Some Aspects of the Inorganic and Organometallic Chemistry of Dipyrrins

Abstract

Oligopyrroles and their complexes with metals and metalloids are often intensely coloured, and as such, are well-studied as dyes and components of organic light-harvesting systems. Of particular interest in this thesis are (aza-)dipyrrins, conjugated dipyrrolic frameworks that act as convenient and highly tunable LX-type bidentate ligands. Boron complexes of (aza-) dipyrrins, called BODIPYs, are extremely well-studied as robust and flexible fluorescent dyes. Dipyrrinato complexes of other metals have also been studied for a plethora of applications from photomedicine and photovoltaics to catalysis and chemical sensors. In Chapter 2, the electron-donating capability of thirteen (aza-)dipyrrinato ligands are evaluated for the first time using 13C NMR spectroscopy and a probe based on an air-stable Pd-NHC complex, which is one of the few systems appropriate for the measurement of LX-type bidentate systems such as these. The measured donor strengths are rationalized based on chemical structure, and compared to other common ligands for which donor strength has been measured via this system. In Chapter 3, the design of an improved synthetic route toward aza-dipyrrins is detailed. Aza-dipyrrins and their complexes have significant potential as long-wavelength chromophores, but currently available synthetic routes severely limit the range of substitution patterns that can be accessed. An improved synthesis is described where the aza-dipyrrin core is built from acyclic precursors, avoiding the generation of unstable intermediates that render traditional methods inflexible. Preliminary experimentation with the aim of validating this method is also included, and considerations for future development are discussed. Chapter 4 describes the serendipitous discovery of a previously unreported class of azobispyrrole during the attempted synthesis of aza-dipyrrins with enhanced fluorescence. These azobispyrroles were found to exhibit unusual behaviour, lacking observable photoisomerization but instead displaying strong solid-state fluorescence. Finally, Chapter 5 details the design and synthesis of BODIPYs bearing 2-tellurophenyl substituents. The inclusion of the tellurophene moiety affords substantial bathochromic shifts in the optical profiles of these BODIPYs. BODIPYs bearing this substituent at the alpha-position(s) have potential as bifunctional probes for mass and fluorescence cytometry, and those bearing the substituent at the beta-position(s) may be used as photosensitizers for photodynamic therapy. This work contains the first examples of BODIPYs bearing tellurophene moieties.