Tunable excited-state intramolecular proton transfer reactions with N-H or O-H as a proton donor: A theoretical investigation

Excited-state intramolecular proton transfer (ESIPT) reactions occurring in the S-1 state for five molecules, which possess five/six-membered ring intramolecular N-H center dot center dot center dot N or O-H center dot center dot center dot N hydrogen bonds bearing quinoline or 2-phenylpyridine moiety, have been described in detail by the time-dependent density functional theory (TD-DFT) approach using the B3LYP hybrid functional. For the five molecules, the constrained potential energy profiles along the ESIPT reactions show that proton transfer is barrierless in molecules possessing six-membered ring intramolecular H-bonds, which is smoother than that with certain barriers in five-membered ring H-bonding systems. For the latter, chemical modification by a more strong acid group can lower the ESIPT barrier significantly, which harnesses the ESIPT reaction from a difficult type to a fast one. The energy barrier of the ESIPT reaction depends on the intensity of the intramolecular H-bond, which can be measured with the topological descriptors by topology analysis of the bond critical point (BCP) of the intramolecular H-bond. It is found that when the value of electron density rho(r) at BCP is bigger than 0.025 a.u., the corresponding molecule might go through an ultrafast and barrierless ESIPT process, which opens a new scenario to explore the ESIPT reactions. (C) 2017 Elsevier B.V. All rights reserved.