Excited States of C70 and the Intersystem Crossing Process Studied

All measurements were made at 22 °C. Picosecond ... 10, 30 ps fwhm), optical delay lines, a polychromator (Acton .... (a) 200 ps; (b) 600 ps; (c) 1 n...
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10518

J. Phys. Chem. 1996, 100, 10518-10522

Excited States of C70 and the Intersystem Crossing Process Studied by Picosecond Time-Resolved Spectroscopy in the Visible and Near-IR Region 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 System DiVision, Hamamatsu Photonics K. K., 812 Joko-cho, Hamamatsu 431-31, Japan ReceiVed: NoVember 7, 1995; In Final Form: February 14, 1996X

The photophysical properties of C70 excited states were investigated by some time-resolved spectroscopic techniques. The absorption bands of singlet excited state (1C70*) and triplet excited state (3C70*) in the visible region were observed by time-resolved absorption spectroscopy using a streak camera. The intersystem crossing rate constant kisc from 1C70* to 3C70* was determined to be 1.25 × 109 s-1 by the analysis of the absorptiontime profiles considering the overlap of the decay of 1C70* and the growth of 3C70*, where the curve fitting was carried out by using the decay rate constant of 1C70* (1.61 × 109 s-1) determined by the picosecond time-resolved emission spectroscopy. The picosecond time-resolved near-IR spectra of 3C70* were obtained by the pump-probe technique, using a probe beam based on the broad-band optical parametric generation (OPG) of a β-barium borate (BBO) crystal pumped by 532-nm laser pulse. The lowest transition band of the T-T absorption of 3C70* was found in the near-IR region (960 nm) by the new technique.

Introduction The excited-state behavior of fullerenes has been investigated by time-resolved spectroscopy based on laser flash photolysis.1-17 The intersystem crossing process for C60 has been observed directly by picosecond laser flash photolysis experiments by Ebbesen et al., and Gevaert and Kamat, in the region from 400 to 950 nm.1,2,8,9 In the case of C60, the absorption band of C60 triplet excited state (3C60*) in the 740-nm region is separated clearly from the absorption band of C60 singlet excited state (1C60*) in the 920-nm region, and the isosbestic point was observed in the picosecond time-resolved absorption spectra.8,9 The decay of 1C60* at 920 nm closely matched the growth of 3C * at 747 nm. The lifetime of 1C * determined from the 60 60 decay of 1C60* transient absorption was 1.2 ns, which is similar to the values by fluorescence decay measurements. On the other hand, in the case of C70, the direct observation of the intersystem crossing process is not simple because singlet and triplet excited states of C70 exhibit similar spectral features in the visible region and the spectral overlap limits detailed investigation of 1C70* and 3C70* in the subnanosecond time domain.8 In addition, Tanigaki et al. mentioned that the spectral shapes of 1C70* and 3C * indicate further absorption bands at even longer wave70 length than 930 nm and that the absorption bands which they observed in the region from 400 to 930 nm do not correspond to the lowest transition from these states.2 The electron transfer between fullerenes and electron donors has been also investigated by laser flash photolysis.10-22 In our previous study, in the nanosecond time-resolved near-IR absorption measurements for C70, an unknown transient absorption band in the 960-nm region has been observed.20 The transient absorption band was effectively quenched by poly(methylphenylsilylene), an electron donor, and reasonably assigned to the lowest energy T-T (triplet-triplet) absorption band of 3C *. However, the detailed characteristics of the transient ab70 sorption band in the near-IR region are not clear. In this paper, we report the excited-state behavior of C70 studied by picosecond X

Abstract published in AdVance ACS Abstracts, June 1, 1996.

S0022-3654(95)03287-4 CCC: $12.00

time-resolved spectroscopy. In this study, we employed two kinds of spectroscopic techniques to observe picosecond timeresolved absorption spectra in the region from visible to nearIR. The first is the picosecond time-resolved absorption spectroscopy using a streak camera in the visible region. The optical measurements using a streak camera makes three-dimensional observation of transient absorption spectra possible. This technique is effective for investigating the complicated system where the transient absorption bands of several intermediates overlap as in the case of C70 excited states. The second is the pump-probe measurements, using a probe beam based on broad-band optical parametric generation (OPG), which is a new technique for the picosecond time-resolved near-IR absorption measurements.23 The intersystem crossing from 1C70* to 3C70* was observed by using these two kinds of techniques. The singlet excited state of C70 was also investigated by picosecond time-resolved emission spectroscopy using a streak camera. Experimental Section Materials and Sample Preparation. The purified C70 sample (>99% pure) was obtained from Texas Fullerrene Corp. The C70 was dissolved in spectroscopic grade benzene or toluene (Nakarai tesque), 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. Picosecond Time-Resolved Absorption Spectroscopy Using a Streak Camera. Picosecond time-resolved absorption spectra in the region from 400 to 850 nm were observed by using a picosecond laser flash photolysis system which consists of an active/passive mode-locked Nd:YAG laser (Continuum, PY61C10, 30 ps fwhm), optical delay lines, a polychromator (Acton Research Corp., SpectraPro-150), and a streak scope (Hamamatsu Photonics, C2830) with a high speed streak unit (HAMAMATSU, M2547) and a CCD 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.24,25 © 1996 American Chemical Society

