Redox properties of octacyano-substituted zinc phthalocyanine ((CN)8PcZn). New charge-transfer complex

Transition Metal Phthalocyanines as Redox Mediators in Li-O2 Batteries: A Combined Experimental and Theoretical Study of the Influence of 3d Electrons in Redox Mediation

Redox mediation is an innovative strategy for ensuring efficient energy harvesting from metal-oxygen systems. This work presents a systematic exploratory analysis of first-row transition-metal phthalocyanines as solution-state redox mediators for lithium-oxygen batteries. Our findings, based on experiment and theory, convincingly demonstrate that d5 (Mn), d7 (Co), and d8 (Ni) configurations function better compared to d6 (Fe) and d9 (Cu) in redox mediation of the discharge step. The d10 configuration (Zn) and non-d analogues (Mg) do not show any redox mediation because of the inability of binding with oxygen. The solution-state discharge product, transition-metal bound Li2O2, undergoes dissociation and oxidation in the charging step of the battery, thus confirming a bifunctional redox mediation. Apart from the reaction pathways predicted based on thermodynamic considerations, density functional theory calculations also reveal interesting effects of electrochemical perturbation on the redox mediation mechanisms and the role of the transition-metal center.

Synthesis, characterization, photophysicochemical properties and theoretical study of novel zinc phthalocyanine containing four tetrathia macrocycles

In this work, Zn(II) phthalocyanine derivative (TTU-Pc) bearing 13-membered tetrathia macrocycles was synthesized, and the novel Zn(II) phthalocyanine derivative was fully characterized by elemental analysis and general spectroscopic methods such as MALDI-TOF mass, FT-IR, UV-vis and [Formula: see text]H-NMR. The synthesized phthalocyanine derivative has quite limited solubility in most of the common organic solvents. Fluorescence measurement was conducted for this Zn(II)phthalocyanine to estimate its fluorescence quantum yields. The singlet oxygen generation ability was also examined to investigate its photosensitizer properties. General trends were described for quantum yields of fluorescence, photodegradation and singlet oxygen quantum yields of this compound. The electrochemical properties of the molecule were investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV). In addition, the lowest energy structure, the electronic structure and frontier molecular orbitals were calculated in DFT and the excitation spectrum was obtained by TDDFT calculations. We found that our computational and experimental results were in agreement.

Highly planar diarylamine-fused porphyrins and their remarkably stable radical cations

Oxidative fusion of a meso-phenoxazino Ni(ii) porphyrin at high temperature gave a doubly phenoxazine-fused porphyrin as a highly planar diarylamine-fused porphyrin. One-electron oxidation of the corresponding β,β-dichlorinated compound gave a remarkably stable radical cation.

Oxidative fusion reactions of meso-phenoxazino Ni(ii) porphyrin were found to be temperature dependent, giving rise to either a doubly phenylene-fused product at room temperature or a singly phenoxazine-fused product at 70 °C. The latter was further oxidized to a doubly phenoxazine-fused Ni(ii) porphyrin, which was subsequently converted to the corresponding free base porphyrin and Zn(ii) porphyrin. Compared to previously reported diphenylamine-fused porphyrins that displayed a molecular twist, doubly phenoxazine-fused porphyrins exhibited distinctly different properties owing to their highly planar structures, such as larger fluorescence quantum yields, formation of an offset face-to-face dimer both in solution and the solid state, and the generation of a mixed-valence π-radical cation dimer upon electrochemical oxidation. One-electron oxidation of the phenoxazine-fused Ni(ii) porphyrin with Magic Blue gave the corresponding radical cation, which was certainly stable and could be isolated by separation over a silica gel column but slowly chlorinated at the reactive β-positions in the solid state. This finding led to us to examine β,β′-dichlorinated phenoxazine-fused and diphenylamine-fused Ni(ii) porphyrins, which, upon treatment with Magic Blue, provided remarkably stable radical cations to an unprecedented level. It is actually possible to purify these radical cations by silica gel chromatography, and they can be stored for over 6 months without any sign of deterioration. Moreover, they exhibited no degradation even after the CH₂Cl₂ solution was washed with water. However, subtle structural differences (planar versus partly twisted) led to different crystal packing structures and solid-state magnetic properties.

