Thin film studies of planar transition metal complexes
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At present the field of molecular electronics - also known as molecular semiconductors, organic semiconductors, plastic electronics or organic electronics - is dominated by organic materials, both polymeric and molecular, with much less attention being focused on transition metal based complexes despite the advantages they can offer. Such advantages include tuneable frontier orbitals through the ligand/metal interaction and the ability to generate stable paramagnetic species. Devices containing radical materials are particularly interesting in order to examine the interplay between conduction and spin - an effect which is not yet properly understood but can give rise to exotic behaviour. A series of homoleptic, bis-ligand Ni(II) and Cu(II) complexes were prepared using three structurally related phenolic oxime ligands, 2-hydroxy-5-t-octylacetophenone oxime (t-OctsaoH), 2-hydroxy-5-n-propylacetophenone oxime (n-PrsaoH) and 2- hydroxyacetophenone oxime (HsaoH). The complexes were characterised by single-crystal X-ray diffraction, cyclic voltammetry, UV/Vis spectroscopy, field-effect-transistor measurements, DFT/TD-DFT calculations and in the case of the paramagnetic species, EPR and magnetic susceptibility. Variation of the substituent on the ligand from t-octyl to n-propyl to H enabled electronic isolation of the complexes in the crystal structures of M(t-OctsaoH)2, which contrasted with π-stacking interactions observed in the crystal packing of M(n-PrsaoH)2 and of M(HsaoH) (M = Ni, Cu). This was further evidenced by comparing the antiferromagnetic interactions observed in samples of Cu(n-PrsaoH)2 and Cu(HsaoH)2 with the ideal paramagnetic behaviour for Cu(t-OctsaoH)2 down to 1.8 K. Despite isostructural single crystal structures for M(n-PrsaoH)2, thin-film X-ray diffraction and SEM revealed different morphologies depending on the metal and the deposition method employed. However, the complexes of M(n-PrsaoH)2 and M(HsaoH) failed to demonstrate significant charge transport in an FET device despite displaying the ability to form π- stacking structures. A series of planar Ni(II), Cu(II) and Co(II) dibenzotetraazaannulenes (dbtaa) and dinapthotetraazaannulenes (dntaa) were synthesised and studied crystallographically, optically, electrochemically and magnetically. Thin films of each of these complexes have been prepared by vacuum deposition to evaluate the field-effect transistor (FET) performance as well as the morphology and crystallinity of the film formed. Single crystal data revealed that Ni(dbtaa) and Cu(dbtaa) are isomorphous to each other, with Co(dbtaa) displaying a different crystallographic packing. The electrochemistry and UV/Vis absorption studies indicate the materials are redox active and highly coloured, with molar extinction coefficients as large as 80,000 M-1cm-1 in the visible region. The paramagnetic Cu(II) and Co(II) complexes display weak 1-dimensional antiferromagnetic interactions and were fit to the Bonner-Fisher chain model. The data revealed that the Co(II) species possesses much stronger magnetic exchange interactions compared with the Cu(II) complex. Each of the materials formed polycrystalline films when vacuum deposited and all showed ptype field-effect transistor behaviour, with modest charge carrier mobilities in the range of 10-5 to 10-9 cm2 V-1 s-1 . SEM imaging of the substrates indicates that the central metal ion, and its sublimation temperature, has a crucial role in defining the morphology of the resulting film. Structurally related Cu(II) and Ni(II) dithiadiazoletetraazaannulene (dttaa) macrocycles were synthesised and studied in the context of their thin film electrochemical, conducting and morphological properties. Both the Ni(II) and Cu(II) complexes were found to be volatile under reduced pressure, which allowed crystals of both materials to be grown and the single crystal structures solved. Interestingly, the crystal packing of these heterocyclic macrocycles varies depending on whether the central metal ion is Cu(II) or Ni(II), which is in contrast to the analogous dibenzotetrazaannulenes complexes. Soluble Ni(II) analogues containing benzoyl groups on the meso- positions of the macrocycle (dttaaBzOR) were also prepared and contrasted with the insoluble Ni(dttaa) complexes in terms of their solution optical and electrochemical properties. Thin film electrochemical studies of Cu(dttaa) and Ni(dttaa) showed chemically reversible oxidative processes but on scanning to reductive potentials the films disintegrated almost immediately as the bulky counter tetrabutylammonium cation entered the thin film. FET studies undertaken on polycrystalline films of both complexes, using various device configurations and surface treatments, failed to realise any gate effect. Thin film XRD measurements indicate that films of both complexes formed by vacuum deposition are crystalline and contain a mixture of molecular alignments, with molecules aligning “edge on” and “face down” to the substrate. SEM imaging failed to effectively resolve the morphology of the films implying the sizes of the crystallites are small, which may help to explain the lack of FET effect. A series of bis-ligand diimine Ni, Cu and Pd complexes have been synthesised from the ligand 4,5-bis(dodecyloxy)benzene-1,2-diamine (dbdaH2). The same ligand was also used to prepare a series of soluble Cu(II) and Ni(II) tetraazaannulene macrocycles. All the bis-ligand diimine complexes were found to suffer from instability in air due to the ease at which the complexes are oxidised. The Ni complex, Ni(dbda)2, was found to display a NIR transition in the region of 971 to 1024 nm depending on the polarity of the solvent that the molecule is dissolved in. Solution electrochemistry studies of Ni(dbda)2 reaffirmed the facile nature of the first oxidative process, with the HOMO energy calculated at -4 eV by hybrid-DFT. This compound failed to yield semiconducting behaviour in an FET device despite the use of surface treatments aimed at promoting suitable molecular alignment across the conducting channel.