Structural, photophysical and magnetic properties of transition metal complexes based on the dipicolylamino-chloro-1,2,4,5-tetrazine ligand.

The ligand 3-chloro-6-dipicolylamino-1,2,4,5-tetrazine (Cl-TTZ-dipica) , prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)-amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-dipica-MCl2]2, with M = Zn(II), Cd(II), Mn(II) and Co(II), when reacted with the corresponding metal chlorides. The dinuclear structure of the isostructural complexes was disclosed by single crystal X-ray analysis, clearly indicating the formation of [M(II)-(µ-Cl)2M(II)] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as a substituent on the tetrazine ring, engage respectively in specific anion-π intramolecular and intermolecular interactions with the electron-poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm(-1)) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm(-1)), the spin Hamiltonian being defined as H = -JSA·SB.

Polynuclear Sm(III) polyamidoamine-based dendrimer: a single probe for combined visible and near-infrared live-cell imaging.

We report herein the synthesis of a luminescent polynuclear dendritic structure (Sm(III)-G3P-2,3Nap) in which eight Sm(III) ions are sensitized by thirty-two 2,3-naphthalimide chromophores. Upon a single excitation wavelength, the dendrimer complex exhibits two types of emission in the visible and in the near-infrared (NIR) ranges. Sm(III)-G3P-2,3Nap was non-cytotoxic after 24 h of incubation and up to 2.5 µM. The ability of the Sm(III)-based probe to be taken up by cells was confirmed by confocal microscopy. Epifluorescence microscopy validated Sm(III)-G3P-2,3Nap as a versatile probe, capable of performing interchangeably in the visible or NIR for live-cell imaging. As both emissions are obtained from a single complex, the cytotoxicity and biodistribution are inherently the same. The possibility for discriminating the sharp Sm(III) signals from autofluorescence in two spectral ranges increases the reliability of analysis and reduces the probability of artifacts and instrumental errors.