Fine-tuned dual fluorescence behavior of N-substituted aniline-imidazopyridine based switches: Mechanistic understanding, substituent and solvent effects

In order to understand the fine-tuned photo-physical behaviors of the N(X)-H center dot center dot center dot N systems, the excited state intramolecular proton transfer (ESIPT) switching in the N-substituted X1-5-NHIPA molecules (NHIPA = 2-(imidazo[1,2 alpha]pyridin-2-yl)aniline and X = H, COCH3, CH3C6H4SO2, C6F5SO2, and COCF3) were investigated in detail in gas phase and three solventmedia at TD-PBE0/6-311++G(d,p) and TD-M06-2X/6-311++G(d,p) levels of theory. ESIPT reactions at S-1 state were approximately without energy barrier, exergonic processes and were quantitatively demonstrated to be mainly sensitive to substituents and solvent media. The X-NHIPA (X = CH3C6H4SO2, C6F5SO2 and CF3CO) compounds were predicted to undergo fast, irreversible proton transfer at S-1 state and, in turn, exhibit tautomer emission with anomalous large Stokes shift in the gas phase. In the toluene solvent, except for C6F5SO2-NHIPA that showed exclusively a tautomer emission with a long wavelength, all other X-NHIPA molecules were predicted to involve in the reversible ESIPT and hence exhibit a dual normal and tautomer emissions behavior, in good agreement with the experimental observations. In polar solvents, it is expected that all compounds show dual normal and tautomer emissions. The nearly equal intensities of the normal and tautomer emissions can lead to the generation of the white lights with the potential in lighting applications. (C) 2019 Elsevier B.V. All rights reserved.