Singlet and triplet states in the cis—trans photoisomerization of 4-cyanostilbenes in solution

Abstract

The quantum yields of fluorescence φ f, cis→trans photoisomerization φ c→ t and trans→cis photoisomerization φ t→ 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 φ T and lifetime τ T of the transient increase, reaching constant values in rigid media where φ t→ c is almost zero. The viscosity independence of the transient spectrum and the temperature dependences of φ T and τ T 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.