Unveiling and regulating the solvent-polarity-associated excited state intramolecular double proton transfer behavior for 1-bis(benzothiazolyl)naphthalene-diol fluorophore

Unraveling an excited state intramolecular double proton transfer pathway in 2,5-bis(benzoxazole-2-yl)benzene-1,4-diol derivatives

Organic molecules exhibiting excited-state intramolecular double proton transfer (ESIDPT) have garnered significant research interest, largely driven by their prevalence in nature and the unique luminescent properties associated with this phenomenon. This study presents a detailed theoretical investigation into the dynamic mechanism of both single and dual cooperative Proton transfer (PT) in the first excited singlet (S1) state of 2,5-bis(benzoxazole-2-yl)-1,4-dihydroxybenzene derivatives. The analysis meticulously incorporates solvent effects, specifically those of dichloromethane, to provide a comprehensive understanding of the PT processes. The occurrence of the ESIDPT process was confirmed by integrating infrared (IR) vibrational spectra, frontier molecular orbital analysis, and reduced density gradient (RDG) analysis. Additionally, the potential energy surfaces (PESs) for the ground (S0) and S1 states revealed a synergistic interaction between single and dual ESIPT processes within the S1 state. Furthermore, variation in the charge distribution, resulting from the coupling of photoinduced electron transfer with ESIPT involving the DPA group, led to distinct PT propensities for the O1 and O4 atoms. Consequently, these configurations are unable to undergo simultaneous proton transfer, as demonstrated by the construction of a potential energy surface. In addition, the corresponding PT PESs show that the ESIPT reaction for Zinhbo-9 is much easier for the Zinhbo-5 modified with a t-Bu group, which affect O4-H5···N6 hydrogen bond. These theoretical computations provide a robust explanation of the observed experimental phenomena and suggest potential pathways for future advancements and applications of ESIDPT molecules.

Theoretical insights on the double ESIPT mechanism and fluorescence properties of H2BIo chromophore

2-{[3-(1H-benzoimidazol-2-yl)-2-hydroxy-5-methylbenzylidene] amino}-benzoic acid (H2BIo) based on proton transfer can serve as the fluorescent probe for detecting heavy metal ions. The excited-state intramolecular proton transfer (ESIPT) reaction mechanism of the H2BIo chromophore with an intramolecular asymmetric double hydrogen bond in different solvents are investigated. The reaction barrier of the ESIPT along hydrogen bond O1-H2···N3 is higher than that of ESIPT along O4-H5···N6, which indicates that the double ESIPT is a stepwise process. The time-evolving non-adiabatic excited-state dynamic simulations shows that the sequence ESIPT reactions on a time scale: the ESIPT along O1-H2···N3 is faster than the ESIPT along O4-H5···N6. The analyses of electron structure and spectra indicate that the double ESIPT couples with electron transfer, significantly enhances the fluorescence signal, thereby improving the performance of the fluorescent probe in detecting heavy metal ions.

Exploring and Regulating Heteroatom-Electronegativity-Associated with Ring Aromaticity and Excited State Intramolecular Proton Transfer Mechanism for Benzothiazole-Based Fluorophore

A novel fluorophore 2-(2'-hydroxy-5'-benzaldehyde phenyl) benzothiazole (HBBT) with excited state intramolecular proton transfer (ESIPT) characteristics, showing good selectivity for Cu+/Cu2+ ions had been synthesized experimentally (Molecules 2022, 27, 7678). However, its ESIPT mechanism and fluorescent performance related to atomic substituents have not been investigated systematically. In this work, two HBBT derivatives, 2-(2'-hydroxy-5'-benzaldehyde phenyl)benzoimidazole (HBBI) and 2-(2'-hydroxy-5'-benzaldehyde phenyl)benzopyrrole (HBBP), were obtained by respectively using -NH and -CH2 groups in place of the sulfur atom in the thiazole ring. The absorption/emission spectra and ring aromaticity as well as ESIPT processes of HBBT and its derivatives were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT). The simulated absorption and fluorescence wavelengths of HBBT agreed well with the corresponding values obtained in the experiment. According to the analyses of geometry structures, electron densities, and infrared vibration frequencies, the intramolecular hydrogen bond becomes stronger upon light excitation. The frontier molecular orbitals were analyzed via establishing potential energy curves, and the ESIPT behavior was described deeply. Obviously, the NH-substitution makes ring 4 more aromatic, while the CH2-substitution changes ring 4 from aromatic to antiaromatic. The ESIPT process helps to alleviate the excited state antiaromaticity. The greater the antiaromaticity of the S1 state normal form, the higher the barrier of ESIPT.

