Click and shift: the effect of triazole on solvatochromic dyes

Solvatochromic dyes are prime candidates for exploring complex polarity-dependent biological processes. The design of novel dyes for these applications typically presents long synthetic routines. Herein we report a concise synthesis of azido-functionalised push-pull fluorenes, belonging to the most potent groups of fluorosolvatochromic compounds. These fluorenes can be attached to multiple alkynes via the highly versatile copper-catalysed azide-alkyne cycloaddition (CuAAC). Our study focuses on the beneficial effect of the triazole, formed by CuAAC, comparing the simple push-pull fluorene FR1 with the novel model compound FR1TP. While the triazole in FR1TP is not conjugated to the dye’s π-system, the heterocycle surprisingly produces a bathochromic emission shift of around 50 nm. This effect, caused by triazole-induced LUMO stabilisation, was also observed for two other model compounds FR2TP and FR1TM. All compounds display photophysical properties that are highly desired for in vivo imaging dyes, including remarkable two-photon absorption properties and solid-state fluorescence. The experimental results are supported by theoretical calculations. We anticipate that our results will enable synthesis of new high-performance fluorosolvatochromic dyes for diverse applications.