Conducting poly(azomethine)esters: synthesis, characterization and insight into the electronic properties using DFT calculations

Abstract

A poly(azomethine)ester was synthesized via solution phase polycondensation of isophthaloyl chloride and preformed 4-((2-(4-hydroxibenzylideneamino)ethylimino)methyl)phenol (SB). Various aliphatic and aromatic moieties were incorporated in the parent chain to process them and for examining their effect on the conducting properties. To provide an insight into the electronic properties, DFT calculations at the 6-31G/B3LYP level were carried out. Emphasis was placed on exploring the Frontier electron density, energy gaps and electrostatic potential maps to predict their conducting behaviour and electrophilic/nucleophilic reactivity. Our results based on experimental data and theoretical studies showed that in spite of some discrepancies, the electronic properties can be approximated theoretically to design a material having the desired properties for organic electronic devices. The material was characterized by FTIR, ¹H NMR spectroscopic studies and elemental analysis.

---

Introduction

Conducting polymers are the most recent generation of material that has facilitated the understanding of the fundamental chemistry and physics of π bonded macromolecules. The desire to develop new conducting macromolecules with anticipated properties began to attract interest from synthetic chemists in the 1980s. Conducting materials like poly(azomethine)s, in spite of having a wide variety of applications, have a disadvantage that they are insoluble in most common organic solvents owing to their strong chain–chain interactions.^1–4 To overcome this problem of insolubility, different tactics are used such as the introduction of flexible aliphatic spacers in the main chain, pendent alkyl groups along the backbone, co-polymerization of different soft groups (aliphatic, alkyl or siloxanes) and composite formation.^5–8 Another approach is the supra-molecular modifications/dopant engineering of material employing organic sulfonic acids, organic esters of phosphoric acids, sulfophthalic acid, or various substituted phenols.^9–11 In addition, charge injection into the conjugated, semiconducting macromolecular chains by doping a suitable metal into a polymer matrix could lead to enhanced electrical properties. Similarly, polyaniline (PANI) blends could be made to improve the conductivity of material.^12–17 However, the chemistry and physics of these polymers in their non-doped semiconducting state are also of great interest because they provide a route to plastic electronic devices. These novel properties enable a number of applications including LEDs, electrochromic material, photo-detectors, and photovoltaic cells.

Nowadays, these polymers can be designed theoretically employing density functional theory (DFT) calculations of their optimized molecular geometries. Performing DFT calculations on conducting polymers is challenging because of their size (large number of atoms); however, they can be approximated using oligomeric/repeat unit model compounds. Addition of different sequences to the chain creates wide possibilities for the modification of their properties.¹⁷

To the best of our knowledge, there are only few articles describing the structural elucidation and predictive study of electrical properties (complemented by experimental data) of newly synthesized macro chains.¹⁷ Thus, we report herein, the synthesis and characterization of poly(azomethine)ester (PAME) and a variety of aliphatic/aromatic terpolymers. The structure–property relation in terms of electrical conductivity is discussed. G0W3 was employed for the 3D electron density mapping of the chains. ΔE along with HOMO/LUMO coefficients were calculated and compared with the experimental data.

Synthesis of the monomer SEM-EDX of a neat and doped material is given in the ESI,† whereas the thermal behaviour and molecular weight by laser light scattering will be discussed in our future article.

Results and discussion

The Schiff base containing terminal diol was prepared according to a previously published procedure given in ESI.^18,19† PAME (PI) was obtained in good yield using low temperature solution phase polycondensation of monomers isophthaloyl chloride and 4-((2-(4-hydroxibenzylideneamino)ethylimino)methyl)phenol (SB), as shown in Scheme 1. A wide variety of terpolymers were prepared by adding different diol-based moieties in the macrochain using a one-pot three-reactant reaction in an in situ process to improve the solubility of a material, as shown in Scheme 2. The material was made soluble by protonating it with p-sulphonic acid. The reaction was carried out at atmospheric pressure and low temperature to avoid any side reaction and decomposition of thermally sensitive monomers.¹⁹ The FTIR and ¹H NMR spectroscopic studies were used to confirm the functionalities present in the synthesized monomer (SB), polymer (PI) and terpolymers (PIF, PIB, PIH, PIPr, and PISi).

