We demonstrate a new approach to solution-processable dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) derivatives that can afford good thin-film transistors having mobilities higher than 0.1 cm2 V-1 s-1. The key molecular design strategy is the introduction of one branched alkyl group at the edge of the DNTT core, which improves solubility while retaining semiconducting characteristics in the thin-film state. Dialkylation, i.e., the introduction of two branched alkyl groups on the DNTT core, had a detrimental effect on the semiconducting properties. Although the physicochemical properties of the mono- and dialkylated derivatives at the molecular level were almost the same, the thin-film absorption spectra and the ionization potentials (IPs) were markedly different, indicating that the intermolecular interaction in the thin-film state was affected by the number of alkyl groups. Indeed, the packing structures of the monoalkylated DNTTs in the thin-film state, which were estimated from the XRD patterns, were similar to that of parent DNTT, indicating the existence of the lamella structure with the herringbone packing motif. In sharp contrast, the XRD patterns of the dialkylated DNTT thin films showed poor crystallinity, and the packing structures were significantly different from that of parent DNTT. All the results of structural characterization in the thin-film state and evaluation of device characteristics of the DNTT derivatives with branched alkyl groups indicate that the introduction of a branched alkyl group in the molecular long-axis direction is an effective way to solubilize the rigid, largely π-extended organic semiconducting core without interfering with the semiconducting characteristics in the thin-film state.
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