Estimation of Ground-State and Singlet Excited-State Dipole Moments of Substituted Schiff Bases Containing Oxazolidin-2-one Moiety through Solvatochromic Methods

Absorption and fluorescence studies on novel Schiff bases (E)-4-(4-(4-nitro benzylideneamino)benzyl)oxazolidin-2-one (NBOA) and (E)-4-(4-(4-chlorobenzylidene amino)benzyl)oxazolidin-2-one (CBOA) were recorded in a series of twelve solvents upon increasing polarity at room temperature. Large Stokes shift indicates bathochromic fluorescence band for both the molecules. The photoluminescence properties of Schiff bases containing electron withdrawing and donating substituents were analyzed. Intramolecular charge transfer behavior can be studied based on the influence of different substituents in Schiff bases. Changes in position and intensity of absorption and fluorescence spectra are responsible for the stabilization of singlet excited-states of Schiff base molecules with different substituents, in polar solvents. This is attributed to the Intramolecular charge transfer (ICT) mechanism. In case of electron donating (?Cl) substituent, ICT contributes largely to positive solvatochromism when compared to electron withdrawing (?NO₂) substituent. Ground-state and singlet excited-state dipole moments of NBOA and CBOA were calculated experimentally using solvent polarity function approaches given by Lippert–Mataga, Bakhshiev, Kawskii-Chamma-Viallet and Reichardt. Due to considerable π- electron density redistribution, singlet excited-state dipole moment was found to be greater than ground-state dipole moment. Ground-state dipole moment value which was determined by quantum chemical method was used to estimate excited-state dipole moment using solvatochromic correlations. Kamlet-Abboud-Taft and Catalan multiple linear regression approaches were used to study non-specific solute-solvent interaction and hydrogen bonding interactions in detail. Optimized geometry and HOMO-LUMO energies of NBOA and CBOA have been determined by DFT and TD-DFT/PCM (B3LYP/6-311G (d, p)). Mulliken charges and molecular electrostatic potential have also been evaluated from DFT calculations.     

The Spectrochemical Properties of Noncubic Transition Metal Complexes in Solutions. VII. Angular Overlap Treatment of the trans-bis(Salicylidene-o-aminopyridine)copper(II) Complex in Various Solvents.

The electronic absorption spectra of trans-[Cu(sap)₂], where (Hsap = salicylidene-o-aminopyridine (Schiff base)), were measured in various solvents at room temperature. The d-d transition energies is used to derive the angular overlap model (AOM) parameters in C_2h symmetry. The experimental curves were resolved by Gaussian analysis. A comparison of the spectra, ligand field parameters and stereochemistries in various solutions was made. The effect of the solvents upon the σ, π-bonding, and bite angle of the bidentate ligand is discussed.     

Structural characterization,luminescence and electrochemical properties of the Schiff base ligands

In this study, we prepared two Schiff base ligands N-(4-hydroxy phenyl)-2,4-di-methoxy benzaldimine (TS¹) and N-(4-hydroxy phenyl)-2,5-di-methoxybenzaldimine (TS²) which were characterized by structural, spectroscopic and analytical methods. The ligands TS¹ and TS² were obtained as single crystals from ethanol solution. X-ray diffraction data for two compounds showed that the bond lengths are within the normal ranges. The electrochemical properties of the Schiff base ligands were studied in different solvents and at various scan rates. The luminescence properties of the ligands TS¹ and TS² in different solvents and at different pH values have been investigated. The results show that the ligands exhibit more efficient luminescence properties in CH₃CN and n-butanol.     

