Design, Synthesis, And Characterization Of Aluminum(Iii) Porphyrin Assemblies For Use In Photochemical Cells

Abstract

A series of axially-coordinated aluminum(III) porphyrins were synthesized and evaluated as potential photosensitizers of a ruthenium-based water oxidation catalyst. The porphyrins themselves are 5,10,15,20-tetraphenylporphyrins, and differ in the degree of fluorination on the peripheral phenyl groups. These aluminum(III) porphyrins readily assemble into catalytic dyads through formation of a covalent ester linkage between the central aluminum atom of the porphyrin and a terminal carboxyl group on the ruthenium catalyst. The aluminum center is also able to act as a Lewis acid, forming the final triad by way of a coordinate bond with a suitable Lewis base. Catalytic dyads were successfully synthesized from all three porphyrins in the series; a set of control compounds were also prepared. The dyads and reference molecules were then characterized, with molecular structure and successful formation of the dyads being confirmed with proton NMR spectroscopy, optical properties assessed with respect to UV-Vis absorption and fluorescence spectroscopy, and redox potentials being assessed by both cyclic and differential pulse voltammetry. Formation of the final triad was achieved by titration of the catalytic dyads with a C60 fullerene functionalized to act as a Lewis base; absorption and fluorescence spectra were monitored during titration, allowing for confirmation of the triad formation, as well as calculation of binding constants. The characterization data were used to construct energy level diagrams, laying the groundwork for a theoretical abstraction of these molecule’s functioning. the catalytic systems as synthesized, as well as how they might function in a prototypical photochemical cell. ii Analysis of the results reveal these materials to be promising candidates as photoactivated water oxidation catalysts. The absorption and electrochemical data demonstrate that, when the catalytic dyads are formed, the electronic structure of the constituent parts is preserved. The fluorescence spectra of the dyads show significant quenching relative to the reference porphyrins. Control studies allowed for the exclusion of intermolecular processes as being the source of this quenching, and therefore the optically excited porphyrin must be able to interact with the attached ruthenium catalyst, either through energy or electron transfer. Based on the negligible overlap of the spectra of the catalyst with that of the porphyrins, energy transfer is unlikely. The most likely source of the fluorescence quenching is therefore electron transfer across the ester-bond. The formation and persistence of such a radical ion pair is a fundamental prerequisite for the material to function as a water oxidation catalyst, as it is on this charge-separated species that water oxidation proceeds. Coordination of the dyads with a fullerene ligand was similarly demonstrated, with the resulting triad exhibiting complete fluorescence quenching. The fullerene ligand itself was chosen specifically for its suitability as an electron acceptor, and once again the most likely cause of this quenching is intramolecular electron transfer.