Regioselective E(trans)–Z(cis) photoisomerization in naphthyldiene derivatives

Naphthyldiene derivatives, 1–4, carrying electron-donating groups at one end and electron-withdrawing groups at the other, were synthesized to study the photoisomerization process. All the compounds showed efficient photoisomerization upon direct excitation leading to the formation of 4–Z isomer with high selectivity. Triplet sensitization studies indicated inefficient E–Z isomerization process. Room temperature fluorescence of 1 and 2 displayed fine structure in hexane solvent and the same was replaced by broad or structureless fluorescence in acetonitrile and methanol solvents. A mechanism involving a polarized or charge transfer singlet excited state is proposed for the observed photoisomerization in these naphthyldiene derivatives.