Theoretical investigation of the excited-state intramolecular double proton transfer process of 2,2'-(benzo[1,2-d:4,5-d']bis(thiazole)-2,6-diyl)diphenol

In this work, the excited state intramolecular double proton transfer (ESIDPT) mechanism of 2,2'-(benzo[1,2-d:4,5-d']bis(thiazole)-2,6-diyl)diphenol (BTAP) is proposed using density functional theory (DFT) and time-dependent DFT (TDDFT). The changes in bond lengths, bond angles and IR vibrational spectra associated with two intramolecular hydrogen bonds of BTAP upon photoexcitation indicate that the hydrogen bonds are strengthened in the excited state, facilitating the ESIDPT process. Investigation of the constructed S1-state potential energy surface proposes that BTAP prefers a stepwise ESIDPT mechanism. Electronic spectra and frontier molecular orbitals (FMOs) are also presented to illustrate the luminescent properties of BTAP.

Computational Investigation about the Effects of Solvent Polarity and Chalcogen Element Electronegativity on ESIPT Behaviors for the Et2N-Substituted Flavonoid

Inspired by the outstanding nature of flavonoid derivatives in the fields of chemistry and medicine, in this work we mainly focus on exploring the photo-induced properties of the novel Et2N-substituted flavonoid (ENF) fluorophore theoretically. Considering the potential photo-induced properties in different solvents and the chalcogen atomic electronegativity-associated photoexcitation, by time-dependent density functional theory (TDDFT) methods we primarily explore the intramolecular hydrogen bonding interactions and photo-induced charge redistribution behaviors. Via comparing geometrical data and the infrared (IR) spectral shifts-associated hydroxy moiety of ENF, we confirm that the intramolecular hydrogen bond O-H···O should be enhanced with facilitating an excited-state intramolecular proton-transfer (ESIPT) reaction. Particularly, the charge reorganization around hydrogen bonding moieties further reveals the tendency of ESIPT behavior. Combined with the construction of the potential energy surface and the search for reaction transition states, we finally confirmed the solvent-polarity-regulated behaviors as well as the chalcogen elements' electronegativity-dependent ESIPT mechanisms for the ENF fluorophore. We sincerely wish our work could accelerate the further development and applications of flavonoid derivatives.

TDDFT Study on the ESIPT Properties of 2-(2'-Hydroxyphenyl)-Benzothiazole and Sensing Mechanism of a Derived Fluorescent Probe for Fluoride Ion

The level of fluoride ions (F-) in the human body is closely related to various pathological and physiological states, and the rapid detection of F- is important for studying physiological processes and the early diagnosis of diseases. In this study, the detailed sensing mechanism of a novel high-efficiency probe (PBT) based on 2-(2'-hydroxyphenyl)-benzothiazole derivatives towards F- has been fully investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. F- attacks the O-P bond of PBT to cleavage the dimethylphosphinothionyl group, and the potential products were evaluated by Gibbs free energy and spectroscopic analyses, which ultimately identified the product as HBT-Enol1 with an intramolecular hydrogen bond. Bond parameters, infrared vibrational spectroscopy and charge analysis indicate that the hydrogen bond is enhanced at the excited state (S1), favoring excited state intramolecular proton transfer (ESIPT). The mild energy barrier further evidences the occurrence of ESIPT. Combined with frontier molecular orbital (FMO) analysis, the fluorescence quenching of PBT was attributed to the photoinduced electron transfer (PET) mechanism and the fluorescence turn-on mechanism of the product was attributed to the ESIPT process of HBT-Enol1.

Synchronous or stepwise Mechanism? a theoretical study on the Excited-State double proton transfer properties of shikonin and acetylshikosin

The excited state double proton transfer (ESDPT) mechanism of shikonin (Shk) and its derivative acetylshikosin (AcShk) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The potential energy curves scanned along the coordinates of proton transfer indicate a preference for the ESDPT reaction to occur step by step. The AcShk molecule possesses an additional reaction pathway in comparison to the Shk molecule. Furthermore, efforts have been made to compute the absorption and fluorescence peak, which exhibits favorable conformity with the experimental findings of the system investigated. The fluorescence spectra in cyclohexane and acetonitrile solvents indicate that the solvent polarity affects the location of the ESDPT fluorescence peak in both Shk and AcShk systems. The fluorescence spectra concentrated in the green light region (504 nm ∼ 550 nm) are obtained, which has the potential to promote human health through disinfection and boosting the immune system.

