Photochemical reaction of lucigenin : electron-transfer reduction of 10,10′-dimethyl-9,9′-biacridinediium by counter anions, followed by photocyclization

Abstract

Lucigenin (1) is a charge-transfer complex between 10,10′-dimethyl-9,9′-biacridinediium (DMBA²⁺) and two nitrate anions. When the complex (1) was irradiated in the charge-transfer absorption band (>510 nm) in deaerated solutions, DMBA²⁺ was converted into the reduced form, 10,10′-dimethyl-9,9′-biacridylidene (2). On the other hand, irradiation of (1) with visible light (>420 nm) gave a red product assumed to be 7,16-dimethylbenzo[1,2,3-kI:6,5,4-k′I′] diacridine (4). A mechanism is suggested which involves electron-transfer photoreduction of DMBA²⁺ by counter anions, followed by photocyclization of (2) to give a dihydrophenanthrene-type intermediate (3), and subsequent dehydrogenation.