The Kinetics of Electron Transfer from CdS Nanorods to the MoFe Protein of Nitrogenase

Combining the remarkable catalytic properties ofredox enzymes with highly tunable light absorbing properties ofsemiconductor nanocrystals enables the light-driven catalysis ofcomplex, multielectron redox reactions. This Article focuses onsystems that combine CdS nanorods (NRs) with the MoFe protein ofnitrogenase to drive photochemical N2reduction. We used transientabsorption spectroscopy (TAS) to examine the kinetics of electrontransfer (ET) from CdS NRs to the MoFe protein. For CdS NRs withdimensions similar to those previously used for photochemical N2reduction, the rate constant for ET from CdS NRs competes withother electron relaxation processes, such that when a MoFe protein isbound to a NR, about one-half of the photoexcited electrons aredelivered to the enzyme. The NR-MoFe protein binding isincomplete with more than one-half of the NRs in solution not having a MoFe protein bound to accept electrons. The quantumefficiency of ET (QEET) in these ensemble samples is similar to previously reported efficiencies of product (NH3and H2) formation,suggesting that the enzyme utilizes the delivered electrons without major loss pathways. Our analysis suggests that QEET, andtherefore the photochemical product formation, is limited at the ensemble level by the NR-MoFe protein binding and at the single-complex level by the competitiveness of ET. We characterized ET kinetics for several CdS NRs samples with varying dimensions andfound that for CdS NRs with an average diameter of 4.2 nm the ET efficiency dropped to undetectable levels, defining a maximumNR diameter that should be used to photochemically drive the MoFe protein. The work described here provides insights into thedesign of systems with increased control of photochemical N2reduction catalyzed by the MoFe protein of nitrogenase.