Theoretical Unveiling Photoinduced Excited State Behaviors for BP(OH)DCEt Fluorophore: Effects of Solvent Polarity.

In this work, the influence of solvent polarities on the excited-state intramolecular proton transfer (ESIPT) process of BP(OH)DCEt fluorophore has been systematically investigated in three solvents with distinct polarities (acetonitrile, chloroform, and cyclohexane) through DFT and TDDFT methodologies. We mainly focus on elucidating the related excited-state double proton transfer (ESDPT) mechanism in BP(OH)DCEt. We analyze geometric configurations, infrared (IR) vibrational spectra, and core-valence bifurcation (CVB) indexes to verify the enhancement of the dual hydrogen bonds in the excited state. Meanwhile, we detected the HOMO and LUMO orbitals to investigate the effects of charge redistribution on the ESIPT/ESDPT process. The reactional potential energy surfaces (PESs) are scanned and transition state (TS) forms are searched to testify the stepwise ESDPT mechanism for BP(OH)DCEt systems in three solvents. We also propose that the increase of solvent polarity can promote the occurrence of the step-by-step ESDPT reaction processes for the BP(OH)DCEt system based on the calculated S-state potential energy barriers in the surrounding environment.