Photochemical E (trans) -> Z (cis) isomerization in substituted 1-naphthylacrylates

Substituted naphthylacrylates, 1-3, not showing rotamerism have been synthe sized with a view to study photochemical E (trans) --> Z (cis) isomerizatio n. Photostationary state composition of the isomers upon direct excitation, triplet sensitized isomerization, quantum yield of isomerization, and stea dy state and time-resolved fluorescence behavior have been studied for thes e naphthylacrylates. The direct excitations of the compounds yield high Z ( similar to 80%) isomer composition, whereas the triplet sensitization resul ts in less Z (similar to 20%) isomer composition. This indicates that the s inglet pathway is very efficient in converting the E isomer to the Z isomer . The naphthylacrylates 1 and 2 exhibit structured fluorescence at room tem perature in hexane and upon changing the solvent to CH3CN; the structure of the fluorescence is lost, indicating that the singlet excited-state develo ps a polar character in a polar environment. The polar nature of the single t excited state becomes more clear in the case of 3 from its fluorescence s olvatochromism. The naphthylacrylates did not exhibit excitation wavelength -dependent fluorescence at room temperature suggesting that the ground stat e conformers (rotamers) are not involved. Fluorescence lifetimes measured f or these compounds displayed biexponential behavior, which is explained usi ng a two-state model.