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Chirality in amorphous and crystalline materials - experimental aspects David Avnir

Chirality in amorphous and crystalline materials - experimental aspects David Avnir Institute of Chemistry, The Hebrew University Summer School on Chirality Mainz, August, 15-17, 2011, sponsored by. Main general questions to be addressed:.

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Chirality in amorphous and crystalline materials - experimental aspects David Avnir

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  1. Chirality in amorphous and crystalline materials - experimental aspects David Avnir Institute of Chemistry, The Hebrew University Summer School on Chirality Mainz, August, 15-17, 2011, sponsored by

  2. Main general questions to be addressed: #How is it possible to induce chirality in a material? # How is it possible to extract chiral activity from a material? Our main road: SiO2-based amorphous materials and crystalline metals

  3. Amorphous silica

  4. How is it possible to induce chirality in a material? The classical approach: Attach covalently a chiral molecule to the surface of the (porous) material Often, a silylating reaction

  5. Photophysical Recognition of Chiral Surfaces The excited chiral surface: Silica derivatized with R- or S-BNP The quencher: DMP, R-Q or S-Q • With • E. Wellner • M. Ottolenghi • J. Am. Chem. Soc., 111, 2001 (1989)

  6. Stern-Volmer quenching analysis For the R-surface (shown): S-Q/R-Q = 1.3 For the S-surface: R-Q/S-Q = 1.2 The S-quencher recognizes better the R-surface

  7. The second, newer approach Dope the material with a chiral molecule

  8. DOPING OF SILICA IS MADE POSSIBLE • BY THE SOL-GEL POLYCONDENSATION • Si(OCH3)4 + H2O (SiOmHn)p + CH3OH (unbalanced) • Variations on this theme: • the metals, semi-metals and their combinations • the hydrolizable substituent • the use of non-polymerizable substituents • organic co-polymerizations (Ormosils) • non-hydrolytic polymerizations H+ or OH-

  9. Organic functionalization by physical entrapment of molecules within sol-gel matrices Monomers, oligomers Sol Sol Gel Gel Xerogel Xerogel • * Small molecules • * Polymers • * Proteins • * Nanoparticles monomer oligomer sol - - particle entrapped molecule

  10. Doping the material with a chiral molecule: # A chiral catalyst # A protein # A chiral surfactant

  11. Entrapment of a chiral catalyst ee = 78% (BPPM) The advantages # Covalent bonding chemistry is not needed # Working with a hydrophobic catalyst in water # Recyclability With F. Gelman J. Blum J. Molec. Catal., A: Chem., 146, 123 (1999)

  12. Doping the material with a chiral surfactant (1R,2S)-(-)-N-dodecyl-N -methylephedrinium bromide (DMB)

  13. The experiment: Inducing Circular Dichroism in Congo-Red Within Silica Sol The chiral inducer: DMB The achiral probe: CR

  14. CR-DMB in solution (blue line) and CR solution (red line) The ICD spectra of co-entrapped CR-DMB in hydrophilic and hydrophobic silica sols CR-DMB@SG sol (red line) and CR-DMB@OSG sol (blue line) Has the silica matrix become chiral? S. Fireman

  15. Second experiment with doped surfactant: NMR detection of diastereomeric interactions within phenylated-silica sols and gels S-BINAP 1R,2S-DMB The possible interactions: DMB/S-BINAP DMB/R-BINAP With S. Fireman S. Marx

  16. 5.94 5.99 S-BINAP R-BINAP In solution ppm ppm 6.1 6.0 5.9 5.8 In the sol 5.98 6.00 S-BINAP interacts better with the chiral surfactant ppm ppm 6.1 6.0 5.9 5.8 6.146 6.132 In the gel ppm 6.2 6.1 6.0 5.9 5.8 31P-NMR spectrum of BINAP-DMB diastereomers: Looking inside the sol and the gel of silica

  17. What have we seen so far? # Covalent attachment of a chiral molecule # Physical entrapment of a chiral dopant Is it possible to induce structural chirality in a material? Make a hole which is chiral - imprint the material; make a chiral silicate skeleton Dickey, 50’s

  18. General methodology for chiral imprinting of sol-gel based thin-films With S. Marx S. Fireman

  19. Silica thin-film chiral imprinting Where is “Smart porosity” needed? for evaluating ee, for chiral separations, for selective sensing, for chiral catalysis

  20. TMOS PTMOS MTMOS Two different cases: I. Selectivity towards an enantiomer of the imprinting molecule Propranolol The functional monomers Film thickness: 700 nm Chem. Mater. ,15, 3607 (2003)

  21. R B S S R S B The enantioselectivity adsorption experiment } Imprinted films Adsorbed molecules are leached out S R Immersed in solutions of R or S, for adsorption, and radio-assay; or: Fluorescence measurement Fluorescence: (lex = 288nm; lem= 335 nm) Radio ligand binding of 3H-S-Propranolol

