The Frontier of Molecular Spintronics Based on Multiple‐Decker Phthalocyaninato TbIII Single‐Molecule Magnets

Ever since the first example of a double‐decker complex (SnPc₂) was discovered in 1936, MPc₂ complexes with π systems and chemical and physical stabilities have been used as components in molecular electronic devices. More recently, in 2003, TbPc₂ complexes were shown to be single‐molecule magnets (SMMs), and researchers have utilized their quantum tunneling of the magnetization (QTM) and magnetic relaxation behavior in spintronic devices. Herein, recent developments in Ln^III‐Pc‐based multiple‐decker SMMs on surfaces for molecular spintronic devices are presented. In this account, we discuss how dinuclear Tb^III‐Pc multiple‐decker complexes can be used to elucidate the relationship between magnetic dipole interactions and SMM properties, because these complexes contain two TbPc₂ units in one molecule and their intramolecular Tb^III?Tb^III distances can be controlled by changing the number of stacks. Next, we focus on the switching of the Kondo signal of Tb^III‐Pc‐based multiple‐decker SMMs that are adsorbed onto surfaces, their characterization using STM and STS, and the relationship between the molecular structure, the electronic structure, and the Kondo resonance of Tb^III‐Pc multiple‐decker complexes.

     

Switching of Single‐Molecule Magnetic Properties of TbIII–Porphyrin Double‐Decker Complexes and Observation of Their Supramolecular Structures on a Carbon Surface

Double‐decker complexes based on single‐molecule magnets (SMMs) are a class of highly promising molecules for applications in molecular spintronics, wherein control of both the ligand oxidative states and the 2D supramolecular structure on carbon materials is of great importance. This study focuses on the synthesis and study of 2,3,7,8,12,13,17,18‐octaethylporphyrin (OEP)–Tb^III double‐decker complexes with different electronic structures comprising protonated, anionic, and radical forms. Magnetic susceptibility measurements revealed that only the anionic and radical forms of the OEP–Tb^III double‐decker complexes exhibited SMM properties. The barrier heights for magnetic moment reversal were estimated to be 207 and 215 cm^?1 for the anionic and radical forms, respectively. Scanning tunneling microscopy (STM) investigations revealed that these OEP–Tb^III complexes form well‐ordered monolayers upon simple dropcasting from dilute dichloromethane solutions. All three complexes form an isomorphic pseudo‐hexagonal 2D pattern, regardless of the differences in the electronic structures of their porphyrin–Tb cores. This finding is of interest for SMM technology as ultrathin films of these materials undergoing chemical transformations will not require any detrimental reorganization. Finally, we demonstrate self‐assembly of the protonated 5,15‐bisdodecylporphyrin (BDP)–Tb^III double‐decker complex as an example of successful supramolecular design to achieve controlled alignment of SMM‐active sites.     

Enhanced Single‐Chain Magnet Behavior via Anisotropic Exchange in a Cyano‐Bridged MoIII–MnII Chain

Anisotropic magnetic exchange is of great value for the design of high performance molecular nanomagnets. In the present work, enhanced single‐chain magnet (SCM) behavior is observed for a Mo^III–Mn^II chain that exhibits anisotropic magnetic exchange. Self‐assembly of the pentagonal bipyramidal [Mo(CN)₇]^4? anion and the Mn^II unit with a tridentate ligand results in a neutral double zigzag 2,4‐ribbon structure which exhibits SCM behavior with a high relaxation barrier of 178(4) K. Open magnetic hysteresis loops are observed below 5.2 K, with a coercive field of 1.5 T at 2 K. Interestingly, this SCM can be considered to be a result of a step‐wise process based on our previously reported Mn₂Mo single‐molecule magnets (SMMs).     

Control of the Spin Dynamics of Single‐Molecule Magnets by using a Quasi One‐Dimensional Arrangement

Magnetic dipole interactions are dominate in quasi one‐dimensional (1D) molecular magnetic materials, in which TbNcPc units (Tb³⁺=terbium(III) ion, Nc^2?=naphthalocyaninato, Pc^2?=phthalocyaninato) adopt a structure similar to TbPc₂ single‐molecule magnets (SMMs). The magnetic properties of the [TbNcPc]^0/+ (neutral 1 and cationic 2 ) with 1D structures are significantly different from those of a magnetically diluted sample ( 3 ). In particular, the magnetic relaxation time (τ) of 2 in the low‐temperature region is five orders of magnitude slower than that of 3 . Furthermore, the coercivity (H_C) of 2 remained up to about 20 K. The single‐ion anisotropy of Tb³⁺ ions in a 1D structure and the magnetic dipole interactions acting among molecules determines the direction of the magnetic properties. These results show that the spin dynamics can be improved by manipulating the arrangement of SMMs in the solid state.     

