The first observation of four-electron reduction in [60]fullerene-metal cluster self-assembled monolayers (SAMs)

Self-assembled monolayers (SAMs) of a mu 3-eta 2:eta 2:eta 2-C60 triosmium cluster complex Os3(CO)8(CN(CH2)3Si(OEt)3)(mu 3-eta 2:eta 2:eta 2-C60) (2) on ITO or Au surface exhibit ideal, well-defined electrochemical responses and remarkable electrochemical stability being reducible up to tetranionic species in their cyclic voltammograms.     

Two metal centers bridging two C60 cages as a wide passage for efficient interfullerene electronic interaction

The reaction of Ir4(CO)8(PMe3)4 with excess C60 in refluxing 1,2-dichlorobenzene, followed by treatment by CNR (R = CH2C6H5) at 70 degrees C, affords a fullerene-metal sandwich complex Ir4(CO)3(mu4-CH)(PMe3)2(mu-PMe2)(CNR)(mu-eta2,eta2-C60)(mu4-eta1,eta1,eta2,eta2-C60) (1), which exhibits an interesting structural feature of two metal atoms bridging the two C60 centers as well as the first example of a mu4-eta1,eta1,eta2,eta2-C60 bonding mode. Compound 1 has been characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction study. A cyclic voltammetry study reveals strong electronic communication between the two C60 centers in 1, which is due to the presence of a wide channel of two metal centers between the two C60 cages for efficient electronic interaction.     

Crystal structure and growth mechanism of unusually long fullerene (C60) nanowires

Exceptionally long C60 nanowires, with a length to width aspect ratio as large as 3000, are grown from a 1,2,4-trimethylbenzene solution of C60. They have been formed to possess a highly unusual morphology, with each nanowire being composed of two nanobelts joined along the growth direction to give a V-shaped cross section. The crystal structure of these nanowires is found to be orthorhombic, with the unit cell dimensions of a = 10.2 A, b = 20.5 A, and c = 25.6 A. Structural and compositional analyses enable us to explain the observed geometry with an anisotropic molecular packing mechanism that has not been observed previously in C60 crystal studies. The nanowires have been observed to be able to transform into carbon nanofibers following high-temperature treatment, but the original V-shaped morphology can be kept unchanged in the transition. A model for the nanowire morphology based upon the solvent-C60 interactions and preferential growth directions is proposed, and potentially it could be extended for use to grow different types of fullerene nanowires.     

C(60)-exTTF-C(60) Dumbbells: cooperative effects stemming from two C(60)s on the radical ion pair stabilization

[structure: see text] The presence of a second C(60) cage in C(60)-exTTF-C(60) triads [exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-anthraquinone] has beneficial effects on the stabilization of the radical ion pair formed upon irradiation in comparison with the related C(60)-exTTF dyad. Although C(60)-exTTF-C(60) ensembles show no electronic interaction between the electroactive units in the ground state, their irradiation leads to C(60)(*)(-)-exTTF(*)(+)-C(60) species with lifetimes on the order of 600 ns in benzonitrile; these lifetimes are twice those determined for the analogous C(60)-exTTF dyad.     

Porphyrin-triarylamine conjugates: strong electronic communication between triarylamine redox centers via the porphyrin dication

A set of porphyrin-triarylamine hybrids have been synthesized in good yield by Sonogashira palladium-catalyzed cross-coupling reactions between the zinc complex of 5,15-diethynyl-10,20-dimesitylporphyrin and the appropriate iodophenyldiarylamines. The crystal structure of porphyrin 1 shows that the dihedral angle between the acetylene-bonded benzene rings and the porphyrin macrocycle is 20.0 degrees. Such a structural characteristic enables effective electronic perturbations within the molecule. The electronic spectra are red-shifted and display a broad Soret band and an intense Q band relative to those of meso-substituted tetraarylporphyrins. These conjugates display four oxidations and one reduction. All the electrochemical reactions involve one-electron transfer. The first and second oxidations are reversible and can be assigned to the porphyrin-centered reactions. The third and fourth ones, separated by about 270 mV, correspond to the triarylamine units. The comproportionation constant (Kc) is calculated to be 3.67x10(4). The electron coupling between the triarylamine moieties, at a separation of >23 A, is remarkably strong. The electrochemical results and the absorption spectra show that the electronic characteristics of these porphyrins can be significantly modulated by the triarylamine substituents via the conjugated carbon-carbon triple bond. Variations of the substituents on the triarylamines can fine-tune the electronic properties of these molecules.     

