Strength Of Intramolecular Hydrogen Bond Research Articles

Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective.

As reversible fluorescent probes, HTP-1 and HTP-2 have favourable applications for the detection of Zn2+ and H2S. Herein, the impact of solvent on the excited-state intramolecular proton transfer (ESIPT) of HTP-1 and HTP-2 was comprehensively investigated. The obtained geometric parameters and infrared (IR) vibrational analysis associated with the intramolecular hydrogen bond (IHB) indicated that the strength of IHB for HTP-1 was weakened in the excited state. Moreover, structural torsion and almost no ICT behaviour indicated that the ESIPT process did not occur in HTP-1. Nevertheless, when the 7-nitro-1,2,3-benzoxadiazole (NBD) group replaced the H atom, the IHB strength of HTP-2 was enhanced after photoexcitation, which inhibited the twisting of tetraphenylethylene, thereby opening the ESIPT channel. Notably, hole-electron analysis and frontier molecular orbitals revealed that the charge decoupling effect was the reason for the fluorescence quenching of HTP-2. Furthermore, the potential energy curves (PECs) revealed that HTP-2 was more inclined to the ESIPT process in polar solvents than in nonpolar solvents. With a decrease in solvent polarity, it was more conducive to the ESIPT process. Our study systematically presents the ESIPT process and different detection mechanisms of the two reversible probe molecules regulated by solvent polarity, providing new insights into the design and development of novel fluorescent probes.

Effect of external electric fields on the ESDPT process and photophysical properties of 1,8-dihydroxy-2-naphthaldehyde.

In this work, by capitalizing on the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, it has been systematically studied that the excited state double intramolecular proton transfer (ESDPT) process and the photophysical properties of 1,8-dihydroxy-2-naphthaldehyde (DHNA) are affected by the distinct external electric fields (EEFs). The obtained intramolecular hydrogen bond (IHB) parameters containing bond lengths and angles, as well as infrared (IR) vibrational spectra demonstrate that IHB strength changes in the distinct EEFs. Moreover, not only do the potential energy surfaces (PESs) indicate that the ESDPT process of DHNA is stepwise, but also increasing the positive EEF results in a decrease in the energy barrier accordingly, while vice versa. The absorption and fluorescence spectra also undergo a corresponding red or blue shift in the EEF; for instance, when the EEF changes from +10 × 10-4 a.u. to +20 × 10-4 a.u., the fluorescence peak undergoes a blue shift from 602 nm to 513 nm in the keto2 form. In a nutshell, the ESDPT process of DHNA can be influenced by the EEF, which will serve as a reference in regulating and controlling proton transfer that causes luminescence.

Structure-property relationships in alkoxy substituted benzoates from experimental and computational thermochemistry

The energetics of formation and the phase transitions for isomeric methyl and ethyl methoxy‑benzoates were studied. The vapour pressures of methyl 2‑methoxy-benzoate and methyl 4‑methoxy-benzoate were measured by transpiration method. The high-precision combustion calorimetry was used to derive the liquid-phase enthalpy of formation of methyl 2‑methoxy-benzoate. The new results and the data available in the literature were critically evaluated using the structure-property correlations. Reliable correlations were established between the vaporisation enthalpies and the normal boiling temperatures as well as with the Kovats indices. Moreover, the validity of vaporisation enthalpies of alkyl methoxy‑benzoates was proved using structure-property relationships within families of structurally similar molecules. Mutual validation of the experimental and theoretical gas phase enthalpies of formation was performed using high-level quantum-chemical methods. The enthalpic contributions to the enthalpy of formation in the gas phase, resulting from the nearest and non-nearest neighbour interactions of the substituents in the benzene ring, were developed and used to quantify the intra-molecular hydrogen bond strength in the ortho-substituted species.

