First Row Transition Metal Complexes as Transmissive Filters for Astronomical X-Ray Polarimetry
thesisposted on 19.12.2012, 14:05 by Andrew Paul Armitage
This thesis aims to develop single crystals that can act as transmissive X-ray polarising filters at K-edge energies corresponding to the first row transition metal series (M = Cu–Ti; K-edge range = 8.9–4.9 keV). Discrete square-based pyramidal co-ordination complexes have been targeted in which the apical M–X or M=O bonds display a high degree of intermolecular alignment. Chapter 1 provides an overview of X-ray polarisation dependence, existing X-ray polarimeters and current materials offering X-ray polarisation capabilities. Finally, a molecular design strategy for the thesis is outlined. In Chapter 2, [(4,4′-Me2bipy)2Cu2Br2(μ-C2O4)] (1a) has been subjected to a successful dichroic evaluation at the Cu (8.9 keV) and Br K-edges (13.5 keV). The origins of the X-ray dichroism in 1a have been modelled using two complementary theoretical methodologies; i) TD-DFT and ii) multiple scattering theory, which have identified the importance of the apical Cu–Br bond in SBP 1a. In Chapter 3 the motif [(α-diimine)MBr(β-diketonate)] has been targeted as an all purpose platform for the Cu (3–5, 8.9 keV), Ni (6, 8.4 keV), Co (7, 7.7 keV), Fe (10, 7.1 keV) and Mn (11, 6.4 keV)] K-edges. Highly aligned SBP 3a has been shown to display X-ray dichroism at the Cu K-edge. For 6, 7, 10 and 11, five co-ordinate metal centres can indeed be accessed using suitably tailored pyridylarylimine chelates; however, the geometries of the metal centres can be anywhere between distorted trigonal bipyramidal and distorted square-based pyramidal while the intermolecular alignment is variable. In Chapter 4 macrocyclic tetraphenylporphyrinato (TPP2-) has been employed as a rigid support to fill the basal co-ordination sites in SBP [(TPP)MX] (M = Co, X = Cl (15a); M = Co, X = Br (15b); M = Fe, X = Br (16); M = Mn, X = Br (17)] and [(TPP)MO] [M = V (18), Ti (19)]. Notable X-ray dichroism has been observed at the Co (7.7 keV), Fe (7.1 keV), V (5.5 keV) and Ti (4.9 keV) K-edges in highly aligned 15, 16, 18 and 19, respectively. The observed dichroic features in 15a and 18 have also been modelled using multiple scattering theory. In addition, compact hybrid filters were developed using TPP-modified [(Y2-TPP)FeX] [(Y = Cl, X = Br (20a); Y = Br, X = Cl (20b); Y = X = Br (20c)], which offer multiple K-edges in one molecule. Chapter 5 explores exclusively halides and oxides of the earlier transition metal elements [Cr (5.9 keV), V (5.5 keV), Ti (4.9 keV)], in which the pre-edge has been observed to reveal the most dominant X-ray polarisation dependence. Hence, M(III) complexes [(nacnac)MCl(β-diketonate] [M = Cr, β-diketonate = dbm (21), acac (22); M = V, β-diketonate = acac (23a), dbm (23b); M = Ti, β-diketonate = dbm (24)] have been targeted and selected examples have been deliberately oxidised to generate M(IV) oxides [(nacnac)MO(dbm)] [M = V (25), Ti (26)]. The FEFF code has been used to predict the polarised XAS spectra in an attempt to evaluate the feasibility of materials of this structural type. Chapter 6 gives overarching conclusions of the key themes of the thesis with a summary that highlight the contributions to the area and science in general.