Tuning the photophysical properties of ESIPT active unsymmetrical azine dyes by the change in the substituent and solvent: TD-PBE0 and TD-CAM-B3LYP studies†

In this study, the effects of the substituent and solvent on the photophysical properties of the designed ESIPT active as well as donor–acceptor structured unsymmetrical azine dyes L1–L5 (R1–5 = –H, –NH₂, –OCH₃, –CF₃ and –CN, respectively) were investigated at PBE0/6-31++G(d,p) and CAM-B3LYP/6-31++G(d,p) levels of theory in the gas phase and three solvent media. The structural parameters, relative energies, vibrational spectra, photophysical properties, potential energy curves, natural bond orbital (NBO) charges, charge transfer (CT) indices, electron density properties, and reduced density gradient (RDG) spikes were computed. The results of vibrational spectra, structural parameters and electron density analysis demonstrated that the O–H⋯N H-bonding interaction is strengthened in all dyes upon photoexcitation from the S₀ to S₁ state which can facilitate the ESIPT process. All dyes exhibited both enol and keto emissions, in good agreement with the reported experimental results. The largest Stokes shift for keto emissions in solvent media was observed in MeOH solvent and is in the order 143 nm (L5) > 138 (L4) > 133 (L1) > 120 (L3) > 115 (L2) at the PBE0/6-31++G(d,p) level of theory. Introducing electron-withdrawing groups can increase the absorption and emission wavelengths as well as the red shift in fluorescence emission of L4 and L5, but hinder the occurrence of the ESIPT process compared with L2 and L3. The results demonstrated that the substituent effect is more significant in changing the molecular optical properties than the solvent effect. Our designed ESIPT molecules can simultaneously show enol and keto emissions and thus can be regarded as candidates to design single-molecule white-light emission materials.