Effects of the silicon core structures on the hole mobility of star-shaped oligothiophenes.

Star-shaped compounds with three or four oligothiophene units linked by an organosilicon core were prepared and their hole-transport capabilities were studied. A top-contact type thin film transistor (TFT) with a vapour-deposited film of tris[(ethylterthiophenyl)dimethylsilyl]methylsilane (3T(3)Si(4)) showed field-effect mobility (μ(FET)) of 4.4 × 10(-5) cm(2) V(-1) s(-1), while the device with the carbon centred analogue tris[(ethylterthiophenyl)dimethylsilyl]methane (3T(3)Si(3)C) showed no TFT activity. Intrinsic intramolecular hole mobility of 3T(3)Si(4) and 3T(3)Si(3)C was determined by time-resolved microwave conductivity measurements to be 8 × 10(-2) and 2 × 10(-2) cm(2) V(-1) s(-1), respectively, arising from higher degree of σ-π interaction in 3T(3)Si(4). To know more about the effects of the organosilicon core structures on the intermolecular hole mobility, we calculated internal reorganization energies for hole transfer at the (U)B3LYP/6-311+G(d,p)//(U)B3LYP/6-31G(d) level, which suggested smoother intermolecular charge transfer in the silicon derivative than the carbon and germanium analogues. Star-shaped compounds with quarterthiophene units behave in a different manner from the terthiophene derivatives and tris[(ethylquarterthiophenyl)dimethylsilyl]methane (4T(3)Si(3)C) showed higher TFT mobility of μ(FET) = 1.2 × 10(-3) cm(2) V(-1) s(-1) than its silicon analogue (4T(3)Si(4): μ(FET) = 5.4 × 10(-4) cm(2) V(-1) s(-1)). This is probably due to the more condensed packing of 4T(3)Si(3)C in the film, arising from the shorter Si-C bonding. Compounds with four terthiophene units were also prepared and tetrakis[(ethylterthiophenyl)-dimethylsilyl]silane (3T(4)Si(5)) showed the mobility of μ(FET) = 2.0 × 10(-4) cm(2) V(-1) s(-1), higher than that of 3T(3)Si(4), indicating the potential of tetrakis(oligothiophenyl) compounds as the TFT materials. Tetrakis[(ethylterthiophenyl)dimethylsilyl]germane (3T(4)Si(4)Ge) was less thermally stable and could not be processed to a film by vapour-deposition, but was found to be TFT active in the spin-coated film, although the mobility was rather low (μ(FET) = 7.7 × 10(-7) cm(2) V(-1) s(-1)).