Photochromic heterocyclic fulgides. Part 4. The thermal and photochemical reactions of (E)-isopropylidene-[α-(2- and -(3-thienyl)ethylidene]succinic anhydrides and related compounds

Abstract

The pale yellow (E)-2-[α-(2,5-dimethyl-3-thienyl)ethylidene]-3-isopropylidenesuccinic anhydride undergoes a reversible photochemical reaction to give, in quantitative yield, the deep red 2,4,7,7,7a-pentamethyl-7,7a-dihydrobenzo[b]thiophene-5,6-dicarboxylic anhydride, which is stable at 100 °C. The corresponding (E)-(2,5-dimethyl-3-thienylmethylene) compound does not photocyclise but shows only E–Z photoisomerisation. 2- and 3-Thienyl compounds, lacking a 3- or 2-methyl substituent, respectively, undergo photochemical and/or thermal ring-closure followed by 1,3- and/or 1,5-hydrogen shifts, depending on the reaction conditions.