Predicting atomic dopant solvation in helium clusters: the MgHen_n case

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg doped helium clusters MgHe$_n$ with $n=2-50$. Three high level (MP4, CCSD(T) and CCSDT) MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He$_n$. Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited $^1$P Mg atom and the He moiety has been approximated using the Diatomics-in-Molecules method and the two excited $^1\Pi$ and $^1\Sigma$ MgHe potentials. The shape of the simulated MgHe$_{50}$ spectra show a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact Density Functional Theory has been adapted to the case of doped He$_n$ and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviours

Communication: Nucleation of quantized vortex rings in 4He nanodroplets

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba+ ion created by photoionization of neutral Ba at the surface of a 4He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba+ ion. The process is expected to be quite general and very efficient under standard experimental conditions

Unraveling the degradation mechanism in FIrpic based Blue OLEDs: II. Trap and detect molecules at the interfaces

The impact of organic light emitting diodes (OLEDs) in modern life is witnessed by their wide employment in full-color, energy-saving, flat panel displays and smart-screens; a bright future is likewise expected in the field of solid state lighting. Cyclometalated iridium complexes are the most used phosphorescent emitters in OLEDs due to their widely tunable photophysical properties and their versatile synthesis. Blue-emitting OLEDs, suffer from intrinsic instability issues hampering their long term stability. Backed by computational studies, in this work we studied the sky-blue emitter FIrpic in both ex-situ and in-situ degradation experiments combining complementary, mutually independent, experiments including chemical metathesis reactions, in liquid phase and solid state, thermal and spectroscopic studies and LC-MS investigations. We developed a straightforward protocol to evaluate the degradation pathways in iridium complexes, finding that FIrpic degrades through the loss of the picolinate ancillary ligand. The resulting iridium fragment was than efficiently trapped "in-situ" as BPhen derivative 1. This process is found to be well mirrored when a suitably engineered, FIrpic-based, OLED is operated and aged. In this paper we (i) describe how it is possible to effectively study OLED materials with a small set of readily accessible experiments and (ii) evidence the central role of host matrix in trapping experiments.Comment: 13 pages, 6 figure

Picosecond solvation dynamics of alkali cations in superfluid 4He nanodroplets

The dynamics following the photoionization of neutral Rb and Cs atoms residing in a dimple at the surface of a superfluid 4He1000 nanodroplet has been investigated within time-dependent density functional theory, complementing a previous study on Ba. The calculations reveal that structured high density helium solvation layers form around both the Rb+ and Cs+ cation on a picosecond time scale, forming so-called snowballs. In contrast to the Rb+ ion, Cs+ is not solvated by the 4He1000 droplet but rather desorbs from it as a Cs+Hen snowball. This outcome is partially related to the large size of Cs+ cation in relation to the helium droplet as is revealed by calculations performed using a planar helium surface. The large droplet deformations induced by the solvation of the Rb+ cation is found to lead to efficient nucleation of quantized vortex loops or rings

Exciplexes with Ionic Dopants: Stability, Structure, and Experimental Relevance of M⁺(²P)⁴He_<i>n</i> (M = Sr, Ba)

M⁺(²P)⁴He_<i>n</i> species, possibly involved in the post ²P ← ²S excitation dynamics of Sr⁺ and Ba⁺ in cold ⁴He gas or droplets, are studied employing both high level <i>ab initio</i> calculations to determine the potential energy curves (PEC) and diffusion Monte Carlo (DMC) to obtain information on their ground state structure and energetics. PEC for the excited M⁺(²P)He dimers were obtained using MRCI calculations with extended basis sets. Potential energy surfaces (PES) for M⁺(²P)­He_<i>n</i> were built with the DIM model including spin–orbit coupling via a perturbative procedure. DMC simulations indicated several exciplexes (<i>n</i> > 2) to be stable against He dissociation whatever the ion state, a finding that is at variance with what was previously suggested for Ba⁺(²P_1/2) due to the repulsive nature of the interaction potential obtained in [Phys. Rev. A 2004, 69, 042505]. Our results, instead, support the suggestion made in [J. Chem. Phys. 2012, 137, 051102] for the existence of Ba⁺(²P_1/2)­He_<i>n</i> exciplexes emitted following the excitation of the barium cation solvated into He droplets. In the ²P_1/2 state, the Ba ion also shows a peculiar behavior as a function of <i>n</i> with respect to the location and binding strength of the attached He atoms compared to Sr⁺. Although the latter forms the usual equatorial He ring, Ba⁺ deviates from this geometry for 1 ≤ <i>n</i> ≤ 4, with the radial distribution functions strongly depending on the number of solvent atoms. Finally, a putative species is proposed to explain the emission band at 523 nm that follows D1 or D2 excitation of Ba⁺ in superfluid bulk helium

The (<i>E</i>, <i>Z</i>) Isomerization of <i>C</i>-methoxycarbonyl-<i>N</i>-aryl Chlorohydrazones

Since chlorohydrazones are planar molecules, it is in principle possible to distinguish between their E and Z stereoisomers. Chlorohydrazones are known to preferentially assume the Z configuration around the C=N double bond, and their (E, Z) isomerization is almost suppressed at room temperature. The lack, or rather the difficulty, of such an isomerization has been conveniently addressed by the in-depth theoretical study of seven C-methoxycarbonyl-N-aryl chlorohydrazones (aryl = phenyl, 4-chlorophenyl, 4-bromophenyl, 4-iodophenyl, 2-chlorophenyl, 2-bromophenyl, and 2-iodophenyl). DFT ωB97M-D4/cc-pVTZ calculations of these C-methoxycarbonyl-N-aryl chlorohydrazones, supported by the XRD determination of the molecular structure, provided a complete picture of the isomerization processes in the studied compounds. The analysis of the energetics, molecular geometry, and electronic structure (the latter in the framework of the Quantum Theory of Atoms In Molecules) showed that the Z isomers are thermodynamically favored because, within the low-energy planar isomers with extensive π conjugation, the electrostatic interactions between the dipoles of the C–O, C–Cl, and N–H bonds overcome the stabilization induced by the N–H ··· O bond present in the E isomers. We confirmed that the (E, Z) isomerization occurs by the umklapp mechanism, in which the –NHAr moiety rotates in the molecular plane towards a linear C=N–N configuration and then proceeds to the other isomer. The (E, Z) isomerization is very slow at room temperature because the umklapp interconversion has high barriers (≈110 kJ/mol) despite the extended π electron delocalization present in the transition state