Luminescent Isocyanide Complexes of Mn(I) and Mo(0)

At the current rate, global warming will render large portions of our planet uninhabitable in the foreseeable future. Therefore, finding a clean and renewable energy source that can replace the current fossil reserve-based industry will be one of the major challenges of the 21st century. A worldwide industry based on solar power seems to be the obvious solution to this problem. The sun provides more than enough energy for our current needs and harnessing that power does not produce any pollutants, in stark contrast to fossil sources.
However, current technology is not advanced enough to fully replace conventional energy sources. In the energy sector, solar cells are used to produce electricity, but they are still expensive and lack the required high efficiencies. In other important industries, the required solar powered technologies are still underdeveloped. Among these are important fields such as CO2 reduction, water splitting, or chemical reactions powered by light in general. The products of all these reactions are energy enriched substrates, so-called solar fuels, that can be used in the transport sector or at times when no sunlight is available.
Main investigations in these research fields are focusing on precious metal complexes. However, for a global scale application of photocatalyzed reactions, systems based on abundant, inexpensive elements are required.
The research presented in this thesis focuses on photoactive metal complexes based on inexpensive first- and second-row transition metal complexes of manganese and molybdenum. The underlying photophysical properties as well as current progress on photoactive first-row transition metal complexes will be discussed in chapter 2 of this thesis.
In chapter 3, two new isocyanide complexes of Mn(I) are presented. Both of these air-stable complexes exhibit MLCT luminescence in solution at room temperature. This represents not only the first report on MLCT luminescence from Mn(I) compounds, but these are also among the first examples of any first-row transition metal complex exhibiting MLCT luminescence (apart from Cu(I) complexes, whose electronic structure makes MLCT luminescence easier accessible). Both complexes were successfully employed as photosensitizers in energy and electron transfer reactions. Further interesting photophysical properties of these complexes, as well as their synthesis, will be discussed in this chapter.
The two publications presented in chapter 4 provide new insights into the properties and photocatalytic applications of Mo(0) isocyanide complexes. The introduction of bulky groups on the ligands leads to rigidification of the complex and more efficient shielding of the active metal centre from the chemical surrounding. This structural change increases the excitedstate lifetime and the photoluminescence quantum yield by one order of magnitude compared to a previously reported analogue. These enhanced properties, in combination with the high exited state oxidation potential, enable the application of this complex in challenging photoreactions.
Chapter 5 provides a brief conclusion on low-valent isocyanide complexes of earth-abundant metals as well as possible future research and improvements in this area.
Finally, photoinduced electron transfer in oligo-1,2-naphthylene linked donor-acceptor compounds was investigated. Unusual distance dependencies of the electron transfer rate were found for different lengths of the helical 1,2-naphthylene linker units which can be explained by “shortcuts” through close noncovalent intramolecular contacts (appendix, chapter 6).