Investigation of the octaethylporphin cationic forms in a silicate gel matrix by methods of low-temperature site selective spectroscopy

The spectral-luminescent properties of an octaethylporphin-doped inorganic xerogel prepared from tetraethoxysilane by the sol-gel method have been investigated. With the help of selective excitation and selective monitoring of fluorescence, it has been established that the octaethylporphin molecules, on their embedment into the gel matrix, form two cationic forms, dicationic and monocationic; the longest wavelength absorption band of the latter is shifted to the red. The significant influence of the gel matrix on the energy of the excited electronic Q states (S₁ and S₂) is shown. By the fluorescence line narrowing method at 4.2 K, fine-structure fluorescence and fluorescence excitation spectra of both forms have been obtained; the frequencies of the normal modes in the S₀ and S₁ states have been determined. The data on vibrational frequencies are interpreted on the basis of their juxtaposition with those from the fluorescence line narrowing spectrum of octaethylporphin and resonance Raman spectra of its complexes with copper and nickel. Cases of the appearance of out-of-plane vibrations in the fluorescence spectra have been revealed; their activation is explained by the nonplanarity of the porphyrin macrocycle for the cationic forms.