Excited-state Intramolecular Proton Transfer Reaction Research Articles

Photoexcitation Dynamics of 4-Aminopthalimide in Solution Investigated Using Femtosecond Time-Resolved Infrared Spectroscopy

Excited-state intramolecular proton transfer (ESIPT) reactions are crucial in photoresponsive materials and fluorescent markers. The fluorescent compound 4-aminophthalimide (4-AP) has been reported to exhibit solvent-assisted ESIPT in protic solvents, such as methanol, wherein the solvent interacts with 4-AP to form a six-membered hydrogen-bonded ring that is strengthened upon excitation. Although the controversial observation of ESIPT in 4-AP has been extensively studied, the molecular mechanism has yet to be fully explored. In this study, femtosecond infrared spectroscopy was used to investigate the dynamics of 4-AP in methanol and acetonitrile after excitation at 350 and 300 nm, which promoted 4-AP to the S1 and S2 states, respectively. The excited 4-AP in the S1 state relaxed to the ground state, while 4-AP in the S2 state relaxed via the S1 state without the occurrence of ESIPT. The enol form of 4-AP (Enol 4-AP) in the S1 state was calculated to be ~10 kcal/mol higher in energy than the keto form in the S1 state, indicating that keto-to-enol tautomerization was endergonic, ultimately resulting in no observable ESIPT for 4-AP in the S1 state. Upon the excitation of 4-AP to the S2 state, the transition to Enol-4-AP in the S1 state was found to be exergonic; however, ESIPT must compete with an internal conversion from the S2 to the S1 state. The internal S2 → S1 conversion was significantly faster than the solvent-assisted ESIPT, resulting in a negligible ESIPT for the 4-AP excited to the S2 state. The detailed excitation dynamics of 4-AP clearly reveal the molecular mechanism underlying its negligible ESIPT, despite the fact that it forms a favorable structure for solvent-assisted ESIPT.

ESIPT-induced intersystem crossing leads to tautomer fluorescence quenching for 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one molecule

The excited-state intramolecular proton-transfer (ESIPT) dynamics of 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF) and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) have been investigated using time-dependent density functional theory (TDDFT). Upon photoexcitation, 3NTF exhibits a single fluorescence emission while 3FTF is fluorescence quenched when dissolved in cyclohexane solution. The present study reveals that both species undergo barrierless ESIPT process, and the underlying reason for fluorescence quenching in 3FTF has been elucidated. Specifically, it is concluded that intersystem crossing (ISC) is responsible for the fluorescence quenching in the 3FTF molecule due to the energy gap between the S1 and T2 states is only 0.12 eV plus large S1 → T2 spin–orbit coupling resulting in a strong interaction between the singlet and triplet states. The present study provides a reference for the fluorescence quenching associated with thiol-hydrogen bond molecules, and it is helpful for further research on ESIPT reactions of sulfur-containing molecules.

Theoretical study on precise regulation of ESIPT reaction based on ICT effect of o-carborane derivatives

The unique stereocarborane fluorophore design has attracted widespread attention because of its ability to effectively enhance the performance of thin-film fluorescence sensors. This study provides a detailed theoretical investigation of the excited-state intramolecular proton transfer (ESIPT) reactions of three o-carborane derivatives, NaCBO, PaCBO and PyCBO, using density functional theory and time-dependent density functional theory. We found that as the number of benzene rings in the electron donor groups increases, the interaction between the first excited-state electron donor–acceptor fragments is enhanced, weakening the intramolecular hydrogen bonding strength, which in turn inhibits the ESIPT. Notice that the intramolecular charge transfer (ICT) is proportional to the interaction between electron donor and acceptor fragments with the analysis of electron structure. The higher the ICT level of the target system, the more unstable the keto form generated by ESIPT. Therefore, the ESIPT can be precisely manipulated by rationally adjusting the ICT effect. Furthermore, with increasing solvent polarity, the photophysical properties of the chromophores exhibit non-solvatochromism. This is attributed to the rigid o-carborane structure, which serves as a spatial scaffold to increase the adaptability of the molecule to the environment. This study is expected to provide theoretical guidance and new ideas for preparing high-performance thin-film fluorescence sensing.

