Picosecond time-resolved multiplex coherent anti-Stokes Raman scattering spectroscopy by using a streak camera: Isomerization dynamics of all-trans and 9-cis retinal in the lowest excited triplet state

Abstract

A picosecond time‐resolved multiplex coherent anti‐Stokes Raman scattering (CARS) measuring system using a streak camera was constructed. Picosecond uv laser pulses (50 ps) were used for the photoexcitation and nanosecond laser pulses (7 ns) were used as ω₁ and ω₂ for CARS probing. The multiplex CARS signals were spectrally analyzed by a spectrograph, and time‐resolved and detected by a streak camera. This system enabled us two‐dimensional (the time and frequency domain) detection of CARS signals with picosecond time resolution. The cis–trans photoisomerization dynamics of all‐trans and 9‐cis retinal was investigated. In the case of all‐trans retinal, the CARS signals due to the ‘‘all‐trans‐like’’ T₁ state appeared in accordance with the decay of the S₁ fluorescence. In contrast, the appearance of the all‐trans‐like T₁ signals was markedly delayed in the case of the 9‐cis isomer. This indicates that there exists a T₁ state having ‘‘9‐cis‐like’’ conformation in the picosecond time region. The lifetime of this 9‐cis‐like T₁ state was obtained as 880±150 ps from the analysis of the rise of the all‐trans‐like T₁ state. The mechanism of retinal photoisomerization is discussed on the basis of the obtained picosecond time‐resolved data. It is concluded that the conformational change from the 9‐cis to the all‐trans form in the triplet manifold proceeds from the thermalized (not vibrationally excited) 9‐cis‐like T₁ state.