Carbenes: The Gathering : Photophysics of Transition Metal Carbene Complexes

Abstract: This thesis explores the photophysics of some transition-metal complexes (TMCs) which utilize N-heterocyclic carbenes as ligands. After a historical interlude which traces the development of the broader field of transition metal complexes and their photophysical investigations, there is an overview of theoretical concepts and the spectroscopic methods employed. The focus thereafter is placed on complexes of the type [ML2]n+ (where M=Fe and Mn) which feature the tripodal ‘Scorpionate’ motif, i.e. L=[phenyl(tris(3-methylimidazol-1-ylidene))borate]–. L, like many carbenes, is an exceptional sigma-donor, and by some metrics is the strongest tripodal donor known. It is therefore able to sufficiently destabilize metal-centred states in conjunction with several 3d metals, allowing for the realization of long-lived charge-transfer states on the nanosecond timescale, in sharp contrast to many complexes based on polypyridyl ligand motifs previously investigated.[FeIIIL2]+ features a 2 ns doublet ligand-to-metal charge transfer (LMCT) excited state, which is substantially energetic and is shown to be capable of engaging in photoinduced electron transfer reactions with both donors and acceptors. The strong ligand field imposed on the iron centre furthermore makes possible the occurrence of two metal-centred redox events before the ligand oxidation – this translates to the unusual situation of the LMCT excited state of [FeIIIL2]+ being able to oxidize or reduce its own ground state: a phenomenon called photoinduced symmetry-breaking charge separation. The finding is the first documented case with direct evidence for a transition-metal complex, and the only one which proceeds with a substantial driving force generally. [MnIVL2]2+ features a long-lived LMCT excited state, which is found to be a potent photo-oxidant, capable of oxidizing a range of substrates including solvents such as methanol. Its excited state lifetime of 16 ns also presents a near order of magnitude improvement over the iron counterpart. One possible cause is traced to the spin-forbidden nature of the transition back to the ground state, highlighting the importance of such a design principle for the realization of longer lifetimes, as has been the case previously for excited states based on precious metals. The last half of the thesis features benzothiadiazole-Au-carbene (and phosphine) chromophores that are bright phosphors in room temperature solution – it is found that the carbene is inconsequential to the photophysics in this case, which is instead contingent on the direct linkage of the gold atom to a heteroarene moiety, causing an efficient population of its triplet manifold. Concluding remarks are furnished.