905731-01-3Relevant academic research and scientific papers
Electron spin-spin exchange coupling mediated by the porphyrin π system
Shultz, David A.,Mussari, Christopher P.,Ramanathan, Krishna Kumar,Kampf, Jeff W.
, p. 5752 - 5759 (2006)
The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1-3) substituted with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin 3 is monoclinic, space group P21/n, with a = 12.239(2) A, b = 17.819(3) A, c = 34.445(7) A, α = 90°, β = 97.466(3)°, γ = 90°, and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent with |J| ? |a|. No evidence was observed for conformational modulation of J by rotation about single bonds as shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3 exhibits frozen-solution EPR spectra with zero-field splitting and a Δms = 2 transition characteristic of a triplet state. The intensity of the Δms = 2 transition of 3 was measured as a function of temperature, and the data fit according to a singlet-triplet model to yield J(3,solution) = -75 cm-1 (H = - 2JS1·S2). Polycrystalline samples of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were fit using a modified Bleaney-Bowers equation to give J(3,solid) = -29 cm-1 (H = - 2JS1·S 2). The antiferromagnetic J values are consistent with the π topology of the porphyrin ring.
