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 (mu(FET)) of 4.4 x 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 x 10(-2) and 2 x 10(-2) cm(2) V-1 s(-1), respectively, arising from higher degree of s-p 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 mu(FET) = 1.2 10-3 cm(2) V-1 s(-1) than its silicon analogue (4T(3)Si(4): mu(FET) = 5.4 10(-4) cm(2) V-1 s(-1)). This is probably due to the more condensed packing of 4T3Si3C 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 mu(FET) = 2.0 x 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 (mu(FET) = 7.7 x 10(-7) cm(2) V-1 s(-1)).