Minimizing the Reorganization Energy of Cobalt Redox Mediators Maximizes Charge Transfer Rates from Quantum Dots.

Light-induced charge separation is at the very heart of many solar harvesting technologies. The reduction of energetic barriers to charge separation and transfer increases the rate of separation and the overall efficiency of these technologies. Here we report that the internal reorganization energy of the redox acceptor, the movement of the atoms with changing charge, has a profound effect on the charge transfer rates from donor quantum dots. We experimentally studied and modelled with Marcus Theory charge transfer to cobalt complexes that have similar redox potentials covering 350 mV, but vastly different reorganization energies spanning 2 eV. While the driving force does influence the electron transfer rates, the reorganization energies had a far more profound effect, increasing charge transfer rates by several orders of magnitude. Our studies suggest that careful design of redox mediators to minimize reorganization energy is an untapped route to drastically increase the efficiency of quantum dot applications that feature charge transfer.

Cobalt cage complexes as mediators of protein electron transfer.

A selection of cobalt (III)/(II) macrobicyclic 'sarcophagine' (sar) cage complexes with N3S3 mixed donor sets but differing in a single apical substituent has been chosen to span a redox potential range of +150 to -150 mV vs the normal hydrogen electrode and thus acts as redox buffers in protein spectroelectrochemistry and redox potentiometry. The cobalt(III) cage complexes are all based on the same parent structure [Co(XMeN3S3sar)]3+, where X, the variable apical substituent, is -NO2, -Cl, -OH, -NH2, or -NMe 3+ , and a methyl group occupies the opposite apical position. The X-ray crystal structures of selected members of this series are reported. Changes to the apical substituent X enable the CoIII/II redox potential to be tuned across a range of more than 200 mV by the inductive effects of the functional group. The pH dependence of the redox potential enabled the pK a values of some functional groups to be determined. The complexes were successfully employed as electron transfer mediators in the spectroelectrochemical investigation of a variety of heme proteins.