The dynamics of interchain and intrachain excitation energy transfer taking place in a polyindenofluorene endcapped with perylene derivatives is explored by means of ultrafast spectroscopy combined with correlated quantum-chemical calculations. The experimental data indicate faster exciton migration in films with respect to solution as a result of the emergence of efficient channels involving hopping between chains in close contact. These findings are supported by theoretical simulations based on an improved Forster model. Within this model, the rates are expressed according to the Fermi golden rule on the basis of (i) electronic couplings that take account of the detailed shape of the excited-state wave functions (through the use of a multicentric monopole expansion) and (ii) spectral overlap factors computed from the simulated acceptor absorption and donor emission spectra with explicit coupling to vibrations (considered within a displaced harmonic oscillator model); inhomogeneity is taken into account by assuming a distribution of chromophores with different conjugation lengths. The calculations predict faster intermolecular energy transfer as a result of larger electronic matrix elements and suggest a two-step mechanism for intrachain energy transfer with exciton hopping along the polymer backbone as the limiting step. Injecting the calculated hopping rates into a set of master equations allows the modeling of the dynamics of exciton transport along the polyindenofluorene chains and yields ensemble-averaged energy-transfer rates in good agreement with experiment.

The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energymigration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual pointdipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the ratelimiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains. E nergy transfer is a key process in the working mechanism of a number of opto-electronic devices based on conjugated materials. This is the case for instance in electroluminescent displays where one can take advantage of energy transfer to tune the color of the emitted light when the active layer includes several materials with different optical gaps (1-5). In addition to providing an efficient technique for internal color conversion, polymer-polymer and polymer-dye blends have been shown also to lead to a significant improvement in photoluminescence (PL) and electroluminescence (EL) quantum efficiencies (6-8).Since most conjugated polymers are characterized by the presence of a distribution of segments with distinct conjugation lengths, optical absorption in the inhomogeneously broadened density of states usually leads to unidirectional energy migration to lower energy sites (9). Thus, controlling the flow of excitations across the polymer material is of importance to limit luminescence quenching due to energy transfer to defects.In the case of solar cells, charge generation usually requires the migration of the electronic excitations induced by light absorption toward dissociation zones (such as interfaces in blends made of different materials) (10, 11). Research in this field aims at mimicking the powerful antenna machines that nature has designed through evolution to harvest solar light and funnel the energy to the photosynthetic reaction centers (12).A most striking demonstration of ultrafast energy transfer in conjugated polymers is the recent discovery of highly sensitive biological an...