Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Exploring the Magnetic Interactions in Two Novel Cyanide-Bridged Homo- and Heterometallic Hexadecanuclear Complexes

Two novel isostructural cyanide-bridged hexadecanuclear complexes with the general formula {[Fe(CN)6]6[M{en(Bn)py}]10}2+ [M=Fe (12+), Ni (22+)] have been synthesized. The structural analyses disclose the presence of multivalent Fe centres with different spin states in complex 12+ whereas all the Fe centres share a conserved oxidation state in complex 22+. The DC magnetic study revealed antiferromagnetic interactions between the adjacent metal centres and ferrimagnetic behaviour in 12+. On the other hand, ferromagnetic interactions were observed in complex 22+ due to nearly orthogonal orientation of the interacting orbitals and poor spatial overlap as observed in BS-DFT calculations.

Electron transfer coupled spin transition in cyano-bridged mixed-valence {FeIII2FeII2} molecular squares

The research on electron transfer coupled spin transition regulating the valence state and spin state transition of metal ions is promising and challenging. Herein, we report a cyano-bridged {FeIII2FeII2} molecular square complex, {[Fe(Tp)(CN)3]2 [Fe(bnbpen)]2}(ClO4)2·8CH3OH (1·8CH3OH, bnbpen = N,N'-bis-(2-naphthylmethyl)-N,N'-bis(2-picolayl)-ethylenediamine), and its free of solvents form (1). Combined single-crystal X-ray diffraction, temperature-dependent infrared (IR) spectra, magnetic measurements, and Mössbauer spectra reveal that 1·8CH3OH and 1 exhibit reversible one-step and two-step electron transfer coupled spin transition (ETCST) with temperature change, between the low-temperature state {FeII,LS(μ-CN)FeIII,LS}2 (LS = low spin, HS = high spin) and the high-temperature state {FeIII,LS(μ-CN)FeII,HS}2, respectively.

Ultrafast and persistent photoinduced phase transition at room temperature monitored by streaming powder diffraction

Ultrafast photoinduced phase transitions at room temperature, driven by a single laser shot and persisting long after stimuli, represent emerging routes for ultrafast control over materials' properties. Time-resolved studies provide fundamental mechanistic insight into far-from-equilibrium electronic and structural dynamics. Here we study the photoinduced phase transformation of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material, designed to exhibit a 75 K wide thermal hysteresis around room temperature between MnIIIFeII tetragonal and MnIIFeIII cubic phases. We developed a specific powder sample streaming technique to monitor by ultrafast X-ray diffraction the structural and symmetry changes. We show that the photoinduced polarons expand the lattice, while the tetragonal-to-cubic photoinduced phase transition occurs within 100 ps above threshold fluence. These results are rationalized within the framework of the Landau theory of phase transition as an elastically-driven and cooperative process. We foresee broad applications of the streaming powder technique to study non-reversible and ultrafast dynamics.

Metal-Metal Charge Transfer Properties of a Series of Trinuclear Fe2 Ru and Corresponding Pentanuclear Fe2 Ru2 Ag Cyanido-Bridged Complexes

A series of trimetallic cyanidometal-bridged compounds [Men Cp(dppe)FeII -(μ-NC)-RuII (MeOpy)4 -(μ-CN)-FeII (dppe)CpMen ] - [PF6 ]2 (N[PF6 ]2 , n=0, N =1; n=1, N=2; n=3, N=3; Cp=cyclopentadiene, dppe=1,2-bis(diphenylphosphino)ethane, MeOpy=4-methoxypyridine) and their one- and two-electron oxidized compounds N3+ and N4+ were synthesized and characterized. Meanwhile, a series of corresponding linear cyanido-bridged pentanuclear compounds [Men Cp(dppe)FeIII -(μ-NC)-RuII (MeOpy)4 -(μ-NC)-AgI -(μ-CN)-RuII (MeOpy)4 -(μ-CN)-FeIII (dppe)CpMen ][BF4 ]5 (M[BF4 ]5 , n=0, M=4; n=1, M=5; n=3, M=6) were also obtained and well characterized. The investigations suggest that in the trinuclear system there exists remote interaction between the two Fe centers, but no significant interactions exist across the central silver unit between the metals on the two sides of the silver center in the pentanuclear system. In both the trinuclear N4+ and the pentanuclear M5+ complexes, there exists the neighboring RuII →FeIII MM'CT transitions, and the MM'CT energy in the corresponding trinuclear system is higher than those in the pentanuclear system in which no remote metal-metal interaction occurs. Meanwhile, as the substituted methyl groups on the cyclopentadiene increases, the redox potential of the ruthenium in the trinuclear N4+ series increases, but that in the pentanuclear M5+ complexes decreases.