Tube and cage C(60)H(60): a comparison with C(60)F(60)

Tube C(60)H(60) (5) with fused five-membered rings is more stable than the cage isomer (1) with isolated five-membered rings. Introduction of endo C-H bonds into structure 5 results in further stabilization, but the most stable tube structure with four endo C-H bonds (7) is higher in energy than the most stable cage structure with ten endo C-H bonds (3) by 74.2 kcal/mol. A comprehensive comparison of C(60)H(60) with C(60)F(60) has been made.     

Electronic communication between two amine redox centers bridged by a bis(terpyridine)ruthenium(II) complex

Two bis(terpyridine)ruthenium(II) complexes 2 and 3 appended with one or two di-p-anisylamino groups, respectively, were synthesized and fully characterized. Their electronic properties were studied by electrochemical and spectroscopic analyses. Electronic communication between individual amine sites of 3 was estimated by intervalence charge-transfer band analyses.     

Unusual sandwich platinum(II) complex: [Pt(phen)2]2+ cation between two Pt(phen)(OOCMe)2 molecules

New carboxylate platinum(II) complexes: syn and anti isomers of Pt(phen)(OOCMe)₂ molecular complex, [Pt(phen)(NCMe)₂](O₃SCF₃)₂, as well as unusual sandwich complex [Pt(phen)₂]²⁺ · 2syn-[Pt(phen)(OOCMe)₂] where [Pt(phen)₂]²⁺ cation is inserted between two syn-Pt(phen)(OOCMe)₂ molecules were synthesized and structurally characterized by X-ray diffraction analysis. As distinct from syn- and anti-Pt(phen)(OOCMe)₂ and [Pt(phen)(NCMe)₂](O₃SCF₃)₂ complexes with flat phenanthroline ligand, the phen ligands in [Pt(phen)₂]²⁺ cation have a curved configuration. Comparative DFT analysis of geometry of model structures phen, phen⁺, phenH⁺, and [Ptphen₂] ⁿ⁺ (n = 1, 2) showed that electron removal from phen molecule had no effect on its geometry in both free state and platinum(II) complexes.     

Arylamine-substituted oligo(ladder-type pentaphenylene)s: electronic communication between bridged redox centers

Novel bis(arylamine-substituted) oligo(ladder-type pentaphenylene)s 1-3, with bridge lengths estimated to be 2.2, 4.2, and 6.3 nm, respectively, have been developed, and the model compound 4 with a mono-arylamine substituent was also synthesized. Their absorption spectra in different solvents are almost identical, while distinct bathochromic shifts of the photoluminescence (PL) spectra were observed with increasing solvent polarity due to the polarized excited states. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) spectra display a two-step oxidation of the bridged diamines in compound 1, which suggests that the electron and charge delocalize in mixed-valence (MV) cation 1+* and that both redox centers can communicate through the pentaphenylene bridge. Only unresolved curves in CV and DPV spectra were observed in the first two oxidation processes of diamines 2 and 3, indicating that the bridges are too long for efficient delocalization over the entire molecules and the radical cations localize at each arylamine center. This finding was further supported by chemical oxidation with SbCl5 and studies of the corresponding UV-vis-NIR absorption spectra of compounds 1-4. A significant intervalence charge-transfer (IVCT) band around 5283 cm-1 (1893 nm) was observed in 1+*. This is the first report of such a highly intense IVCT band in the NIR region with intensity similar to that of the visible band of the radicals, enabling further analysis of the CT process and the coupling matrix element V, classifying 1+* as a class II derivative (V = 1.6 kcal/mol). This study may offer an effective way to improve the understanding of charge transfer and charge-carrier transport in various conjugated oligomers or polymers and facilitate their ongoing exploration in optoelectronic applications.     

