Site-specific solvation of the photoexcited protochlorophyllide a in methanol: formation of the hydrogen-bonded intermediate state induced by hydrogen-bond strengthening

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.

Blue- and Red-Shifting C-H⋯O Hydrogen Bonds of Cyclic Ethers with Haloforms: Effect of Ring-Size and Consistency with Bent's Rule

A DFT-based computational study is carried out to delve into the interplay between hyperconjugation and rehybridization effects underlying the formation of blue- or red-shifting H-bonds (HBs) in 1 : 1 complexes of cyclic ethers (HB acceptor) of varying ring-size with haloforms, CHF3 and CHCl3 (HB donor). The calculations reveal that with decreasing angular strain (increasing ring-size) of the cyclic ethers, the extent of blue-shift increases for 1 : 1 complexes with CHF3, while a reverse sequence is observed with CHCl3, eventually leading to a red-shifting HB in the oxepane : CHCl3 complex. It is noted that the trend in the shift of C-H stretching fundamental is not mirrored by the C-H bond length or interaction energies for both the systems studied, that is, the low sensitivity of the changes on the strain on the O-atom of HB acceptor (cyclic ethers) is to be emphasized.

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.

Control of the fluorescence molecule 2-(2'-hydroxyphenyl) benzothiazole derivatives by introducing electron-donating and withdrawing substituents groups

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT), we investigate the fluorescence mechanism of (E)-4-(3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyryl)-1-methylpyridin-1-ium (HBTMY) and the excited-state intramolecular proton transfer process (ESIPT) of hydroxyphenyl. Herein, we introduce two electron-donating (amino and methoxy) and two electron-withdrawing (hydrogen and cyano) groups into HBTMY to study their effects on the fluorescence and the ESIPT process. Structural parameters, infrared vibration frequency, vertical excitation and emission energies as well as frontier molecular orbitals show that the substituents have different impacts on intramolecular hydrogen bonding behavior. The result shows that the fluorescence wavelength of molecules with the amino group could reach the near-infrared area, which favors using this fluorescence in the living cell. As the ability of electron-absorbing groups increases, the forward energy barrier in the potential energy curves decreases sharply making the ESIPT process more familiar to take place. Thus, this work offers a guide for cell imaging and provides strategies to adjust and control fluorescence by introducing substituents.

ESIPT mechanism of triple emission with hydroxy-oxadiazole compound in DMSO: A theoretical reconsideration

The compound in solvents with triple fluorescence feature of excited state intramolecular proton transfer (ESIPT) has a broad prospect in fluorescent probes, dye sensors and molecular synthesis of photosensitive dyes. An ESIPT molecule hydroxy-bis-2,5-disubstituted-1,3,4-oxadiazoles (compound 1a) emits two fluorescence peaks in dichloromethane (DCM) and three fluorescence peaks in dimethyl sulfoxide (DMSO). [Dyes and Pigments 197 (2022) 109927]. Two longer peaks were attributed to enol and keto emission in both solvents and the shortest third peak in DMSO was just attributed simply. However, there is a significant difference in proton affinity between DCM and DMSO solvents which has influence on the position of emission peaks. Therefore, the correctness of this conclusion needs to be further verified. In this research, density functional theory and time-dependent density functional theory method are used to explore ESIPT process. Optimized structures indicate ESIPT occurs through molecular bridge assisted by DMSO. The calculated fluorescence spectra demonstrate two peaks indeed originated from enol and keto in DCM, while interestingly three peaks are originated from enol, keto and intermediate in DMSO. Infrared spectrum, electrostatic potential and potential energy curves further prove existence of three structures. We reveal the mechanisms that compound 1a molecule occurs ESIPT in DCM solvent and undergoes an ESIPT through assisted by DMSO molecular bridge. Additionally, three fluorescence peaks in DMSO are reattributed. Our work is expected to provide an insight for understanding intra- and intermolecular interactions and synthesis of efficient organic lighting-emitting molecule.

