Photoluminescent metal-sulfur clusters derived from tetrathiometalates: metal-to-metal charge-transfer excited states of d0-d10 heterobimetallic sulfido clusters with bulky phosphine ligands

Reactions of MS4(2-) (M = Mo, W) with M'(PCy3)X (M'=Ag/Au, X= ClO4/Cl) and [Cu2(dcpm)2(MeCN)2](ClO4)2 (dcpm = bis(dicyclohexylphosphino)methane) afforded heterometallic sulfido clusters [M'2(PCy3)2(MS4)] (M=Mo, M'=Au: 2; M=W, M'=Ag: 3, Au: 4) and [Cu4(dcpm)4(MS4)](ClO4)2 (M=Mo: 5 x (ClO4)2, W: 6 x (ClO4)2), all of which, except 4, have been characterized by X-ray structure determination. Clusters 5 x(ClO4)2 and 6 x (ClO4)2 feature unusual 16-membered [Cu4P5C4] metallamacrocycles formed on the respective tetrathiometalate anion templates and have unusually long Cu-S bonds and Cu...M distances for metal sulfur clusters that contain a saddle-shaped [Cu4MS4] core. Low-energy absorption bands are observed in their electronic spectra at approximately 562 and 467 nm, respectively, assignable to MMCT transitions; quasireversible reduction waves are observed with E(1/2) = -1.43 (52+) and -1.78 V (62+) versus FeCp2(0/+); and they are emissive either in the solid state or in solution. The emission of 6(2+) can be quenched by both electron acceptors, such as methylviologen, or electron donors, such as aromatic amines, with the excited state reduction potential E(62+*/6+) estimated to be approximately 1.13V versus a normal hydrogen electrode.     

Monte Carlo simulations of liquid crystals near rough walls

The effect of surface roughness on the structure of liquid crystalline fluids near solid substrates is studied by Monte Carlo simulations. The liquid crystal is modeled as a fluid of soft ellipsoidal molecules and the substrate is modeled as a hard wall that excludes the centers of mass of the fluid molecules. Surface roughness is introduced by embedding a number of molecules with random positions and orientations within the wall. It is found that the density and order near the wall are reduced as the wall becomes rougher, i.e., the number of embedded molecules is increased). Anchoring coefficients are determined from fluctuations in the reciprocal space order tensor. It is found that the anchoring strength decreases with increasing surface roughness.     

A de novo enantioselective total synthesis of (-)-laulimalide

An enantioselective total synthesis of the naturally occurring anticancer agent (-)-laulimalide is described. The synthesis is characterized by extensive use of new reaction methodologies based on catalytic asymmetric acyl halide-aldehyde cyclocondensation (AAC) reactions and transformations of the derived enantioenriched beta-lactones. The synthesis also incorporates a unique allenylstannane glycal acetate alkylation and chemoselective ring-closing metathesis reaction.     

Organic triplet emissions of arylacetylide moieties harnessed through coordination to [Au(PCy(3))]+. Effect of molecular structure upon photoluminescent properties

A family of mono- and binuclear Cy(3)P-supported gold(I) complexes containing various pi-conjugated linear arylacetylide ligands, including the two homologous series (Cy(3)P)Au(Ctbd1;CC(6)H(4))(n)()(-)(1)(Ctbd1;CPh) and (Cy(3)P)Au(Ctbd1;CC(6)H(4))(n)()Ctbd1;CAu(PCy(3)) (n = 1-4), have been prepared. X-ray crystal analyses revealed no intermolecular aurophilic interactions in their crystal lattice. The lowest-energy singlet transitions are predominately intraligand in nature and exhibit both phenyl and acetylenic (1)(pipi) character. Strong photoluminescence is detected in solid and solution states under ambient conditions, with lifetimes in the microsecond regime. For complexes with a single arylacetylide group, only phosphorescence from the arylacetylide (3)(pipi) state is observed. Vibrational spacings in the solid-state emission spectra can be attributed to a combination of phenyl ring deformation and symmetric phenyl ring and Ctbd1;C stretches. Additional delayed-fluorescence emission is recorded for complexes with multiple p-arylacetylide units, and this is attributed to a triplet-triplet annihilation process. The phosphorescence energy of these complexes are readily modified by altering the length of the conjugated arylacetylide system, while the intensity of phosphorescence relative to fluorescence decreases when the p-arylacetylide chain is elongated. Information regarding the nature and relative energies of arylacetylide singlet and triplet excited states has been derived from the two homologous series and extrapolated to polymeric arylacetylide species. The (3)(pipi) excited-state reduction potentials E degrees [Au(+)/Au] (Au = 1a, 2, and 4) are estimated to be -1.80, -1.28, and -1.17 V versus SSCE, respectively.     

