Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies.

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH2)3]Mn(NCS)3.

We report the magnetic structure and properties of a thiocyanate-based honeycomb magnet [Na(OH2)3]Mn(NCS)3 which crystallises in the unusual low-symmetry trigonal space group P3̄. Magnetic measurements on powder samples show this material is an antiferromagnet (ordering temperature TN,mag = 18.1(6) K) and can be described by nearest neighbour antiferromagnetic interactions J = -11.07(4) K. A method for growing neutron-diffraction sized single crystals (>10 mm3) is demonstrated. Low temperature neutron single crystal diffraction shows that the compound adopts the collinear antiferromagnetic structure with TN,neut = 18.94(7) K, magnetic space group P3̄'. Low temperature second-harmonic generation (SHG) measurements provide no evidence of breaking of the centre of symmetry.

Direct Detection of Hydrogen Bonds in Supramolecular Systems Using 1H-15N Heteronuclear Multiple Quantum Coherence Spectroscopy.

Hydrogen-bonded supramolecular systems are usually characterized in solution through analysis of NMR data such as complexation-induced shifts and nuclear Overhauser effects (nOe). Routine direct detection of hydrogen bonding particularly in multicomponent mixtures, even with the aid of 2D NMR experiments for full assignment, is more challenging. We describe an elementary rapid 1H-15N HMQC NMR experiment which addresses these challenges without the need for complex pulse sequences. Under readily accessible conditions (243/263 K, 50 mM solutions) and natural 15N abundance, unambiguous assignment of 15N resonances facilitates direct detection of intra- and intermolecular hydrogen bonds in mechanically interlocked structures and quadruply hydrogen-bonded dimers─of dialkylaminoureidopyrimidinones, ureidopyrimidinones, and diamidonaphthyridines─in single or multicomponent mixtures to establish tautomeric configuration, conformation, and, to resolve self-sorted speciation.

Photoconductive arrays on insulating substrates for high-field terahertz generation.

We report on the design, fabrication and characterisation of large-area photoconductive THz array structures, consisting of a thin LT-GaAs active region transferred to an insulating substrate using a wafer-scale bonding process. The electrically insulating, transparent substrate reduces the parasitic currents in the devices, allowing peak THz-fields as high as 120 kV cm-1 to be generated over a bandwidth >5 THz. These results are achieved using lower pulse energies than demanded by conventional photoconductive arrays and other popular methods of generating high-field THz radiation. Two device sizes are fully characterised and the emission properties are compared to generation by optical rectification in ZnTe. The device can be operated in an optically saturated regime in order to suppress laser noise.

On-Chip Terahertz-Frequency Measurements of Liquids.

Terahertz-frequency-range measurements can offer potential insight into the picosecond dynamics, and therefore function, of many chemical systems. There is a need to develop technologies capable of performing such measurements in aqueous and polar environments, particularly when it is necessary to maintain the full functionality of biological samples. In this study, we present a proof-of-concept technology comprising an on-chip planar Goubau line, integrated with a microfluidic channel, which is capable of low-loss, terahertz-frequency-range spectroscopic measurements of liquids. We also introduce a mathematical model that accounts for changes in the electric field distribution around the waveguide, allowing accurate, frequency-dependent liquid parameters to be extracted. We demonstrate the sensitivity of this technique by measuring a homologous alcohol series across the 0.1-0.8 THz frequency range.

PDielec: The calculation of infrared and terahertz absorption for powdered crystals.

The Python package PDielec is described, which calculates the infrared absorption characteristics of a crystalline material supported in a non-absorbing medium. PDielec post processes solid-state quantum mechanical and molecular mechanical calculations of the phonons and dielectric response of the crystalline material. Using an effective medium method, the package calculates the internal electric field arising from different particle morphologies and calculates the resulting shift in absorption frequency and intensity arising from the coupling between a phonon and the internal field. The theory of the approach is described, followed by a description of the implementation within PDielec. Finally, a section providing several examples of its application is given. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter.

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

Laser Feedback Interferometry as a Tool for Analysis of Granular Materials at Terahertz Frequencies: Towards Imaging and Identification of Plastic Explosives.