Excited States of C70

J. Phys. Chem., Vol. 100, No. 25, 1996 10519

Figure 1. Time-resolved absorption spectra obtained by 355-nm laser pulse excitation of 0.1 mM C70 in benzene: (a) 0.0-0.4 ns; (b) 0.40.8 ns; (c) 0.8-1.2 ns; (d) 3.2-3.6 ns.

Picosecond Time-Resolved Near-IR Spectroscopy. The second harmonic (SHG, 532 nm) and the third harmonic (THG, 355 nm) of an active/passive mode-locked Nd:YAG laser (Continuum, PY61C-10, 30 ps fwhm) were used for OPG and a pump beam to excite a sample solution, respectively. The SHG beam (2-3 mJ) was focused by a 10-cm lens on a β-barium borate (BBO) crystal (4 × 7 mm2, 7-mm thickness), which was set within the focusing length (10 cm) to avoid damage. The broad-band OPG beam was split into probe and reference beams, which were focused on optical fiber heads and detected by two coupled sets of a polychromator and a multichannel detector with a 128-element InGaAs linear image sensor (HAMAMATSU, C5890-128). The spectra were obtained by averaging 100-200 pulses of the probe and reference beams on a microcomputer. Picosecond Time-Resolved Emission Spectroscopy. The time-resolved emission spectra were measured by 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). Nanosecond Time-Resolved Near-IR Spectroscopy. Nanosecond laser flash photolysis experiments were performed with a 532-nm laser pulse from a Nd:YAG laser (Quanta-Ray, GCR130, 6 ns fwhm). A Ge-APD module (HAMAMATSU, C5331SPL) was used for the measurements of transient absorption spectra in the visible and the near-IR region from 600 to 1600 nm. The details of the experimental setup are described elsewhere.19 Results and Discussion 1C * and 3C * by Picosecond Time-Resolved Absorption 70 70 Spectroscopy Using a Streak Camera. Figure 1 shows timeresolved absorption spectra obtained by 355-nm laser pulse excitation of 0.1 mM C70 in benzene. The existence of a shortlived species in the 620-nm region and a long lived-species in the 550- and 720-nm region is clear in Figure 1. These species are attributable to 1C70* and 3C70*, respectively.1,2,8,9 The decrease below 550 nm is due to depletion of the C70 ground state by laser pulse excitation. Figure 1 shows that the 620nm band is mainly the decaying component and the 710-nm band consists of the decaying and rising components. As suggested by Kamat et al. and Tanigaki et al., the transient absorption bands of 1C70* and 3C70* are overlapping in this region from 400 to 850 nm, and the spectral overlap limits detailed investigation.2,8,9 In the case of the pump-probe technique, it is usually difficult to solve complicated mixed kinetics because the absorption-time profile is reconstructed by the plots of the absorbance from transient spectra. On the other hand, picosecond time-resolved absorption spectroscopy using a streak camera and the breakdown of Xe gas as a probe

Figure 2. Absorption-time profiles for 0.1 mM C70 in benzene: excitation, 355 nm.

Figure 3. Three-dimensional representation of fluorescence spectra of 0.1 mM C70 in toluene: excitation, 375 nm.

