Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature

A density functional theory study of the molecular structure, reactivity, and spectroscopic properties of 2-(2-mercaptophenyl)-1-azaazulene tautomers and rotamers

Five stable tautomer and rotamers of the 2-(2-Mercaptophenyl)-1-azaazulene (thiol, thione, R1, R2, and R3) molecules were studied using density functional theory (DFT). The geometries of the studied tautomer and rotamers were fully optimized at the B3LYP/6-31G(d,p) level. Thermodynamic calculations were performed at M06-2X/6-311G++(2d,2p) and ωB97XD/6-311G++(2d,2p) in the gas phase and ethanol solution conditions modeled by the solvation model based on density (SMD). The kinetic constant of tautomer and rotamers conversion was calculated in the temperature range 270-320 K using variational transition state theory (VTST) accompanied by one-dimensional wigner tunneling correction. Energy refinement at CCSD(T)/6-311++G(2d,2p) in the gas phase has been calculated. All the studied DFT methods qualitatively give similar tautomer stability orders in the gas phase. The ethanol solvent causes some reordering of the relative stability of 2-(2-Mercaptophenyl)-1-azaazulene conformers. The transition states for the 2-(2-Mercaptophenyl)-1-azaazulene tautomerization and rotamerization processes were also determined. The reactivity, electric dipole moment, and spectroscopic properties of the studied tautomer and rotamers were computed. The hyper-Rayleigh scattering (βHRS), and depolarization ratio (DR) exhibited promising optical properties when nonlinear optical properties were calculated.

Synthesis, structural characterizations and in vitro cytotoxic activities of new sulfonamide-based Schiff base derivatives

Synthesis of five different new compounds (1-5) were carried out. Their structures were characterized by spectroscopic methods such as Fourier transform infrared, Proton nuclear magnetic resonance, Carbon-13 nuclear magnetic resonance and Elemental analysis. Cytotoxic activities of five new sulfonamide based-Schiff base compounds were determined by MTT assay using A-2780 (human ovarian cancer) and MCF-7 (breast cancer) cell lines. LogIC₅₀ values of the sulfonamide derivates compounds were calculated by Graphpad Prism 12 programme after a 24-hour treatment for A2780 and MCF-7 cells. Comet assay experiments were performed to determine DNA damage using LogIC₅₀ concentrations of all compounds in A2780 and MCF-7 cells. All compounds significantly reduced A2780 and MCF-7 cell viability compared to control groups (p < 0.05). In addition, all compounds caused DNA damage in A2780 and MCF-7 cells (p < 0.05). These results show that the synthesized compounds exhibit cytotoxic effects against cancer cells and that the cause of cell death is due to DNA damage.Communicated by Ramaswamy H. Sarma.

Taming the excited state reactivity of imines – from non-radiative decay to aza Paternò–Büchi reaction

This review highlights the excited state characteristics of imines and processes that govern their photochemical and photophysical properties.

J-type aggregation and thermochromic behavior of a schiff base in solution: Role of keto-enol tautomerization

In present work, we have been investigated the thermochromism, keto-enol tautomerization, J-type aggregation and aggregation induced enhanced fluorescence (AIEF) in a single schiff base molecule (HNBzP) along with excited state intramolecular proton transfer (ESIPT) process. It is observed that keto-enol tautomerization plays an important role on thermochromism, J-type aggregation and aggregation induced enhanced fluorescence (AIEF) process in solution. From UV-Vis and fluorescence studies, J-type aggregation is observed and shows the aggregation induce enhanced fluorescence (AIEF) behaviour in solusion. The excited state intramolecular proton transfer (ESIPT) process is investigated in different solvents. Optimized geometry and electronic properties are calculated by DFT method for better understanding at molecular level.

Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium

Ground state intramolecular proton transfer process has been comprehensively investigated in three salicylideneaniline Schiff base derivatives (SB1, SB2, and SB3) using experimental and theoretical methods. It has been confirmed that all the three Schiff base molecules in the ground electronic state exist in the enol form in non-polar and polar aprotic solvents. Keto form is being populated by the polar protic solvent through ground state intramolecular proton transfer (GSIPT) process. Ground state equilibrium between the enol and keto tautomers for SB1 and SB3 is mainly governed by the proton donating ability of the solvent. Ground state equilibria between the enol and keto tautomers of SB2 which is a positional isomer of SB3 is governed by the polarity and proton donating ability of the solvents. Excited state intramolecular proton transfer (ESIPT) process is also evidenced in all the three Schiff base molecules. Theoretical calculations at the B3LYP/cc-pVDZ level in the gas phase and in different solvents using polarisable continuum model (PCM) have failed to establish the GSIPT process. Microsolvation of individual enol and keto conformers has been investigated considering upto three solvent molecules. The energetics of the individual conformers together with the corresponding transition state have been calculated. It has been confirmed that the keto conformer is more stable compared to the enol conformer in microsolvated cluster of three methanol molecules. Lowering of activation energy for the enol to keto tautomerisation in the presence of methanol also supports the experimental observation for GSIPT process. TDDFT/B3LYP/cc-pVDZ single point calculations for microsolvated clusters of enol and keto form of the Schiff base molecules exhibit an excellent agreement with the experimentally obtained absorption spectra. Difference in spectral nature of the Schiff base molecules has been explained using natural bond orbital (NBO) analysis. Quantum theory of atoms in molecules (QTAIM) has also been utilised to understand the GSIPT process in detail.