Surface/interface electronic structure in C(60) anchored aminothiolate self-assembled monolayer: an approach to molecular electronics

Electronic structure in self-assembled monolayers (SAMs) of C(60) anchored 11-amino-1-undecane thiol (C(60)-11-AUT) on Au(111) was studied by means of ultraviolet photoelectron spectroscopy and hybrid density functional theory calculations. Valence band features of the molecular conformation revealed the interface electronic structure to be dominated by sigma(S-Au), localized at the thiolate anchor to Au. Formation of a localized covalent bond as a result of hybridization between N P(z) orbital of -NH(2) group of the thiolate SAM and the pi level of C(60) resulted in a symmetry change from I(h) in C(60) to C1 in C(60)-11-AUT SAM. Appearance of low, but finite amplitude surface electronic states of bonded C(60), much beyond the Fermi level, ruled out Au-C(60) end group contact. The band gap E(g) of the SAM, determined to be 2.7 eV, was drastically reduced from the insulating alkanethiol SAMs ( approximately 8.0 eV) and fell intermediate between the C(60) ground state (N electrons, 1.6 eV) and C(60) solid (N+/-1 electrons, 3.7 eV).     

Structural aspects of two-stage dimerization in an ionic C60 complex with bis(benzene)chromium: Cr(C6H6)2.C60.C6H4Cl2

Single crystals of the ionic C60 complex with bis(benzene)chromium: {Cr(I)(C6H6)2(.+)}.(C60.-)).C6H4Cl2 (1) were obtained. The crystal structure of 1 shows the presence of monomeric C60.- radical anions at 250 K and the formation of single-bonded (C60-)2 dimers at 90 K. The dimerization is realized in two types of the C60.-) pairs with different interfullerene center-to-center distances of 10.052 and 10.279 A arranged in zigzag chains along the a-direction. As a result, two symmetrically independent (C60-)2 dimers found in 1 at 90 K have different environments, intercage C-C bond lengths and C60- center-to-center distances. Such differences should provide different thermal stability of these dimers and result in the appearance of two stages at the dimerization. Indeed, according to SQUID measurements, the magnetic moment of 1 decreases stepwise at the dimerization in two temperature ranges at 240-200 and 200-160 K.     

Photomagnetic properties of an iron(II) low-spin complex with an unusually long-lived metastable LIESST state

A comprehensive study of the photomagnetic behavior of the [Fe(L222N5)(CN)2].H2O complex has been carried out. This complex is characterized by a low-spin (LS) iron(II)-metal center up to 400 K and exhibits at 10 K the well-known Light-Induced Excited Spin State Trapping (LIESST) effect. The critical LIESST temperature (T(LIESST)) has been measured to be 105 K. The kinetics of the transition from the metastable high-spin (HS) state to the low-spin state have been determined and used for reproducing the experimental T(LIESST) curve. This study represents a second example of a fully low-spin iron(II)-metal complex up to 400 K, which can be photoexcited at low temperature with an atypical long-lived metastable HS state. This underlines the preponderant role of the inner coordination sphere for stabilizing the lifetime of the photoinduced HS state.     