Theoretical study on the cascaded double proton transfer mechanism involving two adjacent hydrogen bonds in conjugated organic molecules

The cascaded double proton transfer reaction has attracted the interest of many researchers due to its effective six-level system cycle. Herein, the double proton transfer mechanism of naphthalene-based analog NDH and anthracene-based analog ADH in different solvents is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results indicate that the first occurrence of excited-state intramolecular proton transfer (ESIPT) in NDH and ADH is an ultrafast, barrierless process. Additionally, ADH undergoes a second ESIPT reaction, radiating a wider range of fluorescence spectrum. Theoretical studies reveal that this is due to the significantly increased hydrogen bond strength of ADH compared to NDH in the first excited (S1) state, creating favorable conditions for the second ESIPT. Analysis of electronic properties indicates that ADH exhibits enhanced chemical and kinetic activity, with a more pronounced intramolecular charge transfer (ICT), leading to increased stability of the keto structure (KB2*) after the second ESIPT. Finally, we identify the factors influencing the cascaded double proton transfer reaction. The second ESIPT process is promoted by extending the π-conjugated skeleton of the core molecule horizontally and leveraging the driving force provided by the H3 atom near the proton donor O3–H2. It is expected that these studies will provide theoretical guidance and new insights for the design and synthesis of near infrared (NIR) organic materials.

Substituent effects on the ESIPT process of H2O2 sensing product 1,3-bis(bispyridin-2-ylimino) isoindolin-4-ol: A theoretical study

H2O2 plays an important role in a variety of physiological processes, but its overproduction or accumulation can lead to oxidative damage and related diseases, so it is crucial to establish accurate H2O2 detection methods. Fluorescence sensing as a potential method, the design and modulation of its luminescent properties are crucial to the detection results. In this work, we investigated the sensing mechanism of a novel H2O2 fluorescent probe (P1) and examined the role of substituent effects in modulating the fluorescence properties of its reaction products. The results show that the non-fluorescence property of the probe is due to twisted intramolecular charge transfer (TICT) mechanism resulting from a torsion of the excited state molecular conformation, and the fluorescence turn-on upon recognition of H2O2 by the probe is due to the reaction product (BBYI1) following the excited state intramolecular proton transfer (ESIPT) process. When the substituent is an electron-donating group, the energy barrier of the ESIPT reaction is lowered, promoting luminescence, while the electron-withdrawing group has the opposite effect. In summary, the present work may offer a theoretical foundation for the development of more sensitive fluorescent probes for ESIPT-based H2O2 detection.

Theoretical study on the effect of different positions of -CN group on photophysical properties and forward-reverse ESIPT behaviours of 2,5-bis(benzo[d]thiazol-2-yl)phenol derivatives

Excited state intramolecular proton transfer (ESIPT) compounds have been widely used in many fields due to its unique photophysical properties, but they are strongly affected by substituents. There are many studies on the effect of substituents at the same position on the ESIPT process, but there are few studies on the effects of substituents at different positions on the proton transfer process. In this work, a strong electron-withdrawing cyano group ( – CN) was introduced into different positions of 2,5-bis(benzo[d]thiazol-2-yl)phenol. The effect of different substituted positions on intramolecular hydrogen bond (IHB), photophysical properties and forward-reverse energy barrier of ESIPT reaction have been systematically investigated adopting density functional theory (DFT) and time-dependent DFT. The result showed that the introduction of – CN group at different positions led to a slight red-shift of absorption, and intramolecular charger transfer is the intrinsic reason for this photophysical phenomenon. The position of – CN group obviously influenced the excited state IHB, and then had an effect on ESIPT process. The substitution positions in the left benzo[d]thiazole ring and phenol ring respectively hinder and promote the ESIPT process, while the substitution in the right benzo[d]thiazole ring not only impetus the ESIPT process, but also increase the tautomer fluorescence.

Solvent polarity dependent photo-induced excited state intramolecular proton transfer for triphenylamine centered bis unsymmetrical azine (TPOV): A computational study

The exquisite, regulated characteristics of triphenylamine (TPA) derivatives have served as a profound inspiration in the photochemical field. Herein, the novel TPA-centered bis unsymmetrical azine (i.e., 3-methoxysalicylaldehyde substituted 4-((E)-hydrazonomethyl)-N-(4-((E)-hydrazonomethyl)phenyl)-N-phenylaniline (TPOV)) is investigated regarding its photo-induced behaviors and excited state intramolecular proton transfer (ESIPT) mechanisms. Solvent polar-dependent hydrogen bonding interactions and charge recombination, which is induced by photoexcitation, can greatly promote the ESIPT reactions of the TPOV chemosensor. In order to unveil the detailed excited state reaction mechanism, we theoretically construct the S0-state and S1-state of potential energy surfaces (PESs). Based on the S1-state PESs for TPOV in toluene, chloroform, and acetonitrile solvents, we unveil the single ESIPT reaction of TPOV that happens along with alternative dual intramolecular hydrogen bonds (O1-H2···N3 and O4-H5···N6). The solvent polarity-regulated excited state might clear the path for the creation of innovative luminescent materials.

