Picosecond Time-Resolved Resonance Raman Study of the

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J. Phys. Chem. B 2000, 104, 9288-9300

Picosecond Time-Resolved Resonance Raman Study of the Photoisomerization of Retinal Atsuhiko Shimojima and Tahei Tahara* Institute for Molecular Science (IMS), Myodaiji, Okazaki 444-8585, Japan ReceiVed: March 6, 2000; In Final Form: May 25, 2000

Picosecond time-resolved Raman spectroscopy was applied to the study of the photoisomerization dynamics of all-trans, 9-cis and 13-cis retinal in nonpolar solvents. It was found that picosecond time-resolved spontaneous Raman spectra are obtainable from retinal in solution despite a high fluorescence background when the probe wavelength is in rigorous resonance with the T-T absorption. In the case of photoexcitation of all-trans retinal, the transient Raman bands ascribed to the all-trans T1 state appeared with the intersystem crossing time of ∼30 ps. No Raman signal attributable to the product was recognized within the signal-to-noise ratio, reflecting the low isomerization quantum yield of all-trans retinal. The frequency shifts of the all-trans T1 bands were observed in the early picosecond time region (τ ∼ 16 ps), which manifests the vibrational cooling process in the excited state. In the case of photoexcitation of 9-cis retinal, the all-trans T1 state slowly appeared with a time constant of ∼1 ns (1000 ( 150 ps), which corresponds to the 9-cis f all-trans structural change occurring in the T1 state. In addition, Raman signals due to the 9-cis T1 state (e.g., 1400 cm-1) were recognized in the early delay time and they disappeared in accordance with the appearance of the all-trans T1 state. The data obtained clearly showed that the 9-cis f all-trans photoisomerization predominantly takes place in the T1 state with thermal activation to cross the potential barrier from the 9-cis configuration to the all-trans. In contrast, with photoexcitation of 13-cis retinal, the transient Raman signals attributable to the mixture of the all-trans T1 state and the 13-cis T1 state appeared in a few tens of picoseconds, and no spectral change was observed after 100 ps up to a few nanoseconds. The quantitative analysis indicated that the all-trans T1 state and the 13-cis T1 state appeared with different time constants. It suggests that the 13-cis f all-trans isomerization takes place in the excited singlet state before the intersystem crossing and that the resultant all-trans S1 and 13-cis S1 states are relaxed to the corresponding T1 states separately with time constants inherent to each isomer. The singlet isomerization quantum yield was estimated approximately at ∼0.2 from the obtained picosecond Raman data. These results indicated that the singlet mechanism is a major pathway (or one of major pathways) in photoisomerization of 13-cis retinal. The present time-resolved Raman study showed that the cis f trans photoisomerization mechanism and dynamics of retinal significantly depend on the position of the double bond to rotate.

1. Introduction Cis-trans photoisomerization is one of the most fundamental chemical reactions, and its mechanism and dynamics have been extensively investigated. A prototypical picture of photoisomerization has been drawn on the basis of intensive studies for several typical molecules such as stilbene.1-3 In the case of stilbene, the minimum of the S1 potential surface is located at the perpendicular configuration, as a result of an avoided crossing of the S0 and S1 states along the isomerization coordinate.4 On this potential curve, the trans S1 state generated by photoexcitation is thermally activated to cross the potential barrier into the perpendicular configuration, and then it is relaxed rapidly to the cis and trans S0 state through the potential funnel. The isomerization from the cis isomer proceeds in a similar way and also gives the cis and trans S0 state, whereas the potential barrier on the cis side is very low.5,6 This “rotational singlet mechanism” has been supported by a number of spectroscopic studies, and it forms a basis of our understanding of the cis-trans photoisomerization around the olefinic CdC double bond. The isomerization around the NdN double bond * To whom correspondence should be addressed. Fax: +81-564-542254. E-mail: [email protected].

(e.g., azobenzene) is considered to proceed with a different mechanism.7-9 The photoisomerization of molecules having more than one double bond is more complicated but is sometimes important. Retinal is a molecule that has four CdC double bonds to isomerize in its polyenic backbone. Photoisomerization of this molecule has been attracting much interest not only in photochemistry but also in photobiology because cis-trans isomerization of the retinyl chormophore plays a crucial role in the primary photochemical process in vision and in halophilic bacterial photosynthesis. It is well-known that the photochemical properties of retinal (or retinyl chromopher) significantly change with the change of environments. In protein, for example, the isomerization of the retinyl chromophore takes place only at a specific double bond: the proton pumping in bacteriorhodopsin originates from isomerization between the all-trans- and the 13cis-retinal Schiff base, whereas the 11-cis f all-trans isomerization of the retinal Schiff base attached to opsin triggers vision. The isomerization of free retinal in organic solvents is more complex, and its mechanism is much less understood. It is known that photoisomerization products and quantum yields of free retinal in solution depend on the solvent polarity and that alkane and cycloalkane solvents such as hexane, cyclohexane,

