Modeling electric field-induced quenching in conjugated polymers and oligomers

Abstract

The phenomenon of electric field-induced emission quenching is important in organic light-emitting diodes because operating conditions involve large electric fields. Past experimental work on light-emitting polymers and oligomers showed that field-induced quenching (FIQ) efficiencies are higher in non-rigid molecules such as poly(pphenylene vinylene) or PPV, than in similar, more planar molecules. Based on this relationship, we previously proposed that the applied field enhances internal conversion decay channels. Our further studies built on this idea by examining FIQ in PPV oligomers of varying length using computational methods. Calculations performed at the INDO/S-CI level showed the presence of free electron-hole pair (FEHP) states which are stabilized by the uniform external electric field. These FEHP states undergo an avoided crossing with the fluorescent 1Bu bound exciton state at sufficiently high field magnitudes. The magnitude of the electronic coupling between the FEHP and 1Bu state, determined from these avoided crossings, is found to be a function of the field at which these states cross. This function is universal in that it applies to all FEHP states in PPV. This includes FEHP states on different length oligomers and the multiple FEHP states on a given length oligomer. Combining this universal function with simple models for the surrounding dielectric medium and effects of disorder allowed Marcus theory to be used to develop a model of FIQ. The resulting model yields reasonable quantitative agreement with FIQ magnitudes, dependence on oligomer length, and threshold field strengths at which quenching is observed. Here, the universal curve, relating electronic coupling to the field at which the FEHP and 1Bu states cross, is examined for planar, ordered oligomers of polyfluorene and ladder-type poly(p-phenylene). The dependence is similar to PPV, suggesting that this curve is universal not just across states, but also across this family of conjugated polymers. Given that the electronic couplings are similar, the observed differences in FIQ may be attributed to other factors, including especially the reduced degree of structural disorder present for these more rigid systems.