Reversible photochemistry, photo-oxidation, and fluorescence of dixanthylidene: temperature and external spin-orbit perturbation effects

Abstract

A detailed experimental study of the photochemistry of dixanthylidene over a wide temperature range indicates the existence of three labile photoisomers, B, C, and P, all of which revert thermally to A. Light-stable B, formed from the excited triplet of the fundamental modification A, is thermally stable below –140°. The structure of B involves torsion of ca. 50° about the central double bond. C And its possible precursor P are photolabile isomers formed from excited singlet A. They are photocyclisation products of the 4a,4b-dihydrophenanthrene type; C is photoconvertible into A. The thermal stability of P is much less than that of C. Helixanthen is obtained on thermal dehydrogenation of C with molecular oxygen or iodine. The quantum yield of A [graphic omitted] B drops with the temperature and is very smail at –140° but can be strongly enhanced (up to 220-fold) by the spin-orbit coupling perturbers molecular oxygen, carbon disulphide, and ethyl iodide.