Synthetic Tuning of the Properties of Transition Metal Complexes Toward Solar Energy Conversion Applications

The conversion of sunlight into useful forms of energy is a much-researched topic with many successful means to effect such transformations. Modern solar conversion
devices operate by direct production of electricity via photovoltaic conversion, with DSSCs being a highly-attractive low-cost option. While direct solar conversion to
electricity will have a place in an integrated renewable energy economy, production of solar fuels is still a necessity for transportation and to supply power after the sun sets.
Many solar fuels exist, with hydrogen being a prime target and zinc metal being a relatively new, but attractive option.
This work reports the design of DSSC dyes which address two shortcomings of current dyes: photodegradation via ligand dissociation and the use of rare metals. Iron
(II)-based dyes were synthesized using hemicaging ligands with a ruthenium(II) dye being synthesized as a comparator to current dyes. These dye candidates were assessed
through electrochemical and spectroscopic techniques where they were found to be promising candidates. The dyes await implementation in a device to assess real-world
performance. This work also reports on the tuning of bis-cyclometalated iridium(III) photosensitizers via the inclusion of nitriles on the cyclometalating and diimine ligands.
Properties of the complexes were interrogated via electrochemical and photophysical methods which were correlated to results from density functional theory calculations. Photophysically, the nitriles lower luminescence quantum yield and excited state lifetime
when placed on the diimine ligand, but strongly enhance these properties when placed on the cyclometalating ligand. Extreme cases of this produced complexes with excited state lifetimes as low as 27 ns and as high as 8 μs; similarly, luminescence quantum yields varied between 0.09% and 63% for these complexes. This work also reports on the assessment of the bis-cyclometalated complexes
ability to photosensitize water reduction catalysis using a variety of catalysts and solvents. Inclusion of nitriles on the diimine ligands was found to facilitate direct coordination of the photosensitizers to platinum colloids during water reduction photocatalysis. Systems achieved up to 1300 photosensitizer turnovers and functioned without degradation for over 75 hours under optimized conditions. Inclusion of nitriles on the cyclometalating ligand produced complexes with much greater solvent tolerance; these complexes were able to achieve over 2000 photosensitizer turnovers and function in more environmentally-benign solvents such as alcohols and acetone. Finally, this work reports on the assessment of the bis-cyclometalated iridium (III)
complexes as catalysts for the photoreduction of zinc salts to zinc metal as a solar fuel. All complexes synthesized were competent photocatalysts for this reaction, where they
achieved between 154 to 237 photocatalyst turnovers. Though the newly-synthesized complexes worked, they did not outperform existing photocatalysts and could not replace
the need for acetonitrile.