Effects of aggregation on the excitation transfer in perylene-end-capped polyindenofluorene studied by time-resolved photoluminescence spectroscopy

Abstract

We have investigated the excitation transfer in a system comprising $\mathrm{poly}{(6,6}^{\ensuremath{'}}{,12,12}^{\ensuremath{'}}$-tetra-2ethylhexyl-2,8-indenofluorene) (PIFTEH) chains end-capped with perylene dye molecules, using femtosecond time-resolved photoluminescence (PL) spectroscopy as well as polarized photoluminescence measurements. The transfer of excitons from isolated PIFTEH chains to perylene molecules is completed within the first 30--40 ps after excitation, and we extract a F\"orster radius ${R}_{0}=(1.8\ifmmode\pm\else\textpm\fi{}0.3) \mathrm{nm}$ from the time-resolved PL transients. We have modelled the polarization anisotropy for a guest-host system subject to F\"orster interactions via a Monte Carlo simulation and find that the emission from acceptors becomes unpolarized at sufficiently large acceptor concentrations, permitting an accurate determination of the F\"orster radius from time-integrated photoluminescence anisotropy measurements. While spectral overlap calculations predict a large efficiency for the transfer of excitations to the perylene molecules from sites where the PIFTEH chains aggregate, no transfer is observed experimentally, which we attribute to chain packing effects within the sample prohibiting sufficiently close contact between PIFTEH aggregates and perylene molecules.