Structure of the short wavelength photoisomers of bianthrone analogues

Abstract

New unstable photoisomers absorbing of short wavelengths were observed for 10,10′-dimethyl-Δ9,9′-biacridanylidene, bithioxanthenylidene, bianthrylidene, and 10.10′-dihydroxybianthrylidene. A C_2v symmetry structure, considerably twisted about the four single bonds of the 9,9′-ethylene unit is suggested by a CFF–π electron–Cl geometry calculation. This conclusion is supported by analyses of the low temperature n.m.r. spectra and of the effects of the conformational changes on the electronic transition energies. Complete thermal conversion to the starting isomer was observed in all cases.