4,7-Substituted 1,10-phenanthroline-2,9-dicarboxamides: photophysics of ligands and their complexes with the Eu–Gd–Tb triad

Abstract

The impact of substituents at the 4- and 7-positions of 1,10-phenanthroline-2,9-dicarboxamides on the photophysical properties of the ligands and their coordination compounds with the lanthanide triad—europium, gadolinium, and terbium—was analyzed. This study demonstrates how modification of the electronic nature of ligands through the incorporation of diverse functional groups affects the luminescence properties of their complexes. The introduction of various substituents leads to the appearance of intra-ligand or ligand-to-ligand charge transfer (CT) states. The highest luminescence efficiency was observed for LH·Eu(NO₃)₃ (Q_in = 54.1% and Q_L = 9.6%), suggesting strong luminescence quenching of the CT state. It was found that a relatively low ΔE (∼3000 cm⁻¹) supports direct energy transfer from S₁ to T₁ bypassing the CT state, even though it is outside Reinhoudt's optimal range. The introduction of fluorines leads to the strongest luminescence quenching among all the substituents.