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
  
  • Tetrathiafulvalene-Benzothiadiazoles as Redox-Tunable Donor-Acceptor Systems: Synthesis and Photophysical Study
    F. Pop, A. Amacher, N. Avarvari, J. Ding, L.M. Lawson Daku, A. Hauser, M. Koch, J. Hauser, S.-X. Liu and S. Decurtins
    Chemistry - A European Journal, 19 (7) (2013), p2504-2514
    DOI:10.1002/chem.201202742 | unige:26401 | Abstract | Article PDF
Electrochemical and photophysical analysis of new donor–acceptor systems 2 and 3, in which a benzothiadiazole (BTD) unit is covalently linked to a tetrathiafulvalene (TTF) core, have verified that the lowest excited state can be ascribed to an intramolecular-charge-transfer (ICT) π(TTF)→π*(benzothiadiazole) transition. Owing to better overlap of the HOMO and LUMO in the fused scaffold of compound 3, the intensity of the 1ICT band is substantially higher compared to that in compound 2. The corresponding CT fluorescence is also observed in both cases. The radical cation TTF+. is easily observed through chemical and electrochemical oxidation by performing steady-state absorption experiments. Interestingly, compound 2 is photo-oxidized under aerobic conditions.
  • Thermal hysteresis in spin-crossover compounds studied within the mechanoelastic model and its potential application to nanoparticles
    C. Enachescu, P. Chakraborty, L. Stoleriu, A. Stancu and A. Hauser
    Physical Review B, 84 (13) (2011), p134102
    DOI:10.1103/PhysRevB.84.134102 | unige:17482 | Abstract | Article PDF
 
The recently developed mechanoelastic model is applied to characterize the thermal transition in spin-crossover complexes, with special attention given to the case of spin-crossover nanoparticles. In a two-dimensional system, hexagonal-shaped samples with open boundary conditions are composed of individual molecules that are linked by springs and can switch between two states, namely, the high-spin (HS) and the low-spin (LS) states. The switching of an individual molecule during the spin transition is decided by way of a Monte Carlo standard procedure, using transition probabilities depending on the temperature, the energy gap between the two states, the enthalpy difference, the degeneracy ratio, and the local pressure determined by the elongation or compression of its closest springs. The influence of external parameters, such as temperature sweeping rate and pressure, or intrinsic features of the system, such as the value of its spring constant, on the width of the thermal hysteresis, its shape, and its position are discussed. The particular case of spin-crossover nanoparticles is treated by considering them embedded into a polymer environment, which essentially affects the molecules situated at the edges and faces by decreasing their transition probabilities from HS to LS. Finally, the pressure hysteresis, obtained by varying the external pressure at constant temperature is discussed.
  • Competition Between Photo-Excitation and Relaxation in Spin Crossover Complexes in The Frame of a Mechano-Elastic Model
    C. Enachescu, L. Stoleriu, A. Stancu and A. Hauser
    Physical Review B, 82 (2010), p104114
    DOI:10.1103/PhysRevB.82.104114 | unige:14682 | Abstract | Article PDF
In this paper we use a recently proposed elastic model in order to study the competition between linear photoexcitation and cooperative relaxation in spin-crossover molecular magnets. The difference in molecular size between the two possible spin states, that is, the high-spin and the low-spin states, respectively, induces distortions of the crystal lattice. These determine the elastic interactions between molecules, treated here as connecting springs that are either compressed or extended from their equilibrium length, thus modulating the local probability for the high-spinlow-spin relaxation. The crossover of individual molecules within the lattice is checked by a standard Monte Carlo procedure. Using very simple assumptions and a minimum number of parameters, photoexcitation curves and hysteresis loops under continuous irradiation below the thermal transition temperature can thus be simulated. The formation of clusters is analyzed and the presence of inhomogeneities in the system is investigated.
  
  • Cluster evolution in spin crossover systems observed in the frame of a mechano-elastic model
    C. Enachescu, M. Nishino, S. Miyashita, L. Stoleriu, A. Stancu and A. Hauser
    Europhysics Letters, 91 (2) (2010), p27003
    DOI:10.1209/0295-5075/91/27003 | unige:14746
In this paper we study the cluster formation and evolution in spin crossover systems during the thermal transition in the frame of a mechano-elastic model applied to open boundary hexagonal lattices. The switching processes between the high-spin (HS) and low-spin (LS) state are studied by a method combining a Monte Carlo standard procedure on the spin state and the lattice relaxation. In the present study, we adopt the transition probabilities of the spin state taking into account the energy gap between the two states, the degeneracy ratio and the local pressure determined by the elongations of the closest springs. It is found that clusters of molecules in the same state tend to grow starting from corners, as in available experimental data. Some qualitative differences between the processes of cluster formation for the two hysteresis branches, i.e., HS to LS and LS to HS are pointed out. Moreover, we have studied the dependence of cluster formation on the strength of the elastic interactions, and also on the system size. The size dependence of the ratio between the system size and the maximum cluster length is very weak, which indicates the appearance of macroscopic domains.
  • A Compactly Fused π-Conjugated Tetrathiafulvalene-Perylenediimide Donor-Acceptor Dyad
    M. Jaggi, C. Blum, N. Dupont, J. Grilj, S.-X. Liu, J. Hauser, A. Hauser and S. Decurtins
    Organic Letters, 11 (14) (2009), p3096-3099
    DOI:10.1021/ol901186n | unige:3553 | Abstract | Article HTML | Article PDF
 
The synthesis and structural characterization of a tetrathiafulvalene-fused perylenediimide molecular dyad is presented. Its largely extendedπ-conjugation provides intense optical absorption bands over a wide spectral range. The planar functional molecule exhibits a short-livednonluminescent excited state attributed to intramolecular charge separation.
  
  • Crystalline, Mixed-Valence Manganese Analogue of Prussian Blue: Magnetic, Spectroscopic, X-ray and Neutron Diffraction Studies
    P. Franz, C. Ambrus, A. Hauser, D. Chernyshov, M. Hostettler, J. Hauser, L. Keller, K. Krmer, H. Stoeckli-Evans, P. Pattison, H.-B. Brgi and S. Decurtins
    Journal of the American Chemical Society, 126 (50) (2004), p16472-16477
    DOI:10.1021/ja0465451 | unige:3247 | Abstract | Article HTML | Article PDF
The compound of stoichiometry Mn(II)3[Mn(III)(CN)6]2·zH2O (z = 12−16) (1) forms air-stable, transparent red crystals. Low-temperature single crystal optical spectroscopy and single crystal X-ray diffraction provide compelling evidence for N-bonded high-spin manganese(II), and C-bonded low-spin manganese(III) ions arranged in a disordered, face-centered cubic lattice analogous to that of Prussian Blue. X-ray and neutron diffraction show structured diffuse scattering indicative of partially correlated (rather than random) substitutions of [Mn(III)(CN)6] ions by (H2O)6 clusters. Magnetic susceptibility measurements and elastic neutron scattering experiments indicate a ferrimagnetic structure below the critical temperature Tc = 35.5 K.

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