Geoffrey Pourtois

Interchain vs. intrachain energy transfer in acceptor-capped conjugated polymers.

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 energy-migration 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 point-dipole 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 rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.

STM imaging of a heptanuclear ruthenium(II) dendrimer, mono-add layer on graphite

Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.

The influence of the counterion on the electronic structure in doped phenylene-based materials

The mechanisms of charge storage and transport in doped conjugated organic materials are a topic of interest in both fundamental and application-oriented research. We investigated the valence electronic structure of n-doped p-sexiphenyl (6P), a model molecule for the electroluminescent polymer poly(p-phenylene). Different alkali metals and an alkaline earth metal were deposited stepwise in situ on thin films of 6P. After each step ultraviolet photoemission spectra were recorded using synchrotron radiation. The evolution of characteristic new features within the formerly empty energy gap of 6P was observed. We conclude from our observations that negative bipolarons (dianions) are the only charged species on a 6P surface. However, the lineshape and energetic positions of the new intragap features varied for each dopant used. By comparing the experimental data with quantum-mechanical calculations on model systems, we discuss the mechanisms that could explain this phenomenon.

Theoretical investigation of the chemical structure and vibrational signature at the aluminum-polythiophene interface

We investigate theoretically the chemical structure and vibrational properties of the interface between aluminum and polythiophene, taken as a prototype conjugated polymer, considering both the deposition of the metal on a polymer substrate and the adsorption of the polymer on the metal surface. We perform quantum-mechanical calculations to determine the geometric and electronic structure and vibrational frequencies in a series of molecular model systems for the Al-polythiophene interfaces.

Energy transfer in π-conjugated polymers: Interchain vs. intrachain processes in polyindenofluorene

The energy transfer processes taking place in conjugated polymers are investigated by means of correlated quantum-chemical calculations applied to polyindenofluorene. The calculations go beyond the usual point-dipole model approximation and account for geometric relaxation phenomena in the excited state prior to energy migration. The results indicate a higher efficiency of the interchain transfer process, which is mainly due to larger electronic coupling matrix elements.