Size-controlled nanocrystals (140 nm and 670 nm) and microcrystals (2.5 mm) of the three-dimensional oxalate network [Ru(bpy)3][NaCr(ox)3], ox = oxalate, bpy = 2,2’-bipyridine, were prepared by the reverse micelle technique. The photo-physical properties of the [Cr(ox)3]3- chromophores in the nanocrystals at low temperatures are significantly different from those of the same chromophore in 4 mm crystallites prepared by fast precipitation. For the latter, the absorption in the region of the R lines of the 4A2 2E transition is characterized by a sharp doublet. For the nanocrystals the inhomogeneous broadening of the two lines is considerably larger with tails on the low-energy side. Whereas the 4 mm crystallites at low temperatures just show equally sharp emission from the R1 line, the emission intensity from the nanocrystallites is shifted into the low-energy tail. Time resolved fluorescence line narrowing spectra and luminescence decay curves demonstrate that this is due to efficient directional energy migration from the center of the nanocrystals towards the surface
  
Size-controlled micro- and nanocrystals of a [Ru(bpy)3][NaCr(ox)3] coordination network are prepared using reverse micelles. Compared with the bulk reference, the 2E emission of the Cr(III) ions indicates an efficient directional energy migration toward the surface of the nanocrystals.
 
The Hoffman-type coordination compound [Fe(pz)Pt(CN)4]⋅2.6 H2O (pz=pyrazine) shows a cooperative thermal spin transition at around 270 K. Synchrotron powder X-Ray diffraction studies reveal that a quantitative photoinduced conversion from the low-spin (LS) state into the high-spin (HS) state, based on the light-induced excited spin-state trapping effect, can be achieved at 10 K in a microcrystalline powder. Time-resolved measurements evidence that the HS→LS relaxation proceeds by a two-step mechanism: a random HS→LS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HS→LS transformation has been reached.
  
  • Structure and properties of complex hydride perovskite materials
    P. Schouwink, M.B. Ley, A. Tissot, H. Hagemann, T.R. Jensen, L. Smrcok and R. Cern
    Nature Communications, 5 (2014), p5706
    DOI:10.1038/ncomms6706 | unige:43536 | Abstract | Article HTML
Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H, substituting for the oxide anion in ​BaTiO3. Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical ​tetrahydroborate anion ​BH4 and new synthesis protocols involving rare-earth elements. Photophysical, electronic and ​hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH4-specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 × the unit cell volume in AB(BH4)3. In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing ​BH4−←X (X=Cl−, Br, I) provides a link to the known ABX3 halides.
  • Light-induced spin-state switching in the mixed crystal series of the 2D coordination network {[Zn1-xFex(bbtr)3](BF4)2}: optical spectroscopy and cooperative effects
    P. Chakraborty, C. Enachescu, A. Humair, L. Egger, T. Delgado, A. Tissot, L. Gune, C. Besnard, R. Bronisz and A. Hauser
    Dalton Transactions, 43 (47) (2014), p17786-17796
    DOI:10.1039/C4DT01728E | unige:42340 | Abstract | Article HTML | Article PDF
 
Depending on the iron(II) concentration, the mixed crystals of {[Zn1-xFex(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, 0.01 ≤ x ≤ 1, show macroscopic light-induced bistability between the high-spin and the low-spin state. In the highly diluted system with x = 0.01 and up to x = 0.31, the photoinduced low-spin state always relaxes back to the high-spin state independent of the initial light-induced low-spin fraction. In the highly concentrated mixed crystals with x = 0.67, 0.87 and 1, the strong cooperative effects coupled to a crystallographic phase transition result in light-induced bistability with decreasing critical light-induced low-spin fraction and increasing hysteresis width for increasing iron(II) concentrations. The lower limit for the light-induced bistability is estimated at x ≈ 0.5.
  
