Juan Manuel Herrera

Bifunctional Hybrid SiO2 Nanoparticles Showing Synergy between Core Spin Crossover and Shell Luminescence Properties

It is well-known that certain octahedral coordination compounds with electronic configurations from d to d can undergo spin crossover (SCO) between high-spin (HS) and low-spin (LS) states. The spin state, and consequently the color, size, and magnetic properties of these SCO systems can be tuned through external stimuli such as temperature, pressure, light, magnetic fields, and guest absorption/desorption. Additionally, some SCO compounds exhibit abrupt transitions with large hysteresis loops, which confer a memory effect to these materials. Therefore, SCO systems offer some promising opportunities for application in information processing, data storage, molecular switches, and/or display devices. For integration into functional devices, SCO materials need to be prepared at the nanometer scale, whilst retaining their magnetic and cooperative behavior. As a result, different research groups have focused their studies on the synthesis and physicochemical characterization of spin-crossover nanoparticles (SCONPs). They have demonstrated that the SCO phenomenon is preserved at the nanometer scale and that it is possible to tune the transition temperatures and the size of the hysteresis loop by controlling the particle size. Parallel to this approach, some other research groups have concentrated their studies on the design of new molecular systems that combine spin crossover and other interesting properties such as luminescence. To the best of our knowledge, only one attempt has been made to prepare bifunctional SCO/luminescence nanoparticles. Specifically, Bousseksou and co-workers have obtained nanoparticles of the SCO complex [Fe(NH2Trz)3](tos)2 (NH2Trz = 4-amino-1,2,4-triazole; tos = tosyl) doped with the fluorescent agent Rhodamine 110. In this system, the emission spectrum of the Rhodamine at room temperature overlaps with the A1g!T1g absorption band of the iron(ii) polymer in the LS state and the emission is partially quenched. When the temperature is raised to 320 K (HS regime), the A1g!T1g absorption band bleaches and the emission intensity increases. Herein, we propose an alternative strategy to prepare bifunctional SCO/luminescence nanoparticles which consists of using silica as a matrix for the SCO component (Figure 1). Although silica nanoparticles (SiO2NPs) have been widely used as supports for the fabrication of multifunctional materials that have a vast number of important potential applications in fields such as drug delivery, biosensors, cell labeling, and so forth, as far as we know, no examples of hybrid SiO2NPs SCO systems have been reported so far. Silica is a particularly suitable material for the preparation of SiO2NPs SCO systems because its high porosity allows for the incorporation of SCO compounds and as silica does not absorb light and does not interfere with magnetic fields the SCO compounds inside the SiO2NPs will keep their original optical and magnetic properties. Additionally, the surfaces of these SiO2NPs SCO systems can be functionalized by grafting active species, such as fluorophores, to afford bifunctional SCO/luminescence nanomaterials. The first results of this original approach are reported, herein. As the SCO material we have used a 1D Fe complex {[Fe(HTrz)2(Trz)](BF4)}n (hereafter Fe-Trz; HTrz = 1,2,4-1H-

Studies on bifunctional Fe(ii)-triazole spin crossover nanoparticles: time-dependent luminescence, surface grafting and the effect of a silica shell and hydrostatic pressure on the magnetic properties

Pure and silica wrapped Fe(II)-triazole (FeHTrz) spin-crossover (SCO) nanoparticles have been prepared following a water-in-oil synthetic procedure. The size and shape can be tuned by controlling the Fe(II) and triazole concentrations in the aqueous phase. The magnetic properties of these nanoparticles are strongly affected by the presence of a silica shell embedding the nanostructured FeHTrz polymer. Whereas bare FeHTrz nanoparticles exhibit abrupt and cooperative spin transition with 24–35K-wide thermal hysteresis loops, for the silica derivates the hysteresis width increases up to 37–42 K. This probes the efficiency of the silica shell to promote interparticle interactions and enhance cooperativity effects. Tomographic studies of the FeHTrz@SiO2 nanoparticles reveal a core–shell structure with the pure FeHTrz polymer wrapped into a thin shell of pure silica. Taking advantage of the chemical properties of the silica shell, these hybrid nanoparticles were coated with a dansyl derivate fluorophore whose luminescence properties can be adjusted by the spin state of the SCO polymer. Time-dependent luminescence studies reveal the existence of a non-radiative energy transfer (Forster type) between the organic fluorophore and the Fe(II)-low spin ions. These nanoparticles have also been functionalized with thiol groups allowing them to be deposited onto a gold surface in a controlled manner.

