Unraveling an excited state intramolecular double proton transfer pathway in 2,5-bis(benzoxazole-2-yl)benzene-1,4-diol derivatives

Abstract

Organic molecules exhibiting excited-state intramolecular double proton transfer (ESIDPT) have garnered significant research interest, largely driven by their prevalence in nature and the unique luminescent properties associated with this phenomenon. This study presents a detailed theoretical investigation into the dynamic mechanism of both single and dual cooperative Proton transfer (PT) in the first excited singlet (S₁) state of 2,5-bis(benzoxazole-2-yl)-1,4-dihydroxybenzene derivatives. The analysis meticulously incorporates solvent effects, specifically those of dichloromethane, to provide a comprehensive understanding of the PT processes. The occurrence of the ESIDPT process was confirmed by integrating infrared (IR) vibrational spectra, frontier molecular orbital analysis, and reduced density gradient (RDG) analysis. Additionally, the potential energy surfaces (PESs) for the ground (S₀) and S₁ states revealed a synergistic interaction between single and dual ESIPT processes within the S₁ state. Furthermore, variation in the charge distribution, resulting from the coupling of photoinduced electron transfer with ESIPT involving the DPA group, led to distinct PT propensities for the O₁ and O₄ atoms. Consequently, these configurations are unable to undergo simultaneous proton transfer, as demonstrated by the construction of a potential energy surface. In addition, the corresponding PT PESs show that the ESIPT reaction for Zinhbo-9 is much easier for the Zinhbo-5 modified with a t-Bu group, which affect O4–H5···N6 hydrogen bond. These theoretical computations provide a robust explanation of the observed experimental phenomena and suggest potential pathways for future advancements and applications of ESIDPT molecules.