Liquid-solid phase regulating excited-state intramolecular proton transfer process of HBT-d-NO₂: A QM/MM study

The excited-state intramolecular proton transfer process of 2-(1,3-benzothiazol-2-yl)-4-[2-(4-nitrophenyl)ethynyl]phenol (HBT-d-NO₂) in the different surrounding environment is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The optimized molecular structure provides convincing evidence that the intramolecular hydrogen bond is strengthened in the first excited (S₁) state. The frontier molecular orbitals observed the HBT-d-NO₂ exists obvious intramolecular charge translate phenomenon. The results of the potential energy curve show that HBT-d-NO₂ is difficult to undergo proton transfer in the ground (S₀) state due to the high energy barrier, while it becomes easier in the S₁ state in both liquid and solid phases. By comparison, the energy barrier of ESIPT in the solid phase is higher than that in the liquid phase. We can conclude that the solid phase effectively hinders the ESIPT process compared with that the liquid phase. In this work, we illustrate the influence of liquid and solid phases on the intramolecular proton transfer process, which could promote further developments in biomedical and fluorophore applications.