The reversible photochemistry of dixanthylidenes: Temperature, external spin-orbit perturbation effects and structure of the C and F photoisomers

Abstract

An extensive photochemical study of dixanthylidenes shows the existence of three unstable isomers: B, F and C. The C isomer was shown by low temperature proton n.m.r. to be a photocyclization product at the 1 and 1′ positions. The structure of the F isomer was shown to involve both rotation of 180° about the central double bond and additional torsion about the four single bonds attached to it. The quantum yields for the A → B process are enhanced by external spin-orbit perturbers. The B isomer was shown to be formed from the triplet state. The formation of C, B and F are temperature controlled. Fluorescence quantum yields in these systems are strongly temperature dependent.