Experimental and theoretical comparison of the vibrational and NMR spectra of novel 6-6’-(1E-1’E)-(Propane-1,3Diylbis (Azanylyidene)) Bis (Phenylmethylylidene)) Bis (3-Octyloxy) Phenol), NBO and molecular docking studies

The characterization study of the synthesized novel compound 6-6’-(1E-1’E)-(Propane-1,3Diylbis(Azanylyidene))Bis(Phenylmethylylidene))Bis(3-Octyloxy)Phenol) (C45H58N2O4) was done by FT-IR, Raman and NMR spectra. Vibrational analysis was realized with normal coordinate treatment, and vibrational modes were assigned in the mid-IR region. The 13C and 1H -NMR spectra of the title compound were measured in DMSO and chloroform, and the corresponding chemical shifts were calculated in the different basis sets (6-31++G(d,p), 6-311G(2d,p), 6-311++G(2d,p), TZVPP) of DFT-B3LYP with continuum models. The vibrational and NMR spectroscopies show that the existence of the intramolecular resonance-assisted hydrogen bonds(O-H...N) results in the redshift of OH vibrational modes and its high proton chemical shifts (ca. 16 ppm), as confirmed by X-ray crystal structure and NRT studies.13C NMR chemical shifts of the Schiff base bond are determined as ca. 174 ppm. The calculations indicate that both the intramolecular hydrogen bond strength and the OH-proton chemical shift increase with increasing solvent polarity. The molecular docking studies reveal that the title ligand has the potency of inhibition against the VIM-2 and NDM-1 active sites of metallo-β-lactamase receptor.

Atomic electronegativity-dependent intramolecular hydrogen bond and fluorescence characteristics of novel scaffold-based fluorophore: a TD-DFT study.

In this work, fluorescent properties and excited-state intramolecular proton transfer (ESIPT) processes of 2,5-bis(benzo[d]thiazol-2-yl)phenol (BTP) and its derivatives (BOP and BSeP) with different heteroatom atoms (O and Se) have been systematically explored by the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated absorption and fluorescence emission peaks agree well with the experimental values in acetonitrile. From the data of structures, topological parameters, reduced density gradient analyses, and infrared (IR) vibrational frequencies, the intramolecular hydrogen bonds (IHBs) of BTP and its derivatives are enhanced upon light-excitation. The potential energy curves show that the ESIPT process occurs in BTP and its derivatives after surmounting 0.167-0.306eV energy barrier. The strength of intramolecular hydrogen bond, HOMO-LUMO energy gap, and red-shifted value of absorption and fluorescence emission wavelengths are dependent on the electron-withdrawing ability of heteroatom from O to S and Se. We believe that this work can pave the way for developing a new ESIPT-based fluorophore with better luminescent properties.

Inductive Effects on Intramolecular Hydrogen Bond Strength: An Investigation of the Effect of an Electron-Withdrawing CF3 Group Adjacent to an Alcohol Hydrogen Bond Donor.

This combined experimental and theoretical study seeks to determine the role that inductive effects have on hydrogen bonds by an investigation into the change in intramolecular hydrogen bond strength in 2-amino-1-trifluoromethylethanol (2ATFME) relative to that in 2-aminoethanol (2AE). Toward this end, the rotational spectra of the normal, 13C, and 15N isotopologues have been measured using Fourier transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson A-reduction Hamiltonian. Structural parameters used to characterize the strength of the intramolecular hydrogen bond have been determined from the experimental structures of both 2ATFME and 2AE as well as from MP2/6-311++G(d,p) calculations. A comparison of these parameters in 2ATFME with those of 2AE indicates that the electron-withdrawing trifluoromethyl CF3 group strengthens the hydrogen bond. These include a 4% decrease in the distance between the donor and acceptor heavy atoms of the hydrogen bond, a 6% increase toward linearity of the OH···N angle, and a 23% decrease of the COH···N torsional angle toward planarity in 2ATFME relative to 2AE. This trend toward increased intramolecular hydrogen bond strength in 2ATFME is also observed within the ab initio structures.

Intramolecular proton transfer reaction dynamics using machine-learned ab initio potential energy surfaces

Hydrogen bonding interactions, which are central to various physicochemical processes, are investigated in the present study using ab initio-based machine learning potential energy surfaces. Abnormally strong intramolecular O–H⋯O hydrogen bonds, occurring in β-diketone enols of malonaldehyde and its derivatives, with substituents ranging from various electron-withdrawing to electron-donating functional groups, are studied. Machine learning force fields were constructed using a kernel-based force learning model employing ab initio molecular dynamics reference data. These models were used for molecular dynamics simulations at finite temperature, and dynamical properties were determined by computing proton transfer free-energy surfaces. The chemical systems studied here show progression toward barrier-less proton transfer events at an accuracy of correlated electronic structure methods. Markov state models of the conformational states indicate shorter intramolecular hydrogen bonds exhibiting higher proton transfer rates. We demonstrate how functional group substitution can modulate the strength of intramolecular hydrogen bonds by studying the thermodynamic and kinetic properties.

