Fine-Tuning Metal and Ligand-Centered Redox Potentials of Homoleptic Bis-Terpyridine Complexes with 4'-Aryl Substituents.

Homoleptic transition-metal complexes of 2,2':6',2″-terpyridine (terpy) and substituted derivatives of the form [M(R-terpy)2]2+ display a wide range of redox potentials that correlate well to the Hammett parameter of the terpy substituents. Less is known about the impact of incorporating a phenyl spacer between the functional group responsible for controlling the electron density of terpy and how that translates to metal complexes of the form [M(4'-aryl-terpy)2]2+, where M = Mn, Fe, Co, Ni, and Zn. Herein, we report our studies on these complexes revealed a good correlation of redox potentials of both metal- and ligand-centered events with the Hammett parameters of the aryl substituents, regardless of aryl-substitution pattern (i.e., the presence of multiple functional groups, combinations of withdrawing and donating functional groups). The phenyl spacer results in 60-80% attenuation of electron density as compared to the 4'-substituted terpy analogue, depending on the metal and redox couple analyzed. Density functional theory calculations performed on a simple model system revealed a strong correlation between the Hammett parameters and lowest unoccupied molecular orbital energies of the corresponding substituted pyridine models, thus serving as an inexpensive predictive tool when coupled with electrochemical data. Overall, these data suggest that such ligand modifications may be used in combination with previous approaches to further fine-tune the redox potentials of homoleptic transition-metal complexes, which may have applications in photochemical and electrochemical catalytic processes.

Fundamental Factors Impacting the Stability of Phosphonate-Derivatized Ruthenium Polypyridyl Sensitizers Adsorbed on Metal Oxide Surfaces.

A series of 18 ruthenium(II) polypyridyl complexes were synthesized and evaluated under electrochemically oxidative conditions, which generates the Ru(III) oxidation state and mimics the harsh conditions experienced during the kinetically limited regime that can occur in dye-sensitized solar cells (DSSCs) and dye-sensitized photo-electrosynthesis cells, to further develop fundamental insights into the factors governing molecular sensitizer surface stability in aqueous 0.1 M HClO4. Both desorption and oxidatively induced ligand substitution were observed on planar fluorine-doped tin oxide (FTO) electrodes, with a dependence on the E1/2 Ru(III/II) redox potential dictating the comparative ratios of the processes. Complexes such as RuP4OMe ( E1/2 = 0.91 vs Ag/AgCl) displayed virtually only desorption, while complexes such as RuPbpz ( E1/2 > 1.62 V vs Ag/AgCl) displayed only chemical decomposition. Comparing isomers of 4,4'- and 5,5'-disubstituted-2,2'-bipyridine ancillary ligands, a dramatic increase in the rate of desorption of the Ru(III) complexes was observed for the 5,5'-ligands. Nanoscopic indium-doped tin oxide thin films (nanoITO) were also sensitized and analyzed with cyclic voltammetry, UV-vis absorption spectroscopy, and X-ray photoelectron spectroscopy, allowing for further distinction of desorption versus ligand-substitution processes. Desorption loss to bulk solution associated with the planar surface of FTO is essentially non-existent on nanoITO, where both desorption and ligand substitution are shut down with RuP4OMe. These results revealed that minimizing time spent in the oxidized form, incorporating electron-donating groups, maximizing hydrophobicity, and minimizing molecular bulk near the adsorbed ligand are critical to optimizing the performance of ruthenium(II) polypyridyl complexes in dye-sensitized devices.

Electrochemical Instability of Phosphonate-Derivatized, Ruthenium(III) Polypyridyl Complexes on Metal Oxide Surfaces.

The oxidative stability of the molecular components of dye-sensitized photoelectrosynthesis cells for solar water splitting remains to be explored systematically. We report here the results of an electrochemical study on the oxidative stability of ruthenium(II) polypyridyl complexes surface-bound to fluorine-doped tin oxide electrodes in acidic solutions and, to a lesser extent, as a function of pH and solvent with electrochemical monitoring. Desorption occurs for the Ru(II) forms of the surface-bound complexes with oxidation to Ru(III) enhancing both desorption and decomposition. Based on the results of long-term potential hold experiments with cyclic voltammetry monitoring, electrochemical oxidation to Ru(III) results in slow decomposition of the complex by 2,2'-bipyridine ligand loss and aquation and/or anation. A similar pattern of ligand loss was also observed for a known chromophore-catalyst assembly for both electrochemical water oxidation and photoelectrochemical water splitting. Our results are significant in identifying the importance of enhancing chromophore stability, or at least transient stability, in oxidized forms in order to achieve stable performance in aqueous environments in photoelectrochemical devices.

Reductive electropolymerization of a vinyl-containing poly-pyridyl complex on glassy carbon and fluorine-doped tin oxide electrodes.

Controllable electrode surface modification is important in a number of fields, especially those with solar fuels applications. Electropolymerization is one surface modification technique that electrodeposits a polymeric film at the surface of an electrode by utilizing an applied potential to initiate the polymerization of substrates in the Helmholtz layer. This useful technique was first established by a Murray-Meyer collaboration at the University of North Carolina at Chapel Hill in the early 1980s and utilized to study numerous physical phenomena of films containing inorganic complexes as the monomeric substrate. Here, we highlight a procedure for coating electrodes with an inorganic complex by performing reductive electropolymerization of the vinyl-containing poly-pyridyl complex onto glassy carbon and fluorine doped tin oxide coated electrodes. Recommendations on electrochemical cell configurations and troubleshooting procedures are included. Although not explicitly described here, oxidative electropolymerization of pyrrole-containing compounds follows similar procedures to vinyl-based reductive electropolymerization but are far less sensitive to oxygen and water.