Tuning the visible absorption of manganese(I) tris(diisocyanide) complexes

With the aim of using transition metal complexes as photosensitisers, it is important that they feature visible absorption and high photostability. While arylisocyanide ligands have previously been used to improve the metal-to-ligand charge-transfer (MLCT) lifetimes in 3d⁶ complexes, the absorption features of tris(diisocyanide) manganese(I) complexes remain nonoptimal as they only tail into the visible part of the absorption spectrum. Taking outset in previously reported diisocyanide ligands, we synthesised and characterised a new manganese(I) complex with an electron deficient pyrazinyl spacer in the ligand backbone as a strategy to redshift the MLCT absorption. Here, we find that it is the decoupling between the electronically stabilised pyrazinyl spacer and the rest of the ligand that results in lower-lying MLCT transitions. We observe significant photodegradation from the chelating isocyanide manganese(I) complexes, but we found that increasing the steric bulk of chelating arylisocyanide ligands leads to a drastic increase in photostability. Ultimately, our work provides insight into understanding how arylisocyanide ligands can be optimised to improve the photophysical properties of first-row transition metal complexes.