The reversible formation of a single-bonded (C(60)(-))(2) dimer in ionic charge transfer complex: Cp*(2)Cr.C(60)(C(6)H(4)Cl(2))(2). The molecular structure of (C(60)(-))(2)

A new ionic complex of C60 with decamethylchromocene, Cp*2Cr.C60(C6H4Cl2)2 (1), has been obtained. The fullerides are monomeric in 1 at room temperature, whereas they form a single-bonded (C60-)2 dimer at low temperatures, the structure of which has been studied by the X-ray diffraction on a single crystal at 100 K. The length of the intercage C-C bond is 1.597(7) A and the interfullerene distance is equal to 9.28 A. A phase transition attributed to the reversible C60*- dimerization is observed in the 220-200 K range. The transition is accompanied by changes in the unit cell parameters, the decrease of the magnetic moment from 4.20 muB (S = 3/2, 1/2) to 3.88 muB (S = 3/2) and the appearance of EPR signal from Cp*2Cr+, simultaneously.     

Cu3C4-: a new sandwich molecule with two revolving C2(2-) units

A combined photoelectron spectroscopy (PES) and ab initio study was carried out on a novel copper carbide cluster in the gas phase: Cu(3)C(4)(-). It was generated in a laser vaporization cluster source and appeared to exhibit enhanced stability among the Cu(3)C(n)(-) series. Its PES spectra were obtained at several photon energies, showing numerous well-resolved bands. Extensive ab initio calculations were performed on Cu(3)C(4)(-), and two isomers were identified: a C(2) structure ((1)A) with a Cu(3)(3+) triangular group sandwiched by two C(2)(2-) units and a linear CuCCCuCCCu structure (D(infinity)(h), (1)Sigma(g)(+)). A comparison of ab initio PES spectra with experimental data showed that the sandwich Cu(3)C(4)(-) cluster was solely responsible for the observed spectra and the linear isomer was not present, suggesting that the C(2) structure is the global minimum in accordance with CCSD(T)/6-311+G predictions. Interestingly, a relatively low barrier (0.4-0.6 kcal/mol) was found for the internal rotation of the C(2)(2-) units in the sandwich Cu(3)C(4)(-). To test different levels of theory in describing the Cu(m)C(n)(-) systems and lay foundations for the validity of the theoretical methods, extensive calculations at a variety of levels were also carried out on a simpler copper carbide species CuC(2)(-), where two isomers were found to be close in energy: a linear one (C(infinity)(v), (1)Sigma(+)) and a triangular one (C(2)(v), (1)A(1)). The calculated electronic transitions for CuC(2)(-) were also compared with the PES data, in which both isomers were present.     

Crystal structure and magnetic properties of an ionic C60 complex with decamethylcobaltocene: (Cp*2Co)2C60(C6H4Cl2, C6H5CN)2. Singlet-triplet transitions in the C60(2-) anion

The C(60) complex with decamethylcobaltocene, (Cp(2)Co)(2)C(60)(C(6)H(4)Cl(2), C(6)H(5)CN)(2) (1) (C(6)H(4)Cl(2) = 1,2-dichlorobenzene; C(6)H(5)CN = benzonitrile), has been obtained as single crystals by the diffusion method. The IR and UV-vis-NIR spectra show the presence of the C(60)(2)(-) and the Cp(2)Co(+) ions, which form a three-dimensional framework with channels accommodating solvent molecules. EPR and SQUID measurements show that C(60)(2)(-) has a diamagnetic singlet (S = 0) state in the 2-140 K range. The appearance of a broad EPR signal and the increase in magnetic susceptibility of 1 above 140 K are assigned to a thermal population of a close lying triplet (S = 1) state. The singlet-triplet energy gap for C(60)(2)(-) in solid 1 is estimated to be 730+/-10 cm(-)(1).     

