Synthesis and characterisation of chromium (III) complexes with polypyridyl ligands

Slowing down climate change is probably the biggest current global challenge for mankind. The two main strategies to achieve this are the development of new sustainable energy sources and new or improved technologies that are less energy consuming. Inorganic transition metal complexes are the basis for two technologies that address both energy production and energy saving: dye-sensitised solar cells (DSCs) and light emitting electrochemical cells (LECs). Promising results were obtained for devices based on scarce metal complexes, such as iridium and ruthenium, but the rarity of these metals is a big drawback in terms of their widespread use and commercialisation. Therefore, complexes with more abundant metals, such as zinc and copper, are now being investigated. Since chromium(III) complexes are known to be emitting and kinetically inert, we chose [Cr(2,20-bipyridine)3]3+ as a model for our approach to use chromium(III) complexes as active compounds in LECs. A series of both new and known homoleptic and heteroleptic {Cr(bpy)3}3+ containing complexes were synthesised. However they showed no electroluminescence in the device con guration, even though they exhibited photoluminescence in solution.
The synthesis and characterisation of 25 new chromium(III) complexes with polypyridyl ligands have been presented in this PhD-thesis (7 tris(diimine)chromium(III) complexes and 18 bis(terpyridine)chromium(III) complexes). A new three step synthesis route for the preparation of {Cr(tpy)2}3+ complexes was developed. Starting with anhydrous CrCl3, {Cr(tpy)Cl3} was prepared, followed by exchange of the Cl- counter anions to CF3SO3- to yield {Cr(tpy)(CF3SO3)3}. In the last step, the second tpy ligand is added to get {Cr(tpy)2}3+. The successive addition of the two tpy ligands gives the possibility to synthesise heteroleptic {Cr(tpyA)(tpyB)}3+ complexes. These are the first reported examples of heteroleptic bis(terpyridine)chromium(III) complexes. The structures of several heteroleptic complexes could be confirmed with X-ray analysis. Altogether, 17 crystal structures (homoleptic and heteroleptic, 6 tris(diimine)chromium(III) complexes and 12 bis(terpyridine) chromium(III) complexes) could be determined. Despite the fact that the Cr3+ ion (d3) is widely recognised as a kinetically inert metal centre, we observed a sensitive liability of the complexes in the presence of F- or in an alkaline environment. The residues of these decompositions could be confirmed as {Cr(tpy)F3} and {Cr(tpy)(OH)3} respectively. These findings led to an explanation of the photoluminescence observations done previously. It was already known that the photoluminescence properties of {Cr(tpy)2}3+ are quite different from those of {Cr(bpy)3} 3+. We found that the majority of the observed emission could be assigned as deriving from small quantities of neutral or protonated free ligand. The {Cr(tpy)2}3+ complexes containing at least one N,N -diphenylaniline-tpy showed a broad and solvatochromic absorption. Although the broad absorption makes these complexes strong candidates for DSC applications, all trials were unsuccessful. Also, the application of {Cr(bpy)3}3+ complexes in LEC devices was unsuccessful.