Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield.

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si6H12) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

Breaking Carbon-Chlorine Bonds with the Unconventional Lewis Acid Dodecachlorocyclohexasilane.

c-Si6Cl12 functions as a Lewis acid strong enough to abstract chloride ions from 2 mol of triphenylchloromethane to form the salt [Tr+]2[Si6Cl142-]. This is the first example of a Lewis acid "hole" breaking a carbon-halogen bond.

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies.

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by time-dependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. The identified pathway supports the chemistry proposed for the fabrication of silicon-based materials using CHS as a precursor. In addition, we find dimers have smaller HOMO-LUMO gaps and exhibit a red shift and line-width broadening in the computed photoluminescence spectra compared with a pair of CHS monomers.

Mechanism of Charged, Neutral, Mono-, and Polyatomic Donor Ligand Coordination to Perchlorinated Cyclohexasilane (Si6Cl12).

We report the detailed computational study of several perchlorinated cyclohexasilane (Si6Cl12)-based inverse sandwich compounds. It was found that regardless of the donor ligand size and charge, for example, Cl- and CN- anions or neutral HCN and NCPh nitriles, their coordination to the puckered Si6Cl12 ring results in its flattening. The NBO and CDA studies of the complexes showed that coordination occurs due to hybridization of low-lying antibonding σ*(Si-Cl) and σ*(Si-Si) unoccupied molecular orbitals (UMOs) of Si6Cl12 and occupied molecular orbitals (OMOs) of donor molecules (predominantly lone-pair-related), resulting in donor-to-ring charge transfer accompanied by complex stabilization and ring flattening. It is known that the Si6 ring distortion results from vibronic coupling of OMO and UMO pairs (pseudo-Jahn-Teller effect, PJT). Consequently, the Si6 ring flattening most probably occurs due to suppression of the PJT effect in all of the studied compounds. In this paper, the stabilization energy E(2) associated with donor-acceptor charge transfer (delocalization) was estimated using NBO analysis for [Si6Cl12·2Cl]2-, [Si6Cl12·2(NC)]2-, Si6Cl12·2(NCH), and Si6Cl12·2(NCPh). It was found that the polarizability of the donor might significantly affect the stabilization energy value (Cl- > CN- > HCN). For the neutral complexes, the E(2) value is correlated with the charge on the nitrogen atoms. All of these factors, that is, specific donor E(2) value, charge transfer, complex MO energy diagrams, and so on, should be taken into account when choosing the ligands suitable for forming Si-based 1D compounds and other nanoscale materials.

Toward the Mechanism of Perchlorinated Cyclopentasilane (Si5Cl10) Ring Flattening in the [Si5Cl10·2Cl]2- Dianion.

We report the detailed computational study of flattening of the puckered Si5 ring by suppression of the pseudo-Jahn-Teller (PJT) effect through coordination of two Cl- anions to the molecule forming an inverse sandwich dianion [Si5Cl10·2Cl]2- complex. The PJT effect that causes nonplanarity of the Si5Cl10 structure (Cs) results from vibronic coupling of pairs of occupied molecular orbitals (OMOs) and unoccupied molecular orbitals (UMOs). It was shown that filling the intervenient molecular orbitals of puckered Si5Cl10 with valent electron pairs of Cl- donors suppresses the PJT effect, with the Si5 ring becoming planar (D5h) upon complex formation. In this paper, the stabilization energy E(2) associated with donor-acceptor charge transfer (delocalization) was estimated using NBO analysis for all studied inverse sandwich compounds [Si5Cl10·2X]2- (where X = F, Cl, Br). It was found that the polarizability of the donor ion might significantly affect the stabilization energy value and should be taken into account when choosing the ligands suitable for forming Si-based one-dimensional compounds and other nanoscale materials.

Polymer Coating Materials and Their Fouling Release Activity: A Cheminformatics Approach to Predict Properties.

