Growth Dynamics of Crystalline Ar Hydrate

The formation of a clathrate hydrate crystal is characterized by several steps, each of them distinguished by a different structural arrangement and temporal duration. A precise definition of these different forms is a challenging task, because the entirety of the formation dynamics spans over a time interval ranging from few nanoseconds to several days. Computational methods are powerful and essential to define the nucleation step, but they fail in providing a reliable picture of the long-range order establishment. On the other side, the experimental methods employed in the study of the growth dynamics usually monitor the hydrate growth at the interface with the fluid and thus are limited by the diffusion of the guest molecules through the newly formed hydrate phase. This problem is overcome here by the confinement of an argon hydrate sample in a sapphire anvil cell, allowing monitoring of the melting and crystallization of hydrates under moderate pressures by FTIR and Raman spectroscopies. This approach, besides providing a spectroscopic characterization of this hydrate, allowed the time windows characteristic of the formation of a macroscopic amorphous phase to be identified, possibly coincident with the so-called blob, and its rapid evolution toward the achievement of the local structure. Long-range ordering takes place on a longer time scale, most of it is realized in few hours but still evolving for weeks. No hints for supporting the so-called memory effect are gained through this study

High pressure chemistry of red phosphorus by photo-activated simple molecules

High pressure (HP) is very effective in reducing intermolecular distances and inducing unexpected chemical reactions. In addition the photo-activation of the reactants in HP conditions can lead to very efficient and selective processes. The chemistry of phosphorus is currently based on the white molecular form. The red polymeric allotrope, despite more stable and much less toxic, has not attracted much attention so far. However, switching from the white to the red form would benefit any industrial procedure, especially from an environmental point of view. On the other side, water and ethanol are renewable, environmental friendly and largely available molecules, usable as reactants and photo-activators in HP conditions. Here we report a study on the HP photo-induced reactivity of red phosphorus with water and ethanol, showing the possibility of very efficient and selective processes, leading to molecular hydrogen and valuable phosphorus compounds. The reactions have been studied by means of FTIR and Raman spectroscopy and pressure has been generated using membrane Diamond (DAC) and Sapphire (SAC) anvil cells. HP reactivity has been activated by the two-photon absorption of near-UV wavelengths and occurred in total absence of solvents, catalysts and radical initiators, at room T and mild pressure conditions (0.2–1.5 GPa)

Iminothioindoxyl as a molecular photoswitch with 100 nm band separation in the visible range

Light is an exceptional external stimulus for establishing precise control over the properties and functions of chemical and biological systems, which is enabled through the use of molecular photoswitches. Ideal photoswitches are operated with visible light only, show large separation of absorption bands and are functional in various solvents including water, posing an unmet challenge. Here we show a class of fully-visible-light-operated molecular photoswitches, lminothioindoxyls (ITIs) that meet these requirements. ITIs show a band separation of over 100 nm, isomerize on picosecond time scale and thermally relax on millisecond time scale. Using a combination of advanced spectroscopic and computational techniques, we provide the rationale for the switching behavior of ITIs and the influence of structural modifications and environment, including aqueous solution, on their photochemical properties. This research paves the way for the development of improved photo-controlled systems for a wide variety of applications that require fast responsive functions.</p

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 ¿-¿ interactions upon compression. These intramolecular interactions justify the H4[n]CPP singular optical properties with increasing fluorescence lifetime as a function of pressure

Towards custom built double core carbon nanothreads by stilbene and pseudo-stilbene type systems

International audienceDouble diamond-like wires connected through chromophores which define the electronic properties have been synthesized compressing in a diamond anvil cell crystalline members of the stilbene class

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) mu m) are observed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770265

High-Pressure Optical Properties and Chemical Stability of Picene

Picene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been recently found to behave as high-temperature superconductors upon alkali metal doping. The electronic properties of organic crystals can be finely and largely modified by the density changes obtained by the application of an external pressure. In this work, the role of pressure in tuning the optical properties of crystalline picene has been investigated from room conditions up to 15 GPa through the measurement of UV-visible absorption spectra, two-photon excitation profiles, and one- and two-photon excited fluorescence spectra in a diamond anvil cell. The pressure dependence of the optical band gap was determined, and the frequencies of several vibronic bands belonging to electronic transitions from the ground state (S0) to the four lowest-energy excited singlet states (S1 to S4) were determined as a function of pressure. We evidence a very different density dependence of the transition energy of S0 → S1, which undergoes a remarkable red shift of ∼400 cm-1/GPa, and of the transitions from S0 to the higher excited states, which remain constant in the whole investigated range. This is consistent with a S 1 state of 1La character in solid picene. The high-pressure chemical stability of solid picene was investigated through visible absorption and Fourier transform infrared spectroscopy (FTIR). A chemical transformation involving the bulk picene crystal occurs above ∼23 GPa, giving rise to a disordered material similar to the amorphous hydrogenated carbon obtained in the pressure-induced reactivity of benzene. The combination of electronic and vibrational data allows us to identify the presence of reaction intermediates at ∼10 GPa, preferentially forming at crystal defects. © 2013 American Chemical Society