RESUMO
Prussian blue analogue nanocrystals of the CsINiII[CrIII(CN)6] cubic network with 6 nm size were assembled as a single monolayer on highly organized pyrolytic graphite (HOPG). X-ray magnetic circular dichroism (XMCD) studies, at the Ni and Cr L2,3 edges, reveal the presence of an easy plane of magnetization evidenced by an opening of the magnetic hysteresis loop (coercive field of ≈200 Oe) when the magnetic field, B, is at 60° relative to the normal to the substrate. The angular dependence of the X-ray natural linear dichroism (XNLD) reveals both an orientation of the nanocrystals on the substrate and an anisotropy of the electronic cloud of the NiII and CrIII coordination sphere species belonging to the nanocrystals' surface. Ligand field multiplet (LFM) calculations that reproduce the experimental data are consistent with an elongated tetragonal distortion of surface NiII coordination sphere responsible for the magnetic behavior of monolayer.
RESUMO
Light-induced spin-state switching is one of the most attractive properties of spin-crossover materials. In bulk, low-spin (LS) to high-spin (HS) conversion via the light-induced excited spin-state trapping (LIESST) effect may be achieved with a visible light, while the HS-to-LS one (reverse-LIESST) requires an excitation in the near-infrared range. Now, it is shown that those phenomena are strongly modified at the interface with a metal. Indeed, an anomalous spin conversion is presented from HS state to LS state under blue light illumination for FeII spin-crossover molecules that are in direct contact with metallic (111) single-crystal surfaces (copper, silver, and gold). To interpret this anomalous spin-state switching, a new mechanism is proposed for the spin conversion based on the light absorption by the substrate that can generate low energy valence photoelectrons promoting molecular vibrational excitations and subsequent spin-state switching at the molecule-metal interface.
RESUMO
By imitating FIrpic, seven new platinum(II) complexes with pic (pic = picolinate) ligand have been designed to be guest materials by means of adding different substituents to functionalized ligands (ppy and fpy, ppy = phenylpyridyl-N,C and fpy = 2-(9',9'-diethyl-9H-fluorenyl)pyridyl-N,C). In order to reveal their molecular structures, photophysical properties and structure-property relationships with typical host materials, an in-depth theoretical investigation was performed via quantum chemical calculations. The electronic structures and photophysical properties of these complexes were investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) using the B3LYP functional with LANL2DZ and 6-31G* basis sets. It turns out that electronic structures and photophysical properties can be tuned by substituent modifications on functionalized ligands. This work highlights that the match between guest materials and host materials in typical OLED structures can be weighed by the energy levels of the HOMO and LUMO and the adiabatic triplet energy of each complex. Also, a combined analysis of electronic structures, host-guest match, reorganization energies (λ) and triplet exciton generation fraction (χ(T)) is helpful in exploring triplet emitters with high phosphorescence efficiency in OLEDs, which is an interesting and creative aspect of this work. Thereinto, λ reveals the capability of carrier transport and the balance between holes and electrons, whilst structural parameters and d-orbital splittings show that those complexes that have strong electron-withdrawing and electron-donating groups are nonemissive. Consequently, complexes 3-7 can be better triplet emitters than FIrpic. Moreover, the emission colors could be predicted by the 0-0 transition energy (E(0-0)) instead of the triplet vertical transition energy (E(vert)). Accordingly, complexes 3, 4 and 6 would be efficient phosphorescent materials with different predicted emission colors.
