absorption spectra in the nearIR region obtained by 355 nm laser pulse excitation of 0.5 mM poly(3-octylthiophene) in THF: (a) 250 ns and (b) 2.5 µs.
Figure 3. Absorption-time profiles at 800 nm obtained by 355 nm laser pulse excitation of 0.5 mM poly(3-octylthiophene) in THF: (a) argon-bubbled and (b) in air.
Results and Discussion The triplet-triplet (T-T) absorption spectra of oligothiophene have been reported by Scaiano and Garnier et al.6-10 The transition energy of the T-T absorption band is lowered with increasing the number of monomers in the thiophene sequence. The T-T absorption spectra of tetrathiophene, quaterthiophene, and sexithiophene show absorption maxima at 470, 560, and 680 nm, respectively. Before the picosecond measurements of the excited singlet and triplet states, the T-T absorption band of poly(3-octylthiophene) was confirmed by nanosecond laser flash photolysis. In the measurements, the existence of the transition band in the visible and the near-IR region from 600 to 1600 nm was examined using a Ge-APD as a detector. The formation of a radical cation of poly(3-octylthiophene) by photoejection in a polar solvent was checked. Figure 2 shows time-resolved absorption spectra obtained by 355 nm laser pulse excitation of poly(3-octylthiophene) in THF. The absorption band with maximum at 800 nm was quenched by oxygen as shown in Figure 3, and it is reasonably assigned to the T-T absorption band of the poly(3-octylthiophene) excited triplet state. A similar photoinduced absorption band at 1.5 eV (827 nm) has been reported for poly(3-octylthiophene) in xylene by Heeger et al. and has been assigned to the T-T absorption.20 This absorption band was also observed in a nonpolar solvent, cyclohexane, from which the assignment as a radical cation band by photoejection was rejected. The decay behavior of the transient absorption band depends on the excitation laser power because the T-T annihilation process depends on the concentration of the excited triplet state. The decay kinetics of the poly(3-octylthiophene) excited triplet state are complicated, and the decay curve cannot be fitted by simple first- or secondorder kinetics.25 The half-life τ1/2 of the excited triplet state decreases from 510 to 100 ns with aeration due to the quenching by oxygen as shown in Figure 3. The energy transfer experiments from donors to poly(3-octylthiophene) were carried out. However, it was difficult to elucidate the sensitization of the poly(3-octylthiophene) triplet state because a suitable donor could not be found due to the overlap of the broad absorption band of the poly(3-octylthiophene) with the donor. The observation of the sensitization was disturbed by the initial rise
Studies of Excited States of Poly(3-octylthiophene)
Figure 4. Picosecond time-resolved absorption spectra obtained by 355 nm laser pulse excitation of 0.5 mM poly(3-octylthiophene) in THF: (a) 0.0-1.0 ns, (b) 1.0-2.0 ns, and (c) 4.0-5.0 ns.
Figure 5. Absorption-time profiles for poly(3-octylthiophene) excited states.
of the intense T-T absorption band by the direct excitation of the poly(3-octylthiophene). In Figure 2, there is no absorption band of a radical cation by photoejection at a longer wavelength than that of the T-T absorption band of poly(3-octylthiophene). Even in a more polar solvent, benzonitrile, there is no photoejection of poly(3-octylthiophene). The transition energy of the T-T absorption for poly(3-octylthiophene) is lower than those of oligothiophenes due to the π-conjugation along the polymer main chain. We investigated the photochemical behavior of the excited singlet state of poly(3-octylthiophene) by the picosecond laser flash photolysis technique. Time-resolved absorption spectra and the absorption-time profiles obtained by 355 nm laser pulse excitation of poly(3-octylthiophene) in THF are shown in Figures 4 and 5, respectively. Figure 4 also shows absorption maximum at 800 nm which is assigned to the T-T absorption band. The negative optical density near 570 nm is due to the influence of the emission from the excited singlet state of poly(3-octylthiophene). In the measurements of the time-resolved absorption spectra, the influence of the emission band is canceled by subtracting the emission streak image from the streak image of the probe beam through the excited sample. However, the intense emission of poly(3-octylthiophene) interferes in the 570 nm region. Time-resolved emission spectra are shown in Figure 6. They show emission maxima at 570 nm with emission tails toward 700 nm. In Figure 4 there is a growth of the absorption band above 700 nm with an increasing delay time after laser pulse excitation. The picosecond time-resolved absorption spectroscopy using a streak camera and the breakdown of Xe gas as a probe22,23 has an advantage in obtaining a precise absorption-time profile when compared to obtaining such a profile by pump-probe techniques, where the absorption-time profile is reconstructed by the plots of the absorbance from transient spectra, and so it is usually difficult to solve the mixed kinetics in the absorption-
J. Phys. Chem., Vol. 100, No. 38, 1996 15311
Figure 6. Time-resolved emission spectra obtained by single photon counting measurements using a streak camera: (a) 0-100 ps, (b) 200300 ps, (c) 400-500 ps, (d) 600-700 (e), 800-900 ps, (f) 100011000 ps. Excitation is 423 nm. The inset figure is the logarithmic plot of the emission decay curve at 570 nm and the first-order fitting line.
