Excited-State Hole-Transfer from CdSe Quantum Dots to a Self-Assembled Pd(II) Nanocage Functionalized with Ferrocenes

We have characterized the photoinduced charge-transfer reactivity of a donor−acceptor system consisting of CdSe QDs and a self-assembled truncated tetrahedral nanocage comprised of six 1,1′-bis(diphenylphosphino)ferrocene (dppf)-capped Pd(II) nodes linked via four tripodal 2,4,6-tri(4-pyridyl)-1,3,4-triazine (TPT) ligands. Our steady-state and time-resolved spectroscopic measurements revealed that photoexcited CdSe QDs transfer holes to the peripheral Fe(II) centers of the nanocage (Pd(dppf)TPT), as evidenced by (a) the growth of a long-wavelength induced absorption in transient absorption data, attributable to the ligand-to-metal charge-transfer transition of ferricenium, the one-electron oxidation product of ferrocene, and (b) the accelerated disappearance of CdSe excited states. Hole transfer occurs on a range of time scales (10⁻¹¹ to 10⁻⁸ s), and the resulting charge-separated state, with photogenerated electrons in CdSe QDs and holes localized on Fe(III) centers of oxidized Pd(dppf)TPT, persists for hundreds of nanoseconds to beyond a microsecond. Although electron transfer from photoexcited CdSe QDs to the Pd(II) centers of Pd(dppf)TPT is thermodynamically favorable, it did not occur to any appreciable degree, due to the inaccessibility of Pd(II) centers to surfaces of CdSe QDs. This structure-dependent selectivity for the transfer of photogenerated holes, rather than electrons, from QDs to Pd(dppf)TPT, together with the prolonged charge-separation promoted by CdSe:Pd(dppf)TPT constructs, highlight the potential of QDs, in concert with self-assembled nanocages bearing multiple peripheral redox-active functional groups, as donor−acceptor systems for redox photocatalysis.