10.1021/jp000873f CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000

Photoisomerization of Retinal and 3-methylpentane afford similar environments for the excitedstate dynamics of retinal. The elucidation of the isomerization mechanism of retinal in such “nonpolar” solvents is of fundamental importance, and hence, it has been studied by a variety of photochemical and spectroscopic methods.10,11 Interestingly, even under free environments such as nonpolar solvents, the isomerization of retinal shows very characteristic and unique features. The isomerization from the mono-cis isomers (the 7-cis, 9-cis, 11-cis, and 13-cis isomers) predominantly affords only the all-trans isomer, whereas photoisomerization of all-trans retinal, which is much less efficient, predominantly produces 13-cis retinal. In other words, the cis f trans isomerization occurs at any of the 7-8, 9-10, 11-12 and 13-14 double bonds, whereas the trans f cis isomerization occurs predominantly only about the 13-14 double bond. A question addressed is whether the isomerization mechanism depends on the direction of the isomerization (cis to trans or trans to cis) and the position of the double bond to rotate. The isomerization quantum yields of several mono-cis isomers are significantly higher in nonpolar solvents than that in polar solvents such as methanol.12 Since the intersystem crossing quantum yield is higher in nonpolar solvents, this fact suggests that the isomerization in the T1 state (the triplet mechanism) is important in nonpolar solvents. To obtain more direct information about the isomerization mechanism of retinal, time-resolved spectroscopy has been applied. Menger and Klinger measured nanosecond time-resolved absorption of the 11-cis isomer and observed that the absorbance change due to the generation of S0 all-trans retinal coincided with the decay of the T-T absorption, supporting the triplet pathway of the cis f trans isomerization in nonpolar solvents.13 The identification of the T1 state using transient absorption spectra was attempted to gain deeper insight into the triplet pathway. However, definitive assignments of the transient spectra could not be made because the transient absorption at room temperature is broad and featureless, so that it is difficult to distinguish each isomer in the excited state. In fact, Harriman and Liu measured timeresolved absorption spectra of all-trans, 7-cis, and 11-cis retinal and obtained identical T-T absorption, and they assigned it to the equilibrated mixture of several conformers in the T1 state.14 On the other hand, Veyret et al. claimed that 11-cis retinal gave a T-T absorption that was different from the all-trans T-T absorption.15 Vibrational spectra are much more sensitive to the difference in the molecular structure and are expected to afford clearer information about intermediates. Thus, nanosecond timeresolved Raman spectroscopy was then applied to the study of retinal.16-20 Hamaguchi and co-workers16 measured nanosecond time-resolved Raman spectra at the delay time of 20 ns and clearly showed that the transient spectra obtained from 7-cis, 9-cis, and 11-cis retinal are identical to the transient spectrum obtained from all-trans retinal. The obtained transient spectra were all ascribed to a single transient species, that is, the alltrans T1 state. The photoexcitation of the 13-cis isomer gave the spectrum attributable to the mixture of the 13-cis T1 and the all-trans T1 state. On the basis of these observations, they proposed a photoisomerization mechanism in the triplet manifold in which the deepest potential minimum is located not at the perpendicular configuration but at the all-trans configuration on the T1 potential curve, and all the T1 isomers are relaxed through this common “all-trans” minimum to the all-trans S0 state.10,16,21 Then, to clarify the conversion process from cis to trans that proceeds in a shorter time region, picosecond timeresolved measurements were carried out. Koyama and coworkers measured picosecond time-resolved absorption spectra

J. Phys. Chem. B, Vol. 104, No. 39, 2000 9289 from the all-trans and the four mono-cis isomers.22,23 They observed temporal spectral changes of the T-T absorption for 7-cis, 9-cis, and 11-cis retinal in the picosecond time regions and attributed them to the conformational change from each cis T1 state to the all-trans T1 state. The 13-cis isomer gave a slightly different T-T absorption. An unambiguous observation of the cis f trans structural change has been achieved for 9-cis retinal by Tahara et al. using picosecond 2D-CARS spectroscopy with 90-ps time resolution.24 They clearly observed that the alltrans T1 CARS bands appear with a time constant of ∼900 ps following photoexcitation of 9-cis retinal. This directly confirmed that the 9-cis f all-trans structural change occurs in the T1 state. On the other hand, for the photoisomerization from trans to cis, nanosecond time-resolved infrared measurements demonstrated that the singlet mechanism is important.25 Very recently, Yamaguchi and Hamaguchi claimed that isomerization from the all-trans to the 13-cis isomer proceeds directly from the S2 state to the S0 state through a perpendicular intermediate on the basis of their femtosecond absorption data.26 Obviously, time-resolved vibrational spectroscopy plays a crucial role in the elucidation of the isomerization mechanism of retinal. Picosecond measurements are especially important to clarify the cis f trans isomerization dynamics, as demonstrated by the 2D-CARS study undertaken with 90-ps time resolution.24 Since the intersystem crossing time of retinal is a few tens of picoseconds, it is highly desirable to carry out timeresolved spontaneous Raman measurements with a few picosecond time resolution in order to elucidate the overall dynamics of the cis f trans isomerization process taking place in the excited state. However, early attempts to measure picosecond spontaneous Raman spectra using the probe laser at 532 nm were not successful owing to the hindrance of the fluorescence background.27 At that time, no further trial was made using blue probe laser pulses, because fluorescence intensity was expected to be much higher in the shorter wavelength region. Recently, our femtosecond time-resolved fluorescence study28 revealed that the fluorescence of retinal is very unique and its spectrum is extended to the red region owing to the contribution from the 1nπ* fluorescence. It turned out that the fluorescence intensity increases only by a few times even if the probe wavelength is changed from 532 nm to ∼450 nm, which is in rigorous resonance with the T-T absorption of retinal. It suggested that resonance Raman enhancement may overcome the fluorescence background under the rigorous resonance condition and that we may be able to study the photoisomerization of retinal with use of picosecond spontaneous Raman spectroscopy. In this paper, we choose three isomers of retinal, all-trans, 9-cis, and 13-cis retinal (Figure 1) and report our time-resolved Raman study of their isomerization process. First, for all-trans retinal, we show that picosecond time-resolved spontaneous Raman spectra can be obtained when we use an adequate probe wavelength. Next, we describe the results obtained from 9-cis retinal. We obtained detailed information about the 9-cis f alltrans isomerization dynamics from the time-resolved Raman data. Third, we present the results of 13-cis retinal, which clearly manifests that the mechanism of the 13-cis f all-trans photoisomerization is significantly different from that of the other cis isomers. 2. Experiment The experimental setup used for the present picosecond timeresolved Raman measurements is shown in Figure 2. The system is based on a picosecond mode-locked Ti:sapphire laser (Spectra

9290 J. Phys. Chem. B, Vol. 104, No. 39, 2000

Figure 1. The molecular structure of all-trans, 9-cis, and 13-cis retinal.

Physics Tsunami 3950; 90-MHz repetition rate, 1.8-ps pulse duration, >1 W average power) and a kHz regenerative amplifier (Spectra Physics Spitfire; 1 kHz, 2.0-2.5 ps, and >600 mW). The Ti:sapphire laser was tuned to generate 770-nm pulses in the present experiments. The second harmonic (SHG, 385 nm) of the amplified pulse was generated in an LBO crystal (5 mm), and it was divided into two by a beam splitter. The minor portion (20%) of the SHG beam was used as the pump pulse to photoexcite the sample. The pump wavelength of 385 nm matches a strong ππ* transition of retinal. The rest of the SHG beam (80%) was introduced into a H2 Raman shifter to convert the wavelength. The first Stokes line of the Raman shifter at 458 nm was used for probing Raman scattering. The probe wavelength is in rigorous resonance with the T-T absorption of all-trans retinal (λmax ∼ 445 nm). The spectrum of the probe pulse obtained from the Raman shifter exhibited a slight asymmetrical feature, which caused an asymmetric band shape in the observed Raman spectrum to some extent. The pump and probe pulses were introduced to each optical path to generate a time delay that was controlled by a computer-driven mechanical stage. After passing through variable neutral density filters, the two beams were recombined at a dichroic mirror and were focused on a thin film-like jet stream of a sample solution. The Raman scattering was collected by a camera lens and was analyzed by a polychromator (Jobin Yvon HR-320) equipped with a liquid nitrogen cooled CCD camera (Princeton Instruments Inc. LN/CCD-1100PB). Typical energies of the pump and probe pulses were 2 µJ and 0.1 µJ, respectively, at the sample point. We reduced the probe energy as low as possible in order to minimize the bleaching of the T1 state. In addition, we examined the pump-power dependence of the time-resolved spectra to check the effect due to the multiphoton process. No noticeable effect was observed. The time-zero position and the time resolution were checked by measuring the optical Kerr effect (OKE) of heptane. All measurements were undertaken in aerated solutions at room temperature in order to avoid the effect of the chain-reaction-type isomerization.29-31 The frequency resolution of the Raman measurements is ∼10 cm-1. The time resolution is ∼3 ps, and all the fitting analysis about the temporal change of the transient Raman intensity in this paper was carried out by taking this instrumental response into account. All-trans retinal and 9-cis retinal were purchased from Sigma Chemical Co., and 13-cis retinal was purchased from Aldrich.

