Pressure-Induced Phase Transition in Hydrogen-Bonded Supramolecular Adduct Formed by Cyanuric Acid and Melamine

Pressure-induced SERS enhancement in a MoS2/Au/R6G system by a two-step charge transfer process

Pressure-induced surface-enhanced Raman spectroscopy (PI-SERS) represents a new frontier in the research field of SERS. However, relatively few studies have focused on PI-SERS due to many difficulties, such as easy aggregation of nanoparticles, and difficulty in understanding the interaction mechanisms between probe molecules and the SERS substrate at high pressure. Here we developed an efficient semiconductor-metal SERS substrate (MoS₂/Au) to study PI-SERS. Different from the previously reported monotonous decrease in Raman intensities upon compression, an anomalous Raman enhancement of R6G molecules adsorbed on the MoS₂/Au substrate was observed up to 2.39 GPa, at which the degree of charge transfer (ρ_CT) between the R6G molecules and the MoS₂/Au substrate reaches a maximum. By comparison, it is proposed that the decoration of Au on the SERS system could bring about a two-step charge transfer (CT) process, introduce localized surface plasmon resonance (LSPR), and thus favor the PI-SERS enhancement. Moreover, this charge transfer also causes obvious changes in the optical behaviors of R6G molecules upon compression. This brings new insights into the SERS study and also offers new ideas for the development of SERS application in high pressure studies.

Structural investigation and compression of a co-crystal of indomethacin and saccharin

Indomethacin : saccharin cocrystal has been studied under high pressure conditions and the amide interactions compared with previous high pressure studies.

Pressure-induced phase transition of 4-aminobenzonitrile: the formation and enhancement of N-H⋯N weak hydrogen bonds

A reversible pressure-induced structural phase transition of 4-aminobenzonitrile was found at about 0.3 GPa by conducting in situ high-pressure synchrotron angle-dispersive X-ray diffraction (ADXRD) experiments. The discontinuous changes of Raman modes at 0.2 GPa confirmed the occurrence of phase transition. In situ high-pressure Raman spectra indicated that the molecular arrangement and intermolecular interactions changed abruptly. The process of this phase transition continued up to about 1.0 GPa. When the pressure reached 1.1 GPa, the initial N-H⋯N interaction transformed into a new weak hydrogen bond, which was enhanced by further compression. The ab initio calculations and Hirshfeld surfaces were used to illustrate the above views. This study gives an example that demonstrates that the pressure can induce the formation of hydrogen bonds, which contributes to the development of supramolecular chemistry.

A Novel High-Density Phase and Amorphization of Nitrogen-Rich 1H-Tetrazole (CH2N4) under High Pressure

The high-pressure behaviors of nitrogen-rich 1H-tetrazole (CH₂N₄) have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman scattering up to 75 GPa. A first crystalline-to-crystalline phase transition is observed and identified above ~3 GPa with a large volume collapse (∼18% at 4.4 GPa) from phase I to phase II. The new phase II forms a dimer-like structure, belonging to P1 space group. Then, a crystalline-to-amorphous phase transition takes place over a large pressure range of 13.8 to 50 GPa, which is accompanied by an interphase region approaching paracrystalline state. When decompression from 75 GPa to ambient conditions, the final product keeps an irreversible amorphous state. Our ultraviolet (UV) absorption spectrum suggests the final product exhibits an increase in molecular conjugation.

Luminescence Properties of Compressed Tetraphenylethene: The Role of Intermolecular Interactions

Mechanochromic materials with aggregation-induced enhanced emission (AIEE) characteristic have been intensively expanded in the past few years. In general, intermolecular interactions invariably alter photophysical processes, while their role in the luminescence properties of these AIEE-active molecules is difficult to fully recognize because the pressurized samples possess amorphous nature in many cases. We now report the high-pressure studies on a prototype AIEE-active molecule, tetraphenylethene, using diamond anvil cell technique with associated spectroscopic measurements. An unusual pressure-dependent color, intensity, and lifetime change in tetraphenylethene has been detected by steady-state photoluminescence and time-resolved emission decay measurements. The flexible role of the aromatic C-H···π and C-H···C contacts in structural recovery, conformational modification, and emission efficiency modulation upon compression is demonstrated through structure and infrared analysis.

p-Aminobenzoic acid polymorphs under high pressures

The effect of high pressure on two forms (α, β) of p-aminobenzoic acids (PABA) is studied in a diamond anvil cell using in situ Raman spectroscopy.

