Abstract
In this paper, density functional theory (DFT) and time-dependent DFT (TDDFT) methods were used to investigate substituent effects and excited-state intramolecular double-proton transfer (ESIDPT) in 1, 3-bis (2-pyridylimino)-4, 7-dihydroxyisoindole (BPI–OH) and its derivatives. The results of a systematic study of the substituent effects of electron-withdrawing groups (F, Cl and Br) on the adjacent sites of the benzene ring were used to regulate the photophysical properties of the molecules and the dynamics of the proton-transfer process. Geometric structure comparisons and infrared (IR) spectroscopic analysis confirmed that strengthening of the intramolecular hydrogen bond in the first excited state (S1) facilitated proton transfer. Functional analysis of the reduced density gradient confirmed these conclusions. Double-proton transfer in BPI–OH is considered to occur in two steps, i.e., BPI–OH (N) [Formula: see text] BPI–OH (T1) [Formula: see text] BPI–OH (T2), in the ground state (S0) and the S1 state. The potential-energy curves (PECs) for two-step proton transfer were scanned for both the S0 and S1 states to clarify the mechanisms and pathways of proton transfer. The stepwise path in which two protons are consecutively transferred has a low energy barrier and is more rational and favorable. This study shows that the presence or absence of coordinating groups, and the type of coordinating group, affect the hydrogen-bond strength. A coordinating group enhances hydrogen-bond formation, i.e., it promotes excited-state intramolecular proton transfer (ESIPT).
Highlights
Proton transfer is an essential process in biology and chemistry
Photochemical excited-state intramolecular proton transfer (ESIPT) of an enol to the proton-transferred keto isomer is usually accompanied by significant changes in the photophysical properties; this leads to a large Stokes shift [3,4,5,6,7]
New insights into substituent modification and the excitedstate intramolecular double-proton transfer (ESIDPT) mechanism for symmetric structures of BPI-OH and its derivatives were obtained by using Density functional theory (DFT)/time-dependent DFT (TDDFT) methods
Summary
Proton transfer is an essential process in biology and chemistry It can achieve interactions via formation of specific hydrogen bonds [1, 2]. Photochemical excited-state intramolecular proton transfer (ESIPT) of an enol to the proton-transferred keto isomer is usually accompanied by significant changes in the photophysical properties; this leads to a large Stokes shift [3,4,5,6,7]. In this previous study, the effects of substituents on the ESIDPT process were not considered. Details of the ESIDPT mechanism were clarified by constructing the potential-energy curves (PECs) for the S0 and S1 states
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