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Single-Electron Lanthanide-Lanthanide Bonds Inside Fullerenes toward Robust Redox-Active Molecular Magnets
2019, Liu, Fupin, Spree, Lukas, Krylov, Denis S., Velkos, Georgios, Avdoshenko, Stanislav M., Popov, Alexey A.
A characteristic phenomenon of lanthanide-fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln-Ln bonding orbital in the second half of the lanthanide row limits the Ln2 → fullerene transfer to only five electrons. One electron remains in the Ln-Ln bonding orbital, whereas the fullerene cage with a formal charge of -5 is left electron-deficient. Such Ln2@C80 molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln2@C79N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp2 carbon into an sp3 carbon and yielding the monoadduct Ln2@C80(CH2Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln2@C79N and Ln2@C80(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides.In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds and highlight the consequences of the unpaired electron residing in the Ln-Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln-Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln2 dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln2@C79N and Ln2@C80(CH2Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln3+-e-Ln3+] fragment behaves as a single giant spin. Furthermore, the Ln-Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln-Ln orbital affects the magnetic structure of the molecule. © 2019 American Chemical Society.
Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals
2022, Graf, Lukas, Liu, Fupin, Naumann, Marco, Roth, Friedrich, Debnath, Bipasha, Büchner, Bernd, Krupskaya, Yulia, Popov, Alexey A., Knupfer, Martin
High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure─unknown before─was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.
Covalency versus magnetic axiality in Nd molecular magnets: Nd-photoluminescence, strong ligand-field, and unprecedented nephelauxetic effect in fullerenes NdM2N@C80 (M = Sc, Lu, Y)
2023, Yang, Wei, Rosenkranz, Marco, Velkos, Georgios, Ziegs, Frank, Dubrovin, Vasilii, Schiemenz, Sandra, Spree, Lukas, de Souza Barbosa, Matheus Felipe, Guillemard, Charles, Valvidares, Manuel, Büchner, Bernd, Liu, Fupin, Avdoshenko, Stanislav M., Popov, Alexey A.
Nd-based nitride clusterfullerenes NdM2N@C80 with rare-earth metals of different sizes (M = Sc, Y, Lu) were synthesized to elucidate the influence of the cluster composition, shape and internal strain on the structural and magnetic properties. Single crystal X-ray diffraction revealed a very short Nd-N bond length in NdSc2N@C80. For Lu and Y analogs, the further shortening of the Nd-N bond and pyramidalization of the NdM2N cluster are predicted by DFT calculations as a result of the increased cluster size and a strain caused by the limited size of the fullerene cage. The short distance between Nd and nitride ions leads to a very large ligand-field splitting of Nd3+ of 1100-1200 cm−1, while the variation of the NdM2N cluster composition and concomitant internal strain results in the noticeable modulation of the splitting, which could be directly assessed from the well-resolved fine structure in the Nd-based photoluminescence spectra of NdM2N@C80 clusterfullerenes. Photoluminescence measurements also revealed an unprecedentedly strong nephelauxetic effect, pointing to a high degree of covalency. The latter appears detrimental to the magnetic axiality despite the strong ligand field. As a result, the ground magnetic state has considerable transversal components of the pseudospin g-tensor, and the slow magnetic relaxation of NdSc2N@C80 could be observed by AC magnetometry only in the presence of a magnetic field. A combination of the well-resolved magneto-optical states and slow relaxation of magnetization suggests that Nd clusterfullerenes can be useful building blocks for magneto-photonic quantum technologies.
On the Preparation and Spectroelectrochemical Characterization of Certain 2,5-Bis(het)aryl Substituted Thiophenes
2024, Dmitrieva, Evgenia, Barche, Jens, Popov, Alexey A., Hartmann, Horst
In this work, a series of novel 2,5-bis(het)aryl and 2,5-bis-thienyl substituted thiophenes have been synthesized and characterized by ultraviolet-visible-near infrared (UV-Vis-NIR) absorption and fluorescence spectroscopy as well as cyclic voltammetry. From the electron paramagnetic resonance (EPR)/UV-Vis-NIR spectroelectrochemical data, information about the optical and magnetic properties of the charged species of these compounds have been provided. The spin distributions in the electrochemically generated radical ions were estimated experimentally and compared with theoretical data.
High Blocking Temperature of Magnetization and Giant Coercivity in the Azafullerene Tb 2 @C 79 N with a Single-Electron Terbium–Terbium Bond
2019, Velkos, Georgios, Krylov, Denis S., Kirkpatrick, Kyle, Spree, Lukas, Dubrovin, Vasilii, Büchner, Bernd, Avdoshenko, Stanislav M., Bezmelnitsyn, Valeriy, Davis, Sean, Faust, Paul, Duchamp, James, Dorn, Harry C., Popov, Alexey A.
