We describe preparation and magnetic properties of an organic π-conjugated polymer with very large magnetic moment and magnetic order at low temperatures. The polymer is designed with a large density of cross-links and alternating connectivity of radical modules with unequal spin quantum numbers ( S ), macrocyclic S = 2 and, cross-linking S = ½ modules, which permits large net S values for either ferromagnetic or antiferromagnetic exchange couplings between the modules. In the highly cross-linked polymer, an effective magnetic moment corresponding to an average S of about 5000 and slow reorientation of the magnetization by a small magnetic field (less than or equal to 1 oersted) below a temperature of about 10 kelvin are found. Qualitatively, this magnetic behavior is comparable to that of insulating spin glasses and blocked superparamagnets.

Current interest in very-high-spin organic molecules and polymers is driven by a possibility of attaining purely organic magnetic materials based upon through-bond magnetic interactions (exchange coupling). [1][2][3] The progress in this area is measured by the synthesis of molecules and polymers with increasing values of the spin quantum number (S) in the electronic ground state. [4][5][6][7][8] From both synthesis and materials points of view, polymers are among the most desirable targets. However, all polymers reported to date have only S < 5, 6-8 significantly below S ) 10 obtained for a well-defined π-conjugated molecule (oligomer). 5 We propose a novel design for a high-spin polymer as implemented in polyradical network 1. 9,10 Polyradical 1 consists of S ) 2 macrocyclic modules, which are cross-linked with S ) 1 / 2 connecting modules. 11 In 1, high density of macrocycles should

The synthesis of a chiral, nonracemic π-conjugated system with D 2 point group of symmetry and the corresponding chiral carbodianion is described. Cu(II)-mediated oxidation of (R)-2,2′-dilithio-1,1′-binaphthyl (prepared from (R)(+)-2,2′-dibromo-1,1′-binaphthyl) gives the corresponding dimer, (R)(+)-3, which is a chiral π-conjugated derivative of o-tetraphenylene, a saddle shaped molecule. When partially resolved 2,2′dibromo-1,1′-binaphthyl is used, amplification of the enantiomeric excess (ee) is observed in the coupling product (3). The dihedral angles between the naphthalene moieties in (R)(+)-3 are about 70 °, as determined by the X-ray crystallography. However, CD and UV spectra, and cyclic voltammetric data of (R)(+)-3 are consistent with small, but far from negligible, conjugation (or homoconjugation) between the naphthalene moieties. The melting points of (R)(+)-3 and its racemate differ by more than 200 °C. Reaction of (R)(+)-3 with Li or Na in tetrahydrofuran (THF) gives the corresponding chiral carbodianions 3 2-,2M + (M ) Li and Na). NMR spectroscopy indicates two non-equivalent naphthalene moieties. UV-vis, ESR, NMR, and electrochemical studies show almost complete disproportionation of the radical anion to the carbodianion in THF with Na + as counterions. The free energy barrier for racemization in carbodianion 3 2-,2Na + is lowered by at least 25 kcal/mol compared to that in (R)(+)-3. However, carbodianions 3 2-,2M + (M ) Li and Na) remain configurationally stable at room temperature. Solid 3 2-,2Na + is a diamagnetic insulator.

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 polyradicals 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 >or= 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.