A novel cheminformatics-based approach has been employed to investigate a set of polymer coating materials designed to mitigate the accumulation of marine biofouling on surfaces immersed in the sea. Specifically, a set of 27 nontoxic, amphiphilic polysiloxane-based polymer coatings was synthesized using a combinatorial, high-throughput approach and characterized for fouling-release (FR) activity toward a number of relevant marine fouling organisms, including bacteria, microalgae, and adult barnacles. In order to model these complex systems adequately, a new computational technique was used in which all investigated polymer-based coating materials were considered as mixture systems comprising several compositional variables at a range of concentrations. By applying a combination of methodologies for mixture systems and a quantitative structure-activity relationship approach (QSAR), seven unique QSAR models were developed that were able to successfully predict the desired FR properties. Furthermore, the developed models identified several significant descriptors responsible for FR activity of investigated polymer-based coating materials, with correlation coefficients ranging from rtest2 = 0.63 to 0.94. The computational models derived from this study may serve as a powerful set of tools to predict optimal combinations of source components to produce amphiphilic polysiloxane-based coating systems with effective, broad-spectrum FR properties.

Halide coordination of perhalocyclohexasilane Si6X12 (X=Cl or Br).

The addition of halide anions (X' = Cl(-), Br(-), or I(-)) to perhalocyclohexasilane Si(6)X(12) (X = Cl or Br) led to the formation of complexes comprising [Si(6)X(12)X'(2)](2-) dianions. An upfield shift in the (29)Si NMR spectra was noted upon coordination, and structural determination by X-ray crystallography showed that the dianions adopt an "inverse sandwich" structure where the six cyclic silicon atoms form a planar hexagon with the two halide anions X' located on the 6-fold axis equally disposed above and below the plane of the Si(6) ring. Additionally, these apical X' atoms are within the van der Waals bonding distance to the silicon ring atoms, indicating a strong interaction between X' and silicon atoms. These results detail crystallographic variations within the halogen series providing further insight into the nature of the Lewis acid sites above and below the Si(6)X(12) ring, where interactions with hard Lewis bases such as halide anions are observed. Interestingly, the stereochemistry of the silicon atoms in [Si(6)X(12)X'(2)](2-) is not affected much by the size or electronegativity of the halogen atoms.

Flattening a puckered cyclohexasilane ring by suppression of the pseudo-Jahn-Teller effect.

We report the experimental and theoretical characterization of neutral Si(6)X(12) (X = Cl, Br) molecules that contain D(3d) distorted six-member silicon rings due to a pseudo-Jahn-Teller (PJT) effect. Calculations show that filling the intervenient molecular orbitals with electron pairs of adduct suppresses the PJT effect in Si(6)X(12), with the Si(6) ring becoming planar (D(6h)) upon complex formation. The stabilizing role of electrostatic and covalent interactions between positively charged silicon atoms and chlorine atoms of the subject [Si(6)Cl(14)](2-) dianionic complexes is discussed. The reaction of Si(6)Cl(12) with a Lewis base (e.g., Cl(-)) to give planar [Si(6)Cl(14)](2-) dianionic complexes presents an experimental proof that suppression of the PJT effect is an effective strategy in restoring high Si(6) ring symmetry. Additionally, the proposed pathway for the PJT suppression has been proved by the synthesis and characterization of novel compounds containing planar Si(6) ring, namely, [(n)Bu(4)N](2)[Si(6)Cl(12)I(2)], [(n)Bu(4)N](2)[Si(6)Br(14)], and [(n)Bu(4)N](2)[Si(6)Br(12)I(2)]. This work represents the first demonstration that PJT effect suppression is useful in the rational design of materials with novel properties.

Coordination chemistry of Si5Cl10 with organocyanides.

Organocyanides readily coordinate to decachlorocyclopentasilane (Si(5)Cl(10)) to form "inverse sandwich" compounds 1-3 with a planar Si(5) ring. The products were isolated in high yield and fully characterized by elemental analysis, multinuclear NMR, IR and UV-Vis spectroscopy. While the spectroscopic data suggests the presence of a fairly weak interaction between the Si(5) ring and the coordinative organocyanide ligands, single-crystal X-ray diffraction studies of compound 1 and 2 show µ(5)-coordination of the apical cyano nitrogen atoms to the silicon atoms in the Si(5) ring. Distances between silicon atoms and nitrogen atoms are significantly shorter than a Si-N van der Waals bond but longer than the sum of their covalent radii. Multiple interactions between the cyano groups and equatorial Cl atoms, and intermolecular interactions were observed in the solid state for both compounds 1 and 2.

Combinatorial materials research applied to the development of new surface coatings IX: an investigation of novel antifouling/fouling-release coatings containing quaternary ammonium salt groups.

