Abstract

For the solar-powered water splitting reaction to proceed efficiently, new classes of light-absorbing sensitizers and hydrogen evolution catalysts are required. We have developed numerous molecular compositions based exclusively on coordination compound sensitizers and catalysts capable of reducing protons to hydrogen in pure water and water-rich solvent mixtures using visible light. In addition to the design, synthesis, and characterization of various molecules that serve either as sensitizers or catalysts in these solar energy conversion schemes, we have developed new apparatus’ for parallel high-throughput screening of these photocatalytic compositions, monitoring pressure, GC, and MS in real-time throughout the time course of the photoreactions. This combinatorial approach to solar fuels photocatalysis has already led to significant understanding and optimization of numerous hydrogen-producing compositions, permitting us to focus on the best systems for detailed transient absorption and IR mechanistic studies. This presentation will highlight advances incorporating all first-row transition metal complexes, based on a recently developed class of Cu(I) sensitizers exhibiting unprecedented excited state lifetimes working in concert with Co(III) hydrogen evolution catalysts, as well as champion compositions functioning in pure water based on new classes of Ru(II) and Ir(III) sensitizers and Co(II) polypyridyl catalysts. Details related to the transient spectroscopy of these molecular photocatalytic systems will also be presented.

Another focus of our research program involves the study of sensitized triplet fusion (TF) in solution using highly photostable metal-organic chromophores in conjunction with energetically appropriate organic molecules with large singlet-triplet gaps. Selective excitation of the long-wavelength absorbing sensitizer efficiently generates long-lived triplet states that serve as energy transfer donors. In the presence of appropriate molecular acceptors, diffusion controlled triplet-triplet energy transfer takes place, producing the excited triplet state of the acceptor while regenerating the ground state of the sensitizer. When sufficient numbers of the sensitized triplets are produced, TTA takes place which results in either frequency upconverted light or the formation of desired chemical products. Various combinations of donor and acceptor have been explored and data will be presented on a number of these systems spanning light conversions ranging from the near-visible to the near-IR. This presentation will also describe many examples of upconversion phenomena realized in solid-state polymeric materials along with emerging classes of acceptor/annihilator chromophores. TF processes will be shown to operate at high efficiencies with concomitant linear incident power density response, demonstrated in both theory and experiment using non-coherent photons. Finally, upconversion-based photoaction observed in water splitting photoelectrochemical cells and operational photovoltaics will be discussed.