Synthesis , Characterization of New Polyamides Bearing Triarylamine for Lightemitting Diodes

In this work, new di-acid monomers 4, 4’-di-carboxillic-4”-bromo-2”, 6”-dimethyl triphenylamine (Ma), 4, 4’di-carboxylic -4”-chloro-2”, 6”-dimethyl triphenylamine (Mb) and 4, 4’di-carboxylic -2”,4”dichloro-6”-methyl triphenylamine (Mc) were synthesized by reaction of p-cyanobenzofluride with three different aromatic amines (4-bromo,2,6-dimethyl aniline, 4-chloro,2,6-dimethyl aniline and 2,4 dichloro, 6methyl aniline ) via aromatic nucleophilc substitution method to form three di cyano intermediates 4, 4’-Dicyano-4”-bromo-2”, 6”-dimethyl triphenylamine (Da), 4, 4’-dicyano-4”-chloro-2”, 6”-dimethyl triphenylamine (Db) and 4, 4’-dicyano-2”,4”-dichloro-6”-methyl triphenylamine (Dc) which form final dicarboxylic monomers after alkaline hydrolysis. Finally, these monomers react with two different aromatic di amines, phenylene diamins and benzidine respectively via polycondensation reaction to form final polyamides 4"-bromo-2", 6"-dimethyl-triphenylamine-4, 4'-polyphenylbenzamide (Pa), 4”-chloro-2”,6”dimethyltriphenylamine-4,4'-polyphenylbenzamide (Pb), 2”,4”-dichloro-6”-methyl-triphenylamine 4,4'polyphenylbenzamide (Pc),4"-bromo-2",6"-dimethyl triphenylamine-4,4'polyphenylbiphenylamide (Pd), 2”,4”-dichloro-6”-methyl-triphenylamine-4,4’-polyphenylamide (Pf). The chemical structure of these polymers characterized by FTIR and NMR techniques. All the results of polyamides showed excellent solubility in most polar solvents to form strong thin films. The polyamides possess a good thermal stability with height glass transition temperatures (Tg). Polyamides in DMSO solvent gave strong photoluminescence PL. Thin casting films of these polyamides in cyclic voltammetry (C.V) on glass base of iridium-tin oxide (ITO) as working electrode in dry CH3CN solvent contains 0.1 M of tetrabutylantimoneperchlorate (TBAP) as an Electrolyte gave one redox wave.


Introduction
Polyamides having triphenyl amine which are characterized by their hole transporting groups, are known in the application of polyamides light emitting (PLEDs).These types of polymers have received great attention because of their important applications in many fields, including screens and lighting points [1,2].These polyamides are known as low energy ionization, movement activity and easily oxidize to convert to stable radical cations [3][4][5][6][7].Polyamides with triphenylamine group show electro chromic behavior and mainly used as hole-transport layer in LED devices [8][9][10][11][12][13].All aromatic polyamides are resulted as high-performance polymers due to their high thermal and mechanical stabilities and good chemical properties [14,15].These polymers are recognized as a very rigid because of hydrogen bonding between polymers backbones so when balk group are incorporated between the backbones.This reduces the close packing and interchanges hydrogen bonding which helps to break the rigidity and make the chins in high flexibility [16][17][18].The introduction of withdrawing groups improves the oxidation resistance, optical transparency and solubility in organic solvents and lead to decrease the crystallinity behavior, dielectric constant, water absorption and color [19][20][21][22].Polyamides are called conducting polymer, are important part of electrochromic materials and have being developed [23][24][25][26].Electrochromic materials can convert color reversibly by mean of important result of energy saving and sufficient electrochemical potential or redox steps.

Characterization Technique
The spectrum of FT-IR for all new materials was easured by a Spotlight 400 Perkin-Elmer spectrometer. 1 H and 13 C-NMR data are recorded by a JEOL spectrometer.UV-vis absorption and PL spectrum were performed in (10 -5 M) DMSO on a Cary 60 UV-vis spectrophotometer.The measurement s of X-ray were recorded by Cuka radiation beam (l ¼ 1.541 _A) with Max Flax (CMF) mirror.DSC and TGA were performed on a Perkin Elmer DSC6 instrument under N2 atmosphere with a scan rate of 20 °C/min and an SDT Q600 thermogravimetric analyzer.

