Structural and Surface Characterization of Newly Synthesized D-π-D Type Schiff Base Ligand : ( 1 E , 2 E )-3-[ 4-( Dimethylamino ) phenyl ] prop-2-en-1-ylidene ) phenylamine

A new Schiff base with D-π-D type, (1E,2E)-3-[4-(dimethylamino)phenyl]prop-2-en-1-ylidene)phenylamine, has been successfully synthesized using the reaction of (2E)-3-[4-(dimethylamino)phenyl]acrylaldehyde with aniline. The Schiff base ligand has been characterized by FTIR, UV-visible, and HNMR as well as TG/DTA, SEM, BET, and elemental analyses and mass spectra. Surface properties and acid-base constants of Schiff base ligand were determined by IGC measurements.


Introduction
The coordination compounds including the Schiff base ligands are of significantly important and play a pivotal role in industry, technology, and life processes.Due to their potential applications in various fields, it has always fascinated and inspired chemists in the world.This can be evidenced by the vast prolificity and scope of research papers on the subject in recent times and also by the diversity in which it has found applications.Some Schiff bases and their derivatives represent an interesting class of compounds possessing a wide spectrum of biological activities, such as analgesic, antiviral, antifungal, and anticancer activities.The Schiff base ligands and their complexes have been a fascinating area of research, due to their biological relevance.These compounds can act as pro-or antioxidants.The published opinions on the structure and antioxidant activity relationships are, however, quite inconsistent.It can be assumed from several scientific works that the quality of antioxidant action depends on the type of ligands forming the bioactive complexes [1,2].
In the past few decades, very fast development in the field of organic-based conjugated materials was observed [3,4].Schiff bases and their metal complexes have many applications in medicinal, industrial, pharmaceutical, biological, and chemical fields.For example, they are industrially used as dyes and pigments in textile besides their application in pharmacology for the synthesis of antibiotics, antiallergic, and antitumor substances.The Schiff bases can also exhibit thermochromic and photochromic properties [5][6][7].Moreover, the Schiff base compounds have been regarded as excellent fluorescent materials because of their ability to achieve high thermal stability as well as high photoluminescent efficiency.The conjugated Schiff base systems that contain electronically coupled photo-and/or redox-active sites across an unsaturated organic bridge are of considerable current interest [8][9][10][11][12][13].
Many complicated factors can affect the two-photon excitation (TPE) properties of organic materials.So it is important to investigate the structure and the TPE effect relationships of organic materials.However, most of these compounds employ C=C bonds as the conjugation bridge; the compounds with C=N structure such as the Schiff bases are less studied to our knowledge [14,15].
It can be used as a pigment in some applications because of excellent yellow colour.The structure of the SBL was determined by elemental analysis, FTIR, UV-visible, 1 H NMR, TG/DTA, and LC-MS spectrometry.In addition, IGC was applied to the investigation of adsorption properties of the SBL.The relation of SBL with used polar and nonpolar solvents was tested at temperatures between 30 ∘ C and 60 ∘ C under melting point of SBL.The goals of this paper were (1) to synthesize ad characterize new D--D type Schiff base ligand, (2) to figure out the surface free energy of SBL, (2) to calculate adsorption thermodynamic parameters of SBL, (3) to determine the acid-base quantity of SBL.

BET and SEM Analyses.
BET analysis was carried out to determine the BET surface area and pore size of SBL.The surface area and pore size are very important application parameters as the surface contributes to the potential for interaction between the pigment and the polymer matrix [16].Quantachrome Ins.Quadrasorb SI model BET was used in this study.The measurements were carried out under nitrogen atmosphere.SEM analysis was carried out to determine the shape of SBL with Zeiss model: EVO LS 10.

Inverse Gas Chromatography (IGC) Measurements.
A Hewlett-Packard 6890 Model, series II, gas chromatograph with a thermal conductivity detector was used to measure the retention time of the solvents in this study.The column was stainless steel tubing with 3.2 mm o.d. and 0.5 m in length.The SBL was coated on the support by slow evaporation of chloroform as stirring the Chromosorb W in the SBL solution.Trace amount of solvent was injected into the chromatograph.The column was conditioned at 110 ∘ C for 24 h under helium atmosphere.The probes were of high purity grade n-alkanes such as n-hexane (Hx), nheptane (Hp), n-octane (O), n-nonane (N), and other acidic, basic, and amphoteric probes such as tetrahydrofurane (THF, basic), dichloromethane (DCM, acidic), chloroform (TCM, acidic), acetone (AC, amphoteric), and ethyl acetate (EA, amphoteric) were used without further purification.The all studied solvents and support materials being Chromosorb W (AW-DMCS-treated, 80/100 mesh) were supplied from Merck AG.Inc. Silane treated glass wool used to plug the ends of the column was obtained from Alltech Associates, Inc.