We propose a self-consistent method for the analysis of granular materials at terahertz (THz) frequencies using a quantum cascade laser. The method is designed for signals acquired from a laser feedback interferometer, and applied to non-contact reflection-mode sensing. Our technique is demonstrated using three plastic explosives, achieving good agreement with reference measurements obtained by THz time-domain spectroscopy in transmission geometry. The technique described in this study is readily scalable: replacing a single laser with a small laser array, with individual lasers operating at different frequencies will enable unambiguous identification of select materials. This paves the way towards non-contact, reflection-mode analysis and identification of granular materials at THz frequencies using quantum cascade lasers.

2D Time-Domain Spectroscopy for Determination of Energy and Momentum Relaxation Rates of Hydrogen-Like Donor States in Germanium.

We present measurements of the coherence times of excited states of hydrogen-like arsenic impurities in germanium (Ge:As) using a table-top two-dimensional time-domain spectroscopy (2D-TDS) system. We show that this laboratory system is capable of resolving the coherence lifetimes of atomic-like excited levels of impurity centers in semiconductors, such as those used in solid-state quantum information technologies, on a subpicosecond time scale. By fitting the coherent nonlinear response of the system with the known intracenter transition frequencies, we are able to monitor coherent population transfer and decay of the transitions from the 2p0 and 2p± states for different low excitation pulse fields. Furthermore, by examining the off-diagonal resonances in the 2D frequency-domain map, we are able to identify coherences between excited electronic states that are not visible via conventional single-frequency pump-probe or Hahn-echo measurements.

Effect of molecular size and particle shape on the terahertz absorption of a homologous series of tetraalkylammonium salts.

The absorption coefficient and refractive index have been measured for a homologous series of tetraalkylammonium bromides over the frequency range 0.3-5.5 THz. Spectral features are found to shift to lower frequencies as the molecular mass is increased, as expected. However, to understand the detailed structure of the observed spectral features, density functional perturbation theory calculations have been performed on the first four crystalline compounds in the series. From these calculations, we find that each spectrum is dominated by three translatory modes involving asymmetric motion of the ammonium cation and bromine counterion, although the overall number of active modes increases with increasing molecular size. The experimentally observed absorption is not completely described by the infrared active phonon modes alone. We show that it is also necessary to include the coupling of the phonon modes with the macroscopic field generated by the collective displacement of the vibrating ions, and we have applied an effective medium theory, which accounts for particle shape to allow for this effect in the calculation of the terahertz spectra.

Exploring the Stability and Disorder in the Polymorphs of L-Cysteine through Density Functional Theory and Vibrational Spectroscopy.

Static and dynamic density functional calculations are reported for the four known polymorphs of l-cysteine. Static calculations are used to explore the relative free energies (within the harmonic approximation) of the polymorphs as a function of pressure. An important feature of the structural differences between the polymorphs is shown to be the dihedral angle of the C-C-S-H bond. It is shown that, by varying this angle, it is possible to move between hydrogen bonding motifs S-H···S and S-H···O in all four polymorphs. The energetics for dihedral angle rotation are explored, and the barriers for rotation between the hydrogen bonding motifs have been calculated for each polymorph. Two possible models for the experimental disorder observed in Form I at room temperature are explored using both static and dynamic methods; a domain disorder model, where the disorder is localized, and a dispersed disorder model, where the disorder is randomly distributed throughout the crystal. Molecular dynamics calculations show transitions between the two hydrogen bonding motifs occurring in the dispersed disorder model at 300 and 350 K. In addition, molecular dynamics calculations of Form IV also showed the onset of hydrogen bond disorder at 300 K. Calculations of the predicted infrared and terahertz absorption are performed for both the static and dynamic simulations, and the results are compared with experimental results to understand the influence of disorder on the observed spectra.

Calculation and measurement of terahertz active normal modes in crystalline PETN.

The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover with the COMPASS force field, CASTEP and PWscf. The calculations are compared to each other and to terahertz spectra (0.3-3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.

Dual-frequency imaging using an electrically tunable terahertz quantum cascade laser.

We report dual-frequency transmission imaging of polycrystalline materials using an electrically tunable terahertz (THz) frequency quantum cascade laser (QCL). Using our system we are able to obtain images at both 3.05 THz and 3.24 THz in a single two-dimensional scan of a sample. By taking the difference of the natural logarithms of the transmission coefficients obtained at each frequency, the difference-attenuation coefficient is determined, and evaluated for samples of lactose monohydrate, glucose monohydrate, sucrose, and the high explosive PETN. We also demonstrate difference-intensity imaging at these frequencies by combining amplitude modulation of the QCL bias with lock-in detection. Owing to the specific molecular absorption spectra of these materials in the THz frequency range, the samples can be distinguished using our technique.

