Mechanistic insights into photochromic 3H-naphthopyran showing strong photocoloration.

3,3-Diphenylbenzo[f]chromene (1) represents an important architectural platform for photochromic systems. Since the practical utility of such chromophores is largely dependent upon the kinetics of coloration and decoloration, elucidating the mechanistic details of these processes is of great value. Toward this end, we studied the photochromic reaction of (3-(2-methoxyphenyl)-3-phenyl-3H-benzo[f]chromene (2) by both time-resolved UV-vis and mid-IR spectroscopies. We found that irradiation of 2 at 365 nm generates long-lived colored transoid-cis isomers with lifetimes of 17.1 s and 17.5 min (at 21 °C) and even longer-lived transoid-trans isomers with a lifetime of 16 h. These experimental results were supplemented with ab initio ground-state and excited-state calculations, and the resulting theoretical interpretation may be useful for the design of new photochromic systems with optimized photofunctionality.