Fundamental Characterization, Photophysics and Photocatalysis of a Base Metal Iron(II)-Cobalt(III) Dyad.

A new base metal iron-cobalt dyad is presented, connecting a heteroleptic tetra-NHC iron(II) photosensitizer combining a 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine) with 2,6-bis(3-methyl-imidazol-2-ylidene)-4,4'-bipyridine ligand, and a cobaloxime catalyst. This iron(II)-cobalt(III) assembly is characterized by ground and excited state methods like X-ray crystallography, X-ray absorption spectroscopy, (spectro-)electrochemistry, and steady-state and time-resolved optical absorption spectroscopy, with a focus on the stability of the assembly in solution and the determination of the excited state landscape. NMR and UV-Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal-to-ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet  MLCT states, leading to the population of the 3 MLCT state that exhibits the longest lifetime. Decay into the ground state finally involves a 3MC state. The attachment of cobaloxime to the iron photosensitizer increases the 3MLCT lifetime at the iron center. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogue two-component system, consisting of the iron photosensitizer and Co(dmgH)2(py)Cl. This work thus sheds new light on the functionality of base metal dyads, important for more efficient and sustainable future proton reduction systems.