Enhanced ferromagnetic interaction in metallacyclic complexes incorporating m-phenylenediamidato bridges.

The double stranded Cu(II)2-metallacyclic complex of formula [Cu2(mbpb)2].2H2O (1) and the triple stranded Ni(II)2-metallacyclic complexes of formula [Ni2(Hmbpb)3]PF6.21H2O (2), [Co(H2O)6][Ni2(mbpb)3)]THF.10H2O (3) and {Ag2(H2O)[Ni2(mbpb)3]}.11H2O (4) (where H2mbpb is the bisbidentate dinucleating bridging ligand 1,3-bis(pyridine-2-carboxamide) benzene) have been synthesised and characterised by single-crystal X-ray diffraction. Within the dinuclear molecules, metal ions are bridged by either fully or semideprotonated bisbidentate ligands, which are coordinated through the pyridine and amidato nitrogen donor atoms. In complex 4 the triple stranded dinuclear Ni2 units are connected to the Ag+ cations through O-amidato bridges and Ag-pi(benzene) interactions to afford a 1D bimetallic chain. Cu2 (1) and Ni2 (2-4) complexes exhibit ferromagnetic coupling between the metal ions through the bridging ligand with J(Cu-Cu) = 21.1 cm(-1) and J(Ni-Ni) in the range of 2.9-3.6 cm(-1), respectively. Amongst copper(II) dinuclear complexes bearing m-phenylenediamidato bridges, complex 1 exhibits the stronger ferromagnetic exchange coupling reported so far. DFT calculations firstly confirm that the spin polarisation mechanism is responsible for the ferromagnetic coupling, and secondly allows us to predict stronger ferromagnetic couplings in Cu(II)(2) complexes with larger tetrahedral distortions of the CuN4 coordination environment.

3d−3d−4f Chain Complexes Constructed Using the Dinuclear Metallacyclic Complex [Ni2(mbpb)3]2− [H2mbpb = 1,3-Bis(pyridine-2-carboxamide)benzene] as a Ligand: Synthesis, Structures, and Magnetic Properties

A series of one-dimensional Ni(2)Ln cationic complexes have been prepared by assembling the in situ generated dinuclear mesocate [Ni(2)(mbpb)(3)](2-) building block [H(2)mbpb is the ligand 1,3-bis(pyridine-2-carboxamide)benzene] with Ln(3+) metal ions (Ln(3+) = Gd, Tb, Dy). The crystal-field potentials for the two types of site symmetries found for these 3d-3d-4f complexes (LnO(7) and LnO(8)) are quite different, which has a direct influence on the depopulation of the Stark sublevels, the magnetic anisotropy, and the magnetic properties.

Homometallic DyIII Complexes of Varying Nuclearity from 2 to 21: Synthesis, Structure and Magnetism

The synthesis, structure, and magnetic properties of four DyIII coordination compounds isolated as [Dy2 (LH2 )2 (μ2 -η1 :η1 -Piv)]Cl⋅2 MeOH⋅H2 O (1), [Dy4 (LH)2 (μ3 -OH)2 (Piv)4 (MeOH)2 ]⋅4 MeOH⋅2 H2 O (2), [Dy6 (LH2 )3 (tfa)3 (O3 PtBu)(Cl)3 ]Cl4 ⋅15.5 H2 O⋅4 MeOH⋅5 CHCl3 (3) and [Dy21 (L)7 (LH)7 (tfa)7 ]Cl7 ⋅15 H2 O⋅7 MeOH⋅12 CHCl3 (4) are reported (Piv=pivalate, tfa=1,1,1-trifluoroacetylacetone, O3 PtBu=tert-butylphosphonate). Among these, 3 displays an equilateral triangle topology with a side length of 9.541 Å and a rare pentagonal-bipyramidal Dy3+ environment, whereas complex 4 exhibits a single-stranded nanowheel structure with the highest nuclearity known for a homometallic lanthanide cluster structure. A tentative model of the dc magnetic susceptibility and the low-temperature magnetization of compounds 1 and 2 indicates that the former exhibits weak ferromagnetic intramolecular exchange interaction between the Dy3+ ions, whereas in the latter both intramolecular ferromagnetic and antiferromagnetic magnetic exchange interactions are present. Compounds 1, 3, and 4 exhibit frequency-dependent ac signals below 15 K at zero bias field, but without exhibiting any maximum above 2 K at frequencies up to 1400 Hz. The observed slow relaxation of the magnetization suggests that these compounds could exhibit single molecule magnet (SMM) behavior with either a thermal energy barrier for the reversal of the magnetization that is not high enough to block the magnetization above 2 K, or there exists quantum tunneling of the magnetization (QTM).

