Switchable Mesomeric Betaines Derived from Pyridinium‐Phenolates and Bis(thienyl)ethane

Abstract

AbstractSyntheses of push–pull substituted non‐symmetric bis(thienyl)ethenes (BTEs) possessing a central perfluorocyclopentene core are described. The substituent effects of anisole, phenole, and phenolate as well as pyridine, pyridinium, and N‐methylpyridinium substituents, joined through their 3‐ or 4‐positions to the central BTE core, respectively, cover the range from very strongly electron‐donating [σ(4‐phenolate)=−1.00] to extremely strongly electron‐withdrawing [σ(pyridinium‐4‐yl)=+2.57] in the title mesomeric betaines. The different isomers possessing 4‐yl/4‐yl, 4‐yl/3‐yl and 3‐yl/3‐yl substituents represent different combinations of conjugated and cross‐conjugated partial structures and cause different spectroscopic properties. In addition, through‐space conjugation between the 2‐ and 2′‐position of the thiophenes can be observed which circumvents the charge‐separation of through‐bond cross‐conjugation. The BTE possessing the push–pull chromophore consisting of 3‐anisole and 4‐pyridinium substituents (24) displays the best extinction coefficients within the series of compounds described here (ϵ=33.8/15.7 L/mol ⋅ cm), while the mesomeric betaine possessing an N‐methylpyridinium‐4‐yl and a 4‐phenolate substituent (29) displays considerable bathochromic shifts to λmax=724 nm in its closed form.