Shimojima and Tahara They were kept in the dark at low temperature and were used without further purification. The purity of the sample was checked by HPLC. It was found that the amount of the other isomers was negligible in the sample of the all-trans and the 9-cis isomers, whereas the sample of the 13-cis isomer contained about 2% all-trans isomer as an impurity. Hexane was obtained from Wako Pure Chemical Co. (HPLC grade) and was used after being dried with a molecular shieve. During the measurements, photoproduct (mainly all-trans retinal) is generated and accumulated in the sample because the sample solution is circulated. We evaluated the amount of the photoproduct by analyzing the sample before and after the measurements (∼1 h) with use of HPLC. For the sample solution of all-trans retinal, the amount of the other isomers was negligible before and after the measurement. The 9-cis retinal solution contained ∼5% all-trans isomer after the measurement. In the sample solution of 13-cis retinal, the amount of all-trans retinal increased from ∼2% to ∼6% during the measurement. When a long exposure time was needed in the Raman measurement, we changed the sample solution each hour. In addition, for obtaining a set of time-resolved spectra of the cis isomers, we measured spectra both in increasing and decreasing order of delay time and averaged the spectra in the two sets in order to minimize a systematic error arising from the change of the amount of photoproducts. Therefore, during the measurements, the average amount (the concentration ratio) of “impurity” alltrans retinal in the cis samples was estimated approximately at half the value that was determined after the experiment, which was approximately 3 ∼ 4%. 3. Result and Discussion 3.1. All-trans Retinal. We first describe the experimental results obtained from all-trans retinal. It is known that the photoisomerization of all-trans retinal is inefficient, and its quantum yield has been reported as low as 0.1.15,31,32 Therefore, the majority of the photoexcited all-trans isomer is relaxed to the S0 state directly or through the lowest excited triplet (T1) state, preserving its original conformation. In fact, the product of “inefficient” photoisomerization of all-trans retinal could not be recognized in the present picosecond Raman measurements. However, the experiments and data analysis concerning all-trans retinal are important because the isomerization pathways of the cis isomers are discussed by comparing the Raman data of the cis isomers with those of the all-trans isomer. Figure 3 shows typical spectra obtained from a hexane solution of all-trans retinal. The pump and probe wavelengths are 385 and 458 nm, respectively. In the spectrum measured with only pump irradiation (Figure 3a), the featureless signal due to retinal fluorescence is observed. When we irradiate probe pulses at the delay time of 400 ps, we clearly observe strong Raman signals on the fluorescence background, as shown in Figure 3b. The intensity of the fluorescence background is about five times higher than that of the strongest Raman signal, even under the optimized experimental condition. However, we can obtain a transient Raman spectrum with a very high signal-tonoise ratio by subtracting the fluorescence background (Figure 3c). The transient Raman spectrum that is obtained with the pump and probe irradiation is totally different from the spectrum taken with only probe irradiation. The probe-only spectrum is shown in Figure 3d for comparison. In the probe-only spectrum, Raman bands due to the S0 state of all-trans retinal are seen at 1580, 1332, 1270, 1196, 1162, and 1008 cm-1 in addition to the solvent (hexane) Raman bands. Thanks to the preresonance

Photoisomerization of Retinal

J. Phys. Chem. B, Vol. 104, No. 39, 2000 9291

Figure 2. Schematic diagram of the experimental setup used for picosecond time-resolved spontaneous Raman measurements.

Figure 3. Picosecond time-resolved Raman spectra of all-trans retinal in hexane taken at the delay time of 400 ps (1 × 10-3 mol dm-3; pump laser 385 nm; probe laser 458 nm): (a) spectrum obtained with pump irradiation; (b) spectrum obtained with pump and probe; (c) subtracted spectrum, (b)-(a); and (d) spectrum obtained with probe only. Solvent bands are indicated by asterisks.

effect due to a strong ππ* transition located around 370 nm, the S0 Raman bands of all-trans retinal appear with fairly high intensity despite its low concentration. On the other hand, in the spectrum taken with pump and probe irradiation, new Raman bands due to transient species appear at 1553, 1338, 1299, 1253, 1183, and 1002 cm-1. The intensities of these transient Raman bands are so high that the Raman bands due to the S0 all-trans retinal (e.g., 1580 cm-1) and the solvent (e.g. 1445 cm-1) can be barely recognized. The observed transient Raman bands are safely assigned to the T1 state that has the all-trans configuration

because of the following reasons. First, the intersystem crossing time of all-trans retinal is a few tens of picoseconds,26,33,34 so that the T1 state is expected to be populated at 100 ps. Second, the probe wavelength is in rigorous resonance with the T-T absorption. Third, the photoisomerization of all-trans retinal is inefficient, and hence, the molecule is highly likely to preserve the all-trans conformation in the excited state. The Raman spectrum of T1 all-trans retinal observed in the present picosecond measurement agrees very well with the all-trans T1 spectrum measured at 20 ns in the nanosecond time-resolved Raman study,16 although the intensity pattern is slightly different, reflecting the difference in the resonance condition. The data obtained clearly show that picosecond time-resolved spontaneous Raman spectroscopy is applicable to the study of the excitedstate dynamics of retinal if we tune the probe wavelength to the rigorous resonance condition with the T-T absorption. Picosecond time-resolved Raman spectra in the delay time region from -50 ps to 400 ps are shown in Figure 4. The Raman intensity in each spectrum was normalized using the solvent Raman bands in order to cancel the self-absorption effect due to the T-T absorption. Since the relative intensity of the solvent bands in the time-resolved spectra was essentially the same as that in the neat solvent spectrum, we neglected the spectral distortion arising from the T-T absorption feature and used a constant normalization factor for the whole spectral region in each spectrum. In the spectra taken at the negative time delays (which means that the probe pulse comes to the sample before the pump), only the Raman bands due to the all-trans S0 state and the solvent are observed. With photoexcitation, the intensity of the S0 bands decreases by ∼40%, and then the transient Raman bands attributable to the all-trans T1 state appear in a few tens of picoseconds. The T1 Raman intensity increases with time and reaches its maximum at around 100 ps. We measured time-resolved spectra up to the delay time of 1.6 ns, but no significant change was observed after 100 ps. The lifetime of the T1 state is about a hundred nanoseconds in aerated solutions24 so that the decay of the T1 state is negligible in the time region we measured. Recent time-resolved fluorescence28,34 and absorption26 studies clarified that the three excited singlet states, the 1Bu-like (ππ*, S3). the 1Ag-like (S2), and the nπ* (S1) states, are successively populated after ππ* (S3) photoexcitation. Although


