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

Concerning for the solvent-polarity-dependent conformational equilibrium and ESIPT mechanism in Pz3HC system: A novel insight

In this report, we propose a new insight into the interaction between the solvent-polarity-dependent conformational equilibrium and excited state intramolecular proton transfer (ESIPT) behavior of Pz3HC system in four different polar solvents (polarity order: ACN > THF > TOL > CYC). Using quantum chemistry method, we first announce a coexistence mechanism between Pz3HC-1 and Pz3HC-3 in the ground state in four solvents based on the Boltzmann distribution. In particular, Pz3HC-1 is the principal configuration in non-polar solvent, but Pz3HC-3 is the principal configuration in polar solvent. In addition, the simulated fluorescence spectra interprets the negative solvatochromism effect of Pz3HC-1 and Pz3HC-3 in four solvents. The evidence from intramolecular hydrogen bonding (IHB) parameters and electronic perspective collectively confirms the light-induced IHB enhancement and intramolecular charge transfer (ICT) properties in Pz3HC-1 and Pz3HC-3, which raises the likelihood of the ESIPT process. Combining the calculation of potential energy curve (PEC) and intrinsic reaction coordinate (IRC), we demonstrate that the ESIPT ease of Pz3HC-1 in different polar solvents obeys the order of CYC > TOL > THF > ACN, while the order of ESIPT ease in Pz3HC-3 is opposite. Notably, the ESIPT process of Pz3HC-3 in CYC solvent is accompanied by the twisted intramolecular charge transfer (TICT) process. In addition, we also reveal that the enol* and keto* fluorescence peaks of Pz3HC-3 in CYC solvent are quenched by ISC and TICT process, respectively. Our work not only provides a satisfactory explanation of the novel dynamics mechanism for Pz3HC system, but also brings light to the design and application of new sensing molecules in the future.

Liquid-solid phase regulating excited-state intramolecular proton transfer process of HBT-d-NO2: A QM/MM study

The excited-state intramolecular proton transfer process of 2-(1,3-benzothiazol-2-yl)-4-[2-(4-nitrophenyl)ethynyl]phenol (HBT-d-NO₂) in the different surrounding environment is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The optimized molecular structure provides convincing evidence that the intramolecular hydrogen bond is strengthened in the first excited (S₁) state. The frontier molecular orbitals observed the HBT-d-NO₂ exists obvious intramolecular charge translate phenomenon. The results of the potential energy curve show that HBT-d-NO₂ is difficult to undergo proton transfer in the ground (S₀) state due to the high energy barrier, while it becomes easier in the S₁ state in both liquid and solid phases. By comparison, the energy barrier of ESIPT in the solid phase is higher than that in the liquid phase. We can conclude that the solid phase effectively hinders the ESIPT process compared with that the liquid phase. In this work, we illustrate the influence of liquid and solid phases on the intramolecular proton transfer process, which could promote further developments in biomedical and fluorophore applications.

A DFT/TD-DFT Study on the ESIPT-Type Flavonoid Derivatives with High Emission Intensity

To reveal the influence of different substituents on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of 4′-N, N-dimethylamino-3-hydroxyflavone (DMA3HF), two novel molecules (DMA3HF-CN and DMA3HF-NH2) were designed by introducing the classical electron-withdrawing group cyano (-CN) and electron-donating group amino (-NH2). The three molecules in the acetonitrile phase were systematically researched by applying the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The excited-state hydrogen bond enhancement mechanism was confirmed, and the hydrogen bond intensity followed the decreasing order of DMA3HF-NH2 > DMA3HF > DMA3HF-CN, which can be explained at the electronic level by natural bond orbital, fuzzy bond order, and frontier molecular orbital analyses. Moreover, we found from the electronic spectra that the fluorescence intensity of the three molecules in keto form is relatively strong. Moreover, the calculated absorption properties indicated that introducing the electron-withdrawing group -CN could significantly improve the absorption of DMA3HF in the ultraviolet band. In summary, the introduction of an electron-donating group -NH2 can promote the ESIPT reaction of DMA3HF, without changing the photophysical properties, while introducing the electron-withdrawing group -CN can greatly improve the absorption of DMA3HF in the ultraviolet band, but hinders the occurrence of the ESIPT reaction.

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.

Theoretical study on the sensing mechanism of chalcone-based fluorescence probe for detecting hydrogen sulfide and biothiols

The single fluorescence phenomenon of Comp2 experimentally is explained by the Boltzmann distribution. Pr1 has three distorted dihedral angles under photo-excitation.