Quenching of Triplet States of Organic Compounds by Coordination Complexes

Recent evidence which establishes that electronic energy transfer is the most common mechanism for quenching of organic triplet states by coordination compounds is discussed. It is pointed out that only when energy transfer produces ligand field excited states is there likely to be markedly different behaviour observed from that expected for a typical organic triplet energy acceptor. Transfer to produce internal ligand and ligand to metal or metal to ligand charge-transfer excited states is not expected to show any differences other than the occurrence of spin statistical factors and these probably only when transfer produces excited charge transfer states. New evidence is presented that triplet state quenching by ferrocene, tris(acetylacetonate) iron(III), F&"(acac)3, and tris(dipivaloylmethanate) iron(III), Fe(dpm)3, is due to electronic energy transfer. For these quenchers a high degree of correlation between their spectroscopically determined energy levels and plots of the quenching rate constants as a function of the energy of the triplet state being quenched is demonstrated. It is suggested that the use of such quenching plots in conjunction with stereochemical and spectroscopic information concerning the excited states which are energetically available to accept electronic excitation energy is likely to allow progress to be made in establishing the determining parameters which govern triplet energy transfer to produce ligand field excited states in coordination compounds. The mechanism of quenching of electronically excited triplet states of organic compounds by coordination complexes is still imperfectly understood despite the fact that the early pioneering work of Porter', Linschitz2, Hammond3 and their respective co-workers has been followed by several recent enlightening investigations'9. It has been established that the observed quenching rate constants show no correlation with magnetic susceptibility'3 and even that diamagnetic coordination complexes with low-lying electronically excited states can quench very efficiently3' . Conversely, it has been shown that when no low-lying electronically excited states of paramagnetic coordination complexes are available no quenching is observed5' '. These findings, as well as those few examples where sensitized luminescence of the quenching coordination complexes has been observed4'6 together with the less direct evidence of observations of several photosensitized reactions of coordination compounds by organic triplet states' 2 establish