Vanadyl Phthalocyanine Research Articles

ChemInform Abstract: The Highly Selective Formation of Biaryls by the Cyclization of Arylethynes Catalyzed by Vanadyl Phthalocyanine.

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Phthalocyanine-based photoelectrical cells: effect of environment on power conversion efficiency

We have fabricated single-layer sandwich cells containing photoactive molecular layer (vanadyl phthalocyanine complex) and tested them in various environments. In contrast with dry gases, such as argon, oxygen, and ammonia, which cause the changes mainly in the first quadrant of J–V plots, addition of water vapor resulted in appearance of significant dark short-circuit currents and open circuit voltages. These values were amplified by the presence of mixed water and ammonia vapors. The observed effect is explained by the near-electrode reactions and by the existence of ionic component of conductivity in phthalocyanine layer, which give rise to generation of output electrical power in the dark. The power conversion efficiency under the white light illumination does not follow this trend being the highest in an inert atmosphere (argon). Changing the material of the top metal electrode from aluminum or indium to gold terminates both photovoltaic and dark power generating processes, though sensitivity of the current to vapors holds out.

Morphology and Third-Order Nonlinear Optical Properties of Vanadyl Phthalocyanine Thin Films on Preheated KBr Substrate

Vanadyl phthalocyanine (VOPc) thin films are fabricated using molecular beam epitaxy (MBE) technique on KBr (001) substrate preheated at different temperatures under vacuum. VOPc thin films on the KBr substrate cleaved in air and then preheated in vacuum are investigated using scanning electron microscopy (SEM) images and UV-vis absorption spectra. VOPc thin films depend upon preheating temperature of KBr substrate. Large VOPc grains appear in the background of small crystals on the substrate preheated at low temperatures, while more small grains appear on the substrate preheated at higher temperatures and the film is smoothly and epitaxially grown. The preheating at high temperatures removes adsorbed gasses from the substrate. The effecs of preheating indicate that gas molecules adsorbed on the KBr substrate strongly affect the morphology of the VOPc thin film. The third-order nonlinear optical susceptibilities χ(3) (-3ω; ω, ω, ω) of the epitaxial and pseudomorphic VOPc films were determined using the Maker fringe technique.

A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(1 1 1)

Scanned-energy mode photoelectron diffraction using the O 1s and V 2p emission perpendicular to the surface has been used to investigate the orientation and internal conformation of vanadyl phthalocyanine (VOPc) adsorbed on Au(1 1 1). The results confirm earlier indications from scanning tunnelling microscopy that the V O vanadyl bond points out of, and not into, the surface. The V O bondlength is 1.60 ± 0.04 Å, not significantly different from its value in bulk crystalline VOPc. However, the V atom in the adsorbed molecule is almost coplanar with the surrounding N atoms and is thus pulled down into the approximately planar region defined by the N and C atoms by 0.52 (+0.14/−0.10) Å, relative to its location in crystalline VOPc. This change must be attributed to the bonding interaction between the molecule and the underlying metal surface.

Changes in adsorption and optical properties near the phase transition of super thin vanadyl phthalocyanine Langmuir films

Structural transition in thin vanadyl-phtalocyanine films deposited by Langmuir technology was studied in detail with the help of the vacuum adsorption and optical spectroscopy techniques. The reversible structural transition observed only in super-thin Langmuir films is accompanied by nonreversible changes during heating. This transformation of the film having defects and amorphous regions occurs during the film transition from a metastable state to a more stable one that is characterized by a lower free energy.

The highly selective formation of biaryls by the cyclization of arylethynes catalyzed by vanadyl phthalocyanine

The dimerization of arylethynes catalyzed by vanadium phthalocyanine to give substituted biaryls has been investigated. The reaction yield is always high and for many examples is only slightly affected by the aryl substituents. This fact is also related to the results obtained with metalloporphyrins, which give lower selectivities due to the presence of variable amounts of triphenylbenzenes.

The first liquid crystalline phthalocyanine derivative capable of edge-on alignment for solution processed organic thin-film transistors

Tetraoctyl-substituted vanadyl phthalocyanine (OVPc4C8) as a new NIR-absorbing discotic liquid crystalline material can form highly ordered thin films with edge-on alignment of the molecules and molecular packing mode identical to that in the phase II of OVPc for solution processed OTFTs with mobility up to 0.017 cm(2) V(-1) s(-1).

Structural phase transition in Langmuir films of vanadyl phthalocyanine

The Langmuir-Blodgett films based on vanadyl phthalocyanine (NHSO2C18H37)4 were studied. By the method of piezoresonance quartz balance, the isotherms of adsorption of water, pyridine, and hexane on film surfaces were obtained. A structural phase transition in the Langmuir films of vanadyl phthalocyanine at about 313 K was revealed. The phase transition was observed in ultrathin films and disappeared as the number of layers increased.

