Crystal engineering for pi-pi stacking via interaction between electron-rich and electron-deficient heteroaromatics.

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-pi-stack for 2. That of 5 is of slipped-pi-stack, albeit with a tilt angle between neighboring pi-stacks. The pi-pi interfacial distances of the molecules in the crystal lattice are 3.47 and 3.59 A for 2 and 5, respectively. On the basis of the crystal structure, compound 2, with negligible pi-pi 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 (mu(+)) for 2 (2.32 cm(2) s(-1) V(-1)) compares favorably with that of pentacene (1.93-5.43 cm(2) 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.