Structural and electronic properties of endohedral metallofullerenes

This account presents an overview of our achievements in structural and chemical understanding of endohedral metallofullerenes (EMFs), a new class of metal‐carbon hybrid materials formed by encapsulation of metals inside fullerene cavities. Structural determination of EMFs is of fundamental importance for understanding their intrinsic properties and the formation mechanism, and for broadening their applications. We have developed an effective method for determining the structures of paramagnetic EMFs, and also succeeded in observing the motion of cluster in a di‐metal EMF for the first time. Recently, we unambiguously established the structures of some carbide EMFs which had been wrongly assumed as conventional EMFs previously. More importantly, we have obtained some insoluble EMF species which had never been explored or even expected before. Meanwhile, the chemical properties of various EMFs with different cage structures or different metallic cores have been systematically investigated by means of both covalent and supramolecular considerations, yielding many fascinating results relating to the dictating effect of internal metals. It is noteworthy that all these achievements are based on unambiguous X‐ray results of pristine or functionalized EMFs. DOI 10.1002/tcr.201100038     

Molecular Spintronics Based on Single‐Molecule Magnets Composed of Multiple‐Decker Phthalocyaninato Terbium(III) Complex

Unlike electronics, which is based on the freedom of the charge of an electron whose memory is volatile, spintronics is based on the freedom of the charge, spin, and orbital of an electron whose memory is non‐volatile. Although in most GMR, TMR, and CMR systems, bulk or classical magnets that are composed of transition metals are used, this Focus Review considers the growing use of single‐molecule magnets (SMMs) that are composed of multinuclear metal complexes and nanosized magnets, which exhibit slow magnetic‐relaxation processes and quantum tunneling. Molecular spintronics, which combines spintronics and molecular electronics, is an emerging field of research. Using molecules is advantageous because their electronic and magnetic properties can be manipulated under specific conditions. Herein, recent developments in [LnPc]‐based multiple‐decker SMMs on surfaces for molecular spintronic devices are presented. First, we discuss the strategies for preparing single‐molecular‐memory devices by using SMMs. Next, we focus on the switching of the Kondo signal of [LnPc]‐based multiple‐decker SMMs that are adsorbed onto surfaces, their characterization by using STM and STS, and the relationship between the molecular structure, the electronic structure, and the Kondo resonance of [TbPc₂]. Finally, the field‐effect‐transistor (FET) properties of surface‐adsorbed [LnPc₂] and [Ln₂Pc₃] cast films are reported, which is the first step towards controlling SMMs through their spins for applications in single‐molecular memory and spintronics devices.     

Preferential Formation of Mono‐Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C2n (2n=74–84) Revealing a General Rule

Encapsulating one to three metal atoms or a metallic cluster inside fullerene cages affords endohedral metallofullerenes (EMFs) classified as mono‐, di‐, tri‐, and cluster‐EMFs, respectively. Although the coexistence of various EMF species in soot is common for rare‐earth metals, we herein report that europium tends to prefer the formation of mono‐EMFs. Mass spectroscopy reveals that mono‐EMFs (Eu@C_2n) prevail in the Eu‐containing soot. Theoretical calculations demonstrate that the encapsulation energy of the endohedral metal accounts for the selective formation of mono‐EMFs and rationalize similar observations for EMFs containing other metals like Ca, Sr, Ba, or Yb. Consistently, all isolated Eu‐EMFs are mono‐EMFs, including Eu@D_3h(1)‐C₇₄, Eu@C_2v(19138)‐C₇₆, Eu@C_2v(3)‐C₇₈, Eu@C_2v(3)‐C₈₀, and Eu@D_3d(19)‐C₈₄, which are identified by crystallography. Remarkably, Eu@C_2v(19138)‐C₇₆ represents the first Eu‐containing EMF with a cage that violates the isolated‐pentagon‐rule, and Eu@C_2v(3)‐C₇₈ is the first C₇₈‐based EMF stabilized by merely one metal atom.     

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.     

Chemical,Electrochemical, and Structural Properties of Endohedral Metallofullerenes

Ever since the first experimental evidence of the existence of endohedral metallofullerenes (EMFs) was obtained, the search for carbon cages with encapsulated metals and small molecules has become a very active field of research. EMFs exhibit unique electronic and structural features, with potential applications in many fields. Furthermore, functionalized EMFs offer additional potential applications because of their higher solubility and their ease of characterization by X‐ray crystallography and other techniques. Herein we review the general field of EMFs, particularly of functionalized EMFs. We also address their structures and their (electrochemical) properties, as well as applications of these fascinating compounds.     