Scheme 1 Synthesis of poly(azomethine)ester (PI).

Scheme 2 Synthesis of the ter-poly(azomethine)ester (PIF, PIB, PIH, PIPr and PISi).

PAMEs were found to be insoluble in all the organic and inorganic solvents owing to the presence of bulky aromatic groups in the main chain.^18,19 In an attempt to process the material, different commercial diols (Pr, H, Si, B or F, Scheme 2) containing aromatic, semi-aromatic, silyl and fluoro moieties were assimilated in the parent polymer chains (PI) to improve their solubility in organic or inorganic solvents.¹⁹ Subsequently, five aliphatic/aromatic terpolymers were synthesized, which were still insoluble in any solvent. After various attempts, eventually, the material was solubilized by supra molecular modification employing p-toluene sulphonic acid.⁹ The macrochains become completely soluble in DMSO and sparingly in THF, ethanol and methanol.

The structure of the synthesized PI was confirmed by the presence of characteristic absorption peaks in their respective areas. Significant changes were observed in the spectral properties of initial compounds and products as some of the signals disappeared and some new signals appeared. Stretching vibration for (C=O) and (C–O) appearing in the range of 1720–1750(s) cm⁻¹ and 1101–1200(s) cm⁻¹, respectively, confirmed the presence of ester linkage in PI. An additional peak around 1600–1645(s) cm⁻¹ confirmed the presence of azomethine linkage in the macrochains. Two symmetric peaks in the ester and azomethine region in each terpolymer confirm the successful incorporation of diol moieties in the parent chain. The FTIR spectra showed characteristic peaks related to the added diols in addition to the one common in all the spectra. The aliphatic absorption band (C–H aliphatic) appeared around 3000–2900 cm⁻¹ in the spectra of PIH and PIPr. The characteristic FTIR absorption peaks for the (Si–O–Si) group present in silanol containing polymers like PISi appeared as doublet around 1020 cm⁻¹ and 2900 cm⁻¹. The presence of aromatic C–H in polymers, such as PIF and PIB, was confirmed by the appearance of a peak around 3100 cm⁻¹. Occurrence of C–Cl in the range of 780–540 cm⁻¹ along with the disappearance of the hydroxyl group peak in PAME and its condensation terpolymers revealed that they have an acid chloride group at the terminal.^18–20

¹H NMR spectroscopy was also used for the structural analysis of PAMEs. The study was performed after the protonation of samples with p-toluene sulphonic acid (dopant engineering) in DMSO, using TMS as the internal reference. ¹H NMR studies showed the presence of all types of protons expected for the proposed structures. Resonance in the range of 8.5–7.7 ppm, common for all spectra, was attributed to the presence of the HC=N proton, whereas the signal that appeared in the range of 6.9–7.9 ppm showed aromatic protons. In addition to these signals, which were common in all the spectra, the polymer having aliphatic diols (Pr and H) showed multiple signals in the range of 0.7–2.3 ppm (aliphatic –CH). The polymer (PIB) showed additional signals in the range of 7.0–8.0 ppm (additional aromatic rings of the added diols) and 2.3 ppm (–CH₃ present in bisphenol A). PIF showed resonance around 7.4 to 8.1 (aromatic rings), slightly deshielded owing to the presence of electronegative F (CF₃) in the added diol. The silanol based material exhibited additional resonance peaks at 1.9–2.1 (CH₃ attached to Si). The signal present in the range of 2–2.5 (–CH₃) and 2–2.2 (OH) confirm the protonation in the main chain. OH group appeared at the lower field owing to the magnetic anisotropy of the aromatic ring present in the p-toluene sulphonic acid group.^18–20

The calculations for the elemental analysis of polymers were done on the basis of the structure of repeat units present in the polymer chain. The data for C, H and N content in the polymers were found to be in relatively good agreement with the presumed structure for the condensation terpolymer. The Si in the copolymers having dimethylsiloxane reacts with C at a high temperature to make a ceramic material. Therefore, their elemental analysis was not possible^19,20 and such polymers are currently investigated as precursors to ceramic materials.

