Terahertz time domain spectroscopy (THz TDS) is used to measure the melting kinetics of fructose molecular crystals. Combining single-crystal anisotropy measurements with density functional ...calculations, we assign the phonon frequencies and interrogate how specific phonons behave with melting. While nearly all the low-frequency phonons continuously red-shift with heating and melting, the lowest-energy phonon polarized along the c-axis blue-shifts at the melting temperature, suggesting an initial structural change immediately before melting. We find that the kinetics follow a 3D growth model with large activation energies, consistent with previous differential scanning calorimetry (DSC) measurements. The large activation energies indicate that multiple H-bonds must break collectively for the transition. The results suggest the generality of the kinetics for molecular crystals and that THz TDS with picosecond resolution could be used to measure ultrafast kinetics.
Extreme red shifted SERS nanotags Bedics, Matthew A; Kearns, Hayleigh; Cox, Jordan M ...
Chemical science (Cambridge),
04/2015, Volume:
6, Issue:
4
Journal Article
Peer reviewed
Open access
Surfaced enhanced Raman scattering (SERS) nanotags operating with 1280 nm excitation were constructed from reporter molecules selected from a library of 14 chalcogenopyrylium dyes containing phenyl, ...2-thienyl, and 2-selenophenyl substituents and a surface of hollow gold nanoshells (HGNs). These 1280 SERS nanotags are unique as they have multiple chalcogen atoms available which allow them to adsorb strongly onto the gold surface of the HGN thus producing exceptional SERS signals at this long excitation wavelength. Picomolar limits of detection (LOD) were observed and individual reporters of the library were identified by principal component analysis and classified according to their unique structure and SERS spectra.
We find that crystallographically resolved Ti17O24(OPr i )20 nanoparticles, functionalized by covalent attachment of 4-nitrophenyl-acetylacetonate or coumarin 343 adsorbates, exhibit hole injection ...into surface states when photoexcited with visible light (λ = 400–680 nm). Our findings are supported by photoelectrochemical measurements, EPR spectroscopy, and quantum dynamics simulations of interfacial charge transfer. The underlying mechanism is consistent with measurements of photocathodic currents generated with visible light for thin layers of functionalized polyoxotitanate nanocrystals deposited on FTO working electrodes. The reported experimental and theoretical analysis demonstrates for the first time the feasibility of p-type sensitization of TiO2 solely based on covalent binding of organic adsorbates.
Fast hole hopping in Ti17cat4, a 1 nm diameter molecular polyoxotitanate cluster bearing four catechol ligands (Ti17(μ4-O)4(μ3-O)16(μ2-O)4(cat)4(OPr i )16), was investigated by ultrafast spectroscopy ...and quantum dynamics simulations. The catechol moieties coupled to the TiO2 core of the cluster give rise to a charge-transfer band, the excitation of which promotes an electron from the highest occupied molecular orbital of the ligand to the inorganic core, resulting in the formation of {cat+•,Ti3+}, a vibrationally hot polaronic exciton. Dynamic depolarization measurements indicate that within less than 100 fs the Franck–Condon polaronic state formed at the interface evolves into a fully charge-separated state and the injected electron delocalizes over the quasi conduction band of the cluster. The positive charge (hole) resulting from the injection does not remain static either. The initial hole hopping between the catechol sites occurs with the rate of ∼5 × 1011 s–1 or more and competes with the intramolecular vibrational relaxation. Upon thermalization, the hopping slows and continues at a rate of ∼5 × 1010 s–1. The experimentally observed rate of hole hopping agrees well with the results of quantum dynamics modeling of the wavepacket propagation.
