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

Abstract

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 S0 state via twisting methyl dihedral angles (Φ1 and Φ2). Upon photoexcitation, we find the intramolecular dual hydrogen bonds of anti, anti-syn and syn compounds are enhanced in S1 state. Given asymmetry of anti-syn structure, we present the O1-H2···N3 should be largely affected, which plays more significant roles in excited state process than the O4-H5···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···N3 rather than the O4-H5···N6 one. This work illustrates the interconversion and coexistence of three isomers in S0 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.