Fluorescence emission mechanism for the π-conjugated zwitterion 2,4-Bisimidazolylphenol base on ESIPT: A TDDFT theoretical reconsideration

Molecules with zwitterionic characteristics exhibit significant potential for utilization in nonlinear optics, optoelectronics, and organic lasers owing to their large dipole moments. Recently, the synthesized compound 2,4-bis (4,5-diphenyl-1H-imidazol-2-yl) phenol (2,4-bImP) by Sakai et al. has been noticed for its unique photochromic properties in solvents [J. Phys. Chem. A, 125 (2021), 4784-4792]. The observed fluorescence in chloroform was attributed to the keto tautomer. Based on the excited state intramolecular proton transfer, the photochromism of 2,4-bImP in chloroform was interpreted as zwitterion production. However, the zwitterion with a specific electronic structure can be in resonance with the conventional neutral structure. The impact of the resonance contribution from the zwitterion and the conventional neutral structure on fluorescence attribution was not taken into account in the previous studies. In this investigation, the ESIPT mechanism of the 2,4-bImP in chloroform has been explored using both the density functional theory and the time-dependent density functional theory. The optimized geometric configuration parameters illustrate the molecular resonant properties. The calculated fluorescence spectra on the basis of the optimization results further corroborate that the fluorescence peaks after proton transfer originates from the resonance of the zwitterionic and the neutral configuration. The zwitterionic nature of the molecule was demonstrated by electrostatic potential and atomic dipole modified Hesfeld atomic charge (ADCH) analysis. Furthermore, the characterization of potential energy curves and IR spectrum further verified the resonance of both the zwitterionic and neutral structures. The results reveal that the 2,4-bImP molecule generates the neutral o-quinoid structure and the zwitterionic structure resonance phenomenon following ESIPT. The aforementioned resonance structure offers novel insights into the ascription of fluorescence. These discoveries establish the theoretical foundation for the exploration and development of zwitterions.

Effects of Chalcogen Atoms on Excited-State Double-Proton Transfer Behavior for 3,6-bis(4,5-Dihydroxyoxazo-2-yl)benzene-1,2-diol Derivatives: A Computational Investigation

The impact of the chalcogen atomic electronegativity (O, S, and Se atoms) of new organic molecules on excited-state dynamical reactions is self-evident. Inspired by this kind of distinguished photochemical characteristic, in this work, we performed a computational investigation of chalcogen-substituted 3,6-bis(4,5-dihydroxyoxazo-2-yl)benzene-1,2-diol (BDYBD) derivatives (i.e., BDYBD-O, BDYBD-S, and BDYBD-Se). In this paper, we pay close attention to characteristic BDYBD derivatives that contain intramolecular double hydrogen bonds (O1-H2···N3 and O4-H5···N6). The main goal of this study was to explore how changes in atomic electronegativity affect the way hydrogen bonds interact and how excited molecules affect transfer protons. We go into further detail in the main text of the paper. By fixing our attention to geometrical variations and infrared (IR) vibrational spectra between the S0 and S1 states, exploring hydrogen bonding behaviors using the core-valence bifurcation (CVB) index, and simulating hydrogen bonding energy (EHB) via the atom in molecule (AIM) method, we clarified the photo-induced strengthened dual hydrogen bonding interactions that facilitate the excited-state dual-proton transfer (ESDPT) behavior of BDYBD derivatives. The reorganization of charge stemming from photoexcitation further verifies the tendencies of ESDPT reactions. We relied on constructing potential energy surfaces (PESs) by adopting a restrictive optimization approach, and herein, we finally clarify the gradual ESDPT mechanism of BDYBD derivatives. Particularly, we confirm that the variation in chalcogen atomic electronegativity has a regulatory effect on the ESDPT behavior of BDYBD derivatives; that is, the lower the atomic electronegativity, the more favorable it is for the gradual ESDPT reaction.

Paying Comprehensive Attention to the Temperature-Dependent Dual-Channel Excited-State Intramolecular Proton Transfer Mechanism of Fluorescence Ratio Probe BZ-DAM

The mechanism of fluorescence detection of diethyl chlorophosphate (DCP) based on 2-substituted benzothiazole (BZ-DAM) was studied by a theoretical calculation method. It should not be ignored that both the BZ-DAM and the detection product BZ-CHO have two excited-state intramolecular proton transfer (ESIPT) channels. Density functional theory (DFT) and time-dependent DFT (TDDFT) theory were used to study the photophysical mechanism of two compounds in two channels in (acetonitrile) ACN solvent, and the temperature dependence of the two channels was given. Channel 1 is more likely to exist at low temperatures and channel 2 is more likely to exist at high temperatures. By theoretical analysis of the constructed potential energy curve, the hydrogen bond energy and electron-hole analysis, we confirmed that both molecules undergo ESIPT and intramolecular charge transfer (ICT) processes in channel 1 and ESIPT and twisted intramolecular charge transfer (TICT) coupling processes in channel 2. The formation of product BZ-CHO molecules led to a significant fluorescence blue-shift phenomenon and inhibited the ICT process, which confirmed that BZ-DAM could be used as a fluorescence probe for fluorescence detection. We sincerely hope that this work will not only help to clarify the excited-state dynamics behavior of the BZ-DAM probe but also provide a new idea for designing and optimizing a new chemical dosimeter.