Abstract
Liquid crystals bearing extended π-conjugated units function as organic semiconductors and liquid crystalline semiconductors have been studied for their applications in light-emitting diodes, field-effect transistors, and solar cells. However, studies on electronic functionalities in chiral liquid crystal phases have been limited so far. Electronic charge carrier transport has been confirmed in chiral nematic and chiral smectic C phases. In the chiral nematic phase, consisting of molecules bearing extended π-conjugated units, circularly polarized photoluminescence has been observed within the wavelength range of reflection band. Recently, circularly polarized electroluminescence has been confirmed from devices based on active layers of chiral conjugated polymers with twisted structures induced by the molecular chirality. The chiral smectic C phase of oligothiophene derivatives is ferroelectric and indicates a bulk photovoltaic effect, which is driven by spontaneous polarization. This bulk photovoltaic effect has also been observed in achiral polar liquid crystal phases in which extended π-conjugated units are properly assembled. In this manuscript, optical and electronic functions of these chiral π-conjugated liquid crystalline semiconductors are reviewed.
Highlights
Chiral liquid crystals based on extended π-conjugated units form the N* and smectic C (SmC)*
Phases, in which electronic function originated from the π-conjugated moieties couples with the molecular chirality to produce new optical and electronic functions
Phase, which has twisted structures, circularly polarized (CP) PL has been observed within the wavelength range of the reflection band
Summary
Molecular chirality often breaks the symmetry of liquid crystalline (LC) phases to induce twisted structures or electrical polarization. When the wavelength of the incident light coincides with the helical pitch of the N*. The wavelength reflected from the N* phase, λ is described in Equation (1), where p, n and θ are helical pitch, refractive index, and incident angle, respectively. A DC voltage is applied in the direction perpendicular to the layer normal, the twisted structure is unwounded to induce macroscopic electrical polarization, which does not disappear after the removal of the DC bias. Macroscopic electrical polarization can be induced in the more ordered smectic phase by the tilted director from the layer normal and chirality. Various unique optical and electro-optical phenomena have been studied, these phases are electrically insulative and the author does not mention these phases in this review
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