INTRAMOLECULAR ELECTRON-TRANSFER IN A LINKED VIOLOGEN QUINONE MOLECULE - A RATE-CONSTANT STUDY

We have synthesized a molecule MV2+Q where a methylviologen (MV2+) and a chloronaphthoquinone (Q) moiety are linked to each other via an aminoalkane (-NH[CH2]3-) chain. Using the electron pulse radiolysis technique, we have determined rate constants for intra- and intermolecular electron transfer between electron donor and acceptor site of this molecule in water and sodium dodecyl sulfate (SDS) micellar solution. Measuring the decay of the absorbance at 605 nm as a function of the MV2+Q concentration in water, we found a remarkably slow intramolecular electron transfer from the reduced methylviologen moiety to the quinone site (kintra,Water = 1.4 × 103 s-1). Thus, even at very low MV2+Q concentrations, intermolecular electron transfer (k = 3.8 × 108 M-1 s-1) was competing with the intramolecular reaction, and kintra was determined by extrapolation to zero MV2+Q concentration. A more accurate determination of kintra was possible in SDS micellar solution where, under conditions of single occupancy of micelles with MV2+Q, the observed rate constants were found to be independent of the MV2+Q concentration and the single-exponential decay curves showed the intramolecular electron transfer of isolated MV2+Q molecules. We obtained a rate constant similar to that determined in pure water (kintra,SDS = 1.1 × 103 s-1). © 1990 American Chemical Society.