Conductivity measurements

The material was pressed in pellets of equal dimensions to study the improvement in their conducting behaviour after doping and blending. It is known that the valence electrons bound in sp³ hybridized sigma-bonds have low mobility and do not contribute to the electrical conductivity of the material. However, in conjugated materials, the situation is completely different as they have backbones of connecting sp² hybridized carbon centres. One valence electron on each centre resides in a p_z orbital. When the material is doped by oxidation, it removes some of the delocalized electrons, which have high mobility. Thus, the conjugated p-orbitals form a partially empty one-dimensional electronic band having mobile electrons. Electrical conductivity of neat macrochains, polymer/PANI blends and silver doped materials at room temperature was measured, and the data are given in Table 1.

Table 1 Conductivity measurement of neat, doped and blended material at room temperature

Codes	ΔE eV	Holes	LUMO coefficients
a Neat.b Silver doped.c Polymer blends. Conductivity of PANI at R.T is 1 S cm^−1.
^aPI, N = 4.1 × 10^−14, ^bSD = 3.44 × 10^−5, ^cPB = 1.54 × 10^−3	LUMO = −0.0824, HOMO = −0.1949, ΔE = 0.11	41	−0.032(3C), 0.033(11C), −0.046(13C), 0.030(15C), −0.021(16C), 0.35(37O), 0.002(37O), −0.0002(38O), −0.04(41H), −0.79(42H), −0.71(43H), −0.64(44H), −0.15(45H), 0.0033(47H), −0.0008(48H), 0.016(49H)
^aPIF, N = 4 × 10^−14, ^bSD = 3.4 × 10^−4, ^cPB = 3.5 × 10^−3	LUMO = −0.02051, HOMO = −0.09950, ΔE = 0.08	69	−0.012(41O), −0.0008(49H), 0.0089(50C), 0.077(51C), −0.056(53C), −0.11(62C), 0.35(64C), −0.11(66C), −0.09(71O), 0.58(73C), −0.13(76F), −0.3(77F), 0.27(78F), 0.03(79C), 0.076(81F), −0.012(41O), −0.0006(49H), 0.009(50C), 0.077(51C), −0.05(53C), −0.104(62C), 0.38(64C), −0.103(66C), −0.09(71O), 0.5(73C), −0.16(74C), −0.39(76F), −0.29(77F), 0.25(78F), 0.033(79F), 0.16(81F)
^aPISi, N = 5 × 10^−15, ^bSD = 4.5 × 10^−7, ^cPB = 4.0 × 10^−6	LUMO = −0.13330, HOMO = −0.171254, ΔE = 0.04	113	0.35(11C), 0.37(13C), 0.38(15C), 0.12(16C), 0.002(H19), 0.004(21C), 0.0045(22C), 0.0006(23C), 0.01(24C), 0.00105(25C), 0.00105(25C), 0.20(26C), 0.0006(27H), 0.002(28H), 0.0022(29H), 0.005(30H), 0.8(31C), 0.021(32C), 0.78(34C), 0.02(35C), 0.0025(36C), 0.137(37N), 0.363(38N), 0.005(39O), 0.2(40O)
^aPIPr, N = 5.9 × 10^−16, ^bSD = 6.4 × 10^−8, ^cPB = 6.1 × 10^−7	LUMO = −0.09850, HOMO = −0.14642, ΔE = 0.05	53	−0.005(11C), −0.007(12C), 0.02(13C), −0.05(14C), 0.03(15C), −0.016(16C), −0.007(17H), −0.005(18H), 0.01(19H), −0.0077(20H), 0.01(21C), 0.44(22C), −0.34(23C), 0.23(24C), 0.32(25C), −0.56(26C), −0.009(27H), 0.0049(28H), 0.005(29H), 0.04(30H), 0.29(31C), 0.16(32C)
^aPIH, N = 4.1 × 10^−15, ^bSD = 5.4 × 10^−6, ^cPB = 5 × 10^−7	LUMO = 0.0012, HOMO = −0.04434, ΔE = 0.042	57	0.016(50C), 0.002(51H), 0.0013(52H), 0.002(53H), −0.00143(54H), −0.002(55H), −0.009(56H), 0.0091(57H), 0.0032(58C), −0.0020(59C), 0.00064(60C), −0.0005(61O), −0.0001(62H), −0.00043(63H), −0.0003(64H), 0.0006(65H), −0.0006(66H), 0.00116(67H), −0.00028(68H), −0.00055(69H), 0.00013, −0.049(70C)
^aPIB, N = 1.5 × 10^−13, ^bSD = 1.9 × 10^−4, ^cPB = 1.5 × 10^−2	LUMO = −0.09312, HOMO = −0.12752, ΔE = 0.04	75	−0.00003(1C), −0.0037(2C), −0.003(3C), −0.0036(4C), −0.00036(5C), 0.0036(6C), −0.00006(7H), 0.00013(8H), −0.00062(9H), −0.00014(19H), −0.0004(11C), −0.0004(12C), −0.003(13C), 0.115(21C), −0.064(22C), −0.005(23C), 0.05(24C), −0.038(25C), −0.025(26C), 0.00145(27H), 0.00063(28H), 0.00034(29H), 0.0054(30C), −0.0082(31C), 0.004(32C), 0.001(33C), −0.07(34C), −0.00004(N), −0.0007(36N), 0.02(38O), 0.055(39O), 0.103(40O), −0.011(41O), −0.002(44H), 0.00135(45H), 0.00132(46H), 0.0003(48H), 0.0031(49H), −0.1(51C), −0.114(52C)

