Sub Projects

The main targets are the design, construction and deep understanding of novel molecule-based magnetic materials with optical read-out. This will be achieved by a combination of iron(II)/chromium(II) spin-crossover and luminescent chromium(III) complexes based on the molecular ruby motif. The constituent functional units will be assembled (a) via ion pairing, (b) via coordinative bonds in heterobimetallic complexes or (c) via redox tuning of mixed-valent CrII/CrIII salts. To disentangle the interaction effects on a molecular level, we will employ suitable spectroscopic methods at variable temperature, including time-resolved spectroscopies as well as quantum chemical investigations (benchmarked DFT and post-Hartree–Fock methods) including an adequate representation of the ensemble. The ultimate aim of this project is an optical read-out of the iron(II) HS/LS population with high contrast using different read-out channels, namely quantum yield F, lifetime t and wavelength l of the chromium(III) luminescence.

We plan to synthesize a new class of SCO compounds on surfaces. Specific research goals are as follows. (a) Using new, correspondingly functionalized SCO complexes and employing on-surface coupling we will prepare arrays of coupled and switchable spin centers. (b) We will strive to develop more rugged complexes that are suitable for sublimation and withstand the interaction with metal substrates. (c) We will use fairly direct probes to get insight into the spin states of individual molecules on surfaces. These encompass the spectroscopy of spin excitations and of Yu-Shiba-Rusinov resonances. Switching will be induced by electron current and by controlling the molecular environment. (d) We will explore to which extent switching of a particular molecule in an array may be achieved “remotely” by manipulating neighbor molecules.

We aim to develop a new class of molecular spin switches based on singlet fission in tetrel-centered (Ge, Si, etc.) radical systems. (a) We will synthesize stable and spectrally tunable triaryl tetrel radicals and combine them with tailored anthracene-based singlet fission bridges to create molecular constructs with two coupled spin centers. (b) We will systematically vary the connectivity, distance, and electronic structure of the anthracene units to control singlet fission efficiency and the resulting spin coupling. (c) Using advanced spectroscopic techniques, in particular EPR and time-resolved optical spectroscopy, we will characterize the spin states, coupling mechanisms, and coherence times in both ground and excited states. (d) Finally, we will explore how molecular design governs photo-induced spin entanglement over extended distances and establish structure-property relationships that enable controlled spin-state switching and long-lived high-spin states.

In this joint experimental-theoretical project, we propose a new bottom-up approach to design and investigate assemblies of spin-crossover complexes in a (matrix) environment. By this approach both, the environment and the size of the agglomeration can be controlled, allowing to customize cooperative effects. For this purpose, various new metallopolymers containing Fe(II) SCO complexes will be synthesized. By a tailor-made design, the assembly of the SCO-complexes in the polymeric environment will be adjusted to enable or disable agglomeration of the complexes, thereby tuning the number of SCO complexes, and, correspondingly, the degree of cooperativity. Combined with a detailed characterization of the spin-crossover properties and the underlying mechanisms by spectroscopic and theoretical approaches, it will be possible to reveal the influence of the polymer matrix on the spin-crossover properties as well as to answer the question of how many complexes are required to achieve bistability through cooperative interactions.

We aim to explore interactive spin-state switching in exchange-coupled systems, using trinuclear complexes based on triaminoguanidine ligand frameworks as a platform. This investigation focuses on light-induced switching processes, emphasizing the sequence of individual switching events within triangular spin systems, their dynamics, and the influence of external stimuli such as electric fields. Additionally, we seek to understand how these stimuli can modulate the behavior of the system and enhance the efficiency of the switching processes.

In NV-center-based quantum sensors, the integration of chemical systems, such as spin-crossover (SCO) molecules, offers several chemistry-related objectives. These goals focus on utilizing the unique properties of NV centers in diamond for detecting and analyzing chemical phenomena at the nanoscale. We aim to use spin-based quantum sensors to study individual spin-crossover molecular complexes.

Project summary:

In the present project, we aim at a fundamental understanding of spin switching in polynuclear SCO complexes, including the influence of the environment. We explore intramolecular magnetic coupling and a possible coupling to a ferromagnetic substrate to improve switchability and to obtain a greater switching response. New dinuclear and trinuclear Fe(II) complexes are synthesised and analysed by X-ray absorption spectroscopy and Fourier transform THz electron paramagnetic resonance spectroscopy. These investigations are accompanied and supported by high-level quantum chemical calculations. 

Developing and controlling spin state switches from the molecular level is a fundamental challenge with direct importance for emerging fields such as molecular spintronics. Central questions concern the development of suitable molecular complexes with reliable trigger and read-out options and their immobilisation on surfaces as a first step towards device fabrication. Thorough insights from high-level spectroscopic and theoretical investigations are essential to understand the electronic structures in the different switch positions as well as the switching and read-out processes. This project focuses on redox switches with optical readout, specifically homo- and hetero-dinuclear iron and cobalt complexes with redox-active imino-quinonoid bridges, as well as their immobilisation. Upon redox triggering, the unpaired electrons on the metal ions and the bridge will be strongly antiferromagnetically coupled, leading to a large spin state change. Additionally, these complexes are expected to display spin crossover, electron transfer induced spin state switching, and mixed-valency coupled with spin-state changes. Optical readout will be facilitated by incorporating pyrene groups at the bridging unit in conjunction with magnetic circular dichroism measurements. Click chemistry will be used for surface immobilization of the complexes modified with appropriate anchoring groups. All systems will be thoroughly characterised using spectroscopy (UV-vis, NIR, IR, rR, EPR, MCD, Mössbauer), electrochemistry and spectroelectrochemistry. Quantum chemical studies will focus on electronic structure characterisation and analysis by quantification of Marcus–Hush theory for mixed-valent systems with an existing ab initio approach, which will be expanded to three-center systems. Mutually beneficial collaborations with synthesis, spectroscopy and theory groups will be established for further characterisaton of the systems. We expect that our project will deliver fundamental insights and first steps towards applications for using redox processes as simple and benign switches to drive large spin state changes in surface-immobilised molecular complexes with optical read-out.

The present project is dedicated to elucidating and subsequently tailoring the light-triggered spin state switching in molecular square-planar first row transition metal complexes. In light-driven applications, e.g. in the context of sensing, smart pigments, information processing and storage as well as energy and ET reactions, 3d TMCs typically suffer from rather short-lived excited (spin) states in contrast to their 4d and 5d analogues, which is a consequence of the decreasing ligand field splitting from 5d to 3d metals. Herein, we will address this issue by combining the expertise from synthesis (Wenger), time-resolved spectroscopy (Schwarzer and Wenger) and quantum chemistry (Kupfer) in a highly interdisciplinary approach. Our consortium will derive detailed structure-property relationships and will establish theory-driven design principles to elongate the lifetime of the excited high spin (triplet) state in 3d8 TMCs.

In a combined synthetic, spectroscopic and theoretical approach, this project will develop multifunctional spin-state switches using discrete multinuclear Fe^II cages in solution. Dual stimuli SCO cages will be synthesised and characterised where spin-state switching can be achieved using temperature and a second stimulus, either guest binding to control the conformation through allosteric switching or light to alter the SCO properties through photoswitching. The cage‘s symmetry, stereochemistry of the metal centres, solvent and cooperative effects will be exploited to further tune the SCO properties. Experimental data from mononuclear model systems and the cages will inform the development of theoretical tools for predicting spin-crossover temperatures taking solvent effects into account. These theoretical tools will be exploited for the design of second-generation cage systems with the aim of preparing systems for achieving spatio-temporal control of SCO behavior using light or sensing of chemical compounds through guest binding.