Pressure Effects on Water Dynamics by Time-Resolved Optical Kerr Effect.

Despite water being the most common and most widely studied substance in the world, it still presents unknown aspects. In particular, water shows several thermodynamic and dynamical anomalies in the liquid and supercooled metastable phases, and the natures of these phases are still hotly debated. Here, we report measurements of water using the optical Kerr effect as a function of pressure along two isotherms, at 273 K from 0.1 to 750 MPa and at 297 K from 0.1 to 1350 MPa, reaching the supercooled metastable phase. The structural relaxation and the low frequency vibrational dynamics of water show a peculiar pressure dependence similar to that of other dynamical properties. The data analysis suggests the presence in the water phase diagram of a crossover area that divides two regions characterized by different dynamic regimes, which appear to be related to two liquid forms, one dominated by the high density water and the other by the low density water.

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

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.

Superheating and Homogeneous Melting Dynamics of Bulk Ice.

Homogeneous melting of crystals is a complex multistep process involving the formation of transient states at temperatures considerably higher than the melting point. The nature and persistence of these metastable structures are intimately connected to the melting process, and a precise definition of the temporal boundaries of these phenomena is not yet available. We set up a specifically designed experiment to probe by transient infrared absorption spectroscopy the entire dynamics, ranging from tens of picoseconds to microseconds, of superheating and melting of an ice crystal. In spite of a large excess of energy provided, only about 30% of the micrometric crystal liquefies in the first 20-25 ns because of the long persistence of the superheated metastable phase that extends for more than 100 ns. This behavior is ascribed to the population of low-energy states that trap a large amount of energy, favoring the formation of a metastable, likely plastic, ice phase.

Impact of High Pressure on Metallophilic Interactions and Its Consequences for Spectroscopic Properties of a Model Tetranuclear Silver(I)-Copper(I) Complex in the Solid State.

Structure-property relationships were investigated via combined high-pressure spectroscopic and X-ray diffraction techniques for a model luminescent Ag2Cu2L4 (L = 2-diphenylphosphino-3-methylindole) complex in the crystalline state. The experimental results were contributed by theoretical calculations, compared with the previously evaluated light-induced geometrical changes, and discussed in the context of available literature to date. To the best of our knowledge, this is the first study of this kind devoted to a coinage-metal complex for which the argentophilic interactions are crucial. High-pressure X-ray diffraction and optical spectroscopy experiments showed close correspondence between structural changes and optical properties. The unit-cell angles, absorption edges, emission maxima, decay lifetimes and silver-copper bond trends, all change around 2-3 GPa. A blue-shift to red-shift switch when increasing the pressure was observed for both absorption and emission spectra. This is unique behavior when compared to the literature-reported coinage metal systems. It also occurred that the pressure-induced structural changes differ notably from the geometrical distortions observed for the excited state. Interestingly, shortening of the Ag-Ag bond itself does not ensure the red shift of the absorption and emission spectra. All the optical spectroscopy data are suggestive of an important role of defects, likely related to the lack of a hydrostatic pressure transmitting medium, for pressures higher than 3 GPa.

The role of H-bond in the high-pressure chemistry of model molecules.

Pressure is an extraordinary tool to modify direction and strength of intermolecular interactions with important consequences on the chemical stability of molecular materials. The decrease of the distance among nearest neighbour molecules can give rise to reactive configurations reflecting the crystal arrangement and leading to association processes. In this context, the role of the H-bonds is very peculiar because their usual strengthening with rising pressure does not necessarily configure a decrease of the reaction activation energy but, on the contrary, can give rise to an anomalous stability of the system. In spite of this central role, the mechanisms by which a chemical reaction is favoured or prevented by H-bonding under high pressure conditions is a poorly explored field. Here we review a few studies where the chemical behaviour of simple molecular systems under static compression was related to the H-bonding evolution with pressure. These results are able to clarify a wealth of changes of the chemical and physical properties caused by the strengthening with pressure of the H-bonding network and provide additional tools to understand the mechanisms of high-pressure reactivity, a mandatory step to make these synthetic methods of potential interest for applicative purposes.

Melting dynamics of ice in the mesoscopic regime.

How does a crystal melt? How long does it take for melt nuclei to grow? The melting mechanisms have been addressed by several theoretical and experimental works, covering a subnanosecond time window with sample sizes of tens of nanometers and thus suitable to determine the onset of the process but unable to unveil the following dynamics. On the other hand, macroscopic observations of phase transitions, with millisecond or longer time resolution, account for processes occurring at surfaces and time limited by thermal contact with the environment. Here, we fill the gap between these two extremes, investigating the melting of ice in the entire mesoscopic regime. A bulk ice I h or ice VI sample is homogeneously heated by a picosecond infrared pulse, which delivers all of the energy necessary for complete melting. The evolution of melt/ice interfaces thereafter is monitored by Mie scattering with nanosecond resolution, for all of the time needed for the sample to reequilibrate. The growth of the liquid domains, over distances of micrometers, takes hundreds of nanoseconds, a time orders of magnitude larger than expected from simple H-bond dynamics.

