Photoacoustic Spectral Study of Lanthanide Complexes Doped in Silica Matrix

Lanthanide phenanthroline (phen) complexes \(\mathrm{Eu(phen)}_{2}\mathrm{Cl}_{3}{\cdot } \mathrm{2H}_{2}\mathrm{O}\) and \(\mathrm{Nd(phen)}_{2}\mathrm{Cl}_{3}{\cdot } \mathrm{2H}_{2}\mathrm{O}\) were incorporated into a silica matrix by an ultrasonic assisted sol–gel method. In the region of ligand absorption, the photoacoustic (PA) intensity for a lanthanide complex is the same as in wet gels. Upon heat treatment at 120\(\,^{\circ }\)C, however, the PA intensity of a \(\mathrm{Nd(phen)}_{2}\mathrm{Cl}_{3}{\cdot }\mathrm{2H}_{2}\)O-doped sample is much larger than that of a \(\mathrm{Eu(phen)}_{2}\mathrm{Cl}_{3} {\cdot } \mathrm{2H}_{2}\)O-doped sample. The characteristic emissions of \(\mathrm{Eu}^{3+}\) complex-doped samples were used to interpret the stability of the complex in silica matrices. The luminescence spectra are consistent with the PA results. The study indicates that phen can only coordinate with lanthanide ions in a silica matrix after a suitable heat treatment. Moreover, the covalency parameters and PA bands of f–f transionts of \(\mathrm{Nd}^{3+}\) have been used to study the formation of the complex in a silica matrix.