Solvent effect on the excited-state intramolecular double proton transfer of...

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are used to study the solvatochromic effect and the excited-state intramolecular double proton transfer (ESIDPT) of 1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole (BPI-OH) in different kinds of solvents. The hydrogen bonding parameters and IR spectra reveal that in the excited state, the strength of excited hydrogen bond increase with the decrease of solvent polarity. Furthermore, the reduction density gradient (RDG) analysis confirms the corresponding conclusion. Frontier molecular orbitals (FMOs) are analyzed, illuminating that the smaller the polarity of solvent, the smaller the energy gap between the HOMO and LUMO. The structures of BPI-OH (N) (normal), BPI-OH (T₁) (single), and BPI-OH (T₂) (double) were optimized. Previous reports found the double protons in BPI-OH molecule are transferred step-by-step process BPI-OH(N)→BPI-OH(T₁)→BPI-OH(T₂) in the ground state (S₀) and the first excited singlet state (S₁). Here, the potential energy curves of O₁-H₂ and O₄-H₅ in the S₀ and S₁ states were scanned in four kinds of solvents, respectively. It was found that in S₁ state, BPI-OH(N)→BPI-OH(T₁) was more prone to proton transfer than BPI-OH(T₁)→BPI-OH(T₂). In addition, by comparing the reaction energy barriers of the four kinds of solvents, it can be found that ESIPT is difficult to occur with the increase of solvent polarity. Meanwhile, it was also studied that MeOH as an explicit solvent was more likely to promote the ESIPT process than other implicit solvents.