DNA Intercalating Near-Infrared Luminescent Lanthanide Complexes Containing Dipyrido[3,2-a:2′,3′-c]phenazine (dppz) Ligands: Synthesis,Crystal Structures,Stability, Luminescence Properties and CT-DNA Interaction

In order to create near-infrared (NIR) luminescent lanthanide complexes suitable for DNA-interaction, novel lanthanide dppz complexes with general formula Ln(NO₃)₃(dppz)₂] (Ln = Nd³⁺, Er³⁺ and Yb³⁺; dppz = dipyrido3,2-a:2′,3′-c]phenazine) were synthesized, characterized and their luminescence properties were investigated. In addition, analogous compounds with other lanthanide ions (Ln = Ce³⁺, Pr³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Tm³⁺, Lu³⁺) were prepared. All complexes were characterized by IR spectroscopy and elemental analysis. Single-crystal X-ray diffraction analysis of the complexes (Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Eu³⁺, Er³⁺, Yb³⁺, Lu³⁺) showed that the lanthanide’s first coordination sphere can be described as a bicapped dodecahedron, made up of two bidentate dppz ligands and three bidentate-coordinating nitrate anions. Efficient energy transfer was observed from the dppz ligand to the lanthanide ion (Nd³⁺, Er³⁺ and Yb³⁺), while relatively high luminescence lifetimes were detected for these complexes. In their excitation spectra, the maximum of the strong broad band is located at around 385 nm and this wavelength was further used for excitation of the chosen complexes. In their emission spectra, the following characteristic NIR emission peaks were observed: for a) Nd³⁺: ⁴F_3/2 → ⁴I_9/2 (870.8 nm), ⁴F_3/2 → ⁴I_11/2 (1052.7 nm) and ⁴F_3/2 → ⁴I_13/2 (1334.5 nm); b) Er³⁺: ⁴I_13/2 → ⁴I_15/2 (1529.0 nm) c) Yb³⁺: ²F_5/2 → ²F_7/2 (977.6 nm). While its low triplet energy level is ideally suited for efficient sensitization of Nd³⁺ and Er³⁺, the dppz ligand is considered not favorable as a sensitizer for most of the visible emitting lanthanide ions, due to its low-lying triplet level, which is too low for the accepting levels of most visible emitting lanthanides. Furthermore, the DNA intercalation ability of the Nd(NO₃)₃(dppz)₂] complex with calf thymus DNA (CT-DNA) was confirmed using fluorescence spectroscopy.