time profiles. In Figure 5, the absorption-time profiles clearly show the growth of the T-T absorption band. Such growths of the absorption band are due to the intersystem crossing process from the excited singlet state to the excited triplet state. Another noticeable feature in Figure 5 is the existence of an initial rising component just after laser flash photolysis. The ratio of the initial rising component to the component showing the rather slow growth of the T-T absorption band increases at the longer wavelength region. Such an initial rising component must be assigned to the S1-Sn absorption band. The transient absorption spectra of the excited singlet and triplet states are overlapping as shown in Figure 4. We tried to depict each absorption band separately using kinetic parameters obtained by curve-fitting of the absorption-time profiles. The mixed kinetics, eq 1, considers the decay of the excited singlet state and the growth of the excited triplet state and was applied to the absorption-time profiles:
A(t) ) AS,0 exp(-kSt) + AS,0ΦT
()
T [1 - exp(-kTt)] (1) S
where kS and kT are rate constants for the singlet decay and the triplet growth, respectively, and AS,0 is the initial absorbance of the singlet immediately after the laser excitation. The ΦT is the quantum yield of triplet state formation. The S and T are molar extinction coefficients of the singlet and excited triplet states, respectively. The rate constant kS can be determined from the emission decay analysis:
kS ) (1/τS) ) kE + kNR + kISC
(2)
where τS is the emission lifetime. The kE, kNR, and kISC are rate constants for emission, nonradiative deactivation, and intersystem crossing. The inserted figure in Figure 6 shows the logarithmic plot of the emission decay curve of poly(3octylthiophene) at 570 nm and the first-order fitting line. From the slope of the first-order plot, the kS value is determined to be 1.61 × 109 s-1 (τS ) 623 ps at 22 °C). Figure 7 shows the absorption-time profile at 760 nm and the fitting curve using eq 1 and the kS value determined from the emission decay. The initial rising of the absorption-time profile is well reproduced considering the decay of the excited singlet state. The decay and growth curves are depicted using fitting parameters for the excited singlet and the triplet states, respectively. Similar curve-fitting was carried out for absorption-time profiles in the region from 650 to 830 nm, and the kT values are plotted against the monitor wavelengths in Figure 8. If the growth of the absorption band in Figure 5 is attributable
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Figure 7. Absorption-time profile of poly(3-octylthiophene) excited states at 760 nm, the fitting curve considering the decay of the excited singlet state, and the growth of the excited triplet state. The decay and growth curves are depicted using fitting parameters.
Figure 9. Time-resolved absorption spectra for the excited singlet and triplet states: (a) excited triplet state and (b) excited singlet state.