Investigation of the Adsorption and Self Assembly of Isocyanide Derivatives on Au（111） Surface

The adsorption and self-assembly of isocyanide derivatives on Au（111） surface were investigated by density functional theory （DFF） and molecular dynamics simulation. The calculation for phenyl isocyanide by DFT was based on cluster and slab models. The self-assembled monolayers of 2-isocyanoazulene and 1,3-diethoxycarbony 1- 2-isocyanoazulene on Au（111） were simulated using Au-C force field parameters developed by us. It was found that the top site was the most preferred position, and the isocyanoazulene and its derivatives could form the ordered face to edge self-assembled monolayer on gold surface indeed, and the molecules stood on the gold surface vertically.     

A Complete Guide on the Influence of Metal Clusters in the Diels–Alder Regioselectivity of Ih‐C80 Endohedral Metallofullerenes

The chemical functionalization of endohedral metallofullerenes (EMFs) has aroused considerable interest due to the possibility of synthesizing new species with potential applications in materials science and medicine. Experimental and theoretical studies on the reactivity of endohedral metallofullerenes are scarce. To improve our understanding of the endohedral metallofullerene reactivity, we have systematically studied with DFT methods the Diels–Alder cycloaddition between s‐cis‐1,3‐butadiene and practically all X@I_h‐C₈₀ EMFs synthesized to date: X=Sc₃N, Lu₃N, Y₃N, La₂, Y₃, Sc₃C₂, Sc₄C₂, Sc₃CH, Sc₃NC, Sc₄O₂ and Sc₄O₃. We have studied both the thermodynamic and kinetic regioselectivity, taking into account the free rotation of the metallic cluster inside the fullerene. This systematic study has been made possible through the use of the frozen cage model (FCM), a computationally cheap approach to accurately predicting the exohedral regioselectivity of cycloaddition reactions in EMFs. Our results show that the EMFs are less reactive than the hollow I_h‐C₈₀ cage. Except for the Y₃ cluster, the additions occur predominantly at the [5,6] bond. In many cases, however, a mixture of the two possible regioisomers is predicted. In general, [6,6] addition is favored in EMFs that have a larger charge transfer from the metal cluster to the cage or a voluminous metal cluster inside. The present guide represents the first complete and exhaustive investigation of the reactivity of I_h‐C₈₀‐based EMFs.     

Improving Memory Performances by Adjusting the Symmetry and Polarity of O‐Fluoroazobenzene‐Based Molecules

Three O‐fluoroazobenzene‐based molecules were chosen as memory‐active molecules: FAZO‐1 with a D–A2–D symmetric structure, FAZO‐2 with an A1–A2–A1 symmetric structure, and FAZO‐3 with a D–A2–A1 asymmetric structure. Both FAZO‐1 and FAZO‐2 had a lower molecular polarity, whereas FAZO‐3 had a higher polarity. The fabricated indium–tin oxide (ITO)/ FAZO‐1 /Al (Au) and ITO/ FAZO‐2 /Al (Au) memory devices both exhibited volatile static random access memory (SRAM) behavior, whereas the ITO/ FAZO‐3 /Al (Au) device showed nonvolatile ternary write‐once‐read‐many‐times (WORM) behavior. It should be noted that the reproducibility of these devices was considerably high, which is significant for practical application in memory devices. In addition, the different memory performances of the three active materials were determined to be attributable to the stability of electric‐field‐induced charge‐transfer complexes. Therefore, the switching memory behavior could be tuned by adjusting the molecular polarity.     

Density functional study of the charge on Aun clusters (n=1-7) supported on a partially reduced rutile TiO2(110): are all clusters negatively charged?

It is widely believed that small gold clusters supported on an oxide surface and adsorbed at the site of an oxygen vacancy are negatively charged. It has been suggested that this negative charge helps a gold cluster adsorb oxygen and weakens the O-O bond to make oxidation reactions more efficient. Given the fact that an oxygen vacancy is electron rich and that Au is a very electronegative element, the assumption that the Au cluster will take electron density from the vacancy is plausible. However, the density functional calculations presented here show that the situation is more complicated. The authors have used the Bader method to examine the charge redistribution when a Aun cluster (n=1-7) binds next to or at an oxygen vacancy on rutile TiO2(110). For the lowest energy isomers they find that Au1 and Au3 are negatively charged, Au5 and Au7 are positively charged, and Au2, Au4, and Au6 exchange practically no charge. The behavior of the Aun isomers having the second-lowest energy is also unexpected. Au2, Au3, Au5, and Au7 are negatively charged upon adsorption and very little charge is transferred when Au4 and Au6 are adsorbed. These observations can be explained in terms of the overlap between the frontier molecular orbitals of the gold cluster and the eigenstates of the support. Aun with even n becomes negatively charged when the lowest unoccupied molecular orbital has a lobe pointing in the direction of the oxygen vacancy or towards a fivefold coordinated Ti (5c-Ti) located in the surface layer; otherwise it stays neutral. Aun with odd n becomes negatively charged when the singly occupied molecular orbital has a lobe pointing in the direction of a 5c-Ti located at the vacancy site or in the surface layer, otherwise it donates electron density into the conduction band of rutile TiO2(110) becoming positively charged.     