Synthesis of polyamides (Pa-Pc) 2.5.1. Synthesis of (Pa)
The stirring mixture of 0.37 g (0.03 mole) of p-phenylenediamine,1.50 g (0.03 mole) of the monomer (Ma), 4.0 mL of (TPP), 0.5 g of calcium chloride, 4.5 mL of (NMP) and 1.5 mL of pyridine was heated at 80°C for 4 h.The collected of high viscosity polymer was poured slowly in 100 mL of stirring cold water which formed stringy precipitate collected after filtration, washed by hot water and dried in thermal oven for 24 h [27].(65 % yield).The FTIR spectrum band of amide group was at 3312.25 (N-H stretching), and 1648.07 cm -1 (amide carbonyl).By using same above method, other polyamides (Pb) and (Pc) were prepared.
The synthesis routes of polyamides (Pa-Pc).

Synthesis of polyamides (Pd-Pf)
These polyamides synthesized by same above method but using di-phenylene diamine instead of phenylene diamine Scheme 4.

Polyamides Synthesis
All the new aromatic polyamides bearing triphenyl amine (TPA) group are synthesized via polycondensation technique.The reaction of synthesized dicarboxylic acid monomers with two types of aromatic diamines (p-phenylenediamine and benzidine in the presence of pyridine and triphenylphosphite (TPP) as promote condensation reaction and CaCl2 addition to decrease the hydrogen bonding in the polyamide chains.This leads to increase the solubility of these polyamides.The structural form of the synthesized (Pa-Pf) were confirmed by FTIR spectra which exhibited absorption bands around (3310-3313) cm -1 (N-H stretching) and (1644-1648) cm -1 (C=O stretching of amide group) as shown in Figure 4.The 1 H NMR spectrum of (Pa) showed that the broad peaks of COOH proton groups appeared at 12.37-12.78.The ppm is disappeared with appearance of singlet peak at 10.21-10.The figure also shows the 13 C NMR spectra of the polyamide (Pa), the main peak at 164.63 ppm is attributed to the carbonyl carbon atom of the polyamide.This carbon atom is with bigger chemical shifting resulting from the carbonyl group resonance with attached to a nitrogen atom with highly electronegative. 13C NMR peaks are assigned to the carbon atoms 1, 3, and 4 of the first ring of the TPA at the regions 115.56 (C1), (131.23)C3 and 143.62 (C4) respectively.The second ring has other peaks at the regions 127.84 (C2), 151.12 (C 5 ), 1119.04(6),131.71 (C7) and 124.62 (C8).Tow peaks assigned to the two equivalent carbon atoms 9 at the region 131.71 and of four carbon atoms 10 at the region 116.30 in the phenylene diamine ring group.One peak for tow carbon atoms of the methyl groups at the region 17.61.

Properties of the synthesized polyamides 3.3.1. Thermal properties of the polyamides
The thermal behaviors of all polyamides were checked via DSC and TGA technique.DSC measurements are investigated with ACAN rate of 20 o C /min in N2.They gave high (Tg) in the average of (245.5 -278.8)°C.The thermal decomposition behavior of the synthesized poly (amine amide) is evaluated by TGA analysis with a heating rate of 20 °C /min in nitrogen atmosphere.The lowest onset temperature in which the polyamides started to decompose was 305.6 °C up to 365 °C for samples.Polyamides (Pd), (Pe) and (Pf) revealed onset degradation temperature (T d ) at the range (305-330) °C because of their flexible chain comparison with the rigid chain polyamides (Pa), (Pb) and (Pc) which showed higher onset (T d ) at the range (335-362) °C.The 10% weight loss temperatures (T d 10%) were investigated in the range of (380-420) °C for the flexible polyamides and (442-466) °C for rigid polyamides.The synthesized polyamides exhibited residual yields about (43-54) % at 800°C, Figure 7. and Table 1.shows the thermal data of the polyamides.
gave lower UV-Vis λabs max absorption compared with (Pa), (Pb) and (Pc) because they have extra (π-π*) transitions of further diphenylene with (n-π*) electronic transitions so they need excess absorbance energy for transition than polyamides with phenylene diamine which require low absorption energy Figure 8. Solution of polyamides in DMSO gave PL emission bands in the blue region at the average (438 -462) nm (Figure 9).PL spectra is assignable from the low inter and intra molecular electronic interactions.The large triphenyl unit led to decrease the charge transfer formation within or between polymer chains by steric hindrance.Polyamides (Pa), (Pb) and (Pc) have convergent and higher λPL values rather than in polyamides (Pd), (Pe) and (Pf) diphenylene diamine in which increase the excitation of π-electrons which leads to decrease the λPL.Table 2 shows optical behaviors data of the polyamides.

