Novel n-Type Conjugated Ladder Heteroarenes: Synthesis, Self-Assembly of Nanowires, Electron Transport, and Electroluminescence of Bisindenoanthrazolines

A novel pi-conjugated heptacyclic framework has been synthesized via a new one-step cyclization that results in a new class of n-type organic semiconductors. Single-crystal structures of bisindenoanthrazolines DADF and DADK showed that the pi-conjugated heptacyclic framework is planar and leads to a slipped face-to-face pi-stacking with short intramolecular distances (3.39 angstrom and 3.56 angstrom, respectively). The series of bisindenoanthrazolines have a formal reduction potential of -0.68 to -0.70 V (vs SCE) and an estimated electron affinity (LUMO level) of 3.65-3.72 eV. Electron mobility in evaporated thin films of the bisindenoanthrazolines, measured by the space-charge limited current method, was as high as 3.84 x 10(-4) cm(2)/(Vs) under ambient air conditions. Organic light-emitting diodes based on DADA as the emissive material gave the best performance among the four molecules with a maximum brightness of 7610 cd/m(2), and maximum efficiency of 6.6 cd/A with EQE of 2.0% at a brightness of 936 cd/m(2). Phosphorescent organic light emitting diodes with fac-tris(2-phenylpyridine)iridium (Ir(ppy)(3)) as the green triplet emitter and a bisindenoanthrazoline as the electron transport layer showed a brightness of 62 000 cd/m(2) and luminous efficiency of 39.2 cd/A at a brightness of 4270 cd/m(2). Nanowires of DADF and DADK self-assembled from solution were found to be single-crystalline and their morphology was further investigated by electron microscopy techniques. These results demonstrate the potential of bisindenoanthrazolines as new n-type semiconductors for organic electronics and optoelectronics.