ON/OFF Photo(switching) along with Reversible Spin-State Change and Single-Crystal-to-Single-Crystal Transformation in a Mixed-Valence Fe(II)Fe(III) Molecular System

A mixed-valence Fe(II)Fe(III) molecular system, {[Fe(pzTp)(CN)₃]₂[Fe(bik)₂]₂}·[Fe(pzTp)(CN)₃]₂·4MeOH (1·4MeOH) (bik = bis-(1-methylimidazolyl)-2-methanone, pzTp = tetrakis(pyrazolyl)borate), exhibits single-crystal-to-single-crystal (SC-SC) transformation while increasing the temperature and is converted into {[Fe(pzTp)(CN)₃]₂[Fe(bik)₂]₂}·[Fe(pzTp)(CN)₃]₂ (1). Both complexes exhibit thermo-induced spin-state switching behavior along with reversible SC-SC transformation, where the low-temperature [Fe^III_LSFe^II_LS]₂ phase transforms into a high-temperature [Fe^III_LSFe^II_HS]₂ phase. 1·4MeOH exhibits an abrupt spin-state switching with T_1/2 at 355 K, whereas 1 undergoes a gradual and reversible spin-state switching with a lower T_1/2 at 338 K. Astonishingly, 1 exhibits ON/OFF photo-induced spin-state switching with T_LIESST = 67 K, whereas 1·4MeOH does not show such an effect.

Investigation of the electron transfer properties between metal centers in binuclear and trinuclear cyanido-bridged mixed valence complexes with cis/trans-configuration

To investigate the electron transfer properties between metal centers and their influencing factors in binuclear and trinuclear cyanido-bridged mixed valence complexes with cis/trans-configuration, binuclear cis-[Cp(dppe)Fe(μ-NC)Ru(5,5'-dmbpy)₂(μ-CN)][PF₆]_n (cis-1[PF6]n, n = 1, 2) and trinuclear cis/trans-[Cp(dppe)Fe(μ-NC)Ru(5,5'-dmbpy)₂(μ-CN)Fe(dppe)Cp][PF₆]_n (cis/trans-2[PF6]n, n = 2, 3, 4) (Cp = 1,3-cyclopentadiene, dppe = 1,2-bis(diphenylphosphino)ethane, 5,5'-dmbpy = 5,5'-dimethyl-2,2'-bipyridyl) cyanido-bridged complexes were synthesized and well characterized. The experimental results indicate that the presence of the other terminal fragment Cp(dppe)Fe^III in cis-2[PF6]4 results in higher MMCT energy than that of cis-1[PF6]2. In addition, the trans-configuration is more conducive to electron transfer between metal centers than the cis-configuration in trinuclear cyanido-bridged mixed valence complexes. Moreover, these mixed valence complexes cis-1[PF6]2 and cis/trans-2[PF6]n (n = 3, 4) could be assigned to Class II systems according to Robin and Day.

Effects of cis/trans-Configuration and Ligand Substitution of the Cyanidometal Bridge on Metal to Metal Charge Transfer Properties in Mixed Valence Complexes

To investigate the effects of cis/trans-configuration of the cyanidometal bridge and the electron donating ability of the auxiliary ligand on the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal-bridged mixed valence compounds, two groups of trinuclear cyanidometal-bridged compounds cis/trans-[Cp(dppe)Fe(μ-NC)Ru(4,4'-dmbpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]_n (n=2 (cis/trans-1[PF₆ ]₂ ), 3 (cis/trans-1[PF₆ ]₃ ), 4 (cis/trans-1[PF₆ ]₄ )) and cis/trans-[Cp(dppe)Fe(μ-NC)Ru(bpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]₃ (cis/trans-2[PF₆ ]₃ ) were synthesized and fully characterized. The experimental results indicate that for these one- and two-electron oxidation mixed valence compounds, the trans-configuration compounds are more beneficial for MMCT than the cis-configuration compounds, and increasing the electron donating ability of the auxiliary ligand on the cyanidometal bridge is also conductive to MMCT. Moreover, compounds cis/trans-1[PF₆ ]_n (n=3, 4) and cis/trans-2[PF₆ ]₃ belong to localized compounds by analyzing the experimental characterization results, supported by the TDDFT calculations.

