We have investigated the energy transfer dynamics in films of a conjugated polyindenofluorene host doped with covalently attached perylene guests. By performing time-resolved measurements of the host luminescence decay under site-selective excitation conditions, we have examined the influence of exciton migration within the host on the temporal evolution of the host-guest energy transfer. We find that highly mobile excitons created at the peak of the host's inhomogeneous density of states transfer to guests considerably faster than more localized excitons created in the low-energy tail, indicating a strong contribution of exciton migration to the overall energy transfer. These effects are significantly more pronounced at low temperature ͑7 K͒ than at ambient temperature, suggesting that for the latter, up-hill migration of excitons in the host and a broadening of their homogeneous linewidth may prevent truly site-selective excitation of localized excitons. In the asymptotic long-time limit, the observed dynamics are compatible with long-range single-step Förster energy transfer. However, at early times ͑Շ10 ps͒ after excitation, the behavior notably deviates from this description, suggesting that diffusion-assisted energy transfer is more important in this regime. The measured changes in excitation transfer rates with temperature and excitation energy correlate well with those observed for the dynamic energy shifts of the vibronic emission peaks from the undoped polymer. Our results therefore indicate that energy-transfer rates in polymeric guest-host systems are strongly time-dependent, owing to a contribution both from exciton relaxation through incoherent hopping within the host's density of states and direct Förster energy transfer.

The emission properties of polyindenofluorenes with various proportions of straight‐ and branched‐chain alkyl substituents have been compared. The polymer with straight octyl substituents shows green emission due to formation of aggregates, while the polymer with branched 2‐ethylhexyl substituents shows blue emission. Studies of the film morphology show the presence of ordered structures due to π‐stacking of the polymer chains for the octyl‐substituted polymer, whereas the polymer with branched side‐chains shows no such order. Copolymers show intermediate behavior. A clear correlation is established between the degree of straight‐chain alkyl substitution, the formation of ordered structures in the films, and the amount of long wavelength emission in the solid‐state spectra.

We have investigated the excitation transfer in a system comprising poly(6,6Ј,12,12Ј-tetra-2ethylhexyl-2,8-indenofluorene͒ ͑PIFTEH͒ chains end-capped with perylene dye molecules, using femtosecond time-resolved photoluminescence ͑PL͒ 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 Förster radius R 0 ϭ(1.8Ϯ0.3) nm from the time-resolved PL transients. We have modelled the polarization anisotropy for a guest-host system subject to Förster 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 Förster 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.

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The ultrafast dynamics of photoexcitations have been studied in chiral stacks of conjugated p-phenylene vinylene molecules functionalized with hydrogen-bonding groups. The results indicate that in solution, -interactions between the molecules give rise to fast exciton diffusion along the stacking axis of the assemblies. The chiral nature of the assemblies is found to cause a rotation of the dipole moment of excitons propagating along the stacks as indicated by time-resolved measurements of the photoluminescence polarization anisotropy. The observed exciton diffusion and energy relaxation dynamics in the molecular stacks are shown to be very similar to those found in conjugated polymer films. Moreover, it is demonstrated that through changes in the temperature of the surrounding solvent, the stacks can be dissociated reversibly as shown by a marked reduction in the diffusivity of excitons.