Spectral and luminescent study of reduced forms of Pd-octaethylporphin

Abstract

It is shown by electron absorption spectroscopy methods that the reduction of Pd-octaethylporphin does not touch the unfilled dx2-y2 orbitals of metal. In the reduction products (mono-and dianions), excess electrons are distributed on the lowest vacant molecular eg(π*) orbital of the ligand. This does not contradict the results of the quantum-chemical calculation of electronic states of neutral Pd-porphyrins by the extended Huckel method that are presented in the literature. Differences in electronic absorption spectra of π-anions of Pd and Zn complexes are attributed to the distortion of the plane geometry of the tetrapyrrole skeleton due to the displacement of the Pd(II) ion from the macrocycle plane. Mono-and dianions of their Pd complex lose the ability to be luminescent. The emissive capacity is reconstructed in the product of π-dianion protonation, which is called phlorin-anion (π-monoanion of Pd-octaethylporphin phlorin). As the temperature is decreased from room temperature to 77 K, the fluorescence spectrum of this product becomes narrower, its peak shifts to the blue region, and the quantum yield of luminescence increases. Specific features of the spectral properties of phlorin-anion are described by conformational changes in the excited state. Phlorin-anion was found to quench luminescence of neutral Pd-octaethylporphin molecules at room temperature.