Electron injection into titanium dioxide is investigated for a series of surface bound ruthenium-polypyridyl dyes whose excited state energetics are tuned through chemical modification. The series of twelve complexes are of the form RuII(bpy-A)(L2)2+, where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO2. Functionalization of the ancillary ligand (L) enables tuning of the excited state oxidation potentials. Injection rates are dye dependent, but all are biphasic with time constants in the 3–30 ps and 30–500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to the dyes’ excited state oxidation potentials. The correlation between density of TiO2 states and injection rate are well described using Marcus-Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the 3MLCT manifold. These results indicate that the multi-exponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, they support a picture in which there is a continuous decrease in the injection rate that occurs as the excited state relaxes from its initial Franck-Condon state to the thermally equilibrated 3MLCT state.

Manifold of excited states and density of acceptors: Disentangling excited state electron injection into nanoporous titania

251st National Meeting (2016)

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