The metastable triplet state of zinc porphin and magnesium porphin: A study by E.S.R. in an n-octane crystal at 1·4 K

E.S.R. experiments at 1·3 K are reported for the photo-excited triplet state of zinc porphin (ZnP) and magnesium porphin (MgP) in single crystals of n-octane. The effect of partial and complete deuteriation, and the influence on the E.S.R. spectra of the addition of ethanol or benzene as a second solvent are given. It is found that most of the ZnP molecules occupy one orientation, whereas MgP + ethanol occurs in two orientations for which the molecules are coplanar but differ by a rotation of 90° about the out-of-plane axis. For ZnP, MgP, and free base porphin (H2P) the zero-field principal axes are parallel to within 5° and run through opposite nitrogen atoms. By E.S.R. combined with X-ray diffraction an insight into the orientation of the porphin molecules in the n-octane host is obtained. The fine structure results show a small (ZnP) to negligible (MgP) influence of the spin-orbit coupling of the metal atom. The observed hyperfine structure in ZnP closely resembles that in MgP and originates from four equivalent methine C-H fragments and two inequivalent pairs of nitrogen atoms. The spin densities determined from the hyperfine data are in good agreement with those resulting from a semi-empirical π-electron calculation; they indicate that the main triplet configuration is given by a 3(eg → a 2u ) orbital excitation in terms of Gouterman's four orbital model. The experimental results on ZnP and MgP suggest that a b 1g type of Jahn-Teller distortion determines the direction of the spin axes in these molecules.