has an advantage to obtain a continuous absorption-time profile as shown in Figure 2. It is possible to obtain the rate constants for the decay of 1C70*, which corresponds to the reciprocal of the fluorescence lifetime, and the growth of 3C70*, which corresponds to the rate constant of intersystem crossing (kisc) from 1C70* to 3C70*, by curve-fitting of the absorption-time profile considering the mixed kinetics. However, it is better to determine the rate constant of 1C70* decay by fluorescence decay experiments before the curve-fitting with mixed kinetics. Lifetime of 1C70* by Picosecond Time-Resolved Emission Spectroscopy Using a Streak Camera. The fluorescence lifetime measurements were carried out by a two-dimensional photon counting system, where the photon counting and streak camera technique are combined and the fluorescence lifetime can be determined with a temporal resolution of 5 ps by using deconvolution processing.26 Figure 3 shows the three-dimensional fluorescence spectra obtained from the streak image for the fluorescence decay of 0.1 mM C70 in toluene. The fluorescence maximum at 670 nm is observed, similar to the reported one.4 The sharp signal at 750 nm is the scattering of the fundamental of the Ti:sapphire laser. Figure 3 shows uniform decay of C70 fluorescence. The fluorescence decay curve at 670 nm and the fitting curve using a single exponential equation are shown in Figure 4. The fluorescence lifetime was determined to be 620 ps, which matches reported values.4 The decay rate constant kfluo for 1C70* is determined to be 1.61 × 109 s-1 from the reciprocal of the fluorescence lifetime. By using the kfluo value, the absorption-time profiles in Figure 2 were analyzed by curve-fitting, considering the 1C70* decay (kfluo) and the 3C70* growth by intersystem crossing from

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Watanabe et al.

Figure 4. Fluorescence decay and the first-order fitting curves for 1C70* at 670 nm.

Figure 6. Time-resolved absorption spectra of 1C70* (A) and 3C70* (B) calculated by using fitting parameters for the absorption-time profiles. (a) 200 ps; (b) 600 ps; (c) 1 ns; (d) 2 ns.

Figure 7. White continuum by broad-band OPG using a BBO crystal pumped by the 532-nm pulse of a picosecond YAG laser. BBO-tuning center (nm): (a) 900; (b) 1150; (c) 1400; (d) 1600.

Figure 5. Absorption-time profiles and the fitting curves considering the overlap of the decay of 1C70* and the growth of 3C70* for 0.1 mM C70 in benzene: (a) 620 nm; (b) 710 nm. The decay and growth curves for 1C70* and 3C70*, respectively, are also shown by using the fitting parameters. 1C * 70

(kisc). Figure 5 shows the absorption-time profiles and fitting curves at 620 and 710 nm. In Figure 5, decay and rising curves for 1C70* and 3C70* which were calculated by using fitting parameters, are also shown. The kisc values for the absorption-time profiles at 620 and 710 nm closely matched and were determined to be 1.25 × 109 s-1. This value is smaller than the previously reported value (8.7 × 109 s-1),6 which is larger than the decay rate constant of 1C70*. The kisc value in this study is reasonable because the value is comparable to kfluo for 1C70*. The time-resolved absorption spectra of 1C70* and 3C70* without overlapping each other can be estimated by using parameters obtained by curve-fitting of the absorption-time profiles. Figure 6 shows the time-resolved absorption spectra of 1C70* and 3C70*, which were obtain by curve-fitting of the absorption-time profiles at intervals of 10 nm. The T-T absorption bands of 3C70* are similar to those of previously reported spectra.2,8,9 The S1 f Sn absorption of 1C70* shows an absorption maximum at 570 nm, a shoulder at 630 nm, and an absorption tail extending to the near-IR region. Intersystem Crossing by Picosecond Time-Resolved NearIR Spectroscopy. From Figure 1, it is shown that there is an absorption band at longer wavelength than 850 nm. The picosecond time-resolved absorption measurements in the nearIR region are necessary to observe the lowest transition of the excited state. However, there are two problems in the measurements of picosecond time-resolved absorption spectra in the near-IR region by the pump-probe technique. The first problem is the limitation of the sensitivity of the multichannel detector

for the probe beam. However, this problem can be solved by the recent development of an InGaAs multichannel detector, which has sensitivity from 700 to 1700 nm. The second problem is the limitation of the spectral wavelength range and the intensity of the probe beam in the near-IR region. The probe beam generated by self-phase modulation in a liquid such as a D2O/H2O mixture is unstable in the near-IR region due to the absorption band of the solution itself and the thermal fluctuation caused by passing through the near-IR light. The recent advance of an optical parametric oscillator (OPO) gives us many possibilities in spectroscopic studies. However, the narrow linewidth of the OPO is not convenient for time-resolved absorption spectroscopy by pump-probe measurements using a multichannel detector system. The broad-band optical parametric generation (OPG) can be attained by using a BBO crystal pumped by a 532-nm pulse of a picosecond mode-locked Nd: YAG laser, where the pump beam passes through the BBO with various phase-matching angles. The OPG based on the BBO pumped by 532-nm laser pulse shows narrow tuning angle for the phase-matching.27 The tuning angle for the idler wave is quite narrow in the near-IR region. Therefore, it is expected that the distribution of the incident angle of the SHG beam causes the OPG in wide wavelength range. Figure 7 shows the spectral profiles of the broad-band OPG in the region from 700 to 1700 nm. By tuning the BBO angle and collecting the OPG light using an aberration-corrected camera lens, a broadband probe beam with 200-nm band width can be obtained. The spectral properties of the white continuum strongly depend on the divergence of laser beam and the focusing length on the BBO crystal. Figure 8 shows the picosecond time-resolved absorption spectra for 0.1 mM C70 in benzene. In a subnanosecond time domain, the growth of the absorption band at 960 nm was observed. The absorbances at 1010 nm are plotted against the delay time in the inserted figure in Figure 8. The calculated absorption-time profile for the growth of 3C70* using kisc (1.25 × 109 s-1) is also shown in the inserted figure. The