Mixed-valence {FeII2FeIII4} hexanuclear complexes with thermally induced Fe(III) spin crossover behavior

Cyano-bridged mixed-valence {Fe6} hexanuclear complexes {[Tp4-MeFeII(CN)3]2[FeIII(Tpa)]2[FeIII(OR)(Tpa)]2}·6ClO4·S {Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate, Tpa = Tris(2-pyridylmethyl)amine; R = -CH3 S = 8MeOH 1, R = -C2H5 S = 6EtOH 2 } have been obtained...

Influence of the electronic effect of an ancillary ligand on MMCT and LMCT in localized cyanide-bridged complexes containing non-innocent ligands

For bimetallic MV complexes with non-innocent ligands, the MMCT energy in the localized system decreases significantly while the LMCT energy in the delocalized moiety increases slightly as the electronic effect of ancillary ligands is enhanced.

Manipulating the spin crossover behaviour in a series of cyanide-bridged {FeIII2FeII2} molecular squares through NCE- co-ligands

Manipulating the transition temperature (T1/2) of spin-crossover (SCO) complexes capable of fulfilling practical criteria through different synthetic strategies is one of the main focuses in the field of molecular magnetism....

Out-of-equilibrium dynamics driven by photoinduced charge transfer in CsCoFe Prussian Blue Analogue nanocrystals

In this paper we study the out-of-equilibrium dynamics associated with photoinduced charge-transfer (CT) in cyanide-bridged Co-Fe Prussian blue analogue nanocrystals. In these coordination networks, the structural trapping of the photoinduced...

Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue

We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced Mn^III Fe^II → Mn^II Fe^III process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of β spins drives intervalence Fe→Mn CT towards non-bonding states and results in a slower dynamic.

Tuning metal to metal charge transfer properties in cyanidometal-bridged complexes by changing the auxiliary ligand on the bridge

In order to investigate the influence of the auxiliary ligand of the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal-bridged complexes, two groups of heterotrimetallic cyanidometal-bridged complexes, trans-[Cp*(dppe)Fe-NC-Ru(L)2-CN-Fe(dppe)Cp*][PF6]n (L = bpy, 1(PF6)n; L = 4,4'-dmbpy, 2(PF6)n; n = 2, 3, 4) (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene, dppe = 1,2-bis(diphenylphosphino)ethane, bpy = 2,2'-bipyridine, 4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridyl) were synthesized and fully characterized. The MMCT of the one-electron oxidation mixed valence complexes is mainly attributed to RuII and FeII → FeIII MMCT transitions, and the MMCT of the two-electron oxidation complexes is mainly attributed to RuII → FeIII MMCT transitions. The energy of the MMCT of the four complexes decreases with the increase of the electron donating ability of the auxiliary ligand of the cyanidometal bridge. The IR, EPR, and Mössbauer spectra, and the solvent independence of MMCT characterizations indicate that the one-electron oxidation mixed valence complexes may belong to Class II-III systems, and the two-electron oxidation complexes may be localized at low temperature but delocalized at room temperature on the EPR timescale.

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

Gold-Catalyzed Annulation of 1,8-Dialkynylnaphthalenes: Synthesis and Photoelectric Properties of Indenophenalene-Based Derivatives

A simple gold-catalyzed annulation of 1,8-dialkynylnaphthalenes utilizing a cationic gold catalyst was developed. Such a peri-position of two alkynyl substituents has not been studied in gold catalysis before. Dependent on the substrate, the reactions either follow a mechanism involving vinyl cation intermediates or involve a dual gold catalysis mechanism which in an initial 6-endo-dig-cyclization generates gold(I) vinylidene intermediates that are able to insert into C-H bonds. Indenophenalene derivatives were obtained in moderate to high yields. In addition, the bidirectional gold-catalyzed annulation of tetraynes provided even larger conjugated π-systems. The optoelectronic properties of the products were also investigated.

