Clément Daniel

Exciton migration in a polythiophene: Probing the spatial and energy domain by line-dipole Forster-type energy transfer

We study exciton migration in low molecular weight poly[3-(2,5-dioctylphenyl)thiophene] in dilute solution by means of ultrafast spectroscopy and Monte Carlo simulations of resonance energy transfer using the line-dipole Forster approach. The model includes the build-up of polymer chains, site-selective exciton generation, and diffusion through incoherent energy transfer. Time-resolved, ensemble-averaged experimental data are reproduced, namely photoluminescence spectral migration and stimulated emission anisotropy decays measured by streak camera and femtosecond transient absorption spectroscopy under site-selective excitation conditions. Importantly, the relatively simple line-dipole Forster-type approach beyond the point-dipole approximation reproduces both experiments quantitatively. Since explicit chain conformations are used in the model, the simulations yield a descriptive microscopic picture of exciton migration. The effective conjugation length (l(seg)=2.9 nm, 7.4 monomer units) and the disorder of the chains (Omega=0.8) are yielded as the only fitting parameters. We find an extra component that is not covered by our fits in anisotropy decays at early times for high excitation energies. This is interpreted within the context that the effective conjugation is limited by conformational disorder.

Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

We present femtosecond transient absorption measurements on p-conjugated supramolecular assemblies in a high-pump-fluence regime. Oligo( p-phenylenevinylene! monofunctionalized with ureido-s-triazine ~MOPV! self-assembles into chiral stacks in dodecane solution below 75 °C at a concentration of 4310 24 M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluencedependent decay of 1 1 Bu-exciton spectral signatures and by the sublinear fluence dependence of time- and wavelength-integrated photoluminescence ~PL! intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors.

Optical probing of sample heating in scanning near-field experiments with apertured probes

We have used the inherent thermochromism of conjugated polymers to investigate substrate heating effects in scanning near-field experiments with metal-coated “apertured” probes. Chemically etched and pulled fibers were used to provide near-field excitation of fully converted films of poly(p-phenylene vinylene), PPV, and of poly(4,4′-diphenylene diphenylvinylene). We detect no significant blueshift of the photoluminescence spectra generated with near-field excitation, in comparison to those collected with far-field excitation. We conclude that polymer heating in the region contributing to the luminescence is less than 40K. We also demonstrate that thermolithography of the PPV precursor is not significant by comparing UV (325nm) and red (670nm) illumination.

The effects of supramolecular assembly on exciton decay rates in organic semiconductors

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene (OPV) materials that are functionalized with quadruple hydrogen-bonding groups. These form supramolecular assemblies with thermotropic reversibility. The morphology of the assemblies depends on the way that the oligomers are functionalized; monofunctionalized OPVs (MOPVs) form chiral, helical stacks while bifunctionalized OPVs (BOPVs) form less organized structures. These are therefore model systems to investigate the effects of supramolecular assembly, the effects of morphology, and the dependence of oligomer length on the radiative and nonradiative rates of pi-conjugated materials. The purpose of this work is to use MOPV and BOPV derivatives as model systems to study the effect of intermolecular interactions on the molecular photophysics by comparing optical properties in the dissolved phase and the supramolecular assemblies. A simple photophysical analysis allows us to extract the intrinsic radiative and nonradiative decay rates and to unravel the consequences of interchromophore coupling with unprecedented detail. We find that interchromophore coupling strongly reduces both radiative and intrinsic nonradiative rates and that the effect is more pronounced in short oligomers.

Exciton dynamics in supramolecular assemblies of p-phenylenevinylene oligomers

We have investigated the dynamics of photoexcitations in chiral assemblies of p-phenylenevinylene oligomers functionalized with hydrogen-bonding motifs. In the regime of low excitation densities, the luminescence transients are influenced by the migration of excitons to defect sites, indicative of fast diffusivity of excitons along the molecular assemblies. In addition, at high excitation densities, bimolecular exciton annihilation is shown to result in the fast depopulation of the stacks’ excitonic energy levels.