Excitation energy migration in highly emissive semiconducting polymer blends probed by photoluminescence detected magnetic resonance

The excitation energy migration (EEM) in methyl-substituted ladder-type poly(para-phenylene) (m-LPPP) doped with small amounts of the red emitter poly(perylene-co-diethynylbenzene) (PPDB) was studied by photoluminescence (PL) detected magnetic resonance (PLDMR). It is suggested that the EEM process proceeds via two steps: (1) migration within the host and (2) transfer from the host to the guest. The contributions of the emissions from m-LPPP and PPDB to the PL-enhancing polaron PLDMR at g = 2, which are due to a reduction in the density of polarons acting as singlet exciton (SE) quenching centers, evolve differently with temperature. This provides clear evidence for SE migration in m-LPPP. The triplet exciton (TE) PLDMR at g = 4 shows a distinct peak for each polymer, with the intensity of the PPDB feature being proportional to its concentration. However, the spectral dependence recorded at the peak of each resonance is the same. This rules out the triplet-triplet-annihilation-mechanism in these blends for simple energetic reasons. Instead we propose that the resonance at g = 4 is due to a SE-quenching mechanism similar to that for the polaron resonance at g = 2. At the field-for-resonance the number of TE's decreases, and hence the PL increases.