The kinetics of quenching *Cu(dpp)2+, the 3d-π* excited state of Cu(dpp)2+ (dpp = 2,9-diphenyl-1, 10-phenanthroline), by a series of Cr(acac)3 derivatives and a series of nitroaromatics have been studied in CH2C12 solution at 20 °C. For the nitroaromatic compounds, the quenching rate falls off as the electrode potential of the quencher ranges below that of *Cu(dpp)2+, consistent with an electron-transfer mechanism. The results were analyzed by a variation of the Marcus theory, and the self-exchange rate for the Cu(dpp)22+/*Cu(dpp)2+ system was estimated to be 8 X 109 M-l s-1. The Cr(III) quenchers span a much wider range of electrode potentials and are also capable of undergoing energy-transfer quenching via the low-lying doublet excited states. As a result the quenching rate constant exhibits a more complex dependence on the free energy of electron-transfer quenching. In the exergonic region, the rate plateaus at the diffusion limit; as electron-transfer quenching becomes endergonic, the rate initially decreases and then levels off again at ca. 107 M-l s-1. This behavior is explained as a changeover from electron-transfer to energy-transfer quenching where the latter process occurs at a roughly constant rate throughout the series. When electron-transfer quenching is prominent, the average self-exchange rate for the Cr(III)/Cr(II) couples is estimated to be 2 X 109 M-1 s-1, suggesting that the Cr(II) forms are low spin. The utility of *Cu(dpp)2+ as a redox reagent and novel aspects of the homologous series of Cr(acac)3 derivatives as quenchers are discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry