Weak epitaxy growth and phase behavior of planar phthalocyanines on p-sexiphenyl monolayer film

We systematically investigated the weak epitaxy growth (WEG) behavior of a series of planar phthalocyanine compounds (MPc), i.e., metal-free phthalocyanine (H2Pc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), zinc phthalocyanine (ZnPc), iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), grown on a p-sexiphenyl ( p-6P) monolayer film by selected area electron diffraction (SAED) and atomic force microscopy (AFM). Two types of epitaxial relations, named as incommensurate epitaxy and commensurate epitaxy, were identified between phthalocyanine compounds and the substrate of the p-6P film. The tiny variation of the lattice constant of phthalocyanine compounds can result in different crystal orientations. The change rule of incommensurate and commensurate epitaxy was extracted. The tendency of commensurate epitaxy becomes weaker as the lattice constant b increases, while it gets stronger as the substrate temperature is elevated. Large size and continuous H2Pc films can be obtained by controlling the growth conditions. The WEG method is generally applicable in the whole family of planar phthalocyanine compounds and may be used to fabricate other high-quality organic films.     

Scanning tunneling microscopy study of metal-free phthalocyanine monolayer structures on graphite

Low temperature scanning tunneling microscopy (STM) studies of metal-free phthalocyanine (H2Pc) adsorbed on highly oriented pyrolytic graphite (HOPG) have shown ordered arrangement of molecules for low coverages up to 1 ML. Evaporation of H2Pc onto HOPG and annealing of the sample to 670 K result in a densely packed structure of the molecules. Arrangements of submonolayer, monolayer, and monolayer with additional adsorbed molecules have been investigated. The high resolution of our investigations has permitted us to image single molecule orientation. The molecular plane is found to be oriented parallel to the substrate surface and a square adsorption unit cell of the molecules is reported. In addition, depending on the bias voltage, different electronic states of the molecules have been probed. The characterized molecular states are in excellent agreement with density functional theory ground state simulations of a single molecule. Additional molecules adsorbed on the monolayer structures have been observed, and it is found that the second layer molecules adsorb flat and on top of the molecules in the first layer. All STM measurements presented here have been performed at a sample temperature of 70 K.     

Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.     

Fluorescence of solid metal-free phthalocyanine

Solid metal-free phthalocyanine fluorescence has been studied as a function of polymorphic modification. Pronounced variation in observed fluorescence spectra suggests that fluorescence can be a useful diagnostic tool for study of photophysical properties of solid metal-free phthalocyanine. The observed fluorescence spectra display two bands, one in the 800 nm region, the other in the 900 nm region. The observed lifetimes and temperature dependence suggest that the former band corresponds to the normal molecular fluorescence, whereas the latter arises from excimer emission.     

Electrical conductivity of polycrystalline films of p-sexiphenyl

In this work the results of DC conductivity measurements of polycrystalline p-sexiphenyl thin films are presented. The investigations concerned the effect of temperature, film thickness and electric field on the DC conductivity mechanism. The thickness of the investigated material varied from 0.2 to 2.5 μm. The measurements were carried out for different electrode polarities of the 0 -100 V voltage and at temperatures ranging from 15 to 325 K. Thin films of p-sexiphenyl were obtained by controlled vacuum sublimation on BK-7 glass substrate with gold and aluminium electrodes. Analyzing the obtained results we conclude that injection of the charge carrier from electrodes into the investigated material proceeds by thermionic emission and field emission and it is dependent on temperature and external electric field. The charge carrier transport is controlled by localized states (traps) in the forbidden energy gap. The activation energy calculated from formula ln I = f(1/kT) varied from kT for low temperature up to 1.0 eV.     

The optical properties of a novel metal-free phthalocyanine

In the present work, 4-(4′-dodecyloxycarbonyl) phenoxy phthalonitrile was synthesized and then this phthalonitrile derivative was cyclotetramerized in dodecanol resulting a new metal-free phthalocyanine. The optical properties of this phthalocyanine were investigated. Novel metal-free phthalocyanine thin films were deposited on indium tin oxide-coated glass substrates by spinning method. Surface and microstructural properties of the films were characterized by atomic force microscopy and scanning electron microscopy. Both the transmittance and reflectance spectra of the deposited films were recorded using an NKD analyser. The optical band gap energy, the thickness of thin films, refractive index (n) and extinction coefficient (k) were calculated as 2.7 eV, 300 nm, 1.47 and 0.02, respectively.     

Electronic structure of a vapor-deposited metal-free phthalocyanine thin film

The electronic structure of a vapor-sublimated thin film of metal-free phthalocyanine (H2Pc) is studied experimentally and theoretically. An atom-specific picture of the occupied and unoccupied electronic states is obtained using x-ray-absorption spectroscopy (XAS), core- and valence-level x-ray photoelectron spectroscopy (XPS), and density-functional theory (DFT) calculations. The DFT calculations allow for an identification of the contributions from individual nitrogen atoms to the experimental N1s XAS and valence XPS spectra. This comprehensive study of metal-free phthalocyanine is relevant for the application of such molecules in molecular electronics and provides a solid foundation for identifying modifications in the electronic structure induced by various substituent groups.     

Photochemical transformation of metal-free phthalocyanine in a Shpol'skii matrix

Low lying vibronics within ∼700 cm⁻¹ above the origins of the first excited states of the two isomeric forms of metal-free phthalocyanine (H₂Pc) in mixed solvent matrix were re-examined by fluorescence excitation. Based on the obtained information, direct observation of photoisomeric transformation in the complex was made both from the fluorescence intensity decline under continuous resonance pumping and from the asymmetry of the fluorescence excitation intensity ratios upon two opposite laser scanning directions.     

Time resolved observation of geminate recombination in metal-free phthalocyanine

Delayed-collection-field measurements of carrier generation and delayed fluorescence in metal-free phthalocyanine have been shown to exhibit the same time dependence. This indicates the existence of geminate electron—hole pairs with survival times extending to tens of milliseconds.     

Distribution of metal-free sulfonated phthalocyanine in subcutaneously transplanted murine tumors

Metal-free sulfonated phthalocyanine with the average number of sulfonate groups per molecule 2.4 (H(2)PcS(2.4)) was recently proved to be an efficient photosensitizer for the photodynamic therapy. Fluorescence spectral imaging microscopy was applied here to study localization and relative concentration of H(2)PcS(2.4) with micron-scale resolution in subcutaneously transplanted murine tumors: Ehrlich mammary gland carcinoma (EC), Lewis lung carcinoma (LLC), P388 lymphoid leukemia (P388) and B16 melanoma (B16). The study of cryogenic tissue sections prepared 24 h after H(2)PcS(2.4) intravenous injection revealed that H(2)PcS(2.4) was present in all tissue structures in the monomeric photoactive state. The preferential accumulation of H(2)PcS(2.4) was documented in tumor cells and adjacent non-tumor tissues (skin structures, fatty tissue, connective tissue enriched in fibrous component and infiltrated with fibroblasts and macrophages) for all the studied tumor models. P388 and B16 were stained with H(2)PcS(2.4) less than adjacent skin structures, whereas EC and LLC accumulated H(2)PcS(2.4) alike or higher than particular skin structures. Staining of EC and LLC was similar and ca. 1.4 and 2 times higher than that of B16 and P388, respectively, thus revealing the differences in ability of particular tumor strains to H(2)PcS(2.4) accumulation. The H(2)PcS(2.4) concentration in remote healthy tissues (skin, muscles and connective tissue) was 2-3 times lower as compared with the analogous tissue structures from the tumor area, whereas subcutaneous fatty tissue staining did not depend on the tissue-to-tumor distance. The tissue distribution of H(2)PcS(2.4) predefines the combined action of two photodynamic damage mechanisms: eradication of tumor due to the direct tumor cell destruction and suppression of tumor growth due to the injury of growth supporting system.     

Multiple two-dimensional structures formed at monolayer and submonolayer coverages of p-sexiphenyl on the Au(111) surface

The two-dimensional structures formed by monolayers and submonolayers of p-sexiphenyl (p-6P) molecules evaporated onto the Au(111) surface are investigated using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Five different 2D structures corresponding to different surface coverages are discovered and their 2D structures solved. The trends in the molecular alignment with respect to the underlying gold lattice are discussed. An unusual structure that consists of paired rows of p-6P molecules was discovered. A surface structure with alternating domains of slightly differently packed p-6P molecules was also found. The boundary between these two domains contains systematic molecular vacancies.     

Synthesis and characterization of a novel unsymmetrical metal-free phthalocyanine with donor-acceptor substituents

A novel unsymmetrical metal-free phthalocyanine with a nitro group as an acceptor substituent and three t-butyl groups as donor groups, namely nitro-tri-t-butylphthalocyanine (NtBuPc) was synthesized for the first time by mixed condensation of two corresponding diiminoisoindolines. The NtBuPc can be separated by common column chromatography on silica gel using chloroform-hexane as the elution solvent. The structure was confirmed by elemental analysis, nmr, uv-vis, ir and mass spectroscopy. No contamination of either symmetrical or other related phthalocyanines was observed.     

