PICOSECOND TIME-RESOLVED FLUORESCENCE STUDY OF ALL-TRANS RETINAL - THE EXISTENCE OF 2 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 (tau < 1 ps, lambda(max) approximate to 470 nm) and a slow (tau = 33-34 ps, lambda(max) approximate to 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.