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

Syntheses 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 [sigma(4-phenolate)=-1.00] to extremely strongly electron-withdrawing [sigma(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 (epsilon=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 lambda(max)=724 nm in its closed form.

Automated summary

The synthesis of push-pull substituted non-symmetric bis(thienyl)ethenes with a central perfluorocyclopentene core is described, showcasing the impact of various substituents on their spectroscopic properties. Different isomers exhibit varied conjugated and cross-conjugated structures, resulting in diverse spectroscopic characteristics.