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Title: Electron Transfer in Rhodamine–TiO2 Complexes Studied as a Function of Chalcogen and Bridge Substitution
Many emerging light-harvesting systems for solar-energy capture depend on absorption of light by molecular dyes and subsequent electron transfer to metal-oxide semiconductors. However, the inhomoge- neous electron-transfer process is often misunderstood when analogies from bimolecular electron transfer are used to explain experimental trends. Here, we develop and apply a theoretical methodology that correctly incorporates the semiconductor density of states and the system reorganization energies to explain observed trends in a series of molecular sensitizers. The effects of chalcogen and bridge substitution on the electron transfer in rhodamine− TiO2 complexes are theoretically investigated by combining density functional theory (DFT)/time-dependent DFT calculations and Fermi’s golden rule for the rate constant. It is shown that all dyes exhibit τeT < 4 ps. Dyes with thiophene bridges exhibit shorter τeT (∼1 ps) than dyes with phenylene bridges (∼4 ps). When the planes of the dye core and bridge are fixed at coplanarity, the dye−TiO2 coupling strength is found to increase by a factor of ∼2 when compared with the Franck− Condon geometry. However, the donor energy level of coplanar dyes falls significantly below the TiO2 conduction band edge so that, despite enhanced coupling, electron transfer is slowed to ∼20 ps. Similar results appear for the excited triplet states of these dyes, showing that the intersystem crossing to low more » energy triplet states can increase electron-transfer time constants to 60−240 ps. These results are compared to the results of previous photocatalytic hydrogen generation and dye-sensitized solar cell experiments. « less
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The journal of physical chemistry
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National Science Foundation
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