Theoretical research on excited-state intramolecular proton coupled charge transfer modulated by molecular structure

A Theoretical Study of Solvent Effects on the Structure and UV-vis Spectroscopy of 3-Hydroxyflavone (3-HF) and Some Simplified Molecular Models

Solvent effects on the UV-vis spectra of 3-hydroxyflavone and other structurally related molecules (3-hydroxychromen-4-one, 3-hydroxy-4-pyrone, and 4-pyrone) have been studied by combining time-dependent density functional theory (TDDFT) and the polarizable continuum method (PCM). Among the first five excited states of the four considered molecules, electronic states of n → π* and π → π* nature appear. In general, the stability of the n → π* states decreases as the π space becomes larger in such a way that only for 4-pyrone and 3-hydroxy-4-pyrone are they the first excited states. In addition, they become less stabilized in ethanol solution than the ground state, and this causes blueshift transitions in solution. The opposite trend is found for the π → π* excited states. They are less energetic with the π-system size and when passing from gas phase to solution. The solvent shift also depends strongly on the size of the π systems and on the formation of an intramolecular hydrogen bond; thus, it decreases when going from 4-pyrone to 3-hydroxyflavone. The performance of the three versions (cLR, cLR², and IBSF) of the specific-state PCM method in predicting transition energies are compared.

Stepwise Excited-state Double Proton Transfer and Fluorescence Decay Analysis

This work considers excited state intramolecular proton transfers (ESIPT) occurred in multiple hydroxyl-containing compounds with one proton transfer site in the normal form. If several hydroxyl groups are located close to each other in a molecule, then the ESIPT process can lead to the next one. A proton donor site in the first ESIPT will be a proton acceptor during the second reaction. Therefore, a number of consecutive excited state proton transfers can occur. This work deals with the case of two successive proton transfers occurred in the molecular system. Such process is called as a stepwise excited state intramolecular double proton transfer (stepwise ESIDPT). It leads to the formation of two molecular tautomers. Therefore, fluorescence of such compounds can contain different emission bands correspond to emission of normal form and two tautomers. In this work, a rigorous analysis of fluorescence decay kinetics has been made using the model with three species, including a normal molecular form and two tautomers. The work presents theoretical framework of fluorescence decay analysis of ESIDPT process taking into account three species emission. Theoretically, the stepwise proton transfers can be consisted of more than two ESIPT reactions. It depends on molecular structure and number of involved hydroxyl groups. Here, a formal analysis of fluorescence decay kinetics has been made in the case of a stepwise process consisting of two proton transfers. Moreover, the quantum-chemical calculations have been performed in the case of scutellarein. It is a multiple hydroxyl-containing flavone and, therefore, it can be applied as a model molecule to study stepwise intramolecular proton transfers. The hypothetical scheme of ESIDPT has been proposed for this compound.

Substituent derivatives of benzothiazole-based fluorescence probes for hydrazine with conspicuous luminescence properties: A theoretical study

In the present work, four probe molecules for detecting hydrazine have been designed based on the 2-(4-Acetoxy-3-benzothiazole-2-yl-phenyl)-4-methyl-thiazole- 5-carboxylic acid ethyl ester (HP1) to investigate the influence of the amino and cyano groups on the excited-state intramolecular proton transfer (ESIPT) behavior and photophysical properties. The changes in hydrogen bond strength indicate that the intramolecular hydrogen bond of all probe products is enhanced upon photoexcitation. Frontier molecular orbitals (FMOs) and natural bond orbital (NBO) reveal the driving force of ESIPT. In addition, the potential energy curves and transition state theory explain the reason for the single fluorescence phenomenon in the experiment. The simulated absorption and fluorescence spectra of HP1 and its product (HPP1) are completely consistent with the experimental results, which also verify the viewpoint. Meanwhile the cyano derivative HPP4 exhibits a larger Stokes-shift (201 nm) than that of HPP1 (145 nm) and has the same low energy barrier as HPP1. These excellent properties allow HPP4 to be a fluorescent probe with superior performance than the original molecule. In conclusion, this work can provide a theoretical basis for the design and synthesis of more sensitive fluorescent probes for the detection of hydrazine.

Deciphering the excited state intramolecular charge-coupled double proton transfer in an asymmetric quinoline–benzimidazole system

Asymmetrical H-bonding responsible for charge coupled-excited state intramolecular double proton transfer.