Nitrogen-rich oligoacenes: candidates for n-channel organic semiconductors

The successive replacement of CH moieties by nitrogen atoms in oligoacenes (benzene to hexacene) has been studied computationally at the B3LYP/6-311+G(d,p)//6-31G(d) level of theory, and the effects of different heteroatomic substitution patterns on structures, electron affinities, excitation, ionization, and reorganization energies are discussed. The calculated tendencies are rationalized on the basis of molecular orbital arguments. To achieve electron affinities of 3 eV, a value required to allow for efficient electron injection from common metal electrodes, at least seven nitrogen atoms have to be incorporated into tetracenes or pentacenes. The latter require rather small reorganization energies for electron transfer (<0.20 eV) making these compounds promising candidates for n-channel semiconducting materials. Particularly interesting are heptaazapentacenes 5 and 6 in which the nitrogen atoms are arranged to form self-complementary systems with a maximum number of intermolecular CH-N contacts in planar oligomers. These interactions are expected to facilitate the formation of graphite-like sheet structures with cofacial arrangements of the pi systems and short interlayer distances due to attractive N-C(H) interlayer interactions. This should not only be ideal for charge transfer but also might contribute to improved air stability of these semiconductors. Self-complementarity is maintained in azaacenes containing two cyano groups in the terminal rings. These compounds require lower reorganization energies than the unsubstituted heterocycles (0.13-0.14 eV), show high electron affinities (3.3 eV), and are thus promising candidates for materials applications.