Spectroscopic, electrochemical, thermal properties and electroluminescence ability of new symmetric azomethines with thiophene core
Introduction
During the last decades appreciable amount of research has been focused on organic semiconductors for electronic applications including photovoltaics (OPV), light emitting diodes (OLED) and field effect transistors (FET). Although devices based on organic compound are commercially available, the development of new highly efficient and long-term stable optical and electrical processable organic semiconductors is still a challenge. The most important factors that determine the device performance include but are not limited to the type of materials and applied deposition processes and device design, with the crucial role played by used compounds [1]. Among the rich chemistry of various semiconductors, the conjugated compounds with chain nitrogen-containing, electron poor moieties, such as CH=N Schiff base linker, seem to be attractive for optoelectronics. The main representatives of this family, are azomethines, also named Schiff bases or imines [2], [3], [4]. Azomethines possess the ability to form liquid crystals and coordinating complexes with metals, are characterized with high thermal stability; being isoelectronic to their vinylene analogues and exhibit therapeutic activity, therefore they are widely used in different fields such as analytical chemistry, organic synthesis, medicine, biology, as well as in organic electronics as electrochromic material and photovoltaic (OPV) cell components [4], [5], [6], [7], [8]. They are interesting alternatives to conventional semiconductors due to both the possibility of tuning their properties by the proper choice of the precursors, that is, (di)amine and (di)aldehyde and the synthesis advantages. The strategy of preparation of conjugated compounds generally based on Suzuki-, Stille, Grignard-, Witting- and Kumada-coupling [9]. Compounds synthesized via listed methods need extensive purification to remove the catalysts and undesired byproducts. While azomethines are obtained in mild reaction conditions without metal catalyst, water is the only byproduct, thus purification is rather minimal. It was found that azomethines showed also good conductivity, comparable to a vinyl analogue, which can be useful for electronic applications [10]. It was demonstrated that azomethines bearing various 5-membered aryl heterocycles, especially thiophene derivatives, are perspective materials for optoelectronic applications [3], [4], [5], [11], [12], [13]. Thiophene derivatives are considered to be promising building blocks for most organic functional materials studied for plastic electronics including electrochromic devices (ECD), OLED, OPV cells, and OFET [14]. Considering the enhanced properties of thiophene azometines relative to their homaryl counterparts reported by the Skene research group [4], we have undertaken a synthesis and characterization of new azomethines containing as a core thiophene derivative coupled with various homoaryl units. Their thermal (DSC, TGA), optical (UV–vis, PL), and electrochemical (CV) properties together with their ability to emit light under applied voltage were examined. Additionally, their electronic structure and photophysical properties were calculated by density functional theory (DFT). Recently, we have described the structure-property relationship for the series of symmetrical azomethines with 1,4-phenyl central core end-capped with anthracene, phenanthrene and pyrene units [15]. In connection with this, the properties of two imines reported herein, that is, prepared from 9-anthracenecarboxaldehyde and 1-pyrenecarboxaldehyde, are also discussed in relation to the analogous azomethines that differ in the central core structure reported in our previous article [15]. This comparison underlines the effect of thiophene ring substituted with carboxylic acid ethyl ester groups on thermal, optical and electrochemical characteristics.
Section snippets
Materials
9-Anthracenecarboxaldehyde, 1-pyrenecarboxaldehyde, 3-phenoxybenzaldehyde, 9-ethyl-3-carbazolecarboxaldehyde, 4-[(trimethylsilyl)ethynyl]benzaldehyde,N,N-dimethylacetamide (DMA), dimethylformamide (DMF), N-methyl-pyrrolidone (NMP), methanol, sulfur, ethyl cyanoacetate and triethylamine were purchased from Sigma-Aldrich. 4-(2-Phenyleth-1-ynyl)benzaldehyde was purchased from Chempur GmbH. 2,5-Diamino-thiophene-3,4-dicarboxylic acid diethyl ester (DAT) was synthesized according to publication [16].
Synthesis and structural characterization
Azomethines with central thiophene core were prepared based on condensation reaction between 2,5-diamino-thiophene-3,4-dicarboxylic acid diethyl ester and various aldehydes, such as: 9-anthracenecarboxaldehyde,1-pyrenecarboxaldehyde, 9-ethyl-3-carbazolecarboxaldehyde, 3-phenoxybenzaldehyde, 4-[(trimethylsilyl)ethynyl]benzaldehyde and 4-(2-phenyleth-1-ynyl)benzaldehyde. The chemical structure of the prepared thiophene azomethines is depicted in Fig. 1.
Formation of the desired imine was confirmed
Conclusions
Symmetrical heterocyclic azomethines bearing as the central core the thiophene substituted with carboxylic acid ethyl ester groups and end-capped with anthracene (ThAz-1), pyrene (ThAz-2), carbazole derivative (ThAz-3), diphenyl ether (ThAz-4) units and phenyl linked by triple bond with trimethylsilyl (ThAz-5) or second phenyl ring (ThAz-6) were presented. The results of investigations conducted revealed a role of imine bond end-capped group nature in determining their properties. These
Acknowledgements
This work was supported by the National Research and Development Center (NCBiR) under Grant ORGANOMET no.: PBS2/A5/40/2014. The GAUSSIAN-09 calculations were carried out in the Wrocław Centre for Networking and Supercomputing, WCSS, Wrocław, Poland, http://www.wcss.wroc.pl (Grant no. 18).
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