Divalent Intermediates\nin Lanthanide-Based Photocatalysts:\nSpectroscopic Characterization and Reactivity

The reduction of\nstable trivalent lanthanide species\n(Ln(III))\nby the excited states of organic chromophores is the basis of photocatalytic\ndivalent lanthanide-mediated reduction reactions. While indirect evidence\nof the photochemical formation of the reactive Ln(II) species is abundant,\ndirect spectroscopic evidence of their presence is scarce. Here, nine\nchromophores with absorptions covering the near UV and visible ranges\nwere systematically investigated in the presence of Ln(III) ions to\nevaluate their ability to reduce Eu(III) upon excitation with visible\nlight to the catalytically active Eu(II) species. Irradiated mixtures\nof Eu(III) and the chromophores were characterized using UV–vis\nabsorption and emission and EPR spectroscopy. Several of the chromophore-Eu(III)\ncombinations were competent photocatalysts in the presence of N,N-diisopropylethylamine\nor Zn terminal reductants. These results demonstrate that a variety\nof visible-absorbing chromophores can efficiently generate reactive\nEu(II) from Eu(III) to catalyze Ln(II)-mediated reduction reactions.