Molecular semiconductors based on transition metal complexes
Silber, Georg Thomas
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The field of organic, or molecular, electronics is currently dominated by both polymeric and molecular organic materials, while considerably less research efforts are devoted to transition metal based complexes. Despite this, such compounds can offer advantages including additional tuneability of the spatial distribution and energy levels of the frontier orbitals or stable paramagnetic species by manipulating the metal-ligand interactions which may be accomplished selectively modifying the ligand framework or changing the central metal. A series of Ni(II) and Cu(II) acenaphthenequinone bis(thiosemicarbazonato) complexes were prepared and characterised using X-ray diffraction, cyclic voltammetry, UV/Vis and EPR spectroscopy, as well as magnetic susceptibility and field effect transistor measurements and computational calculations. The observed charge transport properties are discussed in terms of the structural and electronic trends both within the series and in the context of the two more established analogue series, namely the bis(3-thiosemicarbazonato) and the diacetyl bis(3-thiosemicarbazonato) metal complexes. The Ni(II) analogues of the acenaphthenequinone bis(thiosemicarbazonato) family were found to exhibit p-type charge transport with mobilities between 10¯9 and 10¯5 cm2V¯1s¯1 depending on the exocyclic substitutent and resulting packing pattern. The observed results were rationalised in terms of the reorganisation energy and the charge transfer integrals. A series of 4,4`-phenyl-substituted nickel dithiolene complexes was synthesised and characterised. Initially with the aim of investigating the effect of varying the para-substituent of the phenyl ring on the charge transport properties, these efforts were undermined by the poor processability of these molecules by both vapour and solution phase methods. As a result, n-type charge transport could be observed under ambient conditions only for the phenyl and 4-bromo-phenyl substituted analogues, but the device performance was extremely poor. Nonetheless, the calculated reorganisation energies, charge transfer integrals and predicted mobilities were encouraging and may prompt further work on these materials. An all-organic analogue series of 4,4`-(4-halogen-phenyl)-substituted tetrathiafulvalenes was also investigated. The hole transport materials displayed mobilities of between 10¯3 and 10¯7 cm2V¯1s¯1 for both solution and vapour processed devices, depending on the nature of the halogen. These results are discussed in terms of their molecular properties and the calculated charge transport parameters and put in context of the performance of the 4,4`-bis(phenyl)-substituted benchmark analogue. Interestingly, the obtained crystal structure of the bromo-substituted analogue showed the molecule to be in the cis conformation, an observation that is unprecedented for simple, 4-phenyl,5-hydrogen substituted tetrathiafulvalenes, and indicates that both conformers are initially formed. Finally, a series of 4,4`-(2-alkyl)thienyl substituted nickel dithiolene salts and tetrathiafulvalenes was synthesised and characterised. While the charge transport properties of the former were not further investigated due to the low solubility of the neutral species, the tetrathiafulvalenes were incorporated into FET devices via solution processing. All exhibited comparatively high conductivity at room temperature (1.6x10¯3S m¯1), exceeding that of their quarterthiophene analogues. This masked the observed gate effects but indicates potential applications as conducting or charge transfer materials. While the two resolved analogues displayed trans geometry in the single crystal structures, powder diffraction and preliminary DSC measurements indicate that the materials displayed at least one additional phase, which once again likely corresponded to the cis conformer.