Elsevier

Synthetic Metals

Volume 162, Issues 13–14, August 2012, Pages 1169-1176
Synthetic Metals

Multifunctional quinoxaline containing small molecules with multiple electron-donating moieties: Solvatochromic and optoelectronic properties

https://doi.org/10.1016/j.synthmet.2012.04.016Get rights and content

Abstract

Two multifunctional quinoxaline containing small molecules (designated as: SD-1 and SD-2) composed of electron-donating (D) moieties both in vertical and horizontal directions to an electron-accepting (A) quinoxaline at the central position have been synthesized. In both SD-1 and SD-2, the dimethylaminobenzene (DMAB) and triphenylamine (TPA) groups were used as an electron-donor in the vertical direction, and dihexyloxy-functionalized TPA was adapted as an additional donor in the horizontal direction. The unique donor (D)–acceptor (A) structures around the central quinoxaline moiety impart special solvatochromic and optoelectronic features to the SD-1 and SD-2. Photovoltaic cells (PVCs) and organic light-emitting diodes (OLEDs) were fabricated from SD-1 and SD-2 by solution processing (i.e. spin-coating). While PVCs with a structure of ITO/PEDOT:PSS/SD-1 or SD-2:PC71BM/Al show the power conversion efficiencies of 0.31% and 0.45%, respectively, OLEDs with a structure of ITO/PEDOT:PSS/SD-1 or SD-2/LiF/Al exhibit a maximum luminance (efficiency) of 7.42 cd/m2 (0.034 cd/A) and 48.84 cd/m2 (0.032 cd/A) with a turn-on voltage of 3.6 and 2.4 V, respectively. Furthermore, the Commission Internationale de L’Eclairage (CIE) chromacity coordinates of the OLED device with SD-2 were (0.67, 0.32), which are very close the CIE chromacity coordinates (0.67, 0.33) of National Television Society Committee (NTSC) for red color. Owing to their promising stimuli-responsive properties and device performances, these D–A molecules with unique structures can be considered as good candidates for multifunctional sensory and optoelectronic applications.

Highlights

► Quinoxaline-based small molecules were prepared for PVCs and OLEDs. ► The small molecules into two major optoelectronic devices. ► CIE chromacity coordinates of the OLED device with SD-2 were 0.67, 0.32. ► CIE chromacity coordinates are very close to the red color of NTSC (0.67, 0.33).

Introduction

Materials with combined electron-donor (D) and electron–acceptor (A) units connected via π-bridge(s) have been extensively studied for last two decades due to their potential applications in nonlinear optics (NLO) [1], [2], organic thin film transistors (OTFTs) [3], [4], organic light emitting diodes (OLEDs) [5], [6], [7], and photovoltaic cells (PVCs) [8], [9], [10]. One of the most significant features for the D-π–A molecules is their low band gaps induced by efficient electron delocalization via partial intramolecular charge transfer (ICT) between the donor and acceptor units along the conjugated molecular chain [10]. As a consequence, unique solvatochromic properties [11], [12], [13] have been observed for certain D–π–A molecules. Various electron donors, such as triphenylamine [14], [15] carbazole [16], [17] and fluorine [18], [19], and acceptors, including benzothiadiazole [20], [21] and quinoxaline [14], [22], [18] have been used for the design and synthesis of various functional D-π-A compounds. Meanwhile, thienyl, vinyl, and phenyl moieties have been utilized as the π-bridge between donor and acceptor units. Due to excellent electron-donating and nonaggregation properties associated with the non-planar molecular configuration [14], [15], triphenylamine has been an important donor for many applications. Similarly, quinoxaline has been widely used in conjugated polymers as a strong electron–acceptor because of its high electron affinity originated from the two symmetric nitrogen atoms in the pyrazine ring [23], [24], [25].

