The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and photoluminescence excitation was measured. The emission is assigned to forbidden transitions involving the cluster-molecule surface-capping ligands. This assignment is supported by the emission decay which exhibits distributed kinetics with microsecond time scale. The temperature dependence of the emission intensity is quantitatively explained by multiphonon-induced nonradiative relaxation mediated by low-frequency vibrations of the selenophenol capping ligands. Upon irradiation, the emission of all cluster molecules is quenched. Warming up and recooling leads to recovery of the emission (partial or complete) for all but the cluster molecule with 10 Cd atoms. This temporary darkening is assigned to the photoinduced charging of the cluster-molecule surface ligands, resembling the reversible on-off blinking of the emission observed for larger CdSe nanocrystals.

The self-assembly of the tris-bipyridine ligands B I and B II with iron(II) salts yields polynuclear complexes displaying structures of cyclic double-helix type, termed circular helicates [n]cH (of order n). With B I in which the bipyridine units in the ligand are connected by ethylene bridges, penta- or hexanuclear architectures [5]cH and [6]cH are obtained, depending on the anion present during the self-assembly process. The elongated tris-bipyridine ligand B II with oxypropylene bridges forms a tetranuclear circular helicate [4]cH, whose structure has been confirmed by crystal structure determination. The possible oligomeric combinations of tris-bipy ligands and iron(II) ions may be considered to constitute the potential members of a virtual combinatorial library, generated via dynamic combinatorial chemistry, from which a specific real constituent of the virtual set of circular helicates is expressed in given conditions.

Dedicated to Professor Guy Ourisson on the occasion of his 70th hirtlidaj.Helicates, metal complexes that form double" -41 or triple in solution and/or in the solid state, are generated by self-assembly of two or three ligand strands around suitable metal ions. Whereas the double helix of DNA results from the assembly of two complementary chains of nucleotides through hydrogen bonding, helicates form spontaneously from linear ligands composed of a series of discrete binding subunits through metal ion coordination. The process may be considered as resulting from the operation of a programmed system, in which the steric and interactional information stored in the ligands is read out by the metal ions following the algorithm detined by their coordination geometry.[31 Thus homostranded d~u b l e [ ' .~,~I and t r i~l e [~.~] helicates have been obtained from chains of 2,T-bipyridine (bpy) or 2,2 : 6',2"-terpyridine (terpy) groups connected by short covalent bridges, and metal ions having tetrahedral or octahedral coordination geometry. Recently, a heteroduplex helicate based on a tris-bpy strand, a tris-terpy strand, and pentacoordinated Cu" ions has been described.['] The DNA double helix is usually acyclic but can also be closed into a cyclic structure, a circular DNA, as found in some viruses. By analogy, it would be of great interest to devise means of generating closed, circular helicates. We describe herein the self-assembly of such a supramolecular architecture. The trisbpy ligand L and iron(i1) chloride have been found to form spontaneously and quantitatively a circular double helicate, cH (Fig. I), in other words, a double helicate closed into a ring, as indicated by 'H N M R spectroscopy and mass spectrometry as well as by X-ray crystal structure determination.

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lated and exfoliated PMMA and PS±clay nanocomposites foams were prepared using supercritical CO 2 as the foaming agent. Presence of a small amount of clay nanoparticles greatly reduces foam cell size and increases the cell density. Exfoliated nanocomposites yield foams with the smallest cell size and the highest cell density. Cell morphology can be furthered manipulated by adjustment of polymer±clay surface±CO 2 interaction and foaming conditions to achieve microcellular and submicrocellular foams. The high nucleation efficiency can produce microcellular nanocomposite foams at less stringent processing conditions, leading to cost savings and processing flexibility. ExperimentalMaterials: Methylmethacrylate (MMA), Styrene (St) and 2,2¢-azobisisobutyronitrile (AIBN) were purchased from Aldrich. PS (CX 5197) is from AtoFina Petrochemicals, while PMMA (PL25) is from Plaskolite. Two types of organically modified MMT clays were used. Cloisite 20A (20A) is an MMT modified by dimethyl dihydrogenated tallowalkylammoniumonium cations (Southern Clay Products). Na + ±MMT (cation exchange capacity 92.4 milliequivalent/100 g, from Southern Clay Products) was modified using a reactive cationic surfactant 2-methacryloyloxyethylhexadecyldimethylammonium bromide (MHAB) via ion-exchange reaction [16]. The resulting organoclay is denoted as MHABS. Polymer±20A nanocomposites were prepared using a Leistritz ZSE-27 intermesh twin-screw extruder (L/D = 40, d = 27 mm) in co-rotating mode between 200±220 C and 200 rpm (revolutions per minute) screw speed. In-situ polymerization was carried out to prepare PMMA and PS±MHABS nanocomposites. The monomer, MHABS, and AIBN (0.5 wt.-%) were mixed using a high shear mixer. The mixture was reacted isothermally (60 C for styrene, 50 C for MMA) for 20 h, after which the temperature was raised to 105 C for another 30 min. A two-stage method was also used to prepare PS±MHABS nanocomposites. First a 20 wt.-% nanocomposite masterbatch was prepared by in-situ polymerization. It was then blended with neat PS to prepare nanocomposites with desired clay concentration using a DACA microcompounder at 200 C and 250 rpm. Soxhlet extraction was used to extract non-bonded PMMA from PMMA±MHABS nanocomposites, using dichloromethane as the solvent. The unextractable portion consists of MHABS and a substantial amount of PMMA (64 % of the total weight of the unextractable nanocomposite). The resulting material was blended with neat PS to produce PS±(MHABS±PMMA). (PS±MHABS)±PMMA was prepared via blending a PS±MHABS nanocomposite with PMMA. These two materials have the same weight composition (PS/ PMMA/MHABS = 86:9:5).Foaming of Nanocomposites: The foaming agent, bone-dry grade carbon dioxide, was provided by Praxair. Samples were placed in a stainless steel vessel and CO 2 was delivered via a syringe pump. The system was allowed to equilibrate at the foaming temperature and pressure for sufficient time to ensure equilibrium. The pressure was then rapidly released and the foam cells were fixed by cooling with an ice and ...