Spin-Doctoring Cobalt Redox Shuttles for Dye-Sensitized Solar Cells

A new low-spin (LS) cobalt(II) outer-sphere redox shuttle (OSRS) [Co(PY5Me₂)(CN)]⁺, where PY5Me₂ represents the pentadentate ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, has been synthesized and characterized for its potential application in dye-sensitized solar cells (DSSCs). Introduction of the strong field CN^- ligand into the open axial coordination site forced the cobalt(II) complex, [Co(PY5Me₂)(CN)]⁺, to become LS based upon the complex's magnetic susceptibility (1.91 ± 0.02 μ_B), determined by the Evans method. Interestingly, dimerization and subsequent cobalt hexacyanide cluster formation of the [Co(PY5Me₂)(CN)]⁺ monomer was observed upon long-term solvent exposure or addition of a supporting electrolyte for electrochemical characterization. Although long-term stability of the [Co(PY5Me₂)(CN)]⁺ complex made it difficult to fabricate liquid electrolytes for DSSC applications, short-term stability in neat solvent afforded the opportunity to isolate the self-exchange kinetics of [Co(PY5Me₂)(CN)]^2+/+ via stopped-flow spectroscopy. Use of Marcus theory provided a smaller than expected self-exchange rate constant of 20 ± 5.5 M^-1 s^-1 for [Co(PY5Me₂)(CN)]^2+/+, which we attribute to a Jahn-Teller effect observed from the collected monomer crystallographic data. When compared side-by-side to cobalt tris(2,2'-bipyridine), [Co(bpy)₃]³⁺, DSSCs employing [Co(PY5Me₂)(CN)]²⁺ are expected to achieve superior charge collection, which result from a smaller rate constant, k_et, for recombination based upon simple dark J- E measurements of the two redox shuttles. Given the negative redox potential (0.254 V vs NHE) of [Co(PY5Me₂)(CN)]^2+/+ and the slow recombination kinetics, [Co(PY5Me₂)(CN)]^2+/+ becomes an attractive OSRS to regenerate near IR absorbing sensitizers in solid-state DSSC devices.