$4f^n$–$4f^{n-1}d$-transitions in the optical spectra of tris (1,2,4-triphenylcyclopentadienyl) complexes of the lanthanide ions

Tris complexes of Ln$^{3+}$ ions (Ln = La, Ce, Pr), with triphenyl-substituted cyclopentadienyl ligands were studied by optical spectroscopy. Emission of both $5d$–$4f$ and $4f$–$4f$ transitions was observed for Pr$^{3+}$ complex. It was found that the strength of the crystal field formed by triphenyl-substituted cyclopentadienyl ligands in tris complexes is comparable to the case of sulfides, and as a result, red luminescence is observed for the Ce$^{3+}$ compound. The quantum yield of photoluminescence is 20% for the latter.