Tuning the charge transport properties of non-planar zinc(ii) complexes of azadipyrromethenes using solubilizing groups

Abstract

Non-planar zinc(II) complexes of azadipyrromethenes are π-conjugated molecules with high absorption in the visible to near-infrared (NIR) region and the ability to easily accept or donate two electrons. We previously showed that incorporating hexyl or hexyloxy solubilizing groups either on the distal or proximal phenyls of bis[2,6-diphenylethynyl-1,3,7,9-tetraphenyl azadipyrromethene] zinc(II) (Zn(WS3)₂) tuned the structural, thermal and optical properties of Zn(WS3)₂. Here, we analyzed the crystal packing, morphology in thin films, and charge transport properties of these molecules in diodes and organic thin-film transistors. Analysis of the crystal packing suggests that all complexes support isotropic charge transport except for the molecule with hexyloxy groups on the distal phenyls. Grazing-incidence wide angle X-ray diffraction shows that hexyl substitution tends to produce films with small crystallite sizes and poor orientation, while hexyloxy substitution leads to more crystalline and better oriented films. In general, these non-planar molecules are better suited for diodes than field-effect transistors. In diodes, hexyl substitutions promote electron transport, especially when placed on the proximal phenyls, giving a very high μ_e value of 9 × 10⁻⁴ cm² V⁻¹ s⁻¹. Hexyloxy substitutions at either position promote hole transport, with both molecules exhibiting a very high μ_h value of 3–4 × 10⁻³ cm² V⁻¹ s⁻¹. The molecule with hexyloxy groups on the distal phenyls shows an ambipolar character with high μ_e and μ_h values of 3 × 10⁻⁴ and 4 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. This study highlights the importance of the nature and placement of solubilizing groups in optimizing the charge transport properties of these non-planar molecules.