Lichtinduzierte Elektronentransfer-Reaktionen sind chemische Elementarschritte, die in der biologischen Photosynthese eine zentrale Rolle spielen. Ein grundlegendes Verständnis dieser Elementarschritte ist daher für die Umwandlung von Sonnenergie in chemisch gespeicherte Energie in künstlichen Photosynthese-Systemen von Interesse. Chemische Modellverbindungen, in denen ein Elektronen-Donor, eine lichtabsorbierende Einheit (der sogenannte Photosensibilisator) und ein Elektronen-Akzeptor kovalent miteinander verbunden sind, eignen sich besonders gut für grundlegende Untersuchungen, weil in solchen Triaden-Verbindungen die Elektronentransfer-Prozesse direkt beobachtbar sind. Bisherige Untersuchungen an Triaden-Verbindungen fokussierten vor allem auf die lichtinduzierte Übertragung von einzelnen Elektronen. In der biologischen Photosynthese werden jedoch pro Reaktionsumsatz meist mehrere Elektronen benötigt und daher hat die Natur Wege gefunden, mehrere Elektronen zu akkumulieren. In Triaden-Verbindungen ist dies bislang erst selten gelungen. In diesem Forschungsvorhaben sollen daher die Grundlagen der lichtgetriebenen Elektronen-Akkumulation in Triaden-Verbindungen erforscht werden. Frühere Studien an Triaden-Verbindungen verwendeten ausserdem sehr oft Photosensibilisatoren, die aus Edelmetallen bestehen. In diesem Projekt geht es auch darum, auf kostengünstigeren Metallen aufgebaute Triaden-Verbindungen zu untersuchen. Dies scheint aus Gründen der Nachhaltigkeit wünschenswert, und andererseits können von neuartigen Photosensibilisatoren besonders günstige Elektronentransfer-Eigenschaften erwartet werden.

For a sustainable future we need energy- and resource-efficient catalytic processes that can be controlled externally and use catalysts made of earth-abundant elements. Binary inorganic materials, such as transition metal phosphides and chalcogenides, have shown great promise to replace noble metal catalysts in some areas, but catalysis has not been broadly explored. The goal is to develop customizable catalysis by abundant inorganic materials for complex chemistry and added-value chemical products through a fundamental understanding of their interfacial chemistry. Electric fields could provide an external control on catalysis that is complementary to the more common temperature and pressure controls used in chemical industry. Electrostatic effects, such as electric fields are thought to be a major contributor to enzymatic catalysis, but have not been broadly explored in chemical synthesis. Herein, we will probe electric fields as external controls on catalysis of reductive and oxidative transformations by abundant materials using electrochemical methods and in-situ surface-enhanced infrared absorption spectroscopy. These studies will be supported by elucidation of the structure, thermochemistry, and elementary reactivity of the operative surfaces and the fine-tuning of their catalytic properties by chemical tailoring. Together, these strategies could open up great opportunities for the technological application of binary materials in sustainable chemical processes and for chemical synthesis.

Molecular Systems Engineering is a National Centre of Competence in Research (NCCR) funded by the Swiss National Science Foundation (SNSF), and headed by the University of Basel and the ETH Zurich. This NCCR combines expertise from chemistry, biology, physics, bioinformatics, and engineering. The overreaching aim is to develop tools and devices to monitor and manipulate off-equilibrium (bio)chemical systems. These may find applications in the synthesis of high added-value products, as innovative diagnostic tools and for the restoration of a desired cellular or organ function.

