Excited-state Hydrogen Bond Research Articles

Enhancing the antioxidant potential of ESIPT-based naringenin flavonoids based on excited state hydrogen bond dynamics: A theoretical study

Exploring antioxidant potential of flavonoid derivatives after ESIPT process provides a theoretical basis for discovering compounds with higher antioxidant capacity. In this work, employing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods, the antioxidant potential of two citrus-derived naringenin flavonoids after ESIPT process is explored. Based on studies of ESIPT process including IMHB intensity variations, potential energy curves, and transition state, these molecules exist only in enol and keto⁎ forms due to ultra-fast ESIPT. The HOMOs are utilized to explore electron-donating capacity, demonstrating that the molecules in keto⁎ form is stronger than that in enol form. Furthermore, the atomic dipole moment corrected Hirshfeld population (ADCH) and Fukui functions indicate that the sites attacked by the electrophilic free radical of the two molecules in the keto⁎ form are O3 and O5’ respectively, and both are more active than in the enol form. Overall, a comprehensive consideration of the ESIPT process and antioxidant potential of flavonoid derivatives will facilitate the exploration and design of substances with higher antioxidant capacity.

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.

Installing hydrogen bonds as a general strategy to control viscosity sensitivity of molecular rotor fluorophores

AbstractMolecular rotor‐based fluorophores (RBFs) activate fluorescence upon increase of micro‐viscosity, thus bearing a broad application promise in many fields. However, it remains a challenge to control how fluorescence of RBFs responds to viscosity changes. Herein, we demonstrate that the formation and regulation of intramolecular hydrogen bonds in the excited state of RBFs could modulate their rotational barrier, leading to a rational control of how their fluorescence can be activated by micro‐viscosity. Based on this strategy, a series of RBFs were developed based on 4‐hydroxybenzylidene‐imidazolinone (HBI) that span a wide range of viscosity sensitivity. Combined with the AggTag method that we previously reported, the varying viscosity sensitivity and emission spectra of these probes enabled a dual‐color imaging strategy that detects both protein oligomers and aggregates during the multistep aggregation process of proteins in live cells. In summary, our work indicates that installing intracellular excited state hydrogen bonds to RBFs allows for a rational control of rotational barrier, thus allow for a fine tune of their viscosity sensitivity. Beyond RBFs, we envision similar strategies can be applied to control the fluorogenic behavior of a large group of fluorophores whose emission is dependent on excited state rotational motion, including aggregation‐induced emission fluorophores.

Insights into Hydrogen Bonding Effect as Well as Excited State Intramolecular Proton Transfer Associated with Solvent Polarity and Atomic Electronegativity for 2‐Phenyl‐3‐Hydroxybenzo[g]quinolone

AbstractInspired by the excellent photochemical properties of 3‐hydroxyflavone and its derivatives, in this work, the novel 2‐phenyl‐3‐hydroxybenzo[g]quinolone (2P3HBQ) fluorophore is explored about its photo‐induced behaviors. By investigating the photoexcitation characteristics in different solvents as well as the substituted atomic electronegativity (O→S→Se), the solvent‐polarity‐related and atomic‐electronegativity‐dependent photo‐induced hydrogen bond of 2P3HBQ and its derivatives (2P3HBQ‐O, 2P3HBQ‐S and 2P3HBQ‐Se) indicates the S1‐state hydrogen bonding interactions could be enhanced. Particularly, the nonpolar solvent environment and low atomic electronegativity substitution play large roles in strengthening hydrogen bonding effects. Charge reorganization stemming from photoexcitation, strengthening excited state hydrogen bond effects, and the intramolecular charge transfer (ICT) via frontier molecular orbitals (MOs) further reveals the excited state intramolecular proton transfer (ESIPT) tendency. Via the manner of constructing potential energy curves (PECs) with restricted optimization, we elaborate and reveal the novel solvent‐polarity‐regulated and the atomic‐electronegativity‐controlled ESIPT dynamical behaviors.

