Dynamics of the triplet state of phthalocyanine complexes of the platinum metals in zero field

The luminescence properties of phthalocyanine complexes of platinum metals with d⁶ and d⁸ electron configurations were investigated in dilute solutions between room temperature and 1.6°K. For the first time it was possible to study the magnetic sublevels of the triplet state of aromatic molecules in zero field through optical detection methods alone. It was found that the lowest triplet state of these molecules has three zero field components of unequal splitting which depends on the central metal ion and which ranges from approximately 0.1–‐10 cm^−‐1. Intersystem crossing populates solely the highest triplet component which does not radiate to the ground state, but which, in turn, populates the lower lying triplet components. The spin‐lattice relaxation rates of these processes could be estimated to be of the order of 10⁵ sec⁻¹ at liquid helium temperature, and the triplet to ground state ${\mathrm{(S}}_0{\mathrm{\leftarrow \; T}}_1)$ decay rates were obtained. A noticeable solvent dependence of the radiationless rates was observed. The effect of spin‐orbit coupling and radiationless energy transfer as a function of the metal ion in the phthalocyanine complexes is discussed. Surprisingly, Pd and Rh phthalocyanine were found to fluoresce as well as phosphoresce from essentially ligand‐centered,excited states, an unusual occurrence in open d shell transition metal complexes.