Columnar liquid-crystalline assemblies of X-shaped pyrene–oligothiophene conjugates: photoconductivities and mechanochromic functions

Abstract

π-Conjugated liquid crystals have great potential as organic semiconductors owing to their smooth thin-film forming and flexible properties, as well as charge carrier transport ability. In the present study, we have designed and synthesised X-shaped columnar liquid crystals with pyrene as the central core, which are fourfold conjugated with bithiophene moieties, and tethered with eight or twelve alkoxy chains at their extremities. These X-shaped molecules show hexagonal, tetragonal, and rectangular columnar liquid-crystalline (LC) phases over a wide range of temperature. The extended π-conjugation has led to the realisation of a moderately low bandgap. The hole mobilities of both molecules in their various LC states are revealed to be in the order of 10⁻⁴ to 10⁻⁵ cm² V⁻¹ s⁻¹ using the time-of-flight (TOF) photoconductivity measurements. In addition, the mechano-responsive photoluminescent change has been observed for the columnar phase of the X-shaped molecule with eight alkoxy chains. The possible π-stacking configurations are discussed. The introduction of a gelator, N,N′-bis(lauroyl)hydrazine, into the X-shaped molecules leads to the formation of a LC gel that produces a change in the measured hole mobility. The presented strategy could lead to the design of new solution-processable, low bandgap semiconducting materials for optoelectronic devices.