Insights into the excited state hydrogen bond and proton transfer behaviors associated with solvent polarity for NHBQ fluorophore: a theoretical study

Insights into the excited state hydrogen bond and proton transfer behaviors associated with solvent polarity for NHBQ fluorophore: a theoretical study

Solvent effect on ESIPT process of N-(8-Quinolyl) salicylaldimine: A DFT/TD-DFT calculation

• The effects of solvent on ESIPT process of QS are studied by DFT/TD-DFT methods. • The O1–H2⋯N3 Hydrogen bond is strengthened upon photoexcitation. • The ESIPT process becomes favorable when the dielectric constant of the solvent decreases. The investigations of the solvent effect on ESIPT process of N-(8-Quinolyl) salicylaldimine (QS) are carried out using DFT and TD-DFT methods. We choice TOL, THF, ACE and DMSO solvents to simulate the experimental solvent conditions. The normal and tautomeric structures of the QS in the S 0 and S 1 states are optimized by means of DFT and TD-DFT method in four different kinds of solvents. Through the analysis of the critical geometrical parameters related to hydrogen bond interactions, it could be found that QS molecule preform to be planar or nearly planar in the S1 state, and more importantly, the excited-state hydrogen bond is strengthened. By comparing IR vibrational spectra, RDG isosurfaces and scatter graphs in the four solvent conditions, we found that the hydrogen bond is strengthen after photoexcitation, and weakened with the increase of the dielectric constant of the solvents. The calculated absorption and fluorescence spectra are basically agreement with existing evidence from previous experiment. The energy barriers of ground and excited states of QS molecule under the different solvent conditions have been analyzed by established potential energy curves (PECs) in the S 0 and S 1 states, and the corresponding energy barriers are increased with the effect of change from TOL to DMSO solvents. Therefore, with the increasing dielectric constant of solvents, the ESIPT become favored.

Interplay of conformational relaxation and hydrogen bond dynamics in the excited states of fluorescent Schiff base anions.

Time resolved fluorescence spectroscopic investigation of four Schiff base anions has established that their excited state dynamics is governed by several solvent properties: polarity, viscosity and hydrogen bond donating ability. With viscous protic solvents like glycerol, fluorescence lifetimes of anions have been found to be markedly longer than those in ethanol, implying that conformational relaxation of molecules plays a key role in their nonradiative relaxation. Surprisingly, the lifetimes in less viscous aprotic solvents, like acetonitrile, are found to be even longer. The only plausible rationalization of this observation is in the light of hydrogen bond-assisted nonradiative phenomena that are operative in protic solvents. This contention draws support from a time evolution of the emission in the red end of the spectrum in low to moderately hydrogen bond donating protic solvents, with regard to an absence of such a rise time in aprotic solvents and strongly hydrogen bond donating solvents, viz., 2,2,2-trifluoroethanol. Rudimentary quantum chemical calculations provide a preliminary idea about the nature of excited state hydrogen bond redistribution involved in the process.

Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer and hydrogen bond mechanisms of DHP

AbstractIn this present work, using density functional theory and time‐dependent density functional theory, the mechanism of excited state intramolecular proton transfer (ESIPT) of 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1Himidazo‐[4,5‐b] pyridine (DHP) in ether, 1‐butanol, 1‐propanol, and acetonitrile phases was systematically studied. The optimized geometry includes both normal and tautomeric structure. The fluorescence spectrum and absorption spectrum calculated based on geometric structure agree with the experimental data. The data of bond lengths and bond angles related to hydrogen bonds indicate that hydrogen bonds are strengthened in the S1 state. Frontier molecular orbital analysis combined with Hirshfeld's method shows a redistribution of electron density in different solvents. The infrared vibrational spectra show that the hydrogen bond strength decreases with the increase of solvent polarity. Finally, through the analysis of the potential energy surface, it is found that the excited state intramolecular proton transfer becomes more and more difficult as the polarity of the solvent increases.

