PICOSECOND DYNAMICS OF THE CIS-TRANS PHOTOISOMERIZATION OF STILBENES IN SOLUTION

Picosecond and femtosecond time resolved laser experiments were carried out to investigate the photoinduced cis-trans isomerism of stilbenes in solution. The molecular dynamics of this conceptually simple intramolecular rearrangement have been elucidated by direct measurement of the picosecond and subpicosecond fluorescence, excited state absorptions, and the ground state recovery of the stilbene system. The results have confirmed some of the earlier ideas of the photoisomerism process but revealed that the photophysics are more complex than previously proposed. The results of the experiments on stilbene (1,2-diphenylethylene) and stiff stilbene (1,1\u27-biindanylidene) reveal that following ultra-violet excitation into the first electronically excited state, this state exhibits a strong induced absorption with a peak near 580 nm. The absorption subsequently decays on a (TURN)100 ps timescale in transtilbene, (TURN)10 ps in trans stiff stilbene, and 1.4 ps in cis-stilbene. The decay of this absorption signals twisting about the ethylenic bond. Furthermore, the shape of the transient absorption spectrum of trans-stilbene evolves on a 20 ps timescale. This is attributed to thermalization of the excess vibrational energy introduced by the excitation pulse. The absence of spectral evolution at lower excitation energies confirms this interpretation. Several experimental investigations were also carried out to spectroscopically identify intermediate states in the cis-trans isomerization of stilbenes. Following excitation at 264 nm, trans stiff stilbene exhibits a transient absorption at 351 nm as well as the visible absorption. The viscosity and temperature dependence of the 351 nm absorption is consistent with the decay of the excited trans being the precursor of the new transient. Quantitative analysis of the kenetics suggests that after the viscosity dependent formation of the intermediate it decays away with a time constant of 10 ps. The ground state recovery measurements on cis-stilbene and stiff stilbene also indicate that a 10 ps bottleneck is consistent with the observed time evolution. The most evident assignment of this intermediate is the twisted phantom state in the stilbene photochemistry