Recently, solution-processable small molecular semiconductors have received great attention as active materials in optoelectronic devices, such as bulk heterojunction (BHJ) PVCs [26], [27], [28], [29] and OLEDs [30], [31], [32], [33]. Comparing with polymeric active materials, small molecules have several advantages, including their well-defined mono-dispersed chemical structures for reproducible materials synthesis and device performance as well as straightforward structure and property characterization. More specifically, great improvement in the PVCs power conversion efficiencies (up to 4.4% for diketopyrrollopyrrole (DPP) derivatives [34]) has been achieved with D–π–A small molecules since the important work reported by Roncali et al. in 2006 [35], [36], [37]. Along with the extensive studies on small molecules with D–π–A structures in PVCs [26], [27], [28], [29], [34], [35], [36], [37], their applications in OLEDs have also been demonstrated to be promising with the recent availability of red emissive small molecules [30], [31], [32], [33]. Although several multifunctional polymers, which can be used in PVCs and OLEDs as active materials simultaneously, have been investigated [38], [39], [40], [41], there is no report on the multifunctional solution processable small molecules with the same purpose to our best knowledge. Therefore, it is interesting, though still quite challenging, to develop solution-processable small molecules with special D–π–A structures (e.g., SD-1, SD-2) for optoelectronic device (e.g., PVCs, OLEDs) and other applications.

Herein, we report the synthesis of two model compounds with electron-donating moieties in both vertical and horizontal directions to an electron-accepting quinoxaline core. It has been previously demonstrated that the synthesis of two-dimensional D-A structures around quinoxaline core in both vertical and horizontal directions and their applications for dye sensitized solar cells (DSSCs) [14] and OPVs [42]. The high population of active components in one molecule and the efficient formation of D-π-A structures through multi-directions are advantageous for optoelectronic applications. Our synthetic strategy involved that the dimethylaminobenzene (DMAB) and TPA connected with phenylene–vinylene linkage were used as donors in vertical direction for SD-1 and SD-2, respectively. Consequently, dihexyloxy-functionalized TPA was adapted as an additional donor in the horizontal direction for both SD-1 and SD-2 (Fig. 1). A quinoxaline moiety has been selected as an acceptor due to the strong electron-withdrawing capability of its pyrazine ring and the ease with which structural modifications can be performed on 2 and 3-phenyl rings. In this study, we have investigated the interesting solvatochromic properties of SD-1 and SD-2 originated from their unique D-π-A conjugated structures around the quinoxaline central moiety, along with their potential applications in photovoltaic and electroluminescent devices. The observed interesting stimuli-responsive properties and device performance make these newly-synthesized small molecules with unique D–π–A structures attractive for multifunctional applications in various sensing and optoelectronic devices.

Section snippets

Materials and measurements

1,2-Bis(4-dimethylaminophenyl)-1,2-ethandione was purchased from TCI. All other reagents and solvents were purchased from Aldrich. The compounds, 1 [43], 3 [14] and 5 [44] (Scheme 1) were synthesized according to literature procedures. Proton (1H) and carbon (13C) NMR spectra were recorded on a Varian VNMRS 600 spectrometer. UV–vis and photoluminescent spectra were measured using Perkin-Elmer Lambda 35 and LS 35, respectively. Cyclic voltammetry (CV) measurements were performed on a VersaSTAT3

Synthesis and characterization

The molecules, SD-1 and SD-2, consisting of electron-accepting quinoxaline core and multiple electron-donating components, were synthesized in a moderate yield by the modified synthetic procedures illustrated in Scheme 1, Scheme 2. For the preparation of SD-1, 1 and commecially available 1,2-bis(4-dimethylaminophenyl)-1,2-ethandione were reacted by the acid-catalyzed dehydration reaction to yield quinoxaline core 2. For the synthesis of SD-2, 1 and 3, prepared according to the previously

Conclusions

We have synthesized two multifunctional quinoxaline-based molecules with D–π–A structures, i.e. SD-1 and SD-2. Multiple electron-donating components, such as dimethylaminobenzene (DMBA) and bulky triphenylamine derivatives, were introduced both in the vertical and horizontal directions of an electron-accepting quinoxaline core. Due to the push-pull effect originated from the D–π–A structure and unique chemical structures, the SD-1 and SD-2 exhibit stimuli-responsive behaviors, such as

Acknowledgments

We acknowledge financial support from the World Class University (WCU) program supported by National Research Foundation and Ministry of Education, Science and Technology of Korea.

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