Effect of number of intramolecular double bonds on photophysical properties and ESIPT processes for Cha-NH2 and its derivatives: A theoretical study

In recent years, the research on excited-state intramolecular proton transfer (ESIPT) reactions has aroused increasingly attention theoretically and experimentally. For a deep exploration, we designed three derivatives of Cha-NH2 with different numbers of carbon–carbon double bonds: Cha-NH2-1, Cha-NH2-2, and Cha-NH2-3. Using density functional theory and time-dependent density functional theory methods, we systematically studied the photophysical properties and ESIPT processes of Cha-NH2 and its derivatives. Detailed analysis revealed that the number of carbon–carbon double bonds within the molecule has a significant impact on the ESIPT behavior and spectral properties of Cha-NH2 and its derivatives. Specifically, as the number of carbon–carbon double bonds increases, the strength of intramolecular hydrogen bond is gradually weakened, leading to a red shift in the spectrum and a increase in the forward energy barrier for ESIPT, which is unfavorable for the occurrence of the ESIPT reaction. Therefore, this finding opens up broad application prospects for the adjustment and optimization of future ESIPT process.

Theoretical study of atomic electronegativity effects on the excited‐state behavior of fluorescent compounds of citrinin

AbstractThe present work focuses on the light‐induced behavior of citrinin derivatives in relation to atomic electronegativity. A detailed theoretical study on the photophysical properties and excited‐state behavior of fluorescent compounds of citrinin (Cit‐O, Cit‐S, and Cit‐Se, with different atomic electronegativity) has been conducted, and the effect of electronegativity on the proton transfer in this system has been explained. First, the relevant hydrogen bond parameters and infrared vibrational spectra of the optimized geometrical configurations have been insightfully investigated. It is elucidated that the hydrogen bond is strengthened after photoexcitation, and it provides a driving force for excited‐state intramolecular proton transfer (ESIPT). In addition, the frontier molecular orbitals were analyzed, and the intramolecular charge transfer process in all Cit systems, the phenomenon of charge redistribution, facilitates the ESIPT reaction. By constructing potential energy surfaces for different transfer paths, the atomic electronegativity impact on the ESIPT dynamical behavior of the Cit system was determined. This work clarifies the mechanism of the intramolecular proton transfer process in the excited state of citrinin molecules and complements the theoretical study of the atomic electronegativity‐regulated citrinin system, which provides a corresponding theoretical basis for the design and synthesis of new luminescence‐adjustable citrinin systems.

Theoretical study of the substituent effects on the ESIPT mechanism of salicylideneaniline and the TICT photochemistry reactions.

The study of salicylideneaniline (SA) and its derivatives is critical due to their special photophysical properties and environmental sensitivity. In this work, the density time-dependent functional theory (TDDFT) and complete-active-space self-consistent-field (CASSCF) methods were carried out to calculate the substituent effect on excited-state properties and dynamics of SA derivatives. We found the para-substitution triggers the excited-state intramolecular proton transfer (ESIPT) reaction, exhibiting the dual-fluorescent phenomena. However, the meta- and ortho-substitutions impel the non-radiative transition process along the minimum energy conical intersection (MECI), forming the twisted intramolecular charge transfer (TICT) state to prevent ESIPT. This investigation of substituent effects on the photochemical processes and photophysical properties will provide the benchmarks for the design of fluorescent materials.

Anion Effect on the Excited-State Intramolecular Proton Transfer of 4'-N,N-Diethylamino-3-hydroxyflavone in Ionic Liquids.

