Thiazolothiazole Fluorophores Exhibiting Strong Fluorescence and Viologen-Like Reversible Electrochromism

Abstract

The synthesis, electrochemical, and photophysical characterization of N, N ′-dialkylated and N, N ′-dibenzylated dipyridinium thiazolo[5,4- d ]thiazole derivatives are reported. The thiazolothiazole viologens exhibit strong blue fluorescence with high quantum yields between 0.8–0.96. The dioctyl, dimethyl, and dibenzyl derivatives also show distinctive and reversible yellow to dark blue electrochromism at low reduction potentials. The fused bicyclic thiazolo[5,4-d]thiazole heterocycle allows the alkylated pyridinium groups to remain planar, strongly affecting their electrochemical properties. The singlet quantum yield is greatly enhanced with quaternarization of the peripheral 4-pyridyl groups (Φ F increases from 0.22 to 0.96) while long-lived fluorescence lifetimes were observed between 1.8–2.4 ns. The thiazolothiazole viologens have been characterized using cyclic voltammetry, UV–visible absorbance and fluorescence spectroscopy, spectroelectrochemistry, and time-resolved photoluminescence. The electrochromic properties observed in solution, in addition to their strong fluorescent emission properties, which can be suppressed upon 2 e – reduction, make these materials attractive for multifunctional optoelectronic, electron transfer sensing, and other photochemical applications.