Electrochemical properties of octakis(hydroxyethylthio)-substituted phthalocyanines

Cyclic voltammetry of octakis(hydroxyethylthio)-substituted phthalocyanines with 2 H ( H 2 OHETPc ), Zn (II) ( Zn II OHETPc (−2)) and Co (II) ( Co II( OHETPc (−2)) in the phthalocyanine core has been investigated in dimethylsulfoxide ( DMSO ), and some of the results were evaluated together with their absorption spectra in the visible range. The redox processes due to both aggregated and non-aggregated species were observed on the voltammograms of phthalocyanines with Zn (II). The effect of chloride ion on the cyclic voltammetry of Co II OHETPc (−2) has been investigated in the DMSO / ClO 4− couple. The comparison of cyclic voltammetry for Co II OHETPc in the absence and presence of chloride ion showed that chloride ion does not bind to either Co (II) or Co (I) but does bind very strongly to Co (III). The detailed electrochemistry of Co II OHETPc (−2) was investigated in terms of the Co (I)/ Co (II) and Co (II)/ Co (III) redox couples, and various equilibria related to the species coordinated to Co (II) and Co (III) in the phthalocyanine core.

Electrochemical and Adsorption Properties of Novel Phthalocyanines with Four 16-Membered Diazadithia Macrocycles

Cyclic voltammetric measurements of Cu (II) and Ni (II) phthalocyanines with four 16-membered diazadithia macrocycles (1 and 2) and pentanuclear Ni (II) phthalocyanine bearing four Cu (II) ions in diazadithia macrocycles at the peripheral positions (3) have been carried out on platinum in DMSO and in aqueous medium 3 showed strong adsorption properties in DMSO and especially in aqueous medium, possibly as a result of the interaction between the copper ions in the four macrocycles and the platinum working electrode. Interestingly, the oxidation peak of 3 was split into two waves in aqueous medium. This behaviour is discussed in terms of disaggregation of the mixed valence species produced by the oxidation of aggregated species. A mechanism for both the oxidation of the complex and the splitting of the oxidation peak into two waves is suggested.

Formation and Spectroelectrochemical Characterization of Multilayer and Submonolayer Thin Films of 2,3,9,10,16,17,23,24-Octa(2-benzyloxy-ethoxy) Phthalocyninato Copper (CuPc(OC2OBz)8)

We review here the self-assembly, electrochemical and spectroelectrochemical properties of 2,3,9,10,16,17,23,24-octa(2-benzyloxyethoxy) phthalocyaninato copper ( CuPc ( OC 2 OBz )8), where terminal benzyl groups on the eight ethylene oxide side chains assist in forming unusually ordered, mechanically rigid thin films. New spectroscopic characterization of cast CuPc ( OC 2 OBz )8 films is discussed in comparison with cast films of its metal-free analogue H 2 Pc ( OC 2 OBz )8 and similar copper and dihydrogen phthalocyanines with benzyl — terminated triethylene oxide substituents, CuPc (( OC 2 O )3 Bz )8, and H 2 Pc (( OC 2 O )3 Bz )8, which do not demonstrate the same degree of ordering as CuPc ( OC 2 OBz )8. AFM studies of horizontally transferred LB films of CuPc ( OC 2 OBz )8 show column–column distances of ca 2.8 nm and confirm the high degree of ordering previously surmised from spectroscopic characterization of multilayer thin films. The oxidative electrochemistry of multilayer thin films prepared from these Pcs is strongly dependent on the chemical identity of the supporting electrolyte anion and on annealing of the thin films. Compliance of the films to counter-ion transport limits the extent of electrochemical doping. Preliminary studies of the oxidative electrochemistry of isolated CuPc ( OC 2 OBz )8 aggregates (diluted into an electroinactive methyl arachidate thin film) on an electroactive, integrated optic waveguide (EA-IOW) are also presented. Monitoring the change in absorbance at 633 nm on the waveguide surface allows the determination of the onset potential for oxidation of the isolated aggregates, which appears to be less positive in potential versus that observed for multilayer Pc assemblies.

Electrochemical and in situ spectroelectrochemical behavior of 2,3,7,8,12,13,17,18-octakis(hexyl-thio) tetraazaporphyrin (H2OHTTAP) and [Zn(II)OHTTAP]

The characterization of free base porphyrin 2,3,7,8,12,13,17,18-octakis(hexyl-thio) tetraazaporphyrin (H2OHTTAP) and its zinc(II) complexes [ Zn (II)OHTTAP] containing eight thioether groups at the β-pyrrole positions of the macrocycle was reported. Results obtained by cyclic voltammetry and differential pulse voltammetry indicated a five-electron reduction in five steps for each complex. They were oxidized in two single-electron-transfer steps to yield π-cation radicals and dications and reduced in three single-electron-transfer steps to yield π-anion radicals, dianions and trianions, respectively. The redox property of H2OHTTAP was unusual as compared to porphyrins (PPs) and phthalocyanines (Pcs). Each process was monitored by in situ thin-layer spectroelectrochemistry, which indicated that only the ligand was electroactive. The existence of the eight hexylthio groups was responsible for the intrastack interactions and enhanced intracolumnar and intercolumnar electron motions, resulting in improved conductivity.

