S.T. Hoffmann, J.M. Koenen, U. Scherf, I. Bauer, P. Strohriegl, H. Bässler, A. Köhler
J. Phys. Chem. B 115 (2011) 8417-8423
We have studied the temperature dependence of phosphorescence (Ph) and delayed fluorescence (DF) in two series of poly(p-phenylene) derivatives within a temperature range from 10 to 300 K under quasi-stationary conditions. One set of materials consists of the dimer, trimer, and polymer of ethylhexyl-substituted poly(fluorene) (PF2/6) and thus allows us to assess the effects of oligomer length. The second series addresses the influence of energetic disorder and conjugation length by being composed of the polymers alkoxy-substituted poly(p-phenylene) (DOO-PPP), poly(indenofluorene) (PIF), and ladder-type poly(p-phenylene) (MeLPPP). Under low light intensities, the DF features a maximum at a certain temperature Tmax. For the dimer and trimer, the Tmax coincides with the temperature at which the phosphorescence has decayed to 1/2 of the value at 10 K, while Tmax shifts to lower temperature values along the series DOO-PPP, PIF, and MeLPPP and approaches T = 0 K for MeLPPP. By applying conventional kinetic equations we show that the occurrence of a maximum in the DF intensity is the consequence of generalized thermally activated triplet exciton transport toward quenching sites. We find the quenching rates at 0 K to be in the range of 1 s–1 for the polymers, while they are more than an order of magnitude lower for the oligomers.