Restriction of Molecular Twisting on a Gold Nanoparticle Surface

To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold nanoparticle (Au NP) surface, we have designed and synthesized a new coumarin molecule (C3) that exists both as ICT and TICT states in its excited state in a polar environment. On a Au NP surface, an excited C3 molecule only exists as an ICT state owing to restricted molecular rotation of a diethylamino group; as a result, no conversion from the ICT to TICT state was observed. Selection of the preferential state of a molecule with dual emitting states can be helpful for selected biological applications.     

Endohedral Metallofullerenes: New Structures and Unseen Phenomena

Endohedral metallofullerenes (EMFs), namely fullerenes with metallic species encapsulated inside, represent an ideal platform to investigate metal–metal or metal–carbon interactions at the sub-nanometer scale by means of single-crystal X-ray diffraction (XRD) crystallography. Herein, recent progress in the identification of new structures and unprecedented properties are discussed according to the categories of monometallofullerenes, dimetallofullerenes, carbide clusterfullerenes, and nitride clusterfullerenes. In particular, the dimerization and the cage-isomer dependent oxidation state of the inner metal atom are summarized in terms of pristine monometallofullerenes. Metal–metal bonds involving lanthanide–lanthanides or actinide–actinides are discussed based on both experimental and theoretical studies. The cluster–cage matching and/or mutual selections, as well as the rarely seen M=C double bonds, are discovered in M₂C₂@C_2n, U₂C@C₈₀, M₂TiC@C₈₀, and Ti₃C₃@C₈₀. Subsequently, the geometries of different M₃N clusters in various cages are discussed, revealing size-matching between the internal M₃N cluster and the outer cage induced by the planarity of the cluster. Finally, an outlook regarding the future developments of the molecular structures and applications of EMFs is presented.     

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.

     

Analysis of rat testicular proteome following 30‐day exposure to 900 MHz electromagnetic field radiation

The use of electromagnetic field (EMF) generating apparatuses such as cell phones is increasing, and has caused an interest in the investigations of its effects on human health. We analyzed proteome in preparations from the whole testis in adult male Sprague‐Dawley rats that were exposed to 900 MHz EMF radiation for 1, 2, or 4 h/day for 30 consecutive days, simulating a range of possible human cell phone use. Subjects were sacrificed immediately after the end of the experiment and testes fractions were solubilized and separated via high‐resolution 2D electrophoresis, and gel patterns were scanned, digitized, and processed. Thirteen proteins, which were found only in sham or in exposure groups, were identified by MALDI‐TOF/TOF‐MS. Among them, heat shock proteins, superoxide dismutase, peroxiredoxin‐1, and other proteins related to misfolding of proteins and/or stress were identified. These results demonstrate significant effects of radio frequency modulated EMFs exposure on proteome, particularly in protein species in the rodent testis, and suggest that a 30‐day exposure to EMF radiation induces nonthermal stress in testicular tissue. The functional implication of the identified proteins was discussed.     

Protein‐specific force field derived from the fragment molecular orbital method can improve protein–ligand binding interactions

Accurate computational estimate of the protein–ligand binding affinity is of central importance in rational drug design. To improve accuracy of the molecular mechanics (MM) force field (FF) for protein–ligand simulations, we use a protein‐specific FF derived by the fragment molecular orbital (FMO) method and by the restrained electrostatic potential (RESP) method. Applying this FMO‐RESP method to two proteins, dodecin, and lysozyme, we found that protein‐specific partial charges tend to differ more significantly from the standard AMBER charges for isolated charged atoms. We did not see the dependence of partial charges on the secondary structure. Computing the binding affinities of dodecin with five ligands by MM PBSA protocol with the FMO‐RESP charge set as well as with the standard AMBER charges, we found that the former gives better correlation with experimental affinities than the latter. While, for lysozyme with five ligands, both charge sets gave similar and relatively accurate estimates of binding affinities. © 2013 Wiley Periodicals, Inc.     