---

The influence of the structure on the electrical properties of condensation terpolymers was also studied. The material with continuous conjugation in the chain (PI and PIB) showed maximum conductivity; however, PIF has relatively low conductivity owing to the presence of the highly electronegative hexafluoroisopropylidene group in the macrochain. The polymers based on methylene and silyl spacers have the lowest conductivity among all, which might be because of the suspension of conjugation in the chains due to the presence of these moieties. The study also showed that the lower conductivity of the neat material, as the undoped polymers, usually have an energy gap of >2 eV, which is too high for conduction.^11–13 Since most organic polymers do not have intrinsic charge carriers, the required charge carriers could be introduced in the material either by partial oxidation (p-doping) with electron acceptors (e.g. I₂ and AsF₅) or by partial reduction (n-doping) with electron donors (e.g. Na and K). Through a doping process, charged defects (e.g. polaron, bipolaron and soliton) could be introduced in the structures, which then serve as charge carriers. For the presented work, all the polymers were doped with silver involving the removal of electrons from the HOMO of poly(azomethine)esters and thus forming positive hole and counter anions. Elimination of two electrons from the same energy level form a bipolaron, which causes a decrease in the band gap and thus, increase in the conductivity of the material.^12,13

Polymer matrix was also blended with PANI. The trend of conductivity was found to be the same as was in a neat material; however, the material blended with PANI was found to be more conducting as compared to the silver doped material, which was in accordance to the literature, Table 1.

In spite of intensive research, the relationship between morphology, chain structure and conductivity is still poorly understood. However, it is assumed that conductivity should be higher for the higher degree of crystallinity and better alignment of the chains; however, this could not be confirmed for PANI, which is amorphous in nature. To explain whether the trend observed in conductivity can be explained theoretically considering the molecular properties, DFT calculations were carried out.

DFT calculation results

The objective of this study was to understand the relationship between the chemical structure and the conduction properties of polymers, which could help in designing a material with the desired properties. Quantum chemical calculations offer a good understanding of the connection between the molecular properties and the conductivity of a material. All valence MO calculations, for the repeat unit of each poly(azomethine)ester, were carried out to predict the various parameters in the π-conjugated system to explain the observed conductivity trend. It is assumed that electrical properties (conductivity) in the polymer are correlated to the difference in energy (ΔE) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).^16,16–23 The electron(s) from the highest occupied molecular orbital (HOMO) have the ability to populate the lowest unoccupied molecular orbital (LUMO) in the excited state resulting in conduction. The variation in the electrical conduction of the polymers with the calculated band gap is in the order PIB > PI ≈ PIF > PISi > PIH > PIPr; however, the band gap ΔE does not seem to obey the same trend, Table 1. These results showed that the band gaps alone could not explain the observed conductivity results; therefore, other parameters need to be investigated.

