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Relevant academic research and scientific papers

The synthesis and spectroscopic properties of macrocyclic polyethers containing two different aromatic moieties and their [2] catenanes incorporating cyclobis(paraquat-p-phenylene)

The synthesis of six derivatives of bis-p-phenylene-34-crown-10 (BPP34C10) in which one or both of the p-phenylene rings are replaced by other η-electron-rich aromatic ring systems, and their subsequent use as templates for the self-assembly of the tetrac

Liquid crystalline paracyclophanes and ansa compounds - Series of polyether macrocycles incorporating diacetylene, phenyl, biphenyl, p-terphenyl and 2,5-diphenyl-1,3,4-thiadiazole rigid cores

Series of different macrocyclic liquid crystals consisting of two rod-like aromatic cores connected at both terminal ends by polyether chains have been synthesised by various macrocyclisation reactions (template assisted etherification, Yamaguchi macrolactonisation, Glaser-coupling reaction). The nature and length of the rigid core have a dominant impact on the liquid crystalline phases formed. A stepwise increase of the length of one of the rigid cores leads to significant mesophase stabilisation, to the induction of lamellar phases and, in general, to wider mesophase ranges. The same types of mesophases but at much reduced transition temperatures are observed when flexible methylene linkages are incorporated into the macrocycles. Parallel pre-organisation of the calamitic cores and micro-segregation of chemically distinct molecular parts are discussed as the main driving forces for the mesophase behaviour of these macrocyclic compounds. Additionally, chiral macrocyclic liquid crystals exhibiting SmC* phases, cholesteric phases and blue phases have been synthesised. In contrast to the cyclophanes, ansa-compounds incorporating only one aromatic core within the macrocycle show either a monotropic nematic phase only or no liquid crystalline behaviour at all. An architecture combining the cyclophane structure with an ansa-unit and a molecule with a non-parallel pre-organisation of rod-like mesogenic cores as well does not show LC phases. The lack of LC behaviour for these compounds might be due to the molecular architecture resulting in a steric disturbance of packing that, in this way, does not allow for segregation of rigid and flexible units into distinct sub-domains. The Royal Society of Chemistry 2005.

Barium cation complexation by flexible mono- and ditopic receptors, studied by UV absorption and fluorescence spectroscopy

The metal cation (Ba++) binding ability of a family of designed nonconjugated bichromophoric [bis(para-phenylene)-or bis(para-diphenylene)]polyoxamacrocydic coronands has been studied in acetonitrile in the ground state and in the singlet excited state. The association constants (K) at ambient temperature have been determined from UV absorption and fluorescence data, by use of the LETAGROP-SPEFO programme. The ground state complexes exhibit 1:1 or 1:1 and 2:1 (cation/ligand) stoichiometries and a large variety of association constants, as well as diverse cooperative effects. Remarkably, TTO5O5 {1,4,7,10,13,21,24,27,30,33-decaoxa[13.15]-(1,4)benzenophane} shows a strong binding ability (log β = 10.3) with a positive cooperativity. The apparent association constants measured from stationary fluorescence data were found to be lower in some cases than those observed in the ground state; these results strongly suggest that a transitory photodecomplexation between the metal cation and the phenolic oxygen atoms occurs in the S1 state. An X-ray structure analysis was performed on the barium complex of a related podand and the Ba++ coordination number was found to be 10.

Catenated Cyclodextrins

A novel synthetic approach is described for the construction of catenanes in aqueous solution from a partially methylated cyclodextrin (CD)-namely, heptakis(2,6-di-O-methyl-β-cyclodextrin) (DM-β-CD)- and a range of substrate molecules that contain a hydrophobic central core in the form of a 4,4'-disubstituted biphenyl unit (usually bitolyl) carrying two hydrophilic polyether side chains terminated by primary amine functions.In water, the amphiphilic catenane precursors form 1:1 complexes with β-CD and DM-β-CD and 2:1 (guest:host) complexes with the larger γ-CD.Macrocyclizations of the biphenyl-containing substrates with aromatic diacid chlorides in aqueous solution and in the presence of DM-β-CD under Schotten-Baumann conditions afforded - in low yields - a range of - and catenanes.When a consitutionally asymmetrical diamine was employed as the substrate, orientational isomers of a catenane were obtained.A catenane incorporating a macrocyclic tetralactam was found to exist as a mixture of head-to-head and head-to-tail isomers, which could be separated by high pressure liquid chromatography and identified unambiguously by nuclear magnetic resonance spectroscopy.One of the catenanes afforded good single crystals from which the solid state structure was determined by X-ray crystallography.Other techniques which aided the characterization of these novel compounds included ultraviolet/visible and luminescence spectroscopy, dynamic nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry.Generally speaking, the catenated cyclodextrins are soluble in halogenated and aromatic hydrocarbons as well as in hydroxylic solvents.The existence of these new compounds gives us a unique insight into the nature of the noncovalent bonding interactions that cyclodextrins employ in binding substrate molecules. - Keywords: catenanes * cyclodextrins * macrocycles * orientational isomerism