Pressure Dependence of Hydrogen-Bond Dynamics in Liquid Water Probed by Ultrafast Infrared Spectroscopy.

Clarifying the structure/dynamics relation of water hydrogen-bond network has been the aim of extensive research over many decades. By joining anvil cell high-pressure technology, femtosecond 2D infrared spectroscopy, and molecular dynamics simulations, we studied, for the first time, the spectral diffusion of the stretching frequency of an HOD impurity in liquid water as a function of pressure. Our experimental and simulation results concordantly demonstrate that the rate of spectral diffusion is almost insensitive to the applied pressure. This behavior is in contrast with the previously reported pressure-induced speed up of the orientational dynamics, which can be rationalized in terms of large angular jumps involving sudden switching between two hydrogen-bonded configurations. The different trend of the spectral diffusion can be, instead, inferred considering that the first solvation shell preserves the tetrahedral structure with pressure and the OD stretching frequency is only slight perturbed.

Structure and Dynamics of Low-Density and High-Density Liquid Water at High Pressure.

Liquid water has a primary role in ruling life on Earth in a wide temperature and pressure range as well as a plethora of chemical, physical, geological, and environmental processes. Nevertheless, a full understanding of its dynamical and structural properties is still lacking. Water molecules are associated through hydrogen bonds, with the resulting extended network characterized by a local tetrahedral arrangement. Two different local structures of the liquid, called low-density (LDW) and high-density (HDW) water, have been identified to potentially affect many different chemical, biological, and physical processes. By combining diamond anvil cell technology, ultrafast pump-probe infrared spectroscopy, and classical molecular dynamics simulations, we show that the liquid structure and orientational dynamics are intimately connected, identifying the P-T range of the LDW and HDW regimes. The latter are defined in terms of the speeding up of the orientational dynamics, caused by the increasing probability of breaking and reforming the hydrogen bonds.

Connecting the Water Phase Diagram to the Metastable Domain: High-Pressure Studies in the Supercooled Regime.

Pressure is extremely efficient to tune intermolecular interactions, allowing the study of the mechanisms regulating, at the molecular level, the structure and dynamics of condensed phases. Among the simplest molecules, water represents in many respects a mystery despite its primary role in ruling most of the biological, physical, and chemical processes occurring in nature. Here we report a careful characterization of the dynamic regime change associated with low-density and high-density forms of liquid water by measuring the line shape of the OD stretching mode of HOD in liquid water along different isotherms as a function of pressure. Remarkably, the high-pressure studies have been here extended down to 240 K, well inside the supercooled regime. Supported by molecular dynamics simulations, a correlation among amorphous and crystalline solids and the two different liquid water forms is attempted to provide a unified picture of the metastable and thermodynamic regimes of water.

Picosecond optical parametric generator and amplifier for large temperature-jump.

An optical parametric generator and amplifier producing 15 ps pulses at wavelengths tunable around 2 µm, with energies up to 15 mJ/pulse, has been realized and characterized. The output wavelength is chosen to match a vibrational combination band of water. By measuring the induced birefringence changes we prove that a single pulse is able to completely melt samples of ice in the 10⁻6 cm³ volume range, both at room pressure (263 K) and at high pressure (298 K, 1 GPa) in a sapphire anvil cell. This source opens the possibility of studying melting and freezing processes by spectroscopic probes in water or water solutions in a wide range of conditions as found in natural environments.

HOMO-LUMO transitions in solvated and crystalline picene.

The optical properties of picene at ambient conditions have been investigated through the measurement of UV/Vis absorption and fluorescence spectra and of excitation profiles, using one- and two-photon excitation, in solution and in the crystal phase. For solvated picene an assignment of the vibronic structure of the transitions to the four lowest-energy excited singlet states (S(1)-S(4)) has been obtained from the absorption data, and the vibronic structure of the fluorescence spectra has been assigned. The absorption and fluorescence spectra of the solid phase can be interpreted according to the single molecule analysis. Nevertheless, the strong increase of the optical density in the spectral region of the lowest HOMO-LUMO transitions and the frequency shift of absorption and fluorescence bands may be explained by a mixing of the states of adjacent molecules in the crystal. Moreover, peculiar emission features depending on the crystal dimensions (10(-1) to 10(2) µm) are observed.

Changing the dissociative character of the lowest excited state of ethanol by pressure.

