Characterization of reduced porphyrinatozinc(II) complexes by EPR/ENDOR/TRIPLE and optical absorption spectroscopy Original Research Article

The reduction stages of ZnTPP, ZnOEP and different meso-tetraaryl substituted zinc metalated porphyrins have been studied in depth by EPR/ENDOR/TRIPLE and optical absorption spectroscopy. The striking differences between the characteristic g values and peak to peak linewidth (ΔH) for monoanions of free base or Zn metalated porphyrins ranging from g = 1.980 to g = 2.003 and ΔH = 0.5–2.5 mT reported in the literature could be resolved by demonstrating that the ease of reduction leads to easily accessible trianions, sometimes even in admixtures with diamagnetic dianions and paramagnetic monoanions, also evidenced by their typical Vis-NIR absorptions. Thus the monoanions exhibit g values below 2.0000 indicating strong spin orbit coupling as in the fullerenes. Their linewidths ranges from 1.8 to 2.5 mT. The trianions show larger g values around g = 2.0023 and much smaller linewidths than the monoanions with ΔH = 0.5–0.7 mT. From the relative signs of the hyperfine coupling constants and their multiplicity derived from ENDOR/TRIPLE spectroscopy of the trianions an assignment to the molecular positions of the protons could be achieved. No considerable spin density was found on the nitrogen centres. And in accord with theoretical considerations the trifold charging leads to a lowering of symmetry from D4h to D2h (B2g) such that only two (the opposite ones) instead of four meso-positions are equivalent and Goutermanʹs four orbital model no longer holds for the description of the spin density distribution.