Uncovering the dependence of ESIPT behaviors and fluorescence properties of two new benzothiazole-based fluorophores on solvent polarity: A TD-DFT study

We have studied the spectral features and excited state intramolecular proton transfer (ESIPT) processes of 2-(2',4'-dihydroxyphenyl)benzothiazole (OHBT) and 2-(2'-hydroxy-5'-chlorophenyl)benzothiazole (CHBT) using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). To consider the impact of solvent polarity and intermolecular hydrogen bond (H-bond) on the ESIPT behavior and photophysical properties, four solvents including toluene (TL), tetrahydrofuran (THF), methanol (MeOH) and dimethylsulfoxide (DMSO) were used. The simulated absorption and fluorescence wavelengths of OHBT and CHBT are well consistent with the experimental values. According to the results of structures, electron density and infrared (IR) vibrational frequencies, we found that the intramolecular H-bonds in OHBT/CHBT and OHBT-MeOH/CHBT-MeOH are strengthened in the first singlet excited state (S₁), which will be benefical to the ESIPT process. The potential energy curves (PECs) verified that the ESIPT processes in OHBT/CHBT and OHBT-MeOH/CHBT-MeOH can take place much easier because of their lower energy barrier. The influences of solvent polarity on ESIPT behaviors and photophysical properties of OHBT and CHBT are summarized below. As the solvent polarity becomes stronger from TL to DMSO, the energy gaps enlarges a little, the maximum absorption and fluorescence peaks at normal form red-shift slightly, and the strengths of H-bond in S₁ state become weaker, which makes the ESIPT process occur much harder. The formation of intermolecular H-bond between OHBT/CHBT and MeOH is conducive to promote the ESIPT process of OHBT/CHBT.

How Photoactivation Triggers Protochlorophyllide Reduction: Computational Evidence of a Stepwise Hydride Transfer during Chlorophyll Biosynthesis

The photochemical reaction catalyzed by enzyme protochlorophyllide oxidoreductase (POR), a rare example of a photoactivated enzyme, is a crucial step during chlorophyll biosynthesis and involves the fastest known biological hydride transfer. Structures of the enzyme with bound substrate protochlorophyllide (PChlide) and coenzyme nicotinamide adenine dinucleotide phosphate (NADPH) have recently been published, opening up the possibility of using computational approaches to provide a comprehensive understanding of the excited state chemistry. Herein, we propose a complete mechanism for the photochemistry between PChlide and NADPH based on density functional theory (DFT) and time-dependent DFT calculations that is consistent with recent experimental data. In this multi-step mechanism, photoexcitation of PChlide leads to electron transfer from NADPH to PChlide, which in turn facilitates hydrogen atom transfer by weakening the breaking C–H bond. This work rationalizes how photoexcitation facilitates hydride transfer in POR and has more general implications for biological hydride transfer reactions.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696-1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

The Role of Membranes and Lipid-Protein Interactions in the Mg-Branch of Tetrapyrrole Biosynthesis

Chlorophyll (Chl) is essential for photosynthesis and needs to be produced throughout the whole plant life, especially under changing light intensity and stress conditions which may result in the destruction and elimination of these pigments. All steps of the Mg-branch of tetrapyrrole biosynthesis leading to Chl formation are carried out by enzymes associated with plastid membranes. Still the significance of these protein-membrane and protein-lipid interactions in Chl synthesis and chloroplast differentiation are not very well-understood. In this review, we provide an overview on Chl biosynthesis in angiosperms with emphasis on its association with membranes and lipids. Moreover, the last steps of the pathway including the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the biosynthesis of the isoprenoid phytyl moiety and the esterification of Chlide are also summarized. The unique biochemical and photophysical properties of the light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR) enzyme catalyzing Pchlide photoreduction and located to peculiar tubuloreticular prolamellar body (PLB) membranes of light-deprived tissues of angiosperms and to envelope membranes, as well as to thylakoids (especially grana margins) are also reviewed. Data about the factors influencing tubuloreticular membrane formation within cells, the spectroscopic properties and the in vitro reconstitution of the native LPOR enzyme complexes are also critically discussed.