Development of Single‐Molecule Magnets†

Single‐molecule magnets (SMMs) are paramagnetic molecules that can be magnetized below a certain temperature and have potential applications in high‐density information storage, magnetic qubits, spintronic devices, etc. The discovery of the first SMM, Mn₁₂, opened a new era of molecular magnetism and promoted collaborative researches between chemists and physicists for their exotic quantum as well as classical magnetic properties. In the recent past, great efforts have been made to develop strategies for constructing new SMMs with high energy barriers (U_eff) and blocking temperatures (T_B), resulting in great and fast development of SMMs. In this concise review, we highlight the main synthetic approaches and representative results in the design and synthesis of high performance SMMs. We hope to give the readers a basic understanding of SMMs and a snapshot of the representative researches on SMMs from a perspective of synthetic chemists.     

Recent Developments in Lanthanide Single‐Molecule Magnets

Single‐molecule magnets (SMMs) exhibiting slow relaxation of magnetization of purely molecular origin are highly attractive owing to their potential applications in spintronic devices, high‐density information storage, and quantum computing. In particular, lanthanide SMMs have been playing a major role in the advancement of this field because of the large intrinsic magnetic anisotropy of lanthanide metal ions. Herein, some recent breakthroughs that are changing the perspective of the field are highlighted, with special emphasis on synthetic strategies towards the design of high‐performance SMMs.     

Rational Design of a Lanthanide‐Based Complex Featuring Different Single‐Molecule Magnets

The rational synthesis of the 2‐{1‐methylpyridine‐N‐oxide‐4,5‐[4,5‐bis(propylthio)tetrathiafulvalenyl]‐1H‐benzimidazol‐2‐yl}pyridine ligand ( L ) is described. It led to the tetranuclear complex [Dy₄(tta)₁₂( L )₂] ( Dy‐Dy₂‐Dy ) after coordination reaction with the precursor Dy(tta)₃?2 H₂O (tta^?=2‐thenoyltrifluoroacetonate). The X‐ray structure of Dy‐Dy₂‐Dy can be described as two terminal mononuclear units bridged by a central antiferromagnetically coupled dinuclear complex. The terminal N₂O₆ and central O₈ environments are described as distorted square antiprisms. The ac magnetism measurements revealed a strong out‐of‐phase signal of the magnetic susceptibility with two distinct sets of data. The high‐ and low‐frequency components were attributed to the two terminal mononuclear single‐molecule magnets (SMMs) and the central dinuclear SMM, respectively. A magnetic hysteresis loop was detected at very low temperature. From both structural and magnetic points of view, the tetranuclear SMM Dy‐Dy₂‐Dy is a self‐assembly of two known mononuclear SMMs bridged by a known dinuclear SMM.     

Experimental Determination of Magnetic Anisotropy in Exchange‐Bias Dysprosium Metallocene Single‐Molecule Magnets

We investigate a family of dinuclear dysprosium metallocene single‐molecule magnets (SMMs) bridged by methyl and halogen groups [Cp′₂Dy(μ‐X)]₂ (Cp′=cyclopentadienyltrimethylsilane anion; 1 : X=CH₃^?; 2 : X=Cl^?; 3 : X=Br^?; 4 : X=I^?). For the first time, the magnetic easy axes of dysprosium metallocene SMMs are experimentally determined, confirming that the orientation of them are perpendicular to the equatorial plane which is made up of dysprosium and bridging atoms. The orientation of the magnetic easy axis for 1 deviates from the normal direction (by 10.3°) due to the stronger equatorial interactions between Dy^III and methyl groups. Moreover, its magnetic axes show a temperature‐dependent shifting, which is caused by the competition between exchange interactions and Zeeman interactions. Studies of fluorescence and specific heat as well as ab initio calculations reveal the significant influences of the bridging ligands on their low‐lying exchange‐based energy levels and, consequently, low‐temperature magnetic properties.     

