High-Extinction Ruthenium Compounds for Sunlight Harvesting and Hole Transport

Abstract

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 an acetonitrile solution, the extinction of Ru(deeb)2(BTL)(PF6)2 was ϵ = 44 000 ± 1000 M−1 cm−1 at λ = 470 nm. Two quasi-reversible oxidation waves, E1/2 = +0.88 and +1.16 V, and an irreversible reduction, Epr = −1.6 V, were observed versus ferrocene (Fc+/0). At −40 °C, a state was observed with spectroscopic properties characteristic of a metal-to-ligand charge-transfer excited state, τ = 25 ns. This same compound was found to photoinject electrons into TiO2 with a quantum yield Φ = 0.3 ± 0.2 for 532.5 or 417 nm light excitation in a 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.