Broadband terahertz time-domain spectroscopy of drugs-of-abuse and the use of principal component analysis.

Terahertz frequency time-domain spectroscopy has been used to analyse a wide range of samples containing cocaine hydrochloride, heroin and ecstasy--common drugs-of-abuse. We investigated real-world samples seized by law enforcement agencies, together with pure drugs-of-abuse, and pure drugs-of-abuse systematically adulterated in the laboratory to emulate real-world samples. In order to investigate the feasibility of automatic spectral recognition of such illicit materials by terahertz spectroscopy, principal component analysis was employed to cluster spectra of similar compounds.

Absorption-sensitive diffuse reflection imaging of concealed powders using a terahertz quantum cascade laser.

We report diffuse reflection imaging in air of concealed powdered samples using a terahertz quantum cascade laser. The sensitivity of the detection scheme to sub-surface absorption within samples is confirmed using fully-characterized powdered admixtures of polystyrene and polymethyl methacrylate (PMMA). Measurements of the backscattering intensity from these samples are then used in conjunction with Kubelka-Munk scattering theory, as well as several models based on the quasi-crystalline approximation, to extract the absorption coefficient of PMMA. Our research demonstrates the feasibility of high-resolution frequency-domain terahertz imaging for the detection and identification of concealed powders in a reflection geometry.

Spectroscopy of polycrystalline materials using thinned-substrate planar Goubau line at cryogenic temperatures.

We investigate the effect of substrate thickness on the transmission bandwidth of on-chip terahertz-frequency-range planar Goubau lines both experimentally and theoretically. The bandwidth and frequency resolution are improved through substrate thinning and geometry modifications (reducing reflections arising from the THz photoconductive generators and detectors). We demonstrate that the enhanced bandwidth (2 THz) and resolution (3.75 GHz) allows this type of on-chip waveguide to be used for spectroscopic measurements of polycrystalline materials from cryogenic (4 K) to room temperature (292 K) by recording vibrational absorption spectra from overlaid samples of lactose monohydrate.

Identification of a sporozoite-specific antigen from Toxoplasma gondii.

Reduction of risk for human and food animal infection with Toxoplasma gondii is hampered by the lack of epidemiological data documenting the predominant routes of infection (oocyst vs. tissue cyst consumption) in horizontally transmitted toxoplasmosis. Existing serological assays can determine previous exposure to the parasite, but not the route of infection. We have used difference gel electrophoresis, in combination with tandem mass spectroscopy and Western blot, to identify a sporozoite-specific protein (T. gondii embryogenesis-related protein [TgERP]), which elicited antibody and differentiated oocyst- versus tissue cyst-induced infection in pigs and mice. The recombinant protein was selected from a cDNA library constructed from T. gondii sporozoites; this protein was used in Western blots and probed with sera from T. gondii -infected humans. Serum antibody to TgERP was detected in humans within 6-8 mo of initial oocyst-acquired infection. Of 163 individuals in the acute stage of infection (anti- T. gondii IgM detected in sera, or < 30 in the IgG avidity test), 103 (63.2%) had detectable antibodies that reacted with TgERP. Of 176 individuals with unknown infection route and in the chronic stage of infection (no anti- T. gondii IgM detected in sera, or > 30 in the IgG avidity test), antibody to TgERP was detected in 31 (17.6%). None of the 132 uninfected individuals tested had detectable antibody to TgERP. These data suggest that TgERP may be useful in detecting exposure to sporozoites in early T. gondii infection and implicates oocysts as the agent of infection.

Raman spectroscopic analysis of human remains from a seventh century cist burial on Anglesey, UK.

Specimens from human remains exhibiting unusual preservation excavated from a seventh century stone cist burial at Towyn y Capel in Anglesey, UK, have been analysed using Raman spectroscopy with near-infrared laser excitation at 1,064 and 785 nm. Specimens of hair and bone provided evidence for severe degradation and microbial colonisation. The deposits within the stone cist showed that some microbially mediated compounds had been formed. Analysis of crystals found at the interface between the hair and the skeletal neck vertebrae revealed a mixture of newberyite and haematite, associated with decomposition products of the hair and bone. An interesting differential degradation was noted in the specimens analysed which could be related to the air-void and the presence of plant root inclusions into the stone cist. This is the first time that Raman spectroscopy has been used in the forensic archaeological evaluation of burial remains in complex and dynamic environments.