Journal of Porphyrins and Phthalocyanines

2,3,9,10,16,17,23,24-Octapentoxyphthalocyanine and its nickel derivative have been studied by means of electron and X-ray powder diffraction. The results reveal that the metal-free compound forms monoclinic crystals (space group C2/c) with unit cell constants a = 4.202 nm, b = 0.4977 nm, c = 3.345 nm, β = 98.9 ° and Z = 4 molecules per unit cell. 2,3,9,10,16,17,23,24-Octapentoxyphthalocyaninatonickel(II) is likely to be isomorphous to its metal-free analogue, but occurs in a second phase which is also stable at room temperature. This is identified to be a crystalline primitive hexagonal phase with cell constants a = b = 2.50 nm and c = 0.33 nm. It is assumed that the molecules form a columnar structure along the c-axis, whereas the hexagonal cross-section in the ab-plane goes back to the molecular disc shape. Imaging of monoclinic crystals of 2,3,9,10,16,17,23,24-octapentoxyphthalocyanine in an energy-filtering transmission electron microscope (EFTEM) can be performed to resolve 2 nm structures only, owing to specimen damage. According to the diffraction pattern, the micrograph provides a view along [001] and shows directly that the structure can be described in terms of parallel columnar stacks of molecules along the b-axis, the distance between neighbouring stacks being . The results show that the structures of both the metal-free compound and its nickel analogue are dominated by π–π interactions between the macrocycles, which is a marked difference from the structures of 1,4,8,11,15,18,22,25-octasubstituted phthalocyanines.

An optical oxygen sensor based on the photoluminescent quenching of palladium porphyrin with long alkyl chain, palladium 5-(4-undecancarboxylpyridyl)-10,15,20-tritolylporphyrin (PdPC10COOH), a self-assembled film on an alumina plate has been developed. The luminescence intensity of the PdPC10COOH film decreased with increasing oxygen pressure. The ratio I₀/I₁₀₀ as a sensitivity measure of the sensing film is very large, indicating that this film is a highly sensitive device for oxygen pressure. The response times of the sensor are 26 s on going from argon to oxygen and 131 s on going from oxygen to argon.

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 E_A = 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)O₂(-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)O₂(-1).

Electrochemical proton reduction was catalysed by zinc phthalocyanine (ZnPc) incorporated in a Nafion or poly(4-vinylpyridine-co-styrene) (P(VP-St)) film coated on a graphite electrode. The turnover number (TN) of the complex to catalyse H₂ evolution was 10⁴ h^-1. The TN of Nafion[ZnPc] was about two times higher than that of P(VP-St)[ZnPc]. The difference was ascribed to the electron propagation in the film influenced by the interaction of the complex with the matrix and by the counterion migration between the matrix and the electrolyte solution. The electron transfer in the reduction of the ZnPc complex in the matrix was concluded to be the rate-determining step for the proton reduction. The TN was independent of the ZnPc concentration in the matrix at low concentrations although H₂ formation was regarded to be a bimolecular reaction process, which was explained by the electron transfer through the matrix via a monomolecular diffusion which was the rate-determining step.

Pheophorbide a (Pheo) is a well-known hydrophobic photosensitizer used for photodynamic treatment of various diseases. The influence of the surroundings on the electronic properties of photosensitizers mainly accumulating in membrane structures is of relevance for their photoactivity. In this paper the current knowledge about the electronic properties of Pheo in different microheterogeneous environments is summarized and new findings about its incorparation in different model membranes are discussed.

Vinylporphyrins containing one vinyl group at the pyrrole or benzene ring and their complexes with Cu²⁺ and Zn²⁺ have been obtained by the Wittig reaction. The compounds obtained were characterized by physicochemical methods. X-ray diffraction analysis of 5-(4-vinylphenyl)-10,15,20-triphenylporphyrin has been carried out. It is likely that the inclusion of the vinyl group is accompanied by weak electron effects on the macrocycles. The radical-induced copolymerization of meso-tetraphenylporphyrin monomers with the vinyl group in a benzene or pyrrole ring and their copper and zinc complexes with styrene and methacrylate was studied. Porphyrin comonomers decrease the overall polymerization rate and the number-average molecular weight of the products formed compared with the weight of polystyrene obtained under similar conditions. The main reasons for termination of chain growth by vinylporphyrins were revealed and some quantitative parameters of these reactions were obtained. IR and electronic absorption spectra of porphyrin-containing copolymers are discussed. According to the ESR spectra, the copper-containing centres in the copolymers are fairly remote from each other, and the metal-containing polymeric systems are magnetically dilute. The spectral luminescence properties of solutions of zinc 5-(4-vinylphenyl)-10,15,20-triphenylporphyrin–methyl methacrylate copolymers with various contents of porphyrin groups were studied. It was shown that a new long-wave band appears in the absorption spectra of the copolymers, the intensity of which depends on the copolymer composition, and the quantum yield of fluorescence decreases with increasing molar fraction of porphyrin groups.

