Grafting cavitands on the Si(100) surface

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.     

Metal-free synthesis of calixsalen-type cavitands for the recognition of fluoride ion

Novel calixsalen-type cavitands have been synthesized using metal-free synthesis from simple and inexpensive materials, such as ethylenediamine and 5,5′-methylene-bis-salicylaldehyde derivatives. The cavitand 1 containing salen functionality recognizes fluoride ion. Fluoride ions switch on fluorescence on binding with the cavitand 1. Substitution on bis-salicylaldehyde part of calixsalen-type cavitand shows change in recognition behavior. On the attachment of electron withdrawing substituent, such as nitro group, the cavitand lost its fluorescence properties but proved to be a better colorimetric probe showing marked color change from pale yellow to red on addition of tetrabutyl ammonium salt of fluoride ion to the solution of cavitand. The nitro substituted cavitand is highly sensitive and selective for fluoride anion and hence is a promising candidate for development of colorimetric chemosensor. The binding of the cavitands with fluoride ion is investigated using ¹H NMR-titration experiments.     

Self-assembly of nanosize coordination cages on si(100) surfaces

Bottom-up fabrication of 3D organic nanostructures on Si(100) surfaces has been achieved by a two-step procedure. Tetradentate cavitand 1 was grafted on the Si surface together with 1-octene (Oct) as a spatial spectator by photochemical hydrosilylation. Ligand exchange between grafted cavitand 1 and self-assembled homocage 2, derived from cavitand 5 bearing a fluorescence marker, led to the formation of coordination cages on Si(100). Formation, quantification, and distribution of the nanoscale molecular containers on a silicon surface was assessed by using three complementary analytical techniques (AFM, XPS, and fluorescence) and validated by control experiments on cavitand-free silicon surfaces. Interestingly, the fluorescence of pyrene at approximately 4 nm above the Si(100) surface can be clearly observed.     

pH influenced molecular switching with micelle bound cavitands

A series of resorcinarene host-amphiphilic guest complexes have been developed where guest orientation in the host is drastically influenced by pH. Guests appended with a trimethylammonium and a tert-butyl group switch orientation by 180° in response to a buildup of negative charge in the cavitand host.     

Light-actuated resorcin[4]arene cavitands

A light-actuated resorcin[4]arene cavitand equipped with two quinone (Q) and two opposite Ru(II)-based photosensitizing walls was synthesized and investigated. The cavitand is capable of switching from an open to a contracted conformation upon reduction of the two Q to the corresponding SQ radical anions by intramolecular photoinduced electron transfer in the presence of a sacrificial donor. The molecular switch was investigated by cyclic and rotating disc voltammetry, UV–Vis–NIR spectroelectrochemistry, transient absorption, NMR, and EPR spectroscopy. This study provides the basis for the development of future light-activated switches and molecular actuating nanodevices.     

Controlling photochemical geometric isomerization of a stilbene and dimerization of a styrene using a confined reaction cavity in water

Utility of a water-soluble deep cavity cavitand, octa acid, as a reaction medium is illustrated by carrying out photochemical reactions of a stilbene and a styrene included within the octa acid in water. Geometric isomerization of trans-4,4'-dimethyl stilbene is restricted while dimerization of 4-methyl styrene is facilitated within the octa acid cavity. The excited-state chemistry of both systems is different in this medium from that in organic solvents. The change in chemistry is attributed to the supramolecular effects provided by the host cavity.     

Cavitand-based nanoscale coordination cages

This communication reports design, self-assembly, solution, and solid-state characterization of nanoscale coordination cages formed by tetradentate cavitand ligands and appropriate metal precursors. The preorganization of the cavitand ligand in terms of structural rigidity and relative orientation of the pyridyl units leads to the exclusive formation of coordination cages in a wide temperature and concentration range. Desired features of the cage self-assembly process, such as reversibility in the presence of a competitive ligand and self-recognition of the cavitand components, have been assessed.     

Extraction of hydrophobic species into a water-soluble synthetic receptor

A deep, water-soluble cavitand extracts a variety of neutral hydrophobic species into its cavity. Flexible species such as n-alkanes tumble rapidly on the NMR time scale inside the cavity, but this motion is slowed for bulkier guests. Long, rigid guests such as p-substituted aromatics are either static or only tumble at elevated temperatures via flexing motions of the cavitand. Strong selectivity in recognition of long rigid guests is seen. The binding of neutral guests occurs via the classical hydrophobic effect; the process is entropically favored, as shown by isothermal titration calorimetry measurements. Binding affinities are generally on the order of 10(4)-10(5) M(-1). The extent of the hydrophobic stabilization is shown by the binding of long trimethylammonium salts, which bind the alkyl chain in the cavity, rather than the NMe3+ group. Dynamic NMR studies show that self-exchange of neutral guests is independent of guest concentration, and most likely occurs via rate-determining unfolding of the cavitand. In the absence of guests, the cavitand exists in a dimeric velcrand structure.     

