A radical spin on viologen polymers: organic spin crossover materials in water

Juetten, Mark J.; Buck, Alexander T.; Winter, Arthur H.

doi:10.1039/c4cc07119k

Cited by 31 publications

(26 citation statements)

References 46 publications

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“…1 This radical dimerization can be an annoyance when attempting to make ferromagnetic plastics, but is an attractive feature when using these radicals as building blocks for stimuli responsive materials, dynamic covalent assemblies, chemical sensors with optical or magnetic resonance contrast, or organic spin-crossover materials. [2][3][4][5][6][7][8][9][10] A wealth of literature on such meta-stable radical species shows that they form either weak sigma dimers or multi-centered pi dimers (pimers). 11,12 The radical-radical bond found in these pimers is fascinating for its unusual multicenter covalent bonding pattern that brings the atoms closer than the van der Waals radii but much longer than a conventional two-atom bond (>2.8 Å), while straddling the knife edge between van der Waals interactions and conventional chemical bonds in strength and properties.…”

Section: Introductionmentioning

confidence: 99%

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

2020

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Some free radicals are stable enough to be isolated, but most are either unstable transient species or exist as metastable species in equilibrium with a dimeric form, usually a spin-paired sigma dimer or a pi dimer (pimer). To gain insight into the different modes of dimerization, we synthesized and evaluated a library of 15 aryl dicyanomethyl radicals in order to probe what structural and molecular parameters lead to σversus π-dimerization. We evaluated the divergent dimerization behavior by measuring the strength of each radical association by variable-temperature electron paramagnetic resonance spectroscopy, determining the mode of dimerization (σ-or π-dimer) by UV-vis spectroscopy and X-ray crystallography, and performing computational analysis. We evaluated three different hypotheses to explain the difference in the dimerization behavior: (1) that the dimerization behavior is dictated by radical spin densities; (2) that it is dictated by radical polarizability; (3) that it is dictated by London dispersion stabilization of the pimer. However, no single parameter model in itself was predictive. Two-parameter models incorporating either the computed degree of spin delocalization or the radical polarizability as well as computed estimates for the attractive London dispersion forces in the π-dimers lead to improved forecasts of σvs π-dimerization mode, and suggest that a balance of spin delocalization of the isolated radical as well as attractive forces between the stacked radicals, govern the formation of diradical pimers. Disciplines Disciplines Chemistry Comments Comments

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Section: Introductionmentioning

confidence: 99%

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

2020

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“…The equilibrium between monomeric and pimeric species of viologens have been investigated not only to construct supramolecular structures, but also in view of applications in electrochromic devices, as well as spintronics and data‐storage devices: the monoreduced monomer is paramagnetic and the pimer is diamagnetic…”

Section: Introductionmentioning

confidence: 99%

Hierarchical Growth of Supramolecular Structures Driven by Pimerization of Tetrahedrally Arranged Bipyridinium Units

Marchini

Baroncini

Bergamini

et al. 2017

Chemistry A European J

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A shape-persistent molecule, featuring four bipyridinium units, has been synthesized that upon reduction undergoes intermolecular pimerization because of the rigid architecture of the molecule. The pimerization process has been investigated by a variety of techniques, such as absorption measurements, EPR spectroscopy, as well as gamma and pulse radiolysis, and compared with the behavior of a model compound. Computational studies have also been performed to support the experimental data. The most interesting feature of the tetramer is that pimerization occurs only above a threshold concentration of monoreduced species, on the contrary to the model compound. Furthermore, there is an increase of the apparent pimerization constant by increasing the concentration of reduced bipyridinium units. These results have been interpreted by the fact that pimerization is favored in the tetrahedrally shaped molecule because of a cooperative mechanism. Each multiply reduced molecule can indeed undergo multiple intermolecular interactions that enhance the stabilization of the system, also leading to hierarchical supramolecular growth. The resulting supramolecular system formed by such intermolecular pimerization should exhibit a diamond-like structure, as suggested by a simplified modeling approach. The intermolecular nature of the pimerization process occurring in the tetramer has been demonstrated by measuring the corresponding bimolecular rate constant by pulsed radiolysis experiments.

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“…Other examples of reversible solution dimerization in stable neutral free radicals have been reported in the phenalenyl family and in dithiadiazolyl radicals, including the dimerization of metal coordinated 4‐(2′‐pyridyl)‐1,2,3,5‐dithiadiazolyl radical (Scheme ) . Winter and co‐workers have described π‐dimer formation through host–guest interactions in solutions containing a viologen diradical cation . The well‐known galvinoxyl radical (Scheme ) and aza‐analogue are also known to dimerize in the solid‐state and have interesting temperature dependent magnetic profiles .…”

Section: Methodsmentioning

confidence: 99%

“…[14][15][16][17][18][19][20] Winter and co-workersh ave described pdimerf ormation through host-guesti nteractions in solutions containing av iologen diradical cation. [21][22][23] The well-known galvinoxyl radical( Scheme 1) anda za-analogue are also known to dimerize in the solid-state and have interesting temperature dependentm agnetic profiles. [24] Solution EPR studies of galvinoxyl radicala tdifferent temperatures have revealed behavior consistentw ith dimerization; however,t he nature of the diamagnetic species is unclear.…”

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confidence: 99%

Reversible Solution π‐Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical

Bonanno

Poddutoori

Sato

et al. 2018

Chemistry A European J

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Reversible solution π-dimerization is observed in the stable neutral phenoxyl radical 2,6-bis-(8-quinolylamino)-4-(tert-butyl)phenoxyl baqp and is spectroscopically characterized. This behavior, not previously observed for π-extended phenoxyl radicals, is relevant to the formation of long multicenter bonding in the π-dimer at low temperature akin to previously reported phenalenyl radicals. Our experimental data are supported in a quantitative manner by results from density functional theory (DFT) and ab initio molecular orbital theory calculations. Our theoretical results indicate that the solution dimer features strong bonding interactions between the two phenoxyl rings but that the stability of the dimer is also related to dispersion interactions between the flanking nearly parallel quinolyl rings.

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A radical spin on viologen polymers: organic spin crossover materials in water

Cited by 31 publications

References 46 publications

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

Hierarchical Growth of Supramolecular Structures Driven by Pimerization of Tetrahedrally Arranged Bipyridinium Units

Reversible Solution π‐Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical

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