A comprehensive study of the effect of reactive end groups on the charge carrier transport within polymerized and nonpolymerized liquid crystals

Polymerizable liquid crystalline semiconductors, referred to as reactive mesogens (RMs), consist of pi-conjugated cores with reactive end groups decoupled by an aliphatic spacer. These can be polymerized within the mesophase, maintaining the self-assembled morphology and charge transport characteristics. The polymerized films can then be used in organic electronic applications such as charge transport layers in organic light emitting diodes and field effect transistors. We present a systematic study of the effect of reactive end groups on charge transport in calamitic liquid crystals (RMs) using the time-of-flight technique. Several different compounds were synthesized with a variation in both the liquid crystal (LC) mesogenic core group and the functional end groups. The reactive end groups in most cases affect the mesophase charge transport compared to the nonreactive LC mesophase transport. This manifests itself as a reduction in mobility, varying from a factor of 4 in the best case to as large as two orders of magnitude. In the best systems studied, however, the reactive end group effect on the transport, compared to the nonreactive mesophase transport, is negligible. Polymerized reactive mesogens do maintain long-range transport, with comparable mobilities to those of the phase in which they were polymerized over a broad temperature range, including room temperature. The hole and electron mobilities found in polymerized systems are explored using the Holstein small polaron model in the nonadiabatic limit, yielding the relevant polaron binding energies and bandwidths, and using the Bassler Gaussian disorder model, yielding the relevant energetic disorder parameters. (c) 2007 American Institute of Physics.