Long-distance fluorescence quenching by electron transfer in rigid solutions

Fluorescence of aromatic molecules in solid solutions at room temperature and low temperature is quenched by addition of strong electron donors. The efficiency of quenching correlates well with calculated energetics for electron transfer (ET) from the added donors to the excited molecules, D + A* → D+ + A-. The quencher (D) always had a substantially higher-lying excited state than did A to prevent electronic energy transfer (e.g., Foerster transfer). The A* fluorescence decreased exponentially with D concentration, being halved by addition of about 0.1 M D in cases with favorable energetics for electron transfer. These quenching measurements are interpreted as indicating electron transfer over distances up to 15 A (center to center). These maximum quenching distances are in excellent agreement with electron tunneling distances measured for ion-molecule reactions by pulse radiolysis. Formation of donor-acceptor complexes during sample preparation is a possible alternate interpretattion, but several features of the data provide evidence against this alternative