Abstract
The Li+- and K+-complexes of new discotic mesogens, where two n-alkoxy-substituted triphenylene cores are connected by a central crown ether (12-crown-4 and 18-crown-6), provide interesting structural and electronic properties. The inter- and intra-columnar structure was investigated by small and wide angle X-ray scattering. The electronic and ionic transports were studied by temperature dependent photoconductivity and impedance spectroscopy, respectively. Besides a strong increase of the stability and the width of the columnar phases the presence of soft anions (iodide, thiocyanate, tetrafluoroborate) leads to an improved intra-columnar order. The hereby shortened stacking-distance of the triphenylene cores leads to a significant increase of the photoconductivity in the columnar mesophase. Furthermore, the ionic conductivity of the new materials was investigated on macroscopically aligned thin films. The existence of channels for fast cation transport formed by the stacked crown ether moieties in the centre of each column can be excluded. The cations are coordinated strongly and therefore contributing only little to the conductivity. The ionic conductivity is dominated by the anisotropic migration of the non-coordinated anions through the liquid, like side chains favouring the propagation parallel to the columns. Iodide migrates about 20 times faster than thiocyanate and 100 times faster than tetrafluoroborate.
Highlights
Columnar phases of discotic liquid crystals display one-dimensional (1D) photoconductivity according to a seminal discovery by Haarer et al in 1993 [1], turning the investigation of the electronic charge transport in columnar liquid crystals into a very active field of research [2,3]
We reported about bi-centred liquid crystalline crown ethers and the impact of the molecular flexibility and geometry on structure and electronic charge transport properties in their columnar
Our results reveal that the enhanced intra-columnar order in the KSCN and KI complexes of the
Summary
Columnar phases of discotic liquid crystals display one-dimensional (1D) photoconductivity according to a seminal discovery by Haarer et al in 1993 [1], turning the investigation of the electronic charge transport in columnar liquid crystals into a very active field of research [2,3]. The design of discotic mesogens providing good charge transport properties in a suitable temperature range is of major interest [8,9,10]. Among these materials liquid crystalline crown ethers are promising providing a unique entry into supramolecular chemistry due to their known propensity for selective metal salt complexation [11,12,13]. We reported about bi-centred liquid crystalline crown ethers and the impact of the molecular flexibility and geometry on structure and electronic charge transport properties in their columnar
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