IGC Theory.
The IGC technique provides a means of gaining information concerning the surface polarity of particulates and the acid-base nature of particulates.It provides a basis for determining the potential for chemical interaction between the pigment and its medium (polymer matrix or paint solutions, etc.).The material to be investigated is immobilised within a chromatographic colomn which is flushed through with inert gas such as helium or nitrogen.
The stationary phase characterisation is achieved by utilising the partitioning of the sample between the mobile phase and the stationary phase, indicated by the time taken to elute the sample.The probe-probe interactions in IGC experiments are negligible because of being carried out in infinite dilution.The theory and the technique are now quite well described in the literature [16,17].
The adsorbate net retention volumes   were calculated from the following expression: where   is the adsorbate retention time,   is the retention time of air,  is volumetric flow rate measured at column outlet and at ambient temperature   (K),  is the column temperature (K), and  the is James-Martin gas compressibility correction factor [18].The dispersive component of surface energy was determined using both the Dorris-Gray and the Fowkes methods.The adsorption energy for the n-alkanes increases with the number of carbon atoms in the chain.According to Dorris & Gray (1980), the increment, corresponding to the adsorption energy of a methylene group, Δ [CH 2 ] is given by [19] ) , where  , and  ,+1 are the retention volumes of two nalkanes having n and n + 1 carbon atoms in their chain.This parameter is independent of the chosen state of the adsorbed molecule.Thus, at constant temperature, for a series of alkane probes, a plot  ln   versus the number of carbon atoms should give a straight line from which Δ [CH 2 ] can be found.
The methylene adsorption energy can also be defined as [19] Δ where Thus, using (2)-( 4) and the experimentally determined values of  , and  ,+1 , the dispersion component of the surface free energy,    may be calculated.The free energy of adsorption Δ  may also be defined in terms of the retention volume of the probes [20]: is the column temperature and  is a constant for a given column.
Consequently, the equations may be combined to give the Fowkes equation [21]: Thus, for a series of n-alkane probes, a plot of  ln   against (   ) 0.5 will give a slope of 2(   ) 0.5 .Values of (   ) 0.5 and boiling point   ( ∘ C) of apolar solvents are found in the literature [22,23].
The specific component of the free energy is determined from the n-alkane plot of  ln(  ) against (   ) 0.5 .The distance between the ordinate values of the polar probe datum point and the n-alkane reference line gives the specific component of the surface free energy, −Δ   .An equation may be written for this procedure, where  , and  ,ref are the retention volume for the polar probe and the retention volume for the n-alkanes' reference line, respectively.The adsorption of a polar probe onto the adsorbent surface leads to a change in the enthalpy of the system and the entropy of the system.These factors are related to the energy of adsorption by the following: Here, Δ   is the adsorption enthalpy by the Lewis acid-base interactions, Δ   is the adsorption entropy the Lewis acidbase interactions, and  is the column temperature.For each polar probe, Δ   and Δ   can be determined from a plot of −Δ   / against 1/.The surface Lewis acidity and basicity constants,   and   , may be calculated from the follwoing, Here, DN and AN * are Gutmann's donor and modified acceptor numbers, respectively.Values of (   ) 0.5 , boiling point,   ( ∘ C), Gutmann's modified acceptor number, AN * , and donor number, DN, of the polar probes used in this study are taken from [23,24].
and   are obtained from a plot of −Δ   /AN * versus DN/AN * with   as the slope and   as the intercept.Parameters   and   reflect the ability of the examined surface to act as an electron acceptor and electron donor, respectively [20].

