Cobalt Phthalocyanine Molecules Research Articles

Encapsulation of cobalt phthalocyanine molecules in carbon nanotubes

We report on the encapsulation of cobalt phthalocyanine (CoPc) molecules in double and multi-walled carbon nanotubes. Transmission electron microscopy imaging confirms the filling and shows pristine outer walls after cleaning. We also discuss Co 2p photoemission results, which reveal that hybridization interaction with the surrounding nanotube is likely, resulting in a peak attributed to Co(III).

Low Temperature Scanning Tunneling Spectroscopy of Phthalocyanine Molecules on Metal Surfaces

We studied the electronic structure of cobalt-phthalocyanine (CoPc) molecules on an Au(111) surface by scanning tunneling microscopy and spectroscopy at 5 K. The differential conductance (dI/dV) spectrum measured on the Co atom showed a peak related to the d orbital of the Co atom, whereas that measured on the Pc ring showed three peaks related to the highest occupied molecular orbital (HOMO), HOMO-1 and the lowest unoccupied MO (LUMO). The current-voltage characteristics showed dependence on the position measured within the molecule, indicating that the intramolecular position of the contact between the molecule and the electrode made influence on carrier transport through a single molecule.

Kondo Effect of an Adsorbed Cobalt Phthalocyanine (CoPc) Molecule: The Role of Quantum Interference

It has been shown that by distorting a CoPc molecule adsorbed on a Au(111) surface a Kondo effect is induced with a temperature higher than 200 K. We examine a model in which an atom with strong Coulomb repulsion (Co) is surrounded by four atoms on a square (molecule lobes), with two atoms above and below it representing the apex of the STM tip and an atom on the gold surface (all with a single atomic orbital). The Hamiltonian is solved exactly for the isolated cluster, and, after connecting the leads, the conductance is calculated by standard techniques. Quantum interference prevents the existence of the Kondo effect when the orbitals on the square do not interact (undistorted molecule); the Kondo resonance shows up after switching on that interaction. The weight of the Kondo resonance is controlled by the interplay of couplings to the STM tip and the gold surface and between the molecule lobes.

Controlling the Kondo Effect of an Adsorbed Magnetic Ion Through Its Chemical Bonding

We report that the Kondo effect exerted by a magnetic ion depends on its chemical environment. A cobalt phthalocyanine molecule adsorbed on an Au111 surface exhibited no Kondo effect. Cutting away eight hydrogen atoms from the molecule with voltage pulses from a scanning tunneling microscope tip allowed the four orbitals of this molecule to chemically bond to the gold substrate. The localized spin was recovered in this artificial molecular structure, and a clear Kondo resonance was observed near the Fermi surface. We attribute the high Kondo temperature (more than 200 kelvin) to the small on-site Coulomb repulsion and the large half-width of the hybridized d-level.

Manipulating Magnetism in a Single Molecule

One of the key questions facing researchers studying molecular electronic devices is what happens when molecules come in contact with metals. A subtle effect known as the Kondo resonance offers one way to study these contacts. In his Perspective, [Crommie][1] discusses results reported in the same issue by [ Zhao et al. ][2] on the Kondo effect in a single molecule on a gold surface. Normally a cobalt phthalocyanine molecule sits flat on the metal surface, but when hydrogen atoms are pruned off with a scanning tunneling microscope tip, the molecule arches away from the surface. This causes the spins of the electrons in the metal to interact with the spin of the cobalt atom to form a Kondo resonance. The result opens the way for fundamental studies of spin behavior in molecules that may influence future molecular device applications. [1]: http://www.sciencemag.org/cgi/content/full/309/5740/1501 [2]: http://www.sciencemag.org/cgi/content/short/309/5740/1542

Scanning Tunneling Microscopy and Spectroscopy of Phthalocyanine Molecules on Metal Surfaces

We studied the electronic structure of cobalt-phthalocyanine (CoPc) molecules on Au(111) and Cu(100) surfaces by scanning tunneling microscopy and spectroscopy at 5 K. In the differential conductance (dI/dV) spectra, there were some peaks related to the highest occupied molecular orbital (HOMO), the lowest unoccupied MO (LUMO) and the d-orbitals of the Co atom. CoPc molecules on the Cu(100) surface had new electronic states between the peaks related to HOMO and LUMO, while those on the Au(111) surface did not show additional peaks. A dI/dV image indicated that the new states were generated by the hybridization between the LUMO of molecules and the electronic states of the Cu(100) surface.

Encapsulation of cobalt phthalocyanine in zeolite-y: evidence for nonplanar geometry.

Cobalt (II) phthalocyanine (CoPc) molecules have been encapsulated within the supercage of zeolite-Y. The square-planar complex, being larger than the almost spherical cage, is forced to adopt a distorted geometry on encapsulation. A comparative spectroscopic and magnetic investigation of CoPc encapsulated in zeolite-Y and in the unencapsulated state is reported. These results supported by molecular modeling have been used to understand the nature and extent of the loss of planarity of CoPc on encapsulation. The encapsulated molecule is shown to be the trans-diprotonated species in which the center of inversion is lost due to distortions required to accommodate the square complex within the zeolite. Encapsulation also leads to an enhancement of the magnetic moment of the CoPc. This is shown to be a consequence of the nonplanar geometry of the encapsulated molecule resulting in an excited high-spin state being thermally accessible.

