A mechanophysical phase transition provides a dramatic example of colour polymorphism: the tribochromism of a substituted tri(methylene)tetrahydrofuran-2-one

Background

Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed “tribochromism”. We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies.

Results

A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb – which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb.

Conclusions

Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice.

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Graphical abstract Graphical representation of the structural and colour change in the tribochromic compound (III).

     

Hydrogen Sulfide Donors Activated by Reactive Oxygen Species

Hydrogen sulfide (H₂S) exhibits promising protective effects in many (patho)physiological processes, as evidenced by recent reports using synthetic H₂S donors in different biological models. Herein, we report the design and evaluation of compounds denoted PeroxyTCM, which are the first class of reactive oxygen species (ROS)‐triggered H₂S donors. These donors are engineered to release carbonyl sulfide (COS) upon activation, which is quickly hydrolyzed to H₂S by the ubiquitous enzyme carbonic anhydrase (CA). The donors are stable in aqueous solution and do not release H₂S until triggered by ROS, such as hydrogen peroxide (H₂O₂), superoxide (O₂^?), and peroxynitrite (ONOO^?). We demonstrate ROS‐triggered H₂S donation in live cells and also demonstrate that PeroxyTCM‐1 provides protection against H₂O₂‐induced oxidative damage, suggesting potential future applications of PeroxyTCM and similar scaffolds in H₂S‐related therapies.     

Simultaneously bound guests and chiral recognition: a chiral self-assembled supramolecular host encapsulates hydrophobic guests

Driven by the hydrophobic effect, a water-soluble, chiral, self-assembled supramolecular host is able to encapsulate hydrophobic organic guests in aqueous solution. Small aromatics can be encapsulated in the supramolecular assembly, and the simultaneous encapsulation of multiple species is observed in many cases. The molecular host assembly is able to recognize different substitutional isomers of disubstituted benzenes with ortho substitution leading to the encapsulation of two guests, but meta or para substitution leading to the encapsulation of only one guest. The scope of hydrophobic guest encapsulation is further explored with chiral natural products. Upon encapsulation of chiral molecules into the racemic host, diastereomeric host-guest complexes are formed with observed diastereoselectivities of up to 78:22 in the case of fenchone.     

Reversible guest exchange mechanisms in supramolecular host-guest assemblies

Synthetic chemists have provided a wide array of supramolecular assemblies able to encapsulate guest molecules. The scope of this tutorial review focuses on supramolecular host molecules capable of reversibly encapsulating polyatomic guests. Much work has been done to determine the mechanism of guest encapsulation and guest release. This review covers common methods of monitoring and characterizing guest exchange such as NMR, UV-VIS, mass spectrometry, electrochemistry, and calorimetry and also presents representative examples of guest exchange mechanisms. The guest exchange mechanisms of hemicarcerands, cucurbiturils, hydrogen-bonded assemblies, and metal-ligand assemblies are discussed. Special attention is given to systems which exhibit constrictive binding, a motif common in supramolecular guest exchange systems.     

螺旋波激发甲烷等离子体光谱诊断

利用螺旋波激发等离子体化学气相沉积(LPP-CVD)技术,以甲烷和氦气为反应气体产生等离子体.通过采集到甲烷的可见光到紫外发射光谱,对甲烷等离子体进行原位诊断,发现存在CH、Ha及Hβ等碎片粒子的光辐射,同时,分析了不同入射功率、气压下CH粒子以及Hβ、Hγ的相对强度变化情况.结果表明:CH粒子的相对强度随着射频功率是先增大而后减小,随工作气压的增大而逐渐减小;随气压及功率的增加,Hβ、Hγ相对强度变化的总体趋势都是先增加而后减小的.