Fullerene Complexation in a Hydrogen-Bonded Porphyrin Receptor via Induced-Fit: Cooperative Action of Tautomerization and C-H···π Interactions.

A supramolecular chiral hydrogen-bonded tetrameric aggregate possessing a large cavity and tetraarylporphyrin substituents was assembled using alternating 4H- and 2H-bonds between ureidopyrimidinone and isocytosine units, respectively. The aggregation mode was rationally shifted from social to narcissistic self-sorting by changing urea substituent size only. The H-bonded tetramer forms a strong complex with C60 guest, at the same time undergoing remarkable structural changes. Namely, the cavity adjusts to the guest via keto-to-enol tautomerization of the ureidopyrimidinone unit and as a result, porphyrin substituents move apart from each other in a scissor blade-like opening fashion. The rearrangement is accompanied by C-H···π interaction between the alkyl solubilizing groups and the nearby placed porphyrin π-systems. The latter interaction was found to be crucial for the guest complexation event, providing energetic compensation for otherwise costly tautomerization. We showed that only the systems possessing sufficiently long alkyl chains capable of interacting with a porphyrin ring are able to form a complex with C60. The structural rearrangement of the tetramer was quantitatively characterized by electron paramagnetic resonance pulsed dipolar spectroscopy measurements using photogenerated triplets of porphyrin and C60 as spin probes. Further exploring the C-H···π interaction as a decisive element for the C60 recognition, we investigated the guest-induced self-sorting phenomenon using scrambled tetramer assemblies composed of two types of monomers possessing alkyl chains of different lengths. The presence of the fullerene guest has enabled the selective scavenging of monomers capable of C-H···π interaction to form homo-tetrameric aggregates.

Selective Formation of S4- and T-Symmetric Supramolecular Tetrahedral Cages and Helicates in Polar Media Assembled via Cooperative Action of Coordination and Hydrogen Bonds.

We report on the synthesis and self-assembly study of novel supramolecular monomers encompassing quadruple hydrogen-bonding motifs and metal-coordinating 2,2'-bipyridine units. When mixed with metal ions such as Fe2+ or Zn2+, the tetrahedron cage complexes are formed in quantitative yields and full diastereoselectivity, even in highly polar acetonitrile or methanol solvents. The symmetry of the complexes obtained has been shown to depend critically on the flexibility of the ligand. Restriction of the rotation of the hydrogen-bonding unit with respect to the metal-coordinating site results in a T-symmetric cage, whereas introducing flexibility either through a methylene linker or rotating benzene ring allows the formation of S4-symmetric cages with self-filled interior. In addition, the possibility to select between tetrahedral cages or helicates and to control the dimensions of the aggregate has been demonstrated with a three-component assembly using external hydrogen-bonding molecular inserts or by varying the radius of the metal ion (Hg2+ vs Fe2+). Self-sorting studies of individual Fe2+ complexes with ligands of different sizes revealed their inertness toward ligand scrambling.

A Tautoleptic Approach to Chiral Hydrogen-Bonded Supramolecular Tubular Polymers with Large Cavity.

A new strategy towards tubular hydrogen-bonded polymers based on the self-assembly of isocytosine tautomers in orthogonal directions is proposed and experimentally verified, including by 1 H fast magic-angle spinning (MAS) solid-state NMR. The molecular tubes obtained possess large internal diameter and tailor-made outer functionalities rendering them potential candidates for a number of applications.

Synthesis, molecular structure, and chiroptical properties of dibenzylidene derivatives of bicyclo[3.3.1]nonane and brexane.

Synthesis of several enantiomerically pure unsaturated bicyclo[3.3.1]nonane and related brexane (tricyclo[4.3.0.0(3,7) ]nonane) derivatives bearing exocyclic benzylidene substituents from readily available (+)-(1S,5S)-bicyclo[3.3.1]nonane-2,6-dione was accomplished. Molecular geometry and chiroptical properties of compounds with enone and styrene chromophores were studied by X-ray diffraction analysis, molecular modeling, and circular dichroism (CD) spectroscopy. Difunctional 3,7-dibenzylidenebicyclo[3.3.1]nonanes, such as and , exhibited intense CD couplets, arising from the exciton coupling between the two unsaturated chromophores. The observed negative sign of the exciton couplets is congruent with the negative twist (negative chirality) defined by the two interacting transition dipoles. The sign of the Cotton effect corresponding to the π→π* transitions in the CD spectra of monoenone and tricyclic brexane acetate was correlated with the intrinsic dissymmetry (helicity) of the styrene chromophore.