1. An Interfacial Electron Transfer Switch: Ruthenium-dppz Compounds Anchored to Nanocrystalline TiO 2 Mauricio Arias, Ana Maria Leiva, and Barbara Loeb* Pontificia Univesidad Católica de Chile Alvaro Delgadillo and Gerald J. Meyer* Departments of Chemistry and Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218 ( DMR-0303411) -Molecular approaches to solar energy conversion based on dye-sensitized solar cells have confirmed efficiencies > 10%. - Understanding the factors that control electron injection into the semiconductor nanoparticles is a key step toward improving efficiencies. - Two novel dye molecules, based on the dppz ligands shown, provide insights into the dye sensitization process and allow electron injection yields to be systematically tuned.

2. Excited State Injection - Excited state electron injection into a semiconductor provides a molecular basis for the conversion of light into potential energy. - It is well established that such processes occur on the femto- to pico-second time scale, but there is no information on how to control the rate constants or yields. - Aqueous interactions of the phenazine nitrogens of dppz are known to quench the excited state giving rise to the celebrated ‘light switch’ effect for DNA sensing. -The Ru(II) dppz excited states on TiO 2 are quenched by water. The injection yield decreases with the addition of water to a limiting value of  inj = 0.3. - A model is proposed wherein 30% of the injection occurs from singlet excited states and the remainder occurs from the triplet excited state. Studies are underway to further test this model. Figure Caption. Time resolved absorption difference spectra of 1 anchored to 20 nm TiO 2 nanocrystallites dispersed in a mesoporous thin film. The spectra are shown at delay of 10 ns ( ■ ), 100 ns (●), 500 ns ( ▲ ), and 2  s ( ▼ ) and are assigned to the TiO 2 (e - )/Ru III charge separated state. The inset shows absorption transients monitored at 500 nm in 0.1 M LiClO 4 acetonitrile solution with 0.5 M H 2 O (black) and without (gray). The amplitudes of the signal directly report on the excited state injection yields.

3. Summary and Significance - Novel Ru(II) dye molecules have been prepared that allow excited state injection yields to be widely tuned and mechanistically probed for the first time. - The global solar efficiency of cells based on these dyes is poor due to low injection, < 1 % under simulated sunlight. - The results suggest that blocking the phenazine nitrogens from water will increase the efficiencies substantially. Such studies are underway ion our laboratories.