Ultrafast Two-Electron Transfer in a CdS Quantum Dot-Extended-Viologen Cyclophane Complex

Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis(4,4'-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis(methylene)) (ExBox(4+)) to form both the ExBox(3+center dot) and the doubly reduced ExBox(2(+center dot))) states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes similar to 5 ps, which leads to a mixture of redox states of the acceptor ligand. The doubly reduced ExBox(2(+center dot)) state has a lifetime of similar to 10 ns, while CdS+center dot:ExBox(3+center dot) recombines with multiple time constants, the longest of which is similar to 300 ps. The long-lived charge separation and ability to accumulate multiple charges on ExBox(4+) demonstrate the potential of the CdS:ExBox(4+) complex to serve as a platform for two-electron photocatalysis.