Straightforward Stepwise Excited State Dual Proton Transfer Mechanism for 9-10-HBQ System

Abstract

A new molecule 9,10-dihydroxybenzo[h] quinoline (i.e. 9-10-HBQ) is focused in the present work about its excited state proton transfer (ESPT) mechanism. Though comparing potential energy barriers, it is found that the ultrafast ESPT process could occur in the $S_1$ state without potential energy barrier along with hydrogen bond $O_3$-$H_4···N_5$ forming 9-10-HBQ-PT1 structure, subsequently, the second proton transfers via another intramolecular hydrogen bonded wire $O_1$-$H_2···N_3$ with a low potential energy barrier (about 7.69 kcal/mol) in the $S_1$ state forming 9-10-HBQ-PT2 configuration. After completing excited state dynamical process, the $S_1$-state could turn back to $S_0$ state with occurring reversed ground state proton transfer forming initial 9-10-HBQ structure.