Electron spin-spin exchange coupling mediated by the porphyrin π system

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 P2(1)/n, with a = 12.239(2) angstrom, b = 17.819(3) angstrom, c = 34.445(7) angstrom, beta = 97.466(3)degrees, gamma = 90, and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent with vertical bar J vertical bar = vertical bar a vertical bar. 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 Delta m(s) = 2 transition characteristic of a triplet state. The intensity of the Delta m(s) = 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) -2J (S) over cap (1), (S) over cap (2)). 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= -2J (S) over cap (1).(S) over cap (2)). The antiferromagnetic J values are consistent with the pi topology of the porphyrin ring.