46158-AC3
Photogeneration of Reduced Catalysts with Vibrationally Hot Electrons
New molecular strategies for efficient solar energy conversion are critically needed. The scientific objective of this proposed 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 recently reported a novel new compound 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 ? = 44,000 + 1,000 M-1 cm-1 at ? = 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, ? = 25 ns. This same compound was found to photo-inject electrons into TiO2 with a quantum yield ? = 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 In unpublished work, we have found condition in which “hot” excited stet injection can be used to reduce surface bound hemes to the formal oxidation state of I. It is this state which reacts with protons to yield a hydride intermediate capable of both proton and CO2 reductions. 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.
