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1.
Inorg Chem ; 2020 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-32857507

RESUMO

Photoswitchable 11 nm nanocrystals with the coordination network Cs{Co[Fe(CN)6]} were obtained using a template-free method. The nanocrystals were recovered from the colloidal solutions as solid materials surrounded by cetyltrimethylammonium (CTA) cations or embedded in the organic polymer polyvinylpyrrolidone (PVP). Complementary magnetic, spectroscopic, and structural techniques, including EPR spectroscopy, reveal a majority (∼70%) of the low-spin and photoactive diamagnetic CoIIIFeII pairs located in the core of the nanocrystals and a mixture of CoIIFeII and CoIIFeIII species present mainly within the shell of the objects. While bulk compounds with similar vacancy concentration do not exhibit noticeable photoinduced charge transfer, the observed photoactivity of the nanocrystals is ascribed to their nanometric size. The relaxation temperature of the photoinduced state shifts upward by ∼55 K when PVP is replaced by CTA. This is ascribed to the larger rigidity of the dense CsCoFe_CTA material, whose metastable state is lower than that for CsCoFe_PVP, leading to a larger relaxation energy barrier and, therefore, to a higher relaxation temperature.

2.
Artigo em Inglês | MEDLINE | ID: mdl-32358911

RESUMO

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn3+ compound [Mn(3,5-diBr-sal2 (323))]BPh4 , 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc-P1 transition.

3.
Nat Chem ; 12(5): 429-430, 2020 May.
Artigo em Inglês | MEDLINE | ID: mdl-32346133
4.
J Am Chem Soc ; 142(2): 931-936, 2020 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-31880442

RESUMO

A one-dimensional coordination solid 1c is synthesized by reaction of a bispyridyl dithienylethene (DTE) photochromic unit with the highly anisotropic dysprosium-based single-molecule magnet [Dy(Tppy)F(pyridine)2]PF6. Slow magnetic relaxation characteristics are retained in the chain compound 1c, and photoisomerization of the bridging DTE ligand induces a single-crystal-to-single-crystal transformation that can be monitored using photocrystallography. Notably, the resulting chain compound 1o exhibits faster low-temperature relaxation than that of 1c, which is apparent in magnetic hysteresis data collected for both compounds as high as 4 K. Ab initio calculations suggest that this photomodulation of the magnetic relaxation behavior is due to crystal packing changes rather than changes to the crystal field splitting upon ligand isomerization.

5.
Angew Chem Int Ed Engl ; 58(34): 11811-11815, 2019 Aug 19.
Artigo em Inglês | MEDLINE | ID: mdl-31233272

RESUMO

Molecular magnetic switches are expected to form the functional components of future nanodevices. Herein we combine detailed (photo-) crystallography and magnetic studies to reveal the unusual switching properties of an iron(III) complex, between low (LS) and high (HS) spin states. On cooling, it exhibits a partial thermal conversion associated with a reconstructive phase transition from a [HS-HS] to a [LS-HS] phase with a hysteresis of 25 K. Photoexcitation at low temperature allows access to a [LS-LS] phase, never observed at thermal equilibrium. As well as reporting the first iron(III) spin crossover complex to exhibit reverse-LIESST (light-induced excited spin state trapping), we also reveal a hidden hysteresis of 30 K between the hidden [LS-LS] and [HS-LS] phases. Moreover, we demonstrate that FeIII spin-crossover (SCO) complexes can be just as effective as FeII systems, and with the advantage of being air-stable, they are ideally suited for use in molecular electronics.

6.
Adv Mater ; 31(25): e1901361, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-31034107

RESUMO

Using ultrafast optical absorption spectroscopy, the room-temperature spin-state switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover (SCO) complex [Fe(HB(tz)3 )2 ] (tz = 1,2,4-triazol-1-yl) are studied. These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out-of-equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high-frequency all-optical switches working at room temperature.

7.
Phys Chem Chem Phys ; 21(12): 6606-6612, 2019 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-30854539

RESUMO

We present a study of a molecular material, [Feiii(3-MeO-SalEen)2]PF6, undergoing cooperative reversible photo-induced transition between low-spin state and high-spin state. By using temporally multiscale pump-probe laser spectroscopy, we explore the key parameters that influence the low-spin to high-spin conversion efficiency through long range elastic intermolecular interactions during the so-called elastic step, where crystalline volume expansion takes place. We rationalize our findings using Monte Carlo simulations, and a mechano-elastic model. The experimental results and the simulations support the existence of a fast mechanism by which molecules cooperatively switch through coupling to the lattice strain. The efficiency of the coupling process is shown to depend on several parameters including the initial thermal population and the instantaneous photo-induced population among others. Far below the crossover temperature, the elastic self-amplification occurs above a threshold photo-excitation. On approaching the thermal crossover, the threshold disappears and the photo-elastic conversion increases.