Theoretical investigations on forward-backward ESIPT processes of three fluorophores deriving from 2-(2'-hydroxyphenyl)thiazole

The photophysical properties and excited-state intramolecular proton transfer (ESIPT) processes for 2-(2'-hydroxyphenyl)-4-chloromethylthiazole (1), 2-(2'-hydroxyphenyl)-4-phenylthiazole (2), 2-(2'-hydroxyphenyl)-4-hydroxymethyl-thiazole (3) were studied at the TD-B3PW91/6-31 + G(d, p)/IEFPCM level. The structures of 1-3 were fully optimized and the corresponding structural parameters, infrared spectra and electron densities in the ground (S₀) and the first excited (S₁) states were analyzed. The calculated absorption and fluorescence wavelengths of 1-3 reproduced the experimental data. The potential energy curves of the S₀ and S₁ states were built and the ESIPT processes were clarified. Our results showed that the intramolecular H-bonds of 3 and 2 in the S₁ state were the strongest and the weakest, respectively, and then the ESIPT potential barriers of 3 and 2 were the lowest and highest, respectively. Among the three phenol-thiazole type probes, the compound 2 with phenyl ring group at the 4 position of the thiazole ring had the larger π-conjugation, and had the higher ESIPT potential barrier at the same time. The corresponding compound 1 and 3 with CH₂Cl and CH₂OH had the lower ESIPT barrier.

Photocatalysis as the 'master switch' of photomorphogenesis in early plant development

Enzymatic photocatalysis is seldom used in biology. Photocatalysis by light-dependent protochlorophyllide oxidoreductase (LPOR)-one of only a few natural light-dependent enzymes-is an exception, and is responsible for the conversion of protochlorophyllide to chlorophyllide in chlorophyll biosynthesis. Photocatalysis by LPOR not only regulates the biosynthesis of the most abundant pigment on Earth but it is also a 'master switch' in photomorphogenesis in early plant development. Following illumination, LPOR promotes chlorophyll production, plastid membranes are transformed and the photosynthetic apparatus is established. Given these remarkable, light-induced pigment and morphological changes, the LPOR-catalysed reaction has been extensively studied from catalytic, physiological and plant development perspectives, highlighting vital, and multiple, cellular roles of this intriguing enzyme. Here, we offer a perspective in which the link between LPOR photocatalysis and plant photomorphogenesis is explored. Notable breakthroughs in LPOR structural biology have uncovered the structural-mechanistic basis of photocatalysis. These studies have clarified how photon absorption by the pigment protochlorophyllide-bound in a ternary LPOR-protochlorophyllide-NADPH complex-triggers photocatalysis and a cascade of complex molecular and cellular events that lead to plant morphological changes. Photocatalysis is therefore the master switch responsible for early-stage plant development and ultimately life on Earth.

Revised excited-state intramolecular proton transfer of the 3-Aminophthalimide molecule: A TDDFT study

The spectroscopic properties of 3-Aminophthalimide (3AP) molecule were investigated [Chem. Phys. 2002, 283, 249, New J. Chem. 2018, 42, 1181]. The result was that the 3AP molecule was exhibiting excited-state intramolecular proton transfer (ESIPT). In the research, we revised previous result using time-dependent density functional theory (TDDFT) method. The fluorescence spectrum shows that the only fluorescence peak is from initial enol form, which is different from the traditional case of ESIPT. The red shift of characteristic peaks in infrared vibration spectra is not induced by ESIPT process. The change in the vibration mode of the amino group causes the red shift of characteristic peak in the infrared spectrum. Energy curves indicate that the barrier (19.71 kcal/mol) is anomalously high in the first excited state. In addition, there are not stable points to lead the ESIPT to form a keto isomer. Together, these results demonstrate that there is not an ESIPT process happening of 3AP molecule.

Substituent control of photophysical properties for excited-state intramolecular proton transfer (ESIPT) of o-LHBDI derivatives: a TD-DFT investigation

The substituted effect on excited-state intramolecular proton transfer (ESIPT) of o-LHBDI derivatives (4R-o-LHBDI) was investigated by DFT and TD-DFT methods. The structures of 4R-o-LHBDI (R: OH, NH₂, CN, NO₂, CF₃) were fully optimized, and the H-bond distances, bond angles, and infrared spectra of the atoms involved in PT process in the S₀ and S₁ states were analyzed. The absorption and fluorescence spectra were calculated, and the potential energy curves in both S₀ and S₁ states were constructed. Moreover, the effects of different substituents on the ESIPT mechanism of 4R-o-LHBDI (R: OH, NH₂, CN, NO₂, CF₃) were studied. The results indicate that ESIPT in the 4R-o-LHBDI is a little harder to proceed than that in o-LHBDI since the ESIPT barrier of 4R-o-LHBDI is slightly bigger than that value of o-LHBDI. When the substituent has stronger electron-withdrawing ability or weaker electron-donating ability, the ESIPT process has the smaller potential barrier. Graphical abstract.

