Fine-tuning the photophysical properties of Imidazo[1,2-a]pyridine-based dyes by the change in substituent and solvent media: DFT and TD-DFT studies

In the present work, photophysical properties of the nine excited-state intramolecular proton transfer (ESIPT) active 2-(2 -hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP)-based dyes A1-A9 (X= H, Me, F, Cl, OMe, OH, CN, NO2 and CF3) were investigated at PBE0/6-31++G(d,p) and M06-2X/6-31++G(d,p) levels of theory in the gas phase and solvent media. The structural parameters, relative energies, vibrational spectra, photophysical properties, potential energy curves, natural bond orbital (NBO) charges, hole and electron isosurfaces, electron density properties and reduced density gradient (RDG) spikes were computed. The H-bonding is strengthened at the S1 state by comparing infrared vibrational spectra, the relevant bond length and bond angle, electron density properties and RDG analysis. The results revealed that ESIPT in the A1-A7 and A9 in the gas phase is a barrier-less process, while there is an energy barrier for the A8 (X=NO2). In polar solvents, it is predicted that the ESIPT process only in A5 and A6 is a barrier-less process. The A2, A4 and A5 exhibited the greatest redshift in the keto fluorescence emission in tetrahydrofuran solvents. The significant Stokes shift observed in the S-1-K emission of certain dyes makes these molecules highly appealing for various applications such as chemosensors, fluorescent probes, laser dyes and optoelectronic devices. {GRAPHICAL ABSTRACT}