Influence of terminal alkyl groups on the structure, electrical and sensory properties of thin films of self-assembling organosilicon derivatives of benzothieno[3,2-b][1]benzothiophene

The synthesis and systematic investigation of organosilicon derivatives of benzothieno[3,2-b][1]benzothiophene (BTBT) with different lengths of terminal alkyl groups (from C2 to C13), or without them, capable of self-assembly at the water–air interface are described. All novel compounds were obtained with high yields and purity, which was proved by 1H, 13C and 29Si-NMR spectroscopy, gel permeation chromatography and elemental analysis. The study of their phase behavior by differential scanning calorimetry and polarizing optical microscopy showed that all the compounds are crystalline at room temperature. However, at elevated temperatures those with terminal alkyl substituents form enantiotropic smectic A or C mesophases, while the compound without terminal alkyl groups forms a monotropic nematic mesophase. The increase of the alkyl chain length from C2 to C8 leads to higher temperatures of both phase transitions: from the crystal to the disordered smectic mesophase and to the isotropic melt. X-ray diffraction measurements confirm the presence of disordered SmA and SmC phases and the ordered SmE or SmK phase, and suggest models of their packing depending on the terminal alkyl chain length. The investigation of the electrical properties of monolayer OFETs based on these compounds and their sensing responses to NO2 revealed the dependence of charge carrier mobility values and sensitivity parameters mainly on the surface morphology of organic semiconductors. A comparison of the semiconducting and sensing properties of different compounds investigated revealed a weak correlation between these parameters, while the best characteristics were observed for the OFETs based on the siloxane dimer of BTBT with C8 terminal groups.