Figure 4. Picosecond time-resolved Raman spectra of all-trans retinal in hexane in the delay time range from -50 to 400 ps (1 × 10-3 mol dm-3; pump laser 385 nm; probe laser 458 nm). The Raman intensity at each delay time has been normalized using the solvent band intensity. Solvent bands are indicated by asterisks.

the S3 and S2 states decay within 1 ps, the S1 state is populated before the intersystem crossing in the time region of a few tens of picoseconds. However, no transient Raman signal attributable to the S1 state was recognized in the spectra, implying that the Raman intensity of the S1 state is much weaker than that of the T1 and the S0 state under the present probing condition. This is not surprising, because the transient absorption of the S1 state is much weaker and broader than that of the T1 state.26 To derive quantitative information about the excited-state dynamics of all-trans retinal, we carried out a fitting analysis of the two spectral regions where prominent Raman bands are located. Since we performed the same analysis for the spectra taken from the cis isomers, we describe some details of the procedure here. Figure 5 depicts the results of the band decomposition for the spectra at the delay time of -10, 0, 10, and 30 ps. These spectra correspond to the spectrum before the pump irradiation (-10 ps), the spectrum immediately after the pump irradiation (0 ps), and the spectra when the T1 Raman bands increase (10 and 30 ps). In this analysis, Raman bands due to the T1 state, the S0 state, and the solvent are taken into account. We used a few Lorentzian functions to simulate each Raman band, so that the parameters representing the band shape consist of the bandwidths, the relative amplitude, and the relative position of component Lorenzian functions. We first determined the parameters (the peak position and the band shape) of the S0 and the solvent Raman bands separately for the probe-only spectrum and the neat solvent spectrum. Then, we carried out a preliminary fitting for each time-resolved spectrum to obtain parameters for the T1 Raman bands. Neither the band shape nor peak frequency of the T1 Raman bands exhibited a noticeable change after 30 ps, whereas the significant frequency shift was recognized in the earlier delay time. We averaged the band-shape parameters obtained for the delay time range from 30 to 400 ps. With the averaged band-shape parameters fixed, we made the final fitting for each time-resolved Raman spectrum

Shimojima and Tahara

Figure 5. Result of the spectral decomposition analysis of picosecond time-resolved Raman spectra obtained from all-trans retinal (----- T1; s S0; -‚-‚- solvent).

Figure 6. The temporal change of the two transient Raman bands observed with photoexcitation of all-trans retinal: (upper) intensity change, (lower) vibrational frequency change.

to determine the intensity and the peak position of the T1 Raman bands as a function of the delay time. Although a slight band narrowing was recognized for the T1 Raman bands in the early delay time region, we neglected this effect to minimize the number of the adjustable parameters. The upper trace in Figure 6 depicts the intensity change of the T1 Raman bands at 1553 cm-1 (CdC stretch) and 1183 cm-1 (C-C stretch), which was obtained by the above-described analysis. As clearly seen, the rise curves of these two transient Raman bands are essentially the same. The fitting analysis for the obtained rise curve was made on the basis of the following



relaxation scheme of the photoexcited all-trans isomer: hν

kISC

all-trans S0 98 f all-trans S1 9 8 all-trans T1 trans V kr + kIC ΦISC

(1)

Here, kr and kIC are the rate constants of the radiative decay and the internal conversion, respectively, and kISC is the rate of the intersystem crossing. In this scheme, the internal conversion process before the generation of the S1 state is omitted because this process finishes within 1 ps. The T1 decay process is also neglected. The following expression for the population change of the all-trans T1 state is obtained from the rate equation: -kASt ) [trT1trans(t)] ) [trS1(0)] Φtrans ISC (1 - e

Φtrans ISC ≡

kISC , kAS ≡ kr + kIC + kISC kAS

(2)

trans is the quantum yield of the intersystem crossing where ΦISC and 1/kAS () τAS) is the lifetime of the all-trans S1 state. The rise curve of the two T1 Raman bands were well reproduced by this functional form with the τAS value of ∼25 ps. The obtained time constant is in good agreement with the S1 f T1 intersystem crossing time that has been determined by time-resolved absorption and fluorescence spectroscopy.26,33-37 The lower trace in Figure 6 shows the frequency change of the T1 Raman bands. In the first 30 ps after photoexcitation, both of the two T1 Raman bands exhibit small but noticeable frequency upshifts. The frequencies of the T1 CdC stretch band shifts from 1545 cm-1 (5 ps) to 1553 cm-1 (g30 ps), while the T1 C-C stretch band shifts from 1179 cm-1 (5 ps) to 1183 cm-1 (g30 ps). The observed frequency shifts are well fitted with single-exponential functions having time constants of about 16 ps (14 ps for the CdC stretch band and 18 ps for the C-C stretch band). It is known that the vibrational cooling process (the energy dissipation from the solute molecule to the surrounding solvent molecules) takes place in this time scale in hydrocarbons38-41 and that Raman bands of the vibrationally excited molecule (hot bands) generally appear at lower frequency than that of the “cold” molecule. Since the absorbed photon energy is still localized in the solute molecule in the early picosecond time region, it is highly likely that the T1 state generated by the intersystem crossing initially holds plenty of excess vibrational energy in the early delay time. Therefore, the observed frequency shift is attributable to the vibrational cooling process in the T1 state. Temporal frequency shifts of Raman bands reflecting the vibrational cooling process have also been observed for other molecules such as trans-stilbene41 and trans-azobenzene.9 Although the quantum yield is low, all-trans retinal photoisomerizes to the cis isomer (mainly 13-cis ) to some extent. In this sense, the cis isomer either in the S0 state or in the excited state is expected to appear in the spectra. However, we were not able to observe any signal attributable to the cis isomer, and hence, we did not need to take it into account in our analysis. The contribution from the photoproduct was very small in the case of the photoexcitation of all-trans retinal, so that it was below the signal-to-noise ratio of the present measurements. In Figure 7, we show “net” picosecond time-resolved Raman spectra of all-trans retinal, in which only photoinduced change is shown. Not only the solvent Raman bands but also the signals corresponding to the S0 state that is not photoexcited (60%) have been subtracted. In the following sections describing the

