Spectroscopic and photochemical study of bianthrone

A spectroscopic and photochemical study of 2,7' dimethylbianthrone and 4,5' dimethylbianthrone was carried out. Absorption and emission spectra are reported for 2,7' dimethylbianthrone, 4,5' dimethylbianthrone, 3,6' dimethylhelianthrone, 3,6' dimethylmesonaphthobianthrone, and the photochromic form of 2,7' dimethylbianthrone. The photochemistry of 2,7' dimethylbianthrone was extensively studied. A free radical mechanism is proposed in which the initial reactive species is an excited state of the photochromic form of bianthrone and a secondary reactive species is the photocyclized helianthrone. The influence of the viscosity and steric effects of rigid glasses on the spectra and photochemistry is also considered.