Effect of Substituent Groups on the Strength of Intramolecular Hydrogen Bonds in 2,4-Dihydroxybenzophenone UV Absorbers

2,4-Dihydroxybenzophenone is the most widely used molecule in the benzophenone group of UV absorbers. It is known that the UV absorption ability is dependent on the substituents. Numerous studies have shown that the strength of intramolecular hydrogen bonds is the main factor affecting this type of UV absorber. However, the effect of substituents on the formation and nature of the hydrogen bonds has not been well studied. In this work, the effect of the type of substituent and the substitution position on the absorption intensity of 2,4-dihydroxybenzophenone molecules is verified both experimentally and theoretically. The effect of substituents on the intramolecular hydrogen bonding of 2,4-dihydroxybenzophenone was investigated by DFT calculations. The results indicate that the addition of different substituents leads to various changes in the strength of the hydrogen bonding in 2,4-dihydroxybenzophenone. On the X-substitution site or the Y-substitution site, halogen groups and electron-absorbing groups such as -CN and -NO2 increase the strength of the hydrogen bond, while electron-giving groups such as -N(CH3)2 and -OCH3 decrease the strength of the bond. For the same substituent, the one at the Y site has a higher effect on hydrogen bonding than that at the X site. By NBO analysis, it was found that the substituents would cause charge redistribution of the individual atoms of 2,4-dihydroxybenzophenones, thus affecting the formation and strength of the hydrogen bonds. Moreover, when the substituent is at the Y substitution site, the oxygen atom of the carbonyl group is less able to absorb electrons and more charge is attracted to the oxygen atom of the hydroxyl group, resulting in a larger charge difference between the two oxygen atoms and an increase of bond energy. Finally, a multiple linear regression analysis of the NPA charge number of the atoms involved in the formation of the hydrogen-bonded chelated six-membered ring was performed with the energy of the hydrogen bond and the percentage of influencing factors estimated, which were found to jointly affect the strength of hydrogen bonding. The aim of this study is to provide theoretical guidance for the design of benzophenone-based UV absorbers that absorb UV light of specific wavelength bands.

Tautomerism in pyridinyl methyl β-diketones in the liquid and the solid state; a combined computational and experimental study

Three 1-(n-pyridinyl)butane-1,3-diones (nPM) (n = 2, 3, and 4) molecules were synthesized and their molecular structure, tautomerism, conformational stability, and intramolecular hydrogen bonding (IHB) of cis-enol forms, was investigated by a combination of computational and experimental methods. Density functional theory (DFT) calculations were used and the Atoms in Molecules (AIM) analysis indicated an intramolecular hydrogen bond strength as 17.9–20.3 kcal/mol using the B3LYP/6–311++G(d,p) level of theory. The calculated structures, NBO, and intramolecular hydrogen bond strengths for the stable cis-enol forms of nPM molecules were compared with each other and with those of acetylacetone (AA) and benzoylacetone (BA). The single crystal X-ray structure of 4PM confirmed that the tautomer with the OH group next to the pyridine ring best describes the structure in the solid-state. The vibrational bands of the title molecules were assigned to the corresponding normal modes, and the equilibrium between the two cis enol tautomers was estimated by means of IR, Raman and UV/Vis spectroscopy.

Molecular liquids versus ionic liquids: Thermodynamic insights into the interplay between inter-molecular and intra-molecular hydrogen bonding

Inter- and intra-molecular hydrogen bonding (HB) was studied in series of molecular and ionic liquids, containing a five-membered aliphatic ring (hydroxy-alkyl-pyrrolidines) and a five-membered aromatic ring (hydroxy-ethyl-imidazole), as well as in a corresponding ionic liquids, also containing a five-membered aromatic ring (hydroxy-alkyl-imidazolium) with the hydroxyl attached to the alkyl chain of the cation. To quantify these types of hydrogen bonds, we measured the enthalpies of vaporisation using a number of experimental methods, including thermogravimetric analysis, quartz crystal microbalance, and gas chromatographic method. The method of homomorphs was applied to quantify the strength of inter-molecular hydrogen bonding. The group-additivity based “centerpiece” method was used to quantify the intra-molecular HB strength in both molecular and in ionic liquids. Two new methods based on the experimental vaporisation enthalpies and a well-balanced thermodynamic equations were developed and applied to estimate the intra-molecular hydrogen bond strength, and results of the three methods agreed well. The interesting interplay between intra- and intermolecular hydrogen bonding was observed and discussed for each ionic liquid.