Excited States of C70

Figure 8. Time-revolved near-IR spectra obtained by the pump-probe measurements for 0.1 mM C70 in benzene: excitation, 355 nm. Delay time (ps): (a) 0; (b) 200; (c) 1000; (d) 1500. The inserted figure shows the plots of the optical density at 1010 nm and the calculated absorption-time profile for the growth of 3C70* using kisc (1.25 × 109 s-1).

Figure 9. Nanosecond time-resolved absorption spectra obtained by 532-nm laser pulse excitation of 0.1 mM C70 in benzene: (b) 100 ns, (O) 1 µs.

absorption band at 960nm can be assigned to the T-T absorption of 1C70*, because the growth obeys the intersystem crossing kinetics from 1C70* to 3C70*. The shift of the absorption maxima within 200 ps is due to the overlapping of 1C *. The lowest transition energy of the T-T absorption of 70 3C * (960-nm maximum) is lower than that of 3C * (740-nm 70 60 maximum). This may be caused by the reduced symmetry of C70 relative to C60, which is evident in the ground-state absorption spectra (S0 f S1) of C70 and C60. The forbidden S0 f S1 transition of C70 in the visible region is much stronger than that of C60.6 T-T Absorption Band of 3C70* in the Near-IR Region by Nanosecond Time-Resolved Near-IR Spectroscopy. The T-T absorption band at 960 nm was observed by the nanosecond laser flash photolysis in the near-IR region using a GeAPD. Figure 9 shows the nanosecond time-resolved absorption spectra obtained by 532-nm laser pulse excitation (54 mJ/cm2) of 0.1 mM C70 in benzene. Two absorption maxima are observed at 680 and 960 nm, which show uniform decay. In the region from 580 to 800 nm, the distortion of the absorptiontime profiles was corrected by considering the overlap of the C70 emission. The absorption-time profile at 960 nm is shown in Figure 10. If first-order kinetics is assumed for the decay curve, the decay rate constant kT1 is determined to be 2.05 × 105 s-1. From the reciprocal of the kT1 value, the triplet-state lifetime is determined to be 4.9 µs. On the other hand, the lifetime at 680 nm is determined to be 7.0 µs if first-order kinetics is assumed. This disagreement is caused by the influence of T-T annihilation, which depends greatly on the laser excitation energy, the concentration of C70, and so on. For example, lifetimes for the decay of 680-nm band at laser power densities of 12, 16, 27, and 54 mJ/cm2 are 296, 126, 20, and 7 µs, respectively. The T-T annihilation is enhanced at the higher triplet concentration by the higher laser power excitation. T-T

J. Phys. Chem., Vol. 100, No. 25, 1996 10521

Figure 10. Absorption-time profile at 960 nm obtained by 532-nm laser pulse excitation of 0.1 mM C70 in benzene: Inserted figure is the first-order plots of the decay curve.

