Turn-on type fluorescent photochromism of a diarylmaleimide-S,S,S',S'-tetraoxide.

In recent years, fluorescence-switchable molecules have garnered significant attention as fluorescent dyes for super-resolution fluorescence microscopy, which is increasingly demanded in the field of biochemical imaging. Among such molecules, diarylethene-S,S,S',S'-tetraoxide derivatives have proven particularly promising due to their ability to achieve high contrast fluorescence switching. Diarylethenes incorporating perfluorocyclopentene as the ethene bridge have become the standard scaffold due to their excellent fatigue resistance and thermal stability. However, their inherently low polarity necessitates extensive functionalization with hydrophilic groups to render them suitable for use in aqueous environments, such as in fluorescent labeling for super-resolution fluorescence microscopy. To address this limitation, we designed a novel class of oxidized diarylmaleimide (DAM) derivative featuring a maleimide as the ethene moiety, offering increased polarity and synthetic flexibility. Specifically, DAM 1, composed of 2,4-dimethyl-5-phenylthiophene and N-methylmaleimide, and its oxidized counterpart DAM 2, bearing two S,S-dioxidized thiophene rings, were synthesized and characterized. DAM 1 exhibited reversible photochromism upon irradiation with 436-nm and 550-nm light, accompanied by weak turn-off fluorescence and a cyclization quantum yield that depended strongly on solvent polarity. In contrast, DAM 2 exhibited two types of photoisomerization, one-way and partial two-way, depending on the polarity of the solvent, and exhibited turn-on fluorescence behavior. These findings suggest that DAM-based systems can be promising alternatives to conventional perfluorocyclopentene-based diarylethenes for aqueous fluorescence imaging applications, due to their high polarity compared to them and tunable-photoresponsive properties.