Single Guest Molecules Research Articles

Polymethylated [4.1.1] Octanes Leading to Zeolite SSZ-50

In this communication, we report on the discovery of novel zeolite compositions, SSZ-50. The zeolite has the RTH topology but can be made over a large silica-to-alumina range including no aluminum at all. The surprising capability to produce a broad compositional range comes from the use of a single organo-cation guest molecule in the zeolite synthesis. The molecule is a specific derivative from within a family of 2-aza [4.1.1] bicyclo octanes that were prepared employing a sequence of organic synthesis steps from a starting ketone. Other cage-based zeolites like SSZ-35,-36,-39 and MTN arose from the use of the other derivatives in this series. We also comment on the tendency of a variety of polymethylated organo-cations to produce RTH, the closely related ITE, or the intergrowth structure, SSZ-36.

Molecular mechanisms of photo-induced spectral diffusion of single terrylene molecules in p terphenyl

An empirical molecular model of crystals of p-terphenyl doped with terrylene is developed and compared with recent experiments on single guest molecules. The model provides an assignment of the experimental electronic origins to the substitution sites and an interpretation of the photo-induced frequency jumps of single terrylene molecules. The Stark shifts of terrylene in the different sites are estimated by a coupled Hartree–Fock semi-empirical calculation. The topology of the many-dimensional energy surface of the X1 electronic origin is explored with simulated annealing.

Recent highlights in hemicarcerand chemistry.

Since Donald J. Cram's first synthesis of a carcerand, which permanently entrapped a single guest molecule, many other carcerands and hemicarcerands have been synthesized and studied. Slowly, we begin to understand the role of the template in the formation of hemicarceplexes and carceplexes, why some hemicarceplexes are more stable than others, and how guests enter and exit the inner phases of molecular containers. In this Account we discuss our new insight in the chemistry of molecular containers, as well as recent highlights in through-shell chemistry and in the inner-phase stabilization of reactive intermediates.

Inner-Phase Stabilization of Reactive Intermediates

Molecular container compounds are spherical, hollow molecules with inner cavities (inner phases) that are large enough to accommodate a single guest molecule. These inner phases are superb environments for the stabilization and spectroscopic investigation of important reactive intermediates. The surrounding host protects the incarcerated reactive intermediate from dimerization or from a reaction with bulk-phase reactants that are too large to enter the inner phase by passing through a portal in the host shell.

Elucidation of optical switching of single guest molecules in terrylene/ p-terphenyl mixed crystals

An empirical molecular model is used to explore the ground-state potential energy surface of a mixed crystal in which reversible, reproducible, photo-induced frequency jumps of single molecules were recently reported. In the model, which accounts for many observed features of terrylene in p-terphenyl, optical switching of the guest transition frequency corresponds to relaxation of host molecules in the second solvation shell.

The Inner Phase of Molecular Container Compounds as a Novel Reaction Environment

The conceptual idea of molecular container compoundsand their synthesis has opened an entirely new andvery interesting research field: the chemistry of andwithin molecular container compounds and theircomplexes. Molecular containers have inner phases justlarge enough to accommodate a single guest molecule.Beginning with Donald J. Cram's first synthesis of acarcerand, which permanently entrapped a single guestmolecule, many other containers such ashemicarcerands, molecular lantern, self-assembledcapsules and fullerenes have been synthesized andstudied. Not only is the design and development of newcontainer compounds an ongoing challenge for organicchemists, but also the systematic investigation ofchemical reactions within their inner phases. Theresults of a large variety of inner phase reactionsspanning acid-base, reduction, oxidation, nucleophilicsubstitution, addition, thermal, photochemical andpericyclic reactions have provided us with moreinsight into the relationship between bulk phase andinner phase reactants and the mechanism of thetransfer of electrons and photons through theinsulating shell of a container molecule. They havealso led to very spectacular applications of molecularcontainer compounds such as the stabilization ofreactive intermediates by incarceration. Thesehighlights of inner phase chemistry and the currentefforts and successes towards using molecularcontainers as catalytic reaction vessels are presentedand discussed.