Figure 8. Rate constant kT for the growth of excited triplet state obtained by curve-fitting using eq 1.
to the intersystem crossing from the first excited singlet state S1 to the first excited triplet state T1, the rate constant kT should be identical with kS. As shown in Figure 8, the kT values are in good agreement with kS (1.61 × 109 s-1) in the region from 720 to 830 nm. This result is the additional experimental evidence for the assignment of the absorption band located at 800 nm to the excited triplet state. At the wavelengths shorter than 720 nm, kT and kS values are in disagreement. This deviation must be caused by the interference of the intense emission. The high quantum yields of triplet state formation ΦT (0.880.99) have been reported for alkylthiophene oligomers.26 The sum of the emission quantum yield ΦE and the triplet quantum yield ΦT for alkylthiophene oligomers is unity within experimental accuracy, which suggests that the fluorescence emission and intersystem crossing to the excited triplet state are the only modes of decay from the excited singlet state.26 The ΦE of poly(3-octylthiophene) was determined to be 0.0418 in THF at 22 °C (370 nm excitation) using quinine bisulfate in a 0.1 N H2SO4 aqueous solution. The rate constant for emission kE (where kE ) ΦE kS) is calculated to be 0.0673 × 109 (s-1). In eq 2, the rate constant for nonradiative deactivation kNR is most temperature dependent. If the emission quantum yield at 77 K is assumed to be ΦE,0 ) kE/(kE + kISC), the quantum yield for nonradiative deactivation ΦNR is obtained by (1 - ΦE/ΦE,0). Using the emission lifetime at 77 K, τS,0 ) 1/(kE + kISC), the kNR is expressed by (1/τS - 1/τS,0). The time-resolved emission spectra of poly(3-octylthiophene) were measured at 77 K using a rigid glass of 2-methyltetrahydrofuran. The emission band of poly(3-octylthiophene) splits into two bands at 550 and 625 nm at 77 K. The emission lifetimes are 431 ps at 550 nm and 599 ps at 625 nm. The thermochromism of poly(3-octylthiophene) has been reported, and the emission bands may be attributed to a random coil structure and a linear chain structure, respectively.13 The quantitative comparison of emission lifetimes is difficult due to the spectral change. However, this result suggests that the rate constant for nonradiative activation kNR is negligible compared to the rate constants kE and kISC because the emission lifetime does not show any decrease with increasing
temperature. Such properties are similar to alkylthiophene oligomers.26 Therefore, the rate constant for intersystem crossing kISC and the quantum yield ΦT can be determined to be 1.54 × 109 s-1 and 0.958, respectively. The large ΦT value of poly(3-octylthiophene) is due to the intraannular heavy-atom effect of the sulfur atom. Recently, Heeger et al. have reported the intersystem crossing process of poly(3-octylthiophene) using subpicosecond timeresolved spectroscopy by the pump-probe technique in the region from 1.5 eV (827 nm) to 1.2 eV (1033 nm).20 The absorption-time profiles reported by them are rather different from ours. In their measurements, the growth of the absorption-time profiles was not observed directly, but the mixed order kinetics gave the intersystem crossing rate constant (kISC-1 ) 1.2 ns). Such differences may be caused by the high excitation density and the annihilation of the excited state. The first direct and clear observation of the growth of the T-T absorption band as shown in Figure 5 was done by using the picosecond time-resolved absorption spectroscopy using a streak camera. The curve-fitting using eq 1 was carried out for absorption profiles from 650 to 830 nm at intervals of 10 nm. Using the fitting parameters, the T-T and S1-Sn absorption bands are depicted. In Figure 9, part a, the T-T absorption spectra obtained by the curve-fitting show an absorption maximum similar to that of nanosecond time-resolved absorption spectra in Figure 2. Figure 9, part b, shows the S1-Sn absorption spectra of poly(3-octylthiophene). The absorption spectra suggest the existence of a lower energy transition at 830 nm. The S1-Sn absorption bands in the near-IR region are overlapping with the bands of polaron and bipolaron bands which have been discussed for the polythiophene films, and further spectroscopic studies on the short-lived intermediates are necessary to establish the photophysical process of π-conjugated polymer system. However, there are difficulties for the investigation of the intermediates in the near-IR and IR region because of the limitation of the sensitivity of the picosecond detector system and the spectral feature of the probe beam. The development of a transient measurement system in the near-IR and IR region is important.27
