Charge-Carrier Transport in Thin Films of π-Conjugated Thiopheno-Azomethines
Graphical abstract
Highlights
► Highly π-conjugated thiophenoazomethines were prepared for mobility measurements. ► Organic field effect transistors were prepared with the thiophenoazomethines. ► The polythiophenozomethine exhibited ambipolar transport properties. ► The thiophenoazomethine triad exhibited uniquely p-type transport properties. ► The triad’s mobility increased to 3 × 10−5 cm2/V s with annealing.
Introduction
In the rapidly evolving field of organic electronics [1], organic π-conjugated materials making use of azomethines (–N = C–) could be interesting alternatives to materials based on more conventional coupling protocols (i.e. –C = C–). This is in part due to the straightforward synthesis of azomethines that does not require stringent reaction conditions, unlike their carbon analogues [2], [3]. Azomethines are advantageous because undesired by-products are not formed during the synthesis and water is the unique side-product. Therefore, relatively pure highly π-conjugated materials can easily be obtained both by minimal purification and mild reaction conditions. Organic π-conjugated materials making use of azomethine (–N = C–) couplings are additionally interesting because they withstand both chemical oxidation and reduction [2], [3], [4], [5], [6]. Moreover, their optical and electrochemical properties can readily be tailored contingent on the judicious choice of complementary heterocyclic amines and aldehydes [7].
Despite the synthetic advantages of azomethines, their use in the field of organic electronics has not yet been pursued. This is primarily because π-conjugated materials making use of azomethine couplings are misconceived to be hydrolytically and oxidatively unstable [8], [9]. This has been exacerbated by previously reported homoaryl azomethines that exhibited incompatible electrochemical and optical properties for use in organic electronics [10], [11], [12]. These collective aspects have limited any interest in using azomethines as functional materials in organic electronics.
While cursory property evaluation of azomethines suggests that they have limited usefulness, they nonetheless have been exploited to prepare reversible dynamic materials [13]. The latter can undergo component exchange resulting in materials whose properties can be readily varied including their fluorescence emission, fluorescence yield, absorbance, and solubility in a given solvent [14], [15], [16]. Azomethines have further found uses as end-capping groups for oligothiophenes. The use of aryl azomethines increased the self-assembly of oligothiophenes and enhanced their charge transport properties [17]. Conjugated azomethines have also recently been used as a simple means to adjust the molecular wire spacing between gold contacts, leading to the discovery of different electron migration mechanisms from “tunneling” to “hopping”, in such wires [18], [19], [20]. Organic π-conjugated materials making use of azomethines have lately been used as the photoactive and emissive layers in organic photovoltaic devices and light-emitting diodes [21], [22], [23]. While the performance of these working devices was poor, it nonetheless successfully illustrates the compatibility of the heteroatomic materials properties for use in organic electronic devices. It would therefore be beneficial to identify the electronic processes that are responsible for the poor device performance. This knowledge could in turn be used to design and prepare new azomethines with improved device performance. For this reason, we were motivated to investigate the charge carrier transport properties of azomethines. Determining the transport properties is crucial for assessing the suitability of easily prepared thiopheno-azomethine based materials for use in organic electronic devices, especially given that they have optical and electrochemical properties that are compatible for use in organic electronics [6], [7], [24]. This is of particular interest given that previous studies exclusively examined homoaryl azomethines. The targeted thiopheno-azomethines (1 and 2, Scheme 1) should therefore possess enhanced electronic properties because of their high degree of conjugation owing to the intrinsic coplanarity of the heterocycles with the azomethine bond [7], [25]. This is in contrast to their homoaryl analogues that are highly twisted from planarity [26].
In this work, we present the charge carrier transport properties of thin films of two thiopheno-azomethines, characterized in the field-effect transistor configuration. Both a thiopheno-azomethine oligomer (1) and an analogous polymer (2) were investigated. The transport properties of the films were correlated with their morphological properties that were assessed by fluorescence hyperspectral imaging and atomic force microscopy. We also investigated the effect of the degree of conjugation on the transport properties of the films by comparing the properties of the triad 1 versus its polymer counterpart, 2. The transport properties of the azomethines were validated by measuring the hole mobilities of an all-carbon vinylene triad (3).
Section snippets
Materials and general methods
Reagents and solvents were received from commercial sources and they were used as received, unless otherwise stated. Anhydrous and deoxygenated solvents were obtained with an activated alumina column system. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded at room temperature with a 400 MHz spectrometer. All the samples were dissolved in deuterated solvents and the spectra were referenced to the solvent line relative to Tetramethylsilane (TMS). The syntheses of 4, 5 and 7 were
Materials synthesis
The synthesis of the targeted thiopheno-azomethines was done according to the methods outlined in Scheme 1. The preparation of both materials was done by judicious control of the stoichiometry of the common reagents and the solvent. For example, a 2:1 stoichiometry of 5 and 4, respectively, in ethanol selectively led to the triad 1 in high yield. Similarly, the polymer 2 was obtained in high yield with a 1:1 stoichiometry of 4 and 5 in chloroform. It should be noted that both of the products 1
Conclusions
Thin films of 1–3 deposited on HMDS-treated SiO2 substrates showed interesting charge carrier transport properties. Transistor characterization of the three compounds in thin film form indicated that 1 and 3 have p-type semiconducting behavior. On the other hand, 2 showed ambipolar behavior. Thermal treatment of the films prepared from 1 improved its charge transport properties. Overall, the results show that azomethines have charge transport properties. While the measured transport properties
Acknowledgements
The authors acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada for Discovery, Strategic Research, and Research Tools and Instruments grants, the Canada Foundation for Innovation, the Centre for Self-Assembled Chemical Structures (CSACS) and the Regroupement Québecois sur les Matériaux de Pointe (RQMP). WGS thanks the Humboldt Foundation and the Royal Society of Chemistry for fellowships allowing the manuscript to be drafted. Dr. A Alouhabi is
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