In situ single‐crystal diffraction experiments provide researchers with the opportunity to study the response of crystalline systems, including metal–organic frameworks and other nanoporous ...materials, to changing local microenvironments. This paper reports a new environmental control cell that is remarkably easy to use, completely reusable, and capable of delivering static or dynamic vacuum, liquids or gases to a single‐crystal sample. Furthermore the device is nearly identical in size to standard single‐crystal mounts so a full unrestricted range of motion is expected for most commercial goniometers. In situ single‐crystal X‐ray diffraction experiments performed under dynamic gas‐flow conditions revealed the cell was capable of stabilizing a novel metastable intermediate in the dehydration reaction of a previously reported metal–organic framework.
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We have synthesized a new class of highly efficient nonlinear optical (NLO) chromophores based on the novel tricyanopyrroline (TCP) electron acceptor. Molecular linear and nonlinear optical ...properties of the prototypical chromophores were measured and calculated to understand structure−property relationships. One such chromophore showed molecular first hyperpolarizability (β) of (8700 ± 702) × 10-30 esu at excitation wavelengths of 1.9 μm, and another showed macroscopic electro-optic (EO) coefficients (r 33) of 51 pm/V at 1.55 μm with 20 wt % chromophore loading when poled with 65 V/μm in amorphous polycarbonate film. Synthetic strategies for extending the utility of the acceptor and chromophores are discussed.
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In the title 1:2 co-crystal, C
H
N
·(C
H
O
·C
H
O
)
, l-ascorbic acid (LAA) and 4,4'-bi-pyridine (BPy) co-crystallize in the chiral space group
2
with two mol-ecules of LAA, and one mol-ecule of bpy ...in the asymmetric unit. The structure was modeled in two parts due to possible proton transfer from LAA to the corresponding side of the bpy mol-ecule having an occupancy of approximately 0.25 and part 2 with an occupancy of approximately 0.75. In this structure, LAA forms hydrogen bonds with neighboring LAA mol-ecules, forming extended sheets of LAA mol-ecules which are bridged by bpy mol-ecules. A comparison to a related and previously published co-crystal of LAA and 3-bromo-4-pyridone is presented.
Mesoscale Chiroptics of Rhythmic Precipitates Gunn, Erica; Sours, Ryan; Benedict, Jason B. ...
Journal of the American Chemical Society,
11/2006, Volume:
128, Issue:
44
Journal Article
Peer reviewed
Rhythmic precipitates of centrosymmetric phthalic acid were analyzed by a square-wave mechanically modulated circular extinction imaging microscope. Spherulites were bisected into square-millimeter ...sized heterochiral domains that are a consequence of circular intensity differential scattering of left and right circularly polarized light. The dissymmetry and chiral amplification indicated optically was confirmed in the microtexture established by atomic force and scanning electron microscopies.
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Superoxide reductases (SORs) are cysteine-ligated, non-heme iron enzymes that reduce toxic superoxide radicals (O2 -). The functional role of the trans cysteinate, as well as the mechanism by which ...SOR reduces O2 -, is unknown. Herein is described a rare example of a functional metalloenzyme analogue, which catalytically reduces superoxide in a proton-dependent mechanism, via a trans thiolate-ligated iron-peroxo intermediate, the first example of its type. Acetic-acid-promoted H2O2 release, followed by Cp2Co reduction, regenerates the active Fe(II) catalyst. The thiolate ligand and its trans positioning relative to the substrate are shown to contribute significantly to the catalyst's function, by lowering the redox potential, changing the spin state, and dramatically lowering the ν Fe - O stretching frequency well-below that of any other reported iron-peroxo, while leaving ν O - O high, so as to favor superoxide reduction and Fe−O, as opposed to O−O, bond cleavage. Thus we provide critical insight into the relationship between the SOR structure and its function, as well as important benchmark parameters for characterizing highly unstable thiolate-ligated iron-peroxo intermediates.
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The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti17(μ4-O)4(μ3-O)16(μ2-O)4(OPr i )20 (in short Ti17) was studied using femtosecond ...pump–probe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17 •– , are blue-shifted in comparison with bulk TiO2 and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17 * and Ti17 •– indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the ∼1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to play a special role in this regard. Solvent polarity has only a minor influence on the spectral behavior of Ti17 *. Exciton recombination in Ti17 is faster than in anatase nanoparticles or mesoporous films. The kinetics exhibits three components, ranging from less than 1 ps to 100 ps, which are tentatively assigned to the geminate recombination within the core of the cluster and to the decay of the surface stabilized charge transfer exciton. A persistent long-lived component with τ > 300 ps may indicate the involvement of intraband dark states, i.e., triplet excitons 3 Ti17 *.
