Synthesis, properties, and semiconducting characteristics of electron-transporting three-dimensional π-conjugated compounds containing dicyanomethylene-substituted difluorocyclopenta[b]thiophene

https://doi.org/10.1016/j.jfluchem.2014.07.023Get rights and content

Highlights

  • Three-dimensional electron-transporting π-conjugated compounds have been successfully synthesized.

  • The physical measurements of these compounds revealed the electronic properties and frontier-orbital energy levels.

  • All the synthesized compounds showed electron-transporting characteristics.

  • Organic photovoltaic device containing our developed compound Tetra-TT-BCN as an acceptor showed photovoltaic response.

Abstract

The design and synthesis of three-dimensional π-conjugated compounds containing dicyanomethylene-substituted difluorocyclopenta[b]thiophene for application as acceptor materials in organic photovoltaic devices are reported. Photophysical and electrochemical measurements, as well as semiconducting performance evaluations were performed to investigate the characteristic properties derived from the three-dimensional structure.

Introduction

Over the past decade, π-conjugated systems have attracted great attention because of the potential application in organic electronic devices such as organic field-effect transistors (OFETs) [1], [2] and organic photovoltaics (OPVs) [3], [4]. The active layer of OPVs has been typically fabricated with the bulk heterojunction (BHJ) structure that consists of a blend of electron-donating and electron-accepting materials [5]. Most of polymer-based BHJ OPVs utilize a π-conjugated polymer, such as poly(3-hexylthiophene) (P3HT) and low band gap conjugated polymers, as the electron donor and a fullerene derivative represented by [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor [6]. On the other hand, new acceptor materials as alternatives to the fullerene derivatives are limited [7], owing to the difficulty of molecular design that fulfills the matched lowest unoccupied molecular orbital (LUMO) energy level with donor materials, electron-transporting characteristics in the BHJ film, and solution processability.

We recently reported the synthesis of new electron-transporting π-conjugated systems containing dicyanomethylene-substituted difluorocyclopenta[b]thiophene (BCN) and revealed their function as n-channel OFET and OPV acceptor materials [8], [9], [10]. On the other hand, in terms of structural modifications, we and others demonstrated that a three-dimensional (3-D) arrangement of π-conjugated molecules has an advantage to construct the isotropic electron-transporting pathways in the BHJ films, leading to promising photovoltaic performance [11], [12]. Under this situation, we herein designed and synthesized new 3-D electron-transporting π-conjugated compounds based on the combination of a tetrahedral-covalent Si atom at the center and oligothiophenes having dicyanomethylene-substituted difluorocyclopenta[b]thiophene at the four arms (Tetra-T-BCN, Tetra-TT-BCN) and elucidated their properties and OPV performance (Fig. 1).

Section snippets

Synthesis

The synthetic route of the target 3-D acceptor compounds Tetra-T-BCN and Tetra-TT-BCN are shown in Scheme 1. Tetrastannyl compound 2 was obtained by the lithiation of tetra(2-thienyl)silane (1) [13] using n-BuLi in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) followed by quenching with tributyltin chloride. Tetra-T-BCN was synthesized by a palladium-catalyzed Stille coupling between 2 and 4.8 equiv. of 3 [9] under microwave irradiation (180 °C, 5 min) in a yield of 39%. The

Conclusion

In summary, towards the application to acceptor materials in OPVs, new 3-D electron-transporting π-conjugated compounds containing dicyanomethylene-substituted difluorocyclopenta[b]thiophene units have been synthesized. The photophysical and electrochemical measurements of these compounds revealed the electronic properties and frontier-orbital energy levels. Despite the unfavorable 3-D structure for the carrier transport in OFETs, both compounds showed electron-transporting characteristics.

General information

All reactions were carried out under a nitrogen atmosphere. Solvents of the highest purity grade were used as received. Unless stated otherwise, all reagents were purchased from commercial sources and used without purification. Microwave irradiation was performed by a Biotage Initiator Ver. 2.5. The microwave power output was set at 400 W. The reaction temperature was kept at 180 °C, and internal temperature during the reaction was monitored by IR sensor. Column chromatography was performed on

Acknowledgments

This work was supported by a Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by cooperative Research with Sumitomo Chemical Co., Ltd. We acknowledge Mr. Takeo Makino for the support of synthesis. S. J. acknowledges research fellowship of Japan Society for the Promotion of Science for Young Scientists. Thanks are extended to the CAC, ISIR, for assistance in obtaining elemental analyses.

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