Synthesis of group 10 metal complexes for potential electronic applications

• Main Author: Blackburn, Octavia Ann
• Other Authors: Coe, Benjamin
• Published: University of Manchester 2011
• Subjects: 547
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.606873

The unique properties of dichroic dyes allow a guest-host display to be fabricated whereby an image is formed by liquid crystal (LC)-assisted orientational changes of the dyes upon application of an electric field. This technology is currently of interest for use in electronic paper, which aims to provide the consumer with the advantages of electronic media whilst enjoying portable paper-like images. Essential to this approach is the availability of chromophores with especially high dichroic ratios (DRs). This thesis concerns the relatively unprecedented use of metal ions in constructing new dichroic dyes. Previous studies have shown that DRs can be augmented by elongation of a dye, which can be facilitated by using metal ions to link simple organic chromophores. Linear connection of azobenzene moieties has been achieved by the synthesis of twenty new N2O2-chelated NiII and PdII complexes, resulting in approximately two-fold elongation. Single crystal X-ray structures have been obtained for two pro-ligands and five complexes, the latter showing strictly square-planar arrangements about the metal centres, except for a NiII complex of a ‘salen-like’ ligand. 1H NMR spectra of NiII complexes display broadening attributed to some distortion from square-planar in solution. UV–visible spectra of the complexes display intense bands in the range 409–434 nm with some metal-to-ligand charge-transfer (MLCT) character, as confirmed through Zerner’s intermediate neglect of differential overlap (ZINDO) and time-dependent density functional theory (TD-DFT) calculations. DR measurements in several LC matrices show that complexation to NiII results in an increased DR whereas PdII appears to have a detrimental effect. Side-by-side connection of azobenzene chromophores yields more intricately shaped complexes which are intended to show improved alignment within an LC through ‘mutual packing’. Eight new chloride-/acetate-bridged dipalladium complexes of push–pull azobenzene ligands have been synthesised. The complexes possess complicated UV–visible spectra, which are interpreted with the aid of TD-DFT calculations. The azobenzene-based band at low energy has superior dichroism to the metal/bridge-based band at higher energy, although no improvement in DR with respect to the pro-ligands is observed. Elongated analogues of these dinuclear systems are provided by three new chloride-bridged tetrapalladium complexes of 1,4-bis[trans-phenylazo]benzenes (BPAB) and an azoazoxybenzene derivative. Single crystal X-ray structures have been obtained for two pro-ligands and one complex, showing severe departures from planarity caused by the methyl groups of the central aryl rings. UV–visible spectra differ for BPAB and azoazoxybenzene complexes, which TD-DFT calculations show is due to charge-transfer character in the latter. DR results follow the same trends as for the dinuclear complexes, and in both cases these results correlate with the TD-DFT-calculated directions of the transition dipole moments (μ12). Six new PtII NCN-pincer complexes of 1,3-bis(N-methylimino)benzene ligands have been synthesised with various 5-substituents. The UV–visible spectra show intense high-energy ILCT bands for aryl- and styryl-substituted complexes with several low energy bands of mixed parentage. Structured emission is observed in solution at room temperature, with quantum yields of 0.1–0.2% and lifetimes of 159–170 ns, except for styryl-substituted complexes where emission is quenched by trans–cis isomerisation. DFT calculations indicate the emission to be of 3LLCT/3MLCT origins. Single crystal X-ray structures of two complexes reveal symmetric planar PtNCN cores with non-ideal N–Pt–N angles, and short intermolecular Pt…Pt distances (3.5469(4) Å) for an unsubstituted complex. Absorption DR results are negligible and measurement of dichroic luminescence is complicated by luminescence of the LC.