Optical charge transfer in the ion pair paraquat2+[Au(CN)2]−

In aqueous solution paraquat2+ (N,N′-dimethyl-4,4′-bipyridinium2+) and [Au(CN)2]− form an ion pair (K∼90 M−1) which shows an outer-sphere [Au(CN)2]− to paraquat2+ charge transfer (CT) absorption at λmax=331 nm. A band analysis yields a potential of E0∼3.2 V for the one-electron oxidation of [Au(CN)2]−. Accordingly, Au(I) is a very weak CT donor.