Concomitant polymorphism and the martensitic-like transformation of an organic crystal.

Crystalline polymorphism is a phenomenon that occurs in many molecular solids, resulting in a diverse range of possible bulk structures. Temperature and pressure can often be used to thermodynamically control which crystal form is preferred, and the associated transitions between polymorphic phases are often discontinuous and complete. N-Methyl-4-carboxypyridinium chloride is a solid that undergoes an apparent continuous temperature-dependent phase transition from an orthorhombic to a monoclinic polymorph. However, a hybrid characterization approach using single-crystal X-ray diffraction, terahertz time-domain spectroscopy, and solid-state density functional theory reveals the transformation to be actually a slowly changing ratio of the two discrete polymorphic forms. The potential energy surface of this process can be directly accessed using terahertz radiation, and the data show that a very low barrier (43.3 J mol-1) exists along the polymorph transformation coordinate

Quantification of cation-anion interactions in crystalline monopotassium and monosodium glutamate salts.

Crystalline salt compounds composed of metal cations and organic anions are becoming increasingly popular in a number of fields, including the pharmaceutical and food industries, where such formulations can lead to increased product solubility. The origins of these effects are often in the interactions between the individual components in the crystals, and understanding these forces is paramount for the design and utilisation of such materials. Monosodium glutamate monohydrate and monopotassium glutamate monohydrate are two solids that form significantly different structures with correspondingly dissimilar dynamics, while their chemistry only differs in cation identity. Crystals of each were characterised experimentally with single-crystal X-ray diffraction and terahertz time-domain spectroscopy and theoretically using solid-state density functional theory simulations, in order to explain the observed differences in their bulk properties. Specifically, crystal orbital overlap and Hamiltonian population analyses were performed to examine the role that the individual interactions between the cation and anion played in the solid-state structures and the overall energetic profiles of these materials

Investigation of the terahertz vibrational modes of ZIF-8 and ZIF-90 with terahertz time-domain spectroscopy.

We present experimental and computational evidence that gate-opening modes for zeolitic imidazole frameworks can be observed at terahertz frequencies. Our work highlights the critical importance to correctly optimise the crystal structure prior to computational lattice dynamics analysis. The results support the hypothesis that the low energy vibrational modes do indeed play a significant role in host-guest interactions for ZIFs, such as gas loading.C.O.T. thanks Becas Chile and the Cambridge Trust for funding; D.F.-J. thanks the Royal Society (U.K.) for funding through a University Research Fellowship. T.M.K. would like to acknowledge the Royal Society as well as the Royal Society of Chemistry for support.This is the accepted manuscript. The final version is available at via http://dx.doi.org/10.1039/C5CC06455

Defining the origins of multiple emission/excitation in rhenium-bisthiazole complexes

The underlying mechanism of the unusual emissive behavior of [Re(CO) 3 -1,1-bis-4-thiazole-(1,4)-diaminobutane)] bromide (4-BT) has been investigated. Synthesis and spectroscopic characterization of structurally similar isomers ([Re(CO) 3 -1,1-bis-2-thiazole-(1,4)-diaminobutane)] bromide (2-BT)) and the location of triplet states, solid state and low temperature spectroscopic measurements, and DFT calculations show that the photophysical properties are not due to photoisomerization as previously hypothesized. The results show that the unusual emissive behavior is not observed in structural isomers, is specific to the previously reported complex, 4-BT, and may arise from vibrational energy relaxation and vibrational cooling. Translation of the unusual emissive behavior to the solid state offers an interesting platform allowing this complex to be potentially utilized as a probe, sensor or photonic device

Measuring the Elasticity of Poly-l-Proline Helices with Terahertz Spectroscopy

The rigidity of poly‐l‐proline is an important contributor to the stability of many protein secondary structures, where it has been shown to strongly influence bulk flexibility. The experimental Young's moduli of two known poly‐l‐proline helical forms, right‐handed all‐cis (Form I) and left‐handed all‐trans (Form II), were determined in the crystalline state by using an approach that combines terahertz time‐domain spectroscopy, X‐ray diffraction, and solid‐state density functional theory. Contrary to expectations, the helices were found to be considerably less rigid than many other natural and synthetic polymers, as well as differing greatly from each other, with Young's moduli of 4.9 and 9.6 GPa for Forms I and II, respectively.M.T.R. and T.M.K acknowledge the support of a grant from the National Science Foundation (CHE-1301068). T.M.K. would like to acknowledge the Royal Society International Exchanges Scheme as well as the Royal Society of Chemistry JWT Jones Travelling Fellowship for support. J.S. and J.A.Z. would like to acknowledge the U.K. Engineering and Physical Sciences Research Council (EP/J007803/1) for funding

Coating and Density Distribution Analysis of Commercial Ciprofloxacin Hydrochloride Monohydrate Tablets by Terahertz Pulsed Spectroscopy and Imaging

Terahertz pulsed spectroscopy was used to qualitatively detect ciprofloxacin hydrochloride monohydrate (CPFX·HCl·H2O) in tablets, and terahertz pulsed imaging (TPI) was used to scrutinize not only the coating state but also the density distribution of tablets produced by several manufacturers. TPI was also used to evaluate distinguishability among these tablets. The same waveform, which is a unique terahertz absorption spectrum derived from pure CPFX·HCl·H2O, was observed in all of the crushed tablets and in pure CPFX·HCl·H2O. TPI can provide information about the physical states of coated tablets. Information about the uniformity of parameters such as a coating thickness and density can be obtained. In this study, the authors investigated the coating thickness distributions of film-coated CPFX·HCl·H2O from four different manufacturers. Unique terahertz images of the density distributions in these commercial tablets were obtained. Moreover, B-scan (depth) images show the status of the coating layer in each tablet and the density map inside the tablets. These features would reflect differences resulting from different tablet-manufacturing processes

The significance of the amorphous potential energy landscape for dictating glassy dynamics and driving solid-state crystallisation.

The fundamental origins surrounding the dynamics of disordered solids near their characteristic glass transitions continue to be fiercely debated, even though a vast number of materials can form amorphous solids, including small-molecule organic, inorganic, covalent, metallic, and even large biological systems. The glass-transition temperature, Tg, can be readily detected by a diverse set of techniques, but given that these measurement modalities probe vastly different processes, there has been significant debate regarding the question of why Tg can be detected across all of them. Here we show clear experimental and computational evidence in support of a theory that proposes that the shape and structure of the potential-energy surface (PES) is the fundamental factor underlying the glass-transition processes, regardless of the frequency that experimental methods probe. Whilst this has been proposed previously, we demonstrate, using ab initio molecular-dynamics (AIMD) simulations, that it is of critical importance to carefully consider the complete PES - both the intra-molecular and inter-molecular features - in order to fully understand the entire range of atomic-dynamical processes in disordered solids. Finally, we show that it is possible to utilise this dependence to directly manipulate and harness amorphous dynamics in order to control the behaviour of such solids by using high-powered terahertz pulses to induce crystallisation and preferential crystal-polymorph growth in glasses. Combined, these findings provide compelling evidence that the PES landscape, and the corresponding energy barriers, are the ultimate controlling feature behind the atomic and molecular dynamics of disordered solids, regardless of the frequency at which they occur

The 2017 Terahertz Science and Technology Roadmap

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies