The role of exciton diffusion in energy transfer between polyfluorene and tetraphenyl porphyrin

Singlet exciton migration has been studied in films of the conjugated polymer polyfluorene (PF) by doping the samples with a fluorescent probe molecule, tetraphenyl porphyrin (TPP). Energy transfer in such systems has often been described in terms of Förster transfer, a dipole-dipole mechanism. TPP emission from the films was measured in steady-state, as a function of temperature and dopant concentration. The intensity of the TPP emission was found to be constant up to $150\mathrm{K}$, and then to increase with temperature. Therefore, the energy transfer cannot be occurring solely by Förster transfer as that process is temperature-independent. Instead, energy transfer between PF and TPP is considered to take place via thermally-activated exciton diffusion through the polymer followed by Förster transfer between the polymer and dopant. Moreover, TPP emission as a function of dopant concentration could not be described by Förster transfer alone, but could be well fitted at low temperature $\left(15\mathrm{K}\right)$ and room temperature by the Yokota-Tanimoto model, which combines diffusion and Förster transfer. Diffusion lengths of $11\pm 2\mathrm{nm}$ $\left(15\mathrm{K}\right)$ and $20\pm 2\mathrm{nm}$ (room temperature) were found. The nonzero exciton diffusion at low temperature is believed to be due to downhill migration to low energy polymer segments.