The two-pronged approach of heteroatoms and substituents to achieve a synergistic regulation of the ESIPT process in amino 2-(2'-hydroxyphenyl)benzoxazole derivatives

Amino 2-(2'-hydroxyphenyl)benzazole derivatives are a class of molecules with excellent photophysical properties. Most of them can be applied as a fluorescent probe via the excited-state intramolecular proton transfer (ESIPT) process. In this work, we focus on the effects of heteroatoms (O, S) and substituents (acetylacetone, hydrogen) in the derivatives. Using DFT/TDDFT methods with the B3LYP-D3BJ functionals, the absorption and emission peaks are in good agreement with the experimental data. Results of optimized structures, infrared vibrational spectra, and reduced density gradient present the existence of the ESIPT process in the S₁ state in these molecules, it also indirectly shows that the heteroatom S is more than O, and the substituent acetylacetone is more than hydrogen has stronger hydrogen bonds. The proton transfer (PT) potential energy curves (PECs) qualitatively show that it is easier for the heteroatom S to induce ESIPT than that of O. The same for the substituent acetylacetone than that of hydrogen. Under the joint influence of the simultaneous stacking of heteroatom S and acetylacetone substituent, the energy barrier of the PT process can be effectively lowered, realizing a synergistic strategy, which can provide some guidance for the design of fluorescent materials.

Computational Insights into Sensing Mechanism for Al3+ in a New Acylhydrazone Fluorescent Probe Based on Excited-State Intramolecular Proton Transfer (ESIPT) and Twisted Intramolecular Charge Transfer (TICT)

The work explored the fluorescent properties of probe N'-(2, 4-dihydroxy-benzylidene)pyridine-3-carbohydrazide (HL) and its sensing mechanism for the Al³⁺ ion in detail. HL has two competing deactivation processes: ESIPT and TICT. Upon light-excitation, only one proton can transfer, and the SPT1 structure is generated. The SPT1 form is highly emissive, which is inconsistent with the colorless emission observed in the experiment. Then a nonemissive TICT state was obtained by rotating the C-N single bond. The energy barrier of the TICT process is lower than that of the ESIPT process, which indicates that probe HL will decay to the TICT state and quench the fluorescence. When Al³⁺ is recognized by probe HL, strong coordinate bonds are formed between HL and Al³⁺, and then the TICT state is prohibited, and the fluorescence of HL is turned on. Al³⁺ as a coordinated ion can effectively remove the TICT state but cannot influence the photoinduced electron transfer (PET) process of HL.

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.

Theoretical Investigation of the Excited-State Dynamics Mechanism of the Asymmetric Two-Way Proton Transfer Molecule BTHMB

An asymmetric two-way proton transfer molecule 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with the function of white-light emission was synthesized in a recent experiment (Bhattacharyya, A.; Mandal, S. K.; Guchhait, N. J. Phys. Chem. A 2019, 123, 10246). The particularity of this molecule is that there are two possible forms, one of which contained a six-membered H-bonded network toward a N atom (BTHMB-NH) present in the molecule as a proton acceptor and the other was toward an O atom (BTHMB-OH). Unfortunately, the experimental work lacked the theoretical explanation about the determination of the BTHMB-NH form and its excited-state intramolecular proton transfer (ESIPT) process under different solvents. Therefore, this study has explored these two points by means of the time-dependent density functional theory (TDDFT) method. The calculated relative energy and potential energy profile (PEP) of the transformation between BTHMB-NH and BTHMB-OH forms illustrated that BTHMB-NH was more stable, and the transfer from BTHMB-NH to BTHMB-OH was almost impossible at both S₀ and S₁ states under all solvents due to high potential energy barriers (PEBs) (11.67-21.59 kcal/mol). These calculated results provided the theoretical explanation and verification for the conclusion that the BTHMB molecule exists in the BTHMB-NH form in the experiment. Subsequently, the constructed PEPs of the ESIPT process for BTHMB-NH have proved that it was prone to the ESIPT process due to low PEBs (0.11-0.28 kcal/mol) at the S₁ state. In particular, as the solvent polarity increased, the intensity of the intramolecular hydrogen bond (IHB) (O₃-H₄···N₅) increased and the ESIPT process was more likely to occur. In addition, the twisted intramolecular charge-transfer (TICT) process was studied to explore the possible fluorescence quenching pathway of BTHMB-NH. Based on the PEPs of BTHMB-NH-T as a function of the N₅-C₆-C₇-C₈ dihedral angle at the S₀ and S₁ states, it is seen that the S₀ state TICT process was inhibited due to the large PEBs (16.45-23.93 kcal/mol). Although the S₁ state PEBs have been greatly reduced, they were still maintained at about 3.60 kcal/mol (3.60-3.84 kcal/mol), and hence, this process was still relatively difficult to occur. Due to the fact that BTHMB can be regarded as a standard in future designs involving red light and solvent-specific white-light emitters, a certain amount of investigative work on the ESIPT process was done in detail, and it paved the way for future research on the directionality of ESIPT in double ESIPT probes.