Figure 7. Picosecond time-resolved Raman spectra of all-trans retinal in hexane in the delay time range from -10 to 400 ps (1 × 10-3 mol dm-3; pump laser 385 nm; probe laser 458 nm). The Raman intensity at each delay time has been normalized using the solvent band intensity. The Raman bands due to the S0 state that is not photoexcited as well as the solvent bands have been subtracted.

cis isomers, we show only this type of “net” time-resolved Raman spectra, which were obtained after the subtraction procedures. 3.2. 9-cis Retinal. Next, we describe the results obtained from 9-cis retinal. Figure 8 shows picosecond time-resolved Raman spectra of a hexane solution of 9-cis retinal. The Raman bands due to the S0 state that is not photoexcited, as well as the solvent bands, have been subtracted. The Raman intensity at each delay time is normalized using the solvent bands in order to cancel the self-absorption effect owing to the transient absorption. In the spectra before photoexcitation, the Raman bands due to the 9-cis S0 state are observed. With photoexcitation, the S0 Raman bands vanish (because S0 Raman bands due to the notphotoexcited molecules have been subtracted), and then several transient Raman bands grow with an increase of the delay time. The prominent transient Raman bands are located at 1596, 1549, 1336, 1293, 1251, 1180, and 1003 cm-1. They are essentially the same as those of the all-trans T1 state that we observed with photoexcitation of all-trans retinal. Especially, the spectrum taken at 1.6 ns is indistinguishable from the transient spectrum of the all-trans T1 state. It implies that we see the appearance of the all-trans T1 state after the photoexcitation of 9-cis retinal in these picosecond time-resolved Raman spectra. In the case of the photoexcitation of 9-cis retinal, however, the appearance of the all-trans T1 state is significantly delayed. The intensities of the all-trans T1 Raman bands continue growing even after 1 ns. This presents a striking contrast to the photoexciation of all-trans retinal, where the all-trans T1 state appears with the intersystem crossing time as short as 25 ps. Since the intersystem crossing time of 9-cis retinal is almost the same as that of the all-trans isomer,42 the observed slow rise of the all-trans T1 state represents the generation of the all-trans isomer as a result of the 9-cis f all-trans isomerization taking place in the T1 state.


Figure 8. Picosecond time-resolved Raman spectra of 9-cis retinal in hexane in the delay time range from -10 to 1.6 ns (1 × 10-3 mol dm-3; pump laser 385 nm; probe laser 458 nm). The Raman intensity at each delay time has been normalized using solvent band intensity. The Raman bands due to the S0 state that is not photoexcited as well as the solvent bands have been subtracted.

Figure 9. The temporal intensity change of the two transient Raman bands observed with photoexcitation of 9-cis retinal.

The temporal intensity changes of the two prominent transient Raman bands at ∼1550 cm-1 (the CdC stretch) and ∼1180 cm-1 (the C-C stretch) were evaluated by the fitting analysis, and they are plotted in Figure 9. Obviously, both of these transient Raman bands exhibit slow rises in the picosecond time region, reflecting the subnanosecond 9-cis f all-trans conversion process taking place in the T1 state.24 However, when we see these rise curves carefully, we readily notice that they are not simple exponential rises. The following two points need to be noted. First, the two curves contain a minor fast rise component in addition to the major subnanosecond slow rise. Second, the fast component is more pronounced in the rise of the CdC stretch band, so that the two rise curves look significantly different, especially in the early delay time. These two features of the rise curves cannot be rationalized if we assume that these two transient bands contain only contribution

Shimojima and Tahara from the all-trans T1 state that is produced by the 9-cis f alltrans isomerization. They imply that there exists additional minor contributions from other transients in the spectral region of these two transient Raman bands. There are two transient species that can give rise to the minor fast component in these rise curves. The first one is the alltrans T1 state that is produced by photoexcitation of “impurity” all-trans retinal. As described in the Experimental Section, alltrans retinal is produced as a product of photoisomerization of the 9-cis isomer, and it is accumulated as an impurity in the sample during the measurements. The photoexcitation of coexisting all-trans retinal generates the all-trans T1 state with the intersystem crossing time, which adds an additional fast rise component to the rise curve of the all-trans T1 state. The contribution from this “impurity” all-trans retinal is small but not negligible. This component, however, appears in all the alltrans T1 Raman bands with the same relative magnitude, so that it does not make the two rise curves look different. The second one, which is more important, is the 9-cis T1 state that is the precursor of the 9-cis f all-trans isomerization occurring in the T1 state. With photoexcitation of 9-cis retinal, the excitedstate having the 9-cis conformation is first generated. The present spontaneous Raman data, as well as the reported 2D-CARS data,24 indicates that the intersystem crossing from the S1 state to the T1 state occurs with preservation of its original 9-cis conformation, and then the structural change from 9-cis to alltrans takes place in the T1 state. Therefore, the T1 state having the 9-cis conformation should exist in the early picosecond time region. If this 9-cis T1 state exhibits weak Raman bands and they are overlapped with the CdC stretch and/or the C-C stretch bands of the all-trans T1 state, the contribution of the 9-cis T1 state gives rise to a minor fast component in the rise curves of the transient Raman bands. The observed difference of the two rise curves can be explained in terms of the difference of the magnitude of the 9-cis T1 contribution to each band. The data suggesting the existence of weak 9-cis T1 Raman signals have already been obtained in the previous 2D-CARS study.24 In the CARS spectra taken with 90-ps time resolution, no apparent signal attributable to the 9-cis T1 state was recognized, so that the CARS data were analyzed by assuming that the 9-cis T1 CARS signal is negligibly weak. The CARS spectra in the C-C stretch region were successively reproduced with this assumption, but the spectra in the CdC stretch region were not. This result was ascribed to a weak 9-cis T1 band that is overlapped with the CdC stretch band of the all-trans T1 state. In conclusion, when we quantitatively discuss the two rise curves shown in Figure 9, we need to take into account not only the major contribution from the all-trans T1 state that is produced by the isomerization of 9-cis retinal but also minor contributions from the 9-cis T1 state as well as the all-trans T1 state that originates from the “impurity” all-trans isomer. Therefore, we carried out fitting analysis of the two rise curves by taking these three components into consideration. The relevant photochemical dynamics of 9-cis retinal can be represented by the following scheme: hν

k′ISC

kiso

9-cis S0 98 f 9-cis S1 9 8 9-cis T1 98 all-trans T1 (3) 9cis V k′r + k′IC ΦISC By solving rate equations, we obtain the following expressions for the population change of the 9-cis T1 state (precursor of the isomerization) and the all-trans T1 state (product):


