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

Abstract

A novel π-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 π-conjugated heptacyclic framework is planar and leads to a slipped face-to-face π-stacking with short intramolecular distances (3.39 Å and 3.56 Å, 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 × 10−4 cm2/(V s) 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/m2, and maximum efficiency of 6.6 cd/A with EQE of 2.0% at a brightness of 936 cd/m2. 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/m2 and luminous efficiency of 39.2 cd/A at a brightness of 4270 cd/m2. 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.