Synthesis, Characterization, Thermochromism, and Photochromism of Aromatic Aldehyde Hydrazone Derivatives

The Schiff bases N-(5-phenylthiazole-2-yl)-2-hydroxylnaphthaldehydehydrazone (1), N-(4′-chloro-5-phenylthiazole-2-yl)-2-hydroxylnaphthaldehydehydrazone (2), and N-(4′-nitro-5-phenylthiazole-2-yl)-2-hydroxylnaphthaldehydehydrazone (3) were synthesized. These compounds were characterized by using IR,1H NMR,13C NMR, and MS. The photochromism of the compounds was investigated by IR and UV-visible spectrometry which is time variable under irradiation of 254 nm UV light. The thermochromism of the compounds was studied using temperature-variable IR, UV-visible spectrometry, TG, and differential scanning calorimetry (DSC). The results suggested that compound2showed thermochromism properties and compounds2and3displayed photochromism properties. The relationship between the substituents species and photochromic or thermochromic properties of these compounds was revealed as well.

A pair of Schiff base enantiomers studied by absorption, fluorescence, electronic and vibrational circular dichroism spectroscopies and density functional theory calculation

A pair of enantiomeric Schiff bases were synthesized and characterized, in particular their absolute configurations were determined by vibrational circular dichroism spectroscopy.

Photophysical properties of Schiff's bases from 3-(1,3-benzothiazol-2-yl)-2-hydroxy naphthalene-1-carbaldehyde

A series of novel Schiff's bases have been synthesized from 3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde. The presence of hydroxyl group ortho to the benzothiazolyl group as well as the imine linkage lead to the occurrence of excited state intramolecular proton transfer process. The computational strategy was used to study the ESIPT process of the synthesized Schiff's bases, which revealed surprisingly that the keto form predominantly exists in the ground state contradicting the ESIPT process. Density functional theory and time dependent density functional theory have been used to investigate the structural parameters and photophysical properties in different solvents of one of the Schiff's bases. The experimental results correlate well with the computed results. All Schiff's bases show good thermal stability.

Tautomerism and spectroscopic properties of the immunosuppressant azathioprine

The molecular structure and the relative stabilities of the four possible tautomers of the immunosuppressant azathioprine (AZA) are calculated by DFT/B3LYP method using different basis sets. The results of the energy analysis and thermodynamic treatment of the obtained data are used to predict the relative stabilities of the AZA tautomers. The effect of solvents such as DMSO and water on the stability of the AZA tautomers was studied using the polarized continuum method (PCM) at the same level of theory. The calculation predicted that, the total energies of all tautomers are decreased indicating that all tautomers are more or less stabilized by the solvent effect. The vibrational spectra of AZA are calculated using the same level of theory and the results are compared with the experimentally measured FTIR spectra. Good correlation is obtained between the experimental and calculated vibrational frequencies (R(2)=0.997). The electronic spectra of AZA in gas phase and in methanol as solvent are calculated using the TD-DFT method. The calculations predicted bathochromic shift in all the spectral bands in presence of solvent compared to the gas phase. Also the NMR spectra of all tautomers are calculated and the results are correlated with the experimental NMR chemical shifts where the most stable tautomer gives the best correlation coefficient (R(2)=0.996).

Proton transfer assisted charge transfer phenomena in photochromic Schiff bases and effect of -NEt2 groups to the anil Schiff bases

Photochromic Schiff bases 5-diethylamino-2-[(4-diethylamino-benzylidene)-hydrazonomethyl]-phenol (DDBHP) and N,N'-bis(4-N,N-diethylaminosalisalidene) hydrazine (DEASH) with both the proton and charge transfer moieties have been synthesized, and their photophysical properties such as excited state intramolecular charge transfer (ICT) and proton transfer (ESIPT) processes have been reported on the basis of steady-state and time-resolved spectral measurement in various solvents. The ground-state six-membered intramolecular hydrogen bonding network at the proton transfer site accelerates the ESIPT process for these compounds. Both the compounds show large Stokes-shifted emission bands for proton transfer and charge transfer processes. The hydrogen bonding solvents play a crucial role in these photophysical processes. Excited-state dipole moment of DDBHP and DEASH calculated by the solvatochromic method supports the polar character of the charge transfer excited state. Introduction of -NEt(2) groups to the reported salicylaldehyde azine (SAA) Schiff base results an increase in fluorescence lifetime from femtosecond to picosecond time scale for the proton transfer process.

