Reversible photochemistry of 10,10′-dimethylbiacridan: internal and external heavy atom effects, and the structure of photoisomer F

Abstract

An extensive study of the photochemistry of 10,10′-dimethyl-(3a), 1,1′,10,10′-tetramethyl-(3b), and of 2,2′,7,7′-tetrabromo-10,10′-dimethyl-biacridan (3c) revealed the formation of two light-stable photoisomers E and F, from the fundamental modification A. The photoisomerization quantum yields ϕ_A→E and ϕ_A→F are enhanced by oxygen, xenon, alkyl halides and also by carbon disulphide. Flash photolysis studies of both direct and sensitized photoisomerization A → E suggest that E is formed through the triplet manifold. The formation of the E isomer involves rotation by 180° about the 9,9′-double bond as well as additional rotation of the benzene rings about the single bonds attached to the central double bond. ¹H N.m.r. spectra of photoisomer F measured at low temperatures, and geometry calculations by the CFF-π electron-Cl method suggest that compared with A the structure of F involves torsion about both the central double bond and about the single bonds attached to it.