Design,Synthesis, Photophysical Properties,Biological Estimation and Molecular Docking Studies of Novel Schiff Base Derivatives as Potential Urease Inhibitors

A quinoline functionalized two novel fluorescent Schiff bases, N-(quinolin-2-ylmethylene) anthracen-1-amine (SB1) and 2-(quinolin-2-ylmethyleneamino) benzene thiol (SB2) were synthesized and confirmed by using ¹H NMR, IR and GC-MS techniques. The spectroscopic properties were examined by absorption spectroscopy and fluorescence spectroscopy. The absorption and fluorescence spectra of the probes (SB1 and SB2) were measured in a variety of solvents. Both the compounds were tested for urease inhibitory activity. The synthesized compound SB2 proved to be the most effective screening for enzyme inhibitory activity with IC₅₀?=?0.111 μM than SB1 (IC₅₀?=?0.287 μM). Molecular docking studies were performed to delineate the binding affinity and conformational positions of chemical compounds within the active region of the target protein. In-vitro analysis depicts the potency of SB1 in free radical scavenging as compared to the reference drug vitamin C.     

Fluorescence Properties and Electrochemical Behavior of Some Schiff Bases Derived from N-Aminopyrimidine

A series of Schiff bases (L ₁ , L ₂ and L ₃ ) were prepared by refluxing aromatic aldehydes with N-Aminopyrimidine derivatives in methanol and ethanol. The structures of synthesized compounds were characterized by FTIR, ¹H NMR, ¹³C NMR and microanalysis. The electrochemical behaviors of the Schiff base ligands were also discussed. Moreover, the evaluation of absorption and emission properties of the structures were carried out in five different solvents. The products show visible absorption maxima in the range of 304–576 nm, and emission maxima from 636 to 736 nm in all solvents tested.     

Electrochemical and theoretical investigation on the corrosion of aluminium in acidic solution containing some Schiff bases

The effect of newly synthesised three Schiff bases—2-[2-aza-2-(5-methyl(2-pyridly))vinyl]phenol, 2-[2-aza-2-(5-methyl(2-pyridly))vinyl]-4-bromophenol, 2-[2-aza-2-(5-methyl(2-pyridly))vinyl]-4-chlorophenol—on the corrosion behaviour of aluminium in 0.1 M HCl were investigated using potentiodynamic polarisation, electrochemical impedance spectroscopy and linear polarisation methods. Polarisation curves indicate that all studied Schiff bases were acting as mixed type inhibitors. All measurements show that inhibition efficiencies increase with increase in inhibitor concentration. This reveals that inhibitive actions of inhibitors were mainly due to adsorption on aluminium surface. Adsorption of these inhibitors follows Langmuir adsorption isotherm. Thermodynamic parameters of adsorption (K_ads, ΔG_ads) of studied Schiff bases were calculated using Langmuir adsorption isotherm. The variation in inhibition efficiency values depends on the type of functional groups substituted on benzene ring. It was found that the presence of bromine and chlorine atoms in the molecular structure of studied Schiff bases facilitate the adsorption of molecule on aluminium surface.The correlation between the inhibition efficiencies of studied Schiff bases and their molecular structure has been investigated using quantum chemical parameters obtained by MNDO semi-empirical SCF-MO methods. These results indicate that adsorption of studied Schiff bases depends on the charge density of adsorption centres and dipole moments.     

Pressure effects on thermal isomerizations in highly viscous media. The first clear-cut example of viscosity-induced retardation of “slow” thermal reactions

Abstract

Kinetic effects of pressure on thermal Z/E isomerizations of 4-(dimethylamino)-4′-nitroazo-benzene (DMNAB) and N-[4-(dimethylamino)benzylidene]-4-nitroaniline (DMBNA) were studied by flash photolysis in three viscous solvents; glycerol triacetate (GTA), 2-methyl-2,4-pentanediol (MPD) and “Traction Fluid B” (TFB). In all cases studied, the pressure effects observed at the beginning of pressurization were qualitatively similar to the ones observed in less viscous solvents. The results strongly suggest that the conventional understandings of the kinetic effects of pressure based on the transition state theory (TST) are valid in these thermal unimolecular reactions. At higher pressures, however, pressure-induced viscosity increase resulted in strong retardation of both of the isomerizations. The diffusion-controlled rate constants obtained by substituting the observed and the TST-expected rate constants to I/k_obs = l/k_TST + l/k_dif showed inverse fractional dependence on the solvent viscosity.     

