ELECTRON-TRANSFER IN LINKED VIOLOGEN QUINONE MOLECULES - RATE-CONSTANT ENHANCEMENT WITH INCREASED CHAIN-LENGTH

We synthesized a homologous series of molecules (MVnnQ) where a methylviologen (MV) and a aminochloronaphthoquinone (Q) are linked to each other via a flexible chain. Using the electron pulse radiolysis technique, we have measured time-resolved spectra and rate constant for intra- and intermolecular electron transfer between donor and acceptor site of the MVnnQ molecules in water and in SDS micellar solution. For comparison, we also irradiated a solution containing a 1:1 mixture of methylviologen and aminochloronaphthoquinone and measured spectra and intermolecular ET reactions between the separated electron donor and acceptor molecules. The intramolecular electron transfer rate constants of all MVnnQ molecules were surprisingly low both in water and in aqueous SDS micellar suspension. The intramolecular rate constants measured in water increase with increasing number of intervening bonds, leading to the conclusion that electron transfer occurs by a through-space rather than through-bond mechanism. The intramolecular rate constants virtually lose their chain length dependence in SDS suspensions where because of an extended configuration of the micellized MVnnQ molecules through-space interaction is not favored.