Influence of Mn as a redox-active central metal on the electrical conduction behaviour of phthalocyanine thin films

Thin films of phthalocyaninatomanganese ( PcMn ) in the thickness range of 100 nm have been prepared by vapour deposition on quartz glass substrates. The films were characterized in situ during film growth and following film deposition by measurements of the electrical conductivity under DC applied electric fields parallel to the substrate surface. The dependence of the conductivity on the average film thickness was determined and the mechanism of film growth is discussed. Without breaking the vacuum, a temperature gradient was established again parallel to the substrate surface and a thermopower was detected. Its dependence on the size of the temperature gradient gave a Seebeck coefficient of –780 μV K-1 at 460 K. From the negative sign it is evident that electrons are the majority carriers in PcMn in a freshly prepared thin film. The temperature dependence of the Seebeck coefficient gave the thermal activation of charge carrier generation as ΔE = 0.19 eV , which is discussed in terms of the position of the Fermi energy in the films. The temperature dependence of the electrical conductivity gave an activation energy EA = 0.38 eV considerably higher, indicating a thermal activation of charge carrier transport and hence supporting a hopping mechanism rather than delocalized transport. Under exposure to oxygen the conductivity showed a fast small increase followed by a slow large decrease, which is discussed considering surface as well as bulk interactions. Hole conduction was measured for the completely oxidized films by a positive Seebeck coefficient. Optical spectroscopy performed ex situ was used to allow further discussion of redox interactions of the films. Evidence was found for the presence of Pc (-2) Mn (+3) O 2(-1) as well as Pc (-2) Mn (+3) O (-2) Mn (+3) Pc (-2) and Pc (-2) Mn (+2) in freshly prepared films. Films exposed to air for as long as several months were completely oxidized to Pc (-2) Mn (+3) O 2(-1).

Electrochemical, spectroelectrochemical and ESR spectroscopic characterization of 2,3- and 3,4-cobalt tetrapyridoporphyrazine isomers in non-aqueous media

Two cobalt porphyrazines, 2,3-tetrapyridoporphyrazine and 3,4-tetrapyridoporphyrazine, were examined in N,N-dimethylformamide, dimethyl sulfoxide and pyridine solutions as to their electrochemical, spectroelectrochemical and ESR spectroscopic properties. These results were compared with those of the unsubstituted cobalt phthalocyanine. At high concentrations, aggregation was observed for each investigated compound in the three solvents. The intensity of ESR signals of each derivative depends upon the extent of aggregation in its solution. The g values shift towards high field with an increase in the strength of the axial ligand and the number of axial ligands on the cobalt center. Both tetrapyridoporphyrazine complexes undergo one oxidation and three reductions in N,N-dimethylformamide, dimethyl sulfoxide or pyridine solution. Thin-layer UV-visible spectroelectrochemical results confirmed that the first oxidation and first reduction of both compounds are metal-centered while the second and third reductions are ring-centered. An overall electron transfer mechanism for both porphyrazine derivatives is proposed.

Effect of alkyl group length on the conductivity in the 2,3,7,8,12,13,17,18-octakis(alkyl-thio)tetraazaporphyrins and redox properties of the metallooctakis(hexyl-thio)tetraazaporphyrins

A series of 2,3,7,8,12,13,17,18-octakis(alkyl-thio)tetraazaporphyrins (H2OATTAP) with different alkyl chain lengths have been synthesized. Cyclic voltammetry and differential pulse voltammetry have been used to investigate the effect of the controlled lengths of the eight peripheral thioether tails on the redox behavior of the molecules. The electrochemical reduction of octakis(hexyl-thio)tetraazaporphyrins, MOHTTAP (where M = Cu , Ni ), was studied in 1,2-dichloroethane at a platinum electrode. The Cu derivative was oxidized in one single-electron-transfer step to yield a π-cation radical and reduced in three single-electron-transfer steps to yield a π-anion radical, dianion and trianion, respectively. For the Ni derivative, electron transfer reactions involving both the central metal atom and the macrocyclic ring were observed. Electron transfer pathways are proposed based upon voltammetric and in situ spectroelectrochemical results.

The key role of peripheral substituents in the chemistry of phthalocyanines and their analogs

The preparation and optical properties of peripherally substituted phthalocyanines and their analogs (i.e. naphthalocyanines, anthracyanines, aza-analogs of phthalocyanines, and tetraazaporphyrins) have been widely discussed. This review highlights methodologies that have been published in poorly known and mostly unavailable Russian journals.

