Crystal Engineering for π−π Stacking via Interaction between Electron-Rich and Electron-Deficient Heteroaromatics

Abstract

New dipolar compounds containing alternating electron-rich thieno[3,2-b]thiophene units and electron deficient units have been synthesized. Compounds with 5-pyrimidinyl (compound 2) or benzothiazole (compound 5) as the electron-deficient unit were structurally characterized by the single-crystal X-ray diffraction method. The arrangement of the molecules is found to be one-dimensional slipped-π-stack for 2. That of 5 is of slipped-π-stack, albeit with a tilt angle between neighboring π-stacks. The π−π interfacial distances of the molecules in the crystal lattice are 3.47 and 3.59 Å for 2 and 5, respectively. On the basis of the crystal structure, compound 2, with negligible π−π slip along the short axis of the molecules, has a calculated electronic coupling value (0.153 eV) twice as large as that of the largest coupling of pentacene. Accordingly, the theoretically estimated hole mobility (μ+) for 2 (2.32 cm2 s−1 V−1) compares favorably with that of pentacene (1.93−5.43 cm2 s−1 V−1), despite of the larger reorganization energy for hole transport in 2. The symmetric intrastack S···C contacts found between the thieno[3,2-b]thiophene and pyrimidinyl units explain the unique features of the crystal structure of 2 and the resulting large electronic coupling