Results and Discussion
The new Schiff base ligand was synthesized from the reaction of (2E)-3-[4-(dimethylamino) phenyl]acrylaldehyde with aniline, in a good yield, as shown in Scheme 1.The IR spectrum of the SBL shows the characteristic Schiff base stretching band at 1662 cm −1 .This intense band is assigned to the -C=N stretching frequency of the ligand and is characterized for the azomethine moiety of most of the Schiff base compounds.The absorption band of the C=O in (2E)-3-[4-(dimethylamino) phenyl]acrylaldehyde disappeared in the infrared spectrum of the ligands, which indicates that the condensation has occurred.The alkyl and aryl bands of the ligand are observed at 2809-2899, 1364-1448, and 1149-1151 cm −1 , respectively.
Elemental analysis of the Schiff base ligand shows good agreement with the proposed structures.The 1 H NMR The SBL is stable at room temperature but hygroscopic.The SBL is soluble in common polar organic solvents, such as ethanol, methanol, and chloroform but partially soluble in nonpolar organic solvents, such as benzene and hexane.
The UV-Vis spectral data for the SBL ligand show lowenergy band at approximately 249 nm and high energy band at approximately 389 nm, due to n- * and - * transitions of the azomethine group in the ligands [25,26] (Figure 1).
According to UV spectra, the peaks recorded in the spectrum are broadened and slightly red shifted (Figure 1).
In the mass spectra, the molecular ion peak of the L appeared at m/z (100%): 250.937 [M] + .Mass spectral data confirmed the proposed structure of the Schiff base ligand (Figure 2).
According to BET results, the surface area and pore size of SBL were determined as 32.4 m 2 ⋅g −1 and 3.056 nm, respectively.This ligand has a potential to be used in nanotechnology paint applications.The SEM micrograph of SBL was given in Figure 3.
The shape of SBL was determined as a rod-like structure from SEM micrograph.The retention diagrams of nonpolar and polar solvents on SBL were plotted net retention volumes,   , of solvents on SBL which were calculated from IGC measurements between 30 ∘ C and 60 ∘ C by using (1) versus inverse temperature.According to (2), Δ [CH 2 ] is independent of the chosen reference state of adsorbed molecule.The  ln   versus carbon number of n-alkanes were plotted in Figure 4.
The slope of the fitted line is equal to . The variation of    according to the Dorris-Gray approach and  [CH 2 ] with temperature were calculated from (3) and (4), respectively.According to Fowkes, calculated values of  ln   were plotted against (   ) 0.5 .An example of the pattern of results obtained was given in Figure 5 for the isotherm at 303 K.
The results of Δ [CH 2 ] ,  [CH 2 ], and    according to the Dorris-Gray and the Fowkes approaches were given in Table 1.The specific component of the surface free energy, −Δ   , is calculated using the difference between the calculated value of  ln   and that which was derived using the equation of the linear fit of the n-alkane reference line.By plotting the values of −Δ   / against 1/, the adsorption enthalpy, Δ  , and the adsorption entropy, Δ   were determined for each studied polar probe.The values of   and   were calculated using (9).The plotting −Δ   /AN * versus DN/AN * with   as the slope and   as the intercept (Figure 6).
The values of   and   are found to be 0.1 and 0.6, respectively.According to the values obtained for   and   , the surface of SBL is the basic character between 30-60 ∘ C.

Conclusions
In this study, a new Schiff base ligand with D--D type has been successfully synthesized using the reaction of (2E)-3-[4-(dimethylamino) phenyl]acrylaldehyde with aniline and characterized by FTIR, UV-visible, 1 H NMR, elemental analysis, and mass spectrum for structural characterization and BET, SEM, and IGC for surface characterization.The big surface area and small pore size of SBL were determined   from the BET result.It can be used in nanotechnology paint applications.In addition, IGC was applied to the investigation of adsorption properties of the SBL.The relation of SBL with used polar and nonpolar solvents was tested at temperatures between 30 ∘ C and 60 ∘ C in which SBL does not show any thermal transition.The base constant of newly synthesized SBL was determined as 0.64, while acid constant is 0.1.

Figure 4 :
Figure 4: The plot of  ln   versus carbon number of n-alkanes.

Figure 5 :
Figure 5: A plot of  ln   versus (   ) 0.5 for n-alkanes and polar probes on SBL at 303 K.

Table 1 :
The adsorption energy of a methylene group, Δ [CH 2 ] , the surface free energy of a surface consisting of methylene groups,  [CH 2 ] and dispersion component of surface free energy,    , values calculated by the Dorris-Gray and the Fowkes approaches for  determined at studied temperatures.