Donor-specific photoisomerization in visible squaraine dyes regulates the fluorescence quantum yield over two orders of magnitude

Isomerization reactions in photoexcited polymethine dyes represent a familiar non-radiative deactivation channel that competes with light emission. Here, absorption and emission spectra are reported for a comprehensive series of synthesized 1,2- and 1,3-substituted, methine-bridged squaraines utilizing indoline-, benzothiazole- and quinoline-derived donor heterocycles. Absorption mirror emission spectra and Stokes shifts are small and largely solvent-independent, implying minor geometric differences between the ground and excited state from which emission occurs. Indoline- and benzothiazole-based 1,2-squaraines show fluorescence quantum yields of ≈5%, increasing to ≈40% for 1,3-squaraines. However, emission is almost completely quenched for any compound that incorporates the strong quinoline donor. Efficient non-radiative deactivation is supported by quantum chemical calculations along the twisting motion of the lateral quinoline group in the polymethine bridge. Design rules are derived from calculations using a simple three-state model, and facile photoisomerization is predicted for asymmetric squaraines with very different donors, as well as for symmetric dyes with strong donors.