calixarenes and tetrathiafulvalene

Four upper rim bis-monopyrrolotetrathiafulvalene-calix[4]arene conjugates 2a,b and 3a,b have been efficiently synthesized using a modular construction approach. The new compounds feature a molecular tweezer architecture with a quasi-parallel arrangement of redox-active tetrathiafulvalene (TTF) arms, which serve as the guest binding centers. Complexation studies using UV/vis binding titrations revealed a high affinity of the calixarene-TTF receptors for planar electron-deficient guests, leading to formation of deeply colored charge-transfer complexes in solution. The binding efficiency of the receptors depends on the flexibility of the calixarene scaffolds and the electronic nature of the TTF arms: the highest binding efficiency is shown by receptor 2b, featuring a highly preorganized molecular structure and an electron-rich TTF moiety.

Topics: Calixarenes; Electrons; Heterocyclic Compounds; Models, Molecular; Molecular Structure; Phenols

The study of preorganization in receptors, particularly in cooperative receptors, and their reversible control by external stimuli is important for elucidating design strategies that can lead to increased sensitivity and external control of molecular recognition. In this work we present the design, synthesis, and operation of an asymmetric tetrathiafulvalene (TTF)-calix[4]pyrrole receptor appended with a pyridine moiety. (1)H NMR spectroscopy was employed to demonstrate that intramolecular complexation between the receptor and the pyridine moiety leads to a preorganized receptor. Absorption and (1)H NMR spectroscopy along with a computational investigation were used to demonstrate the ability of the receptor to complex the substrate 1,3,5-trinitrobenzene (TNB) and that the receptor can be reversibly modulated between negative and positive cooperativity by employing external stimuli in the form of Zn(II). Fitting procedures incorporating multiple datasets and fitting to multiple equilibria simultaneously have been employed to quantitatively determine the preorganization effects.

Topics: Allosteric Site; Calixarenes; Heterocyclic Compounds; Hydrogen Bonding; Magnetic Resonance Spectroscopy; Molecular Structure; Porphyrins; Pyridines; Trinitrobenzenes; Zinc

A new calix[4]arene 1 with tetrathiafulvalene (TTF), quinone, and crown ether units in the lower rim was designed and synthesized with the aim to investigate the possibility to tune the metal-ion promoted electron transfer by coordination of the crown ether unit with additional metal ions. Both absorption and electron spin resonance (ESR) spectroscopic studies clearly indicate that electron transfer occurs efficiently from TTF to the quinone units in the presence of Sc(3+)/Pb(2+)/Zn(2+). Moreover, the intramolecular electron transfer within 1 induced by Zn(2+) can be switched off by addition of Na(+) and further turned on by addition of either Sc(3+) or Pb(2+).

Topics: Calixarenes; Cations; Crown Ethers; Electron Transport; Heterocyclic Compounds; Ions; Metals, Heavy; Molecular Structure; Phenols; Quinones; Sodium

Two new cone- and 1,3-alternate-calix[4]arenes (cone-1 and 1,3-alt-1), bearing four modified TTF (tetrathiafulvalene) substituents on the upper rim, have been synthesized. The binding ability of these two sets of conformers for various anions, including F(-), Cl(-), Br(-), I(-), PF6(-), ClO4(-), HSO4(-), CH3COO(-), H2PO4(-), and HP2O7(3-), was tested in organic media by monitoring the changes in their UV/vis and (1)H NMR spectra as a function of added anion, as well as via cyclovoltammetry (CV) (all anions studied as their respective TBA salts). On the basis of the present findings, we propose that incorporation of four TTF units within an overall calix[4]arene-based recognition framework produces a preorganized receptor system that displays a modest preference for the pyrophosphate (HP2O7(3-)) anion.

Topics: Anions; Calixarenes; Heterocyclic Compounds; Magnetic Resonance Spectroscopy; Molecular Conformation; Molecular Structure; Phenols

Binding of chloride anion to a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor results in ET to Li(+)@C(60) to produce the radical pair (TTF-C4P(•+)/Li(+)@C(60)(•-)), the structure of which was characterized by X-ray crystallographic analysis. The addition of tetraethylammonium cation, which binds more effectively than Li(+)@C(60)(•-) as a guest within the TTF-C4P cavity, leads to electron back-transfer, restoring the initial oxidation states of the donor and acceptor pair.

