Temperature dependence of spherical electron transfer in a nanosized [Fe14] complex

Active Polarization Engineering between Symmetry Inequivalent Polar States Using Electron Transfer: A Nonferroelectric Approach

ConspectusCompounds with polarization switching properties have a wide range of applications including ferroelectric memories, pyroelectric sensors, and piezoelectric actuators. Ferroelectric compounds are primarily focused owing to their ability to switch between two or more symmetry-equivalent polarization states. Besides ferroelectrics, numerous compounds with polar structures exhibit polarization changes in response to external stimuli such as temperature and pressure; however, such effects are normally too small to be considered in practical applications.Recently, we proposed a strategy to achieve polarization switching via electron transfer in polar crystals. The strategy consists of the synthesis of molecules exhibiting intramolecular electron transfer, combined with crystal engineering to align these molecules so that the molecular dipole moments are not canceled at the lattice level. Consequently, vectorial (or directional) electron transfer results in a significant polarization change comparable to what is found for conventional ferroelectrics. Chemically, since the functional motifs are molecules, operational parameters such as the working temperatures and polarization change can be fine-tuned by adjusting the energy levels of the electron donor and acceptor sites and their separation, which enables a more active control of polarization than ferroelectrics. From a physical perspective, the key difference between these systems and ferroelectrics is that the polarization switching occurs between symmetry inequivalent and nondegenerate polar states. As a direct result, they can be switched by various external stimuli other than electric fields, including temperature, magnetic fields, pressure, and light, owing to their different physical properties such as entropy, magnetization, volume, absorption, etc. Moreover, although thermally- and photoinduced ferroelectrics have been reported, they typically form domain structures with different polarization direction due to a symmetry-related degenerate ground state, causing macroscopic polarization to be largely canceled. In contrast, our compounds, which lack accessible symmetry equivalent states, can achieve a perfect polarization alignment without polarization domains upon temperature changes, photoirradiation, or magnetic field.In this Account, we discuss the synthesis of polarization switching compounds, i.e., dinuclear [CoGa], [FeCo], and [CrCo] complexes, via a chirality-assisted method. The thermally induced polarization switching behavior, or pyroelectric effect, is then explained, highlighting the large polarization change (2.9 μC cm-2) in the [CoGa] complex, which is comparable to the widely used infrared (IR) detector material, triglycine sulfate (TGS). We then discuss the optical polarization memory effect and photoenergy conversion properties, which are a consequence of the photoinduced valence tautomeric behavior with a long-lived photoinduced metastable state. Furthermore, a magnetoelectric effect in the [FeCo] complex is described. The change in polarization is, to the best of our knowledge, the largest one induced by a magnetic field in molecular compounds to date. Notably, the polarization changes induced by temperature variation, photoirradiation, and magnetic field were detected as an electric current without the need of an electric field because polarization domains are not formed, unlike ferroelectric materials.

Polarization Switching from Valence Trapping in an Oxo-Bridged Trinuclear Iron Complex

Switching electric polarization by external stimuli constitutes a technical foundation for various applications. Here, we reported the observation of polarization-switching behavior in an oxo-bridged mixed-valence complex [Fe3O(piv)6(py)3] (piv = pivalate, py = pyridine). Detailed variable-temperature Mössbauer spectral analyses unambiguously confirm the occurrence of an electron localization-delocalization transition between two inequivalent Fe sites. Given that the compound crystallizes in a polar space group, the change in the molecular dipole moments leads to a pyroelectric effect observed during this transition, indicating thermally induced polarization switching behavior. As the complex exhibits asymmetry in the valence-active sites and antiferromagnetic interaction between them, the possibility of magnetoelectric coupling in this compound is also discussed on the basis of the recent prediction of polarization switching through modulating the degree of electron delocalization by magnetic fields in the mixed-valence systems.

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.

