Effects of surface-anchoring mode and aggregation state on electron injection from chalcogenorhodamine dyes to titanium dioxide

We used transient absorption spectroscopy to characterize excited-state electron injection from a 2,7-bis(dimethylamino)-9-(5-phosphonothien-2-yl) selenoxanthylium dye (3-Se) into TiO₂. Dye 3-Se adsorbed to TiO ₂ via the phosphonic acid group as a mixture of H-aggregates and monomers. Injection of electrons from photoexcited 3-Se into TiO₂ yielded the dication radical (3-Se⁺) and an associated transient absorption at wavelengths shorter than 540 nm, the amplitude of which was proportional to the quantum yield of electron injection (φ_inj). Our data revealed that φ_inj from H-aggregated 3-Se was (2.0 ± 1.3)-fold greater than from monomeric 3-Se; therefore, H-aggregation increased the efficiencies of both light-harvesting and electron injection. Comparison with our reported data for the analogous carboxylic acid-functionalized dye (1-Se) revealed that φ_inj via the carboxylate linkage was (2.3 ± 1.1)-fold greater than via the phosphonate linkage. Thus, electron-injection reactivity is sensitive to both the aggregation state and the surface-anchoring mode of these chalcogenorhodamine dyes. The decrease of φ_inj for 3-Se is offset by its enhanced stability and persistence on TiO₂, rendering the phosphonic acid-functionalized and H-aggregated dye a particularly attractive sensitizer.