Heterocycle Effects on the Liquid Crystallinity of Terthiophene Analogues

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).

Keywords: 2D charge transport pathways (or 2D lamellar arrays); lamellar structures; liquid crystals; organic semiconductors; self-assembly; structure-property relationships.

Figure 1

The structures of compounds Th3(n), Th-Oxd(n) and Th-Thd-Th(n).

Scheme 1

Synthetic route to Th-Oxd-Th(10) and Th-Thd-Th(10). (i) (1) n-BuLi, THF, −78 °C; (2) C₁₀H₂₁Br, RT, 81%; (ii) (1) n-BuLi, THF, −78 °C; (2) CO₂, RT, 69%; (iii) SOCl₂, reflux; (iv) H₂NNH₂·H₂O, Et₃N, NMP, RT, 37%; (v) SOCl₂, reflux, 64%; (vi) Lawesson’s reagent, toluene, reflux, 51%.

Scheme 2

Synthetic route used to prepare the complete series of Th-Thd-Th(n) derivatives (n = 4, 6, 8, 12); (i) H₂NNH₂·H₂O, sulfur, propanol, 150 °C, high pressure, 88%; (ii) (1) n-BuLi, t-BuOK, THF, −78 °C; (2) C₁₀H₂₁Br, RT, 29%–69%.

Figure 2

POM of Th₃(10) showing schlieren (a) and focal-conic (b) textures of the SmC phase at 93.5 °C, modified textures of the SmF phase at 91.5 °C (c), and changes upon cooling to the crystalline phase at 65.0 °C (d).

Figure 3

POM of Th-Thd-Th(10) showing schlieren and focal-conic textures of the SmC phase at 125.0 °C (a), modified textures of the CrJ phase at 91.0 °C (b), and changes upon cooling to the crystalline phase at 79.0 °C (c).

Figure 4

XRD of Th-Thd-Th(10) at 126 °C, in the SmC phase (a); at 88 °C, in the CrJ phase (b); and at 25 °C, in the crystalline phase (c).

Figure 5

Phase behaviour of Th-Thd-Th(n) (this work) and Th₃(n) (*literature) [74].

Figure 6

The optimized structures of the compounds studied, showing differences in bend angle, θ, and transverse dipole moment. See Supplementary Materials for calculation details.

Figure 7

Crystal structure of Th-Thd-Th(10) showing lamellar order (a) and pseudo-hexagonal intra-layer packing (b). Legend: Alkyl chains in grey, aromatic core in red, hydrogens omitted for clarity.

Figure 8

Packing of adjacent molecules in Th-Thd-Th(10) (a) and Th₃(8) (b) as viewed along the long axis of the molecule (left) and the short axis of the molecule (right). Legend: sulphur in yellow, nitrogen in light blue, carbon in grey, hydrogens omitted for clarity.

Figure 9

Absorption (a) and emission (b) spectra of Th₃(10), Th-Oxd-Th(10) and Th-Thd-Th(10) in CHCl₃.

Figure 10

Cyclic voltammograms of 1.0 mM solutions showing reduction of Th-Thd-Th(6) (red) and Th-Oxd-Th(10) (blue) in THF with 0.1 M TBAP supporting electrolyte and oxidation of Th-Thd-Th(6) (red, dotted) in MeCN with 0.1 M TBAF supporting electrolyte.