Protein recognition by a self-assembled deep cavitand monolayer on a gold substrate

This paper details the first use of a self-folding deep cavitand on a gold surface. A sulfide-footed deep, self-folding cavitand has been synthesized, and its attachment to a cleaned gold surface studied by electrochemical and SPR methods. Complete monolayer formation is possible if the cavitand folding is templated by noncovalent binding of choline or by addition of space-filling thiols to cover any gaps in the cavitand adsorption layer. The cavitand is capable of binding trimethylammonium-tagged guests from an aqueous medium and can be deposited in 2 × 2 microarrays on the surface for characterization by SPR imaging techniques. When biotin-labeled guests are used, the cavitand:guest construct can recognize and immobilize streptavidin proteins from aqueous solution, acting as an effective supramolecular biosensor for monitoring protein recognition.     

A Multifunctional Photoswitch: 6π Electrocyclization versus ESIPT and Metalation

A terthiazole‐based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady‐state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time‐dependent photochemical outcome and displays efficient ESIPT‐based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.     

Expanding cavitand chemistry: the preparation and characterization of [n]cavitands with n>=4

The preparation of cavitands composed of 4, 5, 6, and 7 aromatic subunits ([n]cavitands, n=4-7) is described. The simple, two-step synthetic procedure utilized readily available starting materials (2-methylresorcinol and diethoxymethane). The two cavitand products having 4 and 5 aromatic subunits exhibited highly symmetric cone conformations, while the larger cavitands (n = 6 and 7) adopt conformations of lower symmetry. 1H NMR spectroscopic studies of [6]cavitand and [7]cavitand revealed that these hosts undergo exchange between equivalent conformations at room temperature. The departure of these two cavitands from cone conformations is related to steric crowding on their Ar-O-CH2-OAr bridges and is predicted by simple molecular mechanics calculations (MM2 force field). X-ray diffraction studies on single crystals of the [4]cavitand, [5]cavitand, and [6]cavitand hosts afforded additional experimental support for these conclusions.     

Detection of reactive tetrahedral intermediates in a deep cavitand with an introverted functionality

Labile hemiaminal intermediates are stabilized by binding in a deep cavitand with an introverted aldehyde functionality. The aldehyde is attached to the cavitand via an anthracene spacer that rotates rapidly about the cavitand rim. The half-lives of these hemiaminals vary from 30 min to over 100 h at ambient temperature, due to hydrogen bonding with the organized peptide-like framework at the cavitand rim. The intermediates are sufficiently long-lived to allow study by 2D NMR techniques requiring many hours of acquisition time. Mechanistic analysis of the dehydration step shows first-order kinetics. The analogous "extroverted" reaction was also performed, where the addition took place outside the cavitand, displaying standard steady-state kinetics; no hemiaminal was observed. The cavitand shows strong selectivity based not on binding affinity but upon the rate of the product-forming step. A 10:1 ratio of product imines was obtained, while the initial binding ratio was 1:1. The cavitand acts as a mimic of enzymes in that it uses weak binding forces to stabilize reactive intermediates and isolates them from the medium. The synthetic environment allows direct detection and analysis of the intermediates, as opposed to natural systems that must be analyzed indirectly.     

Synthesis and characterization of upper and lower rim functionalized [6]cavitands

A [6]cavitand has been selectively derivatized on both the lower and upper rims. On the lower rim, two out of six potential sites were oxidized to produce a 1,4 substituted [6]cavitand bisketone, which was converted to a corresponding diol as well as a bisacetate [6]cavitand. The crystal structures of the bisketone and the diol were solved. On the upper rim, all six ArCH(3) groups were selectively brominated to ArCH(2)Br groups to produce the hexabromomethyl [6]cavitand, which was converted to the corresponding hexabenzylthiol and hexabenzylthioacetate [6]cavitands. The conformational properties of all compounds are discussed.     

Design and self-assembly of ditopic and tetratopic cavitand complexes

The self-assembly of open ditopic and tetratopic cavitand complexes has been investigated by using monofunctionalized cavitand ligands and suitable metal precursors. In the case of ditopic complexes, self-assembly protocols, leading exclusively to the formation of both thermodynamically stable cis-Pt square-planar complexes 8 and 9 and the kinetically inert fac-Re octahedral complex 14, have been elaborated. The use of cis-[Pt(CH3)CN)2Cl2] as metal precursor led to the formation of monotopic trans-10 and ditopic trans-11 cavitand complexes, while cis-[Pt(dmso)2Cl2] afforded both cis-13 and trans-11 isomers. The self-assembly of tetratopic cavitand complexes has been achieved by using mononuclear [Pd(CH3CN)4(BF4)2] and dinuclear [M2(tppb)(OTf)4] (19: M = Pt; 20: M = Pd) metal precursors. Only the tetratopic dinuclear complexes 21 and 22 were stable. The ligand configuration with two phosphorus and two cavitand ligands at the metal centers is the most appropriate to build tetratopic cavitand complexes with sufficient kinetic stability.     

