Development of N-alkyl-substituted bis(pyrrolo[3,4-d])tetrathiafulvalenes as organic semiconductors for solution-processible field-effect transistors

A series of N-alkyl-substituted bis(pyrrolo[3,4-d])tetrathiafulvalenes (PyTTFs, alkyl = n-butyl, n-octyl, n-dodecyl, n-cetyl, and n-icosyl, 3-7) were synthesized as highly soluble tetrathiafulvalene (TTF) derivatives. Their solid-state structures were characterized by X-ray diffraction, and their suitability for use as semiconductors for solution-processible organic field-effect transistor (OFET) devices was evaluated. Whereas the solubility of the TTF derivatives was enhanced with the introduction of alkyl groups, very long alkyl groups, such as the n-icosyl group, reduced the solubility probably due to intermolecular hydrophobic interactions between the very long alkyl groups. The solid-state structure was also influenced by the length of the alkyl groups; molecules 5-7 having n-dodecyl or longer alkyl groups tended to assume two-dimensional (2-D) molecular ordering both in the bulk single crystals and in the spin-coated thin films. In contrast, 3 and 4, having short n-butyl and n-octyl groups, did not take on a 2-D interactive structure in the solid state. Consistent with the solid-state structure of the PyTTF derivatives, field-effect transistor (FET) characteristics of the solution-processed OFETs markedly depended on the length of the alkyl groups. In contrast to spin-coated thin films of 3 and 4, which were relatively inferior semiconducting layers (μ_FET = ∼10^-5 cm² V^-1 s ^-1 or no reproducible field effect), OFET devices consisting of spin-coated thin films of 5-7 showed typical p-channel FET characteristics, namely, hole mobilities of ∼10^-2 cm² V^-1 s^-1 and current on/off ratios of ∼10⁴. The results indicate that an appropriate combination of a π-conjugated core with long alkyl groups could provide soluble organic semiconductors that are applicable to solution-processible OFETs.