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) 2 Cl 2 ]Cl ( 1 ) and [Tb(ptpy)Cl 2 ]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 Ln 3+ luminescence. The bright luminescence in 1 and 2 suggests favorable photo-induced energy transfer happens from the ptpy antenna to the Ln 3+ to fill long-lived emissive excited states of Eu 3+ and Tb 3+ 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 5 D 0 → 7 F J and 5 D 4 → 7 F J f → f transitions originating from Eu 3+ and Tb 3+ ions respectively. Inner-sphere hydration number ( q ) in 1 and 2 was determined from the excited states lifetime measurements in H 2 O and D 2 O confirming possible ligand-displacement reactions with H 2 O 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 ( K b ~10 5  M −1 ). Complexes also showed strong interaction to the serum albumin proteins (BSA and HSA) ( K ~10 5  M −1 ) through tryptophan fluorescence quenching studies due to perturbation in secondary structure of protein.