9-cis T1: [9cisT19cis(t)] ) [9cisS1(0)] Φ9cis ISC all-trans T1: [trT19cis(t)] )

{ (


(

)

k9S (e-kisot - e-k9St) (4) k9S - kiso

)

}

1 (k e-k9st - k9s e-kisot) k9s - kiso iso (5)

[9cisS1(0)] Φ9cis ISC 1 +

where

Φ9cis ISC ≡

k′ISC , k9S ≡ k′r + k′IC + k′ISC k9S

9cis ΦISC is the intersystem crossing quantum yield of the 9-cis isomer, and 1/k9S () τ9S) is the lifetime of the 9-cis S1 state. The population change of the all-trans T1 state that is generated from the impurity all-trans isomer is expressed by formula (2). Thus, we obtain the following functional form for the temporal behavior of the transient Raman bands that contain the three contributions:

I(t) ) A9cis[9-cisT19cis(t)] + Atrans[trT19cis(t)] + k9S (e-kisot - e-k9St) + Atrans[trT1trans(t)] ) C0 k9S - kiso

{ (

C1 1 +

(

)

)

}

1 (k e-k9St - k9s e-kisot) + k9s - kiso iso C2 (1 - e-kASt) (6)

For the rise curve of the C-C stretch band, we first attempted to reproduce it by setting the 9-cis T1 contribution (C0) at zero because it was reported that the 9-cis T1 CARS intensity was negligibly small in the C-C stretch region. However, the C2 value obtained with this restriction was too large (meaning too large a contribution from the impurity), which implies that this restriction is not adequate. Generally speaking, weak Raman bands appear with enhanced relative intensities in “linear” spontaneous Raman spectra, in comparison with those in CARS spectra. Thus, it is highly likely that the 9-cis T1 contribution has noticeable intensity even in the C-C stretch band region in the obtained spontaneous Raman spectra. Therefore, we made fitting of the two rise curves simultaneously without this limitation about C0. The lifetimes of the 9-cis S1 (1/k9S) and the all-trans S1 (1/kAS) were fixed at 34 ps42 and 25 ps, respectively, and the isomerization time constant (τISO ) 1/kiso), the amplitude factors (C0, C1), and the common relative amplitude of the “impurity” contribution (C2/C1) were treated as adjustable parameters. The best-fitted curves are shown with solid lines in Figure 9, and they are decomposed into the three components in Figure 10. The calculated curves successfully reproduced the experimental data. The parameters of the bestfitted curves were as follows: τISO ) 1010 ps, C0:C1:C2 ) 0.061: 1:0.11 for C-C stretch band, and C0:C1:C2 ) 0.20:1:0.11 for the CdC stretch band. These values obtained are very reasonable. Nevertheless, we should note that the errors in the C0 and C2 values are large, since the change of the C0 value can be compensated by the change of the C2 value to some extent with assistance of the slight change of the τiso value. Taking into account this uncertainty of the parameters, the time constant of the 9-cis f all-trans structural change in the T1 state should be expressed as 1000 ( 150 ps.

Figure 10. Result of the fitting analysis for the temporal intensity change of the two transient Raman bands observed with photoexcitation of 9-cis retinal (s slowly rising all-trans T1; -‚-‚- fast rising alltrans T1; ----- 9-cis T1; s the sum).

The amplitude of the third term, C2, may be worth brief discussion. The C2/C1 ratio is proportional to ([trS1(0)] trans 9cis )/([9cisS1(0)]ΦISC ), which means that it is proportional to ΦISC the ratio of the ground-state concentration, the photoexcitation efficiency, and the intersystem crossing yield of the all-trans and 9-cis isomers. As described in the Experimental Section, the ratio of the average amount of all-trans retinal during the measurement can be estimated approximately at ∼3%. The photoexcitation efficiency (i.e., the extinction coefficient) of the all-trans retinal is about 1.5 times larger than that of 9-cis retinal at the pump wavelength of 385 nm. Although the quantum yield of the intersystem crossing of retinal varies depending on the report,15,26,43-46 it seems fair to assume that there is no big difference between the two isomers. Taking these three factors into consideration, we can estimate that the contribution from “impurity” all-trans retinal is 4∼5%. The C2/C1 value determined by the fitting is ∼10%, which is about double the estimated value. This difference might suggest the existence of a minor additional pathway of the 9-cis f all-trans isomerization other than the triplet mechanism. For example, even if there exists a minor mechanism with which 9-cis retinal changes to all-trans in the excited singlet state (and then it is relaxed to the T1 state by intersystem crossing), the temporal behavior of the generated all-trans T1 state is indistinguishable from that due to the “impurity” all-trans isomer. As described already, it was difficult to determine the C2 value accurately so that, probably, the large C2 value is mainly attributed to the error in the analysis. Nevertheless, the possibility of the existence of the minor singlet mechanism may be also worth mentioning. Now, we can conclude that the 9-cis T1 state gives weak Raman signals, both in the CdC stretch and the C-C stretch band region. However, it was quite difficult to recognize its contributions without quantitative analysis because they are overlapped with much stronger all-trans T1 Raman bands. We searched for a transient Raman signal that is solely assignable


Figure 11. Picosecond time-resolved Raman spectra of 13-cis retinal in hexane in the delay time range from -10 to 400 ps (1 × 10-3 mol dm-3; pump laser 385 nm; probe laser 458 nm). The Raman intensity at each delay time has been normalized using the solvent band intensity. The Raman bands due to the S0 state that is not photoexcited as well as the solvent bands have been subtracted.

