Probing bistability in FeII and CoII complexes with an unsymmetrically substituted quinonoid ligand

The generation of molecular platforms, the properties of which can be influenced by a variety of external perturbations, is an important goal in the field of functional molecular materials. We present here the synthesis of a new quinonoid ligand platform containing an [O,O,O,N] donor set. The ligand is derived from a chloranilic acid core by using the [NR] (nitrogen atom with a substituent R) for [O] isoelectronic substitution. Mononuclear FeII and CoII complexes have been synthesized with this new ligand. Results obtained from single crystal X-ray crystallography, NMR spectroscopy, (spectro)electrochemistry, SQUID magnetometry, multi-frequency EPR spectroscopy and FIR spectroscopy are used to elucidate the electronic and geometric structures of the complexes. Furthermore, we show here that the spin state of the FeII complex can be influenced by temperature, pressure and light and the CoII complex displays redox-induced spin-state switching. Bistability is observed in the solid-state as well as in solution for the FeII complex. The new ligand presented here, owing to the [NR] group present in it, will likely have more adaptability while investigating switching phenomena compared to its [O,O,O,O] analogues. Thus, such classes of ligands as well as the results obtained on the reversible changes in physical properties of the metal complexes are likely to contribute to the generation of multifunctional molecular materials

Frequency-swept Excitation in Distance Measurements by EPR: Spin 1/2 systems

In the toolbox of structural biology, pulsed Electron Paramagnetic Resonance (EPR) experiments have proven their worth in determining distance distributions in the nanometer range. For systems without longrange order, such information can be difficult to obtain by other methods. The distance range accessible by EPR depends on the surroundings of the electron spins. In biologically relevant environments, the distance range is typically limited to 1.5-5 nm, and often particularly short in lipids. Sensitivity is often the limiting factor in accessing distances of sufficient length. Recent advances in technology provide access to amplitude and frequency modulation for EPR. Application of such pulses has for example already led to large sensitivity gains in distance measurements of GdIII–GdIII spin pairs. In such systems with spin > 1/2, the larger transition moment simplifies spin state transfers. However data analysis can be complicated by high-spin effects. This study instead focuses on spin 1/2 systems. The overarching objective of this work was to explore the use of frequency-swept pulses in distance measurements. The developed methodologies were subsequently used in application projects and put in the context of other distance determination methods. In a first scenario, the detection pulses of a ’pump–probe’ experiment were left as monochromatic pulses while the pump band was frequency-swept. These pulses were employed in a variant of the Double Electron Electron Resonance (DEER) experiment with increased sensitivity. Dynamical decoupling by optimal timing of refocusing reduces losses due to interactions with the environment in the sequence called 5-pulse DEER. Remarkably, we found that the increase in sensitivity holds true for various spin environments, including biologically relevant ones. The frequency-swept pump band enabled suppression of a known artefact contribution. Additional artefacts were found and identified as being caused by overlap of the excitation bands. Tuning inversion efficiency and spectral separation of the frequency-swept pump pulse allowed for finding an optimal pulse setup with respect to sensitivity and purity. A data processing algorithm for removing a remaining artefact contribution based on shifting the artefact with respect to the main signal was rigorously tested; before the method, using the optimal pulse setup, was applied to biologically relevant systems. In a second scenario, frequency-swept excitation of both the pump and the probe band was explored. These experiments were performed on a spin 1/2 system with a broad spectrum: Cu(II). The gain by the use of frequency-swept pulses in the individual bands was assessed independently as well as in combination. Interestingly, we found the sensitivity of dipolar traces with frequency-swept observer pulses to be lower than would be predicted by the enhanced echo intensity. Optimized frequency-swept pump pulses yielded similar sensitivities as a different experiment in which the pumped spin is flipped by relaxation. This finding is rather remarkable, seeing that frequency-swept excitation of Cu(II) can compete with the ’infinite bandwidth’ of relaxation. In the third scenario, frequency-swept excitation was employed for one, unified band exciting both pumped and probed spins in an experiment abbreviated as SIFTER. This approach enabled suppression of orientation selection in systems with correlated geometries, an effect which can confound distance determination. We assured reliable determination of distances by performing SIFTER on a series of ’molecular rulers’. Furthermore, the decay characteristics of SIFTER were found to be prolonged to a similar extent as those of 5-pulse DEER. The prolonged dipolar evolution time allowed to estimate a distance of ca. 7 nm, which is, to the best of our knowledge, the longest distance so far measured by EPR between spins in a protonated lipid environment. In the last part of the thesis, applications of the developed methodologies are presented. First, the ability of SIFTER to suppress orientation selection is exploited to verify distance information in stiff compounds intended to study Förster Resonance Energy Transfer (FRET): The EPR data provided the basis for modeling molecular rulers with nitroxide or fluorescence labels. These predictions allowed comparison of the accuracy of distance determination by the two methods as well as to provide experimental proof for the seminal theory by Förster with respect to orientation-dependence. Finally, 5-pulse DEER is applied for structure determination of the protein Pin1. The prolonged distance range allowed for characterization of interdomain distances, which provide valuable complementary information to short-range NMR restraints for this system

High-Bandwidth Q-Band EPR Resonators

Tschaggelar R, Breitgoff FD, Oberhänsli O, Qi M, Godt A, Jeschke G. High-Bandwidth Q-Band EPR Resonators. Applied Magnetic Resonance. 2017;48(11-12):1273-1300

UWB DEER and RIDME distance measurements in Cu(II)-Cu(II) spin pairs.