Many of the most widely used photoactive complexes known to date are made from precious elements such as Ru, Ir, Pt or Au. Their replacement by more earth-abundant metals is of long-standing interest, and the first half of this proposed research project aims to explore the possibility of obtaining fundamentally new luminescent and photoredox-active complexes with long-lived excited-states. The plan is to synthesize and investigate a broad range of metal complexes made from earth-abundant metals in various oxidation states that until now have received either extremely limited or no attention at all in photophysical and photochemical contexts. Specifically, it is planned to explore: (i) homoleptic d6 MLCT luminophores and photoredox catalysts made from Cr(0), Mn(I), or Mo(0) with novel chelating isocyanide ligands, (ii) heteroleptic d10 MLCT emitters made from Ni(0) and a combination of chelating diphosphine and diisocyanide ligands, (iii) d-d emitters based on six-coordinate V(II) or Mn(IV) (d3) complexes, (iv) d0 LMCT luminophores based on Ti(IV) or Zr(IV) with chelating ligands made from pyrrolic and phenolic binding units.While the focus of this project is on obtaining fundamental insight into the basic photophysics and photochemistry of these new metal complexes, the development of such compounds made from earth-abundant elements is of significant interest in the contexts of lighting devices, solar cells, sensors, photoredox catalysis in organic chemistry, or for sensitization of reactions leading to the conversion of (solar) light into chemically stored energy, i. e., for so-called solar fuels.The second half of this proposed project aims at gaining fundamental insight into multi-photon, multi-electron transfer reactions going conceptually far beyond traditional work on photoinduced single electron transfer in donor-acceptor compounds. Artificial photosynthesis will have to rely on multi-electron conversions such as water splitting and CO2 reduction, but currently many studies use sacrificial redox reagents to perform such reactions under irradiation with visible light. This approach will not permit sustainable light-to-chemical energy conversion, and therefore it is highly desirable to explore the basics of multi-photon, multi-electron transfer reactions, as well as the light-driven accumulation of multiple redox equivalents without sacrificial reagents. Toward this end, a series of very carefully designed donor-acceptor compounds will be synthesized and investigated by various photophysical methods. Electrochemical potential inversion, proton-coupled electron transfer (PCET), and redox relays will play an important role in the various sub-projects of this research endeavor.

2,2'-bipyridine and related alpha-diimine ligands form luminescent complexes with a variety of d6 and d8 metals, many of which are very well investigated. The phosphorous analog of 2,2'-bipyridine is called 2,2'-biphosphinine and has been known since 1991. Several transition metal complexes with 2,2'-biphosphinine have been synthesized but the photophysical and photochemical properties of these complexes have remained essentially unexplored. For well-selected complexes with bi- and tridentate phosphinine ligands there is good reason to expect similar (emissive) metal-to-ligand charge transfer (MLCT) excited states as for many alpha-diimine complexes of d6 and d8 metals. However, 2,2'-biphosphinine tends to stabilize metals in lower oxidation states than 2,2'-bipyridine, and some complexes of 2,2'-biphosphinine adopt a trigonal-prismatic geometry rather the octahedral coordination usually encountered in homoleptic 2,2'-bipyridine d6 metal complexes. In addition, 2,2'-biphosphinine is more redox non-innocent than 2,2'-bipyridine. One can therefore expect certain analogies but also some pronounced differences between the photophysics of 2,2'-biphosphinine and 2,2'-bipyridine complexes. The goal of the first part of this proposal (sub-project 1) is to synthesize new luminescent metal complexes with bi- or tridentante phosphinine ligands and to obtain a detailed fundamental understanding of their photophysical and photochemical properties. The work will not be limited to 2,2'-biphosphinine and its derivatives but will encompass bi- and tridentate chelating ligands including for example pyridylphosphinines and phenylphosphinines. This research has the potential to lead to new luminescent materials, photo- or electrocatalysts for CO2 reduction, photosensitizers for dye-sensitized solar cells, potent photooxidants for electron transfer studies in proteins, DNA intercalators, or chemical sensors for volatile organic compounds. The second part of the proposed research (sub-project 2) concentrates on novel bipyridine and terpyridine ligands which are expected to show catechol-like redox properties (Figure 2). The ultimate goal is to develop new transition metal complexes which can undergo multiple oxidations in a reversible fashion at relatively modest electrochemical potentials. Such complexes are of interest as catalysts for a variety of different multi-electron conversions including for example water oxidation or CO2 fixation. Common alpha-diimine complexes such as [Ru(2,2'-bipyridine)3]2+ exhibit simple one-electron redox chemistry, and they usually cannot undergo multi-electron reactions. The ligands in Figure 2 are expected to be highly redox-active, and when coordinated to metal complexes unusually rich multi-electron (photo)redox chemistry might result. Multiple oxidations of these complexes should be possible over a relatively small potential window due to the fact that the respective ligands can release protons upon oxidation, thereby avoiding the formation of highly charged species. Initial work will focus mostly on substitution-inert d6 metal complexes, but in the mid- to long-term more earth-abundant 3d metals will be investigated.