The azafullerene Tb 2 @C 79 N is found to be a single-molecule magnet with a high 100-s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy-axis single-ion magnetic anisotropy are strongly coupled by the unpaired spin of the single-electron Tb−Tb bond. Relaxation of magnetization in Tb 2 @C 79 N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time τ QTM =16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
Substrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2ScN@C80 on Metals and Insulators
2020, Krylov, Denis S., Schimmel, Sebastian, Dubrovin, Vasilii, Liu, Fupin, Nguyen, T.T. Nhung, Spree, Lukas, Chen, Chia-Hsiang, Velkos, Georgios, Bulbucan, Claudiu, Westerström, Rasmus, Studniarek, Michał, Dreiser, Jan, Hess, Christian, Büchner, Bernd, Avdoshenko, Stanislav M., Popov, Alexey A.
Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy2ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2ScN@C80 adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy2ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
Wave-shaped polycyclic hydrocarbons with controlled aromaticity
2019, Ma, Ji, Zhang, Ke, Schellhammer, Karl Sebastian, Fu, Yubin, Komber, Hartmut, Xu, Chi, Popov, Alexey A., Hennersdorf, Felix, Weigand, Jan J., Zhou, Shengqiang, Pisula, Wojciech, Ortmann, Frank, Berger, Reinhard, Liu, Junzhi, Feng, Xinliang
Controlling the aromaticity and electronic properties of curved π-conjugated systems has been increasingly attractive for the development of novel functional materials for organic electronics. Herein, we demonstrate an efficient synthesis of two novel wave-shaped polycyclic hydrocarbons (PHs) 1 and 2 with 64 π-electrons. Among them, the wave-shaped π-conjugated carbon skeleton of 2 is unambiguously revealed by single-crystal X-ray crystallography analysis. The wave-shaped geometry is induced by steric congestion in the cove and fjord regions. Remarkably, the aromaticity of these two structural isomers can be tailored by the annulated direction of cyclopenta[b]fluorene units. Isomer 1 (Eoptg = 1.13 eV) behaves as a closed-shell compound with weakly antiaromatic feature, whereas its structural isomer 2 displays a highly stable tetraradical character (y0 = 0.23; y1 = 0.22; t1/2 = 91 days) with a narrow optical energy gap of 0.96 eV. Moreover, the curved PH 2 exhibits remarkable ambipolar charge transport in solution-processed organic thin-film transistors. Our research provides a new insight into the design and synthesis of stable functional curved aromatics with multiradical characters. © The Royal Society of Chemistry.
Robust Single Molecule Magnet Monolayers on Graphene and Graphite with Magnetic Hysteresis up to 28 K
2021, Spree, Lukas, Liu, Fupin, Neu, Volker, Rosenkranz, Marco, Velkos, Georgios, Wang, Yaofeng, Schiemenz, Sandra, Dreiser, Jan, Gargiani, Pierluigi, Valvidares, Manuel, Chen, Chia-Hsiang, Büchner, Bernd, Avdoshenko, Stanislav M., Popov, Alexey A.
The chemical functionalization of fullerene single molecule magnet Tb2@C80(CH2Ph) enables the facile preparation of robust monolayers on graphene and highly oriented pyrolytic graphite from solution without impairing their magnetic properties. Monolayers of endohedral fullerene functionalized with pyrene exhibit magnetic bistability up to a temperature of 28 K. The use of pyrene terminated linker molecules opens the way to devise integration of spin carrying units encapsulated by fullerene cages on graphitic substrates, be it single-molecule magnets or qubit candidates. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
On the Electrochemical Reduction of 4-(Thiazol-2-ylazo)-Substituted 1-Chloronaphthalenes: Formation and Characterization of Stable Radical Anions
2020, Dmitrieva, Evgenia, Popov, Alexey A., Yu, Xiuling, Hartmann, Horst
The electrochemical reduction of two chloro-substituted 4-(thiazol-2-ylazo)has been studied by means of spectroelectrochemistry and simulated with the DFT method. Whereas the 1-chloro-4-(4-chlorothiazol-2-ylazo)forms both a stable radical anion and a dianion, the dianion of 1-chloro-4-(thiazol-2-ylazo)is instable. In the radical anion of both compounds, the spin densities are high not only at the azo moiety but also at C3 in the naphthalene and at C5 in the thiazole moiety. This is in agreement with former experimental results demonstrating the remarkable reactivity of these positions towards thiols which can act as nucleophiles as well as electron donors. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Perfluoroalkylfullerenes
2015, Boltalina, Olga V., Popov, Alexey A., Kuvychko, Igor V., Shustova, Natalia B., Strauss, Steven H.
New chemical derivatives that possess the greatest variety of addition patterns than any other class of fullerene derivatives represent an important addition to the existing classes of perfluorocarbons, that is, compounds that are composed only of the two types of atoms, carbon and fluorine. These include aromatic and aliphatic perfluorocarbons such as perfluorodecalin, perfluorononane, hexafluorobenzene, etc., which are important as fluorous solvents used in medicine. The propensity of perfluoroalkylfullerenes (PFAFs) to readily crystallize from organic solutions upon slow evaporation in open air provided a straightforward access to their molecular structures via X-ray crystallography. Another crucial aspect that ensures future success in the characterization of numerous PFAFs of higher fullerenes and endohedral metallofullerenes is the possibility to apply HPLC methodologies to the separation of product mixtures. PFAFs, especially those of C60 and C70, are unique fullerene derivatives in terms of the number of structurally characterized derivatives with different number of RF groups and different addition patterns.
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