Synthesis, spectroscopic characterizations and quantum chemical computational studies of (Z)-4-[(E)-p-tolyldiazenyl]-6-[(2-hydroxyphenylamino)methylene]-2-methoxycyclohexa-2,4-dienone

In this study, the molecular structure and spectroscopic properties of title compound were characterized by X-ray diffraction, FT-IR and UV-vis spectroscopies. These properties of title compound were also investigated from calculative point of view. The X-ray diffraction and FT-IR analyses reveal the existence of keto form in the solid state. UV-vis spectra were recorded in different organic solvents. The results show that title compound exists in both keto and enol forms in DMSO, EtOH but it exists in enol form in benzene. In addition, the title compound in DMSO showed new absorption band at 436 nm due to the high ionizing effect of this solvent. The geometry optimization of title compound in gas phase was performed using DFT method with B3LYP applying 6-311G(d,p) basis set. TD-DFT calculations starting from optimized geometry were carried out in gas phase to calculate excitation energies of title compound. The non-linear optical properties were computed with the same level of theory and title compound showed a good second order nonlinear optical property. In addition, thermodynamic properties were obtained in the range of 100-500 K.

Synthesis, spectroscopic characterizations and quantum chemical computational studies of (Z)-4-[(E)-(4-fluorophenyl)diazenyl]-6-[(3-hydroxypropylamino)methylene]-2-methoxycyclohexa-2,4-dienone

In this study, the molecular structure and spectroscopic properties of the title compound were characterized by X-ray diffraction, FT-IR and UV-vis spectroscopies. These properties were also investigated using DFT method. The most convenient conformation of title compound was firstly determined. The geometry optimizations in gas phase and solvent media were performed by DFT methods with B3LYP adding 6-31G(d) basis set. The differences between crystal and computational structures are due to crystal packing in which hydrogen bonds play an important role. UV-vis spectra were recorded in different organic solvents. The results show that title compound exists in both keto and enol forms in DMSO, EtOH but it tends to shift towards enol form in benzene. The polar solvents facilitate the proton transfer by decreasing the activation energy needed for Transition State. The formation of both keto and enol forms in DMSO and EtOH is due to decrease in the activation energy. TD-DFT calculations starting from optimized geometry were carried out in both gas and solution phases to calculate excitation energies of the title compound. The non-linear optical properties were computed at the theory level and the title compound showed a good second order non-linear optical property. In addition, thermodynamic properties were obtained in the range of 100-500K.

3-[(E)-(4-Ethyl-phen-yl)imino-meth-yl]benzene-1,2-diol

The title compound, C₁₅H₁₅NO₂, adopts the enol–imine tautomeric form. The dihedral angle between the two benzene rings is 48.1 (1)°. Intra­molecular O—H⋯N and O—H⋯O hydrogen bonds generate S(6) and S(5) ring motifs, respectively. In the crystal, mol­ecules are linked into centrosymmetric R₂²(10) dimers via pairs of O—H⋯O hydrogen bonds and the dimers may interact through very weak by π–π inter­actions [centroid–centroid distance = 4.150 (1) Å]. The ethyl group is disordered over two orientations, with occupancies of 0.587 (11) and 0.413 (11).

Proton transfer reaction of a new orthohydroxy Schiff base at room temperature and 77 K

One new orthohydroxy Schiff base, 2-(N-benzyl-alpha-iminoethyl)naphthol (BEIN) has been synthesized. The proton transfer reaction of BEIN has been investigated by means of absorption, steady state and time resolved fluorescence spectroscopy in different solvents at room temperature and 77K. The behavior of BEIN in ethanol and water, has been studied in neutral, acidic and basic conditions. Excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted ( approximately 11,000 cm-1) fluorescence in solid crystalline media at room temperature. We present the observation of phosphorescence both in non-polar and protic solvents at 77K. The observed decay dynamics of the phosphorescence and delayed fluorescence indicates that the triplet state can be attributed to the cis-keto form. The molecular structures are determined by B3LYP/6-31G** calculation. From theoretical study it is suggested that the strengthening of hydrogen bond result from the steric repulsion of the phenyl ring. The presence of benzene ring increases the proton transfer barrier in case of BEIN compared to previously studied 7-ethylsalicylidenebenzylamine (ESBA).