Electrochemistry and spectroelectrochemistry of tetra-α-substituted metallophthalocyanines

Four tetra-α-substituted phthalocyanines, represented as [( OR )4 Pc ] M where OR is 2,2,4-tri-methyl-3-pentoxy and M = Co (II), Ni (II), Cu (II) or Zn (II), were investigated as to their electrochemistry, UV-visible and ESR spectroscopy. The electron-donating groups on the four α-positions of the phthalocyanine macrocycles lead to a 40 nm red-shift in the position of the most intense band in the UV-visible spectrum as compared to the unsubstituted ( Pc ) M derivative with the same central metal ion under the same solution conditions. The cobalt, nickel and copper complexes of [( OR )4 Pc ] M are soluble in nonaqueous media and show no evidence for aggregation at the millimolar concentration levels needed for electrochemical measurements. The Zn (II) complex aggregates in DMF at these concentrations but monomers are formed upon diluting the solution or by adding a complexing axial ligand such as pyridine. The first reduction and first oxidation of [( OR )4 Pc ] Co (II) leads to formation of a Co (I) and Co (III) phthalocyanine and this contrasts with the first reduction and first oxidation of the Ni (II), Cu (II) and Zn (II) derivatives where electron addition and electron abstraction occur at the conjugated macrocycle, leading to π-anion and π-cation radicals, respectively.

Phosphonic acid functionalized asymmetric phthalocyanines: synthesis, modification of indium tin oxide, and charge transfer

Metalated and free-base A(3)B-type asymmetric phthalocyanines (Pcs) bearing, in the asymmetric quadrant, a flexible alkyl linker of varying chain lengths terminating in a phosphonic acid (PA) group have been synthesized. Two parallel series of asymmetric Pc derivatives bearing aryloxy and arylthio substituents are reported, and their synthesis and characterization through NMR, combustion analysis, and MALDI-MS are described. We also demonstrate the modification of indium tin oxide (ITO) substrates using the PA functionalized asymmetric Pc derivatives and monitoring their electrochemistry. The PA functionalized asymmetric Pcs were anchored to the ITO surface through chemisorption and their electrochemical properties characterized using cyclic voltammetry to investigate the effects of PA structure on the thermodynamics and kinetics of charge transfer. Ionization energies of the modified ITO surfaces were measured using ultraviolet photoemission spectroscopy.

Zinc phthalocyanine and silver/gold nanoparticles incorporated MCM-41 type materials as electrode modifiers

Mercaptopropyl functionalized ordered mesoporous silica spheres were prepared (MPS). Ag or Au nanoparticles (NPs) were anchored onto the MPS materials (Ag-MPS or Au-MPS). Further, zinc phthalocyanine (ZnPc) was adsorbed into the channels and surface (MPS-ZnPc, Ag-MPS-ZnPc, Au-MPS-ZnPc). Diffuse reflectance studies revealed the successful incorporation of Ag or Au NPs inside the silica spheres with and without ZnPc. TEM images showed the uniform distribution of Ag or Au NPs in the silica spheres of different size ranging from 4 to 22 nm or 6 to 31 nm, respectively. XRD pattern showed average crystallite particle size of 18 or 28 nm for Ag or Au NPs respectively which were reduced to 14 or 16 nm on introduction of ZnPc which oxidizes the metal NPs partially. Chemically modified electrodes were prepared by coating the colloidal solutions of the silica materials on the glassy carbon (GC) electrodes. Electrocatalytic reductions of O(2) and CO(2) at the modified electrodes were studied. The presence of Ag or Au NPs was found to increase the electrocatalytic efficiency of ZnPc toward O(2) reduction by 290% or 70% based on the current density measured at -0.35 V and toward CO(2) reduction by 150% or 120% based on the current density measured at -0.60 V respectively. Catalytic rate constants were increased 2-fold for O(2) reduction and 8-fold for CO(2) reduction due to Ag or Au NPs, respectively, which act as nanoelectrode ensembles. The synergic effect of ZnPc and metal NPs on the electrocatalytic reduction of O(2) is presented.

