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

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, λmax≈470 nm) and a slow (τ=33–34 ps, λmax≈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 S2 state and it relaxes to the optically forbidden S1 state within 1 ps. The fast and slow fluorescence components are assigned to the S2 and S1 states, respectively.