Comparison of the ht Ultraviolet-Visible Spectra of the Copper(Ii) Complexes of Bidentate Schiff Base Ligands

Abstract

The bis(salicylaldiminato)copper(II) complexes of the ligand series of salicylaldimines derived from the condensation of n-alkyl or n-alkyloxy substituted aromatic amines with 2,4-dihydroxybenzaldehyde were synthesized. a series of Schiff base ligands with the general formula 4-X-N-(2,4-dihydroxybenzylidene)-aniline and differing only in substituents were synthesized. Some of these compounds have already been reported in literature. X=OCH₃[1],OC₂H₅[2],C₄H₉[1]. the copper(II) complexes of these ligands having the general formula, copper, bis[o-[N-(p-X-phenyl) formimidoyl]-4-hydroxyphenolato] were also synthesized. the ligands and their complexes, a total of 24 in number, are studied systematically with ultraviolet-visible spectroscopy to examine the effect of various n-alkyl- and n-alkyloxy substituents on the ligands and on their complexes. in the electronic spectrum of these compounds, the bands observed in the 200-450 run region which involve charge transfer π-π? transition were interpreted.     

Fluorescence Characteristics of Schiff Bases Derived from Amino- and Aminoalkylpyridines

The fluorescence characteristics of the Schiff bases 2-(3-pyridylmethyliminornethyl)phenol (1), 2-(2 pyridyliminomethyl)phenol (2), N.N-bis(salicylidene)-2,3-pyridinediamine (3), N,N'-bis(salicylidene)-2,6-pyridinediamine (4) and 2-(2-amino-4-methoxymethyl-6-methyl-3-pyridylmethyliminomethyl)phenol (5) were studied in various solvents at different pH values. Corresponding quantum efficiencies were determined. Compound 1, which showed a tendency towards tautomeric mterconversion to ketoamine in polar protic solvents, was not fluorescent at pH < 8. The fluorescence of other compounds was very sensitive to solvent polarity and the pH of the medium. Compounds 2-4, preferably present as enolimines in all solvents, were not fluorescent in non-polar and moderately polar solvents, whereas weak emission was observed in polar solvents, like methanol, dimethylformamide and dioxane/water 1/1 (0.001 < Q < 0.072). A significant increase in Stokes shifts and in quantum efficiencies was noted as a result of increasing polarity of dioxane/water mixtures, indicating specific interactions with polar water molecules. The emission was promoted at acidic pH values where a pyridinium cation was formed (0.061 < Q < 0.519, in dioxane/water 1/1 at pH 3.4). Compound 5, which was a tautomeric mixture of enoiimine and cyclic diamine in all solvents, was fluorescent in polar as well as in non-polar media. The quantum efficiency varied dependent on the solvent and pH (0.023 <Q< 0.435). The cyclic diamine, i. e. the more rigid structure was supposed to be responsible for the fluorescence in non-polar and aprotic solvents as well as at neutral, and weakly basic pH values.     

Synthesis,spectroscopic, thermal and electrical conductivity studies of three charge transfer complexes formed between 1,3-di[(<Emphasis Type="Italic">E</Emphasis>)-1-(2-hydroxyphenyl)methylideneamino]-2-propanol Schiff base and different acceptors

Charge-transfer complexes (CTC) resulting from interactions of 1,3-di[(E)-1-(2-hydroxyphenyl) methylideneamino]-2-propanol Schiff base with some acceptors such as iodine (I₂), bromine (Br2), and picric acid (PiA) have been isolated in the solid state in a chloroform solvent at room temperature. Based on elemental analysis, UV-Vis, infrared, and ¹H NMR spectra, and thermogravimetric analysis (TG/DTG) of the solid CTC, [(Schiff)(I₂)] (1), [(Schiff)(Br₂)] complexes with a ratio of 1:1 and [(Schiff)(PiA)₃] complexes with 1:3 have been prepared. In the picric acid complex, infrared and ¹H NMR spectroscopic data indicate that the charge-transfer interaction is associated with a hydrogen bonding, whereas the iodine and bromine complexes were interpreted in terms of the formation of dative ion pairs [Schiff⁺, I₂^∙−] and [Schiff⁺, Br₂^∙−], respectively. Kinetic parameters were obtained for each stage of thermal degradation of the CT complexes using Coats–Redfern and Horowitz–Metzger methods. DC electrical properties as a function of temperature of these charge transfer complexes have been studied.     

