New heterocyclic conjugated azomethines containing triphenylamine units with optical and electrochemical responses towards the acid environment

Highlights

• Heterocyclic azomethines containing ortho-linked triphenylamine units are developed.

• They exhibit high organo-solubility/film forming ability and high thermostability.

• Their opto-electronic behavior is modulated through the structural pattern.

• Different spectral changes occurred by doping with hydrochloric and trifluoroacetic acids.

Abstract

A series of heteroaromatic azomethines containing phenyl, pyridine, thiophene or furan ring and ortho-catenated triphenylamine (TPA) core was synthesized and used to investigate the effect of the structural variation on opto-electronic and acid-sensing properties. The twisted structure induced by the TPA core enabled good solubility in organic solvents, including chloroform, acetone or ethyl acetate. Meanwhile, they preserved good thermal stability and showed high glass transition temperatures. The modulation of optical and electronic properties of azomethines was most likely due to two effects: the degree of coplanarity induced by the heterocyclic moiety and the electronic effect of the π-rich heterocycle that is electron-withdrawing (pyridine) or electron-acceptor (thiophene and furan). A detailed study was accomplished with respect to the acid recognition capability promoted by the electronic and basicity character of the azomethine center. The spectroscopic and electrochemical responses to acid environment were followed by cyclic voltammetry, FTIR, fluorescence and UV–vis spectroscopy. Different spectral changes occurred when doping was performed with hydrochloric and trifluoroacetic acids. The sensing properties toward the environmental pH modification were mostly accompanied by fluorescence quenching and driven by different recognition principles with respect to the acid dopant type.

Introduction

Conjugated polymers are extensively studied nowadays for their excellent electronic, optoelectronic and other appealing characteristics, such as flexibility, light-weight, or low fabrication cost in response to the need of overcoming the drawbacks generated by inorganic materials [1]. Among them, polyazomethines (polyimines or Schiff base polymers) are of special interest due to the expected similar properties to their vinyl counterparts which may endow these polymers with a large applicative potential in electronics, optoelectronics, and photonics [[2], [3], [4], [5]]. These polymers are known for their high thermal stability, liquid crystalline properties, metal complexation ability, semiconducting and interesting photo-optical behavior, among others [[6], [7], [8], [9]]. Moreover, the generation of -N = CH- linkage is more facile compared to −CH = CH- bond formation for which rigorous reaction conditions are required along with an extensive purification to eliminate the by-products for optimal properties and performances in the foreseen applications [10]. The preparation of azomethine systems is both simple and versatile under mild reaction conditions from wealthy monomer sources and the water is the only by-product. Therefore the azomethines can be claimed as environmentally friendly materials [[11], [12], [13], [14]].

Protonation as one of the most important noncovalent interaction between molecules is a key synthetic tool which may promote appealing polymer properties as a result of novel polymer architectures induced by self-assembling processes [15]. Polyazomethines is a family of conjugated polymers that may be subjected to structural variation by protonation and complexation owing to the electronic and basicity character of −CH = N- unit [16]. Since azomethine type systems are capable of being protonated or complexed with metal ions, iodine or acids with a clear response in the UV–vis or fluorescence spectroscopy, they are possible alternatives to currently used color switching materials. Thus, a large palette of colors and multiple color shades are possible to be generated with these easy accessible materials [12]. Materials suitable for such color changes have found applications in stealth technologies, smart windows, electronic displays, camouflage textiles, sensors or telecommunication switchers, among others [7,17]. The color switching behavior of azomethine derivatives have been examined with several Lewis acids and dopants, such as FeCl₃, iodine, nitric acid, metansulfonic acid, camphorsulfonic acid or trifluoroacetic acid [12,16,18].

Despite the benefits offered by their synthetic pathway, ecological features and engaging multifunctional properties, like many other conjugated polymers, polyazomethines show limited solubility in common organic solvents which hamper their processing and characterization for use in appropriate opto-electronic devices. Over the years, different strategies have been adopted toward processable azomethine-based materials, such as symmetrical or unsymmetrical substitution of the aromatic rings with alkyl or alkoxy groups [19] or incorporation of flexible or bulky, packing disruptive units into the main polymer chains, like cardo-structures or tetraphenylethene [20,21]. One of this, often approached, consisted in the use of propeller-shape triphenylamine (TPA) as building block for the development of photo- and electroactive imine derivatives. This moiety may reduce the aggregation and crystallization tendency of azomethine derivatives, whilst endows these systems with hole transporting characteristics [22,23]. In the pursuit of novel organic semiconducting materials, triphenylamine (TPA) derivatives proved to be one of the most important group of modern electroactive functional materials [24] largely exploited in the newest technologies, such as organic photovoltaics, both polymer solar cells [2] and dye-sensitized solar cells [25,26], light-emitting diodes [27], electrochromic devices [28] or memory devices [29].

Superior optoelectronic properties can be however achieved by embedding additional chromophoric groups into imine-based molecules by generating an extra means of “tuning” the materials’ properties. Organic chemist have developed various building blocks, such as fluorene, carbazole, naphthalene, thiophene and its fused derivatives, benzothiadiazole and its derivatives, perylene and naphthalene diimides, etc. which were employed to design large families of materials according to individual demands for specific applications [10,30,31]. In this regard, heterocycles have been often introduced into organic semiconducting materials, and less into azomethine derivatives to benefit of their chemical, thermal, thermooxidative and photochemical stability [25,26,32,33]. Although enhanced properties can be obtained with TPA-based azomethine derivatives, the combination of TPA and heterocyclic units in the same azomethine molecular architecture is almost unknown [[34], [35], [36]] and the opto-electronic behavior of such systems remains unexplored.

Recently, we have reported on the development of highly soluble aromatic oligomers containing ortho-catenated TPA core and various chromophoric units [23], which exhibit acid sensing properties. We have observed that the quenched fluorescence of the neutral states was turned-on owing to the pH-induced conformational changes. Since heterocycles are involved in the conjugated electronic systems, it appears challenging to use to explore now the effect of different heterocycles on the photophysical and electrochemical properties of fully conjugated ortho-catenated TPA azomethine derivatives. In fact, to the best of our knowledge, there are no previous reports on the development of heterocyclic TPA-based azomethines in which the TPA core is connected via ortho linkages. This is of particular interest for understanding the structure-property relationships for the future design and development of high performance functional materials for potential applications in opto-electronic devices or sensors. To this scope, novel oligomeric azomethines were synthesized by solution polycondensation reaction of therephthalaldehyde, pyridine-, thiophene- or furan- based dialdehydes with a CF₃-grafted ortho-catenated triphenylamine (TPA) diamine. The influence of this particular structural pattern with respect to the previously reported TPA oligo- and polyazomethines on the thermal, optoelectronic and electrochemical properties modulation was thoroughly explored. Besides the molecular engineering, the azomethines characteristics were tuned by protonation or H-bond formation with proper dopants, such as hydrochloric acid or trifluoroacetic acid. Therefore, a special concern was addressed to the optical transitions between the neutral and doped states in both solution and thin films immobilized on various substrates. The sensing properties towards the change of the environmental pH were mostly accompanied by fluorescence quenching and driven by different recognition principles with respect to the acid dopant type.