Mixed ionic/electronic conduction of organic semiconductors is greatly influenced by their microstructures. In the present work, two thieno[3,2-b]thiophene based liquid crystalline (LC) semiconductors, BTTT/PEO3 and BTTT/APEO6 with the same number of ion solvation sites but varying in the position, were synthesized by Stille coupling reaction. The influence of different side chain architectures on the chain mobility and on the molecular close-packing configuration was investigated. The side chain position greatly influenced the mesophase thermal stability as well as the molecular packing patterns, as determined by differential scanning calorimetry (DSC) and Grazing-Incidence Wide-Angle Scattering (GIWAXS) respectively. From GIWAXS measurements, the asymmetric BTTT/APEO6 led to a tighter packing within the mesogen layer, corresponding with a higher melting point. With the addition of an ionic dopant, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), BTTT/APEO6 tolerated a salt concentration up to r=0.1 ([Li<sup>+</sup>]/EO), whereas BTTT/PEO3 could not form an ordered structure at the same salt concentration. From the ionic conductivity measurements, the size of the side chain is a primary factor in enabling efficient charge transport above the melting temperature. This comparative study provides important guidance considerations for conductivity in amphiphilic oligothiophene-based LCs with enhanced mesophase stability and structural order.

The effect of side chain position on the structural arrangement and conducting properties of Thieno[3,2-b]thiophene-based conjugated liquid crystals

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