Origin of significant solvent effects on 1J(CC) spin–spin coupling in some acetylenes: hydrogen bonding and solvent polarity

It is demonstrated that some acetylenes, those of the R? C?CH structure, display anomalously high sensitivity to solvent effects of their ¹J(C?C) coupling while R? C?CR acetylenes fail to show that. The solvent‐induced variation in the latter coupling does not exceed 3 Hz; this seems to be the upper limit of variation of any J(CC) and J(CH) coupling in the molecular system studied which included: acetylene (in 13 solvents), phenylacetylene (in 12 solvents), 1‐phenylpropyne, and 2‐hexyne (two solvents each), and the only exceptions are ¹J(C?C) in acetylene, which is shown to vary within about 13 Hz, and that in phenylacetylene where the range amounts to about 8 Hz. These apparent anomalies are explained in the present study in terms of two effects of prime importance, solvent polarity and the solute‐to‐solvent hydrogen bonds where the CH moiety in R? C?CH acetylenes acts as a donor of hydrogen bonds to acceptor sites in the solvent concerned. Copyright © 2009 John Wiley & Sons, Ltd.     

A New Candidate Laser Dye Based 1,4-Bis[β-(2-Naphthothiazolyle)Vinyl]Benzene. Spectroscopic Behavior,Laser Parameters and Excitation Energy Transfer

A new candidate laser dye based 1,4-bis[β-(2-naphthothisolyl) vinyl] benzene (BNTVB) were prepared, and characterized in various organic solvents. The center polarity is less sensitive than electronic absorption. A red shift was noticed in the fluorescence spectra (ca. 40 nm) with increment in the solvent’s polarity, this means that BNTVB’s polarity appreciates upon excitation. The dipole moment of ground state (μ_g) and the excited singlet state dipole moment (μ_e) are determined from Kawski – Chamma and Bakshiev–Viallet equations using the disparity of Stokes shift with solvent polarity function of ε (dielectric constant) and n (refractive index) of the solvent. The result was found to be 0.019D and 5.13D for ground and exited state, in succession. DFT/TD-DFT manners were used to understand the electronic structures and geometric of BNTVB in other solvents. The experimental and theoretical results showed a good agreement. The photochemical quantum yield (Ф_c) of BNTVB was calculated in variable organic reagents such as Dioxane, CHCl₃, EtOH and MeOH at room temperature. The values of φ_c were calculated as 2.3?×?10^?4, 3.3?×?10^?3, 9.7?×?10^?5 and 6.2?×?10^?5 in Dioxane, CHCl₃, EtOH and MeOH, respectively. The dye solutions (2?×?10^?4 M) in DMF, MeOH and EtOH give laser emission in the blue-green region. The green zone is excited by nitrogen pulse 337.1 nm. The tuning range, gain coefficient (α) and cross – section emission (σ_e) of laser were also estimated. Excitation energy transfer from BNTVB to rhodamine-6G (R6G) and N,N-bis(2,6-dimethyphenyl)-3,4:9,10-perylenebis-(dicarboximide) (BDP) was also studied in EtOH to increase the laser emission output from R6G and BDP when excited by nitrogen laser. The dye-transfer power laser system (ETDL) obeys the Foster Power Transmission (FERT) mechanism with a critical transmission distance, R_o of 40 and 32 ? and k_ET equals 2.6?×?10¹³ and 1.06?×?10¹³ M^?1 s^?1 for BNTVB / R6G and BNTVB / BDP pair, respectively.     

