Regulated stepwise ESDPT mechanism associated with chalcogen substitutions in BDIBD derivatives

Inspired by the brilliant photochemical and photophysical properties of organic molecules containing chalcogenide substitutions that could be potentially applied across various disciplines, in this work, the effects of the atomic electronegativity of chalcogens (O, S, and Se) on hydrogen bond interactions and excited state proton transfer (ESPT) are mainly focused. We present characteristic oxygen-hydroxybenzazole-substituted 2,5-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,4-diol (BDIBD) derivatives that contain intramolecular double hydrogen bonds. The main objective of this study was to explore in detail the influence of the change of chalcogen atomic electronegativity on dual hydrogen bond interaction and ESPT behavior. By comparing the structural changes and infrared (IR) vibrational spectra of BDIBD derivative (BDIBD-O, BDIBD-S and BDIBD-Se) fluorophores in S₀ and S₁ states, combined with the preliminary detection of hydrogen bond interaction via the core-valence bifurcation (CVB) index and predicted hydrogen bonding energy (E_HB), we conclude that dual hydrogen bonds should be strengthened in the S₁ state, which is favorable for the occurrence of ESPT reactions. The charge recombination behavior of hydrogen bonds, induced by photoexcitation, further illustrates this point. By constructing potential energy surfaces (PESs) based on restrictive optimization and by searching the transition state (TS) structure, we finally elucidate stepwise excited-state double proton transfer (ESDPT). Specifically, we confirm that a change in atomic electronegativity has a regulatory effect on the ESDPT behavior in BDIBD derivatives, that is, lower atomic electronegativity is more conducive to stepwise ESDPT.