Development of high-birefringence thieno[3,2-b]thiophene-based liquid crystalline molecules with an alkyl and alkylsulfanyl group: structural design and optical performance

Abstract

A series of novel liquid-crystalline compounds based on a thieno[3,2-b]thiophene core was synthesised to investigate the relationship between molecular structure and optical as well as dielectric properties. Structural modifications involved variations in terminal polar groups (isothiocyanate or cyano), lateral halogen substituents, ethynyl linkers, and terminal thioalkyl chains. Thermal analysis revealed stable nematic phases for most compounds, with some derivatives exhibiting smectic A phases over broad temperature ranges. Optical studies demonstrated high birefringence values exceeding 0.36 for selected materials, primarily attributed to enhanced molecular polarizability anisotropy introduced by the conjugated core and polar substituents. Incorporation of ethynyl linkers and isothiocyanate terminal groups effectively increased transition temperatures and modified mesophase stability. The results confirm that precise molecular design involving heteroaromatic cores and sulfur-containing chains enables tuning of both mesophase behavior and optical anisotropy, offering strategies for the development of high-performance photonic materials.