Unraveling photo-excited behaviors and proton transfer mechanisms for coexisting 5-methoxy-salicylaldhyde azine isomers

In view of the paramount importance of excited state relaxation for organic fluorophores, in this work, we mainly focus on deciphering the ground-state and excited state dynamical behaviors for the novel 5-methoxy-salicylaldhyde azine compound. Based on constructing potential energy surface (PES) and calculating Boltzmann distribution ratio, we firstly present three isomers of 5-methoxy-salicylaldhyde azine isomers (i.e., anti, anti-syn and syn) could coexist in S-0 state via twisting methyl dihedral angles (Phi(1) and Phi(2)). Upon photoexcitation, we find the intramolecular dual hydrogen bonds of anti, anti-syn and syn compounds are enhanced in S-1 state. Given asymmetry of anti-syn structure, we present the O1-H2 center dot center dot center dot N3 should be largely affected, which plays more significant roles in excited state process than the O4-H5 center dot center dot center dot N6 one. Charge redistribution around hydrogen bonding moieties deriving from photoexcitation illustrates the nature of strengthening dual hydrogen bonds and promotes the excited state proton transfer (ESPT) behaviors for these three isomers. Insights into excited state PESs along with proton transfer paths, we confirm the excited state intramolecular single proton transfer (ESISPT) mechanism are supported kinetic and thermodynamic for anti, anti-syn and syn compounds. Especially for anti-syn form, the ESISPT process is inclined to proceed along with O1-H2 center dot center dot center dot N3 rather than the O4-H5 center dot center dot center dot N6 one. This work illustrates the interconversion and coexistence of three isomers in S-0 state, based on which the specific ESISPT mechanisms of them are elaborated theoretically. We hope this work could pave the way for designing and developing novel applications based on 5-methoxy-salicylaldhyde azine fluorophore. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study focuses on investigating the ground-state and excited state behaviors of 5-methoxy-salicylaldhyde azine compound, revealing the strengthening of intramolecular hydrogen bonds and excited state proton transfer behaviors in different isomers. Through analyzing potential energy surfaces and calculating Boltzmann distribution ratios, we elucidate the chemical reaction mechanisms occurring in these isomers in the excited state.