Energy transfer processes along a supramolecular chain of π-conjugated molecules

We have investigated the energy transfer dynamics in a supramolecular linear polymer chain comprising oligofluorene (OF) energy donor units linked by quadruple hydrogen-bonding groups, and oligophenylene (OPV) chain ends that act as energy acceptors. Using femtosecond spectroscopy, we followed the dynamics of energy transfer from the main chain of OF units to the OPV chain ends and simulated these data taking a Monte Carlo approach that included different extents of electronic wave function delocalization for the energy donor and acceptor. Best correlations between experimental and theoretical results were obtained for the assumption of electronic coupling occurring between a localized donor dipole moment and a delocalized acceptor moment. These findings emphasize that geometric relaxation following initial excitation of the donor needs to be taken into account, as it leads to a localization of the donor's excited state wave function prior to energy transfer. In addition, our simulations show that the energy transfer from the main chain to the ends is dominated by an interplay between slow and spatially limited exciton migration along the OF segments comprising the main chain and the comparatively faster hetero-transfer to the end-cap acceptors from directly adjoining OF segments. These results clearly support the description of host-guest energy transfer in linear polymer chains as a two-step mechanism with exciton diffusion in the host being a prerequisite to energy transfer to the guest.