Abstract
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 = dipyrido[3,2-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+: 4F3/2 → 4I9/2 (870.8 nm), 4F3/2 → 4I11/2 (1052.7 nm) and 4F3/2 → 4I13/2 (1334.5 nm); b) Er3+: 4I13/2 → 4I15/2 (1529.0 nm) c) Yb3+: 2F5/2 → 2F7/2 (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.
Highlights
Lanthanide chemistry is a field of inorganic chemistry that is constantly evolving, in recent decades, as a growing number of synthesized compounds are widely used in different areas.One of the most fascinating aspects of trivalent lanthanides is their unique luminescence properties [1,2,3].They show characteristic narrow line-like emission peaks, ranging from the UV-visible (UV/Vis) to Molecules 2020, 25, 5309; doi:10.3390/molecules25225309 www.mdpi.com/journal/moleculesMolecules 2020, 25, 5309 near-infrared (NIR), and in suitable hosts even in the mid-infrared (MIR) region, exhibiting relatively long luminescence lifetimes
We have focused our work on the design of lanthanide complexes with dipyrido[3,2-a:20,30 -c]phenazine as its triplet level is ideally suited for efficient sensitization of Nd3+, Er3+ and Yb3+ ions [17]
The synthesis of the ligand precursor 1,10-phenanthroline-5,6-dione and of the dipyrido[3,2-a: 20,30 -c]phenazine ligand are presented in the supporting information (Schemes S1 and S2)
Summary
Lanthanide chemistry is a field of inorganic chemistry that is constantly evolving, in recent decades, as a growing number of synthesized compounds are widely used in different areas.One of the most fascinating aspects of trivalent lanthanides is their unique luminescence properties [1,2,3].They show characteristic narrow line-like emission peaks, ranging from the UV-visible (UV/Vis) to Molecules 2020, 25, 5309; doi:10.3390/molecules25225309 www.mdpi.com/journal/moleculesMolecules 2020, 25, 5309 near-infrared (NIR), and in suitable hosts even in the mid-infrared (MIR) region, exhibiting relatively long luminescence lifetimes. One of the most fascinating aspects of trivalent lanthanides is their unique luminescence properties [1,2,3]. They show characteristic narrow line-like emission peaks, ranging from the UV-visible (UV/Vis) to Molecules 2020, 25, 5309; doi:10.3390/molecules25225309 www.mdpi.com/journal/molecules. A downside of trivalent lanthanides is that their f -f transitions are parity forbidden, and their absorption coefficients are very low. This results in low emission intensities unless the compound is excited with high power sources, e.g., lasers. The design of lanthanide complexes with organic ligands, which are strongly absorbing chromophores, is an ideal way to overcome this drawback
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