Revised the excited-state intramolecular proton transfer direction of the BTHMB molecule: A theoretical study

The spectroscopic properties of 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde molecule were investigated [J. Phys. Chem. A 2019, 123, 10246-10253]. The result shows that the excited-state intramolecular proton transfer (ESIPT) was driven toward the N center over the O center. In this research, the density functional theory and time-dependent density functional theory method were used to calculate molecule structures. Through our calculations, the ESIPT process toward N atom is proved to be feasible. Moreover, the emission peak we obtained of ESIPT process from the OH proton to aldehyde O atom is located at 564 nm, which is attributed to 500 nm in previous research. From the potential energy curves, the 0.35 kcal/mol energy barrier indicates the ESIPT process could occur when excited to S1 state from the OH proton to aldehyde O atom. In addition, the frontier molecular orbitals analysis and IR spectrum were also calculated. Finally, we revise the direction of BTHMB molecule, the two directions of ESIPT are both feasible when excited to S₁ state.

Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer mechanism of new 3-hydroxy-4-pyridylisoquinoline compound

The new 3-hydroxy-4-pyridylisoquinoline compound is attractive and promising lead structure in drug discovery. The pronounced sensitivity of its emission property toward solvent polarity effect was presented in experiment (J. Org. Chem, 2019, 84, 3011). Nevertheless, the experiment was lack of solvent polarity effect on the excited state intramolecular proton transfer (ESIPT) mechanism in detail. In this study, the ESIPT process of this molecule in different polarity solvents were comprehensively expounded by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In order to ensure the accuracy of the experiment and roundly explore in theoretical level, two ESIPT pathways (1 and 2) based on the N1 and N2 forms of studied molecule were proposed, among which the ESIPT pathway 1 was derived from experiment. The calculated electronic spectrum of both N1 and N2 forms were rather comparable with the experiment. The calculated intramolecular hydrogen bond (IHB) parameters and infrared (IR) vibration spectra determined the enhancement of IHBs at the S₁ state under different solvents for both N1 and N2 forms. The frontier molecular orbitals (FMOs) analysis proved that the intramolecular charge transfer (ICT) taken place during photoexcitation. The potential energy curves (PECs) at the S₀ and S₁ states were constructed to illustrate the solvent polarity effect on ESIPT mechanism. According to potential energy barriers (PEBs) on the PECs at S₁ state, it is concluded that the ESIPT pathway 1 was forbidden with exceedingly high PEBs (24.585-25.322 kcal/mol), while the ESIPT pathway 2 was feasible with enough low PEBs (0.100-0.510 kcal/mol), which suggested the inconsequence of the experiment. Based on the PEBs of ESIPT pathway 2 in different solvent, the effect of solvent polarity on ESIPT mechanism was depicted. The results are as follows: the S₁ state IHB intensity was enhanced with increasing solvent polarity; the extent of ICT was decreased with the increment of solvent polarity; the S₁ state PEB was decreased as the solvent polarity increased. Indeed in short, the ESIPT reaction became more and more likely as the solvent polarity enhanced. We believe that this investigation will be useful to the utilization and development of property for such photochemical substances.

Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S₀) and excited (S₁) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.

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.

Regular tuning of the ESIPT reaction of 3-hydroxychromone-based derivatives by substitution of functional groups

The electron-withdrawing ability of an atom and length of substitution groups would affect the ESIPT reaction and photophysical properties of 3-hydroxychromone-based derivatives.

Photophysics of proton transfer in hydrazides: a combined theoretical and experimental analysis towards OLED device application

Effect of conjugation to support ESIPT with impossible double proton transfer in structurally favored species.

The effects of the heteroatom and position on excited-state intramolecular proton transfer of new hydroxyphenyl benzoxazole derivatives: a time-dependent density functional theory study

The effects of the heteroatom and position on excited-state intramolecular proton transfer (ESIPT) of 2-[4′-(N-4,6-dichloro-1,3,5-triazi-n-2-yl)2′hydroxyphenyl]benzoxazole (4THBO) have been investigated via time-dependent density functional theory studies.

The effects of different heterocycles and solvents on the ESIPT mechanisms of three novel photoactive mono-formylated benzoxazole derivatives

The detailed effects of different heterocycles and solvents on the dynamical ESIPT mechanisms of three novel mono-formylated benzoxazole derivatives A–C in two different surroundings have been expounded by the TDDFT method at the B3LYP/TZVP level.

Tunable ESIPT reaction and antioxidant activities of 3-hydroxyflavone and its derivatives by altering atomic electronegativity

3-HTF easiest to carry out ESIPT reaction would exhibit the highest efficient antioxidant activity among the three compounds.

The dual-luminescence mechanism of the ESIPT chemosensor tetrasubstituted imidazole core compound: a TDDFT study

The dual-luminescence mechanism of the tetrasubstituted imidazole core (TIC) compound was theoretically explored by considering the excited-state intramolecular proton transfer (ESIPT) process in the present study.