Molecular Modelling Studies on the Thiourea/ Ferrocene Clathrate

Computer modelling studies by means of molecular dynamics, molecular mechanics and Monte Carlo simulations have been carried out on the thiourea:ferrocene clathrate. The known crystal structure of this host:guest system was used as a model system containing four ferrocene guest molecules in a thiourea host channel of 16 Å in length. Molecular dynamics and molecular mechanics calculations indicated that the close packing of ferrocene molecules in the channel occurred despite strong steric repulsive interactions associated with neighbouring ferrocene molecules in the channel when they are positioned along the crystallographic sites. The neighbouring ferrocene molecules rotate and tilt with respect to each other in order to reduce the intermolecular repulsions. A second model containing a single ferrocene guest molecule in the thiourea host channel has also been studied. The guest adopts either a parallel alignment where the molecular five-fold axis coincides with the channel trigonal axis, or a perpendicular orientation or an intermediate orientation. Results from the molecular dynamics and molecular mechanics calculations show that the parallel positions are favoured. Monte Carlo simulations indicate a population distribution of 60%/40% for ferrocene in the parallel/perpendicular orientations. The ferrocene molecule alternates between the two orientations in the thiourea channel in order to avoid the repulsion inherent from the ideal parallel or perpendicular arrangement. Energy calculations show that only small movements off the channel trigonal axis are possible.

Biomimetic studies using artificial systems. V. Design and synthesis of novel water-soluble biscyclophanes.

Novel water-soluble bis-cyclophanes (3a-c) with quaternary ammonium nitrigens, composed of two untis of QCP44 (2) connected by appropriate bridges, were synthesized as hosts having two discrete hydrophobic cavities to bind two guest molecules simultaneously or a single guest molecule via double recognition in neutral water. Glutaryl, terephthaloyl and isophthaloyl groups were used as spacers to bridge the two cyclophane units. THe bridge formation was carried out using 5 as an intermediate, in which three of the four nitrogens of CP44 (1) were protected. The details of the synthesis and characterization of duplex hosts (3a-c) as well as of the reference compounds (9-11) are described.

A study of the inclusion properties of 1,2-bis(diphenylphosphinoselenoyl)ethane and related host molecules

The title compound (I) has been found to form stable adducts when recrystallised from various pure solvents. Most significantly, host (I) exhibits a remarkably high selectivity for certain para-disubstituted aromatic molecules. For example, recrystallisation of (I) from an equimolar mixture of each of the xylenes, and ethylbenzene leads to incorporation of 97.5% of p-xylene. An X-ray investigation of the inclusion compound of (I) with p-xylene has been carried out. Crystals are triclinic, space group P1, with a= 8.958(1), b= 14.822(2), c= 15.496(2)A, α= 81.20(1)°, β= 83.05(1)°, γ= 79.38(1)°, with three host and a single guest molecule in the unit cell. The structure was refined by least-squares methods to a final R of 0.036 for 2 558 significant diffractometer data. The adduct is of the true clathrate type, the highly ordered p-xylene guest being accommodated in an effectively closed cage. Compounds (II)–(V) which are structurally related to (I) have also been prepared and found to exhibit inclusion behaviour, but to a lesser degree than (I).

Theory of the Vibronic Transitions of Substitutional Impurities in Molecular Crystals

A theory for the vibronic states of a single guest molecule in the vicinity of bands of single exciton states of the host crystal is presented. It is assumed that the lattice is rigid, and that the wavefunctions of isolated molecules are Born–Oppenheimer separable. Expressions are given for the excitation energies, transition probabilities, and polarization ratios of a general guest which are much simpler and easier to interpret than any given before. It is shown that the guest transitions exhibit anomalous polarization ratios (Rashba effect) near the host bands. The use of guests to locate exciton band edges and find guest–host interactions is discussed.