Coupling between tautomerism and radiationless deactivation in porphycenes

Abstract Quantum yields of fluorescence of porphycenes – porphyrin isomers – can vary by orders of magnitude, even for very similar derivatives, such as meso-dimethyl- vs. meso-tetramethylporphycene. In weakly emitting porphycenes the fluorescence intensity strongly depends on viscosity and can be recovered by placing a molecule in a rigid environment. We postulate that the efficient nonradiative deactivation is due to the quantum effect, delocalization of the inner protons. The delocalization, which increases with the strength of intramolecular hydrogen bonds may induce structural changes that lead to distortion from planarity and, as a result, efficient S0 ← S1 internal conversion. The effect seems to be general, as indicated by good correlation between the quantum yield of fluorescence and the distance between H-bonded nitrogen atoms, the latter being a reliable measure of hydrogen bonding strength. Based on the available photophysical and X-ray data, such correlation was found so far for over 20 differently substituted porphycenes.

Synthesis of Synthetic Musks: A Theoretical Study Based on the Relationships between Structure and Properties at Molecular Scale.

Synthetic musks (SMs), as an indispensable odor additive, are widely used in various personal care products. However, due to their physico-chemical properties, SMs were detected in various environmental media, even in samples from arctic regions, leading to severe threats to human health (e.g., abortion risk). Environmentally friendly and functionally improved SMs have been theoretically designed in previous studies. However, the synthesizability of these derivatives has barely been proven. Thus, this study developed a method to verify the synthesizability of previously designed SM derivatives using machine learning, 2D-QSAR, 3D-QSAR, and high-throughput density functional theory in order to screen for synthesizable, high-performance (odor sensitivity), and environmentally friendly SM derivatives. In this study, three SM derivatives (i.e., D52, D37, and D25) were screened and recommended due to their good performances (i.e., high synthesizability and odor sensitivity; low abortion risk; and bioaccumulation ability in skin keratin). In addition, the synthesizability mechanism of SM derivatives was also analyzed. Results revealed that high intramolecular hydrogen bond strength, electrostatic interaction, qH+ value, energy gap, and low EHOMO would lead to a higher synthesizability of SMs and their derivatives. This study broke the synthesizability bottleneck of theoretically designed environment-friendly SM derivatives and advanced the mechanism of screening functional derivatives.

Rationalizing hydrogen bond solvation with Kamlet-Taft LSER and molecular torsion balances.

The strength of a weak intramolecular hydrogen bond was quantified using molecular torsion balances and found to vary between -0.99 kcal mol-1 and +1.00 kcal mol-1 due to solvation. Analyzing the results using Kamlet-Taft's Linear Solvation Energy Relationship enabled partitioning of the hydrogen-bond strength into physically meaningful solvent parameters through a linear equation: ΔGH-Bond = -1.37 - 0.14α + 2.10β + 0.74(π* - 0.38δ) kcal mol-1 (R2 = 0.99, n = 14), where α and β represent the solvent hydrogen-bond acceptor and donor parameters, respectively, and π* associates with the solvent nonspecific polarity/dipolarity parameter. Based on the coefficient of each solvent parameter (derived by linear regression), the β electrostatic term emerged as the dominant contributor to solvent effects on hydrogen bonding. This finding aligns with the natural attributes of hydrogen bonds as electrostatic interactions, but the contributions from the nonspecific interactions of the solvent (e.g., dispersion) are also important. Hydrogen bond solvation affects molecular properties and functions, and this study provides a predictive tool to harness the efficiency of hydrogen bonds.

Experimental and computational thermochemistry: how strong is the intramolecular hydrogen bond in alkyl 2-hydroxybenzoates (salicylates).

