Pavel Lhoták

Urea derivatives of calix[4]arene 1,3-alternate: an anion receptor with profound negative allosteric effect

Abstract The complexation behaviour of tetrakis(phenylureido)calix[4]arene in the 1,3-alternate conformation has been studied using NMR titration experiments. It was found that this receptor with two possible binding sites exhibits a strong negative allosteric effect which leads to the exclusive complexation of only one anion.

(Thia)calix[4]arene–porphyrin conjugates: novel receptors for fullerene complexation with C70 over C60 selectivity

Molecular tweezers, (thia)calix[4]arene–porphyrin conjugates, are constructed from the (thia)calix[4]arene unit serving as a scaffold and from two and/or four porphyrin units. These molecules form stable complexes with fullerenes in a toluene solution and exhibit selectivity towards C70. The observed fullerene–porphyrin contacts suggest cooperative behaviour of closely separated porphyrin units attracting C60 or C70. Measurements show efficient quenching of porphyrin fluorescence emission.

Novel biscalix[4]arene-based anion receptors

Novel biscalix[4]arene derivatives where two calixarene units are connected via one or two ureido bridges on the upper rim has been prepared. These compounds represent well preorganised cavities with interesting complexation abilities towards anions. The structure of bis-ureido derivative was proved by X-ray crystallography.

Unprecedented meta-substitution of calixarenes: direct way to inherently chiral derivatives.

Electrophilic aromatic substitution in the calix[n]arene series is a well-established procedure leading exclusively to para-substituted derivatives. An unprecedented regioselectivity of the mercuration reaction leading to the meta-substituted calix[4]arenes is described. These compounds represent a new type of substitution pattern in classical calixarene chemistry and open the door for the straightforward synthesis of inherently chiral receptors based on calixarenes.

Conformational flexibility of a novel tetraethylether of thiacalix[4]arene. A comparison with the “classical” methylene-bridged compounds

Abstract Conformational analysis of a novel tetraethylether of thiacalix[4]arene by means of NMR spectroscopy is presented. Equilibrium between three dominant conformers paco, 1,3-alt and cone exists in CDCl3 solution at room temperature. Observed chemical exchange between conformers indicates higher internal flexibility of the title compound in comparison with similar methylene bridged analogues of tetraethylethers of calix[4]arene and p-tert-butylcalix[4]arene. The cone conformer experiences additional internal motions.

Photophysical Properties and Photoinduced Electron Transfer Within Host–Guest Complexes of 5,10,15,20‐Tetrakis(4‐N‐methylpyridyl)porphyrin with Water‐soluble Calixarenes and Cyclodextrins¶

Abstract We report the formation of host–guest complexes between water-soluble calix[n]arene-p-tetrasulfonates (n = 4, 6, 8) or 2-hydroxypropyl-cyclodextrins (α-, β-, γ-) and the tetratosylate salt of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (TMPyP). The binding constants ranging between 102 and 105 M−1 were calculated from the absorption and fluorescence changes. Calix[4]arene-p-tetrasulfonate has a high binding affinity and forms with TMPyP a 1:1 complex, whereas other calixarenes bind two molecules of TMPyP. Electrostatic attraction is the dominating binding mode. Binding to calixarenes leads to a considerable decrease of the quantum yields of the triplet and excited singlet states and to shortening of the singlet and triplet lifetimes of TMPyP. The quenching mechanism is attributed to electron transfer between calixarene phenolates and excited TMPyP. Photoinduced electron transfer within a novel supramolecular complex calixarene/TMPyP (electron donor)/methyl viologen (electron acceptor) has been proven by absorption and fluorescence measurements. Electrostatic attraction between the cationic donor and cationic acceptor, on the one hand, and the anionic host, on the other, overcomes the electrostatic repulsion forces. In contrast, the interaction of cyclodextrin with TMPyP is hydrophobic in nature and only slightly influences the photophysical properties of TMPyP. The different behavior of TMPyP bound to either of the hosts has been assigned to the specific effects of the dominant binding modes, viz. the electrostatic attraction for calixarenes and the hydrophobic interactions for inclusion complexes with cyclodextrins.

Uncommon Regioselectivity in the Thiacalix[4]arene Series: Gross Formylation of the Cone Conformer

Thiacalix[4]arene immobilized in the cone conformation undergoes a direct Gross formylation reaction (Cl(2)CH-O-CH(3)/SnCl(4)/CH(2)Cl(2)) to give the upper-rim formylated thiacalixarene. Albeit using excess of the formylation agent and various reaction temperatures, only one formyl group is introduced into the meta position of the thiacalixarene skeleton. The surprising regioselectivity indicates dramatically different reactivity of the thiacalix[4]arene system when compared with a classical calix[4]arene analogue, which yields exclusively para isomers. The introduction of functional groups into the meta position represents an exceptional substitution pattern in thiacalixarene chemistry, which imparts an interesting conformational behavior to these compounds. The systematic NMR study revealed that the pinched cone-pinched cone equilibrium is remarkably shifted toward one pinched cone structure depending on the substitution.

High sensitive calixarene-based sensor for detection of dopamine by electrochemical and acoustic methods.

We synthesized 25,26,27,28-tetrakis(11-sulfanylundecyloxy)calix[4]arene (CALIX) sensitive to dopamine and confirmed its structure by (1)H NMR and mass spectrometry. Chemisorption of CALIX molecules or their mixtures with 1-dodecanethiols (DDT) or hexadecanethiols (HDT) resulted in formation of compact low permeable monolayers as revealed by cyclic voltammetry at presence of redox probe [Fe(CN)(6)](3-/4-). These self-assembled monolayers (SAMs) served as sensor for dopamine. Thickness shear mode acoustic method (TSM) has been used for study the interaction of dopamine with calixarene SAM. The admittance spectra of TSM transducer have been measured and used for simultaneous determination of the changes in series resonant frequency, f(S), and motional resistance, R(m), respectively. Addition of dopamine resulted in substantial decrease of f(S) and increase of R(m), which is evidence on increased viscoelastic contribution into the acoustic properties of the sensing layer. Limit of detection (LOD) for dopamine was 50 pM, which is much better in comparison with so far reported lowest LOD for dopamine-sensitive electrochemical sensors (20 nM). The sensor allowed discrimination between dopamine and epinephrine.

Meta Nitration of Thiacalixarenes

Nitration of thiacalix[4]arene, immobilized in the 1,3-alternate conformation, leads regioselectively to meta-substituted products. Depending on the reaction conditions, mono- and dinitro-derivatives can be isolated in acceptable yields. This unique substitution pattern is inaccessible in classical calixarene chemistry, and yields inherently chiral compounds, which makes thiacalixarenes very attractive as building blocks or molecular scaffolds.

NMR and X-ray analysis of 25,27-dimethoxythiacalix[4]arene: unique infinite channels in the solid state

Abstract The conformational behaviour of 25,27-dimethoxythiacalix[4]arene was studied using NMR techniques and X-ray analysis. The title compound prefers a cone conformation in solution, while in the solid state it adopts a unique 1,2-alternate conformation thus creating a novel type of molecular channel held together by π–π interactions.