Enhancing single-molecule magnet behavior of linear Co<Superscript>II</Superscript>-Dy<Superscript>III</Superscript>Co<Superscript>II</Superscript> complex by introducing bulky diamagnetic moiety

Single-molecule magnets (SMMs) are regarded as promising candidates for ultrahigh-density storage, quantum information processing and molecular spintronics. It is a crucial challenge for chemists to modulate magnetic dynamics of SMMs. Here, we successfully synthesized two 3d-4f polynuclear compounds [Co₂Dy(TTTT^Cl)₂(MeOH)]NO₃·3MeOH (1) and [Co₂Dy(TTTT^Cl)₂ (MeOH)][Co(HTTTT^Cl)](NO₃)₂·2.5MeOH·2H₂O (2), where H₃TTTT^Cl=2,2′,2′′-(((nitrilotris(ethane-2,1-diyl)) tris(azanediyl)) tris(methylene))tris-(4-chlorophenol). On applying the approach by co-crystallization of bulky diamagnetic moiety, the effective energy barrier enhances from 401 K (1) to 536 K (2), which are both among the highest d-f heterometallic SMMs.     

Can Non‐Kramers TmIII Mononuclear Molecules be Single‐Molecule Magnets (SMMs)?

In recent years, plentiful lanthanide‐based (Tb^III, Dy^III, and Er^III) single‐molecule magnets (SMMs) were studied, while examples of other lanthanides, for example, Tm^III are still unknown. Herein, for the first time, we show that by rationally manipulating the coordination sphere, two thulium compounds, 1 [(Tp)Tm(COT)] and 2 [(Tp*)Tm(COT)] (Tp=hydrotris(1‐pyrazolyl)borate; COT=cyclooctatetraenide; Tp*=hydrotris(3,5‐dimethyl‐1‐pyrazolyl)borate), can adopt the structure of non‐Kramers SMMs and exhibit their behaviors. Dynamic magnetic studies indicated that both compounds showed slow magnetic relaxation under dc field and a relatively high effective energy barrier (111 K for 1 , 46 K for 2 ). Magnetic diluted 1 a [(Tp)Tm_0.05Y_0.95(COT)] and 2 a [(Tp*)Tm_0.05Y_0.95(COT)] even exhibited magnetic relaxation under zero dc field. Relativistic ab initio calculations combined with single‐crystal angular‐resolved magnetometry measurements revealed the strong easy axis anisotropy and nearly degenerated ground doublet states. The comparison of 1 and 2 highlights the importance of local symmetry for obtaining Tm SMMs.     

Novel high‐performance polymer memory devices containing (OMe)2tetraphenyl‐p‐phenylenediamine moieties

The functional polyimide (OMe)₂TPPA‐6FPI ( PI ) and the polyamide (OMe)₂TPPA‐6FPA ( PA ) consisting of electron‐donating N,N′‐bis(4‐aminophenyl)‐N,N′‐di(4‐methoxylphenyl)1,4‐phenylenediamine [(OMe)₂TPPA‐diamine] for memory application were prepared in this study. These polyimide and polyamide memory devices were fabricated with the sandwich configuration of indium tin oxide (ITO)/polymer/Al, and could be switched from the initial low‐conductivity (OFF) state to the high‐conductivity (ON) state with high ON/OFF current ratios of 10⁷ and 10⁹, respectively. PI exhibited dynamic random access memory (DRAM) performance, whereas PA showed static random access memory (SRAM) behavior. To get more insight into the memory behaviors of these two different types of polymer memory devices, molecular simulation on the basic unit was carried out. Furthermore, the differences of highest occupied molecular orbital (HOMO) energy level, lowest unoccupied molecular orbital (LUMO) charge density isosurfaces, dipole moment, and linkage conformation between PI and PA were found to affect the volatile memory behavior. Both polymer memory devices revealed excellent stability with long operation time of 10⁴ s at continuous applied voltage of ‐2 V. The effect of polymer thickness on the volatile memory behavior of PA was also investigated in this study. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Magnetic Memory in an Isotopically Enriched and Magnetically Isolated Mononuclear Dysprosium Complex

The influence of nuclear spin on the magnetic hysteresis of a single‐molecule is evidenced. Isotopically enriched Dy^III complexes are synthesized and an isotopic dependence of their magnetic relaxation is observed. This approach is coupled with tuning of the molecular environment through dilution in an amorphous or an isomorphous diamagnetic matrix. The combination of these approaches leads to a dramatic enhancement of the magnetic memory of the molecule. This general recipe can be efficient for rational optimization of single‐molecule magnets (SMMs), and provides an important step for their integration into molecule‐based devices.     

