Carrier transport characteristics of glass-forming chiral liquid crystalline dimers based on oligo(phenylenevinylene) units

Abstract

We synthesized chiral dimeric chiral nematic (N*) liquid crystals bearing tri(p-phenylene vinylene) units. The mixture of dimeric compounds and monomeric chiral liquid crystals exhibited a glassy N* phase at room temperature and the reflection band could be tuned by the change of the temperature from which the rapid cooling of the mixture started. When the reflection band covered the fluorescence peak, high quality circularly polarized emission with a dissymmetry factor of 1.4 was observed. Hole mobilities were determined over wide temperature ranges by the TOF method and the hole transport characteristics in the N* phase were analyzed using the Gaussian disorder model. The preexponential factors in the N* phase of these compounds, corresponding to a virtual mobility without energetic and spatial disorders, were one or two order of magnitudes higher than those of amorphous organic semiconductors. This should be attributed to the large π-conjugated units of these compounds. The energetic and spatial disorders were larger than those of amorphous organic semiconductors in spite of the local uniaxial molecular orientation in the N* phase. This result should be caused by the large anisotropic polarizability of the π-conjugated units with a large aspect ratio.