Luminescent Eu3+ and Tb3+ complexes of 4-aminophenyl terpyridine (ptpy): Photophysical aspects, DNA and serum protein binding properties

Abstract

Two lanthanide complexes namely [Eu(ptpy)2Cl2]Cl (1) and [Tb(ptpy)Cl2]Cl (2) where ptpy is 4′-(4-aminophenyl)-2,2′:6′,2′′-terpyridine were synthesized, and photophysical aspects, speciation in solution, excited state properties, and binding interactions with DNA and protein were studied. Here ptpy acts as light-harvesting photosensitizing antenna moiety for Ln3+ luminescence. The bright luminescence in 1 and 2 suggests favorable photo-induced energy transfer happens from the ptpy antenna to the Ln3+ to fill long-lived emissive excited states of Eu3+ and Tb3+ and respective f→f transitions. The time-gated luminescence spectral profile of complexes 1 and 2 show characteristic sharp multiple emission bands assigned to the 5D0→7FJ and 5D4→7FJf→f transitions originating from Eu3+ and Tb3+ ions respectively. Inner-sphere hydration number (q) in 1 and 2 was determined from the excited states lifetime measurements in H2O and D2O confirming possible ligand-displacement reactions with H2O in solution (q=1.35 (1), 1.86 (2)). The studies on binding of the compounds with DNA suggesting partial intercalation with planar ptpy ligands present in 1 and 2 with the planar DNA base pairs (Kb~105M−1). Complexes also showed strong interaction to the serum albumin proteins (BSA and HSA) (K~105M−1) through tryptophan fluorescence quenching studies due to perturbation in secondary structure of protein.