Many photophysical and photochemical processes which are relevant for light-to-chemical energy conversion occur on very rapid timescales. Time-resolved UV-Vis absorption spectroscopy has become an indispensable tool in modern photochemistry. Several ongoing Ph. D. theses and postdoctoral research projects in the main applicant's group ask for a transient absorption spectrometer with picosecond time resolution and an appropriate laser source. Among these projects are for example the investigation of photoinduced multi-electron transfer reactions in order to spatially separate multiple electrons from multiple holes, which is of key importance for producing chemical fuels with sunlight as energy input (projects 1 and 2). Similarly, picosecond transient absorption spectrosocopy will permit mechanistic studies of photoinduced proton-coupled electron transfer (PCET) reactions which will greatly further our current fundamental understanding of this important class of reactions (project 3). The activation of small inert molecules such as H 2 O, CO 2 or N 2 will invariably rely on multi-electron, multi-proton chemistry hence the proposed photochemical studies are important in the greater context of solar energy conversion.

The NCCR Molecular Systems Engineering combines competences from life sciences, chemistry, physics, biology, bioinformatics and engineering sciences. More than 100 researchers and support personnel distributed into four work packages and 31 projects work together to address systems engineering challenges by integrating novel chemical and biological modules into molecular factories and cellular systems for the production of high added-value chemicals and applications in medical diagnostics and treatment. Molecular systems engineering relies on the combination of both chemical- and biological modules. In this approach, complex dynamic phenomena emerge as the result of the integration of molecular modules designed to interact in a programmed way with their complex environment. In this manner, it should be possible to create molecular factories and cellular systems whose properties are more than the sum of the attributes of the individual modules. The commitments of the leading houses, the University of Basel and ETH Zurich, also include new (joint) professorships, and extensive training of a new generation of scientists and technologists, leading to a long-term paradigm shift in molecular sciences and a new structure of the Swiss research landscape.

The goal of this project is to provide solar fuels to molecular factories. Mastering light-induced charge accumulation and phototriggered multi-electron transfer are the key challenges for the first phase of this project.

Lead Protonengekoppelte Elektronenübertragungsreaktionen spielen eine wichtige Rolle in der Photosynthese. Die lichtinduzierte Übertragung einzelner Elektronen von einem Reaktionspartner auf einen anderen ist mittlerweile recht gut verstanden. Im Hinblick auf eine künstliche Photosynthese scheint jedoch ein besseres Verständnis von protonengekoppelten Elektronenübertragungsreaktionen wünschenswert. Inhalt und Ziel des Forschungsprojekts Im Rahmen dieses Forschungsvorhabens sollen grundlegende Aspekte des lichtinduzierten protonengekoppelten Elektronentransfers an geeigneten chemischen Modellsystemen, die eigens dafür hergestellt werden, systematisch mit optisch-spektroskopischen und elektrochemischen Methoden untersucht werden. Ein wichtiges Ziel dabei ist die Ermittlung der Triebkraft- und Distanzabhängigkeit der Geschwindigkeitskonstanten von protonengekoppelten Elektronenübertragungen. Des Weiteren sollen molekulare Systeme hergestellt und untersucht werden, in denen nach Lichtanregung nicht nur einzelne, sondern zwei oder mehrere Elektronen von einem Reaktionspartner auf einen anderen übertragen werden können. Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts Dieses Forschungsvorhaben soll neue Erkenntnisse im Hinblick auf die Umwandlung von Sonnenenergie in chemisch gespeicherte Energie liefern. Ein mögliches langfristiges Ziel dieser Grundlagenforschung ist die künstliche Photosynthese. Keywords Photochemistry, electron transfer, laser spectroscopy, electrochemistry, time-resolved spectroscopy.