Effect of fluorosubstitution and central metals on the molecular structure and vibrational spectra of metal phthalocyanines

Cobalt and Iron Phthalocyanine Derivatives: Effect of Substituents on the Structure of Thin Films and Their Sensor Response to Nitric Oxide

In this work, we study the effect of substituents in cobalt(II) and iron(II) phthalocyanines (CoPcR₄ and FePcR₄ with R = H, F, Cl, tBu) on the structural features of their films, and their chemi-resistive sensor response to a low concentration of nitric oxide. For the correct interpretation of diffractograms of phthalocyanine films, structures of CoPcCl₄ and FePcCl₄ single crystals were determined for the first time. Films were tested as active layers for the determination of low concentrations of NO (10-1000 ppb). It was found that the best sensor response to NO was observed for the films of chlorinated derivatives MPcCl₄ (M = Co, Fe), while the lowest response was in the case of MPc(tBu)₄ films. FePcCl₄ films exhibited the maximal response to NO, with a calculated limit of detection (LOD) of 3 ppb; the response and recovery times determined at 30 ppb of NO were 30 s and 80 s, respectively. The LOD of a CoPcCl₄ film was 7 ppb. However, iron phthalocyanine films had low stability and their sensitivity to NO decreased rapidly over time, while the response of cobalt phthalocyanine films remained stable for at least several months. In order to explain the obtained regularities, quantum chemical calculations of the binding parameters between NO and phthalocyanine molecules were carried out. It was shown that the binding of NO to the side atoms of phthalocyanines occurred through van der Waals forces, and the values of the binding energies were in direct correlation with the values of the sensor response to NO.

Octafluoro-Substituted Phthalocyanines of Zinc, Cobalt, and Vanadyl: Single Crystal Structure, Spectral Study and Oriented Thin Films

In this work, octafluoro-substituted phthalocyanines of zinc, vanadyl, and cobalt (MPcF₈, M = Zn(II), Co(II), VO) were synthesized and studied. The structures of single crystals of the obtained phthalocyanines were determined. To visualize and compare intermolecular contacts in MPcF₈, an analysis of Hirshfeld surfaces (HS) was performed. MPcF₈ nanoscale thickness films were deposited by organic molecular beam deposition technique and their structure and orientation were studied using X-ray diffraction. Comparison of X-ray diffraction patterns of thin films with the calculated diffractograms showed that all three films consisted of a single crystal phase, which corresponded to a phase of single crystals. Only one strong diffraction peak corresponding to the plane (001) was observed on the diffraction pattern of each film, which indicated a strong preferred orientation with the vast majority of crystallites oriented with a (001) crystallographic plane parallel to the substrate surface. The effect of the central metals on the electronic absorption and vibrational spectra of the studied phthalocyanines as well as on the electrical conductivity of their films is also discussed.

Boosting the Electrocatalytic Conversion of Nitrogen to Ammonia on Metal-Phthalocyanine-Based Two-Dimensional Conjugated Covalent Organic Frameworks

The electrochemical N₂ reduction reaction (NRR) under ambient conditions is attractive in replacing the current Haber-Bosch process toward sustainable ammonia production. Metal-heteroatom-doped carbon-rich materials have emerged as the most promising NRR electrocatalysts. However, simultaneously boosting their NRR activity and selectivity remains a grand challenge, while the principle for precisely tailoring the active sites has been elusive. Herein, we report the first case of crystalline two-dimensional conjugated covalent organic frameworks (2D c-COFs) incorporated with M-N₄-C centers as novel, defined, and effective catalysts, achieving simultaneously enhanced activity and selectivity of electrocatalytic NRR to ammonia. Such 2D c-COFs are synthesized based on metal-phthalocyanine (M = Fe, Co, Ni, Mn, Zn, and Cu) and pyrene units bonded by pyrazine linkages. Significantly, the 2D c-COFs with Fe-N₄-C center exhibit higher ammonia yield rate (33.6 μg h^-1 mg_cat^-1) and Faradaic efficiency (FE, 31.9%) at -0.1 V vs reversible hydrogen electrode than those with other M-N₄-C centers, making them among the best NRR electrocatalysts (yield rate >30 μg h^-1 mg_cat^-1 and FE > 30%). In situ X-ray absorption spectroscopy, Raman spectroelectrochemistry, and theoretical calculations unveil that Fe-N₄-C centers act as catalytic sites. They show a unique electronic structure with localized electronic states at Fermi level, allowing for stronger interaction with N₂ and thus faster N₂ activation and NRR kinetics than other M-N₄-C centers. Our work opens the possibility of developing metal-nitrogen-doped carbon-rich 2D c-COFs as superior NRR electrocatalyst and provides an atomic understanding of the NRR process on M-N_x-C based electrocatalysts for designing high-performance NRR catalysts.