Selective formation of a self-assembling homo or hetero cavitand cage via metal coordination based on thermodynamic or kinetic control

The selective formation of a homo or hetero cavitand cage composed of two molecules of tetra(4-pyridyl)-cavitand (1), tetrakis(4-cyanophenyl)-cavitand (2), or tetrakis(4-pyridylethynyl)-cavitand (3), and four molecules of Pd(dppp)(OTf)(2) (4) or Pt(dppp)(OTf)(2) (5) has been studied. A 1:1:4 mixture of 1 with more steric restriction, 2 with less coordination ability, and 4 or 5 specifically self-assembled into a hetero cavitand cage 6 or 7, respectively. In contrast, a 1:1:4 mixture of 2, 3, and 4 in CDCl(3) at room temperature assembled into the most labile homo cyanophenyl cavitand cage 8 and the most stable homo pyridylethynyl cavitand cage 9 in a 1:1 ratio. Upon heating at 50 degrees C, the thermodynamic equilibrium was shifted to a 1:1:1 mixture of 8, 9, and a hetero cavitand cage 10. When 1 equiv of 3 was added to 8 at room temperature, 8, 9, and 10 were formed initially in a 1:1:3 ratio and finally shifted to a 1:1:1 ratio. In the Pt-system, upon addition of 1 equiv of 3 to homo cyanophenyl cavitand cage 11 in CDCl(3) at room temperature, the ratio of hetero to homo cavitand cage (13/12) initially attained was 8.7 and remained above 5.6 at room temperature. Upon heating at 50 degrees C, 13 was finally converted to 11 and 12. Thus, the selectivity for the self-assembly of the homo or hetero cavitand cage is controlled by the balance between kinetic and thermodynamic stabilities of cages based on a combination of factors such as coordination ability and steric demand of the cavitands.     

A Tetraferrocenyl‐Resorcinarene Cavitand as a Redox‐Switchable Host of Ammonium Salts

Tetraannulation of a resorcinarene‐octaamino cavitand with ferrocenecarboxaldehyde allows the preparation of a tetrabenzimidazole‐resorcinarene cavitand with four ferrocenyl moieties directly linked to the C2 atom of the imidazole units. Oxidation of the four ferrocenyl moieties produces important structural modifications of the molecule, as indicated by DFT calculations performed for the neutral and tetraoxidized forms of the cavitand. By means of ¹H NMR spectroscopic analysis, the encapsulating properties of the new tetraferrocenyl‐resorcinarene cavitand toward a series of ammonium salts were evaluated, and a clear cutoff point in binding affinity with respect to size was observed. Cyclic voltammetric studies allowed us to estimate the relative association constants for the neutral and oxidized forms of the cavitand, thus indicating that the guest was bound to the neutral (reduced) state of the cavitand and was released from the oxidized form. These redox‐addressable conformational and binding properties of the resorcinarene‐tetraferrocenyl cavitand constitute all the necessary features of a redox‐switchable molecular gripper. By means of mass‐spectrometric analysis, we could unambiguously confirm the molar stoichiometry of the host–guest complex (1:1) and assess the strong guest encapsulation, as indicated by triggering the covalent coupling between host and guest in the gas phase.     

Water inside a hydrophobic cavitand molecule

The structure and dynamics of water inside a water-soluble, bowl-shaped cavitand molecule with a hydrophobic interior are studied using molecular dynamics computer simulations. The simulations find that the number of inside water molecules is about 4.5, but it fluctuates from being completely empty to full on a time scale of tens of nanoseconds. The transition from empty to full is energetically favorable and entropically unfavorable. The water molecules inside have fewer hydrogen bonds than the bulk and in general weaker interactions; the lower energy results from the nearest-neighbor interactions with the cavitand atoms and the water molecules at the entrance of the cavitand, interactions that are lost upon dewetting. An analysis of translational and rotational motion suggests that the lower entropy of the inside water molecules is due to decreased translational entropy, which outweighs an increased orientational entropy. The cavitand molecule acts as a host binding hydrophobic guests, and dewetting can be induced by the presence of a hydrophobic guest molecule about 3 A above the entrance. At this position, the guest displaces the water molecules which stabilize the inside water molecules and the empty cavitand becomes more stable than the full.     

Templated assembly of water-soluble nano-capsules: inter-phase sequestration, storage, and separation of hydrocarbon gases

In aqueous solution, a deep-cavity cavitand was shown to self-assemble into dimeric nano-capsules via the sequestration of gaseous-phase hydrocarbons. The sequestration, assembly, and entrapment process was shown to be dependent on the hydrocarbon. Thus, by way of example, butane could be selectively sequestered from a propane-butane mixture.     

Recognition and catalysis in allylic alkylations

[structure: see text] A cavitand outfitted with a chelated palladium atom catalyzes allylic alkylation reactions. Molecular recognition by the cavitand distinguishes between closely related structures and results in subtle substrate specificities.     

Synthesis of polyhydroxy cavitands and intramolecular inclusion of their octaester derivatives

A facile and efficient synthesis of novel cavitands containing eight hydroxyl groups was accomplished in eight steps beginning from commercially available resorcinol and the corresponding aldehydes. The synthesis combines two classical approaches to cavitand chemistry and yields the target octol cavitand molecules in gram quantities with no chromatographic separations. A variety of octaester cavitand derivatives were then prepared from the parent octols and their spectral properties are reported.