Direct Observation of the Rate for Cis->Trans and Trans->Cis Photoisomerization of Stilbene with Picosecond Laser Photolysis

Article abstract of DOI:10.1246/bcsj.55.85

The rates for direct cis->trans and trans->cis photoisomerizations of stilbene in solution were measured with a newly developed monitor light source for picosecond laser photolysis in the UV region. The rate constant of the trans->cis isomerization is 1.2*10¹⁰ s^-1 in hexane at 295 K. The fluorescence decay rate constant of the S₁ state of trans-stilbene has the same value. Thus, the decay of the S₁ state is the rate limiting step and the lifetime of an intermediate perpendicular state (¹p) has to be very short. The cis->trans isomerization is very fast and the half life of the excited cis-isomer is less than a few picosecond. These observations strongly indicate that the isomerization reaction proceeds through the potential surface of the singlet excited states.