[Show abstract][Hide abstract] ABSTRACT: Raman spectroscopy and in situ Raman spectroelectrochemistry were applied to study the lithium vapor doping of C70@SWCNTs (peapods). A strong degree of doping was proved by the vanishing of the single walled carbon nanotubes (SWCNT's) radial breathing mode (RBM) and by the attenuation of the tangential (TG) band intensity. In contrast to potassium vapor doping, the strong downshift of the frequency of the TG band has not been observed for Li-doping. The Li vapor treated peapods remained partly doped even if they were exposed to humid air. This has been reflected by a reduced intensity of the nanotube and the fullerene modes and by the change of the shape of the RBM band as compared to that of the undoped sample. The modes of the intratubular fullerene were almost unresolved after the contact of the Li-doped sample with water. A lithium insertion into the interior of a peapod and its strong interaction with the intratubular fullerene is suggested to be responsible for the air-insensitive residual doping. This residual doping was studied by spectroelectrochemical measurements. The TG band of the Li doped peapods is partly upshifted during the anodic doping, which points to the different state of C70@SWCNTs and C60@SWCNTs studied previously.
[Show abstract][Hide abstract] ABSTRACT: Raman spectroscopy and in situ Raman spectroelectrochemistry have been applied to the study of the lithium vapor doping of C60@SWCNTs (peapods; SWCNT=single-walled carbon nanotube). A strong degree of doping was proven by the disappearance of the radial breathing mode (RBM) of the SWCNTs and by the attenuation of the tangential (TG) band intensity by two orders of magnitude. The lithium doping causes a downshift of the Ag(2) mode of the intratubular C60 by 27 cm(-1) and changes the resonance condition of the encapsulated fullerene. In contrast to potassium vapor doping, the strong downshift of the TG band was not observed for lithium doping. The peapods treated with lithium vapor remained partially doped even when they were exposed to humid air. This was reflected by a reduction in the intensity of the nanotube and the fullerene modes and by the change in the shape of the RBM band compared with that of the undoped sample. The Ag(2) mode of the intratubular fullerene was not resolved after contact of the lithium-doped sample with water. Lithium insertion into the interior of a peapod and its strong interaction with the intratubular fullerene is suggested to be responsible for the air-insensitive residual doping. This residual doping was confirmed by in situ spectroelectrochemical measurements. The TG band of the lithium-doped peapods did not undergo an upshift during the anodic doping, which points to the formation of a stable exohedral metallofullerene peapod.
[Show abstract][Hide abstract] ABSTRACT: The electrochemical reduction behaviour of a new promising energetic material 2,2-dinitroethene-1,1-diamine (FOX-7) was investigated in acetonitrile, dimethylformamide and dimethylsulfoxide at mercury, platinum, gold and glassy carbon electrodes. Instead of expected consumption of four to eight electrons necessary for reduction of nitro groups, only two electrons are consumed. In acetonitrile, the starting form of FOX-7 is the olefinic, nearly planar structure polarized by the push-pull effect, the first electron attacks one of the nitro groups. The electrochemical as well as in situ UV-vis-NIR and EPR spectroelectrochemical results are in agreement with autoprotonation reduction mechanism. The radical intermediate was intercepted and identified by EPR spectrometry where an alternation line-width (AL) effect was observed.
Full-text · Article · Nov 2014 · Journal of Solid State Electrochemistry
[Show abstract][Hide abstract] ABSTRACT: The low-temperature magnetic properties of the endohedral metallofullerene HoSc2N@C80 have been studied by superconducting quantum interference device (SQUID) magnetometry. Alternating current (ac) susceptibility measurements reveal that this molecule exhibits slow relaxation of magnetization in a small applied field with timescales in the order of milliseconds. The equilibrium magnetic properties of HoSc2N@C80 indicate strong magnetic anisotropy. The large differences in magnetization relaxation times between the present compound and the previously investigated DySc2N@C80 are discussed.
No preview · Article · Oct 2014 · Chemistry - A European Journal
[Show abstract][Hide abstract] ABSTRACT: The first intermediates of electrochemical reduction of nitro compounds in nonaqueous DMF are stable radical anions. In the series of mono-, di-, tri- and tetranitro calixarenes each nitro group represents a reduction center, therefore the question about the spin state of intermediary anions arises. In this communication, the voltammetric and coulometric investigation of these compounds is performed together with the spectral measurements (ESR and UV-vis). The in-situ spectroelectrochemical approach proved that during reduction, in all polynitro radicalic intermediates the electrons remain unpaired and thus relatively stable mono-, di-, tri- and tetraradical mono-, di-, tri- and tetraanions, respectively, can be electrochemically generated in aprotic DMF from polynitrocalixarenes. This finding confirms that the nitrophenyl units in polynitrocalixarenes are completely independent and no mutual electronic communication takes place among them.