Hydrogen bonding (HB) is a fascinating phenomenon that exhibits unusual properties in organic and biomolecules. The qualitative manifestation of hydrogen bonds is known in numerous chemical processes. However, quantifying HB strength is a challenging task, especially in the case of intra-molecular hydrogen bonds. It is qualitatively well established that the alkyl 2-hydroxybenzoates have strong intra-HB. The thermochemical methods suitable for the determination of intra-HB strength were the focus of this study. The experimental gas phase formation enthalpies for alkyl 2-hydroxybenzoates (including methyl, ethyl, n-propyl and n-butyl) at 298.15 K were derived from a combination of vapour pressure measurements and high-precision combustion calorimetry and validated by the quantum chemical methods G3MP2 and G4. The intra-HB strength in methyl 2-hydroxybenzoate was determined from the evaluated gas-phase enthalpies of formation by comparing the energies of cis- and trans- conformers, by well-balanced reactions, the "para-ortho" method and the "HB and Out" method. All these methods give a common level of intra-molecular hydrogen bond strength of -43 kJ mol-1. The intra-HB strength was found to be independent of the chain length of the alkyl 2-hydroxybenzoates.

A theoretical study of solvent effect on the excited state intramolecular proton transfer of 3‐hydroxyflavone

AbstractIn order to study the effect of solvents polarity on the excited state intramolecular proton transfer (ESIPT) processes of 3‐hydroxyflavone (3HF) in detail, the ESIPT processes of 3HF in toluene, dichloromethane (DCM), and acetonitrile (ACN) were investigated using theoretical methods. The stable configurations of 3HF in ground (S0) and excited (S1) states were optimized. The critical configuration parameters, infrared (IR) vibrational spectra, absorption and fluorescence spectra, frontier molecular orbitals (MOs), charge distribution, and potential energy curves were calculated. The intramolecular hydrogen bonds are obviously enhanced and the electronic density redistributes when excited to S1 state. The strengthening of intramolecular hydrogen bonds and the redistribution of electronic density are beneficial to the ESIPT processes. The analysis of potential energy curves show that the proton transfer processes are feasible in thermodynamics in S1 state, but not in S0 state. In addition, the intramolecular hydrogen bonds gradually weaken when the solvents polarity increases from toluene, DCM to ACN. As the solvents polarity increases, the ESIPT processes are more and more difficult to occur. It can be concluded that the strength of intramolecular hydrogen bonds will be affected by the solvents polarity, and the strength of intramolecular hydrogen bonds are directly related to the difficulty of the ESIPT processes.

Rational design strategies for nonconventional luminogens with efficient and tunable emission in dilute solution

The design and synthesis of organic nonconventional luminescent (NL) materials with efficient emission have attracted significant attention. However, these materials usually suffer from the fact of low photoluminescent quantum efficiency (Φ), poor tunability of emission wavelength, and non-luminescence in dilute solution, limiting their further application. Herein, rational molecular design strategies for NL compounds with efficient and tunable emission in dilute solution were presented via theoretical calculation and practical verification. Firstly, increasing the ratio of π → π* transition in the excited state to suppress the n → π* transition, inhibit intersystem crossing, and avoid the generation of triplet excitons which can be easily quenched by water or oxygen. Secondly, introducing strong intramolecular hydrogen bonds to suppress intramolecular rotation and inhibit non-radiative transitions. Besides, D-π-A structure can effectively adjust the luminescence color. Based on the proposed design strategies, two novel compounds named LJ-1 and LJ-2 were synthesized, both compounds can emit bright green fluorescence in dilute solution with Φ of up to 59.8 %, and their photophysical properties are pH-dependent. The results showed that pH had a great impact on the strength of intramolecular hydrogen bonds and the degree of conjugation. For example, the emission wavelength of LJ-1 exhibited red-shift as the increase of pH value. Furthermore, LJ-1 can be used for visual detection of Ag+ in water solution in naked eye and the precise localization of lysosomes. We believe that our work will greatly broaden the application range of nonconventional luminogenic compounds in future.

Radical scavenging capacity, UV activity, and molecular docking studies of 2ʹ, 5ʹ, 3, 4-Tetrahydroxychalcone: An insight into the photoprotection

