Quantitative analysis of the singlet exciton-polaron interaction in para-phenylene-type ladder polymers

Abstract

The steady state photoinduced absorption (PA), photoluminescence (PL), PL detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) N<SUB>O</SUB> dependence of the PA, PL, and PLDMR signals are analyzed by means of a new rate equation model. It describes the dynamics of singlet excitons (SE's) and polarons in all three experiments with the same set of parameters. This yields the first quantitative analysis of the interaction of SE's and polarons in conjugated polymers. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin 1/2 magnetic resonance conditions reduce the total polaron and triplet exciton (TE) populations. Since the sublinear N<SUB>O</SUB> dependence of the positive (PL-enhancing) spin 1/2 PLDMR and the polaron PA band are essentially the same, we conclude that that PLDMR is due to reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. It also suggests that it is a very significant process in luminescent (pi) - conjugated materials and organic light-emitting devices, which needs to be taken into account especially at high excitation densities such as in lasing action.