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 high ly sensitive streak camera. The observed fluorescence consists of a fa st (tau < 1 ps, lambda(max) approximate to 470 nm) and a slow (tau = 3 3-34 ps, lambda(max) approximate to 540 nm) component. The lifetime of the slow component agrees well with the reported rise time of the tri plet-triplet absorption. It is concluded that the photoexcitation firs t produces the optically allowed S-2 state and it relaxes to the optic ally forbidden S-1 state within 1 ps. The fast and slow fluorescence c omponents are assigned to the S-2 and S-1 states, respectively.