Intrinsic structural factors and solvent effects on excited-state dynamics and photophysical properties of BIDP derivatives with dual-ESIPT units

When multiple excited-state intramolecular proton transfer (ESIPT) units are incorporated into a molecule, the excited-state dynamics become more complex and sensitive to both intrinsic structure and solvent environment. However, a systematic understanding of how intrinsic and extrinsic factors regulate these dynamics remains unclear. Herein, quantum-chemical calculations were performed to elucidate the effects of the number of ESIPT units, coupling position, symmetry, and solvent polarity on the photochemical behavior and photophysical properties of BIDP derivatives with dual-ESIPT units. The results show that structural symmetry alteration can significantly change H-bond strength and ESIPT efficiency, compared with other intrinsic factors. Topological analysis of the electron localization function along proton-transfer channels offers mechanistic insights into the reduction in proton-transfer barriers for excited-state molecules with mirror-symmetric structures. The DLPNO-STEOM-CCSD method enables accurate assessment of energies in different electronic states for the keto tautomer. Furthermore, calculated radiative and nonradiative decay rate constants support that emission from keto-form BIDP derivatives exhibits anti-Kasha characteristics. In addition, high-polarity solvents impede the ESIPT process by preferentially stabilizing the more polar enol structures compared with the corresponding transition state structures. We expect that this theoretical work will provide valuable insights into the design and development of photochromic materials featuring dual ESIPT units.