Porphyrazines with Annulated Diazepine Rings. 2. Alternative Synthetic Route to Tetrakis-2,3-(5,7-diphenyl-1,4-diazepino)porphyrazines: New Metal Complexes, General Physicochemical Data, Ultraviolet−Visible Linear and Optical Limiting Behavior, and Electrochemical and Spectroelectrochemical Properties

Following a previous report on the synthesis and physicochemical characterization of a novel class of porphyrazines carrying peripherally annulated seven-membered rings, i.e., tetrakis-2,3-(5,7-diphenyl-1,4-diazepino)porphyrazine [Ph(8)DzPzH(2)].4H(2)O and its metal derivatives [Ph(8)DzPzM].xH(2)O (x = 2-7, M = Mg(II)(H(2)O), Cu(II), and Zn(II)), a new more convenient procedure is reported here, allowing the preparation in high yields of the Li(I) and Na(I) derivatives of formulas [Ph(8)DzPzLi(2)].5H(2)O and [Ph(8)DzPzNa(2)].6H(2)O, which can be directly converted into other metal derivatives under mild conditions (room temperature) and in good yields. The series studied has been extended to include the Mn(II) and Co(II) complexes also reported here for the first time. Physicochemical characterization of the new "diazepinoporphyrazines" was based on fast atom bombardment (FAB) mass spectrometry and X-ray powder patterns, infrared (IR), electron paramagnetic resonance (EPR), and room-temperature magnetic susceptibility measurements. A detailed discussion of the UV-vis spectra emphasizes the role played by the external diazepine rings in electron delocalization through their tautomeric or protonated forms present in neutral, basic, and acidic media. The nonlinear optical effect of optical limiting for the different species [M = 2H, Mg(II)(H(2)O), Mn(II), Co(II), Cu(II), and Zn(II)] has also been measured. It has been observed that the extent of the optical limiting depends on the specific M center. The observed nonlinear optical features are analyzed and discussed in terms of the electronic and magnetic properties exhibited by some of the metal ions and taking into account the model of the excited-state absorption in which the nature of M determines the kinetics of formation of the highly absorbing state of the specific complex examined. As evidenced by the detailed electrochemical and spectroelectrochemical study carried out on this new class of macrocycles, one of the most important aspects is the facilitated electron delocalization for the oxidized and reduced species allowed by a 1H-6Htautomerism taking place on the peripheral diazepine rings.

Spectroscopy and electronic structure of electron deficient zinc phthalocyanines

The effect of introduction of perfluoro alkyl groups into phthalocyanines, as evidenced by the spectroscopic properties of 1,4,8,11,15,18,22,25-octa-fluoro-2,3,9,10,16,17,23,24-octa-perfluoro isopropyl zinc phthalocyanine, ZnF(64)Pc(-2) and its ring-reduced radical anion species, [ZnF(64)Pc(-3)](-), are reported. A combination of UV-visible absorption and magnetic circular dichroism (MCD) spectroscopy, ESI and MALDI-TOF mass spectrometry, cyclic and differential pulse voltammetry, and complete theoretical calculations using INDO/S and DFT techniques reveals that the substitution of all sixteen hydrogen atoms in protio ZnPc(-2) by eight F and eight i-C(3)F(7) groups red shifts the Q and pi --> pi transitions and narrows the HOMO-LUMO gap while simultaneously preventing ring photooxidation and stabilizing the radical anion. The [ZnF(64)Pc(-3)](-) species, which is in equilibrium in solution with the neutral complex when a reducing agent is present, is unusually stable. The above effects are attributed to the strong electron withdrawing properties of the peripheral substituents, which render ZnF(64)Pc extremely electron deficient.

Electrochemistry and Spectroelectrochemistry of Strongly Electron-Withdrawing Metallated Octanitrophthalocyanines

The spectral and electrochemical properties of metallated octanitrophthalocyanines, [MII{(NO2)8pc}] where M = Zn, Cu or Co, were studied by electrochemical and spectroelectrochemical methods as well as electronic absorption and magnetic circular dichroism spectroscopy. Aggregation is observed for several species. The octanitrophthalocyanines are easier to reduce and more difficult to oxidize than most other metallophthalocyanines. Indeed, the first ring-localized reduction potential observed with these phthalocyanine species, is the most positive of the metallated, neutral phthalocyanines in the literature. These data are added to existing Hammett free energy relationship database to extend design protocols for substituted metallophthalocyanines. Optical spectra for several redox series are presented and related to the existing literature.

Molecule liftoff from surfaces

Molecular dynamics simulations have been used to model the kiloelectronvolt particle bombardment of organic layers on metal substrates such as occurs in the analytical techniques of secondary ion mass spectrometry and fast atom bombardment mass spectrometry. Vignettes of insights gained from the simulations along with comparisons to experimental data are presented in this Account. Topics include intact molecular ejection vs fragmentation, prediction of reaction pathways, influence of the substrate, and quantitative predictions of energy and angular distributions.