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.

To investigate charge polarity of transistors based on donor/acceptor cocrystals, charge-transfer complexes of wide donor molecules such as perylene and pyrene are investigated. As-grown single-crystal transistors are deposited from solutions, which attain perfect reproducibility in comparison with the conventional pasted single-crystal transistors. Perylene complexes of dicyanoquinonediimines (DCNQI) with substituents X = CH 3 , Cl, and Br show holedominant ambipolar characteristics. Owing to the large molecular width, the donor/acceptor orbitals are not orthogonal, and ambipolar transport is realized. In contrast, pyrene works similarly to ordinary acenes, and not only the tetracyanoquinodimethane (TCNQ) complexes but also the DCNQI complex show only electron transport. The existence of the short-axis offset in the molecular overlap is the criterion that transforms electron transport to ambipolar transport.