Photochemistry of nickel salen based complexes and relevance to catalysis

A series of four nickel salen complexes have been synthesised and their photochemical properties studied. These complexes emit in hexane at λem 580 nm with similar quantum efficiencies. Fluorescence decay occurs on the nanosecond time scale and its intensity is quenched by oxygen except for the complex with the shortest lifetime. For all complexes laser flash photolysis in CH2Cl2 or hexane generates a transient (lifetime on the μs time scale) that was assigned to a Ni-to-ligand electron transfer triplet excited state. In an attempt to show the relevance of photochemical data to catalysis, epoxidation of β-methylstyrene was carried out in the presence of the four complexes. It was found that the activity of the complexes correlates with the lifetime of the triplet charge-separated state. This relationship was interpreted as reflecting the intrinsic stability of the oxidised Ni ion in the different complexes, which is the common link between the photochemical and catalytic experiments.