Solution-processable electronic and redox-active liquid crystals based on the design of side chains

Abstract

For the development of solution-processable functional materials, it is significant to increase the solubilities of the materials for organic solvents, retaining ordered structures of functional π-conjugated units during an evaporation process of solvents. For this purpose, not only a design of π-conjugated functional units but also a design of side chains is indispensable. In addition to alkyl side chains utilized for liquid crystals and conjugated polymers, oligosiloxane, oligoethylene oxide, perfluoroalkyl side chains are discussed in this review. Oligosiloxane side chains remarkably increase the solubilities of π-conjugated liquid crystals and conjugated polymers while the side chains promote self-organization of π-conjugated units to enhance their functions. In the thin films of the liquid crystals and conjugated polymers bearing cycloterasiloxane rings, liquid crystalline nanosegregated structures can be immobilized by an exposure to acid vapors. Oligoethylene oxide chains also promote nanosegregation and coordinate with ionic species. This property provides liquid crystals with electrochemical functions. Perfluoroalkyl side chains reinforce nanosegregated structures to enhance device performances. The electroluminescence devices using π-conjugated liquid crystals and polymers can emit linearly polarized light. Field-effect transistors comprising liquid crystalline active layers provide flexible devices with high performance. Solar cells based on liquid crystalline nanosegregated structures have a potential for high performance devices, due to high carrier mobility in the liquid crystal phases. The π-conjugated liquid crystals bearing oligoethylene oxide chains and ionic moieties form thin films in which electrons and ions are conducted separately, resulting in a new kind of electrochemical devices.