Study of Long-Lived Charge-Separated States in Iron Based Dyads

Details

Supervisors

Troian-Gautier, Ludovic

Faculty

Faculté des sciences

Degree label

Master [120] en sciences chimiques, à finalité approfondie

Abstract

enMore than ever, the need to turn towards greener chemistry approaches, cheaper industrial processes and the use of renewable resources is being felt. Therefore, why not turn to energy sources that are free (or almost), abundant and environmentally friendly, such as light? Nowadays, numerous applications in the field of photochemistry (the branch of chemistry dealing with the transformation of matter through light) are flourishing to allow us to harvest light in order to produce electricity, treat diseases, produce all sorts of chemical compounds, or even treat wastewater. Yet not everything is rosy: indeed, many of these photochemical systems are based on extremely rare heavy metals, sometimes toxic, and whose exploitation raises geopolitical and ethical issues, such as ruthenium, iridium, rhodium or osmium. Faced with these challenges, some questions naturally arise: could we do better by using more renewable resources? Concretely, why not use more abundant metals, such as iron, for example? Beyond its high natural abundance, its price and its low environmental impact, iron appears as some kind of Holy Grail capable of remedying all these problems. However, from a photochemical point of view and despite all these qualities, iron suffers from numerous disadvantages. Due to an intrinsically weaker ligand field than its analogues from the 4th and 5th rows of the periodic table, namely ruthenium and osmium, the excited-state properties of iron-based photosensitisers are generally very limited. One of these properties, the excited-state lifetime, is one of the most critical. Whereas it is common for a photosensitiser based on a rare metal to exceed tens of nanoseconds (10⁻⁹ s) or even the microsecond (10⁻⁶ s), iron complexes often struggle to reach the picosecond (10⁻¹² s). These limitations have of course not overcome the determination of researchers, who see the immense potential that the democratisation of iron-based photosensitisers would represent. Over the last 15 years, enormous advances have been made in this field, led by pioneering researchers such as Wärnmark, Lomoth, Persson and Sunsdström. Their research has enabled the discovery of new concepts and rationalisations leading to a drastic increase in excited-state lifetimes. Thus, after having been forgotten for some time, iron-based photosensitisers came back to the forefront in 2013, where the symbolic milestone of the picosecond was reached. We are now witnessing drastic advances over the years, from hundreds of picoseconds in 2018 to the consecration of the nanosecond in 2019. More recently still, our group has used a well-known concept, molecular dyads, to reach tens of microseconds of excited-state lifetime. More than ever, photochemists seem to be on the verge of laying their hands on their Holy Grail. This Master's project is therefore rooted in this particular context, and it aims to study the factors influencing the lifetime of the charged separated state (CSS) of iron-based molecular dyads, as a function of several parameters, such as the oxidation potential of the electron donor, its rigidity, or the distance separating it from the electron acceptor. After an introduction to photochemistry (section 1.1) and to the particular problem of iron-based photosensitisers (section 1.2), the different objectives and methods used in the scope of this project will be addressed in detail in section 2. In section 3, the results of this project will be addressed in details, starting with the synthesis (section 3.1) of the molecular dyads but also of reference compounds. Then, the steady state properties will be described in section 3.2 (UV-Vis time resolved emission lifetimes and emission quantum yields). Section 3.3 will focus on the electrochemical properties of the synthesised references (oxidation and reduction potentials, spectroelectrochemistry) as well as on the case of one of the synthesised dyads. Furthermore, fs-spectroscopy has been extensively used to study the behaviour of the molecular dyads (section 3.4). The results obtained within this master’s thesis will be studied at the light of the Marcus-Levich-Jortner theory in section 3.5. Bimolecular quenching measurements were carried out in order to determine cage escape quantum yields as well as electron transfer rate constants, which will conclude the section 3. Finally, section 4 will conclude this manuscript and address perspectives with respect to this work. We hope that this work will allow for a better understanding of these states that are charge-separated states, and who knows, perhaps lay our hands on that long-coveted Holy Grail?