Efficient catalysis of the photoisomerization of stilbene derivatives by zinc porphyrin complexation

Abstract Very high quantum yields have been obtained for the cis – trans photoisomerization of 1-(1-naphthyl)-2-(4-pyridyl)ethylene (NPE) using zinc(II)-5,15-(3,5-di- tert -butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (ZnP) as sensitizer and catalyst. The quantum yield was found to increase with the concentration of the zincporphyrin to a maximum of about 85. At higher concentrations of the porphyrin the quantum yield decreases. The increase in quantum yield at low ZnP concentrations is explained as being due to energy transfer processes. Triplet energy is transferred from a complex of trans -isomer and ZnP to a ground state complex of cis -isomer and ZnP. Formation of excimers and exciplexes is probably responsible for the decrease in quantum yield at high concentrations of ZnP. Ligand exchange in complexes seems to have only a minor effect on the quantum yields.