Adsorption of cobalt phthalocyanine on Si(1 0 0)2×1 and Si(1 0 0)2×1:H surfaces studied by scanning tunneling microscopy and spectroscopy

Ultra-high vacuum, scanning tunneling microscopy (STM) and spectroscopy (STS) are used to study isolated cobalt phthalocyanine (CoPc) molecules adsorbed on both the Si(1 0 0)2×1 surface and single dangling bonds of the Si(1 0 0)2×1:H surface. Three major adsorption configurations are observed on the Si(1 0 0)2×1 surface consisting of a fourfold molecular image characteristic of CoPc but possessing different central structural features. However, images obtained on the Si(1 0 0)2×1:H surface correspond to molecular rotation of CoPc parallel to the surface. STM, STS, current image tunneling spectroscopy and molecular modeling are used to investigate the different interactions between CoPc and these two Si substrates, as well as their effects upon the electronic charge distribution and the geometric configurations of the adsorbed CoPc molecules.

Overoxidized Polypyrrole/Cobalt Tetrasulfonated Phthalocyanine Modified Ultramicro‐Carbon‐Fiber Electrodes for the Electrooxidation of 2‐Mercaptoethanol

We report on the catalytic electrooxidation of 2-mercaptoethanol on ultramicro-carbon-fiber electrode (UMCF) modified with overoxidized polypyrrole-doped cobalt-tetrasulfonated phthalocyanine (CoTSPc) film. The easily prepared electrodes are stable and do not passivate after repetitive use. The sensitivity of the modified UMCF electrode towards the amperometric detection of 2-mercaptoethanol was tested with differential pulse voltammetry (DPV) and differential normal pulse amperometry (DNPA). A calculated detection limit as low as 8610 ˇ5 M was obtained by DPV. Our results show that UMCF electrodes are promising for the detection of low concentrations of thiols as mass transport limitations are minimized. The detection of thiol-derivated substances is an important field of research as they can be present as contaminants in fuels (1) and biological fluids (2) and they may be useful as markers of food deterioration (3, 4). During the last decade, several authors (5-17) have reported the catalytic electrooxidation of various thiols on electrodes modified with metallophthalocyanines. In general, electrodes of this type can be used as sensors (15-25) and various configurations have been developed such as carbon paste (15-17) and screen-printed carbon electrodes (19-25) containing cobalt phthalocyanines and derivatives. The advantage of the reported cobalt phthalocyanine-based electrodes is that the thiols (namely 2-mercaptoethanol, L-cysteine, reduced gluthathione etc.) can be monitored at considerable lower potentials compared with the nonmodified electrodes. Superior selectivity and sensitivity were also achieved. We and others have reported that much advantage can be obtained from the activity of cobalt phthalocyanines if they are electropolymerized forming conductive films that coat an electrode surface (14, 18), or if they are dispersed within a conductive polymeric matrix (12, 13). In addition, such an approach should allow the conception of various shapes and geometrical designs of electrode surfaces and a better control of their modification. In this work, we combine for the first time the use of electrochemically formed polypyrrole-doped cobalt tetra- sulfonated phthalocyanine (CoTSPc) with ultramicro-carbon-fiber electrode configuration (UMCF) to achieve the electrocatalytic oxidation of 2-mercaptoethanol (2-ME) in alkaline aqueous solu- tion and provide a miniaturized electrochemical sensor that is easily fabricated from low cost materials, providing an ultramicroprobe that may be developed for applications in biological systems. We have selected 2-ME as the examined analyte target since its elec- potential scan between ˇ0.2 and 0.7 V, at room temperature, starting at the lower potential limit. The incorporation of CoTSPc within the polypyrrole matrix (PPy) is based on the ion exchange properties of the oxidized polymer (26): the multianionic tetra- sulfonated cobalt phthalocyanine molecules are introduced into polypyrrole film as counterions (or ''doping'' ions) during the electrochemical growth of the polymer matrix. Note that in this operation, only CoTSPc was used as supporting electrolyte. The obtained film is denoted as PPy=CoTSPc. In order to avoid having a large electrical charge in the potentiodynamic response of the polypyrrole-based electrodes which creates a large background current that hinder the characterization of the electrocatalytic effects of the complex, the polymer was overoxidized by succes- sive potential scans betweenˇ0.2 and 0.4 V in 0.1 M NaOH (in the absence of pyrrole monomers) until the total suppression of its electroactivity. The obtained modified UMCF electrode is then denoted as OPPy=CoTSPc. In general, these electrodes are much more stable than those obtained by simple adsorption of phtha- locyanine monolayers on carbon or graphite surfaces since the