Specific features of the formation of color centers in nanoporous glasses doped with copper β-diketonate through different solvents

The EPR and optical spectra of Cu2+ ions impregnated in the nanoporous Vycor glass in the form of copper β-diketonate (Cu(hfac)2) dissolved in supercritical CO2, ethanol, and benzene are investigated. It is revealed that the spectra recorded immediately after the impregnation of the samples represent a superposition of two EPR spectra with hyperfine structures. This indicates that there are two structural forms of Cu(hfac)2 molecules located in fixed positions in pores. After long-term storage of the samples in air, the EPR spectra irrespective of the solvent type represent a superposition of three spectra. Each spectrum contains four hyperfine-structure components with a large splitting. The form of the third spectrum depends on the duration of storage of the sample and can be associated with water absorbed from the atmosphere and adsorbed on the pore walls. The bonds between the impregnated Cu(hfac)2 molecules and the pore walls are weak, which is confirmed by the experiments with immersion of the impregnated samples in a pure solvent. The EPR spectra of Cu2+ ions introduced into oxide glasses through a batch with subsequent melting differ substantially from the spectra of the silica glasses impregnated with Cu(hfac)2 solutions, even though the former spectra also correspond to Cu2+ complexes with symmetry D4h.