Syntheses based on physical methods, such as pressure and light, are extremely attractive to prepare novel materials from pure molecular systems in condensed phases. The structural and electronic modifications induced by selective optical excitation can trigger unexpected chemical reactions by exploiting the high density conditions realized at high pressure. The identification of the microscopic mechanisms regulating this reactivity, mandatory to design synthetic environments appealing for practical applications, requires a careful characterization of both structural and electronic properties as a function of pressure. Here, we report a spectroscopic study, by FTIR and Raman techniques, of the ambient temperature photoinduced reactivity of liquid C(2)H(5)OD up to 1 GPa. The results have been interpreted by comparison with those relative to the fully hydrogenated isotopomer. The dissociation along the O-H (D) coordinate is the primary reactive channel, but the different reactivity of the two isotopomers with rising pressure highlights a dramatic pressure effect on the energy surface of the first electronic excited state. Dissociation along the O-H (D) coordinate becomes the reaction rate-limiting step due to an increase with pressure of the binding character along this coordinate.

Pressure-induced fluorescence of pyridine.

Two-photon excitation profiles and fluorescence spectra have been measured as a function of pressure in a diamond anvil cell up to 15.5 GPa in crystal phases I and II and in the glassy form of pyridine. The fluorescence emission intensity increases by about 6 orders of magnitude in going from the liquid to the crystalline phases at 3 GPa and further increases with pressure. This is explained by an energy inversion of the lowest (1)B(1) (nπ*) and (1)B(2) (ππ*) excited states likely due to the involvement of the lone pair of the N atom in intermolecular CH···N bonds. These interactions characterize the crystal phases and are stabilized by pressure. The glassy form, accordingly, is characterized by a much weaker fluorescence. Excimer emission is also observed. Comparison of the emission of several samples with different compression and annealing histories, the lack of reversibility in the excimer emission with decompression, and the larger relative intensity of the excimer band in the glassy form suggest that excimer formation occurs at crystal defects. This results support the conclusions of a previous investigation proposing that pressure-induced reactivity of pyridine is limited to crystal defects and agrees with the present knowledge of the solid-state chemistry of aromatic crystals.

Structure and reactivity of pyridine crystal under pressure.

In this work we have performed an extensive high pressure study of the condensed phases of pyridine by Raman and IR spectroscopy. We have evidenced three different polymorphs, two crystalline, and one glassy and established the pressure conditions in which they exist as stable or metastable phases by several compression/decompression experiments both on annealed and not annealed samples. Crystallization and phase transitions are found to be kinetically driven. The vibrational spectra are extremely complex due to the low symmetry of the crystals, which implies a large number of crystal components. This complexity required a careful analysis of both IR and Raman data that led to the identification of 20 out of 21 external modes expected for phase II. We did not find any conclusive indication of phase transitions on compressing phase II thus indicating that phase II is likely the stable phase at the onset pressure of the chemical transformation of pyridine. The latter starts at 18 GPa and relevant differences from the well characterized benzene reaction suggest that it is likely driven by crystal defects.

Photoinduced reactivity of liquid ethanol at high pressure.

The room temperature photoinduced reactivity of liquid ethanol has been studied as a function of pressure up to 1.5 GPa by means of a diamond anvil cell. Exploiting the dissociative character of the lowest electronic excited states, reached through two-photon absorption of near-UV photons (350 nm), irreversible reactive processes have been triggered in the pure system. The active species are radicals forming along two main dissociation channels involving the split of C-O and O-H bonds. The characterization of the reaction products has been performed by in situ FTIR and Raman spectroscopy. At pressures of a few megapascals, molecular hydrogen is the main reaction product, an important issue in the framework of environmentally friendly synthesis of this energetic vector. In the gigapascal range, the main products are ethane, 2-butanol, 2,3-butanediol, 1,1-diethoxyethane, and some carbonylic compounds. The relative amount of these species changes with pressure reflecting the nature of the radicals formed in the photodissociation process. As the pressure increases, the processes requiring a greater molecularity are favored, whereas those requiring internal rearrangements are inhibited. Disproportion products like CH(4), H(2)O, and CO(2) increase when the amount of ethanol decreases due to the reaction, becoming the main products only when ethanol is exhausted.

Nitromethane decomposition under high static pressure.

The room-temperature pressure-induced reaction of nitromethane has been studied by means of infrared spectroscopy in conjunction with ab initio molecular dynamics simulations. The evolution of the IR spectrum during the reaction has been monitored at 32.2 and 35.5 GPa performing the measurements in a diamond anvil cell. The simulations allowed the characterization of the onset of the high-pressure reaction, showing that its mechanism has a complex bimolecular character and involves the formation of the aci-ion of nitromethane. The growth of a three-dimensional disordered polymer has been evidenced both in the experiments and in the simulations. On decompression of the sample, after the reaction, a continuous evolution of the product is observed with a decomposition into smaller molecules. This behavior has been confirmed by the simulations and represents an important novelty in the scene of the known high-pressure reactions of molecular systems. The major reaction product on decompression is N-methylformamide, the smallest molecule containing the peptide bond. The high-pressure reaction of crystalline nitromethane under irradiation at 458 nm was also experimentally studied. The reaction threshold pressure is significantly lowered by the electronic excitation through two-photon absorption, and methanol, not detected in the purely pressure-induced reaction, is formed. The presence of ammonium carbonate is also observed.