Electrochemical Properties
Cyclic voltammetry (CV) was used to verify the redox properties of synthesized polyamides.The method supports on synthetic a working electrode as anode by coating a special glass plate containing (TiO) by a thin layer of the polyamide.This electrode is a part of the electrolytic cell with external cathode electrode Ag/AgCl.The used electrolyte in this proccese is dry CH3CN contains 0.1 M of tetrabutylammonium perchlorate (Bu4NH4ClO4).All polyamides showed the half-wave at oxidation potential in the range E1/2 = (1.13-1.28)V with scan rate at 0.2 V/s to remove two unpaired electrons from the nitrogen atom at each repeating triarylamine unit to form stable polymide 2+ [29].By change the potential scans between 0.0 -1.51 V lead to conversion the pale yellowish color to the blue (Figure 11.HOMO and LUMO) energy levels of the compatible polyamides is calculated from the oxidation half-wave potential E1/2 and the onset absorption wavelength of the UV-Vis absorption (λabs onset) of the solution [30].The result data are listed in Table 3.The external ferrocene/ferrocenium (Fc/Fc+) redox standard E1/2 was 0.41 V vs Ag/AgCl in CH3CN and the HOMO energy for the Fc/Fc+ standard was 4.80 eV.Energy gab (Eg) can be calculated by the equation: Eg = 1242/ λabs, onset (1) Where 1242 is a constant, λabs onset is the onset wavelength which can be calculated by intersection of two tangents on the absorption edges as shown in Figure 10.  3.

Conclusion
New amorphous polyamides having triarylamine group in their backbone structure have been successfully synthesized from the reaction of aromatic dicarboxylic acid with aromatic diamines.All these polymers were characterized by excellent thermal properties such as glass transition (Tg) was in the range (245.5-278.8)°C, its thermal stability was also very high.Polyamides with mono phenylene diamine showed a high Tg value due to its high hardness of single phenylene amine compared with than di phenylene diamine with lower stiffness and rigidity.The optical properties (Uv-Vis) and photoluminance spectroscopy were analyzed for the solutions of all polyamides in concentration10 -5 M in DMSO which showed strong UV-Vis absorption at the ranges (301.4 -364.3)nm and PL emission ranges at about (438-462) nm.All polyamides showed halve oxidative wave at a scan rate of 0.2 V / s to convert the original yellowish color to the blue.

Figure 3 .
Figure 3.shows the proton of carboxylic group appeared as a very broad singlet peak and hardly observable resulting from the substitution of carboxylic group hydrogen atoms by deuterium of DMSO solvent and H-bonding formation, also the disappearance of the cyano carbon at 118.62 ppm and appearance of the carboxylic peak at 166.72 ppm.
28 after polymerization is assigned to the (Pa) amide group protons.The doublet peaks at 7.23-7.36ppm and 7.86-7.98ppm are resulting from the four protons b and c in the triphenylamine group respectively.Singlet peak in the region 7.72 ppm is assign to four equivalent protons d of the phenylene diamine groups.Singlet peak in the region 7.45 is resulting from 2 protons a in the ring of triphenylamine group Figure 5.

Figure 5 :
Figure 5: 1 H and 13 C NMR spectrum of the polyamide (Pa).

Figures 6
shows the (Tg) values of all polyamides (Pa) -(Pd) and (Pe) -(Pf) respectively.In general, if the diamine component in the polyamides decreases leads to decrease the stiffness and the (Tg) values.Polyamides with phenylene diamine showed high Tg value resulting from increasing of chain rigidity compared with polyamides with diphenylene diamines.It is agreeing with the values of glass transitions (Tg) of the polyamides (Pa), (Pb) and (Pc) with the rigged chain which have higher (Tg) than other polyamide (Pd), (Pe) and (Pf) with lower stiffness chain because of their flexible diamine diphenylene component.

Table 3 :
electrochemistry data for all the synthesized polyamides.