The excited-state intramolecular proton transfer (ESIPT) reaction of 4'-N,N,-diethylamino-3-hydroxyflavone (C2HF) was studied using time-resolved fluorescence measurements in ionic liquids (ILs) of various anions with a fixed cation (1-ethyl-3-methylimidazolium [C2mim]+). C2HF showed an ESIPT reaction from the normal excited state (N*; keto form) to the tautomer excited state (T*; enol form) where both states are emissive. The ESIPT rate and yield were obtained by analyzing the time-resolved fluorescence spectra measured using the optical Kerr gate method. Both the ESIPT rate and yield decreased with increasing hydrogen-bond accepting ability of the anion. According to density functional theory calculations, the complex formation energy between C2HF and the anion became significantly negative with increasing the hydrogen-bond accepting ability of anion. The pseudoequilibrium constant between N* and T* ([T*]/[N*]) in the electronic excited state decreased with increasing hydrogen-bond accepting ability of the anion, while it increased with increasing the alkyl-chain length of alkyl sulfonate. The excitation wavelength dependence of the ESIPT rate and yield was studied for C2HF in [C2mim][C6H13SO3]. The ESIPT yield decreased by nearly a factor of 2 with increasing excitation wavelength from 360 to 425 nm, although the change in the ESIPT rate was small. The solvation heterogeneity due to the alkyl chain in the anion was considered to be the reason for the excitation wavelength dependence.

Revealing the ESIPT process in benzoxazole fluorophores within polymeric matrices through theory and experiment.

The impact of the polymeric matrix on the photophysical characteristics of monomeric dyes responsive to excited-state intramolecular proton transfer (ESIPT) was investigated through UV-Vis absorption as well as steady-state and time-resolved emission spectroscopies. For this purpose, two benzoxazole monomers (M1 and M2) with acryloyl groups at different positions in their molecular structures were employed to facilitate covalent bonding within a styrene chain. Our findings reveal significant variations in their excited-state properties due to the proximity of the acryloyl groups, which affects the energy barrier of the ESIPT reaction, the emission wavelength, and the balance between the normal and tautomeric forms. The experimental results were corroborated through theoretical investigations at the DFT/TDDFT level, specifically using the B3LYP-D3/def2-TZVP methodology. Three notable observations emerged: donor/acceptor groups at the meta/para positions induced electron distribution changes, causing red-shifted emission for M2; in the polymer film, particularly in PM1, intramolecular hydrogen bond deactivation favored N* emission over T* emission; and the zwitterionic character of the T* species. This study underscores the advantages of functionalization in polymers, which can lead to colorless films and prevalent N* or T* emission, and contributes valuable insights into molecular design strategies for tailoring the photophysical properties of polymeric materials.

Detection mechanism and solvent effects of the 3‐hydroxy‐2‐(4‐(pyrrolidin‐1‐yl)phenyl)benzo[g]quinolin‐4(1H)‐one (PBQ) probe

AbstractHydroxy‐2‐(4‐(pyrrolidin‐1‐yl)phenyl)benzo[g]quinolin‐4(1H)‐one (PBQ) is a ratiometric fluorescent probe based on excited‐state intramolecular proton transfer (ESIPT). PBQ‐1 is the reaction product following its exposure to phosgene. Density functional theory (DFT) and time dependent density functional theory (DFT) have been used to study the excited state dynamics of PBQ and PBQ‐1 in different solvents. The results show that the reaction of PBQ with a transition from charge‐transfer excitation to local excitation before and after the reaction. It becomes more difficult for PBQ in the excited state to transfer proton with increasing solvent polarity. The product PBQ‐1 undergoes a molecular structure twist, and the angle of twisting decreases with increasing solvent polarity, resulting in a lower degree of rotational freedom of the hydroxyl group (5‐OH) at the 5th carbon position, which makes it more susceptible to ESIPT reactions. Therefore, PBQ‐1 is more susceptible to ESIPT as solvent polarity increases. Our theoretical calculations also elucidate the cause of the blue shift of PBQ fluorescence and the impact of the twisting intramolecular charge transfer phenomenon on the solvent effect. Furthermore, our study provides the theoretical guidance for the designing probe based on excited state intramolecular proton transfer.

The effect of the number of conjugated C=C bonds on the ESIPT and ICT reactions of SNCN derivatives

The electronic structure of the molecule is significantly influenced by the number of conjugated C=C bonds. In this work, the influence of the conjugated C=C bonds of the SNCN derivatives on the excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) properties are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculation level is proved to be reasonable by calculating electronic spectra. The hydrogen bond parameters, infrared vibrational frequency (IR), reduction density gradient (RDG) isosurface, topological analysis and potential energy curves of SNCN derivatives in ground state (S0) and the first excited state (S1) are analyzed. According to theoretical research results, ESIPT reaction has a higher likelihood of occurring in the S1 state. Moreover, the ESIPT reaction becomes more challenging to occur with the number of conjugated C=C bonds rising. Finally, the analyses of the frontier molecular orbitals (FMOs), dipole moment and charge transfer transition confirm that the ICT effect is aided by the increased number of conjugated C=C bonds. This work indicates that the number of conjugated C=C bonds can regulate the ESIPT and ICT processes, which provides guidance for the study of fluorescent groups with similar characteristics.