Unraveling the effect of two different polar solvents on the excited-state intramolecular proton transfer of 4'-methoxy-3-hydroxyflavone fluorescent dye

The fluorescence properties of 4'-methoxy-3-hydroxyflavone (M3HF) dye in different solvents were investigated through experimental (Phys. Chem. Chem. Phys., 2018, 20, 7885) and theoretical (Org. Chem. Front., 2019, 6, 218) methods. However, the intermolecular hydrogen bonds between M3HF and solvents were ignored. In this work, we investigated the effect of methanol (MeOH) and N,N-dimethylformamide (DMF) solvents on the excited-state intramolecular proton transfer (ESIPT) of M3HF fluorescent dye. In excited state (S₁), the intramolecular hydrogen bonds are significantly strengthened, which can facilitate the ESIPT processes. The calculated absorption and fluorescence spectra agree well with the experimental date. The fluorescence spectra of M3HF and ESIPT tautomers (T^⁎) were found to be sensitive to the solvent polarity. Upon photo-excitation, the electron density of the M3HF molecular is redistributed, which can provide driving force for the ESIPT. The polar solvents MeOH (hydrogen bond donor) and DMF (hydrogen bond acceptor) can form different types of intermolecular hydrogen bonds with M3HF. The two different bonding modes of intermolecular hydrogen bonds are expected to weaken the intramolecular hydrogen bond of M3HF to varying degrees. The analysis of the potential energy curves indicate that the ESIPT processes of M3HF can be hindered by the intermolecular hydrogen bonds. The intermolecular hydrogen bond of M3HF-DMF complex is weaker than that of M3HF-MeOH complex, while the potential barrier of the ESIPT process in DMF solvent is higher than that of in the MeOH solvent. This is principally because, in DMF solvent, the hydroxyl group H₁ atom of M3HF can be captured by the O₃ atom of DMF and form O₃H₁ bond with O₃ atom in the intermediate process of ESIPT. There appears an energy barrier hopping point on the potential energy curve of M3HF in DMF solvent but does not appear in MeOH solvent.

On the excitation dependence of fluorescence spectra of meso-tetrapyridyl zinc (II) porphyrin and its relation with hydrogen bonding and outlying decoration

Zinc porphyrins are potential candidates for boosting the advancement of various technological applications, including those exploring the molecule's radiative emissions. In this work, the excitation dependence of fluorescence spectra from 5,10,15,20-meso-tetrapyridyl zinc(II) porphyrin dissolved in a binary solvent mixture of CHCl₃: MeOH, is reported. Important modifications in the profiles of the fluorescence bands are observed after exciting the molecules in a broad wavelength range from 350 to 565 nm. We attribute such modifications to the existence of two distinct relaxation pathways, related to two quasi-degenerated potential energy surfaces (PES) in the ZnTPyP's first excited state whose population rates changes for different excitation wavelengths. We also observed that by changing the CHCl₃:MeOH proportion in the binary mixture, a quenching mechanism mediated by the MeOH hydrogen bondings and ZnTPyP takes place, which allows for tuning the excitation dependence of the aforementioned relaxations pathways. Moreover, our data confirm that the addition of outlying RuCl(dppb)(bipy) ruthenium complex linked to the pyridyl moieties of the ZnTPyP ring is also an excellent strategy to modify the excitation dependence of the fluorescence relaxation pathways.

