TRANSIENT AND PHOTOPRODUCT STUDIES OF BIANTHRONE AND SUBSTITUTED BIANTHRONES

The photochromic transients of bianthrone, 2,2′-dimethylbianthrone, 4,4′-dimethylbianthrone, and 2,2′-dibromobianthrone as well as their photolysis products and the transients of the photoproducts have been investigated from 20 to 55° in benzene solvent. The photochromic state is produced via the excited singlet state and thermally decays by first-order kinetics (k1 = 3-11 sec-1 at 25°) with Ea = 14-15 kcal mol-1. The helianthrone and naphthodianthrone photoproducts have triplet transients which decay back to the ground state by second-order diffusion-controlled kinetics (k2 = 1.3-1.7 × 109 1. mol-1 sec-1). 4,4′-Dimethylbianthrone is photochemically stable and only reversibly cycles through the photochromic state.