A bio-inspired organic semiconductor 5,5 0-diphenylindigo shows excellent and well-balanced ambipolar transistor properties; its hole and electron mobilities are 0.56 and 0.95 cm 2 V À1 s À1 , respectively. The enhanced performance is attributed to the extended p-p overlap of the phenyl groups as well as the characteristic packing pattern that is a hybrid of the herringbone and brickwork structures. The ambipolar transistor characteristics are analyzed considering its operating regions, where a large unipolar saturated region appears due to the difference of the electron and hole threshold voltages. Scheme 1 Structures of indigo derivatives.

5,5'-Diiodoindigo (4) exhibits excellent ambipolar transistor properties with the hole/electron mobilities of µ h /µ e = 0.42/0.85 cm 2 V −1 s −1 .The halogen substituted indigos show systematically decreasing energy gaps from F to I due to the spreading highest occupied molecular orbitals, and decreasing tilt angles in the crystals. In addition, the iodine-iodine interaction provides extraordinarily large interchain interaction.However, the X-ray diffraction suggests that the indigo molecules are arranged approximately perpendicular to the substrate in the thin films, probably due to the extra iodine-iodine interaction. The remarkable performance is ascribed to this characteristic supramolecular interaction. Scheme 1. Synthesis of indigo derivatives. Fig. 4. Crystal structures of 1-4 viewed along the (a) c and (b) b axes. (c) Schematic illustration of the stacking structure. Halogen-halogen contacts of (d) 3 and (e) 4. The distances are 3.611 Å and 3.875 Å, respectively.

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In order to improve the ambipolar performance of indigo-based semiconductors, we have investigated halogen-substituted (1-4) and phenyl-substituted (5) indigo derivatives at the 5-position. We show that introduction of iodine atoms, namely 5,5'-diiodoindigo (4), leads to the strong halogen-halogen interaction (iodine-iodine interaction) that gives a significant effect on the molecular packing. Thanks to the supramolecular network coming from the extra iodine-iodine interaction, the molecules are arranged approximately perpendicular to the substrate in the thin film. This results in remarkable transistor performance of the maximum hole and electron mobilities (µh/µe) = 0.42/0.85 cm 2 V-1 s-1 , which are one of the highest among small-molecule ambipolar organic transistors. Furthermore, introducing phenyl groups, 5 improves the transistor performances up to the maximum mobilities µh/µe = 0.56/0.95 cm 2 V-1 s-1. We have found that the phenyl groups destroy the standard molecular packing of indigo to achieve a unique structure that is a hybrid of the herringbone and brickwork structures.