No preview · Article · Sep 2014 · Electrochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Low-temperature x-ray magnetic circular dichroism measurements on the endohedral single-molecule magnet DySc2N@C80 at the Dy M4,5 edges reveal a shrinking of the opening of the observed hysteresis with increasing x-ray flux. Time-dependent measurements show that the exposure of the molecules to x-rays resonant with the Dy M5 edge accelerates the relaxation of magnetization more than off-resonant x-rays. The results cannot be explained by a homogeneous temperature rise due to x-ray absorption. Moreover, the observed large demagnetization cross sections indicate that the resonant absorption of one x-ray photon induces the demagnetization of many molecules.
[Show abstract][Hide abstract] ABSTRACT: A detailed spectroelectrochemical and theoretical study of thienyl derivatives based on pyrene, namely 1-(2-thienyl)-pyrene (1), 1,6-bis(2-thienyl)-pyrene (2), 1,4-bis(2-thienyl)-pyrene (3), 1,3,6-tris(2-thienyl)-pyrene (4) and 1,3,6,8-tetra(2-thienyl)-pyrene (5), and their oxidation products with focus on the formation of their corresponding cationic states are presented. Regioregular polymeric systems were prepared by means of electrochemical polymerization from monomers 2-4. The redox behavior of the corresponding polymers was studied by cyclic voltammetry and in situ EPR/UV-vis-NIR spectroelectrochemistry with the aim of obtaining details on the type of charge carriers within the polymer film. Quantum chemical calculations of the optimal geometries and optical transitions for the neutral and charged forms were performed in order to reveal the influence of electrochemical oxidation on the molecular structure of the monomers and corresponding dimers and/or polymers.
No preview · Article · May 2014 · Electrochimica Acta
[Show abstract][Hide abstract] ABSTRACT: In all metallofullerenes known before this work, metal atoms form single highly polar bonds with non-metal atoms in endohedral cluster. This is rather surprising for titanium taking into account the diversity of organotitanium compounds. Here we show that the arc-discharge synthesis of mixed titanium-lutetium metallofullerenes in the presence of ammonia, melamine or methane unexpectedly results in the formation of TiLu2C@Ih-C80 with an icosahedral Ih(7) carbon cage. Single-crystal X-ray diffraction and spectroscopic studies of the compound reveal an unprecedented endohedral cluster with a μ3-carbido ligand and Ti-C double bond. The Ti(IV) in TiLu2C@Ih-C80 can be reversibly reduced to the Ti(III) state. The Ti=C bonding and Ti-localized lowest unoccupied molecular orbital in TiLu2C@Ih-C80 bear a certain resemblance to titanium alkylidenes. TiLu2C@Ih-C80 is the first metallofullerene with a multiple bond between a metal and the central, non-metal atom of the endohedral cluster.
[Show abstract][Hide abstract] ABSTRACT: The magnetic properties of nitride clusterfullerenes GdxSc3-xN@C80 (x = 1-3) are studied as a function of the number of Gd atoms in the cluster. The intracluster interaction of the Gd ions in Gd2ScN@C80 and Gd3N@C80 is ferromagnetic with exchange constants of -1.2 K ± 0.1 K and -0.6 K ± 0.1 K, respectively. At 2 K, the magnetization curves exhibit noticeable deviations from the Heisenberg model pointing to a non-negligible anisotropy.
Full-text · Article · Mar 2014 · Dalton Transactions
[Show abstract][Hide abstract] ABSTRACT: The typical design of chiral electroactive materials, hinging on attaching chiral pendants to an electroactive
polyconjugated backbone, generally results in modest chirality manifestations. We propose electroactive chiral polyheterocycles, where chirality is not external to the electroactive backbone but inherent to it, resulting from a tailored torsion generated by the periodical presence of atropisomeric, conjugatively active biheteroaromatic scaffolds, e.g. 3,3’-bithianaphthene. As the stereogenic element coincides with the electroactive site, films of impressive chiroptical activity and outstanding enantiodiscrimination properties are obtained. Moreover, chirality manifestations can be finely and reversibly tuned by the electric potential, as progressive injection of holes forces the two thianaphthene rings to co-planarize to favour delocalization. Such deformations, revealed by CD spectroelectrochemistry, are elastic and reversible, suggesting the image of a breathing system.
No preview · Article · Mar 2014 · Angewandte Chemie International Edition in English
[Show abstract][Hide abstract] ABSTRACT: A series of various thienyl derivatives of pyrene were synthesized by Stille cross-coupling procedure. Their structures were characterized by (1)H NMR, (13)C NMR and elemental analysis. The spectroscopic characteristics were investigated by UV-vis absorption and fluorescence spectra. Based on quantum chemical calculations, the energy levels of investigated molecules with respect to the pyrene molecule were also discussed.