Ferromagnetic Archimedean polyhedra {Fe24M18} (M = Fe, Ni, and Mn) with tunable electron configurations

Three symmetric nanocages {Fe24M18} that mimic the Archimedean polyhedra, namely pseudo-rhombicuboctahedron, were synthesized. Their electron configurations depend highly on the changes of metal ions and the deprotonation of auxiliary ligands.

Synthesis, properties and crystal structure of novel Copper(II) ammine complex with [Pd(CN)4]2− building blocks

The title compound [Cu4(NH3)12-(µ2-CN)8-Pd4(CN)8] was prepared from the aqueous solution Cu2+ – N,N,N′,N′-tetraethylethane-1,2-diamine – [Pd(CN)4]2−, where the addition of excess amounts of ammonia caused the dissolution of the precipitate formed during reaction. Using this method, we prepared and structurally characterized the new complex of Cu(II) containing ammine ligand with tetracyanidopalladate(II) anion incorporated within the molecule. The crystal structure of compound is molecular; the unit cell contains centrosymmetric octanuclear molecules in which all central atoms lies on mirror plane. Generally, the structure of the molecule can be divided into central cyclic part and terminal pendants. The structure contains nitrogen atoms derived from the ammonia molecules coordinated to Copper(II) atoms. They are involved in the formation of N-H···N hydrogen bonds type.

Manipulating the spin crossover behavior in a series of {FeIII2FeII} complexes

Three cyanide-bridged {Fe₂Fe} complexes are reported to exhibit excellent SCO properties which are highly dependent on the compact degree of the π-π stacking, the loss of lattice solvents as well as the electron-donor strength of Tp^R.

A cyanide-bridged FeIII-MnII heterobimetallic one-dimensional coordination polymer: synthesis, crystal structure, experimental and theoretical magnetism investigation

A new cyanide-bridged Fe^III-Mn^II heterobimetallic coordination polymer (CP), namely catena-poly[[[N,N'-(1,2-phenylene)bis(pyridine-2-carboxamidato)-κ⁴N,N',N'',N''']iron(III)]-μ-cyanido-κ²C:N-[bis(4,4'-bipyridine-κN)bis(methanol-κO)manganese(II)]-μ-cyanido-κ²N:C], {[FeMn(C₁₈H₁₂N₄O₂)(CN)₂(C₁₀H₈N₂)₂(CH₃OH)₂]ClO₄}_n, (1), was prepared by the self-assembly of the trans-dicyanidoiron(III)-containing building block [Fe(bpb)(CN)₂]^- [bpb^2- = N,N'-(1,2-phenylene)bis(pyridine-2-carboxamidate)], [Mn(ClO₄)₂]·6H₂O and 4,4'-bipyridine, and was structurally characterized by elemental analysis, IR spectroscopy, single-crystal X-ray crystallography and powder X-ray diffraction (PXRD). Single-crystal X-ray diffraction analysis shows that CP 1 possesses a cationic linear chain structure consisting of alternating cyanide-bridged Fe-Mn units, with free perchlorate as the charge-balancing anion, which can be further extended into a two-dimensional supramolecular sheet structure via inter-chain π-π interactions between the 4,4'-bipyridine ligands. Within the chain, each Mn^II ion is six-coordinated by an N₆ unit and is involved in a slightly distorted octahedral coordination geometry. Investigation of the magnetic properties of 1 reveals an antiferromagnetic coupling between the cyanide-bridged Fe^III and Mn^II ions. A best fit of the magnetic susceptibility based on the one-dimensional alternating chain model leads to the magnetic coupling constants J₁ = -1.35 and J₂ = -1.05 cm^-1, and the antiferromagnetic coupling was further confirmed by spin Hamiltonian-based density functional theoretical (DFT) calculations.

