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 π-system of the dye, 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. The experimental results are supported by theoretical calculations. We anticipate that our findings will enable synthesis of new high-performance fluorosolvatochromic dyes for diverse applications.