Sensitised luminescence of heterometallic d-f lanthanide complexes

Abstract

In this thesis the potential for a variety of covalently bridged bis-tpy transition metal complexes to act as an antenna for sensitised Near Infrared (NIR) luminescence from different Ln(III) cations has been explored. In total, a series of nine metalloligands were synthesised and fully characterised, and were then complexed with differing Ln(III) cations (Ln = Nd, Er, Yb and Lu) to form stable hetero bimetallic d-f complexes. A combination of the experimental steady state and time resolved photophysical properties for each of the metalloligands and their corresponding d-f complexes, and also supported by TD-DFT analysis, has provided several insights into the antenna process. Synthetic modifications to the bridging ligand scaffold, the transition metal coordination environment and the identity of the transition metal ion allowed various aspects of the electronic energy transfer which leads to sensitised NIR emission via the antenna effect to be elucidated.The mechanism of the observed electronic energy transfer was initially determined by constructing a series of 1,4-para phenylene spaced back-to-back tpy based Ru(II) antenna chromophores. A detailed analysis of the distance dependence between the Ru(II) based 3MLCT excited state donor and various 4f * metal centred acceptors in terms of the observed compared to calculated rates of energy transfer conclusively determined that the energy transfer between the antenna and the Ln(III) centre was mediated via a Dexter-type superexchange mechanism. After determining the superexchange mechanism was responsible for the observed energy transfer, the influence of the connectivity between the donor and acceptor was further investigated with a series of Ru(II) metalloligands wherein the 1,4-para phenylene linkage was substituted for either a meta 1,3-phenylene linkage, or an ethynyl bridge. In this way, while maintaining a similar intramolecular separation between the two metals, the influence of the differing connectivity between donor and acceptor was shown to have a large effect on their resulting electronic coupling, and therefore, greatly influenced the observed rates and hence the resulting efficiencies for the observed energy transfer. Using a comparative approach, the efficiency of the energy transfer was also explored by extending the lifetime of the antenna donor excited state. This was achieved synthetically by altering the coordination environment around the Ru(II) centre, moving from bis tridentate to tris bidentate polypyridyl based ligands. It was shown that this structural modification can clearly influence both the rate but more significantly the efficiency of energy transfer between the metal centres. The influence of the identity of the transition metal cation on the observed energy transfer in the back-to-back tpy d-f systems was explored. By exchanging the Ru(II) centre for the isoelectronic Ir(III) or Os(II) cations, the photophysical properties of the respective metalloligands and their bimetallic complexes are not surprisingly very different. This has a large bearing on the ability for each of these types of chromophore to act as an antenna for sensitised NIR luminescence from each Ln(III) cation. In general, the rates and efficiencies for the observed energy transfer using the Ir(III) based systems were superior, although this came at a significant cost of necessitating higher energy excitation wavelengths. By contrast, the Os(II) based analogues could be excited at much lower energy wavelengths, but generally displayed slower energy transfer rates which has been rationalised using the classical Marcus approach to electron transfer, applied in this case to the observed Dexter superexchange mechanism. Lastly, the most significant results are summarised, and by taking into consideration both the synthetic requirements and the resulting photophysical performance of the investigated series of metalloligands, several suggestions in terms of potential future work towards designing a heterobimetallic d-f system with applications in biological imaging are proposed.