Effect of heteroaromatic annulation with five-membered rings on the photochromism of 2H-[1]-benzopyrans

Photochromic properties (colorability, UV-VIS spectra of photo-merocyanines, rate constant of thermal bleaching) of chromenes annulated with different five-membered azaheterocycles or with furan cycles were investigated under flash photolysis. All the compounds, i.e., the 7H-pyrano[2,3-g]benzazoles 4-6, the 7H-pyrano[3,2-e]benzazoles 7-10, the 7H-pyrano[3,2-e]indoles 11-12, the 3H-pyrano[3,2-a]carbazole 13, the 8H-pyrano[2,3-e]indole 14 and the 3H-furano[3,2-f]chromenes 15-18 undergo reversible photochromic reaction at room temperature. They are compared with naphthopyranic parents 1-3. Whatever the structure of the heteroaromatic moiety, the annulation of the benzopyrans leads to the splitting into two bands of the electronic absorption spectra of the photoinduced forms with a significant bathochromic shift, a feature of interest for variable optical absorption systems. Unfortunately, diaryl-substituted azolo-fused chromenes exhibit decreased colorabilities. Furofused benzopyrans are particularly interesting with respect to naphthopyran parents owing to the bathochromically shifted and broadened absorption spectra of photoinduced forms and slightly enhanced colorabilities. (C) 1998 Elsevier Science S.A. All rights reserved.