Role of the Outer Metal of Double Metal Cyanides on the Catalytic Efficiency in Styrene Oxidation

The catalytic efficiency of double metal cyanide (DMC) has been shown to be very effective in heterogeneous catalysis. The catalytic activity of the outer divalent cations (Mn, Co, Ni, and Cu) of a family of hexacyanocobaltates was examined in the oxidation reaction of styrene, as a model molecule, using tert Butyl Hydroperoxide (TBHP, Luperox®) as an oxidizing agent. The most electronegative outer cations showed the best conversions, with 95% for copper, followed by nickel with 85% conversion of the monomer at atmospheric pressure and temperature of 75 °C. The evidence showed that the catalytic activity and selectivity towards oxidized products are strongly linked to the accurate choice of the outer cation in the DMC together with the oxidizing agent.

The syntheses, crystal structures, electrochemical and magnetic properties of tri-nuclear cyanide-bridged complexes [cis-MII(bpy)2(CN)2]2MnIII(salcy) (PF6) (M = Fe, Ru, Os)

Three cyanide-bridged heteronuclear complexes, [cis-M(bpy)₂(CN)₂]₂Mn(salcy) (PF₆) (M = Fe, 1; M = Ru, 2; M = Os, 3; bis(salicylideneiminato) dianion) were synthesized and fully characterized. Complexes 1 and 2 show low temperature antiferromagnetic order.

Charge transfer and slow magnetic relaxation in a series of cyano-bridged FeIII4MII2 (M = FeII, CoII, NiII) molecules

The synthesis, single-crystal structures and magnetic properties of three new cyano-bridged complexes [FeIII4MII2] (M = Fe^II, Co^II, Ni^II) are reported.

Wide Magnetic Thermal Memory Effect (∼55 K) Above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = 1/2 Spin Chain Molecule Crystal

One-dimensional (1D) S = 1/2 Heisenberg antiferromagnetic (AFM) chain system shows frequently a spin-Peierls-type transition owing to strong spin-lattice coupling. From high-temperature phase (HTP) to low-temperature phase (LTP), the spin chain distortion leads to the reduction in lattice symmetry in LTP, called the symmetry breaking (SB) phase transition. Herein, we report the first example of 1D S = 1/2 AFM molecular crystal, [Et₃( n-Pr)N][Ni(dmit)₂] (Et₃( n-Pr)N⁺ = triethylpropylammonium, dmit^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate), which shows a structural phase transition with lattice symmetry increase in LTP, which is contrary to the SB phase transition. Particularly, the structure phase transition leads to magnetically bistable state with T_C↑ ≈ 375 K, T_C↓ ≈ 320 K, and surprisingly large thermal hysteresis (∼55 K). Additionally, LTP and HTP coexist in a temperature region near T_C but not at T_C in this 1D spin system. The large hysteresis is related to the huge deformation of anion stack, which needs high activation energy for the structure transformation and magnetic transition between LTP and HTP. This study would not only provide new insight into the relationship of spin-Peierls-type transition and structure phase transition but also offer a roadmap for searching molecular-scale magnetic bistable materials, which are in huge demand in future electronic, magnetic, and photonic technologies.

A new semiconducting 1D Cu(i)–Cu(ii) mixed-valence coordination polymer with Cu(ii) dimethylpiperidine–dithiocarbamate and a tetranuclear Cu(i)–Br cluster unit

A new 1D semiconducting mixed-valence Cu(i)–Cu(ii) coordination polymer was synthesized and characterized using impedance measurements.

Synthesis, crystal structure and magnetic properties of a cyanide-bridged heterometallic {CoIIMnIII} chain

The assembly reaction between the low-spin [Co^II(dmphen)(CN)₃]^- metalloligand and the [Mn^III(salen)(H₂O)]⁺ complex cation yielded the one-dimensional compound {[Mn^III(salen)(μ-NC)₂Co^II(dmphen)(CN)]·2H₂O}_n (1), which behaves as a ferrimagnetic chain, the intrachain magnetic coupling being J = -1.71(1) cm^-1.

Spin crossover and reversible single-crystal to single-crystal transformation behaviour in two cyanide-bridged mixed-valence {FeIII2FeII2} clusters

Two mixed-valence {FeIII2FeII2} clusters exhibit spin crossover behaviour, in which a distorted {Fe₄(μ-CN)₄} core and deviations of bent ∠Fe−NC angles play a key role in tuning the transition temperature.