Experimental and theoretical study of energy and charge transfer in nanotube/chromophore compounds. – TRANCHANT

In the framework of the energy challenge and renewable energy sources, an important research activity is devoted to new low-cost photovoltaic materials. Carbon nanotubes grafted with photo-active dyes could lead to an important breakthrough in this domain due to their exceptional transport properties and high aspect ratio. The proposal TRANCHANT aims at studying the key microscopic mechanisms at work in those systems, namely charge transfer and excitation energy transfer between the chromophore and the nanotube. The project includes a research program dedicated to (i) the synthesis of new donor/acceptor compounds and to (ii) various strategies for grafting nanotubes (covalent and non-covalent approaches), together with (iii) a complementary spectroscopic and (iv) theoretical analysis of the two types of elementary processes that determine the ultimate functionality, i.e., excitation energy transfer (EET) and charge transfer (CT). These phenomena will be investigated by means of a series of optics measurements, including optical absorption, luminescence (both on ensembles and at the single molecule level), spectro-electrochemistry and transient absorption spectroscopy. These experimental studies will be closely coupled to theoretical modelling and numerical simulations of the supra-molecular EET and CT phenomena. The theoretical modelling will allow us, in turn, to tailor new molecular assemblies for optimized EET/CT oriented functionality.

Covalent grafting is well-known for deteriorating the intrinsic properties of carbon nanotubes (transport or optical properties) because it partly destroys the pi conjugated electronic system. Non-covalent grafting preserves this unique structure but, on the other hand, leads to less stable compounds. Starting from these two basic approaches, we will elaborate new strategies. In particular, for covalent grafting, we will keep the number of links to the nanotube's wall to a minimum, while putting a large number of chromophore in interaction with the nanotube by using hyperbranched or dendritric dye molecules. For the non-covalent approach, we will enhance the interaction between the nanotube and the dye by means of surfactants that will confine the compound in micelles.

An added strength of the proposal is to allow an unprecedented analysis of the microscopic mechanisms of energy and charge transfer thanks to the use of chirally enriched samples. The resulting optical spectra are considerably simplified as compared to those of the regular statistical ensembles and will allow new insights into the transient species subsequent to the photo-excitation of the compound. In addition, spectro-electro-chemical measurements under illumination or in the dark, on raw molecules and nanotubes or on the compound will allow to identify all relevant species and pick up their signatures in time-resolved experiments.

Throughout the project, theoretical analysis will be closely connected to the experimental part. Electronic structure and quantum dynamical simulations will be undertaken which complement the available spectroscopic results. While a considerable number of theoretical studies have been carried out for the intrinsic properties of carbon nanotubes, the detailed investigation of EET and CT phenomena (and their competition) on ultrafast time scales poses a considerable challenge. The spatially delocalized nature of the relevant exciton states, the possible involvement of exchange (overlap) effects, and the rôle of conformational disorder lead to a situation which is significantly different, e.g., from conventional Foerster transfer.

Overall, TRANCHANT thus combines synthesis, steady-state and transient spectroscopy, spectro-electrochemistry, and theoretical modeling such to obtain a new perspective on the design and optimization of carbon nanotube-dye assemblies.