Actual Symmetry of Symmetric Molecular Adducts in the Gas Phase, Solution and in the Solid State

This review discusses molecular adducts, whose composition allows a symmetric structure. Such adducts are popular model systems, as they are useful for analyzing the effect of structure on the property selected for study since they allow one to reduce the number of parameters. The main objectives of this discussion are to evaluate the influence of the surroundings on the symmetry of these adducts, steric hindrances within the adducts, competition between different noncovalent interactions responsible for stabilizing the adducts, and experimental methods that can be used to study the symmetry at different time scales. This review considers the following central binding units: hydrogen (proton), halogen (anion), metal (cation), water (hydrogen peroxide).

Supramolecular architectures of 4-hydroxytetraphenylporphyrin with aza-donors

Molecular adducts of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (1) with aza-donors like 4,4'-bipyridine (a), 1,2-bis(4-pyridyl)ethane (b), trans-1,2-bis(4-pyridyl)ethylene (c), 4,4'-trimethylene-dipyridine (d), phenazine (e), 1,10-phenanthroline (f), 1,7-phenanthroline (g) and 4,7-phenanthroline (h) have been prepared. All the molecular complexes are crystallized along with the solvent of crystallization, except in the complex with the aza-donor b. Detailed structural analysis of the obtained complexes has been carried out by single crystal X-ray diffraction. The three dimensional structures of the molecular adducts are facilitated by directional hydrogen bonding features of hydroxyl groups with aza donors as well as solvent molecules, leading to the formation of different types of supramolecular architectures like sheets, tapes, host-guest assembly etc. For example, in the complex of 1 and aza donor a, which crystallizes as a hydrate, the porphyrin molecules interact with water and 4,4'-bipyridine through O-H...O and O-H...N hydrogen bonds, which leads to the formation of molecular sheets in two dimensional arrangement. An important noteworthy observation is that the molecular complexes are crystalline even after removal of the solvents by heating, as characterized by thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD). Further, all the complexes are found to be fluorescence sensitive, perhaps due to the porphyrin molecules.