Role of LMCT states in luminescence excitation processes in europium indolecarboxylates

The luminescence excitation energy transfer in europium and terbium indole-3-carboxylates, indole-3-acetates and indole-3-propionates as well as ternary indolecarboxylates containing 1,10-phenanthroline and 2,2'-bipyridine molecules have been studied. The luminescence excitation spectra, the lifetimes of the $^5D_0$ (Eu$^{3+}$) and $^5D_4$ (Tb$^{3+}$) states, and the luminescence intensity are analyzed. The decisive role of ligand-metal charge transfer (LMCT) states in the quenching of the luminescence of europium aromatic carboxylates containing a $\pi$-excessive pyrrole or indole fragment is demonstrated. Most europium compounds are characterized by quenching due to the depopulation of the $^5D_0$ state of the Eu$^{3+}$ ion through the low-energy LMCT state. But in some ternary compounds, the LMCT state being of higher energy participates in the nonradiative depopulation of the excited electronic states of the ligand.