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

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Abstract
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.