Mechanistic Insights from Transient Spectroscopy leading to Hydrogen Evolving Photocatalysis and Photon Upconversion in Wate

Solar energy conversion represents a crucial component of the renewable energy resources that is aimed to reduce carbon-based emissions in the atmosphere. Utilizing sunlight to produce H2 through reduction of water provides an environmentally sensible and naturally abundant fuel. The generation of hydrogen gas from water using solar photons remains a formidable challenge that requires multi-faceted approaches in order to develop new photocatalytic molecular compositions.;To address this issue, we have recently developed new homogenous molecular compositions capable of directly converting water, with the aid of solar photons, into hydrogen, a combustible fuel and precious high-energy chemical feedstock. For the water splitting reaction to proceed economically for large-scale applications, efficient light absorbing sensitizers and hydrogen evolution catalysts are required.;The formulations examined in this dissertation were water-soluble ruthenium(II) photosensitizers and cobalt(II) proton reduction catalysts in conjunction with ascorbic acid as an electron donor. Subsequent to light absorption, the detailed photoinduced electron transfer mechanisms leading to hydrogen production and all rate parameters were investigated by pump-probe nanosecond transient absorption spectroscopy. Charge separated cage-escape efficiencies were extracted from transient absorption, spectroelectrochemistry, and quenching rate constants between the donor and quencher in the photocatalytic cycle. The optimized compositions were also investigated using steady state and time-resolved spectroscopy as well as conventional analytical tools (HPLC, ESI-MS, etc).;In addition to the design and synthesis of various molecules that serve as either photosensitizers or catalysts in these solar energy conversion schemes, we have designed an apparatus for parallel high-throughput screening of these photocatalytic compositions. This combinatorial approach to solar fuel photocatalysis has already led to significant advances in the fundamental understanding of hydrogen gas generation from pure water.;The second project discussed in this dissertation demonstrated the direct observation of photochemical upconversion performed homogenously in pure water in the absence of hydrophobic or surfactant additives. Photon upconversion represents a phenomenon that yields high-energy emission from low-energy absorption through sensitized triplet-triplet annihilation (TTA).;The current investigation achieved this goal using combinations of water soluble Ru(II) metal-to-ligand charge transfer (MLCT) sensitizers [Ru(bpy) 3]2+ (bpy = 2,2`- bipyridine) and [Ru(BPS)3] 4- (BPS = bathophenanthroline disulfonate), in concert with 9- anthracenecarboxylate anion (AnCO2-) and 1-pyrenecarboxylate anion (PyCO 2-).;Triplet-triplet energy transfer was identified as the exclusive quenching pathway using transient absorption spectroscopy where the characteristic T 1→Tn absorption bands for 3AnCO 2 - and 3PyCO2 - were unambiguously identified. Transient absorption spectroscopy was also used to determine the diffusion limited rate constants for TTA and demonstrated that 75% of the initially sensitized aromatic carboxylate triplets decay through bimolecular TTA.;As photochemical upconversion relies on sequential triplet energy transfer reactions, it comes as no surprise that nonpolar media has been exclusively used to host these processes to date. However, the work presented here illustrates that with the proper selection of donor and acceptor molecules, photochemical upconversion can indeed be realized in pure water, potentially enabling chemistry from the resulting high-energy singlet excited state.