Charge transfer control in small molecular architectures – CHACRA

The CHACRA project aims to study the fundamental aspects of charge transfer (CT) processes induced in small molecular assemblies (MA) adsorbed on a functionalized insulating layer. Our recent experimental work concerning the electronic decoupling of individual molecules on thin insulating layer of CaF2 grown on silicon and the study of CT processes in iron tetraphenyl porphyrin homodimers allow us to use the scanning tunneling microscopy (STM) techniques coupled to a luminescence acquisition system in order to induce, analyze and control CT in simple model MA. Our strategy is to gather all the know-how and expertise in France around this topic to crystalize exceptional working conditions to succeed in our goal. For this, we will focus on the study of model MA formed with metalloporphyrins in planar dimers or trimers conformations with or without covalent bonds. The dimer structure represents a typical donor-acceptor (DA) model for which the influence of the conformation and the initial electronic structure of the molecules on the CT efficiency will be studied with a very local injection of charges via the tip of a low temperature (9 K) STM. The trimer structure will be studied as a second step aiming to model the effect of a molecular bridge located in between the DA. In this context, we plan to study the influence of the surface by adjusting the distance between the molecules via local manipulations or by increasing the thickness of the insulating layer. This research context will allow to point out and compare various types of phenomena such as tunnel, resonant, hopping, or superexchange CT processes. Our investigation methods will be based on the sequential detection of a molecular conformation movement coupled or not with the presence of a molecular ion (transfer of a single electron or a hole). All these events will be studied statistically. In parallel, the CT processes will be diagnosed via the study of the luminescence signal emitted by the MA. This consists in the analysis of the optical spectrum, the luminescence decay or the statistical study of the photon emission blinking. In this context, we plan to use [Ln]TPP that can provide a specific luminescence signal with characteristic lifetimes. To form covalent dimers on the insulating surface we plan to use the expertise of the ICMMO group to synthesis metalloporphyrins having various C-Br reactive groups and use them as reactive ligands. The synthesis of these molecules is very well known and perfectly well controlled by the ICMMO group and the insertion of transition metals or lanthanides inside them does not represent any particular complexity. The CHACRA project will also include a strong expertise in numerical simulations using the density functional theory (DFT) that takes into account long range interactions such as van der Waals forces for large systems via the use of various codes such as VASP or SIESTA. As a first step, we plan to simulate the electronic structure of the MA adsorbed on the insulating layer to analyze the weak interactions between them. Then we will simulate the formation of molecular cations/anions as they can be created after a CT process in the MA. Finally we want to exploit the power of the Time Dependent DFT method on small systems in order to give a picture of the dynamics of the CT. The very good and recognized synergy between experiments and simulations at the ISMO, IS2M and FEMTO-ST groups combined with the expertise brought by the chemical synthesis of metalloporphyrins at ICMMO allow us to propose a very ambitious and innovative research program that will bring new understanding tracks of CT processes at the nanoscale.