Studies of Excited States of Poly(3-octylthiophene) Conclusion The excited singlet and triplet states of poly(3-octylthiophene) and the intersystem crossing process were observed by picosecond time-resolved spectroscopy using a streak camera. Poly(3-octylthiophene) showed the maximum of the T-T absorption band at 800 nm. The rate constant of intersystem crossing kISC was determined to be 1.54 × 109 s-1 by curve-fitting of the absorption-time profile considering the decay of the excited singlet state and the growth of the excited triplet state. The S1-Sn absorption bands were depicted using the fitting parameters. Acknowledgment. All experiments were carried out in URAS (Ultrafast Reaction Analyzer System) at the Institute for Chemical Reaction Science, Tohoku University. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture. References and Notes (1) Su, W. P.; Schrieffer, J. R.; Heeger, A. Phys. ReV. Lett. 1979, 42, 1698. (2) Scott, J. C.; Pfluger, P; Krounbi, M.; Street, G. B. Phys. ReV. B 1983, 28, 2140. (3) Kaneto, K.; Kohno, Y.; Yoshino, K. Solid State Commun. 1984, 51, 267. (4) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1990, 39, 243. (5) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 600. (6) Scaiano, J. C.; Redmond, R. W.; Mehta, B.; Arnason, J. T. Photochem. Photobiol. 1990, 52, 655. (7) Scaiano, J. C.; Evans, C. H.; Arnason, J. T. J. Photochem. Photobiol., B 1989, 3, 411. (8) Evans, C. H.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 2694. (9) Garcia, P.; Oernaut, J. M.; Hapiot, P.; Wintgens, V.; Valat, P.; Garnier, F.; Delabouglise, D. J. Phys. Chem. 1993, 97, 513.
J. Phys. Chem., Vol. 100, No. 38, 1996 15313 (10) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228. (11) Fujitsuka, M.; Sato, T.; Segawa, H.; Shimidzu, T. Synth. Met. 1995, 69, 309. (12) Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoshino, K. Chem. Express 1986, 1, 635. (13) Yoshino, K.; Nakajima, S.; Gu, H. B.; Sugimoto, R. Jpn. J. Appl. Phys., Part 2 1987, 26, L2046. (14) Garten, F.; Schlatmann, A. R.; Gill, R. E.; Vrijimoeth, J.; Klapwijk, T. M.; Hadziioannou, G. Appl. Phys. Lett. 1995, 66, 2540. (15) Braun, D.; Gustafsson, G.; McBranch, D. J. Appl. Phys. 1992, 72, 564. (16) Osaka, T.; Komaba, S.; Kaneko, N. Chem. Lett. 1995, 1023. (17) Sinclair, M. B.; McBranch, D.; Hagler, T. W.; Heeger, A. J. Synth. Met. 1992, 49-50, 593. (18) Samuel, I. D. W.; Raksi, F.; Bradley, D. D. C.; Friend, R. H.; Burn, P. L.; Holmes, A. B.; Murata, H.; Tsutsui, T.; Saito, S. Synth. Met. 1993, 55-57, 15. (19) Grebner, D.; Helbig, M.; Rentsch, S. J. Phys. Chem. 1995, 99, 16991. (20) Kraabel, B.; Moses, D.; Heeger, A. J. J. Chem. Phys. 1995, 103, 5102. (21) Watanabe, A.; Ito. O. J. Phys. Chem. 1994, 98, 7736. (22) Sumitani, K.; Yoshihara, K. Bull. Chem. Soc. Jpn. 1982, 55, 85. (23) Ito, T.; Hiramatsu, M. ; Hosoda, M.; Tsuchiya, Y. ReV. Sci. Instrum. 1991, 62, 1415. (24) Watanabe, A.; Ito. O.; Mochida, K. Organometallics 1995, 14, 4281. (25) There are intramolecular T-T annihilation and intermolecular T-T annihilation pathways. In Figure 3, the fast decay component within 500 ns corresponds to the intramolecular T-T annihilation, and the slow decay component after 1 µs corresponds to the intermolecular T-T annihilation. The ratio of the intramolecular T-T annihilation to the intermolecular T-T annihilation significantly depends on the excitation laser power. The ratio of the fast decay component to the slow decay component increases with increasing laser power. (26) Rossi, R; Ciofalo, M.; Carpita, A.; Ponterini, G. J. J. Photochem. Photobiol., A 1993, 70, 59. (27) Watanabe, A.; Ito, O.; Watanabe, M.; Saito, H.; Koishi, M. J. Chem. Soc., Chem. Commun. 1996, 117.
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