Cage-Like Local Structure of Ionic Liquids Revealed by a ¹²⁹Xe Chemical Shift

The chemical shift of xenon (at natural abundance) dissolved in a variety of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) has been measured with ¹²⁹Xe NMR spectroscopy. The large chemical shift differences observed are mainly related to the type of anion; the strongest deshielding effect is observed ILs with I^–, Br^–, and Cl^– anions, and the strongest shielding is found for the bis­(trifluoromethanesulfonyl)­imide ([Tf₂N]⁻)-based IL. The measured ¹²⁹Xe chemical shift variations correlate well with the IL structure organization imposed by the anions and with the size of the empty voids due to charge alternation patterns. Descriptors taken from literature data on X-ray and neutron scattering, as well as single-crystal structures where available, support this interpretation. The proposed methodology adds a new investigating tool to the elucidation of the short-range order in ILs. The observed chemical shift trend provides information about how these solvents are organized

Cage-Like Local Structure of Ionic Liquids Revealed by a ¹²⁹Xe Chemical Shift

The chemical shift of xenon (at natural abundance) dissolved in a variety of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) has been measured with ¹²⁹Xe NMR spectroscopy. The large chemical shift differences observed are mainly related to the type of anion; the strongest deshielding effect is observed ILs with I^–, Br^–, and Cl^– anions, and the strongest shielding is found for the bis­(trifluoromethanesulfonyl)­imide ([Tf₂N]⁻)-based IL. The measured ¹²⁹Xe chemical shift variations correlate well with the IL structure organization imposed by the anions and with the size of the empty voids due to charge alternation patterns. Descriptors taken from literature data on X-ray and neutron scattering, as well as single-crystal structures where available, support this interpretation. The proposed methodology adds a new investigating tool to the elucidation of the short-range order in ILs. The observed chemical shift trend provides information about how these solvents are organized

Surface Characterization of TiO₂ Polymorphic Nanocrystals through ¹H‑TD-NMR

Nanocrystals (NCs) surface characterization is a fundamental step for understanding the physical and chemical phenomena involved at the nanoscale. Surface energy and chemistry depend on particle size and composition, and, in turn, determine the interaction of NCs with the surrounding environment, their properties and stability, and the feasibility of nanocomposites. This work aims at extracting more information on the surface of different titanium dioxide polymorphs using ¹H-TD-NMR of water. Taking advantage of the interaction between water molecules and titanium dioxide NCs, it is possible to correlate the proton transverse relaxation times (<i>T</i>₂) as the function of the concentration and the specific surface area (δ_p·<i>C</i>_m) and use it as an indicator of the crystal phase. Examples of three different crystals phase, rutile, anatase, and brookite, have been finely characterized and their behavior in water solution have been studied with TD-NMR. The results show a linear correlation between relaxivity (<i>R</i>₂) and their concentration <i>C</i>_m. The resulting slopes, after normalization for the specific surface, represent the surface/water interaction and range from 1.28 g m^–2 s^–1 of 50 nm rutile nanocrystals to 0.52 for similar sized brookite. Even higher slopes (1.85) characterize smaller rutile NCs, in qualitative accordance with the trends of surface energy. Thanks to proton relaxation phenomena that occur at the NCs surface, it is possible to differentiate the crystal phase and the specific surface area of titanium dioxide polymorphs in water solution

Solid-State Sensitized Upconversion in Polyacrylate Elastomers

The sensitized triplet–triplet annihilation-based upconversion in bicomponent systems is currently considered the most promising strategy for increasing the light-harvesting ability of solar cells. Flexible, manageable, inexpensive up-converting devices become possible by implementing this process in elastomers. Here, we report a study combining optical spectroscopy data of the light conversion process with the nano- and macroscopic viscoelastic characterization of the host material embedding the active dyes, in order to find a rationale for the fabrication of efficient solid-state upconverting systems. By using the poly­(<i>n</i>-alkyl acrylates) as a model of the monophasic elastomers, we demonstrate that the yield of the bimolecular interactions at the base of the upconversion process, namely, energy transfer and triplet–triplet annihilation, is mainly determined by the glass transition temperature (<i>T</i>_g) of the polymer. By employing the polyoctyl acrylate (<i>T</i>_g = 211 K), we achieved a conversion yield at the solid state larger than 10% at an irradiance of 1 sun, showing the potential of the elastomer-based upconverting materials for developing real-world devices

Understanding Cage Effects in Imidazolium Ionic Liquids by ¹²⁹Xe NMR: MD Simulations and Relativistic DFT Calculations

¹²⁹Xe NMR has been recently employed to probe the local structure of ionic liquids (ILs). However, no theoretical investigation has been yet reported addressing the problem of the dependence of the chemical shift of xenon on the cage structure of the IL. Therefore, we present here a study of the chemical shift of ¹²⁹Xe in two ionic liquids, [bmim]­[Cl] and [bmim]­[PF₆], by a combination of classical MD simulations and relativistic DFT calculations of the xenon shielding constant. The bulk structure of the two ILs is investigated by means of the radial distribution functions, paying special attention to the local structure, volume, and charge distribution of the cage surrounding the xenon atom. Relativistic DFT calculations, based on the ZORA formalism, on clusters extracted from the trajectory files of the two systems, yield an average relative chemical shift in good agreement with the experimental data. Our results demonstrate the importance of the cage volume and the average charge surrounding the xenon nucleus in the IL cage as the factors determining the effective shielding

Unraveling Triplet Excitons Photophysics in Hyper-Cross-Linked Polymeric Nanoparticles: Toward the Next Generation of Solid-State Upconverting Materials

The technological application of sensitized upconversion based on triplet–triplet annihilation (TTA) requires the transition from systems operating in liquid solutions to solid-state materials. Here, we demonstrate that the high upconversion efficiency reported in hyper-cross-linked nanoparticles does not originate from residual mobility of the embedded dyes as it happens in soft hosts. The hyper-reticulation from one side blocks the dyes in fixed positions, but on the other one, it suppresses the nonradiative spontaneous decay of the triplet excitons, reducing intramolecular relaxations. TTA is thus enabled by an unprecedented extension of the triplet lifetime, which grants long excitons diffusion lengths by hopping among the dye framework and gives rise to high upconversion yield without any molecular displacement. This finding paves the way for the design of a new class of upconverting materials, which in principle can operate at excitation intensities even lower than those requested in liquid or in rubber hosts

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Strongly Fluorescent Quaternary Cu–In–Zn–S Nanocrystals Prepared from Cu_1‑<i>x</i>InS₂ Nanocrystals by Partial Cation Exchange

We report a high-yield, low cost synthesis route to colloidal Cu_1‑<i>x</i>InS₂ nanocrystals with a tunable amount of Cu vacancies in the crystal lattice. These are then converted into quaternary Cu–In–Zn–S (CIZS) nanocrystals by partial exchange of Cu⁺ and In³⁺ cations with Zn²⁺ cations. The photoluminescence quantum yield of these CIZS nanocrystals could be tuned up to a record 80%, depending on the amount of copper vacancies