Based on literature, in addition to ΔE, the carrier movement and the movement of holes are also involved in the conduction process, where holes (theoretically) are regions where there is no electron density and the coefficients (mixing ratio) of atoms involved in HOMO are zero. In addition, the coefficients of atoms involved in LUMO are also important as they facilitate the carrier movement up to the point, and it is also known that the higher the coefficient, the greater is the electron density on excitation. Zero or very low LUMO coefficients are visualized as impaired carrier movement.

To explain the results, the coefficients of the atoms involved in HOMO were examined to study the role of intrinsic holes (zero coefficient shows the presence of a hole) as they seem to contribute in the conduction process. Since there is significant involvement of LUMO in conduction, these coefficients were also investigated, as shown in Table 1.

The highest conductivity observed in polymer, PIB, can now be explained by considering its low band gap, number of HOMO (holes) and contribution of LUMO. Both PI and PIF have approximately the same conductivity although both differ in their band gaps. The good electrical conductivity of these polymers may be due to the presence of holes in HOMO and the significant LUMO electron density at the connecting atoms (centers). The polymer PISi showed less conductivity although it has a ΔE value of 0.04, which might be due to the presence of low electron density at some centers in the chain. Thus, low eigen values are responsible for the lower conductivity of polymer PISi as compared to others as expected. The polymers PIPr and PIH have low ΔE (0.05 a.u. and 0.04 a.u., respectively), which indicate an easier excitation of electrons from HOMO to LUMO. However, the conductivity of these polymers was low, i.e., 5.9 × 10⁻¹⁶ and 4 × 10⁻¹⁵, respectively, as compared to the PIB.^22,23

Our calculations showed that a low band gap does not ensure facile conduction as no correlation was found in the band gap and conductivity. The results suggest that the presence of intrinsic holes as well as significant contribution from the atoms in LUMO also contribute to the conductivity (Fig. 1).

Fig. 1 3D HOMO/LUMO orbitals of synthesized material.

The electrostatic potential V(r) maps are well-known for the identification of one species by the other; thus, this methodology can be used to evaluate the electronic distribution around the molecular surface for the macrochains. The electrostatic potential has been used primarily for predicting sites and relative reactivities towards electrophilic attacks, and in the studies of biological recognition and hydrogen bonding interactions. It can be seen that the macromolecules are stable with an almost uniform distribution of charge density. The oxygen and nitrogen atoms are surrounded by a greater negative charge surface, making these sites potentially more favorable for an electrophilic attack (red). Among these two sites, the nitrogen atoms having higher values are more susceptible to attacks, which support the H bonding of p-toluene sulphonic acid in dopant engineering. Similarly, those with minimum electron density are prone to nucleophilic attack (blue color), as shown in Fig. 2. The representative electrostatic potential data for PI are given in Table 2.^20–23

Fig. 2 Electrostatic potential map, PI: yellow color around N and O atoms indicates an electron rich area prone to the electrophilic attack.

Table 2 Electrostatic potential data for PI

Atoms	Electrostatic potential	Atoms	Electrostatic potential	Atoms	Electrostatic potential
C34	0.0027	C14	−0.0005	C13	−0.0065
O39	−0.026	C15	−0.007	O49	0.002
O40	−0.007	C16	−0.015	N32	−0.02
H51	0.0165	C11	−0.0174	H44	−0.028
C23	−0.0013	C12	−0.0106	C48	0.007
C21	0.0038	H18	−0.012	N36	−0.0065
C26	0.001	H17	−0.0174	H50	0.009
C25	−0.00162	H19	0.0074	H46	−0.00023
C29	−0.0025	H20	0.0055	H27	0.0138
H28	0.0072	C33	−0.010