  22. Enantioselectivity towards Propranolol enantiomers

  23. Electrochemical detection of enantioselectivity and molecular selectivity in very thin silica films L-Dopa D-Dopa 70 nm films Current (mA) Current / mA

  24. The more general case: Enantioselectivity towards enantiomers of non-imprinting molecules Why is that important? Because a small, recyclable chiral imprinting molecules can be used and reused S. Fireman S.Marx

  25. Silica imprinted with aggregates of DMB Was capable of separating the enantiomer-pairs of: BINAP Propranolol Naproxen

  26. R Discrimination Ratio R General enantioselectivity in imprinted thin films J. Am. Chem. Soc. 127, 2650 (2005) 20% phenylated silica, 270nm

  27. R R S S R R General enantioselectivity in granules: Comparison of two methods of inducing chirality Before extraction: Chiral dopant (DMB) After extraction: Chiral holes The recognition handedness changes!

  28. Next: If an SiO2 material is made chiral by a foreign molecule which either remains there or not, then: #How are the building blocks of the material affected? #Is it possible that an SiO4 tetrahedron which is neighboring to the chiral event, becomes chiral itself? #Is it possible that the material becomes chiral deeper inside?

  29. Nature has already provided an answer -Yes, it is possible! Quartz

  30. 31 Right Helix 32 Left Helix A:P3121 & B:P3221 SiO4

  31. Silica is composed of randomly distorted SiO4 tetrahedra. Therefore: 1.Each of the chiral SiO4 tetrahedra is a single enantiomer event. # A statistically similar counter enantiomer maybe defined. 2. Silica is a racemic mixture of chiral SiO4 tetrahedra: # Half comprise a homochiral left-handed set, and half a right-handed set. # This is true for ANY handedness definition; but each definition will divide the set differently into two equal halves.

  32. 3. Induction of chirality by any of the methods, will enrich the chiral population of SiO4 tetrahedra with one type of handedness.

  33. The ICD signal of CR adsorbed on DMB@silica Co-doping:CR/DMB@silica Reversal of the ICD signal indicates that the chirality-inducer is different in the two cases. CR adsorbed on DMB@silica The only possibility is chiral skeletal porosity induced by the doped DMB

  34. Inducing chirality in metals

  35. Motivation: Why should one dope metals with organic molecules? * Hybrid materials of metals and organics have been unknown * Most elements are metals * Metals are everywhere – any new methodology of affecting their properties is interesting * The library of organic compounds is huge; the number of metals is small * Placing a molecule in a sea of electrons may affect its properties; and the properties of the metal * Synergetic effects between the metal and the dopant may emerge

  36. Doping through metal synthesis Reducing aqueous solution Synthetic methods: Reduction in the presence of the dopant Aggregation and entrapment Reduction AgNO3 Ag metal Dopant Reducing agent 2AgNO3 + NaH2PO2 + H2O 2Ag + NaH2PO3 + 2HNO3 Hanna Behar-Levy et al, Chem. Mater., 14, 1736 (2002)

  37. CR@Ag 1:100 molar Ag Congo-Red

  38. Scope: The metals Magnetic metal Coin metals Noble metals Alloys: Cu-Pd, Cu-Pt, Au-Ag

  39. Scope: The dopants Small molecules, hydrophilic or hydrophobic: Sudan III Biologicals:D-Tryptophan Complexes: [Rh] Inorganic compounds: H3[P(Mo3O10)4] Polymers, hydrophobic or hydrophilic: Polyacrylonitrile Proteins:Alkaline phosphatase Nanoparticles: Carbon nanofibers

  40. Nafion@Ag PSSA@Au CR@Co CR@Cu The New Materials

  41. Scope: The entrapment range 0.2% (doped metals) - 10% by weight (hybrid materials) For instance for PSSA@Ag: Molar ratio - PSSA-monomer units : Ag = 1:250 Weight ratio - 0.42 carbon w/w% Atomic molar ratio - C : Ag = 1:30

  42. Hierarchical structure: PSSA@Ag H. Behar-Levy, G. Shter, G. Grader, Chem. Mater., 16, 3197 (2004)

  43. Rhodium-complex@silver First taken after a few seconds

  44. compression DMSO Thionin@Ag - Powder Thionin@Ag - Coin Thionin@Ag No extraction with water, although water is a solvent of the dye

  45. Adsorption on Adsorption on Entrapment in Ag commercial Ag Ag 1% 1% 100% Adsorbed Doped Adsorption of CR compared to entrapment

  46. Starting solution: 6.2x10-4 M Supernatant after entrapment:3.5x10-7 M Thionin@Cu-Pt: Entrapment vs adsorption Adsorption: 4% Y. Ben-Efraim

  47. Dopant@metal - the picture of the entrapment * Aggregated crystallite metal system * Porous material * The dopant is tightly entrapped in narrow pores and cages * The molecules are entrapped intact * Adsorption and entrapment are different processes

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