Synthesis, electronic, and morphological properties of tetrahedral oligothiophenes with n-hexyl terminal groups.

A series of tetrahedral oligothiophenes bearing n-hexyl groups at the α-positions of the terminal thiophene rings, (n-C(6)H(13)(C(4)H(2)S)(n))(4)C (Hex-TnTM; n=1-4), has been synthesized by Kosugi-Migita-Stille coupling as a key reaction. Thanks to the improved solubility afforded by the terminal n-hexyl groups, the largest homologue (n=4) was successfully obtained. Whereas the smaller derivatives (n=1, 2) were obtained as liquid substances, the larger derivatives (n=3, 4) were obtained as solids. Hex-T3 TM partially adopts syn conformations between the adjacent thiophene rings in the crystal, probably owing to the packing force. Hex-T3 TM not only appeared in the crystalline state but also the amorphous state, which was stable to up to 80 °C. Regardless of the terminal groups, the derivatives of n=2 exhibited a broad fluorescence with large Stokes shifts compared to the corresponding linear analogues, thereby suggesting the presence of intramolecular interactions between the bithiophene moieties. Interactions between terthiophene branches was also suggested in the radical cations of Hex-T3 TM by cyclic voltammetry measurements.