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 TiO2 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 (ϕinj). Our data revealed that ϕinj 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 ϕinj 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 ϕinj 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.