Exploring electron donor and acceptor effects: DFT analysis of ESIPT/GSIPT in 2-(oxazolinyl)-phenols for photophysical and luminophore enhancement.

Understanding excited-state intramolecular proton transfer (ESIPT) is essential for designing organic molecules to enhance photophysical and luminophore properties in the development of optoelectronic devices. In this context, an attempt has been made to understand the impact of substituents on the ESIPT process of 2-(oxazolinyl)-phenol. Electron donating (EDG: -NH2, -OCH3, and -CH3) and electron withdrawing (EWG: -Cl, -Br, -COOH, -CF3, -CN, and -NO2) substitutions have been computationally designed and screened through density functional theory (DFT) and time-dependent density-functional theory (TDDFT) calculations. Furthermore, the ground state intramolecular proton transfer and ESIPT mechanisms of these designed luminophores are explored using the transition state theory. The results reveal that molecules with EDG show higher absorption and emission peaks than molecules with EWG and also indicate that the mobility of charge carriers in 2-(oxazolinyl)-phenol derivatives is significantly influenced by substituents. We found that the EWGs decrease the reorganization energy and increase the vertical ionization potential and electron affinity values, as well as the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, compared to the EDG substituted molecules. Significantly, the excited state (S1) of the keto emission (K) form shows notably larger values for the EDG substitutions. The intersystem crossing pathway efficiency weakens with reduced spin-orbit coupling matrix element in the enol form with electron-donating substituents and vice versa in the keto form during S1-T3 transitions. Our research links intramolecular proton transfers and triplet generation, making these substituted molecules appealing for optoelectronic devices. Introducing EDGs, such as -NH2, boosts the ESIPT reaction in 2-(oxazolinyl)-phenol. This study guides designing ESIPT emitters with unique photophysical properties.

Insights into the solvent-polarity-associated hydrogen bonding interactions and ESIPT reactions for DBOX fluorophore: a theoretical investigation

Inspired by the exquisite design of distinguished optical materials, novel organic molecules with remarkable characteristics of excited-state intramolecular proton transfer (ESIPT) have emerged as a prominent and captivating research topic. In this study, our primary focus lies in exploring the intricate dynamics of the excited state in 2-(5-(benzo[d]thiazol-2-yl)-1,3,4-oxadiazol-2-yl)-5-(diethylamino)phenol (DBOX), an extraordinary reversible molecular fluorophore that undergoes single-crystal-to-single-crystal transition. By investigating four distinct aprotic solvents with varying degrees of polarity, we unequivocally validate the significant impact of solvent polarity on photo-induced hydrogen bonding interactions, charge redistribution, and associated excited-state intramolecular proton transfer (ESIPT) phenomena. Through meticulous comparison and precise quantification of reaction barriers in diverse solvent environments, our groundbreaking findings strongly suggest that solvents with low polarities effectively facilitate the ESIPT reaction for DBOX fluorophore.

Theoretical exploring effects of solvent polarity and atomic electronegativity on excited state behaviour for BY4TP fluorophore

As widely acknowledged, the realm of novel organic molecules boasting extraordinary attributes pertaining to excited-state intramolecular proton transfer (ESIPT) has emerged as a captivating subject matter. In this context, our primary focus lies in delving into the excited-state behaviour exhibited by 2-benzoxazol-2-yl-4-triethylsilanylethynyl-phenol (BY4TP), an alluring derivative derived from 2-(2-hydroxyphenyl)benzoxazole (HBO). Relying on the examination of four distinct aprotic solvents with varying degrees of polarities, we can unequivocally affirm that solvent polarity exerts a profound influence on the intricate interplay of hydrogen bonding interactions, charge redistribution and reorganisation, as well as associated ESIPT phenomena by light. Through meticulous comparison and precise measurement of reaction barriers across diverse solvent environments, our groundbreaking findings indicate that lowly polar solvents serve as efficacious facilitators for promoting the occurrence of the ESIPT reaction in BY4TP fluorophore. By considering atomic-electronegativity-regulated hydrogen bonding effects and excited state behaviours for BY4TP-S and BY4TP-Se, we also present low atomic electronegativity with Se substation promotes ESIPT reaction. We ardently anticipate that this study will provide invaluable insights into the behaviour exhibited by BY4TP upon excitation and under the influence of solvent polarity and atomic electronegativity, while simultaneously paving new avenues for future research endeavours and applications encompassing novel HBO derivatives.

