Organic spin clusters. A dendritic-macrocyclic poly(arylmethyl) polyradical with very high spin of S=10 and its derivatives: Synthesis, magnetic studies, and small-angle neutron scattering

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Synthesis and characterization of organic spin clusters, high-spin poly(arylmethyl) polyradicals with 24 and 8 triarylmethyls, are described. Polyether precursors to the polyradicals are prepared via modular, multistep syntheses, culminating in Negishi cross-couplings between four monofunctional branch (dendritic) modules and the tetrafunctional calix[4]arene-based macrocyclic core. The corresponding carbopolyanions are prepared and oxidized to polyradicals in tetrahydrofuran-d(8). The measured values of S, from numerical fits of magnetization vs magnetic field data to Brillouin functions at low temperatures (T = 1.8-5 K), are S = 10 and S = 3.6-3.8 for polyradicais with 24 and 8 triarylmethyls, respectively. Magnetizations at saturation (M-sat) indicate that 60-80% of unpaired electrons are present at T = 1.8-5 K. Low-resolution shape reconstructions from the small-angle neutron scattering (SANS) data indicate that both the polyradical with 24 triarylmethyls and its derivatives have dumbbell-like shapes with overall dimensions 2 x 3 x 4 nm, in agreement with the molecular shapes of the lowest energy conformations obtained from Monte Carlo conformational searches. On the basis of these shapes, the size of the magnetic anisotropy barrier in the polyradical, originating in magnetic shape anisotropy, is estimated to be in the milliKelvin range, consistent with the observed paramagnetic behavior at T greater than or equal to 1.8 K. For macromolecular polyradicals, with the elongated shape and the spin density similar to the polyradical with 24 triarylmethyls, it is predicted that the values of S on the order of 1000 or higher may be required for "single-molecule-magnet" behavior, i.e., superparamagnetic blocking (via coherent rotation of magnetization) at the readily accessible temperatures T > 2 K.
Publisher
AMER CHEMICAL SOC
Issue Date
2004-06
Language
English
Article Type
Article
Keywords

GROUND-STATE; M-PHENYLENE; BIOLOGICAL MACROMOLECULES; CONJUGATED SYSTEMS; BIPHENYL UNIT; DIRADICALS; EXCHANGE; MODEL; CARBOPOLYANIONS; POLYCARBENES

Citation

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, v.126, no.22, pp.6972 - 6986

ISSN
0002-7863
DOI
10.1021/ja031548j
URI
http://hdl.handle.net/10203/85121
Appears in Collection
NE-Journal Papers(저널논문)
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