No preview · Article · Feb 2014 · Journal of Fluorescence
[Show abstract][Hide abstract] ABSTRACT: Redox behavior of endohedral metallofullerenes, in particular their oxidation process, can be classified as a fullerene-based or endohedral species-based process according to the mechanism of the electron transfer. Here we report on the phenomenon of the strain-driven electrochemical behavior achieved by encapsulating the cerium-containing clusters into a series of carbon cages ranging from C78 to C88. The Ce-based mixed metal nitride clusterfullerenes CexM3-xN@C2n (x = 1, 2; M = Sc or Y; 2n = 78-88) were synthesized and characterized. The magnitude of the inherent strain caused by the limited inner space of the carbon cage for the relatively large nitride clusters can be varied by choosing different scaffold metals (Sc, Lu, or Y) to tailor the size of the encaged CexM3-xN cluster and by matching the nitride cluster with different fullerene cages in the size range from C78 to C88. The redox properties of CexM3-xN@C2n were investigated by cyclic and square wave voltammetry. The mechanism of the electrochemical oxidation of Ce-based mixed metal nitride clusterfullerenes, in particular whether the fullerene-based oxidation or the Ce(III) → Ce(IV) process is observed, is found to be dependent on the scaffold metal and the size of the fullerene cage. The endohedral oxidation of Ce(III) to Ce(IV) was observed for a number of compounds as revealed by the negative shift of their oxidation potentials with respect to the values measured for the non-Ce analogues. Experimental studies are supported by DFT calculations. We conclude that the prerequisites for the Ce-based endohedral oxidation process are suitable inherent cluster-cage strain and sufficiently high oxidation potential of the fullerene cage.
[Show abstract][Hide abstract] ABSTRACT: Gold electrodes as one of the most common electrode materials e.g. in devices or chemical sensors are the subject of detailed studies on their surfaces with respect to passivation by very thin oxide layers. Here, we show that the passivating effect of oxide layers on gold can be switched on and off by changing the temperature of the electrode at a fixed potential. Formation and deletion of passivating oxide layers as fast processes can be changed rapidly at heated gold electrodes. Consequently, thermal switching can be performed with high frequency. Examples are given where homogeneous redox reactions are changed by switching the passive films e.g. for the oxidation of ferrocyanide, hydrogen peroxide or ferrocene. Furthermore, the reversible anodic oxidation of gold in the presence of chloride ions can be switched thermally as well. The processes contributing to passivation or depassivation, respectively, have been studied by standard as well as novel thermoelectrochemical methods like TPV (temperature pulse voltammetry). It is shown that temperature variation is an additional parameter to enhance the performance of electrochemical processes at solid electrodes.
No preview · Article · Jan 2014 · Journal of Solid State Electrochemistry
[Show abstract][Hide abstract] ABSTRACT: Poly(2-methoxynaphthalene) (P2MN) belongs to the category of fused ring organic semiconductors that due to their planar structure have very interesting optical and electrical properties like a reduced band gap and easiness of undergoing both oxidation and reduction.
Here, we present a spectroelectrochemical study by means of simultaneous recording of electron spin resonance–UV–vis–near infrared spectra (ESR–UV–vis–NIR) to probe the nature of the charge carriers in both p- and n-doped P2MN. As shown earlier P2MN is both p- and n-dopable, however, further information of the nature of both types of charge carriers is needed. In this work we demonstrate that the dominant charge carrier in p- and n-doped P2MN is positive and negative polarons, respectively.
The material under study constitutes an example of an ambipolar organic conjugated polymer capable to conduct both holes and electrons. This is a unique property worthy to be further studied since only a few organic conducting polymers can reversibly be both p- and n-doped.
No preview · Article · Jan 2014 · Electrochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Infrared spectra of La2@C80 and a series of Ce-based endohedral metallofullerenes (EMFs), including Ce@C82, Ce2@C72, Ce2@C78, and Ce2@C80 are reported. DFT calculations are used for their thorough analysis and assignment. The vibrations of the fullerene cages in all studied EMFs differ from those of their empty, charged cage analogues. Furthermore, EMFs with the same carbon cage but different type of encapsulated species also show significant differences in their cage vibrational patterns. These phenomena are explained by a different coupling of the vibrational modes as well as by the different charge distributions in EMFs and empty, isostructural fullerene anions.
Full-text · Article · Dec 2013 · Fullerenes Nanotubes and Carbon Nanostructures
[Show abstract][Hide abstract] ABSTRACT: A single molecule magnet (SMM) can maintain its magnetization direction over
a long period of time [1,2]. It consists in a low number of atoms that
facilitates the understanding and control of the ground state, which is
essential in future applications such as high-density information storage or
quantum computers [3,4]. Endohedral fullerenes realize robust, nanometer sized,
and chemically protected magnetic clusters that are not found as free species
in nature. Here we demonstrate how adding one, two, or three dysprosium atoms
to the carbon cage results in three distinct magnetic ground states. The
significantly different hysteresis curves demonstrate the decisive influence of
the number of magnetic moments and their interactions. At zero field the
comparison relates tunneling of the magnetization, with remanence, and
frustration. The ground state of the tridysprosium species turns out to be one
of the simplest realizations of a frustrated, ferromagnetically coupled
No preview · Article · Oct 2013 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Electrochemical and NMR spectroscopic studies prove the endohedral oxidation of CeIII in nitride clusterfullerenes CeM2N@C80 (M = Sc, Y, Lu). The redox potential of the endohedral CeIV/CeIII couple systematically varies with the ionic radius of the second cluster metal. DFT computations show that this metal dependence is caused by the release of strain when CeIV with a small ionic radius is formed. In particular, after endohedral oxidation the pyramidal CeY2N cluster becomes planar in the [CeM2N@C80]+ cation.