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 TiO2. Monolayers of 1-Se on nanocrystalline TiO2 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 (ϕinj) were calculated from the measured amplitudes of these signals. The value of ϕinj for H-aggregated 1-Se was approximately 2-fold greater than ϕinj for nonaggregated 1-Se and approximately 3-fold greater than ϕinj 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.