Thiazolothiazole Fluorophores Exhibiting Strong Fluorescence and Viologen-Like Reversible Electrochromism

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 (ΦF 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.