Excited-State Electron Transfer from CdS Quantum Dots to TiO2 Nanoparticles via Molecular Linkers with Phenylene Bridges

We have characterized electron transfer from photoexcited CdS quantum dots (QDs) to TiO2 nanoparticles (NTs) through bifunctional phenylene-containing linkers (L). Three L were examined: 4-mercaptobenzoic acid (4MBA), 2-mercaptobenzoic acid (ZMBA), and 4-mercaptophenylacetic acid (4MPAA). Only 4MBA and 4MPAA promoted the attachment of US QDs to TiO2 NPs. Electron injection and interfacial charge recombination were characterized by steady-state emission quenching, nanosecond time-resolved emission, and nanosecond transient absorption measurements. The electron injection efficiency was influenced by the electronic coupling through L and/or the interparticle distance. Electron injection through 4MBA occurred on multiple time scales. The fast component (<10(-8) s) was attributed to injection of electrons from excitonic states, whereas the slower component (10(-7)-10(-6) s) was attributed to injection from trap states. Only the slower component was significant for electron injection through 4MPAA. The fast component of injection through 4MBA was approximately as efficient as through short-chain mercaptoalkanoic acids. Our findings reveal that the excited-state deactivation pathways and interfacial electron-transfer reactivity of tethered assemblies of NPs can be tuned by varying the properties of molecular linkers.