Synthesis and characterization of benzo[1,2-b:3,4-b':5,6-b'']trithiophene (BTT) oligomers

Two dimers (2 and 3), dendritic tetramer (4), hexamer (5), and decamer (6) of benzo[1,2-b:3,4-b':5,6-b'']trithiophene (BTT), a potential π-core unit with C(3h) symmetry, were synthesized, characterized, and evaluated for possible use as organic semiconductors. Single crystal X-ray analyses of the dimers (2 and 3) revealed that they have planar molecular structures with dihedral angles of almost 180° between two BTT units. In accordance with the rigid and planar molecular structure, the unsubstituted dimer (2) is poorly soluble, whereas the octyl-substituted dimer (3) has improved solubility. Although the solubility of the dendritic tetramer (4) is decreased, further extended systems, i.e., the dendritic hexamer (5) and decamer (6), have solubilities better than that of 4. With increasing numbers of BTT units in the molecule, the experimentally determined energy levels of HOMO shift upward slightly and the HOMO-LUMO energy gaps become smaller, but the extent of HOMO destabilization and reduction of the HOMO-LUMO gap are not significant. Taking into account the energy levels of the frontier orbitals, 3-6 could be useful as p-channel organic semiconductors rather than n-channel. In fact, the spin-coated thin film of 3 with edge-on molecular orientation acted as an active channel of field-effect transistors that showed hole mobilities as high as 0.14 cm(2) V(-1) s(-1), indicating that the BTT core is a useful π-conjugated system for application to organic semiconductors, although 4-6 gave FET characteristics rather inferior to those of 3, owing to their amorphous nature in the thin film state.