Quenching of triplet states by Schiff base nickel(II) complexes
Rate constants for the quenching of triplet states of several molecules by certain Schiff bases and their Ni(II) chelate complexes have been measured using the technique of laser photolysis. In the case of the parent ligands the dependence of the quenching efficiency on the energy of the triplet states being quenched suggests triplet–triplet energy transfer as the quenching mechanism and allows estimates of the energies of the triplet states of the ligands which are often too short-lived to be calculated. The observed variation in rate constants for quenching by diamagnetic trans-planar Schiff base Ni(II) complexes is also explained as being due to triplet–triplet energy transfer with high energy triplet donors exciting triplet ligand states and low energy donors exciting triplet ligand field states in the quencher. It is suggested that the much lower efficiency of quenching by paramagnetic, tetrahedral and octahedral Ni(II) complexes of low energy triplet donors is due to the stereochemical changes and demonstrates the need for good overlap with d orbitals before spin-allowed exchange transfer of high efficiency can occur to lower lying ligand field states in coordination complexes.