Abstract
Liquid crystalline self-assembly offers the potential to create highly ordered, uniformly aligned, and defect-free thin-film organic semiconductors. Analogues of one of the more promising classes of liquid crystal semiconductors, 5,5”-dialkyl-α-terthiophenes, were prepared in order to investigate the effects of replacing the central thiophene with either an oxadiazole or a thiadiazole ring. The phase behaviour was examined by differential scanning calorimetry, polarized optical microscopy, and variable temperature x-ray diffraction. While the oxadiazole derivative was not liquid crystalline, thiadiazole derivatives formed smectic C and soft crystal lamellar phases, and maintained lamellar order down to room temperature. Variation of the terminal alkyl chains also influenced the observed phase sequence. Single crystal structures revealed the face-to-face orientation of molecules within the layers in the solid-state, a packing motif that is rationalized based on the shape and dipole of the thiadiazole ring, as corroborated by density functional theory (DFT) calculations. The solution opto-electronic properties of the systems were characterized by absorption and emission spectroscopy, cyclic voltammetry, and time-dependent density functional theory (TD-DFT).
Highlights
Organic semiconductors are attractive materials due to their light weight, mechanical flexibility, tunability through chemical modification, and compatibility with solution processing techniques that allow for low-cost device fabrication [1,2,3,4]
They have proven suitable for applications such as light emitting diodes (OLEDs) [5,6,7,8], field-effect transistors (OFETs) [9,10,11,12,13], and photovoltaics (OPVs) [14,15,16,17], an organic semiconductor’s performance is limited by its supramolecular ordering
Liquid crystals (LCs) represent a promising alternative to common crystalline solid organic semiconductors, owing to their ability to self-assemble into ordered self-healing structures capable of uniform alignment over large areas, and with supramolecular order that can be maintained in the solid-state [31,32,33,34,35,36,37]
Summary
Organic semiconductors are attractive materials due to their light weight, mechanical flexibility, tunability through chemical modification, and compatibility with solution processing techniques that allow for low-cost device fabrication [1,2,3,4]. Smectic LCs have emerged as more attractive candidate materials since their layered structure permits conduction in two dimensions, potentially leading to more consistent performance [45,46,47,48,49] Tuning both the opto-electronic properties and self-assembly of smectic LC materials represents an ongoing challenge in their exploitation as organic semiconductors. Of these systems [72]
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