A Novel Bat‐Shaped Dicyanomethylene‐4H‐pyran‐Functionalized Naphthalimide for Highly Efficient Solution‐Processed Multilevel Memory Devices

Small‐molecule‐based multilevel memory devices have attracted increasing attention because of their advantages, such as super‐high storage density, fast reading speed, light weight, low energy consumption, and shock resistance. However, the fabrication of small‐molecule‐based devices always requires expensive vacuum‐deposition techniques or high temperatures for spin‐coating. Herein, through rational tailoring of a previous molecule, DPCNCANA (4,4′‐(6,6′‐bis(2‐octyl‐1,3‐dioxo‐2,3‐dihydro‐1H‐benzo[de]isoquinolin‐6‐yl)‐9H,9′H‐[3,3′‐bicarbazole]‐9,9′‐diyl)dibenzonitrile), a novel bat‐shaped A‐D‐A‐type (A‐D‐A=acceptor–donor–acceptor) symmetric framework has been successfully synthesized and can be dissolved in common solvents at room temperature. Additionally, it has a low‐energy bandgap and dense intramolecular stacking in the film state. The solution‐processed memory devices exhibited high‐performance nonvolatile multilevel data‐storage properties with low switching threshold voltages of about −1.3 and −2.7 V, which is beneficial for low power consumption. Our result should prompt the study of highly efficient solution‐processed multilevel memory devices in the field of organic electronics.     

Fine‐tuning the Local Symmetry to Attain Record Blocking Temperature and Magnetic Remanence in a Single‐Ion Magnet

Remanence and coercivity are the basic characteristics of permanent magnets. They are also tightly correlated with the existence of long relaxation times of magnetization in a number of molecular complexes, called accordingly single‐molecule magnets (SMMs). Up to now, hysteresis loops with large coercive fields have only been observed in polynuclear metal complexes and metal‐radical SMMs. On the contrary, mononuclear complexes, called single‐ion magnets (SIM), have shown hysteresis loops of butterfly/phonon bottleneck type, with negligible coercivity, and therefore with much shorter relaxation times of magnetization. A mononuclear Er^III complex is presented with hysteresis loops having large coercive fields, achieving 7000 Oe at T=1.8 K and field variation as slow as 1 h for the entire cycle. The coercivity persists up to about 5 K, while the hysteresis loops persist to 12 K. Our finding shows that SIMs can be as efficient as polynuclear SMMs, thus opening new perspectives for their applications.     

Synthesis,Characterization, and Non‐Volatile Memory Device Application of an N‐Substituted Heteroacene

N‐substituted heteroacenes have been widely used as electroactive layers in organic electronic devices, and only a few of them have been investigated in organic resistive memory devices. Here, a novel N‐substituted heteroacene 2‐(4′‐(diphenylamino)phenyl)‐4,11‐bis((triisopropylsilyl)ethynyl)‐1H‐imidazo[4,5‐b]phenazine ( DBIP ) has been designed, synthesized, and characterized. Sandwich‐structure memory devices based on DBIP have been fabricated and the devices show non‐volatile and stable memory character with good endurance performance.     

Substrate‐Independent Magnetic Bistability in Monolayers of the Single‐Molecule Magnet Dy2ScN@C80 on Metals and Insulators

Magnetic hysteresis is demonstrated for monolayers of the single‐molecule magnet (SMM) Dy₂ScN@C₈₀ deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy₂ScN@C₈₀ adsorbed on Au(111) were studied by STM. X‐ray magnetic CD studies show that the Dy₂ScN@C₈₀ monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy₂ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene‐surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene‐SMMs on conducting substrates and facilitates their application in molecular spintronics.     

Heterometallic Fe2II–UV and Ni2II–UV Exchange‐Coupled Single‐Molecule Magnets: Effect of the 3 d Ion on the Magnetic Properties

Uranium‐based compounds have been put forward as ideal candidates for the design of single‐molecule magnets (SMMs) with improved properties, but to date, only two examples of exchange‐coupled 3d–5f SMM containing uranium have been reported and both are based on the Mn^II ion. Here we have synthesized the first examples of exchange‐coupled uranium SMMs based on Fe^II and Ni^II. The SMM behavior of these complexes containing a quasi linear {M?O?U?O?M} core arises from intramolecular Fe?U and Ni?U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. The measured values of the relaxation barrier (53.9±0.9 K in the UFe₂ complex and of 27.4±0.5 K in the UNi₂ complex) show clearly the dependency on the spin value of the transition metal, providing important new information for the future design of improved uranium‐based SMMs.     