8.
Inorg Chem ; 57(16): 9880-9891, 2018 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-30080034

RESUMO

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L = 5,5'-dimethyl-2,2'-bipyridine (bipy-CH3, 1), L = dimethyl-2,2'-bipyridyl-5,5'-dicarboxylate (MeObpydc, 2), L = diethyl-2,2'-bipyridyl-5,5'-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2'-bipyridine-5,5'-dicarboxylate ( i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

9.
Chem Sci ; 9(25): 5623-5629, 2018 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-30061995

RESUMO

Molecule-based spin state switching materials that display ambient temperature transitions with accompanying wide thermal hysteresis offer an opportunity for electronic switching, data storage, and optical technologies but are rare in existence. Here, we present the first 2D Hofmann-type materials to exhibit the elusive combination of ambient temperature spin crossover with wide thermal hysteresis (ΔT = 50 and 65 K). Combined structural, magnetic, spectroscopic, and theoretical analyses show that the highly cooperative transition behaviours of these layered materials arise due to strong host-host interactions in their interdigitated lattices, which optimises long-range communication pathways. With the presence of water molecules in the interlayer pore space in the hydrated phases, competing host-host and host-guest interactions occur, whilst water removal dramatically increases the framework cooperativity, thus affording systematic insight into the structural features that favour optimal spin crossover properties.

10.
Chemistry ; 24(55): 14760-14767, 2018 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-29985534

RESUMO

Spin-transition compounds are coordination complexes that can present two stable or metastable high-spin and low-spin states at a given temperature (thermal hysteresis). The width of the thermal hysteresis (difference between the maximum and minimum temperature between which the compound exhibits bi-stability) depends on the interactions between the coordination complexes within the compound, and which may be modulated by the absence or presence of solvent within the structure. The new compound [Fe(3-bpp)2 ][Au(CN)2 ]2 (1, 3-bpp=2,6-di-(1H-pyrazol-3-yl)pyridine) was synthesized and its properties were compared with those of the solvated compound [Fe(3-bpp)2 ][Au(CN)2 ]2 ⋅2 H2 O (1.H2 O) already described. 1 has a two-steps thermal hysteresis of 45 K, in contrast to the compound 1.H2 O which exhibits a gradual conversion without hysteresis. This hysteretic transition is accompanied by a reversible reconstructive structural transition and twinning. This stepped behaviour is also observed in the photomagnetic properties despite the low efficiency of photoswitching. Single-crystal photocrystallographic investigations confirm this low conversion, which we attributed to the high energy cost to form the high-spin structure, whose symmetry differs from that of the low-spin phase.

11.
Chemistry ; 24(59): 15696-15705, 2018 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-29968946

RESUMO

Directing the functionality of molecules, materials and biophysical systems is challenging both from fundamental and applied standpoints. For example, understanding the elementary processes responsible for light-induced transformations require watching electronic and structural reorganizations on their intrinsic timescales. The X-ray free electron lasers (X-FEL) represent a new generation of incredibly short and ultra-bright X-ray source, which open new possibilities for developing the multidisciplinary field of ultrafast science. Experiments around X-FEL provide probes, sensitive to electronic and structural reorganizations, able to monitor transformations on the femtosecond timescale (1 fs=10-15  s). Recent years have seen terrific successes in providing a detailed view on light-induced processes, compared to what was understood from conventional optical pump-probe spectroscopy. This Concept article aims at illustrating, through recent studies mainly focussing on light-induced excited spin state trapping, how these X-FEL based techniques can help understanding light-activated functions, by monitoring elementary electronic and structural processes that may occur beyond the Born-Oppenheimer approximation.


Assuntos
Complexos de Coordenação/química , Lasers , Elétrons , Cinética , Estrutura Molecular , Fenômenos Físicos , Espectrometria por Raios X , Termodinâmica , Raios X
12.
IUCrJ ; 5(Pt 3): 361-372, 2018 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-29755752

RESUMO

A mixed-valence conducting cation radical salt of the unsymmetrically substituted o-Me2TTF donor molecule (TTF is tetrathiafulvalene) was obtained upon electrocrystallization in the presence of the non-centrosymmetric NO3- anion. It crystallizes at room temperature in the monoclinic P21/c space group, with the anion disordered on an inversion centre. The donor molecules are stacked along the a axis. A 90° rotation of the longest molecular axis of o-Me2TTF generates a chessboard-like structure, preventing lateral S⋯S contacts between stacks and providing a strongly one-dimensional electronic system, as confirmed by overlap interaction energies and band structure calculations. A strong dimerization within the stacks explains the semi-conducting behaviour of the salt, with σroom temp = 3-5 S cm-1 and Eactivated = 0.12-0.14 eV. An X-ray diffuse scattering survey of reciprocal space, combined with full structure resolutions at low temperatures (250, 85 and 20 K), evidenced the succession of two structural transitions: a ferroelastic one with an anion-ordering (AO) process and the establishment of a (0, ½, ½) superstructure below 124 (±3) K, also visible via resistivity thermal dependence, followed by a stack tetramerization with the establishment of a (½, ½, ½) superstructure below 90 (±5) K. The latter ground state is driven by a spin-Peierls (SP) instability, as demonstrated by the temperature dependence of the magnetic susceptibility. Surprisingly, these two kinds of instability appear to be fully decoupled here, at variance with other tetra-methyl-tetra-thia-fulvalene (TMTTF) or tetramethyl-tetra-selena-fulvalene (TMTSF) salts with such non-centrosymmetric counter-ions.