The solution chemistry and solid-state structures of the Co(II), Fe(II), and Ni(II) complexes of 7,13-bis(carbamoylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L) are reported as members of a ...new class of paramagnetic chemical exchange saturation transfer (paraCEST) MRI contrast agents that contain transition metal ions. Crystallographic data show that nitrogen and oxygen donor atoms of the macrocyclic ligand coordinate to the metal ions to generate complexes with distorted pentagonal bipyramidal geometry for Co(L)Cl2·2H2O or Fe(L)(CF3SO3)2. The Ni(II) complex Ni(L)(CF3SO3)2·H2O features a hexadentate ligand in a distorted octahedral geometry. The proton NMR spectra of all three complexes show highly dispersed and relatively sharp proton resonances. The complexes were further characterized by monitoring their dissociation under biologically relevant conditions including solutions containing phosphate and carbonate, ZnCl2, or acidic conditions. Solutions of the paraCEST agents in 20 mM N-(2-hydroxyethyl)piperazine-N'-ethanesulfonic acid (pH 7.4) and 100 mM NaCl showed highly shifted and intense CEST peaks at 59, 72, and 92 ppm away from bulk water for Co(L)(2+), Ni(L)(2+), and Fe(L)(2+), respectively at 37 °C on a 11.7 T NMR spectrometer. CEST spectra with corresponding rate constants for proton exchange are reported in 4% agarose gel (w/w), rabbit serum, egg white, or buffered solutions. CEST phantoms of 4 mM complex in buffer, 4% agarose gel (w/w), or rabbit serum on a 4.7 T MRI scanner at 37 °C, are compared. The most substantial change was observed for the reactive Ni(L)(2+), which showed reduced CEST contrast in rabbit serum and egg white. The complexes with the least highly shifted CEST peaks (Co(L)(2+) and Ni(L)(2+)) showed a reduction in CEST contrast in 4% agarose gel (w/w) compared to that in buffered solutions, while the CEST effect for Fe(L)(2+) in 4% agarose gel (w/w) was not substantially different.
Fe(II) , Co(II) and Ni(II) complexes of two tetraazamacrocycles (1,4,8,11-tetrakis(carbamoylmethyl)-1,4,8,11-tetraazacyclotetradecane (L1) and ...1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (L2) show promise as paraCEST agents for registration of temperature (paraCEST=paramagnetic chemical exchange saturation transfer). The Fe(II) , Co(II) and Ni(II) complexes of L1 show up to four CEST peaks shifted ≤112 ppm, whereas analogous complexes of L2 show only a single CEST peak at ≤69 ppm. Comparison of the temperature coefficients (CT ) of the CEST peaks of Co(L2)(2+) , Fe(L2)(2+) , Ni(L1)(2+) and Co(L1)(2+) showed that a CEST peak of Co(L1)(2+) gave the largest CT (-0.66 ppm (o) C(-1) at 4.7 T). NMR spectral and CEST properties of these complexes correspond to coordination complex symmetry as shown by structural data. The Ni(L1)(2+) and Co(L1)(2+) complexes have a six-coordinate metal ion bound to the 1-, 4-amide oxygen atoms and four nitrogen atoms of the tetraazamacrocycle. The Fe(L2)(2+) complex has an unusual eight-coordinate Fe(II) bound to four amide oxygen atoms and four macrocyclic nitrogen atoms. For Co(L2)(2+) , one structure has seven-coordinate Co(II) with three bound amide pendents and a second structure has a six-coordinate Co(II) with two bound amide pendents.
Crystallization of nicotine, an oil prone to degradation at room temperature, has been demonstrated to be an effective means of creating nicotine-based materials with tunable thermal properties and ...improved resistance to photo-induced degradation. Herein, we show that both isomers of enantiomerically pure tartaric acid are highly effective salt formers when combined with nicotine. Both salts exhibit enhanced photostability, and with a melting point of 143.1 °C, the salt prepared using d-(−)-tartaric acid possesses one of the highest melting points for a crystalline nicotine solid reported to date.
Interfacial electron transfer (IET) between a chromophore and a semiconductor nanoparticle is one of the key processes in a dye-sensitized solar cell. Theoretical simulations of the electron transfer ...in polyoxotitanate nanoclusters Ti17O24(OPri)20 (Ti 17 ) functionalized with four p-nitrophenyl acetylacetone (NPA-H) adsorbates, of which the atomic structure has been fully established by X-ray diffraction measurements, are presented. Complementary experimental information showing IET has been obtained by EPR spectroscopy. Evolution of the time-dependent photoexcited electron during the initial 5 fs after instantaneous excitation to the NPA LUMO + 1 has been evaluated. Evidence for delocalization of the excitation over multiple chromophores after excitation to the NPA LUMO + 2 state on a 15 fs time scale is also obtained. While chromophores are generally considered electronically isolated with respect to neighboring sensitizers, our calculations show that this is not necessarily the case. The present work is the most comprehensive study to date of a sensitized semiconductor nanoparticle in which the structure of the surface and the mode of molecular adsorption are precisely defined.
Organic photochromic molecules including diarylethenes are of particular interest for their numerous potential applications including high-density optical data storage and light-activated switches. ...In this report, we examined the temperature dependence of the light-drive photocyclization reaction in a classic diarylethene. The steady-state populations were monitored spectroscopically and by temperature dependent in situ photocrystallography, the latter being the first reported example of this technique. The observed decrease in the steady-state population with decreasing temperature suggests this classic diarylethene possesses an excited-state potential energy surface topology similar to previously reported “inverted” diarylethenes.
PDF