Polysiloxane coatings containing chemically-bound ("tethered") quaternary ammonium salt (QAS) moieties were investigated for potential application as environmental-friendly coatings to control marine biofouling. A combinatorial/high-throughput approach was applied to the investigation to enable multiple variables to be probed simultaneously and efficiently. The variables investigated for the moisture-curable coatings included QAS composition, ie alkyl chain length, and concentration as well as silanol-terminated polysiloxane molecular weight. A total of 75 compositionally unique coatings were prepared and characterized using surface characterization techniques and biological assays. Biological assays were based on two different marine microorganisms, a bacterium, Cellulophaga lytica and a diatom, Navicula incerta, as well as a macrofouling alga, Ulva. The results of the study showed that all three variables influenced coating surface properties as well as antifouling (AF) and fouling-release (FR) characteristics. The incorporation of QAS moieties into a polysiloxane matrix generally resulted in an increase in coating surface hydrophobicity. Characterization of coating surface morphology revealed a heterogeneous, two-phase morphology for many of the coatings investigated. A correlation was found between water contact angle and coating surface roughness, with the contact angle increasing with increasing surface roughness. Coatings based on the QAS moiety containing the longest alkyl chain (18 carbons) displayed the highest micro-roughness and, thus, the most hydrophobic surfaces. With regard to AF and FR properties, coatings based on the 18 carbon QAS moieties were very effective at inhibiting C. lytica biofilm formation and enabling easy removal of Ulva sporelings (young plants) while coatings based on the 14 carbon QAS moities were very effective at inhibiting biofilm growth of N. incerta.

Combinatorial materials research applied to the development of new surface coatings I: a multiwell plate screening method for the high-throughput assessment of bacterial biofilm retention on surfaces.

Combinatorial, high-throughput capabilities have been established to aid in the rapid development of new and effective antifouling marine coatings for naval applications. A biological screening process involving marine bacteria was developed that allows for rapid and effective quantification of bacterial biofilm growth and retention on large numbers of coating surfaces in parallel. The screening process involves (1) multiwell plate modifications for coating deposition, (2) deposition of combinatorial coating libraries via an automated liquid dispensing robot, (3) coating thickness measurements of cured coatings, (4) preconditioning of coatings via immersion in deionized water, (5) bacterial incubation, (6) plate processing, and (7) data analysis for identification of promising candidates. The details of the method developed are described in this document.

Silicones containing pendant biocides for antifouling coatings.

The preparation of biocide-incorporated silicone coatings for antifouling/fouling release applications is described. The biocide Triclosan (5-chloro-2-(2, 4-dichlorophenoxy) phenol) was modified with alkenyl moieties and incorporated into a silicone backbone through covalent bonds. The presence of the biocide on the coating surface was expected to deter fouling organisms from attaching to the surface of the coating. Allyl glycidyl ether was used to provide crosslink functionalities. Resins were cured using vinyl-terminated polydimethylsiloxane for hydrosilyl functionality and 1, 3-cyclohexane-bis (methylamine) for epoxy crosslinking functionality. Coatings were characterized by static water contact angle measurements and dynamic mechanical thermal analysis. Synthetic control over the incorporation of crosslink functionalities within the polymer resin allowed tuning of the surface of the coating and of mechanical properties. Resistance to macrofouling was tested by static immersion tests in the Indian River Lagoon at the Florida Institute of Technology from 15 October 2003 to 13 November 2003. Preliminary results showed that the coatings prepared from biocide-incorporated silicones with the appropriate bulk modulus significantly reduced macrofouling.

Tris(benzylthiolato)bismuth. Efficient Precursor to Phase-Pure Polycrystalline Bi(2)S(3).

Details of the synthesis, physical and spectroscopic characterization, and thermal decomposition of tris(benzylthiolato)bismuth, (BnS)(3)Bi, Bn = CH(2)C(6)H(5), are presented. Results from pyrolysis of (BnS)(3)Bi demonstrate that this compound is a convenient precursor to phase-pure, polycrystalline Bi(2)S(3) with low carbon and hydrogen contamination under mild thermal conditions (ca. 275 degrees C). Flow-tube pyrolysis produces small ( approximately 1 &mgr;m) spherical particles, whereas sealed-tube pyrolysis produces 6-&mgr;m diameter spherical particles composed of radiating acicular crystallites. Bi(2)S(3) was characterized by X-ray powder diffraction and scanning electron microscopy.