Add time:09/09/2019         Source:sciencedirect.com

The quantum yields of fluorescence φf, cis→trans photoisomerization φc→t and trans→cis photoisomerization φt→c, and photostationary trans/cis ratios of 4-cyanostilbene, 4,4′-dicyanostilbene, 4-cyano-4′-methoxystilbene and 4-cyano-4′-dimethylaminostilbene were measured in several solvents as functions of quencher (ferrocene, azulene and oxygen) concentration, temperature and viscosity. The results suggest a singlet pathway for the direct trans→cis photoisomerization of the cyanostilbenes.Photosensitized cis—trans isomerization and triplet energy transfer measurements were performed at room temperature for 4-cyano-4′-methoxystilbene. A transient was observed be laser flash photolysis for trans isomers of 4-cyanostilbene, 4,4′-dicyanostilbene and 4-cyano-4′-methoxystilbene at high viscosities. On decreasing the temperature and thus increasing the viscosity the yield φT and lifetime τT of the transient increase, reaching constant values in rigid media where φt→c is almost zero. The viscosity independence of the transient spectrum and the temperature dependences of φT and τT lead to the identification of the transient as the planar trans configuration of the lowest triplet state. Contribution of this triplet state and possibly of a higher excited triplet state to the trans→cis photoisomerization is suggested to be very small even at low temperatures.

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