annihilation obeys second-order kinetics and the slope of thesecond-order plots gives (2k2/), where k2 and  are the second-order rate constant and the molar excitation coefficient of the triplet. Therefore, the apparent rate constants by assuming the first-order kinetics without considering the second-order kinetics exhibit wavelength dependence because of the influence of the molar excitation coefficient. For example, the apparent rate constants at 680, 960, 1080, and 1160 nm are 1.43 × 105, 2.05 × 105, 1.50 × 105, and 0.75 × 105 s-1, respectively. The apparent rate constant at 680 nm is close to that at 1080 nm because the similarity of the optical density at 680 and 1080 nm. For this reason, reported lifetimes of 3C70* show disagreements.28 Conclusion The intersystem crossing rate constant kisc from 1C70* to 3C70* is determined to be 1.25 × 109 s-1 by picosecond time-resolved absorption spectroscopy using a streak camera. The system using a streak camera is quite effective to investigate the mixed kinetics in the subnanosecond time domain. The picosecond time-resolved near-IR spectra of 3C70* was obtained by the pump-probe technique using a probe beam based on broadband OPG. The lowest transition band of the T-T absorption of 3C70* was found in the near-IR region (960 nm) by the new technique. The kinetics for the growth of the 960-nm band for 3C * confirms the k 70 isc obtained by the streak system. The broad-band OPG as a probe provides the possibilities of new time-resolved spectroscopy using a multichannel detector in the near-IR region. Acknowledgment. All experiments were carried out in URAS at the Institute for Chemical Reaction Science, Tohoku University. References and Notes (1) Ebbesen, T. W.; Tanigaki, K.; Kuroshima, S. Chem. Phys. Lett. 1991, 181, 501. (2) Tanigaki, K.; Ebbesen, T. W.; Kuroshima, S. Chem. Phys. Lett. 1991, 185, 189. (3) Palit, D. K.; Sapre, A. V.; Mittal, J. P.; Rao, C. N. R. Chem. Phys. Lett. 1992, 195, 1. (4) Williams, R. M.; Verhoeven, J. W. Chem. Phys. Lett. 1992, 194, 446. (5) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95, 11. (6) Arbogast, J. W.; Foote, C. S. J. Am. Chem. Soc. 1991, 113, 8886. (7) Arbogast, J. W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc. 1992, 114, 2227. (8) Gevaert, M.; Kamat, P. V. J. Phys. Chem. 1992, 96, 9883. (9) Sauv, G.; Dimitrijevic, N. M.; Kamat, P. V. J. Phys. Chem. 1995, 99, 1199. (10) Sension, R.; Szarka, A. Z.; Smith, G. R.; Hochstrasser, R. M. Chem. Phys. Lett. 1991, 185, 179.

10522 J. Phys. Chem., Vol. 100, No. 25, 1996 (11) Osaki, T.; Tai, Y.; Tazawa, M.; Tanemura, S.; Inukawa, K.; Ishiguro, K.; Sawaki, Y.; Saito, Y.; Shinohara, H.; Nagashima, H. Chem. Lett. 1993, 789. (12) Nonell, S.; Arbogast, J. W.; Foote, C. S. J. Phys. Chem. 1992, 96, 4169. (13) Ghosh, H. N.; Pal, H.; Sapre, A. V.; Mittal, J. P. J. Am. Chem. Soc. 1993, 115, 11722. (14) Palit, D. K.; Ghosh, H. N.; Pal, H.; Sapre, A. V.; Mittal, J. P. Chem. Phys. Lett. 1992, 198, 113. (15) Dimitrijevic, N. M.; Kamat, P. V. J. Phys. Chem. 1992, 96, 4811. (16) Samanta, A.; Kamat, P. V. Chem. Phys. Lett. 1992, 199, 635. (17) Kamat, P. V. J. Am. Chem. Soc. 1991, 113, 9705. (18) Watanabe, A.; Ito. O. J. Chem. Soc., Chem. Commun. 1994, 1285. (19) Watanabe, A.; Ito. O. J. Phys. Chem. 1994, 98, 7736. (20) Watanabe, A.; Ito. O. Jpn. J. Appl. Phys. 1995, 34, Suppl. 1, 194. (21) Watanabe, A.; Ito. O.; Mochida, K. J. Organometallics 1995, 14, 4281.

Watanabe et al. (22) Ito, O;. Sasaki, Y.; Yoshikawa, Y.; Watanabe, A. J. Phys. Chem. 1995, 99, 9838. (23) Watanabe, A.; Ito, O.; Watanabe, M.; Saito, H.; Koishi, M. J. Chem. Soc., Chem. Commun. 1996, 117. (24) Sumitani, K.; Yoshihara, K. Bull. Chem. Soc. Jpn. 1982, 55, 85. (25) Ito, T.; Hiramatsu, M.; Hosoda, M.; Tsuchiya, Y. ReV. Sci. Instrum. 1991, 62, 1415. (26) Nishimura, H.; Yamazaki, T.; Yamazaki, I.; Watanabe, M.; Koishi, M. J. Spectrosc. Soc. Jpn. 1991, 40, 155. (27) Fix, A.; Schro¨der, T.; Wallenstein, R. Laser Optelektronik 1991, 23, 106. (28) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 4093.

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