Particle tuning and modulation of the magnetic/colour synergy in Fe(II) spin crossover-polymer nanocomposites in a thermochromic sensor array

Thermochromic thin films of the spin crossover (SCO) polymer [Fe(NH2trz)3](BF4) are prepared using a variety of organic polymers as hosts. The formation of different polymeric networks is confirmed macroscopically by the colour changes related to an SCO phenomenon induced by thermal variation, and the results are correlated with electron microscopy and energy dispersive X-ray spectroscopy. Large particles of the SCO material are observed in SCO/polymer hybrid systems with hydrophobic polymers, while more dispersed nano-crystals appear in the hydrophilic matrices, leading to the transformation of the particles into fibrous structures. Subsequently, submicrometer-size SCO fibrous nanoparticles undergo colourimetric spin transitions near room temperature while grains with sizes larger than several microns move their transitions to lower temperatures. The difference in properties between the SCO/polymer hybrid materials is not only due to the differences in the size and shape of the SCO crystals in each polymer but also to the nature of the polymer and solvent interactions. The optical changes obtained for each SCO/polymer hybrid material are related to the microscopic origin of the cooperative interactions tracked by using a photographic digital camera. A linear correlation is obtained (colour values versus temperature) when processing all the colourimetric data by artificial neural networks, thus avoiding the uncertainty inherent in the hysteresis loop.

Double and triple stranded mesocates containing the bis(bidentate) bridging ligand 1,3-bis(pyridine-2-carboxamide)benzene. Structure, properties and DNA interaction

The double stranded CuII2 metallacyclic complex of formula [Mn(hfac)2(H2O)2][Cu2(mbpb)2(CH3CN)2] (1) and the triple stranded NiII2, ZnII2 and CoIII2 metallacyclic complexes of formula [M2(Hmbpb)3]X·nH2O (M = ZnII, X = NO3− (2), n = 17; M = NiII, and X = ClO4− (3), n = 15) and [Co2(mbpb)3]·19H2O (4) (H2mbpb is the bisbidentate dinucleating bridging ligand, 1,3-bis(pyridine-2-carboxamide)benzene) have been prepared and structurally characterised. Their X-ray structures show that inside the dinuclear molecules metal ions are bridged by either fully or semideprotonated bisbidentate ligands, which are coordinated through the pyridine and amidato nitrogen donor atoms. In 1, the neutral metallamacrocycle dinuclear entities [Cu2(mbpb)2(CH3CN)2] and the [Mn(hfac)2(H2O)2] molecules are connected by hydrogen bonds to afford a 1D system. These intermolecular interactions overcome the expected intradinuclear weak ferromagnetic interaction leading to an overall weak antiferromagnetic interaction. The Ni2 complex exhibit ferromagnetic coupling between the metal ions through the bridging ligand with JNi–Ni = 3.1 cm−1. DFT calculations were performed to estimate the value of the exchange magnetic coupling inside the dinuclear unit in 1 and to confirm that the spin polarisation mechanism is responsible for the ferromagnetic coupling. AFM studies show that the ZnII2 and NiII2 cationic complexes interact with pBR322 DNA producing supercoiled forms in higher extension as well as kinks and cross linking.

