Photophysics of click-functionalized 2,2'-bithienyl flexible Lewis pairs

Copper-catalyzed click chemistry is used to prepare triazole-functionalized 2,2'-bithienyl flexible Lewis pairs (FlexLPs) that exhibit solvent- and temperature-responsive photophysical behavior. The introduction of triazole endcaps modifies the conformational flexibility of the 2,2'-bithienyl Lewis pair core, leading to solvent-dependent changes in the open-closed equilibrium that differ from those observed for the unsubstituted FlexLP system. A model compound bearing biphenyl end groups shows distinct absorption and emission responses in hydrogen-bond-donating solvents, consistent with solvent-dependent stabilization of different conformers. Variable temperature NMR spectroscopy demonstrates the potential application of these molecular systems as temperature probes based on the linear correlation between the chemical shift of the triazole protons and the temperature. Time-dependent density functional theory (TD-DFT) calculations support the experimental observations by showing that the open and closed conformers possess different geometries, transition energies, oscillator strengths, and charge-transfer character. Together, these results demonstrate that click functionalization provides an effective strategy for tuning the conformational dynamics and environment-responsive optical properties of FlexLP-based photofunctional molecules.