The mobility and decay kinetics of charge carriers in discotic hexabenzocoronenes

The temperature dependence of the mobility and decay kinetics of charge carriers in discotic hexa-peri-hexabonzocoronene derivatives has been measured using the pulse-radiolysis time-resolved microwave conductivity technique (PR-TRMC). For both the racemic and chiral dimethyloctyl, and the isomeric n-decyl substituted derivatives, the mobility decreases at the crystalline solid (K)-liquid crystalline (Col) transition from 0.5±0.1 to 0.30±0.05 cm² V^-1 s^-1 with the transition temperature ca. 30°C lower for the branched chain compounds. The charge recombination kinetics are similar for the branched chain isomers in the Col phase but a faster decay is found for the racemic compound in the K phase. The fact that the mobility values for an asymmetrically substituted butylanthraquinone (C4 AQ) derivative are a factor of 2-3 lower than for the fully hexakis-alkyl substituted compounds is attributed to the similar intracore mobilities for holes and electrons in the latter materials and the electron localization on the AQ moiety in the former. Charge recombination is found to be orders of magnitude faster in the Col phase of the C4 AQ derivative than in the K phase, this is attributed to the motional freedom of the AQ ^- group in the liquid-crystalline phase.