Effects of push-pull electronic substitutions on ESIPT reaction for BH-BA compound

This work mainly focuses on exploring excited state hydrogen bond interactions and excited state intramolecular proton transfer (ESIPT) behaviors for the novel excitation wavelength dependent emission BH-BA fluorophore. Given substitutions of different push and pull electron groups could largely affect molecular properties, we also investigate BH-NH2 and BH-CF3 derivatives incidentally. Via probing into photo-induced hydrogen bonding effects, we infer O4-H5···N6 plays a more dominant role. Photo-related charge redistribution further reveal the ESIPT trend. Constructing potential energy surfaces (PESs), we elucidate the substituent-related ESIPT behavior only proceeds along with O4-H5···N6 hydrogen bonding wire rather than the O1-H2···N3 one.

Theoretically unveiling the effect of solvent polarities on ESDPT mechanisms and photophysical properties of hydroxyanthraquinones.

In this work, we were devoted to explore the effect of solvent polarities on the excited-state intramolecular proton transfer (ESIPT) process of 1-acetamido-4-hydroxyanthraquinone (AcHAQ) in three different polarity solvents (acetonitrile, chloroform, and cyclohexane) based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, and thereby regulating the distribution ratio between the dual excited-state isomers (enol and keto). The calculated geometrical parameters and infrared (IR) vibrational spectra have confirmed the excited-state intramolecular hydrogen bond (IHB) strengthening mechanism. Natural bond orbital (NBO) population analysis indicates that the intramolecular charge transfer (ICT) around IHBs has enhanced IHB, thereby triggering the ESIPT reaction. In addition, results obtained from the scanned potential energy curve (PEC) manifest that ESIPT process prefers to occur along the O3-H2…O1 IHB and energy barriers corresponding to the proton transfer in ACN are the lowest among all the studied solvents.

Modulating the ESIPT dynamics of 3HF derivatives via substitution and solvent effect: A theoretical study

The ground-state and excited-state electronic structure, electronic properties, vertical excitation and emission energies, as well as the potential energy curve of the excite-state intramolecular proton transfer (ESIPT) process for 3-hydroxyflavone (3HF) derivatives 2-(4-(diphenylamine)phenyl)-3-hydroxy-4H-chromen-4-one (PPC), 2-(4-(diethylamino)phenyl)-3-hydroxy-4H-chromen-4-one (EPC) and 2-(4-methoxyl)-3-hydroxy-4H-chromen-4-one (MNC) have been examined with DFT and TDDFT methods. We found that the ESIPT dynamics could be controlled via both the solvent effect and the substituents. Structurally and electronically, the strength of excited-state intramolecular hydrogen bond and the twisted intramolecular charge transfer degree could be tuned by changing the electron donating ability of the substituent, which in turn influences the ESIPT energy barrier. In particularly, the stronger the electron donating ability, the longer the excited-state hydrogen bond distance, accompanied by more twisted intramolecular charge transfer, and the higher energy barrier for ESIPT. Besides, we found that the emission energies for the derivatives are very sensitive to the excited-state intramolecular charge transfer degree, with PPC exhibiting the smallest emission energies, followed by EPC and MNC, which is consistent with experimental observations. Furthermore, the ESIPT energy barrier of 3HF derivatives in solvent increases when the solvent becomes more polar. This work provides significant insight into tuning the ESIPT dynamics in 3HF derivatives which is useful for the design of new organic fluorescent probes with excellent performance.

Effects of Twisted Intramolecular Charge Transfer Behavior on Excited-State Intramolecular Proton Transfer Reactions of Methyl Benzoate Derivatives in Water Solution.

