Observation of the distorted form of Pd-porphin in single site spectra at low temperatures

Highly resolved phosphorescence and excitation of phosphorescence spectra for palladium porphin (PdP) in polycrystalline Shpol'skii matrices at liquid helium temperatures were recorded. Two non-interactive forms of PdP in the ground state have been identified. The energy splitting between the two forms amount up to 78 cm−1 for PdP in an n-octane at 4.2 K. The short-wavelength spectral form is attributed to the structure, where the central Pd(II) ion is in plane of the porphyrin macrocycle, while the long-wavelength form is associated with the nonplanar saddle-type conformation of the PdP. The frequencies of the normal vibrations in the ground electronic state have been measured separately for both forms and the differences in the normal modes of two macrocycle conformations are discussed. The set of temperature activated bands in phosphorescence spectra were found. Analysis of phosphorescence spectra at elevated temperatures and excitation of phosphorescence spectra under direct excitation in the S0→T1 channel make possible the value of zero-field splitting of quasi-degenerate T1,2 state for two forms to be determined. The splitting value in n-octane matrix amount to 40 and 57 cm−1 for planar and nonplanar conformations of PdP, respectively.