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The title compound systematic name: benzoic acid-pyrrolidin-1-ium-2-carboxyl-ate (1/1), C
H
O
·C
H
NO
, is an example of the application of non-centrosymmetric co-crystallization for the growth of a ...crystal containing a typically centrosymmetric component in a chiral space group. It co-crystallizes in the space group
2
2
2
and contains benzoic acid and l-proline in equal proportions. The crystal structure exhibits chains of l-proline zwitterions capped by benzoic acid mol-ecules which form a
(5)
(11) hydrogen-bonded network along 100. The crystal structure is examined and compared to that of a similar co-crystal containing l-proline zwitterions and 4-amino-benzoic acid.
A pair of organoplatinum(II) metallacycles, M1 and M2, was self-assembled by combining one of two donor molecules, cis-Pt(dhim)
2
(C≡CC
5
H
4
N)
2
(D1; dhim = 1,3-dihexyl-2-H-imidazole-2-yelidene) ...and Pt(tbbpy)(C≡CC
5
H
4
N)
2
(D2; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), with an acceptor precursor, Pt(tbbpy)(OTf)
2
(A1; OTf = CF
3
SO
3
−
), respectively. Both donor molecules exhibit an idealized 90° angle between the coordination vectors of their ethynylpyridine moieties. When mixed with A1, coordination-driven self-assembly occurs at 50 °C (4 days in acetonitrile for M1 and 18 h in CH
2
Cl
2
for M2). Both metallacycles were characterized by
1
H NMR, FT-IR, and FT-ICR-MS techniques that support D
2
A
2
self-assembly of molecular squares. The structure of building block D1 was determined by X-ray diffraction, confirming the expected square coordination geometry and 90° orientation of the pyridyl coordination vectors. Photophysical studies of M1 and M2 reveal that the metallacycles display triplet emission bands at 466 and 469 nm, respectively, that originate from transitions localized on building blocks D1 and D2. This phosphorescent behavior is assigned to
3
ILCT and
3
LLCT (π
C≡C
* → π
C≡C
; ILCT = intraligand charge transfer, LLCT = ligand-to-ligand charge transfer) transitions based on previous studies of phenyl analogs to D1 and D2 that indicate that the ethynyl moieties dominate in their contributions to the molecular orbitals involved in absorption and emission.
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d-Sorbitol forms so-called spherulites from under-cooled melts. These polycrystalline formations have optically uniaxial radii. Melts pressed between glasses crystallize as plane sections of spheres. ...Dyes that are soluble in molten sorbitol become oriented as the crystallization front passes through the melt so as to form disks with large linear dichroism in the absorption bands of the dyes. The dyeing of spherulites is thus a general method of solute alignment. The linear optical properties of sorbitol spherulites containing the azo dye amaranth were analyzed in detail so as to correct a persistent confusion in the literature regarding the orientational dependence of linear dichroism. In cases involving thin film dichroism of multilayered samples requiring many corrections of intensity data in non-normal incidence, some authors have taken transmittance and others absorbance as having a cosine-squared angular dependence on the plane of the electric vector of linearly polarized light. Plane sections of doped spherulites present all orientations of an electric dipole oscillator in spatially localized region in normal incidence. As such, the samples described herein are ideally suited to resolving this confusion. Images of transmittance of dyed spherulites in polarized light were recorded with a CCD camera and simulated under the assumption that both absorbance and transmittance show a cosine-squared angular dependence but with respect to different angles. Transmittance with a cosine-squared dependence follows azimuthal rotations of the spherulite radii around the wave vector, while absorbance with a cosine-squared dependence follows rotations about axes perpendicular to the wave vector, natural consequences of the properties of the optical indicatrix that are often overlooked. Spherulites obviate the substantial experimental complexities that are engendered in non-normal incidence by sample reorientation. Thus, the principles of anisotropic absorption are given in a complete and intuitive fashion.
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In the title 1:2 co-crystal, C
10
H
9
N
2
+
·(C
6
H
7.75
O
6
·C
6
H
7.25
O
6
)
−
, L-ascorbic acid (LAA) and 4,4′-bipyridine (BPy) co-crystallize in the chiral space group
P
2
1
with two molecules of ...LAA, and one molecule of bpy in the asymmetric unit. The structure was modeled in two parts due to possible proton transfer from LAA to the corresponding side of the bpy molecule having an occupancy of approximately 0.25 and part 2 with an occupancy of approximately 0.75. In this structure, LAA forms hydrogen bonds with neighboring LAA molecules, forming extended sheets of LAA molecules which are bridged by bpy molecules. A comparison to a related and previously published co-crystal of LAA and 3-bromo-4-pyridone is presented.
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