  • Oxadiazole based bipolar host materials employing planarized triarylamine donors for RGB PHOLEDs with low efficiency roll-off
    P. Kautny, D. Lumpi, Y. Wang, A. Tissot, J. Bintinger, E. Horkel, B. Stoeger, C. Hametner, H. Hagemann, D. Ma and J. Frhlich
    Journal of Materials Chemistry C, 2 (11) (2014), p2069-2081
    DOI:10.1039/c3tc32338b | unige:34399 | Abstract | Article HTML | Article PDF
A series of 6 novel triarylamine-containing oxadiazole compounds (o-PCzPOXD, o-ICzPOXD, o-TPATOXD, o-PCzTOXD, o-ICzTOXD, o-CzTOXD) have been designed, synthesized and characterized concerning applications as host materials in PHOLED devices. To further improve the ortho-linkage concept, the impact of incorporating planarized electron-donating triarylamine (TAA) structures on intramolecular charge transfer was examined. The effect was evaluated for two series of electron-accepting oxadiazole scaffolds, realizing ortho-linkage on the benzene (POXD) and the thiophene (TOXD) core. Thermal analysis shows increased glass-transition temperatures for planarized structures indicating an improved morphological stability. A higher degree of planarization also results in significantly increased singlet and triplet energy values, revealing the impact on the intramolecular charge transfer. Employing the developed materials, red (o-TPATOXD: CEmax: 28.8 cd A-1, EQEmax: 16.9%), green (o-PCzPOXD: CEmax: 62.9 cd A-1, EQEmax: 17.1%) and blue (o-PCzPOXD: CEmax: 29.8 cd A-1, EQEmax: 13.4%) devices were achieved showing remarkably low efficiency roll-off for planarized donors. Hence, this is the first report of efficient blue devices for this specific class of host materials. It is proposed that the results correlate with an increasing ortho-linkage effect and decreasing donor strength of the TAA moiety by planarization and, thus, tackling one of the major challenges in PHOLED research: improving both triplet energy and compound stability.
  • Determination of the molecular structure of the short-lived light-induced high-spin state in the spin-crossover compound [Fe(6-mepy)3tren](PF6)2
    P. Chakraborty, A. Tissot, L. Peterhans, L. Gune, C. Besnard, P. Pattison and A. Hauser
    Physical Review B, 87 (21) (2013), p214306
    DOI:10.1103/PhysRevB.87.214306 | unige:28964 | Abstract | Article PDF
In the spin-crossover compound [Fe(6-mepy)3tren](PF6)2, (6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-butenyl}amine, the high-spin state can be populated as metastable state below the thermal transition temperature via irradiation into the metal to ligand charge transfer absorption band of the low-spin species. At 10 K, the lifetime of this metastable state is only 1 s. Despite this, it is possible to determine an accurate excited state structure by following the evolution of relevant structural parameters by synchrotron X-ray diffraction under continuous irradiation with increasing intensity. The difference in metal-ligand bond length between the high-spin and the low-spin state is found to be 0.192 Å obtained from an analysis of the experimental data using the mean-field approximation to model cooperative effects.
A switch in time: A fast precipitation technique was used to prepare 75 nm FeII spin-crossover nanocrystals. Their photoswitching dynamics, based on the light-induced excited spin-state trapping effect, has been investigated by means of optical spectroscopy. A significant variation of the switching proprieties is observed compared to similar but amorphous nanoparticles.
  • Stimuli Responsive Hybrid Magnets: Tuning the Photoinduced Spin-Crossover in Fe(III) Complexes Inserted into Layered Magnets
    M. Clemente-Len, E. Coronado, M. Lpez-Jord, J.C. Waerenborgh, C. Desplanches, H. Wang, J.-F. Ltard, A. Hauser and A. Tissot
    Journal of the American Chemical Society, 135 (23) (2013), p8655-8667
    DOI:10.1021/ja402674x | unige:28703 | Abstract | Article HTML | Article PDF
 