Manifestation of grain boundaries in the transport properties of p-sexiphenyl films

Direct current conductivity of p-sexiphenyl films with 0.2-2.5 microm thickness deposited on glass substrates was investigated at temperatures between 10 and 300 K. The molecular dynamics simulations and quantum chemical calculations of the experimentally observed conductivity have shown that there exist at least three different states of the films effectively contributing to the observed features of the dc conductivity: conductivity caused by proper crystalline states, conductivity originated from amorphous-like inter-grain region and caused by grain's boundary. Comparison of the experimental data and theoretically calculated dependences shows that the main contribution to the observed dc conductivity gives grain boundaries between polycrystalline grains and amorphous-like background, though the latter possesses a relatively low part of the total volume.     

Physics of ultra-thin phthalocyanine films on semiconductors

This paper focuses on the structural arrangements, as seen in scanning tunneling microscopy, chemical and electric properties, as studied especially by high resolution synchrotron radiation photoelectron spectroscopy and energy loss spectroscopy of sub-monolayer up to few monolayer thin films of phthalocyanines adsorbed on narrow band gap III–V semiconductors where they form ordered superstructures, as well as on silicon surfaces. Special emphasis will be put on non-planar metal phthalocyanines where the central atom is either lead or tin. These molecules have been studied in detail from the experimental point of view as well as theoretically by ab initio quantum calculations. Comparison with metal free and metal substituted planar phthalocyanines (H₂Pc, CuPc, CoPc, …) adsorbed on semiconductors or noble metal surfaces is included. Perspectives and novel potential applications are finally outlined.     

Density-of-states function in metal-free phthalocyanine: effect of exposure to chlorodifluoromethane

Changes in the energy structure of local states during the exposure of metal-free phthalocyanine films to chlorodifluoromethane were studied using the technique of thermomodulated space-charge-limited currents. The broadening of the energy distribution of the states is explained on the basis of charge-dipole interactions.     

Surface plasmon resonance on the LB monolayer of solvent-soluble silicon phthalocyanine

Thin films of bis-(p-chlorophenoxy) (tetra-4-nitrophthalocyaninato)silicon, TNPcSi(OPhCl-p)₂, were prepared by using Langmir-Blodgett techniques and the morphology of the film was investigated by AFM (atomic force microscope) analysis. It has been found that the LB films were successfully transferred onto substrates like mica, gold deposited glass, and pure glass with domains aggregated on the substrates. Surface plasmon resonance has been used to investigate the interaction between sulfur dioxide and a monolayer of TNPcSi(OPhCl-p)₂ LB film. It has been found that the monolayer of LB film transferred onto gold deposited glass gave a 0.55° shift of resonant angle and an additional 0.45° shift of resonant angle after exposion in sulfur dioxide\atomospheric ambient for twenty minute. The UV–visible absorption spectrum of the LB films of TNPcSi(OPhCl-p)₂ showed that there is chemical changes in the film after the exposure to the sulfur dioxide ambient. This work has shown a promising application as an optical gas sensor.     

Observation of Davydov splitting in the MCD spectra of α metal-free phthalocyanine

The magnetic circular dichroism and absorption spectra of a metal-free phthalocyanine have been measured at 300 and 8 K. Using both techniques the visible spectrum can be analysed as two transitions to degenerate and two to nondegenerate excited states. These excited states are assigned as the differently split pairs of Davydov components derived from the lowest energy E_u and A_2u states of the monomeric 18π electron phthalocyanine ring.     

Unusual self-assembled nanoaggregation of the thermally stable metal-free slipped-cofacial J-type phthalocyanine dimer

Nanoaggregation of the thermally stable bis(2-hydroxy-9(10),16(17),23(24)-tri-tert-butylphthalocyanine) of J-type was observed by the field emission scanning electron microscopy (FE-SEM). Ordered phases, represented as threads composed of about 200 nm diameter circles were discovered, unlike the parent dimeric magnesium complex, in which similar circles combine together to give visually distinguishable zones with widths of about 600 nm and the same distance between them.     

Investigation on the orderly growth of thick zinc phthalocyanine films on Ag(100) surface

The growth of zinc phthalocyanine (ZnPc) on Ag(100) surface from monolayer to multilayer was investigated by low-energy electron diffraction, x-ray diffraction, and high-resolution electron energy loss spectroscopy (HREELS). At monolayer coverage, ZnPc molecules form an ordered film with molecular planes parallel to the substrate. The same structure is maintained as the film thickness increases. HREELS analysis shows that intermolecular π-π interaction dominates during the film growth from monolayer to multilayer. The π-d interaction between the adsorbates and the substrate is only applicable in the first adlayer. Stronger intermolecular-layer interaction is observed at higher coverages.