Post-Modification of the Electronic Properties by Addition of π-Stacking Additives in N-Heterocyclic Carbene Complexes with Extended Polyaromatic Systems

A series of iridium complexes containing phenanthro[4,5-abc]phenazino[11,12-d]imidazol-2-ylidene and acetonaphtho[1,2-b]quinoxaline[11,12-d]imidazol-2-ylidene ligands have been obtained and fully characterized. These complexes display highly extended polyaromatic systems attached to the backbone of the N-heterocyclic carbene. The presence of this extended polyaromatic system makes the electron-donating character of these ligands sensitive to the presence of π-stacking additives, such as pyrene and hexafluorobenzene. The computational studies predict that the addition of pyrene affords an increase of the electron-donating character of the polyaromatic ligand (TEP decreases), while the addition of hexafluorobenzene has the opposite effect (TEP increases). This prediction is experimentally corroborated by IR spectroscopy, by measuring the shift of the CO stretching bands of a series of IrCl(NHC)(CO)2 complexes, where NHC is the N-heterocyclic carbene ligand with the polyaromatic system. Finally, the energy of the π-stacking interaction of one of the key Ir(I) complexes with pyrene and hexafluorobenzene has been estimated by using the Benesi-Hildebrand treat-ment, based on the δ-shifts observed by 1H NMR spectroscopy.MEC of Spain (CTQ2011-24055/BQU

Gold-catalyzed Cycloisomerization Reactions within Guanidinium M₁₂L₂₄ Nanospheres: the Effect of Local Concentrations

Gold‐catalyzed cycloisomerization reactions have been explored using guanidinium functionalized M12L24 nanospheres that strongly encapsulate gold complexes functionalized with a sulfonate group through hydrogen bonds. As the M12L24 nanospheres can bind up to 24 gold complexes, the effect of local catalyst concentration on the reaction outcome can be easily evaluated. Also, the guanidinium groups of the sphere can weakly interact with the carboxylic group of the substrates, facilitating the pre‐organization of the substrate near to the catalytic active site. Both effects can influence the selectivity and rate of the gold‐catalyzed transformation. Challenging acetate‐containing substrates with internal acetylene functional groups can be cyclized efficiently within the M12L24 nanospheres, where the pre‐organization of the substrate plays a crucial role. For 2‐alkynyl benzoic acids the selectivity of the reaction can be controlled by adjusting the local concentration of gold catalyst in the guanidinium functionalized M12L24 nanosphere

Aggregation-induced heterogeneities in the emission of upconverting nanoparticles at the submicron scale unfolded by hyperspectral microscopy

Transparent upconverting hybrid nanocomposites are exciting materials for advanced applications such as 3D displays, nanosensors, solar energy converters, and fluorescence biomarkers. This work presents a simple strategy to disperse upconverting b-NaYF4:Yb3+/Er3+ or Tm3+ nanoparticles into low cost, widely used and easy-to-process polydimethylsiloxane (PDMS)-based organic–inorganic hybrids. The upconverting hybrids were shaped as monoliths, films or powders displaying in the whole volume Tm3+ or Er3+ emissions (in the violet/blue and green/red spectral regions, respectively). For the first time, hyperspectral microscopy allows the identification at the submicron scale of differences in the hybrids' emission colour, due to variations in the relative intensity of the distinct components of the upconversion spectrum. The effect is attributed to the size distribution of the agglomerates of nanoparticles, highlighting the importance of studying the emission at submicron scales, since this effect is not observable in measurements recorded in larger areas

Photon Up-Conversion with Lanthanide-Doped Oxide Particles for Solar H₂ Generation

Up-conversion (UC) of infrared (IR) photons into visible radiation constitutes a promising strategy to enhance the light harvesting efficiency of photovoltaic and photoelectrochemical devices. In the present study, we integrate Er³⁺/Yb³⁺-codoped yttrium oxide (Y₂O₃) submicrometric particles with outstanding up-conversion properties into mesoporous titanium oxide (TiO₂) structures sensitized with cadmium selenide (CdSe) for solar hydrogen generation. We demonstrate that the incorporation of these up-converting particles (UCP) leads to effective H₂ generation with IR photons. Moreover, based on the analysis of the emission lifetimes, we show that the optical interaction between the emitting UCPs and the CdSe absorber occurs via a radiative emission–reabsorption process. The low cost and toxicity and excellent chemical and thermal stability of our UC phosphors allow envisaging them as real candidates for the new generation of long-term photoelectrochemical devices for solar H₂ generation