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.     

Density Functional Theory Investigations on Vibrational Spectra,Molecular Structure,and Properties of the L‐Serine,L‐Cysteine,and L‐Aspartic Acid Molecules

In this paper, we present a thorough investigation of the conformational space to characterize all possible gas‐phase structures of the neutral L‐serine, L‐cysteine, and L‐aspartic acid molecules. A total of 120 trial structures were generated for L‐aspartic acid and 96 trial structures for L‐serine and L‐cysteine by combining all internal single‐bond rotamers. Various combinations of the Hartree–Fock and density functional theory/B3LYP methods with different bases were used to optimize all possible trial structures. The theoretical studies on the structure, harmonic vibrational spectra, and molecular properties of these amino acids are presented. The assignments of the calculated wave numbers resulting from potential energy distributions were performed using the VEDA 4 program to allow a good interpretation of the theoretical vibrational spectra of the title compounds. The fundamental harmonic frequencies were found to be in good agreement with data in the literature. A natural bond orbital analysis was performed to investigate the charge delocalization throughout the molecules for the three test compounds. Moreover, an extensive discussion of the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap as well as other related molecular properties are reported.     

Binding of propene on small gold clusters and on Au(111): simple rules for binding sites and relative binding energies

We use density functional theory (DFT) to investigate the bonding of propene to small gas-phase gold clusters and to a Au(111) surface. The desorption energy trends and the geometry of the binding sites are consistent with the following set of rules. (1) The bond of propene to gold is formed by donation of electron density from the highest occupied molecular orbital (HOMO) of propene to one of the low-lying empty orbitals [denoted by LUMO1, LUMO2, em leader (LUMO-lowest unoccupied molecular orbital)] of the gold cluster. (2) Propene binds to a site on the Au cluster where one of the low-lying LUMOs protrudes in the vacuum. Different isomers (same cluster, but different binding sites for propene) correspond to sites where different low-lying LUMOs protrude in space. (3) The desorption energy of the lowest energy isomer correlates with the energy of the lowest empty orbital of the cluster; the lower the energy of that LUMO, the higher the desorption energy. (4) If the lowest-lying LUMO protrudes into space at two nonequivalent sites at the edge of a cluster, propene binds more strongly to the site with the lowest coordination. These rules are consistent with the calculated bond energies and geometries for [Au(n)(C(3)H(6))](q), for n=1-5 and n=8 and q=-1, 0, +1. Based on them we have made a number of predictions that have been confirmed by DFT calculations. The bond of propene to gold is strengthened as the net charge of the cluster varies from -1, to zero, to +1. Compared to a gas-phase cluster, a cluster on a support binds propene more strongly if the support takes electron density from the cluster (e.g., a Au cluster on a gold surface) and more weakly if the support donates electron density to the cluster (e.g., a Au cluster on an oxygen vacancy on an oxide surface).     

Synthesis and stability of monolayer-protected Au38 clusters

A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust-Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and scanning electrochemical microscopy to unequivocally demonstrate that Au38 can be reversibly oxidized to charge states z = +1 or +2; however, reduction to z = -1 leads to desorption of the protecting thiolate monolayer. Although this reductive desorption of thiol from the cluster surface is superficially analogous to electrochemical desorption of planar self-assembled monolayers (SAMs) from macroscopic electrodes, the molecular details of the process are likely to be complicated based on the current view that the thiolate monolayer in clusters is in fact composed of polymeric Au-S complexes.     

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.     

A Chemically Soldered Polyoxometalate Single‐Molecule Transistor

Polyoxometalates have been proposed in the literature as nanoelectronic components, where they could offer key advantages with their structural versatility and rich electrochemistry. Apart from a few studies on their ensemble behaviour (as monolayers or thin films), this potential remains largely unexplored. We synthesised a pyridyl‐capped Anderson–Evans polyoxometalate and used it to fabricate single‐molecule junctions, using the organic termini to chemically “solder” a single cluster to two nanoelectrodes. Operating the device in an electrochemical environment allowed us to probe charge transport through different oxidation states of the polyoxometalate, and we report here an efficient three‐state transistor behaviour. Conductance data fits a quantum tunnelling mechanism with different charge‐transport probabilities through different charge states. Our results show the promise of polyoxometalates in nanoelectronics and give an insight on their single‐entity electrochemical behaviour.