Relationship of deformation behavior to thermal transitions of ethylene/styrene and ethylene/octene copolymers

The stress‐strain response of low‐crystallinity ethylene‐octene (EO) and ethylene‐styrene (ES) copolymers with 7–20 mol % comonomer was compared over a temperature range that spanned the glass‐transition and crystal melting regions. Above the onset temperature of the glass transition, the copolymers exhibited elastomeric behavior with low initial modulus, uniform deformation to high strains, and high recovery after the stress was released. In the glass‐transition range, an initial low‐stress elastomeric response was followed by a distinct “bump” in the stress‐strain curve. On the basis of the temperature and rate dependence of the stress‐strain curve, local strain‐rate measurements, local temperature changes, and recovery characteristics, the “bump” was identified as high strain yielding. Hence, the stress‐strain curve sequentially exhibited the features of elastomeric and plastic deformation. Following high strain yielding, strain hardening dramatically increased the fracture strength. This behavior was defined as elastomeric‐plastic. Elastomeric‐plastic behavior in the broad glass‐transition range constituted a gradual transition from elastomeric behavior at higher temperatures to low‐temperature plastic behavior with high modulus and macroscopic necking. Because of the lower glass‐transition temperature of EO, ?40 °C as compared with ?10 °C for ES, the onset of elastomeric‐plastic behavior occurred at a significantly lower temperature. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions that provides the structural basis for elastomeric behavior of low‐crystallinity ethylene copolymers was extended to elastomeric‐plastic behavior by considering a network with a fraction of rigid, glassy chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 142–152, 2002     

Synthesis, emission, and electrochemical properties of luminescent dinuclear zinc(II) chalcogenolate complexes. Dynamic 1H NMR studies and X-ray crystal structure of [(bpy)Zn2(SC6H4-Cl-p)(mu-SC6H4-Cl-p)(mu-OAc)2

A series of novel luminescent dinuclear zinc(II) diimine complexes with bridging chalcogenolate ligands have been synthesized, in which the two zinc atoms were found to exist in different coordination environment. The luminescence and electrochemical behavior of these complexes have been studied. These complexes have also been shown to exhibit dynamic fluxional behavior in solution due to an exchange of the bridging and terminal thiolate ligands. The mechanism and kinetics of which have been investigated by variable-temperature 1H NMR studies. The X-ray crystal structure of [(bpy)Zn2(SC6H4-Cl-p)(mu-SC6H4-Cl-p)(mu-OAc)2] has also been determined.     

Resonance raman spectra and excitation profiles of soret-excited metalloporphyrins

Resonance Raman spectra of nickel, chromium, and copper porphyrins, excited at their Soret maxima, show decreasing scattering intensity due to depolarized modes. This order correlates with diminished ¹Q(0-0) absorption in the porphyrins. The data are interpreted in terms of Jahn-Teller distortions in both Q and B states. General agreement between predicted and observed excitation profiles of nickel etioporphyrin I support a vibronic treatment of the excited states. The effect of tight focusing of the incident beam is to quench most Raman scattering intensity, except the 1384 cm^?1 line of the nickel complex.