Synthesis and fluorescence of dihydro-[1,2,4]triazolo[4,3-a]pyridin-2-ium-carboxylates: An experimental and TD-DFT comparative study

A series of inexpensive and water-soluble fluorescent dihydro-[1,2,4]triazolo [4,3-a]pyridin-2-ium-carboxylates (Safirinium dyes) have been synthesized and characterized by UV-Vis, IR, H-1- and C-13 NMR spectroscopic techniques as well as by single crystal X-ray analysis. Significantly lower fluorescence quantum yields were determined in aprotic polar solvent (DMF). The photophysical properties of zwitterions and hydrochlorides were rationalized theoretically. A combined experimental and DFT/TD-DFT study of absorption and emission spectra have been performed. The calculations were carried out within the vertical non-equilibrium approximation or the state-specific approach using conventional (APFD, B3LYP and PBE0) and long-range-corrected hybrid functionals (CAM-B3LYP and omega B97XD) by means PCM and SMD solvation models. We have found that global hybrid functionals (GH) fail for charge transfer excited states in which charge is moved from the substituent to the heteroaromatic system, while the long range corrected functionals deliver consistent estimates, especially within the state-specific PCM modeling framework. The long-range-corrected hybrid functionals (RSH) were also found to be the most efficient in reproducing the emissive properties of the dyes.