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The excited-state intramolecular proton transfer (ESIPT) reaction in 1-hydroxy-11H-benzo[b]fluoren-11-one (HBF) is found to be highly sensitive to the presence of specific substituents in positions C2, C4 and C8 with respect to the hydroxyl group, leading to different fluorescent behaviors of HBF with different substituents (J. Phys. Chem. C, 2018, 122, 21833). However, a systematic and comprehensive computational study of the influence of alkyl-substitutions at various positions of HBF on the intramolecular hydrogen bond strength, adsorption and fluorescence spectra and finally the proton transfer energy barriers is scare up to now. Therefore, in this work, the excited-state overall perspective of the proton transfer process of HBF and its seven tert -butyl substituted derivatives (Scheme 1) have been theoretically studied at TD-PBE0/TZVP theory level with the DFT and TDDFT methods. It is concluded that alkyl-substitution at positions C2 and C4 of the parent molecule should increase the strength of intramolecular hydrogen bonding O 1 -H⋯O, which decrease both the S 1 -state forward proton transfer energy barriers and the energy differences between forms S 1-PT and S 1 and eventually facilitate the ESIPT process, whereas alkyl-substitution at position C8 of the parent molecule should have slight opposite effect. • Alkyl-substitution at positions C2 and C4 increase the hydrogen bonding strength of O 1 -H⋯O. • Strengthened O 1 -H⋯O decreases the S 1 -state forward proton transfer energy barrier and facilitates the ESIPT process. • Alkyl-substitution at position C8 of the parent molecule has slight negative effect on the ESIPT process. • Balance between acidity of proton and basicity of proton donor and proton accepter subtly tunes the ESIPT equilibrium. [ABSTRACT FROM AUTHOR]

In this present work, combing density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we investigate a novel white light material 2,2′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4-tert-butylphenol) (t-HTTH) about its dual intramolecular hydrogen bonding behaviors and relative excited state intramolecular proton transfer (ESIPT) process in detail. Via testing functional and basis set, the DFT//TDDFT/MPW1PW91/6–311+G(d,p) theoretical level has been decided in three kinds of solvents (cyclohexane (CYH), dichloromethane (DCM) and toluene (Tol)), which can well reappear experimental phenomenon. Exploring molecular electrostatic potential (MEP), primary bond lengths, bond angles and infrared (IR) vibrational spectra for t-HTTH system in both S 0 and S 1 states, we find the double hydrogen bonds O1-H2···N3 as well as O4-H5···N6 should be strengthened in the S 1 state. Investigating the photo-excitation process, we find that the S 0 → S 1 transition corresponds to charge transfer, which provides the driving force for ESIPT. And analyses about frontier molecular orbitals (MOs) and relative energy gaps between HOMO and LUMO orbitals, we deduced the ESIPT process may occur more likely in nonpolar solvents. Constructing potential energy curves, we reveal that the ESIPT reaction results in the dynamic equilibrium in S 1 state between forward and backward processes, which facilitates white light. We not only clarify the excited state hydrogen bond dynamical behavior of t-HTTH material, but also elaborate the ESIPT mechanism and explain previous experimental phenomenon reasonably. [ABSTRACT FROM AUTHOR]