Structural basis for enzymatic photocatalysis in chlorophyll biosynthesis

The enzyme protochlorophyllide oxidoreductase (POR) catalyses a light-dependent step in chlorophyll biosynthesis that is essential to photosynthesis and, ultimately, all life on Earth^1-3. POR, which is one of three known light-dependent enzymes^4,5, catalyses reduction of the photosensitizer and substrate protochlorophyllide to form the pigment chlorophyllide. Despite its biological importance, the structural basis for POR photocatalysis has remained unknown. Here we report crystal structures of cyanobacterial PORs from Thermosynechococcus elongatus and Synechocystis sp. in their free forms, and in complex with the nicotinamide coenzyme. Our structural models and simulations of the ternary protochlorophyllide-NADPH-POR complex identify multiple interactions in the POR active site that are important for protochlorophyllide binding, photosensitization and photochemical conversion to chlorophyllide. We demonstrate the importance of active-site architecture and protochlorophyllide structure in driving POR photochemistry in experiments using POR variants and protochlorophyllide analogues. These studies reveal how the POR active site facilitates light-driven reduction of protochlorophyllide by localized hydride transfer from NADPH and long-range proton transfer along structurally defined proton-transfer pathways.

Effect of hydrogen bond on solvation dynamics of coumarin153 in cyclohexane-phenol solvent mixtures by time-resolved optical Kerr fluorescence

Time-resolved optical Kerr fluorescence system was used to investigate time-resolved red-shift of coumarin 153 in different solvent mixtures. The mixtures included four mole fractions of phenol-cyclohexane solvents (0, 0.013, 0.08, and 0.3), and anisole-cyclohexane solvents with the mole fraction of 0.3. The measured time-resolved fluorescence showed that, in the solvent mixtures containing phenol, the time-dependent frequency shift accelerated with the increase in the mole fraction of phenol-cyclohexane mixtures. However, the time-dependent red-shift in the fluorescence was not observed in the anisole-cyclohexane mixture, the solvent polarity could not influence the spectral Stokes shift compared with phenol. The results indicated that coumarin 153 formed an excited hydrogen bond with phenol, and the excited hydrogen bond was strengthened with an increase in the mole fraction of phenol. And, these processes also suggested that the increase in the phenol ratio improves a large number of hydrogen bond formed between phenol and carbonyl group of coumarin 153, the charge distribution will be faster towards lower the free energy of the system due to the stronger dipole moment. Therefore, the corresponding solvation response in phenol-cyclohexane mixtures with higher mole fractions decays very rapidly.

Computational insights into the mechanism of formaldehyde detection by luminescent covalent organic framework

Luminescent covalent organic frameworks (COFs) as fluorescent sensor materials provide a distinct advantage over other materials. In this work, we investigated the hydrogen bonding between the luminescent COF Ph-An-COF and formaldehyde in its excited electronic state by using density functional theory and time-dependent density functional theory to determine whether this type of COF can be used for formaldehyde detection. Hydrogen bonding significantly changed the nature of the frontier orbital and the luminescent properties. Our study reveals that the hydrogen bonding was strengthened in the excited state and the fluorescence rate coefficient was significantly reduced, which is not favorable for the luminescence of this type of COF and would lead to a luminescence decrease or quenching phenomenon. Therefore, this type of luminescent COF can be used as a potential chemical sensor to detect formaldehyde. This work provides an insight into the design of luminescence covalent organic frameworks.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

Theoretical study on excited-state intramolecular proton transfer process of cyanide group substituted 2-(2-hydroxyphenyl)benzothiazole

In this work, the influence of cyanide group substituted on the excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole (HBT) has been theoretical studied. It is found that the intramolecular hydrogen bonds of HBT and its cyanide substituted derivatives are significantly strengthened in the first excited (S₁) state, while this strengthening tends to decrease with the substitution of cyano group. The natural population analysis (NPA) is performed to interpret this phenomenon and the calculated results show that the introduction of cyano groups induces intramolecular charge transfer, leading to the decrease of electrostatic interaction of intramolecular hydrogen bond in the S₁ state compared to isolated HBT. Therefore, proton transfer of HBT and its derivatives become difficult with the introduction of cyano groups, which is consistent with the calculated potential barriers (1.95, 2.71 and 2.81 kcal/mol respectively) of the three molecules in the S₁ state. This work interprets the hindrance of cyanide group on the proton transfer of HBT and its derivatives in the S₁ state, which will bring new insight into the study of analogous molecular system.