The inter-relationship between triplet energies and spin chemistry

Abstract

Triplet energies play a considerable role in optical spectroscopy, and can be determined from phosphorescence or the quenching thereof. Their role in spin chemistry may not be as obvious, but the triplet state has always had an important function or utility, namely of reaction intermediates such as radical pairs, their precursors, of carbenes, and of the final products. In situ NMR spectroscopy represents a useful tool to explore certain properties of the triplet state, especially in cases with no phosphorescence. The ‘phase’ of CIDNP resonances, i.e., emission or enhanced absorption, reflects the spin selectivity of electron transfer reactions. In radical ion pairs the spin selectivity is determined by the relation between the change of the standard free enthalpy ΔG° during the electron back transfer and the triplet energies (E_T) of the products. If triplet recombination is energetically feasible (ΔG° > E_T), it is typically the more efficient process in agreement with the Marcus theory.