Compression studies of face-to-face π-stacking interaction in sodium squarate salts: Na2C4O4 and Na2C4O4●3H2O

High-pressure Raman scattering and synchrotron X-ray diffraction measurements of sodium squarate (Na(2)C(4)O(4), SS) are performed in a diamond anvil cell. SS possesses a rare, but typical structure, which can show the effect of face-to-face π-stacking without interference of other interactions. At ~11 GPa, it undergoes a phase transition, identified as a symmetry transformation from P2(1)/c to P2(1). From high-pressure Raman patterns and the calculated model of SS, it can be proved that the phase transition results from the distorted squarate rings. We infer it is the enhancement of π-stacking that dominates the distortion. For comparison, high-pressure Raman spectra of sodium squarate trihydrate (Na(2)C(4)O(4)●3H(2)O, SST) are also investigated. The structure of SST is determined by both face-to-face π-stacking and hydrogen bonding. SST can be regarded as a deformation of SS. A phase transition, with the similar mechanism as SS, is observed at ~10.3 GPa. Our results can be well supported by the previous high-pressure studies of ammonium squarate ((NH(4))(2)C(4)O(4), AS), and vice versa. High-pressure behaviors of the noncovalent interactions in SS, SST, and AS are compared to show the impacts of hydrogen bonding and the role of electrostatic interaction in releasing process.

Structure, enhancement and white luminescence of multifunctional Lu₆O₅F₈:20%Yb³⁺,1%Er³⁺(Tm³⁺) nanoparticles via further doping with Li⁺ under different excitation sources

A series of Lu6O5F8:20%Yb(3+),1%Er(3+)(Tm(3+)),x%Li(+) (0 ≤ x ≤ 12) nanoparticles with average size from 20 to 320 nm upon increasing Li(+) concentration were prepared by a coprecipitation method. The detailed crystal structure of Lu6O5F8 as a new matrix is firstly analysed via retrieved refinement of the powder X-ray diffraction (XRD). In addition, the corresponding Powder Diffraction File card information was also obtained through indexing the XRD pattern of the host. Upconversion under excitation at 980 nm, downconversion with Xe lamp as excitation source and cathodoluminescence properties of Lu6O5F8:20%Yb(3+),1%Er(3+)(Tm(3+)),x%Li(+) (0 ≤ x ≤ 12) nanoparticles were compared and studied. It is worthwhile pointing out that according to the effects of Li(+) on emission intensity ratio, white UC emission was achieved in the Lu6O5F8:6%Yb(3+),0.3%Er(3+),0.4%Tm(3+),5%Li(+) compared to Li(+) free sample with the same activator concentration. The reasons behind this behavior were presented and discussed. All in all, Li(+) ion would be a wonderful luminescence intensifier for lanthanide ions, and the multifunctional lanthanide ion-doped Lu6O5F8 nanoparticles have potential application in photoluminescence areas and field emission display devices.

Study of high-pressure Raman intensity behavior of aromatic hydrocarbons: benzene, biphenyl and naphthalene

Raman spectra of benzene, naphthalene and biphenyl have been taken up to a pressure of 13 GPa. The results for benzene and naphthalene indicate that the perturbation of inter-molecular π-π stacking effect on the Raman intensity is neglectable. For biphenyl, all the Raman peaks show intensity enhancement during the compression process, which indicates the planar intra-molecular aromatic conformation play an important role in the intensity increments of Raman bands. For three aromatic compounds, C-H stretching vibration bands located at about 3000 cm(-1) show intriguing intensity changes during compression account for the inter-molecular C-H⋯π interaction.

Pressure-induced phase transition in N-H···O hydrogen-bonded molecular crystal oxamide

The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to ∼14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to ∼17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from P1 to P1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N-H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks.

Effect of high pressure on the typical supramolecular structure of guanidinium methanesulfonate

We report the high-pressure response of guanidinium methanesulfonate (C(NH(2))(3)(+)·CH(3)SO(3)(-), GMS) using in situ Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques up to the pressures of ~11 GPa. GMS exhibits the representative supramolecular structure of two-dimensional (2D) hydrogen-bonded bilayered motifs under ambient conditions. On the basis of the experimental results, two phase transitions were identified at 0.6 and 1.5 GPa, respectively. The first phase transition, which shows the reconstructive feature, is ascribed to the rearrangements of hydrogen-bonded networks, resulting in the symmetry transformation from C2/m to Pnma. The second one proves to be associated with local distortions of methyl groups, accompanied by the symmetry transformation from Pnma to Pna2(1). The cooperativity of hydrogen bonding, electrostatic, and van der Waals interactions, as well as mechanisms for the phase transitions is discussed by means of the local nature of the structure.

Structural properties and halogen bonds of cyanuric chloride under high pressure

The effects of high pressure on cyanuric chloride (C(3)N(3)Cl(3)), a remarkable crystal structure dominated by halogen bonds, have been studied by synchrotron X-ray diffraction and Raman spectroscopy in a diamond anvil cell. The results of high pressure experiments revealed that there was no obvious phase transition up to 30 GPa, indicating that halogen bonding is an effective noncovalent interaction to stabilize the crystal structure. Moreover, cyanuric chloride exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layered crystal packing. Ab initio calculations were also performed to account for the high pressure Raman spectra and the high pressure behavior of halogen bonding.