Novel calamitic liquid crystalline organic semiconductors based on electron-deficient dibenzo[c,h][2,6]naphthyridine: synthesis, mesophase, and charge transport properties by the time-of-flight technique

Abstract

Calamitic liquid crystalline organic semiconductors based on an electron-deficient dibenzo[c,h][2,6]naphthyridine (DBN) core were designed for electron transport materials and synthesized from 6-bromoisatin to form a key intermediate without tedious purification by means of a one-pot reaction including 4 steps, and then the target products were obtained in an additional 3 steps in a total yield of 13%. Two dialkylated DBN derivatives, 2,8-didecyl DBN (C10-DBN-C10) and 2,8-didodecyl DBN (C12-DBN-C12), exhibited a low ordered mesophase of smectic C (SmC) only. Their phase transitions, photophysical properties, and charge carrier transport properties were investigated and compared with their carbon analogs, i.e., chrysene and the DBN isomers of isoquino[8,7-h]isoquinoline derivatives. In the SmC phase, transient photocurrents had well-defined transits, which allowed us to evaluate their exact mobilities. The mobility for negative and positive carriers in the SmC phase depended on both electric field and temperature, which suggested that the conduction mechanism is via electronic hopping transport for both holes and electrons. It was unexpectedly found that C10-DBN-C10 in the SmC phase exhibited quite low mobility on the order of 10⁻⁵ cm² V⁻¹ s⁻¹, which is two orders of magnitude smaller than that for dialkylated chrysenes, and is comparable to the typical mobility in liquid crystalline semiconductors with large dipole moments, even though the DBN core has no dipole moment; this is also true for holes. This new phenomenon has not been reported before, and may provide new insight into how mobility in liquid-crystal phases is determined.