Synthesis and characterization of new polyphenols derived from o-dianisidine: The effect of substituent on solubility, thermal stability, and electrical conductivity, optical and electrochemical properties

Abstract

In this study, we proposed to synthesize soluble polyphenol derivatives containing azomethine bond. For this reason, o-dianisidine was chosen to synthesize Schiff base monomers due to containing dimethoxy groups. Four phenolic Schiff bases were synthesized by condensation reaction of o-dianisidine with salicylaldehyde (2-HBADIAN), 4-hydroxybenzaldehyde (4-HBADIAN), vanillin (MHBADIAN) and 3-ethoxy-4-hydroxybenzaldehyde (EHBADIAN). These monomers were converted to their polyphenol derivatives via oxidative polycondensation reaction (OP). The structures of the obtained compounds were confirmed by FT-IR, UV–vis, ¹H NMR and ¹³C NMR techniques. Progressing of OP was also followed by a time-controlled spectrum mode of a UV–vis spectrophotometer. The molecular weight distribution parameters of the synthesized polyphenols were determined by the size exclusion chromatography (SEC). The synthesized compounds were also characterized by solubility tests, TG-DTA and DSC. Cyclic voltammetry (CV) measurements were carried out and HOMO–LUMO energy levels and electrochemical band gaps ($E_{\text{g}}^{\prime }$) were calculated. Additionally, optical band gaps (E_g) were determined by using UV–vis spectra of the materials. Electrical conductivities of both doped and undoped states of the synthesized materials were measured by four-point probe technique using a Keithley 2400 electrometer showing that P-2-HBADIAN has approximately 130 times higher electrical conductivity than the others. Also, it was stressed that the synthesized polyphenols are semiconductors which have a potential for electronic and optoelectronic applications, with fairly low band gaps.

Introduction

Polyazomethines (PAMs), with a wide range of applications, have had continual increasing interest due to having a lot of useful properties such as high thermal stability and excellent mechanical strength as well as their semi-conductivity and optoelectronic properties [1], [2], [3]. On the other hand, PAMs, especially aromatic derivatives suffer from low solubility. To solve this problem, many kinds of PAMs like poly(azomethine ether)s [4], poly(acrylate-azomethine)s [5], poly(azomethine carbonate)s [6], poly(amide-azomethine-ester)s [7] and poly(azomethine sulfone)s [8] have been synthesized so far. Additionally, PAMs containing methoxy substituents had been presented with fine solubility and high thermal stability [9]. PAMs derived from alcoxy-substituted benzidine had been previously presented as good soluble polymers [10]. The attempts to increase of solubilities of PAMs by side-group functionalization were clearly reviewed by Iwan and Sek [11]. According to this review chemical modification in structures of PAMs by side-group functionalization can weaken interchain interactions and consequently improve the solubilities. In the present study, we synthesized some soluble PAMs derived from o-dianisidine which has dimethoxy substituents at ortho positions of –NH₂ groups of benzidine.

As a result, we obtained good soluble polyphenols containing azomethine groups. Many kinds of oligophenols had been previously investigated with their useful properties [11], [12], [13]. Also, substituent effect on radical-controlled oxidative polymerization rate of phenol monomers had been presented and the probable coupling mechanism of this reaction had been suggested [14]. According to these studies, radicalic oxidative polycondensation reaction of phenol derivatives proceeds by CC and CO coupling mechanism.

We previously reported several oligophenol derivatives of PAMs with a wide range of useful properties like paramagnetism, semi-conductivity, electrochemical cell, and resistance to high energy [15], [16], [17], [18], [19]. Moreover, some kinds of this class polymer are indicated to have antimicrobial activity as well as fine electrical properties [20], [21]. Also, it is known that the synthesis procedure of this class of PAMs is cheap and environmentally harmless [22]. This is because of using cheap oxidants (NaOCl, H₂O₂, air, etc.) in an aqueous medium. So, this method is important for green chemistry and it can be preferable to obtain polyconjugated azomethine polymers.

At the first part of this study, we synthesized the Schiff base monomers by condensation reaction of o-dianisidine with aromatic hydroxyl aldehydes containing different substituents. Then, we converted these compounds to polyphenol derivatives via oxidative polycondensation (OP) reaction using NaOCl as oxidant. We also followed the OP reaction of MHBADIAN monomer using a time-controlled spectrum mode of a UV–vis spectrophotometer. New growing peaks during polymerization were properly obtained. Solubility measurements of the synthesized compounds were carried out showing that the solubilities of the polymers are quite higher than those of the previously synthesized kinds due to containing methoxy groups. However, it is indicated that higher molecular weight of a polymer decreases its solubility even if it contains more solubility-increasing substituents. We characterized the synthesized compounds using FT-IR, UV–vis spectra, ¹H and ¹³C NMR and SEC analyses. TG-DTA technique was used to determine the stabilities of thermal degradation. DSC analyses of the polyphenols were also carried out thus the glass transition temperatures (T_g) were determined. Optical properties were determined by using UV–vis spectra and the optical band gaps were calculated from absorption edges. The HOMO–LUMO energy levels and electrochemical band gap values of the compounds were obtained by using cyclic voltammetry measurements. Additionally, electrical properties of doped and undoped polymers and Schiff bases were determined.