Graduate Thesis Or Dissertation
Photophysical Studies of Chromium Sensitizers Designed for Excited State Hole Transfer to Semiconductors and Sequential Hole/Electron Transfers from Photoexcited Cadmium Sulfide Nanorods to Mononuclear Ruthenium Water-Oxidation Catalysts Public Deposited
This dissertation describes three research projects related to solar cells and solar water splitting with a goal of utilizing solar energy, a renewable energy source. The first project is focused on photophysical studies of four newly-synthesized Cr(III) tris-bipyridyl complexes featuring the 4-dmcbpy (dimethyl 2,2'-bipyridine-4,4'-dicarboxylate) ligand. Static and time-resolved emission results suggest that the complexes store ~1.7 eV of energy for multiple microseconds. Using cyclic voltammetry, it is found that the inclusion of 4-dmcbpy shifts the E1/2 of CrIII/II by +0.2 V from the homoleptic parent complexes without 4-dmcbpy. All four complexes have excited state potentials of CrIII*/II between +1.8 and +2.0 V vs. NHE, placing them among the most powerful photooxidants reported and making them candidates for hole-injection sensitizers.
The second project continues with Cr(III) complexes, but using iminopyridine Schiff base ligands. Two complexes feature hexadentate ligands and the other two are their tris-bidentate analogues. One of each pair contains methyl ester groups for attachment to semiconductors. Cyclic voltammograms show that the hexadentate and tris-bidentate analogues have almost identical reduction potentials, but the addition of ester substituents shifts the reduction potentials by +0.2 V. The absorption spectra of the hexadentate complexes show improved absorption of visible light compared to the tris-bidentate analogues. For freshly prepared sample solutions in CH3CN, time-resolved emission and transient absorption measurements for the Cr(III) tris-bidentate ester complex show a doublet excited state with a 17-19 microsecond lifetime at room temperature, while no emission or transient absorption signals from the doublet states are observed for the hexadentate analogue under the same conditions. The dramatic difference is due to the presence of a nonligated bridgehead nitrogen atom.
The third project features charge transfer interactions between a photoexcited cadmium sulfide nanorod and [Ru(diethyl 2,2'-bipyridine-4,4'-dicarboxylate)(2,2':6',2"-terpyridine)Cl]+, a mononuclear water-oxidation catalyst. Upon photoexcitation, hole transfer from the cadmium sulfide nanorod oxidizes the catalyst (Ru2+ → Ru3+) on a 100 ps to 1 ns timescale. This is followed by electron transfer (10-100 ns) from the nanorod to reduce the Ru3+ center. The relatively slow electron transfer dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is required for water oxidation.
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