Distance-Dependent Electron Transfer in Tethered Assemblies of CdS Quantum Dots and TiO2 Nanoparticles

We have characterized electron injection from photoexcited CdS quantum dots (QDs) to TiO2 nanoparticles as a function of the interparticle separation within molecularly linked assemblies. CdS QDs were tethered to TiO2 nanoparticles through bifunctional mercaptoalkanoic acids (MAAs). Electron injection and interfacial charge recombination were characterized by steady-state emission quenching, nanosecond time-resolved emission, and nanosecond transient absorption. The electron injection yield decreased with increasing MAA chain length and interparticle separation. Electron injection occurred on multiple timescales. A fast component (<10(-8) s) accounted for the majority of injection, while the remainder occurred on the microsecond time scale. We attribute the multiexponential injection kinetics to electron transfer from a range of conduction-band and trap states. Interfacial charge recombination occurred on the microsecond time scale, and the kinetics were independent of the MAA chain length. Our findings reveal that the excited-state deactivation pathways and interfacial electron-transfer reactivity of tethered assemblies of nanoparticles can be tuned systematically by varying the interparticle separation.