Molecular Orientation and Interface Compatibility for High Performance Organic Thin Film Transistor Based on Vanadyl Phthalocyanine

Organic thin film field-effect transistors (OTFTs) with mobility up to 1.0 cm2 V(-1) s(-1) and on/off ratio of 10(6)-10(8) as well as good environmental stability were demonstrated by using vanadyl phthalocyanine (VOPc), a pyramid-like compound with an ultra closely pi-stacked structure. The high performance, remarkable stability, low price, easy availability and nontoxicity of VOPc enabled it to be a promising candidate for OTFTs. Furthermore, we found that the mobility of the devices on OTS-modified Si/SiO2 substrates was 2 orders of magnitude higher than that of devices on Si/SiO2 substrates. Significantly, the relationship between field effect property and insulator surface property was explained from two new aspects of distribution of molecular orientation and interface compatibility, which might provide not only a useful model to explain why the surface modification with OTS could largely improve the field-effect performance but also a guide for rational optimization of device structure for higher performance. In addition, the field effect property of VOPc devices under vacuum, i.e., the oxygen doping effect on the VOPc devices, was measured. We found that the hole mobility decreased by several orders of magnitude with decreasing pressure. At a pressure below 10(-2) Pa, the device on OTS-modified substrates exhibited ambipolar conduction. These results indicated that the oxygen doping exerted essential effect on the field-effect property of VOPc, which was clearly distinct from that observed for pentacene-based OFETs.

Observation of persistent photoconductivity in vanadyl phthalocyanine

Persistent photoconductivity (PPC) in vanadyl phthalocyanine (VOPc) organic light-emitting diodes was investigated using photoconductive time response, photocurrent–voltage characteristics and charge extraction in linearly increasing voltage (CELIV) measurements. The experiments were performed in phase 1 (amorphous) and in phase 2 (crystalline) samples obtained by the physical vapour deposition (PVD) technique over ITO/glass electrodes with an Al covering electrode. The results indicated a photoconductivity with a long decay time in phase 1 VOPc described by a stretched exponential relaxation. The device showed a rectifying behaviour and the mobility of holes was measured by CELIV, following a dispersive model. In crystalline samples the PPC effect was not observed and the dominant mechanism of transport of holes was hopping in a Gaussian density of states.

Spectral characterization of thin films of vanadyl hexadecafluorophthalocyanine VOPcF 16

In this work, structural features and optical properties of hexadecafluorosubstituted vanadyl phthalocyanine (VOPcF 16) films deposited by Organic Molecular Beam Deposition (OMBD) technique have been investigated. Thin films of VOPcF 16 were characterised by ellipsometry, optical absorption and vibrational (IR and Raman) spectroscopy. VOPcF 16 films deposited onto silicon and quartz substrates held at room temperature are well organized and characterised by a predominantly co-facial parallel arrangement of molecules vertical to the surface. Thermal treatment of these films at 220 °C leads to the transition from one phase to another and formation of the polycrystalline VOPcF 16 films with randomly distributed crystallites on the substrate surface.

Temperature dependence of the optical anisotropy of vanadyl phthalocyanine films

The optical anisotropy of vanadyl phthalocyanine films is investigated in the temperature range from 300 K to 10 K by reflection anisotropy spectroscopy in the energy range from 1.5 eV to 5.5 eV. With changing temperature all spectral features show reversible changes in their relative intensity. The most pronounced changes are observed in the sub-structure of the Q band related feature when cooling down from 300 K to 100 K. All changes are recovered upon annealing back to room temperature, indicating a reversible structural modification of the films assisted by the illumination.

Vapor Pressure of Some Metal Phthalocyanines

The temperature dependences of saturated vapor pressures of hexadecafluorinated zinc and vanadyl phthalocyanines (MPcF16, M = Zn, VO) and their unsubstituted analogues were studied by the Knudsen effusion method. The temperature dependences of saturated vapor pressures were used to calculate the thermodynamic parameters of vaporization of phthalocyanines.

Electrical properties in vanadyl-phthalocyanine-based metal-insulator-semiconductor devices

We investigated electrical properties of vanadyl phthalocyanine (VOPc) metal-insulator-semiconductor (MIS) devices by the measurement of capacitance and conductance, which were fabricated on ordered para-sexiphenyl (p-6P) layer by weak epitaxy growth method. The VOPc∕p-6P MIS diodes showed a negligible hysteresis effect at a gate voltage of ±20V and small hysteresis effect at a gate voltage of ±40V due to the low interface trap state density of about 1×1010eV−1cm−2. Furthermore, a high transition frequency of about 10kHz was also observed under their accumulation mode. The results indicated that VOPc was a promising material and was suitable to be applied in active matrix liquid crystal displays and organic logic circuits.

