Aggregation-Induced Increase of the Quantum Yield of Electron Injection from Chalcogenorhodamine Dyes to TiO2

We used transient absorption spectroscopy to characterize excited-state electron injection from a 2,7-bis(dimethylamino)-9-(2-thienyl-5-carboxy)selenoxanthylium dye ( 1-Se ) and a 2,7-bis(dimethylamino)-9-(3-thienyl-2-carboxy)selenoxanthylium dye ( 2-Se ) to TiO 2 . Monolayers of 1-Se on nanocrystalline TiO 2 films consisted of both H-aggregated and nonaggregated dyes, whereas 2-Se underwent little or no aggregation upon adsorption. Two transient spectral signals were correlated with the dication radicals ( 1-Se + and 2-Se + ) generated by electron injection: an absorption at wavelengths shorter than 540 nm and a bleach from approximately 540−650 nm. Relative quantum yields for electron injection (ϕ inj ) were calculated from the measured amplitudes of these signals. The value of ϕ inj for H-aggregated 1-Se was approximately 2-fold greater than ϕ inj for nonaggregated 1-Se and approximately 3-fold greater than ϕ inj for nonaggregated 2-Se . Thus, H-aggregation can increase both the light-harvesting efficiencies and the electron-injection yields of rhodamine derivatives. Our findings suggest that controlled aggregation of organic dyes may represent an attractive strategy for improving the global energy-conversion efficiencies of organic dye-sensitized solar cells and photocatalysts.