---

Experimental

Isophthaloyl chloride (m.p. = 43–44 °C), 4-hydroxybenzalde (m.p. = 112–114 °C), 4-aminophenol (m.p. = 188–190 °C, Fluka), 1,2-ethylenediamine (206 °C, Sigma Aldrich), p-toluene sulphonic acid (monohydrated, m.p. = 98–102 °C, Fluka), 1,3-propan-diol (211–217 °C, Sigma Aldrich), 1,6-hexan-diol (250 °C, Sigma Aldrich), poly(dimethylsiloxane), hydroxyl-terminated (n = 550) (Sigma Aldrich), (1,1,1,3,3,3-hexaflouro)bisphenol propane (160–163 °C, Sigma Aldrich) and bisphenol A (158–159 °C, Sigma Aldrich) were used as received. The solvents dichloromethane, ethanol (Sigma Aldrich), and dimethyl sulfoxide (Sigma Aldrich) used were purified according to the standard reported method.²⁴

Computational details

All MO calculations for the studied compounds were carried out for a repeat unit of each polymer. An exchange functional proposed by Becke in 1988 using a gradient-corrected correlation functional of Lee, Yang and Parr was employed (B3LYP) using the minimal basis set 6-31G, considering the computational cost due to the large size of the repeat unit. Gaussian 03 program package was used to evaluate some of the useful molecular descriptors.

Methods

Synthesis of 4-((2-(4-hydroxibenzylideneamino)ethylimino)methyl)phenol (SB). 4-((2-(4-Hydroxibenzylideneamino)ethylimino)methyl)phenol (SB) was prepared according to the reported procedure.^18,19

Polycondensation

Synthesis of polymers: (PI). Stoichiometric amount (1 : 1) of SB was added to 50 mL dried dichloromethane in a two necked round bottom flask fitted with a reflux condenser, magnetic stirrer, hot plate and gas inlet. The temperature of the reaction mixture was maintained at 0 °C using an ice bath. Isophthaloyl chloride(I) was added to the flask followed by the addition of 3–4 mL triethylamine with constant stirring. The reaction mixture was stirred for 24 h at room temperature and then refluxed for 1 h. Yellow colored precipitates of the obtained polymer were filtered, washed several times with water and ethanol, dried in air and weighed, Scheme 1.^18,19
IP. 398, yellow powder, 88%, FTIR (cm⁻¹, KBr) 3124 (arom-CH), 2956 (aliphatic C–H) 1763 (–CO–), 1634 (–N=CH–), 1076 (–C–O–), 747 (C–Cl). ¹H NMR [δ, deuterated dimethyl sulfoxide (DMSO-d₆)] δ (ppm) (protonated): 8.42 (2H, s, azomethine), 7.4–6.8 (m, aromatic-H), 2.3 (6H, m, methylene), 2.01 (2H, s, alcohol), 1.8–1.2 (1H, s, methylene). Elemental analysis: calcd (C 72.61, H 4.52, N 7.03) found; (C 72.51, H 4.32, N 7.10).