Many methyl benzoate derivatives were found to show intramolecular charge transfer (ICT), intramolecular proton transfer, and other properties, which have extensive applications in lasing media, metal ion sensors, active materials, and fluorescence probe fields. However, the intrinsic relationship and reaction mechanism between the excited-state intramolecular proton transfer (ESIPT) and ICT between methyl benzoate derivatives with different substituents have not been explained. In this paper, the density functional theory and time-dependent density functional theory methods were used to study the ESIPT and ICT behaviors of p-aminosalicylic acid methyl ester and p-dimethylaminosalicylic acid methyl ester in water and obtain the intrinsic interaction between the two behaviors. The bond parameters, infrared spectra, reduced density gradient scatter plots, and topological analyses of these two molecules in the ground state and excited state were analyzed to confirm the enhancement of the excited-state intramolecular hydrogen bonds (IHBs). The simulated absorption and fluorescence spectra of these molecules agreed well with the experimental values. Based on the optimized structure, we also plotted the natural transition orbitals, electron density difference maps, and frontier molecular orbitals (FMOs), which showed the changes of the charge distribution of these molecules intuitively upon photoexcitation. In addition, we also found that the degree of IHB enhancement with -N(CH3)2 substituents was less than that with -NH2, reflecting an inhibition effect of twisted intramolecular charge transfer (TICT) on ESIPT reaction. This conclusion was confirmed by our calculated potential energy curves. This work may better deepen the comprehension of the intrinsic relationship between ESIPT and TICT behavior and sequentially provide better theoretical guidance for the synthesis of fluorescent molecules related to these two behaviors.

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.

Solvent dependence of ESIPT process in 2-(2-carbonmethoxy-3,4-dichloro-6-hydroxyphenyl) compounds

After the synthesis of 2-(2-carbonmethoxy-3, 4-dichloro-6-hydroxyphenyl) (Compound 1), Elena V. Vetrova et al. found that it has different photophysical phenomena in the different solvent environment [Dyes and Pigments 180 (2020) 108417]. However, there is still no detailed theoretical research for this phenomenon. Therefore, based on the density functional (DFT) and time-dependent density functional (TD-DFT) methods, under different solvent conditions, the excited state intramolecular proton transfer process (ESIPT) of Compound 1 is investigated in this work. For Compound 1, the bond parameter analysis of the optimized structure confirms that the excited state hydrogen bond interaction is enhanced. The photophysical properties show that the theoretically simulated vertical transition energy is in good agreement with the experimental spectral value. Based on the optimized geometric structure, the frontier molecular orbital isosurface is drawn, and the redistribution of electron density in different solvents is depicted, which intuitively explain the changes in the photophysical properties of Compound 1. Though the calculated potential energy curves, we further explain that ESIPT process are easier to occur from Heptane, Dioxane, Toluene to THF.

Unveiling the Effect of Solvent Polarity on the Excited State Intramolecular Proton Transfer and Hydrogen Bond Mechanisms of Dhp

Unveiling the Effect of Solvent Polarity on the Excited State Intramolecular Proton Transfer and Hydrogen Bond Mechanisms of Dhp

Unveiling the Effect of Solvent Polarity on the Excited State Intramolecular Proton Transfer and Hydrogen Bond Mechanisms of Dhp

Unveiling the Effect of Solvent Polarity on the Excited State Intramolecular Proton Transfer and Hydrogen Bond Mechanisms of Dhp

Is it possible to switch ESIPT-channel of hydroxyanthraquinones with the strategy of modifying electronic groups?

In this work, to unveil the substituent effect on the excited-state intramolecular proton transfer (ESIPT) reaction of 1,8-Dihydroxyanthraquinone (DHAQ), four derivatives DHAQ-Me, DHAQ-NH2, DHAQ-COOH, and DHAQ-CN were considered using quantum chemistry methods in the acetonitrile (ACN) phase. The excited-state intramolecular hydrogen bonds (IHBs) (O1-H2⋯O3 and O5-H4⋯O3) strengthening mechanism was confirmed by computing primary geometric parameters, infrared (IR) spectra and reduced density gradient (RDG) scatter plots. The strength relationship of the dual IHBs within all the investigated compounds was established based on the calculated binding energies. Charge-recombination triggered ESIPT mechanism has been demonstrated by the analysis of frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population. Besides, the hole-electron analysis indicates that a remarkable intramolecular charge transfer (ICT) behavior has induced the fluorescence intensity to decrease significantly for DHAQ-NH2 in K1 form. DHAQ derivatives with electron-withdrawing groups substituted (DHAQ-COOH and DHAQ-CN) have exhibited a larger Stokes shift, which is a desirable property for manufacturing luminescent materials. From the located transition state (TS) structures and the calculated potential energy curves (PECs), it is noticeable that the dominant channel for ESIPT reactions is always the O1-H2⋯O3 hydrogen bonding wire, which is irrelevant to the types of substituents introduced, although O5-H4⋯O3 is the stronger one for DHAQ-Me and DHAQ-NH2. This theoretical research may well deepen the comprehension of the relationship between IHB and ESIPT behavior and thereby provide valuable guidance to synthesize efficacious ESIPT-based organic chromophores experimentally.

