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Synthetic strategies towards covalently linked multiporphyrin arrays
thesisposted on 14.02.2017, 00:34 authored by Mulholland, Amy Rachel
This thesis explores new synthetic methodology for the construction of multiporphyrin arrays. The formation of new carbon-carbon bonds is a unifying theme in all of the syntheses presented as part of this work. Suzuki cross-coupling chemistry and olefin metathesis were explored in the context of synthetic porphyrin chemistry leading to large, multichromophoric systems. Subsequent analysis of the host-guest chemistry of these systems gave insight into cooperative processes and the association kinetics in supramolecular complexes held together by metal-ligand and π-π interactions. Initial work focused on template-directed strategies for the synthesis of cyclic porphyrin arrays. The unusually high affinity of fullerenes for porphyrins inspired the application of C60 and C70 to the templated synthesis of two different types of cyclic porphyrin trimer. Olefin metathesis was selected as the method of reversibly linking the porphyrinic monomers during synthesis, resulting in a product distribution that is largely under thermodynamic control. The fullerene is also believed to have a secondary, kinetic role in the stabilisation of a 3-porphyrin transition state prior to covalent linkage. Varying the solvent composition during the synthesis of the hexa-linked trimer showed that the product yield varied quadratically with the percentage of toluene in dichloromethane/toluene mixtures. This result illustrated the complex and synergistic relationship between solvation, desolvation and cooperative processes in a templated synthesis driven by π-π interactions. Solvent-related differences in the initiation rate of Grubbs’ catalyst were also considered in light of this result. The second component of this thesis describes the design of an artificial light harvesting device based on a supramolecular cucurbituril-methyl viologen inclusion complex with porphyrin units appended to the periphery. Although the synthetic chemistry leading to a covalently-linked cucurbituril-porphyrin conjugate was unsuccessful, several broadly applicable porphyrinic building blocks were developed as part of this work. Recognition processes in multivalent metalloporphyrin hosts were studied using a binding motif based on a biphenyl-linked bisporphyrin. The synthesis of these porphyrin dimers and dyads proceeded via a stepwise approach in which sequential amide and Suzuki couplings gave rise to both homo- and heteroporphyrin dyads in reasonable yields and without the need for extensive purification. The complexation of the Zn(II)/Zn(II) dimer with 4,4’-dipyridyl, 1,2-bis(4-pyridyl)ethane and DABCO was examined, revealing that a ‘cyclic’ 1:1 complex forms initially and is eventually degraded to the 1:2 complex in the presence of excess ligand. The calculated microscopic binding constants were self-consistent with the Ka values derived from titration of this host with the monotopic ligands pyridine, 4-methylpyridine and quinuclidine. Secondly, the elaboration of this core binding motif with a derivative of C60 was achieved using similar synthetic methodology. Intended as a tunable system where the efficiency of photoinduced electron transfer could be studied as a function of donor-acceptor orientation, the host-guest chemistry of this complex was also explored. The K1 values for the formation of a supramolecular tetrad with the bipyridyl ligands were 30-40% lower cf. the model zinc(II) dimer, reflecting competition from the C60 moiety. The binding stoichiometry of the bisporphyrin-C60 triad-DABCO complex was determined by a combined global non-linear regression analysis of UV-vis and fluorescence spectroscopic titration data, revealing the unusual self-assembly of a 2:2 complex.