EFFECT OF HETEROAROMATIC ANNULATION WITH 5-MEMBERED RINGS ON THE PHOTOCHROMISM OF 2H-[1]-BENZOPYRANS

Photochromic properties (colorability, UV-VIS spectra of photo-merocya nines, rate constant of thermal bleaching) of chromenes annulated with different five-membered azaheterocycles or with furan cycles were inv estigated under flash photolysis. All the compounds, i.e., the 7H-pyra no[2,3-g]benzazoles 4-6, the 7H-pyrano[3,2-e]benzazoles 7-10, the 7H-p yrano[3,2-e]indoles 11-12, the 3H-pyrano[3,2-a]carbazole 13, the 8H-py rano[2,3-e]indole 14 and the 3H-furano[3,2-f]chromenes 15-18 undergo r eversible photochromic reaction at room temperature. They are compared with naphthopyranic parents 1-3. Whatever the structure of the hetero aromatic moiety, the annulation of the benzopyrans leads to the splitt ing into two bands of the electronic absorption spectra of the photoin duced forms with a significant bathochromic shift, a feature of intere st for variable optical absorption systems. Unfortunately, diaryl-subs tituted azolo-fused chromenes exhibit decreased colorabilities. Furofu sed benzopyrans are particularly interesting with respect to naphthopy ran parents owing to the bathochromically shifted and broadened absorp tion spectra of photoinduced forms and slightly enhanced colorabilitie s. (C) 1998 Elsevier Science S.A. All rights reserved.