Theoretical calculation on excited state hydrogen bond kinetics of PRODAN derivative 2b molecule in methanol solution

To explore the mechanism of intermolecular hydrogen bond relating to PRODAN derivative 2b in methanol solvent on photoexcitation process, spectral behavior calculations of PRODAN-2b and its hydrogen bonding complex were studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). After photo excitation, the hydrogen bond between PRODAN-2b and methanol become stronger. It was found that with the increase in the number of methanol molecules, the more hydrogen bonds between PRODAN-2b and methanol were formed and the hydrogen bondings were also enhanced. Further evidence for enhanced hydrogen bond due to intramolecular charge transfer excitation character of PRODAN-2b is presented. This effect leads to the red shift of absorption and fluorescence spectra of PRODAN-2b molecules in methanol solvent with increasing methanol concentration.

Theoretical Investigation of Excited-State Intramolecular Double-Proton Transfer Mechanism of Substituent Modified 1, 3-Bis (2-Pyridylimino)-4,7-Dihydroxyisoindole in Dichloromethane Solution

In this paper, density functional theory (DFT) and time-dependent DFT (TDDFT) methods were used to investigate substituent effects and excited-state intramolecular double-proton transfer (ESIDPT) in 1, 3-bis (2-pyridylimino)-4, 7-dihydroxyisoindole (BPI–OH) and its derivatives. The results of a systematic study of the substituent effects of electron-withdrawing groups (F, Cl and Br) on the adjacent sites of the benzene ring were used to regulate the photophysical properties of the molecules and the dynamics of the proton-transfer process. Geometric structure comparisons and infrared (IR) spectroscopic analysis confirmed that strengthening of the intramolecular hydrogen bond in the first excited state (S1) facilitated proton transfer. Functional analysis of the reduced density gradient confirmed these conclusions. Double-proton transfer in BPI–OH is considered to occur in two steps, i.e., BPI–OH (N) [Formula: see text] BPI–OH (T1) [Formula: see text] BPI–OH (T2), in the ground state (S0) and the S1 state. The potential-energy curves (PECs) for two-step proton transfer were scanned for both the S0 and S1 states to clarify the mechanisms and pathways of proton transfer. The stepwise path in which two protons are consecutively transferred has a low energy barrier and is more rational and favorable. This study shows that the presence or absence of coordinating groups, and the type of coordinating group, affect the hydrogen-bond strength. A coordinating group enhances hydrogen-bond formation, i.e., it promotes excited-state intramolecular proton transfer (ESIPT).

Fine-tuning directionality of ESIPT behavior of the asymmetric two proton acceptor system via atomic electronegativity

The excited state intramolecular proton transfer (ESIPT) processes and photophysical features of 3-(benzo[d]oxazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BOHMB) and 3-(benzo[d]selenazole-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BSeHMB) molecules were investigated in detail by using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The strengthened excited state hydrogen bonds (H-bond) of the title compounds are favorable to ESIPT process according to the analyses of structural parameter, infrared vibration frequency, electron density and reduced density gradient. The atomic substitution changes the intramolecular H-bond O1-H2…O3 and O1-H2…N4 and the fluorescence emission peaks of BOHMB-N and BSeHMB-N in normal and tautomer forms. The potential energy curves indicate that the ESIPT energy barriers of BOHMB-O, BTHMB-O and BSeHMB-O increase as the electron-withdrawing abilities of atoms (from O to S and Se) are gradually weakened. However, the ESIPT energy barriers of BOHMB-N and BTHMB-N follow the totally opposite order. For BOHMB and BSeHMB, ESIPT process prefers to occur in the direction from O-H group to the O atom.