Electric Field Effects on Internal Conversion:  An Alternative Mechanism for Field-Induced Fluorescence Quenching of MEH-PPV and Its Oligomers in the Low Concentration Limit

In a previously published study (J. Phys. Chem. B 2006, 110, 7732−7742), we reported field-induced fluorescence quenching in both poly[2-methoxy,5-(2‘-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrices at high dilution (<0.1% by weight). The observed quenching is not readily explained by field-induced exciton dissociation or by the presence of free charges, two mechanisms that have been invoked to explain this phenomenon by previous authors. A model is developed here that ascribes the observed fluorescence quench in dilute samples to an energetic shift of the relaxed excited state caused by the electric field resulting in increased nonradiative relaxation. To determine whether the relevant nonradiative pathway is intersystem crossing or internal conversion, analytical expressions are derived for each of these two mechanisms. Only the expression derived for the Stark effect on the rate of internal conversion quantitatively predicts the magnitude of quench observed in MEH-PPV and in the oligomeric species.