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.

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.

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, Iminothioindoxyls (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.

Carbon nanothreads are likely the most attracting new materials produced under high pressure conditions. Their synthesis is achieved by compressing crystals of different small aromatic molecules also exploiting the applied...