Excited-state intramolecular proton transfer mechanism for 2-(quinolin-2-yl)-3-hydroxychromone: A detailed time-dependent density functional theory study

In this work, the Excited-state intramolecular proton transfer mechanism of 2-(quinolin-2-yl)-3-hydroxychromone (Q3HC) in 1,2-dichloroethane solvent have been investigated via the density functional (DFT) and the time-dependent density functional theory (TDDFT) method. By calculation, we optimize three configurations (Q3HC-A, Q3HC-B and Q3HC-C) and find the phenomenon of converting between Q3HC-B and Q3HC-A in the S0 and S1 states. Based on comparing the primary bond lengths, bond angles and infrared vibrational spectra involved in the hydrogen bonds between S0 and S1 states, hydrogen bonds strengthening in the S1 state have been testified. The calculated absorption and fluorescence spectra of Q3HC-A, Q3HC-B and Q3HC-C are agreement with the experimental data, and the results also show that the tautomer form of Q3HC-C has a fluorescence spectrum, which is different from the previous report (Svechkarev et al., Journal of Luminescence, 2011, 131, 253-261). Meanwhile, we also constructed the potential energy curves of S0 and S1 states, and further explained the ESIPT mechanism of Q3HC-A, Q3HC-B and Q3HC-C, respectively. These results have shown that the ESIPT of Q3HC-C is a barrierless process, which occurs more easily than that of Q3HC-A and Q3HC-B. Combination with reduced density gradient function, the hydrogen bond of Q3HC-C is strongest, compared with Q3HC-A and Q3HC-B. And it also can provide the real evidence that the ESIPT of Q3HC-C is more favorable than that of Q3HC-A and Q3HC-B.