Mechanism of photochromic transformations of peri-acyloxy-9,10- and 1,4-anthraquinone derivatives

The thermally reversible phototransfer of the acyl group of 9-acetoxy-1,4-anthraquinone and a series of 1-acyloxy-9,10-anthraquinone derivatives was discovered and investigated using low temperature and flash photolysis techniques. The relationship between the reactivity of these compounds and their chemical structure was studied. The correlation diagrams of the energy levels of the initial compounds and the photolysis products were analysed, and an adiabatic excitation-conserving mechanism is proposed for the photochemical migration of the acyl group. Formation of triplet-excited 9-acetoxy-2-methoxy-1,10-anthraquinone was detected experimentally during the course of photochemical isomerization of 1-acetoxy-2-methoxy-9,10-anthraquinone. The influence of temperature, medium and nature of the substituents in the anthraquinone ring and migrant on the rate constant of the thermal migration of the acyl group was studied. Donor substituents in the migrant led to a substantial decrease (up to five orders of magnitude) in the rate constant of migration and acceptor substituents to a substantial increase. The irreversible photochemical reactions of the 1-acetoxy-9,10-anthraquinone 2-dialkylamino derivatives were investigated. A two-stage mechanism involving the participation of excited molecules caused by the action of one photon is proposed.