Effect of substitution on the solid-state fluorescence properties of styrylbenzoxazole derivatives with terminal dicyanomethylene group

Abstract

Three styrylbenzoxazole derivatives with terminal dicyanomethylene group were studied in order to understand how minor modifications brought to the benzoxazole ring influence the photoluminescence (PL) properties. The three compounds, in which several molecular motions are allowed, were weakly fluorescent in organic solution. Remarkably, the unsubstituted (1) and methoxy (2) derivatives showed clear solid-state luminescence enhancement (SLE). Distinct emission characteristics were attributed to the formation of amorphous and crystalline particles owing to scanning electron microscopy. These two compounds were also strongly luminescent as microcrystalline powders. Compound 1 that crystallized in herringbone configuration was a better emitter than the methoxy derivative 2, in which intermolecular interactions are more numerous, according to X-ray diffraction analysis. In contrast, compound 3 was virtually not emissive, confirming that in this series of dyes the presence of the hydroxy group is detrimental to PL emission, as rarely observed among SLE-active fluorophores.