Electronic energy transfer from triplet state acridine to paramagnetic ions

Abstract

Energy transfer from triplet state acridine molecules to paramagnetic complex ions of the first transition series in aqueous solution has been studied by kinetic spectroscopy in absorption following flash photolysis. Paramagnetic ions with electronic states lying below the triplet acridine energy level gave rise to second-order quenching constants in the range 1–60 × 10⁷ l. moles^–1 sec^–1; the magnitudes of the quenching constants of those ions with no accessible levels were less than 0.1 × 10⁷ l. moles^–1 sec^–1. It is concluded that the deactivation of the triplet acridine proceeds almost exclusively by energy transfer in which the quenching species is raised to an electronically excited state. The quenching efficiencies for complex ions involving different saturated ligands with a given metal ion are discussed in terms of the nephelauxetic effect, which is a measure of the covalent nature of the metal-ligand bonds. The degree of the delocalization of the d electrons of the metal over the ligand orbitals directly affects the probability of energy transfer by an orbital to orbital overlap mechanism.