Effect of Thermal Annealing on Hydrogen Bond Configurations of Host Lattice Revealed in VOPc/TCDB Host−Guest Architectures

The two-dimensional (2D) self-assembled 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB) networks connected by hydrogen bonds are shown to accommodate molecules such as vanadyl phthalocyanine (VOPc) on highly oriented pyrolytic graphite (HOPG) surfaces when examined by scanning tunneling microscopy (STM) under ambient conditions. Thermally annealed adlayers were investigated with STM, which demonstrated that thermal annealing could induce a distinctive phase transition from the host−guest architecture VOPc/TCDB (II) (consisting of VOPc dimers or two VOPcs entrapped in one TCDB cavity) to the architecture VOPc/TCDB (I) (consisting of VOPc monomers or one VOPc entrapped in one TCDB cavity). On the basis of density functional theory (DFT) calculations, we suggest that the transition is associated with the transformation of the hydrogen bond configurations in the TCDB dimers. The experimental results combined with the theoretical calculations revealed that monomer-entrapped VOPc/TCDB (I) is thermodynamically stable for the VOPc/TCDB complex architectures.

Effect of gas phase adsorption on the conductivity of Langmuir-Blodgett films of vanadyl phthalocyanine

Frequency dependences are obtained of the conductivity in Langmuir films of modified vanadyl phthalocyanine in vacuum and under the conditions of adsorption of donor and acceptor molecules. It is found that, upon the adsorption of NO2 and CH4O molecules, the film conductivity at f < 10 kHz increases by several orders of magnitude. It is shown that, in vacuum and upon adsorption of acceptor molecules, the dominant charge transfer mechanism is the hopping of holes via localized states. In the case of adsorption of donor molecules, the ionic conductivity mechanism appears to prevail.

Electronic structure of the organic semiconductor vanadyl phthalocyanine (VO-Pc)

The electronic structure of thin films of the organic semiconductor vanadyl phthalocyanine has been studied using synchrotron radiation-excited resonant soft X-ray emission spectroscopy, resonant inelastic soft X-ray scattering, X-ray absorption spectroscopy, and X-ray photoemission spectroscopy. The C, N, V, and O partial densities of states for both the valence and conduction bands have been determined, as well as the core level electronic structure. Good agreement was found between the experimental measurements of the valence and conduction bands, and the results of a density functional calculation.

Comment on: “Magnetic field influence on the molecular alignment of vanadyl phthalocyanine thin films” by: V. Kolotovska et al. [J. Crystal Growth 291 (2006) 166

Comment on: “Magnetic field influence on the molecular alignment of vanadyl phthalocyanine thin films” by: V. Kolotovska et al. [J. Crystal Growth 291 (2006) 166

Nanoparticle Formation of Pentacene by Laser Irradiation in Ethanol Solution

Laser irradiation using a Nd3+:YAG laser (532 nm, 8 ns, 10 Hz) on an ethanol suspension of micrometer-sized pentacene crystals induced absorption spectral changes in the solution, as a result of the formation of pentacene nanoparticles. The nanoparticles were platelike crystals. The nanoparticles formed by irradiation with a fluence of 50 mJ/cm2 for 30 min were 4–13 nm in height and 10–70 nm in width. The threshold laser fluence for the formation was determined to be ca. 20 mJ/cm2, and irradiation with fluences above 80 mJ/cm2 induced the decomposition of the pentacene nanoparticles. The nanoparticle size decreased with an increase in the laser fluence, which was reflected in the longest-wavelength peak position of the absorption bands of the pentacene nanoparticles. On the basis of these results, the mechanism of pentacene nanoparticle formation by laser irradiation is discussed in connection with those of vanadyl phthalocyanine and quinacridone systems reported by Masuhara and coworkers [J. Phys. Chem. A 106 (2002) 2135; Jpn. J. Appl. Phys. 42 (2003) 2725; Jpn. J. Appl. Phys. 45 (2006) 384].

Magnetic field influence on the molecular alignment of vanadyl phthalocyanine thin films

Thin vanadyl phthalocyanine (VOPc) films with thicknesses ranging between 10 and 100 nm were grown by organic molecular beam deposition (OMBD) in high vacuum on H–Si(1 1 1) and quartz substrates. The influence of a magnetic field (1.3 T) applied during the film growth was investigated by means of variable angle spectroscopic ellipsometry (VASE), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction and atomic force microscopy (AFM). The dielectric function was determined by means of VASE. The VOPc layers were found to be uniaxially anisotropic, i.e. exhibit the same optical constants along two of the Cartesian axes, and different optical constants along the third axis. The values of the dielectric function with in-plane of the substrate are larger compared to those out-of-plane. The in-plane and out-of-plane values of the imaginary part of the dielectric function in the region of the Q-band were used to determine the tilt angle of the molecular plane with respect to the substrate plane. Films deposited without magnetic field have the molecular planes tilted with respect to the substrate plane by about 24°, while those deposited in a magnetic field perpendicular to the substrate, the molecules tend to align their molecular planes with respect to the substrate at ca. 3°. All the samples were found to have a triclinic crystallographic structure. From the comparison between the FTIR spectra of VOPc layers and pellets dispersed in KBr, it can be also deduced that the molecules are lying parallel to the substrate in the case of samples grown in the presence of a magnetic field and slightly tilted in those grown without magnetic field.