Intermolecular Charge Separation in Aggregated Rhodamine Dyes Used in Solar Hydrogen Production

Abstract

Various modern solar light-harvesting systems, including those used in photovoltaics and solar fuel production, depend on efficient electron transfer from a surface-bound molecular dye to nanoscopic semiconductor particles. However, the productive electron transfer competes with a variety of other relaxation pathways for the dye, and the dominant pathway can change dramatically depending on its environment. A new sulfur-substituted thiorhodamine dye was synthesized having exceptional light-harvesting qualities for solar energy applications and for solar hydrogen production in particular. The dye was created with a thiophene spacer bearing a phosphonate-ester (1-Ester) or phosphonic-acid (1-Acid) allowing for excellent solubility in MeCN or the ability to functionalize metal oxide semiconductor nanoparticles such as TiO2. While 1-Ester is found to be fully monomeric in MeCN, 1-Acid readily forms H-aggregated dimers which, upon photoexcitation, undergo charge separation to an ion pair (IP) in 1.5 ps. For 1-...