Mixed-stack donor–acceptor crystals show electron or ambipolar transport depending on the symmetry of the frontier orbitals.

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.

n-Channel organic transistors with excellent air stability are realized on the basis of charge-transfer complexes, (BTBT)(TCNQ), (BTBT)(F 2 TCNQ), (BSBS)(F 2 TCNQ), and (BTBT)(F 4 TCNQ), where BTBT is benzothieno[3,2b]benzothiophene, BSBS is benzoseleno[3,2-b]benzoselenophene, and F n TCNQ (n = 0, 2, and 4) are fluorinated 7,7,8,8-tetracyanoquinodimethanes. These complexes consist of mixed stacks of essentially neutral molecules, and the transistors are air stable even after several-month storage in ambient conditions.

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Dithieno[2,3- d;2'3'- d']benzo[1,2- b;4,5- b']dithiophene forms mixed-stack charge-transfer complexes with fluorinated tetracyanoquinodimethanes (F TCNQs, n = 0, 2, and 4) and dimethyldicyanoquinonediimine (DMDCNQI). The single-crystal transistors of the FTCNQ complexes exhibit electron transport, whereas the DMDCNQI complex shows hole transport as well. The dominance of electron transport is explained by the superexchange mechanism, where transfers corresponding to the acceptor-to-acceptor hopping ( t) are more than 10 times larger than the donor-to-donor hopping ( t). This is because the donor orbital next to the highest occupied molecular orbital makes a large contribution to the electron transport owing to the symmetry matching. Like this, inherently asymmetrical electron and hole transport in alternating stacks is understood by analyzing bridge orbitals other than the transport orbitals.