Forster energy transfer and control of the luminescence in blends of an orange-emitting poly(p-phenylenevinylene) and a red-emitting tetraphenylporphyrin

We report on the luminescence of a tetraphenylporphyrin, TPP-d, blended int o poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV. We f ind significant energy transfer from MEH-PPV to the porphyrin, in spite of the low absorption of the porphyrin at the emission wavelength of MEH-PPV, reflected in a Forster transfer radius (2.5 nm) smaller than for materials with larger spectral overlap. The overall photoluminescence, PL, efficiency decreases monotonically with increasing porphyrin content, whereas the por phyrin contribution to the total efficiency, referred as an "apparent" PL e fficiency, exhibits a maximum at 1.4% porphyrin content (by weight). We att ribute this non-monotonic behaviour to the interplay of the exciton transfe r probability and PL quenching, both of which increase with concentration. We also observed the energy transfer under electrical excitation, but notic ed that, at low concentrations, the porphyrin contribution to the electrolu minescence is higher than that observed in PL. This indicates significant e mission from excitons formed directly at the porphyrin sites, which are lik ely to act as charge trapping sites. We also compare the luminescence prope rties of the blends with those of copolymers based on the same host-guest p air.