Theoretical study on the ESIPT and photophysical properties for molecules with different extension π conjugation

In a recent study, Jincheng Si and coworkers designed four candidate linker units with different extension π conjugation sizes and directions. These molecules were used for the synthesis of metal–organic framework (MOF) structures known as Olympic rings, which can detect the Cr2O72- in the aqueous phase. However, the experimental work did not provide a qualitative understanding of the excited-state dynamic mechanism. Therefore, this research focused on investigating the excited state intramolecular proton transfer (ESIPT) process and the photophysical properties of these molecules. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were employed to conduct an in-depth analysis. The molecule structure properties were investigated by analyzing the hydrogen bond parameters and the infrared (IR) vibrational spectra. The calculated absorption and emission spectra were in good agreement with the experimental values, it was proved that the correctness of the optimized structure and the rationality of our calculation method, and we can judge their detection sensitivity by the sizes of the Stokes shift. The Hirshfeld method was used to study the enhancement mechanism of hydrogen bond interaction. Furthermore, the reaction mechanism and ESIPT reactivity of molecules with different extension π conjugation sizes and directions were explained by calculating and plotting the relaxed potential energy curve, electrostatic potentials (ESPs) and frontier molecular orbitals (FMOs). Our research found that extension π conjugation in different sizes and directions can either promote or inhibit the occurrence of the ESIPT reactions and the characteristic of intramolecular charge transfer (ICT), thereby altering the fluorescence performance. Based on our findings, we concluded that the ESIPT activity and photophysical properties of the H4BIHQC were higher than the other three molecules. This discovery enhances the potential application value of MOF as a fluorescence detection tool.

Theoretical insights into photoinduced excited‐state behaviors for the novel CHPPhl fluorophore: Effects of solvent polarity

AbstractInspired by the distinguished photochemical and photophysical properties of novel hydroxyl‐substituted tetraphenylimidazole (HPI) derivatives that could be potentially applied across various disciplines, in this work, effects of solvent polarity on excited‐state hydrogen bond effects and excited‐state intramolecular proton transfer (ESIPT) reaction of 3‐(6,9‐Diphenyl‐1H‐phenanthro[9,10‐d]imidazole‐2‐yl)‐9‐phenyl‐9H‐carbazol‐4‐ol (CHPPhl) are focused. By comparing the structural changes and infrared (IR) vibrational spectra of the E‐HBT fluorophore in polar acetone, moderate polar tetrahydrofuran and non‐polar hexane solvents, combined with the preliminary detection of hydrogen bond interaction by core–valence bifurcation (CVB) index, we can conclude that the hydrogen bond could be strengthened in S1 state, which is favorable for the occurrence of ESIPT reactions. The charge recombination behavior of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimization and searching transition state (TS) form, we confirm change of surrounding solvent polarity has a regulatory effect on the ESIPT behavior for CHPPhl; that is, the lower the solvent polarity is more conducive to the ESIPT reaction.

Theoretical investigation into the effect of atomic electronegativity related chalcogen on ESIPT behaviour for the novel biphenyl-modified 2-(2’-hydroxyphenyl)benzothiazole compounds

Inspired by the remarkable photochemical and photophysical properties of novel 2-(2’-hydroxyphenyl)benzothiazole (HBT) derivatives that could be potentially applied across various disciplines, in this work, effects of atomic electronegativity of chalcogen elements on excited state hydrogen bond effects and excited state intramolecular proton transfer (ESIPT) reaction of the biphenyl-modified HBT derivatives (i.e. HBT-HH-O, HBT-HH-S and HBT-HH-Se) are focused. By comparing the structural changes and infrared (IR) vibrational spectra of the HBT-HH fluorophores in toluene solvent, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that the hydrogen bond could be strengthened in S1 state, which is favourable for the occurrence of ESIPT reactions. The charge recombination behaviour of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimisation and searching transition state (TS) form, we confirm the variations of atomic electronegativity of chalcogen have the regulatory effect on the ESIPT behaviour for HBT-HH derivatives, that is, the lower the atomic electronegativity is more conducive to the ESIPT reaction.