to the 9-cis T1 state in other spectral regions and noticed weak Raman signals around 1400 cm-1 (indicated by arrows in Figure 8). This weak Raman band appears with a time constant of a few tens of picoseconds and disappears in accordance with the appearance of the all-trans T1 Raman bands. The temporal behavior of this signal is that expected for a “pure” 9-cis T1 Raman band. Although its weakness did not allow us to carry out further quantitative analysis of this band, we assigned this 1400-cm-1 band to a Raman band characteristic to the 9-cis T1 state. The Raman signals due to the 9-cis T1 state were recognized clearly for the first time in the present spontaneous Raman measurements. 3.3. 13-cis Retinal. The photoisomerization mechanism of 13-cis retinal has not been well clarified in comparison with the mechanism of the 9-cis isomer. Nanosecond time-resolved Raman spectroscopy16 as well as picosecond absorption spectroscopy23 showed that two kinds of T1 states, the all-trans T1 state and the 13-cis T1 state, exist at 5-20 ns after photoexcitation of 13-cis retinal. This fact was interpreted as that the 13cis f all-trans isomerization takes place in the T1 state, but the reaction is very slow and is not completed even at 20 ns after photoexcitation.10,23 However, these spectroscopic data themselves do not exclude another possibility, that isomerization is completed in the excited singlet manifold before the intersystem crossing and that it does not proceed in the T1 state. In fact, as described in this section, our picosecond Raman data indicate that the latter mechanism is important in the 13-cis f all-trans photoisomerization. Figure 11 shows time-resolved Raman spectra obtained from a hexane solution of 13-cis retinal. The self-absorption effect has been corrected, and the solvent Raman bands have been subtracted. Under this experimental condition, the intensity of the 13-cis S0 bands decreased by 30% immediately after photoexcitation. The unchanged S0 Raman intensity has been


Figure 12. Comparison between the transient Raman spectra obtained from all-trans and 9-cis retinal: (a) spectrum obtained at 400 ps after photoexcitation of all-trans retinal; (b) spectrum obtained at 400 ps after photoexcitation of 13-cis retinal (the spectrum obtained from alltrans retinal is also shown with the broken line for comparison); and (c) subtracted spectrum, (b)-(a) × 0.5.

also subtracted. In the time-resolved Raman spectra, only S0 Raman signals due to 13-cis retinal, e.g., 1578 cm-1, are seen at a negative time delay of -10 ps. The S0 Raman bands disappear with the photoexcitation (0 ps), and then several transient Raman bands appear. Two prominent transient Raman bands are located at 1550 and 1182 cm-1. The intensities of these transient Raman bands reach their maximum within 100 ps, and they do not change afterward. We measured timeresolved Raman spectra up to 1.6 ns, but we did not recognize any noticeable change after 100 ps. The rise time of the transient Raman signal is very similar to a typical intersystem crossing time of retinal. In other words, it looks that the transient Raman signal appears with the intersystem crossing and no noticeable change takes place afterward in the T1 state (at least up to a few nanoseconds) in the case of the photoexcitation of 13-cis retinal. The vibrational frequencies of the two prominent transient Raman bands, 1550 and 1182 cm-1, are very similar to those of the CdC stretch and the C-C stretch bands of the all-trans T1 state. However, the transient spectrum obtained from 13-cis retinal is not identical to the all-trans T1 spectrum. In Figure 12, the transient Raman spectrum taken from 13-cis retinal at 400 ps is compared with that of the all-trans T1 state. The following points are readily recognized as differences between the two spectra: (1) the relative intensity of the CdC stretch band and the C-C stretch band, (2) the band shape (width) of the C-C stretch band, (3) the spectral feature in the region from 1200 to 1400 cm-1, and (4) the high-frequency shoulder of the CdC stretch band, which is assigned to the CdO stretch vibration. It is obvious that the transient Raman spectrum obtained from 13-cis retinal is not simply ascribed to the alltrans T1 state.


Figure 13. The temporal intensity change of the two transient Raman bands observed with photoexcitation of 13-cis retinal.

To consider the transient species appearing in the picosecond time-resolved Raman spectra, we analyzed the rise of the two prominent Raman bands. Figure 13 depicts the intensity change of the 1550-cm-1 band and the 1182-cm-1 band. As clearly seen, these two transient Raman bands show a quite fast rise within 100 ps. More interestingly, the rise curves of these two transient Raman bands are noticeably different from each other. In fact, we obtained time constants of ∼11 ps and ∼19 ps for the rise of the 1550-cm-1 band and the 1182-cm-1 band, respectively, assuming single-exponential rises (the functional form of formula (2)). This difference in the rise time of the two transient Raman bands implies that the observed transient Raman spectra are not due to a single transient species but attributable to a mixture. Since the previous time-resolved spectroscopic study showed that there exists a mixture of the 13-cis T1 state and the all-trans T1 state at 5-20 ns, it is natural to think that the mixture existing in the picosecond time region is also ascribed to the same mixture. This assignment of the transient species was further confirmed by the following spectral analysis. In the previous nanosecond Raman study,16 Hamaguchi et al. obtained the spectrum of the 13-cis T1 state by spectral subtraction and showed that the 13cis T1 state gives a Raman band at ∼1550 cm-1 but does not exhibit any strong band at ∼1180 cm-1. Therefore, it is safely assumed that the 1182-cm-1 band in our picosecond Raman spectra is predominantly due to the all-trans T1 state, even if a small contribution from the 13-cis T1 state is overlapped. Thus, we subtracted the all-trans T1 spectrum from the spectrum shown in Figure 12b after normalization at the peak of the 1182cm-1 band in order to decompose the Raman spectrum of the mixture into the spectra of each transient species. The spectrum obtained by this spectral subtraction is shown in Figure 12c. The obtained spectrum is essentially the same as the reported spectrum of the 13-cis T1 state, although the probing wavelength (458 nm) in the present study is different from that used in the nanosecond measurement (532 nm). This spectral analysis directly manifests that the picosecond time-resolved Raman spectra obtained from 13-cis retinal are ascribed to the mixture of the all-trans T1 state and the 13-cis T1 state. We evaluated the ratio of the all-trans T1 contribution in the two transient Raman bands on the basis of the decomposed spectra. It was estimated approximately as 50% for the 1550-cm-1 band and as 80% for the 1182-cm-1 band. Although the sample solution contains a small amount of all-trans retinal as an impurity, the observed large Raman signals due to the all-trans T1 state cannot be attributed merely to the impurity. It is ascribed to the alltrans T1 state that is generated by isomerization of 13-cis retinal.

