Limitations of conjugated polymers as emitters in triplet–triplet annihilation upconversion†
A series of poly(phenylene-vinylene)s (PPV) was synthesized with two monomer building blocks containing methoxy and 2-ethylhexyloxy (MEH) sidechains and sterically bulky fluorenyloxy sidechains. Variations in monomer incorporation were achieved by adjusting the ratio of the monomer precursors in the synthesis of the PPVs using the Gilch polymerization method. The increased incorporation of the bulky monomer led to expected variation in photophysical properties, in particular, the enhancement of photoluminescence quantum yield. Using palladium(II) tetraphenyltetrabenzoporphyrin (PdTPTBP) as the triplet sensitizer, all polymers displayed upconverted emission in solution with the comonomer ratio for the fluorenyloxy and MEH units at 1 : 10 showing the best performance with upconversion efficiency of 0.135%. This efficiency was up to three times higher than that of two commercially available PPVs, MEH-PPV and Super yellow PPV, under the same experimental conditions. Further analysis of the efficiency of the photochemical processes involved revealed several limitations to the performance of these conjugated polymer emitters. While increasing steric bulk of the sidechains improved the photoluminescence quantum yield, it also caused variation in excited state energies of the polymers through the series. This led to poor alignment with the triplet energy of our chosen sensitizer for some of the polymers. Even when the triplet–triplet energy transfer efficiency was high, the contact triplet pair formation efficiency was found to be very low. We reasoned that the structural conformation of the polymers hindered the chromophore orbital overlap during triplet collisions required for triplet–triplet annihilation.