Charge transfer control in small molecular architectures – CHACRA

The scientific approach of the CHACRA project combines state-of-the-art low-temperature (9K) and low-noise scanning tunneling microscopy (STM) techniques with numerical calculations using quantum chemistry codes for fine characterization of the electronic properties of the studied systems. Additionally, external chemical synthesis allows for the creation of non-commercial molecules with ad-hoc properties, enabling the study of covalent dyads with localized luminescence properties in parts of the device. The operation of the STM at 9K ensures that the ro-vibrational population of the surface + molecule ensemble does not exceed 775 micro-eV in its stationary state. These experimental conditions allow for the spectral resolution of the vibrational modes of the studied molecules, with energy as low as 40 cm-1 (i.e., ~5 meV). Furthermore, we have implemented powerful numerical simulation tools suitable for large systems (> 1000 atoms) to describe the electronic properties of the studied systems and reproduce our experimental observations. These numerical models, based on density functional theory (DFT), describe the stationary states of the systems but do not directly predict the dynamics of excited states and therefore the CT itself. However, we have developed other approaches that combine various models based on Fermi's golden rule and the Marcus-Levich-Jordner model, which incorporate the vibrational modes of the system.

The scanning tunneling microscope (STM) has been the ideal tool for this project because it allows for the mapping of molecules on surfaces as well as their manipulation to form ad-hoc molecular devices. By leveraging the strong localization of tunneling electrons from the STM, we conducted electronic excitations at various points on a single molecule, thereby triggering the charge transfer process in carefully selected molecular devices. The charge transfer analysis is performed sequentially by imaging the device before and after excitation: the movement of the non-excited molecule being the signature of the charge transfer process. We developed various experimental methods that combine local statistical measurement of the charge transfer process with fine statistical analysis of the generated telegraphic noise. The studied molecules belong to the metalloporphyrin family and are precisely assembled into covalent or non-covalent devices on a semi-insulating surface (CaF2/Si(100)).

The strength of this project lies in combining the synthesis of specific molecules, whose properties are initially evaluated to favor certain processes, with numerical calculations that describe with high precision the molecule-molecule and molecule-surface interactions, thus the environment of the studied system.

The major observations made during this project touch on many aspects of the physico-chemistry of charge transfer processes. In the context of non-covalent molecular devices, we observed that charge transfers involving cation formation by the STM are highly efficient and mainly drive the systems tested, potentially challenging the traditional donor/acceptor concept. Moreover, the initial cation formation, which triggers the CT process, prevents energy transfer processes, ensuring a true charge transfer process involving the effective displacement of one or more charges within the system. The localization of electronic excitation via STM showed that the CT process does not follow the known relaxation processes in optically excited systems (Kasha's rule), allowing for enhanced optimization of CT in various existing devices using our approach. These initial observations led us to explore the role of vibrations in the studied molecular device. We observed that controlling these vibrations plays a predominant role in the efficiency of CT processes. Finally, the comparison of CT studies in covalent or non-covalent heterodyads reveals how crucial the environment and initial conformation of the nano devices at the molecular scale are.

The possible perspectives of the CHACRA project are multifaceted. Firstly, it is crucial to continue studying covalent and non-covalent systems in molecular triads through electronic excitation. One of the challenges of this study will be finding the ad-hoc experimental conditions to reduce the attenuation of the luminescence of excited systems (surface). A second potential way to explore is the combination of electronic and optical excitation. Recent work at ISMO shows that it is possible to measure a photoelectron spectrum of a molecular device under the STM tip, which measures the electronic state of the system [ACS Nano 2024, 18, 13, 9656–9669]. Concurrently, many numerical challenges remain, including more relevant relaxation of experimentally observed molecular configurations and a precise description of the dynamics of these systems, including the surface and the studied nano-device. In the long term, the approach developed in the CHACRA project to address the issue of planar molecular contact is an essential research path for applicative stakes.

The CHACRA project has resulted in eight scientific articles in peer-reviewed journals. Four of these have been published, one article has just been accepted, and three others are in preparation. Among these, two articles focus on the study of charge transfer (CT) in non-covalent homodyads, highlighting the importance of anti-Kasha processes and vibrations in the systems studied. Two dedicated theoretical models are being developed for these systems. Another two articles summarize our discoveries on covalent and non-covalent heterodyads, demonstrating the ability to control the efficiency of the CT process in the device, like an ON/OFF switch. Two of the eight manuscripts are devoted to the atomic-scale description of the semi-insulating epitaxial surface structure, one of which reports the discovery of a new periodic structure on the Si(100) surface. Finally, we have also shown that we can control the charge state of the studied molecules and described a synthesized molecule with a lanthanide atom (Er).

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.