Juin 2019. Nouvelle publication de Narcis Avarvari dans le journal ACS Omega

 

 

Large Synthetic Molecule that either Folds or Aggregates through Weak Supramolecular Interactions Determined by Solvent

Cristina Oliveras-González, Mathieu Linares, David B. Amabilino and Narcis Avarvari


ABSTRACT: Weak noncovalent interactions between large disclike molecules in poorly solvating media generally lead to the formation of fibers where the molecules stack atop one another. Here, we show that a particular chiral spacing group between large aromatic moieties, which usually lead to columnar stacks, in this case gives rise to an intramolecularly folded structure in relatively polar solvents, but in very apolar solvents forms finite aggregates. The molecule that displays this behavior has a C3 symmetric benzene-1,3,5-tris(3,3′- diamido-2,2′-bipyridine) (BTAB) core with three metalloporphyrin units appended to it through short chiral spacers.
Quite well-defined chromophore arrangements are evident by circular dichroism (CD) spectroscopy of this compound in solution, where clear exciton coupled bands of porphyrins are observed. In more polar solvents where the molecules are dispersed, a relatively weak CD signal is observed as a result of intramolecular folding, a feature confirmed by molecular modeling. The intramolecular folding was confirmed by measuring the CD of a C2 symmetric analogue. The C3 symmetric BTAB cores that would normally be expected to stack in a chiral arrangement in apolar solvents show no indication of CD, suggesting that there is no transfer of chirality through it (although the expected planar conformation of the 2,2′-bipyridine unit is confirmed by NMR spectroscopy). The incorporation of the porphyrins on the 3,3′-diamino-2,2′-bipyridine moiety spaced by a chiral unit leaves the latter incapable of assembling through supramolecular π−π stacking. Rather, modeling indicates that the three metalloporphyrin units interact, thanks to van der Waals interactions, favoring their close interactions over that of the BTAB units. Atomic force microscopy shows that, in contrast to other examples of molecules with the same core, disclike aggregates (rather than fibrillar one dimensional aggregates) are favored by the C3 symmetric molecule. The closed structures are formed through nondirectional interlocking of porphyrin rings.
The chiral spacer between the rigid core and the porphyrin moieties is undoubtedly important in determining the outcome in polar or less polar solvents, as modeling shows that this joint in the molecule has two favored conformations that render the molecule relatively flat or convex.

 

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Juin 2019. Nouvelle publication de Narcis Avarvari dans le journal Elsevier

 

  Dipicolylamino-methoxy-1,2,4,5-tetrazine ligand and its metal complexes:
  Structural and photophysical studies

  Oleh Stetsiuk, Abdelkrim El-Ghayoury, Andreas Hauser, Narcis Avarvari.

 

The ligand 6-methoxy-N,N-bis(pyridin-2-ylmethyl)-1,2,4,5-tetrazin-3-amine has been prepared by nucleophilic substitution from 3-chloro-6-dipicolylamino-1,2,4,5-tetrazine. The ligand L, together with two neutral zinc(II) and cadmium(II) metal complexes,  respectively formulated as [ZnLCl2] and [CdLCl2]2 , have been crystallized and  analyzed by single crystal X-ray diffraction analysis. The two metal complexes are isostructural and crystallize in the monoclinic system, space group P21/c.
The structure analysis indicates that the amino nitrogen atom is involved in semi-coordination with the metal centers, thus leading to distorted coordination geometries. Photophysical studies of the ligand and its corresponding metal complexes 2 and 3 show a switch-on of the luminescence upon chelation with the diamagnetic Zn(II) and Cd(II) metal ions.

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