Crystalline indole at high pressure: chemical stability, electronic, and vibrational properties.

Vibrational and electronic spectra of crystalline indole were measured up to 25.5 GPa at room temperature in a diamond anvil cell. In particular, Fourier transform infrared (FTIR) spectra in the mid-infrared region and two-photon excitation profiles and fluorescence spectra in the region of the HOMO-LUMO transitions were obtained. The analysis of the FTIR spectra revealed a large red-shift of the N-H stretching mode with increasing pressure, indicating the strengthening of the H-bond between the NH group and the pi electron density of nearest neighbor molecules. The frequencies of four vibronic bands belonging to the (1)L(a) and (1)L(b) systems were obtained as a function of pressure. Comparison with literature data shows that the crystal acts as a highly polar environment with regard to the position of the (1)L(b) origin and of the fluorescence maximum, which are largely red-shifted with respect to the gas phase or to solutions in apolar solvents. A large, and increasing with pressure, frequency difference between the (1)L(b) origin and the blue edge of the fluorescence spectrum suggests that the emitting state is (1)L(a), that is known to be more stabilized than (1)L(b) by dipolar relaxation. Crystalline indole was found to be very stable with respect to pressure-induced reactivity. Only traces of a reaction product, containing saturated C-H bonds, are detected after a full compression-decompression cycle. In addition, differently from many unsaturated compounds at high pressure, irradiation with light matching a two-photon absorption for a HOMO-LUMO transition has no enhancing effect on reactivity. The chemical stability of indole at high pressure is ascribed to the crystal structure, where nearest neighbor molecules, formig H-bonds, are not in a favorable position to react, while reaction between equivalent molecules, for which a superposition of the pi electron clouds would be possible, is hindered by H-bonded molecules. Consistently, no excimer emission was observed except at the cell opening at the end of the compression-decompression run. Extremely limited chemical reactivity and excimer formation likely occur at crystal defects, evidencing the strict connection between the two phenomena.

Role of excited electronic states in the high-pressure amorphization of benzene.

High-pressure methods are increasingly used to produce new dense materials with unusual properties. Increasing efforts to understand the reaction mechanisms at the microscopic level, to set up and optimize synthetic approaches, are currently directed at carbon-based solids. A fundamental, but still unsolved, question concerns how the electronic excited states are involved in the high-pressure reactivity of molecular systems. Technical difficulties in such experiments include small sample dimensions and possible damage to the sample as a result of the absorption of intense laser fields. These experimental challenges make the direct characterization of the electronic properties as a function of pressure by linear and nonlinear optical spectroscopies up to several GPa a hard task. We report here the measurement of two-photon excitation spectra in a molecular crystal under pressure, up to 12 GPa in benzene, the archetypal aromatic system. Comparison between the pressure shift of the exciton line and the monomer fluorescence provides evidence for different compressibilities of the ground and first excited states. The formation of structural excimers occurs with increasing pressure involving molecules on equivalent crystal sites that are favorably arranged in a parallel configuration. These species represent the nucleation sites for the transformation of benzene into amorphous hydrogenated carbon. The present results provide a unified picture of the chemical reactivity of benzene at high pressure.

Crystal structure of nitromethane up to the reaction threshold pressure.

Angle dispersion X-ray diffraction (AXDX) experiments on nitromethane single crystals and powder were performed at room temperature as a function of pressure up to 19.0 and 27.3 GPa, respectively, in a membrane diamond anvil cell (MDAC). The atomic positions were refined at 1.1, 3.2, 7.6, 11.0, and 15.0 GPa using the single-crystal data, while the equation of state (EOS) was extended up to 27.3 GPa, which is close to the nitromethane decomposition threshold pressure at room temperature in static conditions. The crystal structure was found to be orthorhombic, space group P2(1)2(1)2(1), with four molecules per unit cell, up to the highest pressure. In contrast, the molecular geometry undergoes an important change consisting of a gradual blocking of the methyl group libration about the C-N bond axis, starting just above the melting pressure and completed only between 7.6 and 11.0 GPa. Above this pressure, the orientation of the methyl group is quasi-eclipsed with respect to the NO bonds. This conformation allows the buildup of networks of strong intermolecular O...H-C interactions mainly in the bc and ac planes, stabilizing the crystal structure. This structural evolution determines important modifications in the IR and Raman spectra, occurring around 10 GPa. Present measurements of the Raman and IR vibrational spectra as a function of pressure at different temperatures evidence the existence of a kinetic barrier for this internal rearrangement.