氘代溶剂对丙烯酰胺溶剂化作用的NMR研究

用¹H NMR谱研究了丙烯酰胺（AM）分别在氘代氯仿（CDCl₃）和氘代二甲基亚砜（DMSO-d₆）溶剂以及在不同比率的混合溶剂中,AM的烯键C上的反式两个质子的谱峰位置开始相向移动,重叠,后又反向交错以致形成“类似镜像”的现象,用溶剂化作用讨论了成因;混合溶剂中,随着DMSO-d₆摩尔分数的增加,－NH₂质子和溶剂残余水质子的化学位移逐渐都向低场移动,这与－NH₂和DMSO之间形成氢键,－NH₂和水质子之间既有氢键生成又有质子交换有关.     

Acid assisted proton transfer in 4‐[(4‐R‐phenylimino)methyl]pyridin‐3‐ols: NMR spectroscopy in solution and solid state,X‐ray and UV studies and DFT calculations

The behaviour of Schiff bases of 3‐hydroxy‐4‐pyridincarboxaldehyde and 4‐R‐anilines (R?H, CH₃, OCH₃, Br, Cl, NO₂) in acid media has been described. ¹H, ¹³C, ¹⁵N‐NMR chemical shifts allow to establish the protonation site and its influence on the hydroxyimino/oxoenamino tautomerism. DFT calculations, electronic spectra and X‐ray diffraction are in agreement with the NMR conclusions. Copyright © 2007 John Wiley & Sons, Ltd.     

Solvatochromic effect on photophysical properties of Quinoxalin-2(1H)-one and 3-Benzylquinoxalin-2(1H)-one

Quinoxalin-2(1H)-one and its derived 3-Benzylquinoxalin-2(1H)-one were synthesized and characterized by UV–visible spectroscopy. The changes displayed by the photophysical properties of these molecules in different solvents can be explained in terms of a sum of dielectric polarity and hydrogen bonding effects taking part in the stabilisation of the structure. 3-Benzylquinoxalin-2(1H)-one exhibits two fluorescence emission bands (F _a and F _n) in very polar solvents and one band (F _n) in low polar solvents. These bands are assigned on the basis of the absorption and emission solvent effect. The abnormal fluorescence (F _a) observed in very polar solvents is attributed to an intermolecular interaction between solute and solvent molecules in the excited state (exciplex formation).     

Excited-state inter- and intramolecular proton transfer in methyl 3-hydroxy-2-quinoxalinate: effects of solvent and acid or base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical (AM1) and density functional theory using Gaussian 98 program calculations have been used to study the effects of solvent and acid or base concentration on the spectral characteristics of methyl 3-hydroxy-2-quinoxalinate (M3HQ). M3HQ is present as enol in less polar solvents and as keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to the phototautomer, formed by the excited-state intramolecular proton transfer, whereas fluorescence is only observed from keto in the polar solvents. In aqueous and polar solvents the monocation (MC5/MC6) is formed by protonating the carbonyl oxygen atom in the ground (S₀) and the first excited singlet states (S₁). Dication is formed by protonating one of N- atom of MC5/MC6. Monoanion is formed by deprotonating the phenolic proton of enol in the basic solution. pK_a values for different prototropic equilibriums were determined in S₀ and S₁ states and discussed.     

Solvent polarity scales: determination of new ET(30) values for 84 organic solvents

The solvent polarity parameter E_T(30) is newly measured from the solvatochromism of the betaine dye 30 for 84 solvents and re‐measured for 186 additional ones. The results are organized in a database. It is shown that the validity of linear solvation energy relationships used for the determination of secondary E_T(30) values is limited to non‐hydrogen‐bond donor solvents. Relationships with the chain length n are given for the determination of tertiary E_T(30) values of the homologous H(CH₂)_nY solvent series. The parameter E_T(30) is orthogonal to the function of the refractive index (n² ? 1) / (2n² + 1). For non hydrogen‐bond donor solvents, this allows to enter E_T(30) as an almost pure electrostatic parameter in a new linear solvation energy relationship. Copyright © 2014 John Wiley & Sons, Ltd.     