J. Phys. Chem. B, Vol. 104, No. 39, 2000 9297 The present picosecond time-resolved Raman data revealed that the mixture of the all-trans T1 state and the 13-cis T1 state appear with time constants of a few tens of picoseconds after photoexcitation of 13-cis retinal. In addition, the two transient Raman bands having different relative contribution from the two T1 states rise with different time constants, which implies that the 13-cis T1 state and the all-trans T1 state are generated separately. These facts indicate that the 13-cis f all-trans isomerization takes place in the excited singlet state and that the resultant all-trans and 13-cis excited singlet states are relaxed to the corresponding T1 states separately, with different time constants that are inherent to each isomer. This argument is supported by the fact that the apparent rise time of the 1182cm-1 band (∼19 ps) is close to the intersystem crossing time of the all-trans isomer (∼25 ps) although it contains ∼20% contribution from the 13-cis T1 state. The faster rise of the 1550cm-1 band that contains a greater 13-cis T1 contribution implies that the rise time of the 13-cis T1 state is shorter. The picosecond Raman data also showed that the relative population of the generated 13-cis and all-trans T1 states does not change significantly after 100 ps. This means that the 13-cis f alltrans structural change taking place in the T1 state is negligible, at least in the time range of a few nanoseconds. To examine the above arguments more quantitatively, we simulated the rise curve of the two transient Raman bands (shown in Figure 13) on the basis of the following reaction scheme:

{

13-cis S1

hν

13-cis S0 98 9 8 + 13cis Φiso

k13S

9 8 13-cis T1 13cis ΦISC

(7) kAS

8 all-trans T1 all-trans S1 9 trans ΦISC

where 1/k13S () τ13S) and 1/kAS () τAS) are the lifetimes of the 13-cis S1 state and the all-trans S1 state, respectively. Φ13cis iso is the quantum yield of the all-trans S1 state that is generated from 13cis trans and ΦISC are the intersystem crossing 13-cis retinal. ΦISC quantum yields of the 13-cis and all-trans isomers, respectively. (In this model, we do not specify the excited singlet state in which the isomerization takes place.) The temporal intensity change of the transient Raman signal that contains contributions from both the 13-cis T1 state and the all-trans T1 state is represented as follows:

I(t) ) A13cis[13cisT113cis(t)] + Atrans[trT113cis(t)] ) C′0 (1 - e-k13St) + C′1 (1 - e-kASt) (8) On the basis of the spectral decomposition, we already evaluated the all-trans T1 contribution in the two transient bands at a late decay time (k13St, kASt . 1), which corresponds to the following values: C0′:C1′ = 50:50 for the 1550 cm-1 band and = 20:80 for the 1182 cm-1 band. The lifetime of the all-trans S1 state (τAS) can be fixed at 25 ps, which was obtained from the picosecond Raman measurements of all-trans retinal described in the section 3.1. With these parameters fixed, we adjusted only the τ13S value to find the best-fitted functions for the two rise curves simultaneously. The best curves were obtained with the τ13S value of 4 ps, and they are shown in Figure 13 with solid lines. Some arbitrariness in the spectral decomposition procedure as well as the relatively low signal-to-noise ratio of the time-resolved Raman spectra of 13-cis retinal can cause error in this simulation. Nevertheless, the simulated curves fairly well



reproduced the observed rise curves, which strongly supports our interpretation of picosecond time-resolved Raman spectra of 13-cis retinal. As far as the authors are aware, the present picosecond Raman study affords the first clear and direct spectroscopic evidence showing that the isomerization of 13-cis retinal occurs in the excited singlet state (the singlet mechanism). The possibility of the existence of the singlet mechanism has been mentioned,47 but it was considered that the singlet pathway is very minor in nonpolar solvents, even if it exists.48 In this sense, it is important to discuss the isomerization quantum yield that is attributable to the singlet mechanism. Thus, we attempted to evaluate the singlet isomerization quantum yield from the observed picosecond time-resolved Raman spectra, as described below. Since the all-trans S1 state is almost exclusively relaxed to the all-trans S0 state (directly or through the T1 state), Φ13cis iso defined in scheme (7) is considered to be almost equivalent to the singlet isomerization quantum yield of 13-cis retinal. The amount of the all-trans T1 state that is obtained with the photoexcitation of 13-cis retinal can be represented as follows, by assuming the reaction scheme (7): trans [trT113cis] ) N13cisΦ13cis iso ΦISC

(9)

Here, N13cis is the number of the photoexcited 13-cis molecules. If we compare this amount with the amount of the all-trans T1 state that is obtained with photoexciation of all-trans retinal, we have the following expression for their ratio:

R)

[trT113cis] [trT1trans]

)

trans N13cisΦ13cis iso ΦISC

NtransΦtrans ISC

)

N13cis 13cis Φiso (10) Ntrans

We measured the picosecond time-resolved Raman spectra of 13-cis retinal and all-trans retinal under identical experimental conditions (e.g., concentration, pump and probe power, etc.). Then, we decomposed transient Raman spectra obtained from 13-cis retinal into the spectra of the 13-cis T1 state and the alltrans T1 state and compared the all-trans T1 signal intensity with that obtained by the photoexcitation of all-trans retinal. It was found that the intensity obtained with the 13-cis photoexcitation is about 20% of the intensity observed with the alltrans photoexcitation. Taking account of ∼4% contribution from “impurity” all-trans retinal that coexists in the 13-cis sample, we obtained the value of 0.16 as the ratio, R. The ratio of the number of the photoexcited molecules, N13cis/Ntrans, was obtained as 0.75 from the depletion of the S0 Raman bands immediately after photoexcitation: 30% for 13-cis and 40% for all-trans. (Note that this ratio is consistent with the difference in the extinction coefficient of the two isomers at the pump wavelength: 13cis ) 2.8 × 104, trans ) 3.7 × 104) By using these values, we obtained 0.21 as the Φ13cis iso value from formula (10). This evaluated isomerization quantum yield of the singlet mechanism is comparable with the overall isomerization quantum yield of 13-cis retinal reported by Waddel et al. (∼0.20),12 whereas it is about half the value given by Kropf and Hubbard49 and Feis et al.48 (∼0.4). Since there are large errors in the reported isomerization quantum yield of 13-cis retinal, we cannot make a further quantitative discussion. However, our estimation of the singlet isomerization quantum yield indicates that the singlet mechanism is not a minor mechanism but a major pathway (or one of major pathways) in the isomerization of 13-cis retinal. 3.4. Mechanism of cis-trans Photoisomerization of Retinal. Figure 14 shows a schematic diagram of the cis-trans

Figure 14. Schematic diagram of the photoisomerization pathways of the retinal isomers. The time constants obtained in the present study are given in parentheses. See text.

photoisomerization pathways of 9-cis and 13-cis retinal in nonpolar solvents, which is drawn on the basis of spectroscopic data available at present. We now compare the isomerization mechanism of these two isomers. The reaction mechanism of 9-cis retinal can be described as follows. The ground state molecule is initially excited to the S3 state having a ππ* character by direct photoexcitation. This S3 state is relaxed to the S2 state (

Picosecond Time-Resolved Resonance Raman Study of the

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