Photochromic Dihetarylethylenes. 22. Synthesis and Photochromic Properties of Unsymmetrical Alkylthio-substituted 1,2-Bis(dithienyl)perfluorocyclopentenes

Abstract

Certain unsymmetrical 1,2-bis(dithienyl)perfluorocyclopentenes have been synthesized containing alkylthio, bromo, carboxy, and benzothiazolyl substituents in the thiophene rings. It was shown that on irradiation with UV light they all display photochromic properties, with the exception of the derivative containing an alkylthio group in the reaction center of one of the thiophene rings.Previously we synthesized vicinally substituted perfluorocyclopentenes (PFCP) 1a,b in which the fluorinated ring was linked symmetrically with thienyl rings bearing alkyl and alkylthio groups (Scheme 1) [2,3]. We noticed that the presence of alkylthio groups in positions 2 or 5 of the thiophene rings activates the thiophene ring in electrophilic substitution and metallation reactions, which facilitates the subsequent introduction of various substituents into the free positions of the heterocycle. The series of photochromes 1 with substituents in the 4 and 4' positions of the thiophene rings was obtained for the first time by the functionalization of compound 1b [4].Unsymmetrical alkylthio-substituted PFCP have been synthesized in the present work having substituents of various nature in the thiophene rings, containing alkylthio groups either in the reaction center of one of the thiophene rings (product 2a) or in position 5 (products 2b-e) (Scheme 2), and their photochromic properties have been investigated. A convenient starting material for these purposes was the mono-substitution product of perfluorocyclopentene, 1-(2-ethyl-5-ethylthio-3-thienyl)heptafluorocyclopentene (3). We note that dibromide 4d [4], necessary for its synthesis and described by us previously [4], was obtained successfully in higher yield (~80%) on using a bromide-bromate mixture as brominating agent. The unsymmetrical dithienylPFCP 2a-c were obtained by the interaction of 1-(2-ethyl-5-ethylthio-3-thienyl)PFCP (3) with the appropriate bromides 4a-c.