Reports: AC3
46158-AC3 Photogeneration of Reduced Catalysts with Vibrationally Hot Electrons
As global energy consumption continues to rise there is a pressing need for the production of solar fuels. Molecular approaches to practical solar fuel generation will likely require the integration of 1) light absorption, 2) electron transfer, and 3) catalysis. An appealing starting point is with sensitized mesoporous nanocrystalline (anatase) titanium dioxide thin films that have been developed for application in dye-sensitized solar cells. These materials already largely satisfy the first two stated requirements, efficient solar harvesting and excited-state electron transfer. Furthermore, novel ultrafast excited state electron transfer processes that occur at these interfaces can be exploited at the molecular to exceed the well-known Schockley-Queisser limit. The scientific objective of this research is to quantify interfacial electron transfer processes that capture energy that would otherwise be lost to vibrational (or phonon) relaxation. This objective represents a necessary step toward the production of ultra-efficient Class III solar cells. We have synetheszied and characterized a new class of Ru(II) sesnitizers that undergoes both rapid excited state injection into TiO2 and hole transfer away from the semiconductor surface. The compounds Ru(bpy)2(BTL)(PF6)2 and Ru(deeb)2(BTL)(PF6)2, where bpy is 2,2’-bipyridine, deeb is 4,4’-(C2H5CO2)2-bpy, and BTL is 9’-[4,5-bis-(cyanoethylthio)]-1,3-dithiol-2-ylidene]-4’,5’-diazafluorene, were found to have very high extinction coefficients in the visible region. In acetonitrile solution, the extinction of Ru(deeb)2(BTL)(PF6)2 was e = 44,000 ± 1,000 M-1 cm-1 at l = 470 nm. Two quasi-reversible oxidation waves were observed, E½ = +0.88 V and +1.16 V, and an irreversible reduction, Epr = -1.6 V versus ferrocene (Fc+/0). At –40 ºC a state was observed with spectroscopic properties characteristic of a metal-to-ligand charge-transfer excited state, t = 25 ns. This same compound was found to photo-inject electrons into TiO2 with a quantum yield f = 0.3 ± 0.2 for 532.5 or 417 nm light excitation in 0.1 M LiClO4 acetonitrile electrolyte. In regenerative solar cells, a sustained photocurrent was observed with a maximum incident photon-to-current efficiency of 0.4. The photocurrent action and absorptance spectra were in good agreement consistent with injection from a single excited state.
In recently published work, we have found condition in which “hot” excited state injection can be used to reduce surface bound hemes to the formal oxidation state of unity. It is this state which reacts with protons to yield a hydride intermediate capable of both proton and CO2 reductions. This objective was realized by tuning both the iron redox potentials and the TiO2 conduction band edge. Systematic and comparative studies of CO equilibrium and ligand exchange dynamics for hemes in fluid solution and anchored to the mesoporous TiO2 thin films immersed in the same solvent have been conducted. To a first approximation, heme coordination chemistry was remarkably insensitive to the TiO2 environment. Small but significant influences of the mesoporous nanocrystalline TiO2 environment were manifest by the appearance of complex kinetics and factor of two decrease in the equilibrium constant for CO coordination relative to fluid solution. The kinetics were well described by the Kohlrausch-Williams-Watts model that has previously been utilized to quantify ligand exchange dynamics of hemes in proteins. A distribution of distances between 5-coordinate hemes and CO within the TiO2 mesopores was proposed to underlie the non-exponential kinetics. Rapid “geminate” recombination of photoreleased CO resulted in less than unity quantum yields for CO photorelease measured on nanosecond time scales. A kinetic analysis based on the well-established dissociative mechanism for heme ligand exchange suggested that the smaller equilibrium constants for CO-heme coordination within the TiO2 mesopores for CO dissociation and/or smaller rate constants for solvent dissociation. Absent from the heme excited state chemistry was evidence of excited-state injection into the anatase nanocrystallites. Rapid internal conversion from the porphyrin singlet state to dissociative ligand field states was proposed to inhibit excited-state injection, behavior that was promoted by the ethylene spacer between the porphyrin ring and the carboxylic acid anchoring groups. These studies are fundamental and directly relevant to the development of efficient photocatalytic materials for solar hydrogen production and for exceeding the well-known Shockley-Queisser limit of single-junction photovoltaic cells.