Synthesis of terpolymers: (PIF, PIB, PISi, PIH and PIPr). The monomer SB and the commercial diol (R = 1,3-propan-diol, 1,6-hexan-diol, poly(dimethylsiloxane), hydroxyl-terminated (M_n = 550), (1,1,1,3,3,3-hexaflouro)bisphenol propane or bisphenol A) were taken in a two-necked round-bottom flask under inert atmosphere (N₂), Scheme 2. 50 mL dried dichloromethane (CH₂Cl₂) was added to the flask followed by 5 mL triethylamine (Et₃N) at a low temperature using an ice bath with constant stirring. Then, diacid chloride I was added to the reaction mixture under the same reaction conditions. The reactant ratio was 1 : 1 : 2 for SB, the used diol (R), and I. After 24 h, the reaction mixture was refluxed for 1 h and then poured into water for removing the triethyl ammonium chloride salt (Et₃NHCl) from the organic layer. Polymers were obtained as precipitates, which were then washed several times with water and ethanol and then dried in air,^18,19 Scheme 2.
PIF. 712, light yellow powder, 92%, FTIR (cm⁻¹, KBr): 3212 (arom-CH), 2989 (aliphatic CH) 1752 (–CO–), 1756 (–CO–), 1085 (–O–), 1088 (–O–), 1643 (–N=CH–), 751 (C–Cl). ¹H NMR [δ, deuterated dimethyl sulfoxide (DMSO-d₆)] δ (ppm) (protonated): 8.7 (2H, s, azomethine), 7.8–7.4 (aromatic, m), 2.6–1.7 (m, methyl, methylene, alcohol). Elemental analysis: calcd; (C 65.4, H 3.6, N 3.9), found; (C 65.7, H 3.69, N 3.91)
PIB. 608, light yellow powder, 90%, FTIR (cm¹, KBr): 3109 (arom-CH), 2985 (aliphatic-CH), 1732, 1737 (C=O), 1076, 1081 (–CO–), 1609 (–N=CH–), 749 (C–Cl). ¹H NMR [δ, deuterated dimethyl sulfoxide (DMSO-d₆)] δ (ppm) (protonated): 8.6 (2H, s, azomethine), 7.4–7.1 (aromatic, m), 2.3–1.4 (m, methyl, methylene, alcohol). Elemental analysis: calcd; (C 77.01, H 5.69, N 4.6); found; (C 77.0, H 5.69, 4.67).
PIH. 498, light yellow powder, 90%, FTIR (cm¹, KBr): 3173 (arom-CH), 2967 (aliphatic-CH), 1730, 1735 (C=O), 1033, 1035 (–CO–), 1654 (–N=CH–), 743 (C–Cl). ¹H NMR [δ, deuterated dimethyl sulfoxide (DMSO-d₆)] δ (ppm) (protonated): 8.23 (2H, s, azomethine), 7.5–6.8 (m, aromatic), 2.6–1.7 (m, methylene, methyl, alcohol). Elemental analysis: calcd; (C 72.30, H 6.02, N 5.62) found; (C 72.2, H 6.16, N 5.62).
PIPr. 456, light yellow powder, 92%, FTIR (cm¹, KBr): 3193 (arom-CH), 2989, 1732, 1735 (C=O), 1042, 1047 (–CO–), 1654 (–N=CH–), 741 (C–Cl). ¹H NMR [δ, deuterated dimethyl sulfoxide (DMSO-d₆)] δ (ppm) (protonated): 8.3 (2H, s, azomethine), 7.5–7.2 (aromatic, m), 2.4–2.0 (m, methyl, methylene, alcohol). Elemental analysis: calcd. (C 72.01, H 5.26, N 6.14); found; (C 72.1, H 5.34, N 6.23).
PISi. Light yellow powder, 89%, FTIR (cm¹, KBr): 3212 (arom-CH), 2989 (aliphatic-CH), 1752, 1756 (C=O), 1085, 1088 (–CO–), 1643 (–N=CH–), 747 (C–Cl). ¹H NMR [δ, deuterated dimethyl sulfoxide (DMSO-d₆)] δ (ppm) (protonated): 8.2 (2H, s, azomethine), 7.2–6.6 (m, aromatic), 2.1–1.6 (m, methyl, methylene, alcohol).

Doping and blending

Silver was incorporated (by wt%) into the matrix of PAMEs and their terpolymers according to the reported method.¹⁶ EDX analysis was carried out qualitatively to check the silver incorporated in the polymer matrix.

PANI was prepared as reported previously.^16,17 PAME/PANI was taken in the weight ratio percentage of 9 : 1. The powder was ground, blended in a crucible for homogenous mixing and dried in oven at 40 °C for 6 h. The material doped with silver or blended with PANI was compressed into pellets under a 5 ton load for conductivity measurements at room temperature.^16,17

Instrumentation

Melting point was determined on a Mel-Temp. (Mitamura Riken Rogyo, Inc.) using open capillary tubes. FTIR spectra in KBR pellets were recorded on a Bio-Rad FTS-60A series FTIR spectrophotometer. Nuclear magnetic resonance was carried out by Bruker Avance 300 digital NMR using DMSO-d₆ as the solvent and tetramethylsilane as the internal standard. Elemental analyses were obtained on a Vaio-EL instrument. X-ray diffraction was carried out on a Philip X-Pert PRO 3040/60 diffractometer equipped with a Cu Kα radiation source (λ = 1.54 A) operated to characterize the solid samples. SEM-EDX studies were carried out by model JSM 6460 and EDX OXFORD model 7573. Electrical conductivity was measured by Keithley 2400 at room temperature. G03W was employed for density function theory (DFT) calculations at the 6-31G/B3LYP level to evaluate the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and to locate the charge distribution and generate the electron density maps.¹⁵