The insertion of a [Fe(sal2-trien)]+ complex cation into a 2D oxalate network in the presence of different solvents results in a family of hybrid magnets with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compounds [FeIII(sal2-trien)][MnIICrIII(ox)3]·CHCl3 (1·CHCl3), [FeIII(sal2-trien)][MnIICrIII(ox)3]·CHBr3 (1·CHBr3), and [FeIII(sal2-trien)][MnIICrIII(ox)3]·CH2Br2 (1·CH2Br2). The three compounds crystallize in a 2D honeycomb anionic layer formed by MnII and CrIII ions linked through oxalate ligands and a layer of [Fe(sal2-trien)]+ complexes and solvent molecules (CHCl3, CHBr3, or CH2Br2) intercalated between the 2D oxalate network. The magnetic properties and Mössbauer spectroscopy indicate that they undergo long-range ferromagnetic ordering at 5.6 K and a spin crossover of the intercalated [Fe(sal2-trien)]+ complexes at different temperatures T1/2. The three compounds present a LIESST effect with a relaxation temperature TLIESST inversely proportional to T1/2. The isostructural paramagnetic compound, [FeIII(sal2-trien)][ZnIICrIII(ox)3]·CH2Cl2 (2·CH2Cl2) was also prepared. This compound presents a partial spin crossover of the inserted FeIII complex as well as a LIESST effect. Finally, spectroscopic characterization of the FeIII doped compound [Ga0.99Fe0.01(sal2trien)][MnIICrIII(ox)3]·CH2Cl2 (3·CH2Cl2) shows a gradual and complete thermal spin crossover and a LIESST effect on the isolated FeIII complexes. This result confirms that cooperativity is not a necessary condition to observe the LIESST effect in an FeIII compound.
  
Four-dimensional (4D) electron microscopy (EM) uniquely combines the high spatial resolution to pinpoint individual nano-objects, with the high temporal resolution necessary to address the dynamics of their laser-induced transformation. Here, using 4D-EM, we demonstrate the in situ irreversible transformation of individual nanoparticles of the molecular framework Fe(pyrazine)Pt(CN)4. The newly formed material exhibits an unusually large negative thermal expansion (i.e. contraction), which is revealed by time-resolved imaging and diffraction. Negative thermal expansion is a unique property exhibited by only few materials. Here we show that the increased flexibility of the metal–cyanide framework after the removal of the bridging pyrazine ligands is responsible for the negative thermal expansion behavior of the new material. This in situ visualization of single nanostructures during reactions should be extendable to other classes of reactive systems.
  • Single-nanoparticle phase transitions visualized by four-dimensional electron microscopy
    R.M. Van der Veen, O.-H. Kwon, A. Tissot, A. Hauser and A.H. Zewail
    Nature Chemistry, 5 (5) (2013), p395-402
    DOI:10.1038/NCHEM.1622 | unige:27715 | Abstract | Article HTML | Article PDF

The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structural dynamics in real and reciprocal space. Here, we demonstrate the selectivity and sensitivity of the technique by visualizing the spin crossover dynamics of single, isolated metal–organic framework nanocrystals. By introducing a small aperture in the microscope, it was possible to follow the phase transition and the associated structural dynamics within a single particle. Its behaviour was observed to be distinct from that imaged by averaging over ensembles of heterogeneous nanoparticles. The approach reported here has potential applications in other nanosystems and those that undergo (bio)chemical transformations.
  • Unidirectional Photoisomerization of Styrylpyridine for Switching the Magnetic Behavior of an Iron(II) Complex: A MLCT Pathway in Crystalline Solids
    A. Tissot, M.-L. Boillot, S. Pillet, E. Codjovi, K. Boukheddaden and L.M. Lawson Daku
    Journal of Physical Chemistry C, 114 (49) (2010), p21715-21722
    DOI:10.1021/jp106583f | unige:14853 | Abstract | Article HTML | Article PDF
 
The photoreactivity of two iron(II)−styrylpyridine frameworks Fe(stpy)4(NCSe)2 (stpy = 4-styrylpyridine) has been investigated for the very first time in a crystalline solid. A quantitative cis-to-trans isomerization of stilbenoids is shown to occur in the confined environment of the inorganic solid. The photochromic reaction was driven by a visible excitation into the metal-to-ligand charge transfer absorption of the high-spin all-cis complex. The solid-state transformation is accompanied by a unit-cell volume increase and an amorphization. Interestingly, the photoproduct formed by irradiating the high-spin all-cis reactant undergoes a spin conversion when the temperature is decreased. This observation is related to the “ligand-driven light-induced spin change” effect in a constrained environment.

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