Picosecond time-resolved fluorescence study of all-trans retinal. The existence of two fluorescent singlet excited states

Abstract

Picosecond time-resolved fluorescence spectra of all-trans retinal in hexane have been measured with 8 ps time-resolution with use of a highly sensitive streak camera. The observed fluorescence consists of a fast ( τ<1 ps, λ max≈470 nm) and a slow ( τ=33–34 ps, λ max≈540 nm) component. The lifetime of the slow component agrees well with the reported rise time of the triplet-triplet absorption. It is concluded that the photoexcitation first produces the optically allowed S 2 state and it relaxes to the optically forbidden S 1 state within 1 ps. The fast and slow fluorescence components are assigned to the S 2 and S 1 states, respectively.