Prof Laura Herz FRS

Abstract:

Atomistic models based on quantum-chemical calculations are combined with time-resolved spectroscopic investigations to explore the migration of electronic excitations along oligophenylenevinylene-based chiral stacks. It is found that the usual Pauli master equation (PME) approach relying on uncoherent transport between individual chromophores underestimates the excitation diffusion dynamics, monitored here by the time decay of the transient polarization anisotropy. A better agreement to experiment is achieved when accounting for excitation delocalization among acceptor molecules, as implemented in a modified version of the PME model. The same models are applied to study light harvesting and trapping in guest-host systems built from oligomers of different lengths.

Abstract:

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.

Abstract:

Oligo-p-phenylenevinylene (OPV) materials monofunctionalised with ureido-s-triazine form chiral, helical stacks in dodecane solution. Here, we investigate resonance energy transfer dynamics in supramolecular stacks of OPVs consisting of three phenyl rings (MOPV3) doped with similar oligomers containing four phenyl rings (MOPV4). Broad spectral overlap between the MOPV3 fluorophores and MOPV4 chromophores results in efficient energy transfer from MOPV3 to MOPV4. We observe resonance energy transfer following two distinct regimes. The first is evident by growth of MOPV4 photoluminescence on a timescale of similar to50 ps, mediated by rapid exciton diffusion in MOPV3 within the stack. In the second regime, dynamics of localised excitons on nanosecond timescales are dominated by direct resonance energy transfer to MOPV4 chromophores. Global analysis of the photoluminescence decay of MOPV3 in blends with varying MOPV4 composition on times greater than or similar to2 ns is consistent with quasi-one-dimensional resonance energy transfer with Forster radius of 8 nm. (C) 2004 Elsevier B.V. All rights reserved.