Excited states

Abstract

Femtosecond time-resolved fluorescence spectroscopy (up-conversion) was applied to directly determine the lifetime of the S2 state of 0-carotene in n-hexane at room temperature. Upon excitation with a 425-nm pulse, the lifetime was measured to be 195 ± 10 fs throughout the whole emission wavelength, and the emission anisotropy ratio at 539 nm was 0.39 ± 0.02 throughout the emission time. Absence of a dynamic Stokes shift suggests that intramolecular relaxation occur within 50 fs, followed by internal conversion from the S2 to Si state in 195 fs. Carotenoids have important physiological functions in pho-tosynthesis. They work as light-harvesting pigments due to a large oscillator strength in the visible region and efficient singlet energy transfer to chlorophylls. Further, they work as quenchers of chlorophyll triplet state and of singlet molecular oxygen due to their low-lying triplet state.* 1 Carotenoids are, in general, derivatives of polyenes belonging to the Cih point group.2 Two energetically low-lying singlet states are expected; one is closely related to the 2'Ag (Si) state, which is dipole forbidden from the ground state by parity, and the other is related to the 1 *BU (S2) state, which is responsible for a strong visible absorption (Figure l).3 Thus, the following photophysical processes are expected: hv *21 Optical excitation to the S2 state induces an internal conversion to the Si state, followed by relaxation to the ground state. Singlet energy transfer to chlorophylls is a competitive relaxation process from both states,4 and hence the accurate estimation of rate constants is very important. A lifetime of the Si state (l/fc,0) was measured by ground-state recovery experiments, ~ 10 ps for 0-carotene5 (molecular structure is shown below and steady-state spectra in Figure 1), whereas that of the S2 state (l/fc2i) was estimated by the subpicosecond transient absorption of 0-carotene.4b Shreve et al.4b measured kinetics including two relaxation processes and detected a lifetime of the S2 state to be 200 or 250 fs in ethanol or CS2, respectively, and a lifetime of the Si state, 9.5 or 11 ps, respectively. However, a direct measurement of the lifetime of S2 state in the femtosecond time regime is necessary to characterize the excited-state dynamics. I Present address:. 1991, 178, 89-96. (5) (a) Wasielewski, M. R.; Kispert, L. D. Chem. Phys. Lett. 1986,128, 238-243. (b) Frank, H. A.; Farhoosb, R.; Gebhard, R.; Lugtenburg, J.; Gosztola, D.; Wasielewski, M. R. Chem. Phys. Lett. 1993, 207, 88-92. Figure 1. Absorption and corrected fluorescence spectra of 0-carotene in n-hexane at room temperature. The sample concentration was either 2.5 X *5 or 1.3 X 10"* M for absorption or emission measurements, respectively. The emission spectrum was obtained by excitation at 430 nm. Time / ps Figure 2. Typical fluorescence decay kinetics (at 513 nm) of 0-carotene in n-hexane at room temperature. The instrumental response function (IRF) is shown by a broken line. The polarization angle between the excitation and the gate pulses was set to 54.7°. Solid circles indicate the observed photon counts with 10-fs intervals. The solid line is the best-fit curve, obtained by convolution with the present IRF and a single exponential decay of 195-fs lifetime. The-square was adopted as an index of the best fit, and the simulation was performed in 5-fs intervals. The-square was less than 0.5 for the best fit. We adopted the fluorescence up-conversion technique6 to determine directly a lifetime of the S2 state of 0-carotene. In our system, a combination of sum frequency generation and single-photon counting detection ensured a high sensitivity for fluo-rescence in the femtosecond time regime.1 The second harmonics (425 nm) of a mode-locked Ti:sapphire laser (Spectra Physics, Tsunami) was used to excite a sample, and the fluorescence and a fundamental gate pulse (850 nm) were focused onto a phase-matched thin BBO crystal to generate ultraviolet sum frequency light. The instrumental response function, obtained by the cross-correlation measurement between the excitation and the gate pulses, had a Gaussian shape of 200 fs (fwhm, Figure 2, broken line). 0-carotene was purchased from Wako Chemicals, and its all-trans form was purified by high-performance liquid chroma-te) (a) Shah, J. IEEE J. Quantum Electron. 1988, 24, 276-288. (b) Kobayashi, T.; Takagi, Y.; Kandori, H.; Kemnitz, K.; Yoshihara, K. Chem.