A theoretical evaluation of radical scavenging activity and UV absorbing ability of chalcone named 2ʹ, 5ʹ, 3, 4-Tetrahydroxychalcone has been performed in the gas phase and aqueous media. The DFT-B3LYP/6–311+G (d, p) level of theory was utilized for geometry optimization and the calculation of reaction enthalpies, while the CAM-B3LYP/6–311+G (d, p) level of theory was employed for TDDFT analysis. Three potential working mechanisms of radical scavenging activity, hydrogen atom transfer mechanisms (HAT), stepwise electron transfer proton transfer (SET-PT), and sequential proton loss electron transfer mechanisms (SPLET) were investigated within the gas phase and aqueous phase. The results obtained demonstrate that the HAT will be the probable mechanism in the gas phase whereas SPLET will be in the water medium. Using the same level of theory, explicit surface analysis of the title compound was performed. The compound wave function analysis and molecular docking investigations were also performed in the optimized structure. The theoretical UV spectra were recorded in both the solvents and the compound's electronic absorption spectrum was analyzed using the TD-DFT method. The distribution of hole and electrons in the peaks of maximal oscillator strength corresponds to local excitation within the molecule. The Quantum Theory of Atoms in Molecule (QTAIM) was used to compute the strength of intramolecular hydrogen bonds and non-covalent interactions. The global reactivity parameters and electrostatic potential (ESP) were also included to define the reactivity of the whole molecule. The spin density distribution analysis was come upon to be complementary to the radical scavenging activity. Besides the radical scavenging activity of the compound, it can also be used as a potential UVA filter.

Synthesis, molecular structure, conformational, and intramolecular hydrogen bond strength of ethyl 3-amino-2-butenoate and its N-Me, N-Ph, and N-Bn analogs; An experimental and theoretical study

Amino derivatives of ethyl acetoacetate like ethyl (Z)- 3-(amino)but-2-enoate (EAB), ethyl (Z)- 3-(methylamino)but-2-enoate (Me-EAB), ethyl (Z)- 3-(phenylamino)but-2-enoate (Ph-EAB), and ethyl (Z)- 3-(benzylamino)but-2-enoate (Bn-EAB) were synthesized. Their structures, conformations, and intramolecular hydrogen bonding (IHB) is characterized using computational analysis (density functional theory, DFT, methods) at the B3LYP/6-311++G(d,p) level and spectroscopic techniques)IR, UV, and NMR). All the mentioned theoretical and experimental results were compared to those of (Z)-methyl 3-aminobut-2-enoate (MAB). The vibrational spectra of EAB are fully assigned. Vibrational spectroscopy results confirmed the existence of two conformers of title molecules. DFT calculations at the B3LYP/6-311++G(d,p) level show the calculated NH bond lengths in EAB, Me-EAB, Ph-EAB, Bn-EAB, and MAB are 1.015, 1.017, 1.022, 1.018, and 1.014 Ǻ, respectively. Also, according to the natural bond orbital (NBO) results, the σN-H10↔LP(2)O7 interactions for EAB, Me-EAB, Ph-EAB, Bn-EAB, and MAB are 5.21, 6.77, 8.65, 7.18, and 5.11 kcal/mol, respectively. So, the trend in IHB strength is: Ph-EAB > Bn-EAB ∼ Me-EAB > EAB ∼ MAB, which also agrees with the mentioned experimental results. The mentioned trend also is confirmed by the LP(O7)↔σNH steric exchange interaction.

Vibrational spectra, conformation and molecular transport of 5-amino-2,2,6,6-tetramethyl-4-hepten-3-one

In this research, the molecular structure and intramolecular hydrogen bonding (IHB) of 5-amino-2,2,6,6-tetramethyl-4-hepten-3-one (AMTMH) were studied using spectroscopic techniques (IR, Raman, UV, and NMR) and computational analysis (density functional theory (DFT) and the second-order Møller–Plesset (MP2) methods). The results of this study compared with those of 4-amino-3-penten-2-one (APO) and 3-methyl-4-amino-3-penten-2-one (3-MeAPO). The averaged calculated geometrical parameters of AMTMH (obtained by considering the populations of all conformers) agree with the averaged geometrical parameters obtained by X-ray crystallography of two conformations found in the crystal. Comparison of different parameters related to the IHB strength, such as NH stretching, experimental proton chemical shift, and deuterium isotope effects on 13C chemical shifts of AMTMH with those of APO and 3-MeAPO, suggests the following trend for the IHB strength: 3-MeAPO > AMTMH > APO. All normal modes of AMTMH were successfully assigned and compared to the corresponding band frequencies of APO. The appearance of more than one band for several vibrational normal modes is related to the coexistence of different conformers in the sample. The molecular transport results show that the conductivity of the enol form (on-state) to keto form (off-state) changes, which suggests that this system has the potential to function as a molecular switch. In addition, the experimental fluorescence and the calculated TD-DFT results confirmed both aminoketone and iminoenol forms are in the excited state.

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.