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.
We report a molecular design that provides an intravenously injectable organic radical contrast agent (ORCA) that has molecular r1 ≈ 5 mM−1s−1. The ORCA is based on spirocyclohexyl nitroxide radicals and polyethylene glycol chains conjugated to a generation 4 polypropylenimine dendrimers scaffold. The metal-free ORCA has a long shelf-life and provides selectively enhanced MRI in mice for over 1 h.
High spin (S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, ΔE ST ≥ 1.7 kcal mol −1 , leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ~160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J′/k = −14 K, which is associated with the N•••N and N•••O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains *
A series of sterically shielded pyrrolidine nitroxides were synthesized and their reduction by ascorbate (vitamin C) indicate that nitroxide 3 – a tetraethyl derivative of 3-carboxy-PROXYL – is reduced at the slowest rate among known nitroxides, i.e., at a 60-fold slower rate than that for 3-carboxy-PROXYL.
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