Ground and excited state electronic interactions in a bis(phenanthroline) copper(I) complex sandwiched between two fullerene subunits

A new fullerene-substituted phenanthroline ligand has been obtained by reaction of a phenanthroline derivative bearing a 1,3-phenylenebis(methylene)-tethered bis-malonate with C(60) in a double Bingel cyclopropanation. The relative position of the two cyclopropane rings in the resulting bis-methanofullerene derivatives has been determined on the basis of the molecular symmetry (C(s)()) deduced from the (1)H and (13)C NMR spectra. The corresponding Cu(I) complex F-Cu-F has been prepared in good yields by treatment of the ligand with Cu(CH(3)CN)(4)BF(4). In the resulting multicomponent system, both C(60) moieties are in a tangential orientation relative to their bridging phenyl ring, and the central bis(phenanthroline)Cu(I) core is sandwiched between the two carbon spheres. The electrochemical properties of F-Cu-F suggest the existence of ground-state electronic interactions in this multicomponent array based on the mutual effects exerted by the fullerene units to the bis(2,9-diphenyl-1,10-phenanthroline)Cu(I) complex and vice versa. Close vicinity and electronic interactions between the inorganic core and the peripheral fullerene units are also suggested by increased electronic absorption around 430 nm. The distance between the two moieties is estimated to be 4.3 A by molecular modeling studies. The excited-state properties of F-Cu-F have also been investigated. Photoinduced electron transfer from the central chromophore to the external fullerene units occurs but, surprisingly, only following population of the excited states of the central inorganic unit and not of the external carbon spheres. This is mainly attributed to kinetic factors related to the different nature of the two types of excited states involved, namely charge transfer (excitation on the metal-complexed moiety) vs a localized state (excitation on the fullerene units).     

The former "C60F16" is actually a double-caged adduct: (C60F16)(C60)

X-ray diffraction study of the substance originally believed to be C(60)F(16) reveals a double-caged structure, (C(60)F(16))(C(60)); MALDI mass spectra, 19F NMR spectral data and reasons for stability are discussed.     

Mutual orientation of two C60 molecules: an ab initio study

The orientational dependence of the interaction between two C(60) molecules is investigated using ab initio calculations. The binding energy, computed within density functional theory in the local density approximation, is substantially smaller than the one derived from the experimental heat of sublimation of fullerite, which calls into question the nature of inter-C(60) bonding. According to our calculations, the experimentally observed orientation with a C(60) presenting a hexagon-hexagon bond to a pentagonal face of the other C(60) is not really favored. Some other configurations are very close in energy and in fact a pentagon facing a pentagon and a hexagon facing a hexagon-hexagon bond are found to be slightly more favorable situations. Our results are compared to previous ones obtained either with previous empirical intermolecular potentials or to existing ab initio studies of crystalline C(60). In addition, the stacking of C(60) in a crystal and in a decahedral (C(60))(7) cluster is discussed.     

Synthesis, crystal structure and photoconductivity of the first [60]fullerene complex with metal diethyldithiocarbamate: {CuII(dedtc)2}2.C60

The first molecular complex of fullerene C60 with metal dithiocarbamate, namely, {CuII(dedtc)2}2.C60(dedtc: diethyldithiocarbamate) (1) was obtained as single crystals. Butterfly-shaped CuII(dedtc)2 molecules efficiently co-crystallized with spherical fullerene molecules to form a layered structure, in which closely packed hexagonal C60 layers alternate with the layers composed of CuII(dedtc)2 dimers. The formation of the complex with C60 changes geometry and the EPR spectrum of starting CuII(dedtc)2. Magnetic susceptibility of 1 follows the Curie-Weiss law in the 300-1.9 K range with the negative Weiss constant of -2.5 K showing a weak antiferromagnetic interaction between CuII centers in the dimers. The crystals of 1 have low dark conductivity of 10(-11) S cm-1, which is consistent with a neutral ground state of the complex. Illumination of the crystals by white light increases the photocurrent by 20-50 times. The photoconductivity spectrum of 1 has a maximum at 470 nm showing that both intermolecular charge transfer between neighboring C60 molecules and photoexcitation of CuII(dedtc)2 can contribute to photogeneration of free charge carriers. The effect of a weak magnetic field with Bo<0.5 T on the photoconductivity of 1 has been found.