Topics: Calixarenes; Crystallography, X-Ray; Electron Transport; Fullerenes; Heterocyclic Compounds; Ions; Lithium; Models, Molecular; Porphyrins

The importance of noncovalent interactions in the realm of biological materials continues to inspire efforts to create artificial supramolecular polymeric architectures. These types of self-assembled materials hold great promise as environmentally stimuli-responsive materials because they are capable of adjusting their various structural parameters, such as chain length, architecture, conformation, and dynamics, to new surrounding environments upon exposure to appropriate external stimuli. Nevertheless, in spite of considerable advances in the area of responsive materials, it has proved challenging to create synthetic self-assembled materials that respond to highly disparate analytes and whose environmentally induced changes in structure can be followed directly through both various spectroscopic and X-ray diffraction analyses. Herein, we report a new set of artificial self-assembled materials obtained by simply mixing two appropriately chosen, heterocomplementary macrocyclic receptors, namely a tetrathiafulvalene-functionalized calix[4]pyrrole and a bis(dinitrophenyl)-meso-substituted calix[4]pyrrole. The resulting polymeric materials, stabilized by combination of donor-acceptor and hydrogen bonding interactions, undergo dynamic, reversible dual guest-dependent structural transformations upon exposure to two very different types of external chemical inputs, namely chloride anion and trinitrobenzene. The structure and dynamics of the copolymers and their analyte-dependent responsive behavior was established via single crystal X-ray crystallography, SEM, heterocomplementary isodesmic analysis, 1- and 2D NMR, and dynamic light scattering spectroscopies. Our results demonstrate the benefit of using designed heterocomplementary interactions of two functional macrocyclic receptors to create synthetic, self-assembled materials for the development of "smart" sensory materials that mimic the key biological attributes of multianalyte recognition and substrate-dependent multisignaling.

Topics: Calixarenes; Chemistry, Physical; Crystallography, X-Ray; Dinitrobenzenes; Heterocyclic Compounds; Hydrogen Bonding; Magnetic Resonance Spectroscopy; Microscopy, Electron, Scanning; Molecular Conformation; Polymers; Porphyrins

The study of positive homotropic allosterism in supramolecular receptors is important for elucidating design strategies that can lead to increased sensitivity in various molecular recognition applications. In this work, the cooperative relationship between tetrathiafulvalene (TTF)-calix[4]pyrroles and several nitroaromatic guests is examined. The design and synthesis of new annulated TTF-calix[4]pyrrole receptors with the goal of rigidifying the system to accommodate better nitroaromatic guests is outlined. These new derivatives, which display significant improvement in terms of binding constants, also display a positive homotropic allosteric relationship, as borne out from the sigmoidal nature of the binding isotherms and analysis by using the Hill equation, Adair equation, and Scatchard plots. The host-guest complexes themselves have been characterized by single-crystal X-ray diffraction analyses and studied by means of UV-spectroscopic titrations. Investigations into the electronic nature of the receptors were made by using cyclic voltammetry; this revealed that the binding efficiency was not strictly related to the redox potential of the receptor. On the other hand, this work serves to illustrate how cooperative effects may be used to enhance the recognition ability of TTF-calix[4]pyrrole receptors. It has led to new allosteric systems that function as rudimentary colorimetric chemosensors for common nitroaromatic-based explosives, and which are effective even in the presence of potentially interfering anions.

Topics: Calixarenes; Colorimetry; Crystallography, X-Ray; Cyclization; Explosive Agents; Heterocyclic Compounds; Molecular Conformation; Porphyrins

The syntheses of monotetrathiafulvalene-calix[4]pyrrole 5 and bistetrathiafulvalene-calix[4]pyrrole 6, prepared from the acid-catalyzed condensation of monopyrrolo[3,4-d]tetrathiafulvalene (MPTTF, 7) with acetone in the presence of tripyrrane 8 and dipyrromethane 9, respectively, are described. Compound 5 and the previously reported tetrathiafulvalene-calix[4]pyrrole 4 both adopt a 1,3-alternative conformation in the solid state, as determined from X-ray crystallographic analysis. The anion binding properties of the tetrathiafulvalene-calix[4]pyrroles 5 and 6, as well as those of the parent meso-octamethylcalix[4]pyrrole (1), were investigated in acetone using (1)H NMR spectroscopic and isothermal titration calorimetry (ITC) techniques and, within the error limits of the methods, were generally found to give concordant results. On the basis of the results of the ITC studies carried out in 1,2-dichloroethane, increasing the number of tetrathiafulvalene units annulated to the calix[4]pyrrole system serves to enhance the anion binding affinities substantially but at the price of lowered selectivity. Cyclic voltammetry (CV) studies, carried out in 1,2-dichloroethane, provided evidence of an anion-dependent electrochemical response with Cl(-) and Br(-) ions. This response was particularly dramatic in the case of the monotetrathiafulvalene-calix[4]pyrrole 5, with a DeltaE(max) of -145 mV being seen after the addition of approximately 1 equiv of Cl(-) ion.

Topics: Anions; Calixarenes; Crystallography, X-Ray; Electrochemistry; Heterocyclic Compounds; Kinetics; Magnetic Resonance Spectroscopy; Mass Spectrometry; Porphyrins; Titrimetry