Design of a Family of Ln3 Triangles with the HAT Ligand (1,4,5,8,9,12-Hexaazatriphenylene): Single-Molecule Magnetism

A series of trinuclear Ln3 complexes (LnIII = Yb (1), Er (2), Dy (3) and Gd (4)) were prepared from the tris-chelate bidentate ligand 1,4,5,8,9,12-hexaazatriphenylene (HAT). 1 and 2 exhibited field-induced single-molecule-magnet (SMM) behavior with estimated Ueff values of 21.30 and 13.86 K, respectively. Complex 3 behaved as a SMM even at zero field, and two different thermally assisted relaxation processes were detected with Ueff values of 29.6 K (fast relaxation process, FR) and 69 K (slow relaxation process, SR) due to the existence of two magnetically different DyIII centers in the molecule. Ab initio studies reveal that all the Dy3+ centers have almost an Ising ground state. The local anisotropy axes are not coplanar but form angles with the Dy3 plane in the range 58-78°. The magnetic interaction between the anisotropic Dy3+ ions is antiferromagnetic in nature and very weak in magnitude. However, due to the extreme feebleness of the magnetic interaction with regard to the local excitation energies, the magnetization blockade is most probably of single-ion origin. Calculations support the existence of two relaxation processes, which take place through the first excited state following an Orbach/Raman mechanism. Finally, for complex 4, the magnetocaloric effect was simulated using the magnetic parameters extracted from the fit of the magnetization and susceptibility data and demonstrated that the simulated -ΔSm values were almost coincident with those extracted from the integration of the field dependence of the magnetization. The simulated MCE value at 2 K and 5 T (20.46 J kg-1 K-1) makes complex 4 an attractive candidate for cryogenic magnetization.

Photophysical properties of [Ir(tpy)2]3+-doped silica nanoparticles and synthesis of a colour-tunable material based on an Ir(core)–Eu(shell) derivative

In this study, we report the synthesis and characterization of phosphorescent silica nanoparticles doped with the blue-greenish emitting Ir-tpy complex [Ir(tpy)2]X3 (tpy = 2,2′:6′,2′′-terpyridine; X = PF6− or NO3−). Depending on the type of counterion and the solubility of the complex, three different kinds of Ir(tpy)-doped silica nanoparticles were prepared by the Stober, water-in-oil and direct micelle synthetic approaches. The materials prepared through the Stober and the water-in-oil approaches showed enhanced photochemical stability and higher luminescence efficiency compared to the free Ir-tpy complex. In these cases, the silica matrix hampers the diffusion of O2 and restrains the mobility of the complexes resulting in a decrease of the vibration relaxation and restraining the nonradiative decay. Conversely, for the material prepared by the direct micelle method, in which the structure of silica shows some degree of mesoporosity, the luminescence properties of the Ir-tpy complex remained almost unchanged after silica encapsulation. Additionally, the nanoparticles prepared by the Stober method were chosen to functionalize their surface with a red-emitting Eu(hfac)3-alkoxysilane derivative leading to multicoloured luminescent silica nanoparticles in which the colour of the emission could be tuned by changing the excitation wavelength and where an Ir → Eu energy transfer was evidenced.

Two-step spin crossover behaviour in the chiral one-dimensional coordination polymer [Fe(HAT)(NCS)2]∞

Solvated and unsolvated forms of the complex [Fe(HAT)(NCS)2]∞·(nMeOH) (1) (n = 1.5, 0; HAT = 1,4,5,8,9,12-hexaazatriphenylene) were prepared. The structure of 1·(1.5MeOH), measured at 120 K, showed that this system crystallizes in the homochiral P43 tetragonal space group. The solid is constituted of stacks of one-dimensional coordination polymers running along c-axis. All the FeII centres have the same Λ or Δ conformation and are in the LS state at 120 K. In the range of temperatures 10–300 K the magnetic properties of 1·(1.5MeOH) shows the occurrence of reversible spin crossover behaviour. However, above ca. 310 K complete desolvation of 1·(1.5MeOH) to give 1 was observed from crystal structure analysis, magnetic behaviour and thermal analysis. Compound 1 displays a two-step spin crossover behaviour characterised by a plateau 60 K wide. Simulation of the two-step behaviour in the frame of the regular solutions theory afforded, respectively, the critical temperatures (Tci), the interaction parameters (Γi), and the enthalpy (ΔHi) and entropy (ΔSi) variations for steps i = 1 and 2: Tc1(Tc2) = 172 (358) K, Γ1(Γ2) = 1.6 (3.0) kJ mol−1, ΔH1(ΔH2) = 5.7 (18.3) kJ mol−1 and ΔS1(ΔS2) = 33.4 (51.0) J K mol−1.