Time‐dependent density functional theory study on the excited‐state hydrogen bonding strengthening of photoexcited 4‐amino‐1,8‐naphthalimide in hydrogen‐donating solvents

The time‐dependent density functional theory (TDDFT) method was performed to investigate the excited‐state hydrogen bonding dynamics of 4‐amino‐1,8‐naphthalimide (4ANI) as hydrogen bond acceptor in hydrogen donating methanol (MeOH) solvent. The ground‐state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low‐lying electronically excited states for the isolated 4ANi and hydrogen‐bonded 4ANi‐(MeOH)_1,4 complexes were calculated by the DFT and TDDFT methods, respectively. We demonstrated that the intermolecular hydrogen bond C═O···H–O and N–H···O–H in the hydrogen‐bonded 4ANi‐(MeOH)_1,4 is strengthened in the electronically excited state, because the electronic excitation energies of the hydrogen‐bonded complex are correspondingly decreased compared with that of the isolated 4ANi. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electronically excited state of fluorescent dye in hydrogen‐donating solvents exists in many other systems in solution. Copyright © 2013 John Wiley & Sons, Ltd.     

On the Photophysicochemical Properties of Selected Fluoroquinolones: Solvatochromic and Fluorescence Spectroscopy Study

The photophysicochemical properties of selected fluoroquinolones in different solvents of various physical properties, including polarity and hydrogen bonding ability, were investigated using steady state fluorescence spectroscopy. The solvent-dependant fluorescence emission spectra of selected fluoroquinolones, namely ciprofloxacin (CIPR) and enrofloxacin (ENRO), were employed to gain insights concerning its photophysicochemical properties of interests. Interestingly, fluorescence spectra of the selected drugs exhibited structured emission spectra in nonpolar solvents such as hexane, whereas unstructured spectra were observed in more polar solvents such as alcohols and water. Also, a notable bathochromic shift in $ \lambda_{{\max }}^{{em}} $ was observed in fluorescence spectra of both drugs with increasing solvent polarity that resulted in biphasic behavior upon applying the Lippert-Mataga correlation that correspond to general and specific solvent effects. Applying the Lippert-Mataga correlation to the fluorescence spectra of CIPR and ENRO in various solvents was employed to estimate the dipole moment difference between the ground and excited states of them, $ \Delta \mu \left( {{\mu_e} - {\mu_g}} \right) $ , where obtained results revealed the values of 9.4 and 16.2 Debye for the LE and ICT states of ENRO, respectively, and 8.0 and 16.2 Debye for the LE and ICT states of CIPR, respectively. Multiple linear regression analysis (MLRA) based on Kamlet-Taft equating was applied against absorption frequency (ν_abs), emission frequency (ν_em), Stokes shift (?ν), and fluorescence quantum yield (Φ_f), where obtained results revealed excellent correlation (R: 0.916–0.966) that are consistent with other results considering the effect of solvent polarizability, hydrogen bonding ability, and viscosity on the photophysicochemical properties of the studied fluoroquinolones.     

ESR spectral studies of a Cu(II) salicylidene tyrosine complex

A novel Schiff base derived from salicylidene and tyrosine and its copper(II) complex have been synthesized and characterized. The composition of the complex is K[CuL(Ac)] · H₂O, where L = H₁₃C₁₆NO₄. Electron spin resonance (ESR) spectra of the copper(II) complex were investigated at different temperatures and in various solvents. The second-order effect and the relaxation effect were observed in the solution spectrum at room temperature and satisfactorily explained by the spin Hamiltonian. The bonding parameters of the Cu(II) complex were calculated with spectral parameters from ESR spectra at low temperature. Its bonding characterization and stability were discussed. The result shows that both the in-plane σ-bond and the in-plane π-bond in the complex play an important role.