Research

22 pages, 4307 KiB

Open AccessArticle

Linear, Non-Conjugated Cyclic and Conjugated Cyclic Paraphenylene under Pressure

by Miriam Peña-Álvarez, Samuele Fanetti, Naomi Falsini, Giulia Novelli, Juan Casado, Valentín G. Baonza, Mercedes Taravillo, Simon Parsons, Roberto Bini and Margherita Citroni

Molecules 2019, 24(19), 3496; https://doi.org/10.3390/molecules24193496 - 26 Sep 2019

Cited by 3 | Viewed by 3120

Abstract

The n-paraphenylene family comprises chains of phenylene units linked together by C-C bonds that are between single- and double-bonded, and where n corresponds to the number of phenylene units. In this work, we compare the response of the optical properties of different [...] Read more.

The n-paraphenylene family comprises chains of phenylene units linked together by C-C bonds that are between single- and double-bonded, and where n corresponds to the number of phenylene units. In this work, we compare the response of the optical properties of different phenylene arrangements. We study linear chains (LPP), cyclic systems (CPPs), and non-conjugated cyclic systems with two hydrogenated phenylenes (H₄[n]CPP). Particularly, the systems of interest in this work are [6]LPP, [12]- and [6]CPP and H₄[6]CPP. This work combines Raman and infrared spectroscopies with absorption and fluorescence (one- and two-photon excitations) measured as a function of pressure up to maximum of about 25 GPa. Unprecedented crystallographic pressure-dependent results are shown on H₄[n]CPP, revealing intramolecular π-π interactions upon compression. These intramolecular interactions justify the H₄[n]CPP singular optical properties with increasing fluorescence lifetime as a function of pressure. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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11 pages, 2476 KiB

Open AccessArticle

Effect of H₂O on the Pressure-Induced Amorphization of Hydrated AlPO₄-17

by Frederico G. Alabarse, Boby Joseph, Andrea Lausi and Julien Haines

Molecules 2019, 24(16), 2864; https://doi.org/10.3390/molecules24162864 - 07 Aug 2019

Cited by 6 | Viewed by 2511

Abstract

The incorporation of guest species in zeolites has been found to strongly modify their mechanical behavior and their stability with respect to amorphization at high pressure (HP). Here we report the strong effect of H₂O on the pressure-induced amorphization (PIA) in [...] Read more.

The incorporation of guest species in zeolites has been found to strongly modify their mechanical behavior and their stability with respect to amorphization at high pressure (HP). Here we report the strong effect of H₂O on the pressure-induced amorphization (PIA) in hydrated AlPO₄-17. The material was investigated in-situ at HP by synchrotron X-ray powder diffraction in diamond anvil cells by using non- and penetrating pressure transmitting media (PTM), respectively, silicone oil and H₂O. Surprisingly, in non-penetrating PTM, its structural response to pressure was similar to its anhydrous phase at lower pressures up to ~1.4 GPa, when the amorphization was observed to start. Compression of the structure of AlPO₄-17 is reduced by an order of magnitude when the material is compressed in H₂O, in which amorphization begins in a similar pressure range as in non-penetrating PTM. The complete and irreversible amorphization was observed at ~9.0 and ~18.7 GPa, respectively, in non- and penetrating PTM. The present results show that the insertion of guest species can be used to strongly modify the stability of microporous material with respect to PIA, by up to an order of magnitude. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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16 pages, 5912 KiB

Open AccessArticle

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study

by Ian B. Hutchison, Craig L. Bull, William G. Marshall, Andrew J. Urquhart and Iain D.H. Oswald

Molecules 2019, 24(11), 2174; https://doi.org/10.3390/molecules24112174 - 10 Jun 2019

Cited by 3 | Viewed by 4032

Abstract

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure [...] Read more.

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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23 pages, 6474 KiB

Open AccessArticle

The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile

by Nico Giordano, Sergejs Afanasjevs, Christine M. Beavers, Claire L. Hobday, Konstantin V. Kamenev, Earl F. O’Bannon, Javier Ruiz-Fuertes, Simon J. Teat, Rafael Valiente and Simon Parsons

Molecules 2019, 24(10), 2018; https://doi.org/10.3390/molecules24102018 - 27 May 2019

Cited by 11 | Viewed by 4081

Abstract

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up [...] Read more.

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) Å at 5.0 GPa. The π∙∙∙π contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energy calculations (PIXEL) indicate that the π∙∙∙π interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic (

P \bar{1}

) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I···N bond increases in length to 2.928(10) Å and become less linear [<C−I∙∙∙N = 166.2(5)°]; the energy stabilises by 2.5 kJ mol⁻¹ and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm⁻¹. The driving force of the transition is shown to be relief of strain built-up in the π∙∙∙π interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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12 pages, 3088 KiB

Open AccessArticle

High Pressure Crystal Structure and Electrical Properties of a Single Component Molecular Crystal [Ni(dddt)₂] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate)

by Hengbo Cui, Takao Tsumuraya, Hamish H.-M. Yeung, Chloe S. Coates, Mark R. Warren and Reizo Kato

Molecules 2019, 24(10), 1843; https://doi.org/10.3390/molecules24101843 - 14 May 2019

Cited by 5 | Viewed by 3516

Abstract

Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt) [...] Read more.

Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)₂] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)₂], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)₂] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)₂], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)₂] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)₂] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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13 pages, 6639 KiB

Open AccessArticle

Packing Rearrangements in 4-Hydroxycyanobenzene Under Pressure

by Ines E. Collings and Michael Hanfland

Molecules 2019, 24(9), 1759; https://doi.org/10.3390/molecules24091759 - 07 May 2019

Cited by 8 | Viewed by 3016

Abstract

4-hydroxycyanobenzene (4HCB) is a dipolar molecule formed of an aromatic substituted benzene ring with the CN and OH functional groups at the 1 and 4 positions. In the crystalline state, it forms spiral chains via hydrogen bonding, which pack together through [...] Read more.

4-hydroxycyanobenzene (4HCB) is a dipolar molecule formed of an aromatic substituted benzene ring with the CN and OH functional groups at the 1 and 4 positions. In the crystalline state, it forms spiral chains via hydrogen bonding, which pack together through

π - π

interactions. The direct stacking of benzene rings down the a- and b-axes and its

π - π

interactions throughout the structure gives rise to its semiconductor properties. Here, high-pressure studies are conducted on 4HCB in order to investigate how the packing and intermolecular interactions, related to its semiconductor properties, are affected. High-pressure single-crystal X-ray diffraction was performed with helium and neon as the pressure-transmitting mediums up to 26 and 15 GPa, respectively. The pressure-dependent behaviour of 4HCB in He was dominated by the insertion of He into the structure after 2.4 GPa, giving rise to two phase transitions, and alterations in the

π - π

interactions above 4 GPa. 4HCB compressed in Ne displayed two phase transitions associated with changes in the orientation of the 4HCB molecules, giving rise to twice as many face-to-face packing of the benzene rings down the b-axis, which could allow for greater charge mobility. In the He loading, the hydrogen bonding interactions steadily decrease without any large deviations, while in the Ne loading, the change in 4HCB orientation causes an increase in the hydrogen bonding interaction distance. Our study highlights how the molecular packing and

π - π

interactions evolve with pressure as well as with He insertion. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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18 pages, 5247 KiB

Open AccessArticle

Pressure-Dependent Structural and Luminescence Properties of 1-(Pyren-1-yl)but-2-yn-1-one

by Anna Makal, Joanna Krzeszczakowska and Roman Gajda

Molecules 2019, 24(6), 1107; https://doi.org/10.3390/molecules24061107 - 20 Mar 2019

Cited by 4 | Viewed by 2654

Abstract

The crystal structure of 1-(pyren-1-yl)but-2-yn-1-one (

1 a

, a polynuclear aromatic hydrocarbon displaying enhanced luminescence in the solid state, has been re-determined at several pressures ranging from atmospheric up to 3 GPa using a Diamond Anvil Cell (DAC). These experiments were augmented [...] Read more.

The crystal structure of 1-(pyren-1-yl)but-2-yn-1-one (

1 a

, a polynuclear aromatic hydrocarbon displaying enhanced luminescence in the solid state, has been re-determined at several pressures ranging from atmospheric up to 3 GPa using a Diamond Anvil Cell (DAC). These experiments were augmented by periodic DFT calculations at pressures up to 4.4 GPa. UV-Vis fluorescence of

1 a

at non-ambient pressures has also been investigated. The crystal structure consists of infinite

π

-stacks of anti-parallel

1 a

molecules with discernible dimers, which may exemplify aggregates formed by pyrene derivatives in solution and thin films, and is predominantly stabilized by dispersion. The average inter-planar distance between individual molecules within

π

-stacks decreases with pressure in the investigated range. This results in piezochromic properties of

1 a

: a red-shift of sample color, as well as a bathochromic shift of fluorescence with pressure (by ca. 100 nm at 3.5 GPa). Two-component fluorescence spectra support the hypothesis that at least two types of excimers are involved in the electronic excitation processes in crystalline

1 a

. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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13 pages, 1735 KiB

Open AccessArticle

Intrinsic Flexibility of the EMT Zeolite Framework under Pressure

by Antony Nearchou, Mero-Lee U. Cornelius, Jonathan M. Skelton, Zöe L. Jones, Andrew B. Cairns, Ines E. Collings, Paul R. Raithby, Stephen A. Wells and Asel Sartbaeva

Molecules 2019, 24(3), 641; https://doi.org/10.3390/molecules24030641 - 12 Feb 2019

Cited by 8 | Viewed by 4547

Abstract

The roles of organic additives in the assembly and crystallisation of zeolites are still not fully understood. This is important when attempting to prepare novel frameworks to produce new zeolites. We consider 18-crown-6 ether (18C6) as an additive, which has previously been shown [...] Read more.

The roles of organic additives in the assembly and crystallisation of zeolites are still not fully understood. This is important when attempting to prepare novel frameworks to produce new zeolites. We consider 18-crown-6 ether (18C6) as an additive, which has previously been shown to differentiate between the zeolite EMC-2 (EMT) and faujasite (FAU) frameworks. However, it is unclear whether this distinction is dictated by influences on the metastable free-energy landscape or geometric templating. Using high-pressure synchrotron X-ray diffraction, we have observed that the presence of 18C6 does not impact the EMT framework flexibility—agreeing with our previous geometric simulations and suggesting that 18C6 does not behave as a geometric template. This was further studied by computational modelling using solid-state density-functional theory and lattice dynamics calculations. It is shown that the lattice energy of FAU is lower than EMT, but is strongly impacted by the presence of solvent/guest molecules in the framework. Furthermore, the EMT topology possesses a greater vibrational entropy and is stabilised by free energy at a finite temperature. Overall, these findings demonstrate that the role of the 18C6 additive is to influence the free energy of crystallisation to assemble the EMT framework as opposed to FAU. Full article

(This article belongs to the Special Issue High–Pressure Behaviour of Solids: From Molecular Species to 3D-Framework Materials)

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