13.
Chemistry ; 24(20): 5064-5069, 2018 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-29105179

RESUMO

The [Fe(L222 N5 )(CN)2 ] compound, where L222 N5 refers to the macrocyclic Schiff-base ligand, 2,13-dimethyl-3,6,9,-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,- 16-pentaene, is a photomagnetic FeII based coordination compound, which undergoes light-induced excited spin-state trapping (LIESST). The low spin state is hexacoordinated and the high spin state heptacoordinated. This system also serves as complex for the design of trinuclear or one-dimensional compounds made of other types of bricks with diverse coordinated metals. Here its ultrafast spin-state photoswitching dynamics are studied, by combining femtosecond optical spectroscopy and femtosecond X-ray absorption measurements at the XPP station of the X-ray free-electron laser LCLS. DFT and TD-DFT calculations are used to interpret experimental findings. These studies, performed in the solution phase, show that LIESST in [Fe(L222 N5 )(CN)2 ] occurs on the 100 fs timescale under different types of photoexcitation. In addition, coherent oscillations were observed, resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral FeII N6 systems.

14.
Chem Commun (Camb) ; 53(99): 13268-13271, 2017 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-29186217

RESUMO

Light-Induced Excited Spin State Trapping (LIESST) data are reported for seven isostructural solvate salts from the iron(ii)/2,6-di(pyrazol-1-yl)pyridine family. A complicated relationship between their spin-crossover T1/2 and T(LIESST) values may reflect low-temperature thermal and light-induced symmetry breaking, which is shown by one of the compounds but not by two others.

15.
Chem Commun (Camb) ; 53(74): 10283-10286, 2017 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-28868544

RESUMO

The organization of a molecular FeIII complex embedded in a halogen-bonded 2D network is chemically tuned to trigger temperature- and light-induced spin-state switching. We attribute the associated magnetic properties and the unprecedented photoswitching effect to the optimized structural confinement provided by the presence of the supramolecular host framework.

16.
Acta Crystallogr B Struct Sci Cryst Eng Mater ; 73(Pt 4): 660-668, 2017 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-28762976

RESUMO

The complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN)3)2](4,4'-bipy)2]n is reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering, occurring at low temperature and precluding long-range SSCW order.

17.
Chem Rev ; 117(16): 11025-11065, 2017 Aug 23.
Artigo em Inglês | MEDLINE | ID: mdl-28692268

RESUMO

We review the tremendous advances in ultrafast X-ray science, over the past 15 years, making the best use of new ultrashort X-ray sources including table-top or large-scale facilities. Different complementary X-ray-based techniques, including spectroscopy, scattering, and diffraction, are presented. The broad and expanding spectrum of these techniques in the ultrafast time domain is delivering new insight into the dynamics of molecular systems, of solutions, of solids, and of biosystems. Probing the time evolution of the electronic and structural degrees of freedom of these systems on the time scales of femtosecond to picoseconds delivers new insight into our understanding of dynamical matter.

18.
Angew Chem Int Ed Engl ; 56(25): 7130-7134, 2017 06 12.
Artigo em Inglês | MEDLINE | ID: mdl-28510325

RESUMO

Photoexcitation of spin crossover (SCO) complexes can trigger extensive electronic spin transitions and transformation of molecular structure. However, the precise nature of the associated ultrafast structural dynamics remains elusive, especially in the solid state. Here, we studied a single-crystal SCO material with femtosecond electron diffraction (FED). The unique capability of FED allows us to directly probe atomic motions and to track ultrafast structural changes within a crystal lattice. By monitoring the time-dependent changes of the Bragg reflections, we observed the formation of a photoinduced structure similar to the thermally induced high-spin state. The data and refinement calculations indicate the global structural reorganization within 2.3 ps, as the metal-ligand bond distribution narrows during intramolecular vibrational energy redistribution (IVR) driving the intermolecular rearrangement. Three independent dynamical group are identified to model the structural dynamics upon photoinduced SCO.

19.
Nat Commun ; 8: 15342, 2017 05 24.
Artigo em Inglês | MEDLINE | ID: mdl-28537270

RESUMO

The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be treated independently. Here we gain experimental insights, beyond the Born-Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical [Fe(bpy)3]2+ compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules.

20.
Angew Chem Int Ed Engl ; 55(30): 8675-9, 2016 07 18.
Artigo em Inglês | MEDLINE | ID: mdl-27193972

RESUMO

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3 )2 ](4,4'-bipy)2 }n exhibits a thermal high-spin (HS)↔low-spin (LS) staircase-like conversion characterized by a multi-step dependence of the HS molar fraction γHS . Between the fully HS (γHS =1) and LS (γHS =0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the γHS ≈0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the γHS ≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS-LS sequence incommensurate with the molecular lattice. The formation of these different long-range spatially ordered structures of LS and HS states during the multi-step spin-crossover is discussed within the framework of "Devil's staircase"-type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.

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