Magnetoelectric effect generated through electron transfer from organic radical to metal ion

Magnetoelectric (ME) materials induced by electron transfer are extremely rare. Electron transfer in these materials invariably occurs between the metal ions. In contrast, ME properties induced by electron transfer from an organic radical to a metal ion have never been observed. Here, we report the ME coupling effect in a mononuclear molecule-based compound [(CH₃)₃NCH₂CH₂Br][Fe(Cl₂An)₂(H₂O)₂] (1) [Cl₂An = chloranilate, (CH₃)₃NCH₂CH₂Br⁺ = (2-bromoethyl)trimethylammonium]. Investigation of the mechanism revealed that the ME coupling effect is realized through electron transfer from the Cl₂An to the Fe ion. Measurement of the magnetodielectric (MD) coefficient of 1 indicated a positive MD of up to ∼12% at 10^3.0 Hz and 370 K, which is very different from that of ME materials with conventional electron transfer for which the MD is generally negative. Thus, the current work not only presents a novel ME coupling mechanism, but also opens a new route to the synthesis of ME coupling materials.

Thermally induced charge transfer in a quinoid-bridged linear Cu3 compound

Charge transfer always occurs in molecular valence tautomers, leading to the redistribution of electron density and exhibiting electrical, optical, and magnetic properties, and can be further controlled by multiple external stimuli such as temperature, light and electric field. The design of molecule-based materials capable of charge transfer remains a challenge. Herein, a linear Cu₃ compound [(CH₃)₃NCH₂CH₂Br]₂[Cu₃L₄(H₂O)₂] (H₂L = chloranilic acid) (1) with a multi-center donor-acceptor architecture was constructed using the redox-active chloranilic acid quinoid ligand. Temperature-dependent dielectric measurement was performed to capture the charge transfer valence tautomer transition because it is difficult to detect this transition by crystal structure and magnetism analysis. Temperature-dependent XPS and EPR further confirmed that the charge transfer valence tautomer transition is based on the Cu^II-L^2- to Cu^I-L^-˙ multi-center charge transfer. Thus, the present work builds a charge transfer compound with a multi-center donor-acceptor architecture and proves that dielectric measurement is a very effective means to detect charge transfer.

Experimental and theoretical insights into the photomagnetic effects in trinuclear and ionic Cu(ii)–Mo(iv) systems

New ionic and trinuclear copper(ii)–octacyanidomolybdate(iv) systems were developed and tested experimentally and theoretically to improve understanding of the photomagnetic effects.

Influence of Fine Ligand Substitution Modification of the Isocyanidometal Bridge on Metal-to-Metal Charge Transfer Properties in Class II-III Mixed Valence Complexes

The synthesis and characterization of Class II-III mixed valence complexes have been an interesting topic due to their special intermediate behaviour between localized and delocalized mixed valence complexes. To investigate the influence of the isocyanidometal bridge on metal-to-metal charge transfer (MMCT) properties, a family of new isocyanidometal-bridged complexes and their one-electron oxidation products cis-[Cp(dppe)Fe-CN-Ru(L)₂ -NC-Fe(dppe)Cp][PF₆ ]_n (n=2, 3) (Cp=1,3-cyclopentadiene, dppe=1,2-bis(diphenylphosphino)ethane, L=2,2'-bipyridine (bpy, 1[PF₆ ]_n ), 5,5'-dimethyl-2,2'-bipyridyl (5,5'-dmbpy, 2[PF₆ ]_n ) and 4,4'-dimethyl-2,2'-bipyridyl (4,4'-dmbpy, 3[PF₆ ]_n )) have been synthesized and fully characterized. The experimental results suggest that all the one-electron oxidation products may belong to Class II-III mixed valence complexes, supported by TDDFT calculations. With the change of the substituents of the bipyridyl ligand on the Ru centre from H, 5,5'-dimethyl to 4,4'-dimethyl, the energy of MMCT for the one-electron oxidation complexes changes in the order: 1³⁺ <2³⁺ <3³⁺ , and that for the two-electron oxidation complexes decreases in the order 1⁴⁺ >3⁴⁺ >2⁴⁺ . The potential splitting (ΔE_1/2 (2)) between the two terminal Fe centres for N[PF₆ ]₂ are the largest potential splitting for the cyanido-bridged complexes reported so far. This work shows that the smaller potential difference between the bridging and the terminal metal centres would result in the more delocalized mixed valence complex.

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.

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.