Electron spin-spin exchange coupling mediated by the porphyrin pi 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) A, b = 17.819(3) A, c = 34.445(7) A, alpha = 90 degrees , beta = 97.466(3) degrees , gamma = 90 degrees , 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 Deltam(s) = 2 transition characteristic of a triplet state. The intensity of the Deltam(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 = - 2Jŝ1.ŝ2). 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 = - 2Jŝ(1).ŝ(2)). The antiferromagnetic J values are consistent with the pi topology of the porphyrin ring.
Shultz
David A
DA
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA. david_shultz@ncsu.edu
Mussari
Christopher P
CP
Ramanathan
Krishna Kumar
KK
Kampf
Jeff W
JW
D002621:Chemistry / Q000379:methods*; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D008280:Magnetics; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009569:Nitric Oxide / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*; D013053:Spectrophotometry; D013696:Temperature; D015032:Zinc / Q000737:chemistry
D011166:Porphyrins; D009569:Nitric Oxide; D015032:Zinc