Breitgoff FD, Keller K, Qi M, et al. UWB DEER and RIDME distance measurements in Cu(II)-Cu(II) spin pairs. Journal of magnetic resonance (San Diego, Calif. : 1997). 2019;308: 106560.Distance determination by Electron Paramagnetic Resonance (EPR) based on measurements of the dipolar coupling are technically challenging for electron spin systems with broad spectra due to comparatively narrow microwave pulse excitation bandwidths. With Na4[{CuII(PyMTA)}-(stiff spacer)-{CuII(PyMTA)}] as a model compound, we compared DEER and RIDME measurements and investigated the use of frequency-swept pulses. We found very large improvements in sensitivity when substituting the monochromatic pump pulse by a frequency-swept one in DEER experiments with monochromatic observer pulses. This effect was especially strong in X band, where nearly the whole spectrum can be included in the experiment. The RIDME experiment is characterised by a trade-off in signal intensity and modulation depth. Optimal parameters are further influenced by varying steepness of the background decay. A simple 2-point optimization experiment was found to serve as good estimate to identify the mixing time of highest sensitivity. Using frequency-swept pulses in the observer sequences resulted in lower SNR in both the RIDME and the DEER experiment. Orientation selectivity was found to vary in both experiments with the detection position as well as with the settings of the pump pulse in DEER. In RIDME, orientation selection by relaxation anisotropy of the inverted spin appeared to be negligible as form factors remain relatively constant with varying mixing time. This reduces the overall observed orientation selection to the one given by the detection position. Field-averaged data from RIDME and DEER with a shaped pump pulse resulted in the same dipolar spectrum. We found that both methods have their advantages and disadvantages for given instrumental limitations and sample properties. Thus the choice of method depends on the situation at hand and we discuss which parameters should be considered for optimization. Copyright © 2019 The Author(s). Published by Elsevier Inc. All rights reserved

Linear and Kinked Oligo(phenyleneethynylene)s as Ideal MolecularCalibrants for Förster Resonance Energy Transfer

ISSN:1948-718

Linear and Kinked Oligo(phenyleneethynylene)s as Ideal Molecular Calibrants for Forster Resonance Energy Transfer.

Czar MF, Breitgoff FD, Sahoo D, et al. Linear and Kinked Oligo(phenyleneethynylene)s as Ideal Molecular Calibrants for Forster Resonance Energy Transfer. The journal of physical chemistry letters. 2019;10(21):6942-6947.We show that oligo(phenyleneethynylene)s (oligoPEs) are ideal spacers for calibrating dye pairs used for Forster resonance energy transfer (FRET). Ensemble FRET measurements on linear and kinked diads with such spacers show the expected distance and orientation dependence of FRET. Measured FRET efficiencies match excellently with those predicted using a harmonic segmented chain model, which was validated by end-to-end distance distributions obtained from pulsed electron paramagnetic resonance measurements on spin-labeled oligoPEs with comparable label distances

A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier

Single-molecule magnets display magnetic bistability of molecular origin, which may one day be exploited in magnetic data storage devices. Recently it was realised that increasing the magnetic moment of polynuclear molecules does not automatically lead to a substantial increase in magnetic bistability. Attention has thus increasingly focussed on ions with large magnetic anisotropies, especially lanthanides. In spite of large effective energy barriers towards relaxation of the magnetic moment, this has so far not led to a big increase in magnetic bistability. Here we present a comprehensive study of a mononuclear, tetrahedrally coordinated cobalt(II) single-molecule magnet, which has a very high effective energy barrier and displays pronounced magnetic bistability. The combined experimental-theoretical approach enables an in-depth understanding of the origin of these favourable properties, which are shown to arise from a strong ligand field in combination with axial distortion. Our findings allow formulation of clear design principles for improved materials.ISSN:2041-172

Multiple Bistability in Quinonoid-Bridged Diiron(II) Complexes: Influence of Bridge Symmetry on Bistable Properties

Quinonoid bridges are well-suited for generating dinuclear assemblies that might display various bistable properties. In this contribution we present two diiron­(II) complexes where the iron­(II) centers are either bridged by the doubly deprotonated form of a symmetrically substituted quinonoid bridge, 2,5-bis­[4-(isopropyl)­anilino]-1,4-benzoquinone (<b>H</b>_<b>2</b><b>L2′</b>) with a [O,N,O,N] donor set, or with the doubly deprotonated form of an unsymmetrically substituted quinonoid bridge, 2-[4-(isopropyl)­anilino]-5-hydroxy-1,4-benzoquinone (<b>H</b>_<b>2</b><b>L5′</b>) with a [O,O,O,N] donor set. Both complexes display temperature-induced spin crossover (SCO). The nature of the SCO is strongly dependent on the bridging ligand, with only the complex with the [O,O,O,N] donor set displaying a prominent hysteresis loop of about 55 K. Importantly, only the latter complex also shows a pronounced light-induced spin state change. Furthermore, both complexes can be oxidized to the mixed-valent iron­(II)–iron­(III) form, and the nature of the bridge determines the Robin and Day classification of these forms. Both complexes have been probed by a battery of electrochemical, spectroscopic, and magnetic methods, and this combined approach is used to shed light on the electronic structures of the complexes and on bistability. The results presented here thus show the potential of using the relatively new class of unsymmetrically substituted bridging quinonoid ligands for generating intriguing bistable properties and for performing site-specific magnetic switching