Nanostructuring of Rare-earth-based Single-Molecule Magnets as Long-range Ordered Arrays in the Framework of Organic Metal Halide Perovskites

The nanostructuring of single-molecule magnets (SMMs) on substrates, in nanotubes and periodic frameworks is highly desired for the future magnetic recording devices. However, the ability to organize SMMs into long-range ordered arrays in these systems is still lacking. Here, we report the incorporation of magnetic (RECl₂(H₂O)₆)⁺ (RE=rare earths) molecular groups into the framework of an organic metal halide perovskite (OMHP)—(H₂dabco)CsCl₃. Intriguingly, we show the incorporated rare-earth groups self-organized into long-range ordered arrays that uniformly and periodically distributed in the A sites of OMHP. The ordered (RECl₂(H₂O)₆)⁺ groups serve as SMMs in the perovskite frameworks, exhibiting large effective magnetic moment, moderate magnetic anisotropy and two-step relaxation behavior. With the additional merit of great structural flexibility and multifunction of OMHPs, the preparation of the first SMMs@OMHP magnetic materials furthers the development of molecular spintronics.     

NMR‐based analysis of structure of heteroleptic triple‐decker (phthalocyaninato) (porphyrinato) lanthanides in solutions

A novel approach for the structural analysis of heteroleptic triple‐decker (porphyrinato)(phthalocyaninato) lanthanides(III) in solutions is developed. The developed approach consists in molecular mechanics (MM+) optimization of the geometry of the complex taking into account the lanthanide‐induced shift (LIS) datasets. LISs of the resonance peaks in ¹H NMR spectra of a series of symmetric complexes [An₄P]Ln[(15C5)₄Pc]Ln[An₄P], where An₄P^2? is 5,10,15,20‐tetrakis(4‐methoxyphenyl)porphyrinato‐dianion, [(15C5)₄Pc]^2? is 2,3,9,10,16,17,24,25‐tetrakis(15‐crown‐5)phthalocyaninato‐dianion and Ln = La, Ce, Pr, Nd, Sm, Eu, are analyzed. Analysis of LISs showed two sets of protons in the molecule with opposite signs of shift. Two‐nuclei analysis of LISs testifies isostructurality of the whole series of investigated complexes in solution despite contraction of the lanthanide ions. Model‐free separation of contact and dipolar contributions of LISs was performed with one‐nucleus technique and did not show changes in contact and dipolar terms within the investigated series. MM+ optimization of the molecular structure allowed the interpretation of features of LIS for each particular group of protons. Parameterization of MM + ‐optimized model of molecule with values of structure‐dependent dipolar contributions of LIS allows the development of the precise structural model of the triple‐decker complex in solution. This approach allows the determination of the geometry and structure of the sandwich macrocyclic tetrapyrrolic complexes together with conformational analysis of flexible peripheral substituents in solutions. The developed method can be applied with minor modifications for the determination of structural parameters of other types of lanthanides(III) complexes with tetrapyrrolic ligands and also supramolecular systems based on them. Copyright © 2010 John Wiley & Sons, Ltd.     

Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple‐Decker Arene‐Bridged Cerium(II) Sandwich

Organometallic multi‐decker sandwich complexes containing f‐elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the Ce^III siloxide complex, [KCeL₄] ( 1 ; L=OSi(OtBu)₃), with excess potassium in a THF/toluene mixture afforded a quadruple‐decker arene‐bridged complex, [K(2.2.2‐crypt)]₂[{(KL₃Ce)(μ‐η⁶:η⁶‐C₇H₈)}₂Ce] ( 3 ). The structure of 3 features a [Ce(C₇H₈)₂] sandwich capped by [KL₃Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of Ce^II ions involved in δ bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi‐decker complex and the first containing non‐classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf=O₃SCF₃) yielded the Ce^IV complex, [CeL₄] ( 2 ), showing that siloxide ligands can stabilize Ce in three oxidation states.