Effects of solvent polarity on excited state behaviors for the two intramolecular proton-transfer-site 4,4'-(hydrazine-1,2-diylidene-bis(methanylylidene))-bis(3-hydroxybenzoic acid) compound

Highlights: • Charge reorganization and excited state hydrogen bonding strengthening reveal the ESPT tendency for HDBB system. • Even though HDBB owns two intramolecular proton-transfer sites, only the ESISPT reaction occurs via photoexcitation. • Nonpolar solvents should be conducive to ESISPT behavior for HDBB compound. Excited state proton transfer (ESPT) behavior of organic fluorophores has been drawing continuously considerable interests, since it owns enormous potential in wide of application fields. Given the paramount importance of excited state relaxations, in this work, we focus on deciphering excited state behaviors for the novel two proton-transfer-site 4,4'-(hydrazine-1,2-diylidene-bis(methanylylidene))-bis(3-hydroxybenzoic acid) (HDBB) system. In aprotic solvents with different polarities, we confirm the dual hydrogen bonds of HDBB should be enhanced in S₁ state. It is a remarkable fact that nonpolar solvents play a more vital role in enhancing dual hydrogen bonds in S₁ state. Probing into photo-induced excitation, we verify charge reorganization around proton acceptor and donor triggers ESPT tendency. Comparing energy gaps of frontier molecular orbitals (MOs), we further predict the ESPT process of HDBB could be facilitated by nonpolar solvents. To clarify the specific excited state behaviors, potential energy surfaces (PESs) in different solvents are constructed along with dual hydrogen bonds to present the overall ESPT processes. Searching transition state (TS) along reaction path, we confirm the excited state intramolecular single proton transfer (ESISPT) mechanism for HDBB. Further, we successfully present the excited state regulation mechanism via solvent polarity, namely, nonpolar solvents contribute to ESISPT reaction for HDBB compound.