Effects of Twisted Intramolecular Charge Transfer Behavior on Excited-State Intramolecular Proton Transfer Reactions of Methyl Benzoate Derivatives in Water Solution

Abstract

Many methyl benzoate derivatives were found to show intramolecular charge transfer (ICT), intramolecular proton transfer, and other properties, which have extensive applications in lasing media, metal ion sensors, active materials, and fluorescence probe fields. However, the intrinsic relationship and reaction mechanism between the excited-state intramolecular proton transfer (ESIPT) and ICT between methyl benzoate derivatives with different substituents have not been explained. In this paper, the density functional theory and time-dependent density functional theory methods were used to study the ESIPT and ICT behaviors of p-aminosalicylic acid methyl ester and p-dimethylaminosalicylic acid methyl ester in water and obtain the intrinsic interaction between the two behaviors. The bond parameters, infrared spectra, reduced density gradient scatter plots, and topological analyses of these two molecules in the ground state and excited state were analyzed to confirm the enhancement of the excited-state intramolecular hydrogen bonds (IHBs). The simulated absorption and fluorescence spectra of these molecules agreed well with the experimental values. Based on the optimized structure, we also plotted the natural transition orbitals, electron density difference maps, and frontier molecular orbitals (FMOs), which showed the changes of the charge distribution of these molecules intuitively upon photoexcitation. In addition, we also found that the degree of IHB enhancement with −N­(CH3)2 substituents was less than that with −NH2, reflecting an inhibition effect of twisted intramolecular charge transfer (TICT) on ESIPT reaction. This conclusion was confirmed by our calculated potential energy curves. This work may better deepen the comprehension of the intrinsic relationship between ESIPT and TICT behavior and sequentially provide better theoretical guidance for the synthesis of fluorescent molecules related to these two behaviors.

Photophysical Properties of (E)-1-(4-(Diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide Compound and Its Triple Fluorescence Emission Mechanism: A Theoretical Perspective

“…The excited-state intramolecular proton-transfer (ESIPT) process was a photophysical process that induced the isomerization of intramolecular hydrogen bonds (IHBs) from enol to keto in organic molecules after photoexcitation. − The presence of ESIPT resulted in a reduction of the first singlet energy level − and a redshift of the emission wavelength; − thus, the ESIPT molecule usually had a large Stokes shift. − Molecules with ESIPT properties were considered to have potential applications in fluorescence probes, − organic luminescent materials, − and biomedicine. − In 1950, Weller first reported the ESIPT process in methyl salicylate . Subsequently, numerous scholars were committed to exploring the molecular mechanism of the ESIPT reaction and its application value both experimentally and theoretically …”

Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective

“…Hydrogen bonds (HBs) are chemical bonds formed by interactions between hydrogen atoms and atoms with higher electronegativity (such as nitrogen, oxygen or fluorine) and are among the most important weak interactions within and between molecules [1][2][3][4][5][6]. Owing to their specific saturation and directivity [7][8][9][10], HBs play an extremely vital role in the fields of chemistry, physics and biology [11][12][13][14][15][16]. Excited-state intramolecular proton transfer (ESIPT) is a photophysical process that promotes the interconversion isomerisation between enol and keto in molecules under photoexcitation [17][18][19][20][21].…”

Substituent effect on photophysical properties and proton transfer process of HBQ derivatives

“…Excited state intramolecular proton transfer (ESIPT) exists in biological and chemical systems and plays an important role in these molecular systems [1][2][3][4][5][6]. The ESIPT reaction process is a super-fast process, and the time required for the reaction is on the subpicosecond time scale, in which the proton transfer pathway is connected by hydrogen bonds and the proton donor and acceptor groups are close to each other [7].…”