Metal phthalocyanines: thin-film formation, microstructure, and physical properties

Metal phthalocyanines (MPcs) are an abundant class of small molecules comprising of a highly conjugated cyclic structure with a central chelated metal ion. Due to their remarkable chemical, mechanical, and thermal stability MPcs have become popular for a multitude of applications since their discovery in 1907. The potential for peripheral and axial functionalization affords structural tailoring to create bespoke MPc complexes for various next generation applications. Specifically, thin-films of MPcs have found promising utility in medical and electronic applications where the need to understand the relationship between chemical structure and the resulting thin-film properties is an important ongoing field. This review aims to compile the fundamental principles of small molecule thin-film formation by physical vapour deposition and solution processing focusing on the nucleation and growth of crystallites, thermodynamic and kinetic considerations, and effects of deposition parameters on MPc thin-films. Additionally, the structure-property relationship of MPc thin-films is examined by film microstructure, morphology and physical properties. The topics discussed in this work will elucidate the foundations of MPc thin-films and emphasize the critical need for not only molecular design of new MPcs but the role of their processing in the formation of thin-films and how this ultimately governs the performance of the resulting application.

Synthesis, Spectroscopic Properties and Redox Behavior Kinetics of Rare-Earth Bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato Metal Complexes with Er, Lu and Yb

Novel bistetrakis-4-[3-(3,4-dicyanophenoxy)phenoxy]phthalocyaninato of complexes erbium, lutetium and ytterbium were synthesized using a template fusion method to prevent any polymerization process. The complexes were separated from the reaction mixtures and characterized by NMR, IR and electron absorption spectroscopy. The spectroscopic properties of the metal phthalocyaninates in chloroform, acetone and tetrahydrofuran were studied. The regular bathochromic shift in the Er-Yb-Lu series was determined. In acetone medium all the complexes obtained were found to exist in an equilibrium state between neutral and reduced forms. The linearity of Lambert-Bouger-Beer curves makes it possible to study the kinetics of redox processes in the presence of phenylhydrazine and bromine. The lutetium complex showed better reducing properties and turned fully into the reduced form, while the erbium and ytterbium ones changed only partially. Upon oxidizing all the phthalocyaninates transformed into a mixture of oxidized and neutral-radical forms. The extinction coefficients and effective redox constants were calculated.

Screen-Printed Electrodes Modified with Metal Phthalocyanines: Characterization and Electrocatalysis in Chlorinated Media

Metal phthalocyanines are well-known sensing phases with applications in different scientific fields due to their interesting properties. Detailed characterization by Raman spectroscopy was performed in order to study the shifting of the vibrational bands related to the coordination sphere of each metal phthalocyanine. In this work, a study involving the use of screen-printed electrodes (SPEs) with various metal phthalocyanines to electrochemically detect and quantify chlorine (Cl₂) gas is presented. The Cl₂ gas was generated in-situ via oxidation of the chloride present in form of aqueous salt solutions. The developed method offers not only the possibility to quantify chlorine, but also to discriminate among several chlorinated species due to the changes observed in the voltammetric profiles associated with the interaction between the specie assayed and the phthalocyanine metallic center. Optimization of detecting parameters was also performed to apply this procedure for the quantification of chlorine generated from commercial chlorine tablets. The development of this proof of concept shows interesting possibilities and easy-to-use applications with novel on metal phthalocyanines based SPE sensors.

New chalcone-substituted metallophthalocyanines: Synthesis, characterization, and investigation of their properties

The synthesis of novel metallophthalocyanines (M = Zn, Mg, and Co) derived from ( E)-3-(3-bromophenyl)-1-(3-hydroxyphenyl)prop-2-en-1-one and ( E)-3-(3-fluorophenyl)-1-(3-hydroxyphenyl)prop-2-en-1-one is achieved. These complexes and the synthesized novel phthalonitrile derivatives are characterized by FTIR, 1H NMR, 13C NMR, UV-Vis, mass spectrometry, and elemental analysis. The aggregation properties of the metallophthalocyanines ZnPc, MgPc, and CoPc are investigated in different solvents and at different concentrations in dimethyl formamide. The electrochemical behavior is also investigated. The cyclic voltammograms give one oxidation reaction for all the bromo-derived metallophthalocyanines and one reduction reaction for all the fluoro-derived metallophthalocyanines. Fe3+, Cd2+, Hg2+, Cu2+, Ni2+, and Co2+ ions are titrated fluorometrically with the phthalocyanines. The bromo- and fluoro-substituted phthalocyanine compounds show different effects on the metal ion titrations.

2D/3D Metallic Nano-objects Self-Organized in an Organic Molecular Thin Film

We present the fabrication and investigation of the properties of nanocomposite structures consisting of two-dimensional (2D) and three-dimensional (3D) metallic nano-objects self-organized on the surface and inside of organic molecular thin-film copper tetrafluorophthalocyanine (CuPcF₄). Metallic atoms, deposited under ultrahigh vacuum (UHV) conditions onto the organic ultrathin film, diffuse along the surface and self-assemble into a system of 2D metallic overlayers. At the same time, the majority of the metal atoms diffuse into the organic matrix and self-organize into 3D nanoparticles (NPs) in a well-defined manner. The evolution of the morphology and electronic properties of such structures as a function of nominal metal content is studied under UHV conditions using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and photoelectron spectroscopy (PES) techniques. Using HR-TEM, we have observed the periodicity of atomic planes of individual silver NPs. The steady formation of agglomerates from individual single nanocrystallites with intercrystallite boundaries is observed as well. PES reveals generally weak chemical interactions between silver and the organic matrix and n-doping of CuPcF₄ at the initial stages of silver deposition, which is associated with charge transfer from the 2D wetting layer on the basis of core-level spectra shift analysis.