Exploring the Mechanisms of Excited-State Intramolecular Proton Transfer in Hydroxyphenyl-Benzimidazole Derivatives: A Theoretical Perspective

Abstract

We conducted a comprehensive theoretical exploration for the ESIPT mechanism in a set of HBI derivatives in THF solution (Cpd-A and Cpd-B, and three isomers: Cpd-1, Cpd-2 and Cpd-3), employing time-dependent density functional theory (TD-DFT) methods for the excited-state calculations. An energy scan for the excited-state proton transfer pathway indicates that both Cpd-A and Cpd-B exhibit a low transition barrier from Enol* to Keto*, with the energy of Keto* much lower than Enol* state, which implies a rapid ESIPT process. The high instability of Enol* state contradicts the experimental observed dual emission bands for Cpd-A, where the emission band predominantly contributed from Keto* state. We propose a new mechanism to account for this dual fluorescence phenomenon of Cpd-A, where the overall fluorescence band can be ascribed to the emissions from both Keto* and a stable rotamer of Cpd-A. Calculations suggest that Cpd-A can exist in both syn and anti rotamers, with a relatively low isomerization barrier of 14.0 kcal/mol. The coexistence of two stable isomers in solution gives rise to the phenomenon of dual emission for Cpd-A. Further investigation for the substituent effects on the ESIPT dynamics of three isomers (Cpd-1, Cpd-2 and Cpd-3) indicates that varying substituent positions can have a substantial impact on both ESIPT process and excited-state charge transfer. The methoxy group substitution proximate to the hydroxy functional group decreases the energy barrier for Enol*-Keto* transition, and also promotes a more substantial charge transfer. Our study demonstrates a new mechanism of the dual fluorescence observed for the target HBI derivatives, offering a novel understanding for the ESIPT mechanism in solvent.

Keywords: ESIPT, TD-DFT, Rotamerization, Dual fluorescence, Charge transfer