Are High-Potential Cobalt Tris(bipyridyl) Complexes Sufficiently Stable to Be Efficient Mediators in Dye-Sensitized Solar Cells? Synthesis, Characterization, and Stability Tests

We report the synthesis of three
new, high-oxidation-potential
cobalt complexes. The ligand lability of cobalt complexes, especially
those of higher redox potential, has been scantly addressed in the
current literature on the use of these complexes in dye-sensitized
solar cells (DSSCs). In characterizing our high-potential complexes,
multiple pieces of evidence reveal a trend of decreasing complex stability
with increasing redox potential. These include the appearance of multiple
waves in the cyclic voltammetry, a color change upon addition of 4-tert-butylpyridine indicating the formation of a new species,
direct NMR evidence of free dissociated ligand in acetonitrile solution,
and potential-independent DSSC recombination currents paralleling
the stability trends as determined by NMR. We take advantage of a
simple quantitative NMR experiment to determine the approximate ligand-binding
equilibrium (stability) constants of our complexes in acetonitrile
at room temperature. With the above information, we propose alternative,
clathrochelating ligand structures for the future study of high potential
cobalt mediators for application in DSSCs.