Unusual side-chain effects on charge-carrier lifetime in discotic liquid crystals.

When hexa-peri-hexabenzocoronene (HBC) carries paraffinic side chains with ester-ether termini (1a-1c), a hexagonal columnar liquid-crystalline (LC) mesophase forms, which is characterized by a very wide temperature range (ca. 0-300 degrees C). The LC materials from these HBC derivatives show characteristic electronic properties with an extremely long photocarrier lifetime. For example, by means of a flash-photolysis time-resolved microwave conductivity technique, the photocarrier lifetime, observed for liquid-crystalline 1a, is as long as 14 ms, which is four-orders of magnitude longer than that of fully paraffinic HBC 4 (0.0011 ms). The maximum transient photoconductivities of liquid-crystalline 1a-1c are comparable to one another and also to those of HBCs without ester-ether terminals. Liquid-crystalline 1a exhibits an obvious photocurrent generation in response to light illumination.