Conclusions

The electrical properties of a newly synthesized, conducting PAME and its terpolymers comprising various soft and hard moieties in the chain were evaluated successfully. The polymers were doped with silver and blended with PANI; it was found that a lesser amount of PANI is required to increase the conductivity of the material. In contrast to some approaches that attempt to lower the band gap by increasing the aromaticity of the backbone, we showed that being aromatic in itself does not ensure a low band gap. However, the data could help in designing a material with a small band gap and more electron density and holes, resulting in significant technological progress in the next generation organic light emitting diodes (OLEDs) and flexible photovoltaic materials.

Acknowledgements

We acknowledge the financial and moral support extended by Higher Education Commission (HEC) Pakistan.

Notes and references

1. L. Marin, V. Cozan and M. Bruma, Polym. Adv. Technol., 2006, 17, 664–672.
2. P. K. Gutch, S. Banerjee, D. C. Gupta and D. K. Jaiswal, J. Polym. Sci., Part A: Polym. Chem., 2001, 39, 383–388.
3. J. Miyake and Y. Chujo, Macromolecules, 2008, 41(15), 5671–5673.
4. L. Marin, V. Cozan, M. Bruma and V. C. Grigoras, Eur. Polym. J., 2006, 42, 1173–1182.
5. G. Wenz, M. B. Steinbrunn and K. Landfester, Tetrahedron, 1997, 53, 15575–15592.
6. A. Haraha, Adv. Polym. Sci., 1997, 133, 92–141.
7. D. Whang and K. Kim, J. Am. Chem. Soc., 1997, 119, 451–452.
8. M. Grigoras and A. Farcas, J. Optoelectron. Adv. Mater., 2000, 525–530.
9. A. Iwan and D. Sek, Prog. Polym. Sci., 2008, 33, 289–345.
10. C. G. B. Garrett and N. B. Hannay, Semiconductors, New York, 1959, ch. 15.
11. A. N. Terenin, Mendeleeva, 1960, 5, 498.
12. A. N. Terenin, Proc. Chem. Soc., 1961, 321.
13. A. N. Terenin, Symposium on Electrical Conduction in Organic Solids, ed. H. Kallman and M. Silver Durham, 1961, p. 39.
14. L. E. Lyons, Physics and Chemistry of the Organic Solid State, Interscience, 1963, p. 745.
15. M. W. Byan and G. J. Cordarol, J. Phys. Chem. C, 2011, 115, 18333–18341.
16. I. Kaya and S. Koça, Int. J. Polym. Mater., 2007, 56, 197–206.
17. L. R. Kumar, V. Sengodan, M. B. Prasad, K. Gopalakrishnan and K. Sethupathi, Mater. Lett., 2002, 167–174.
18. A. Gul, Z. Akhter, A. Bhatti, M. Siddiq, A. Khan, H. M. Siddiqe, N. K. Janjua, A. Shaheen, S. Sarfraz and B. Mirza, J. Organomet. Chem., 2012, 719, 41–53.
19. A. Gul, Z. Akhter, M. Siddiq, S. Sarfraz and B. Mirza, Macromolecules, 2013, 46, 2049–2059.
20. K. Fukui, T. Yonezawa and H. J. Shingu, J. Chem. Phys., 1952, 20, 722–725.
21. K. D. C. Politzer, The Force Concept in Chemistry, Van Nostrand Reinhold Publisher, New York, NY, USA, 1981, ch. 6.
22. S. M. Politzer, Reviews in Computational Chemistry, VCH Publishers, New York, NY, USA, 1991, ch. 7.
23. G. Naray-Szabo and G. G. Ferenczy, Chem. Rev., 1995, 95, 829–847.
24. W. L. F. Armarego and C. L. Chai, Purification of Laboratory Chemicals, Butterworth-Heinenann, London, 2003.

---

Footnote

† Electronic supplementary information (ESI) available: Synthesis of monomer, SEM-EDX of neat and doped material. See DOI: 10.1039/c4ra02443e

---