Tin(IV) sulfide–alkylamine composite mesophase: a new class of thermotropic liquid crystals

Abstract

The synthesis, structural characterization, thermal, electronic, optical birefringence and electrical properties of a new class of semiconducting tin sulfide based thermotropic liquid crystals (LCs), Meso-SnS-n, are described for the first time. The as-synthesized crystalline form of Meso-SnS-1 has a structure that is based upon well registered yet poorly ordered porous tin(iv) sulfide layers between which are sandwiched well organized hexadecylamine bilayers. On warming this material to around 45 °C the hexadecylamine bilayer first becomes disordered while the porous tin(iv) sulfide sheets remain registered. This is followed, around 85 °C, by a transition where both the alkylamine bilayer and the porous tin(iv) sulfide lamellae become liquid crystalline at which point the intralayer but not the interlayer registry is lost. The liquid-crystal organic–inorganic composite phase has either a nematic or smectic C structure. Electrically, the room-temperature ordered phase has a conductivity of 5.3×10^–8 Ω^–1 cm^–1 which increases by more than a 1000 times on transforming to the LC phase where it behaves as a semiconducting metallogen. The conductivity of Meso-SnS-1 cycles reversibly with temperature and displays discontinuities that are coincident with the crystal–semiliquid crystal and semiliquid crystal–liquid crystal thermal transitions that are defined by differential scanning calorimetry and variable-temperature powder X-ray diffraction. Meso-SnS-1 readily forms electrically conducting thin films which are able to reversibly adsorb molecules like H₂O and CO₂ . These properties bode well for the use of this new class of inorganic semiconducting LCs for electro-optical displays and chemical sensing applications.