The mechanism of charge transport by metal complexes confined in polymer matrices is reviewed. There are two possibilities, one where the charge propagates in the matrix by diffusion of redox center molecules, and the other where the charge propagates by hopping between redox center molecules. The mechanism can be decided by investigating the dependence of the charge propagation rate on the redox center concentration, since in the diffusion mechanism the rate is first-order with respect to the concentration, while in the hopping mechanism it is second-order. In the hopping mechanism the charge-hopping distance can be analyzed by assuming a random distribution of the redox center in the matrix. The mechanism of charge transport by typical redox centers such as metal porphyrins, metal phthalocyanines, and methylviologen confined in a polymer membrane is presented and the charge-hopping distance is determined.

Lipidporphyrinatoiron(II) complexes are tetrakis(o-substituted)phenylporphinatoiron(II) derivatives which can be easily dispersed in water by molecular assembling. The most remarkable aspect of lipidporphyrinatoiron(II) assemblies is their reversible binding of dioxygen under physiological conditions (in aqueous media, pH 7.3, 37 °C) like hemoglobin and myoglobin. In these structures the O₂-binding properties are largely influenced by the molecular environment around the coordination site. Tetrakis(o-pivalamido)phenylporphinatoiron(II) with a covalently linked axial imidazole (lipidporphyrinatoiron(II), 1) is incorporated into recombinant human serum albumin (rHSA), providing a totally synthetic O₂-carrying hemoprotein (rHSA–1). Electrospray ionization mass spectrometry revealed the molecular mass of this non-covalent albumin–porphyrin hybrid. The O₂ rebinding after laser flash photolysis represented a three-phase decay, suggesting that each porphyrin is embedded into different cavities in the albumin structure. On the other hand, amphiphilic lipidporphyrinatoiron(II) with four alkylphosphocholine chains (2) is self-organized in aqueous solution to produce bimolecular fibers with a uniform thickness of 10 nm. This fiber also gave a stable O₂ adduct, and the O₂ rebinding after laser flash irradiation showed monophasic kinetics. Up to 20 vol% of methanol, which is a critical concentration for fiber formation, the morphology was gradually dissociated into spherical micelles, and the stability of the dioxygenated species suddenly decreased to 10% of that of the fibers.

The columnar mesomorphism of triphenylene-, phthalocyanine-, tetraazaporphyrin- and porphyrin-based compounds is briefly reviewed and the potential use of this particular supramolecular structure for charge-conducting devices is discussed. Suitably substituted triphenylenes exhibit ideal material properties for photoconducting devices, and well-aligned layers have been shown to give a very high charge carrier mobility of 0.1 × 10^-4m²V^-1s^-1 (time-of-flight measurements). From a molecular point of view, porphyrin systems should be superior to triphenylenes, but the properties of their mesophases do not support the advantageous homeotropic alignment of the columnar stacks. However, pulse radiolysis time-resolved microwave conductivity measurements revealed the highest charge carrier mobility value of 0.28 × 10^-4m²V^-1s^-1 for mesomorphic phthalocyanines.

Lithium metal added to a solution of 4-neopentoxyphthalonitrile in 1-octanol or other long-chain primary alcohols at room temperature resulted in phthalocyanine formation at a reasonable rate in good yield, while preformed lithium 1-octanolate under the same conditions gave 2,9,16,23-tetraneopentoxyphthalocyanine, but in lower yield at a slower rate. The use of lower-molecular-weight alcohols slowly gave a phthalocyanine in lower yields. Reverse micelle formation when using long-chain alcohols is proposed as a possibility for enhanced phthalocyanine formation at room temperature. 2,9,16,23-Tetrasubstituted phthalocyanines and metallated phthalocyanines were prepared at room temperature from 4-neopentoxyphthalonitrile, 4-bis(4-methoxyphenyl)methoxyphthalonitrile, 4-[1-(4-ethoxy-3-methoxyphenyl)-1-phenyl]methoxyphthalonitrile and phthalonitrile using lithium 1-octanolate in 1-octanol or by the addition, to a solution of the phthalonitrile in ethanol, of calcium turnings or, to a solution of the phthalonitrile in methanol, of magnesium, zinc, iron or copper powder. The tetrasubstituted phthalocyanines produced exhibited a non-statistical distribution of regioisomers, indicating that electronic effects become important in room-temperature cyclotetramerization of phthalonitriles to phthalocyanines.

Electrochromic redox reactions of vacuum-evaporated thin films of tetrapyridotetraazaporphyrinatozinc(II) (TPyTAPZn) have been studied in aqueous KCl electrolytes by in situ spectrocyclic voltammetry, which combines cyclic voltammetry and in situ monitoring of the absorption spectrum of the film. Although the voltammetric response of TPyTAPZn thin film electrodes can hardly be understood by the conventional electrochemical approach, the spectral information linked to the voltammetric information elucidated the presence of two-step reductions and reoxidations by each electron per TPyTAPZn molecule, which, however, take place with significant overlap during the voltammetric scan. Reduction of the film turned the color of the film from blue to purple and it changed reversibly to regenerate the original spectrum upon reoxidation. These reversible electrochromic reactions could be repeated many times without causing any noticeable change in the voltammetric and spectral responses. While use of electrolytes with various cations did not affect the redox behavior of the film, the effect of electrolyte pH was clearly seen, changing the ratio of the two reduction and two reoxidation peak magnitudes in the voltammogram. XPS analysis of the reduced film indicated that charge compensation upon reduction of the film occurred by protonation of reduced TPyTAPZn molecules at their aza and/or pyrido nitrogen atoms. A reaction scheme is presented to describe the observed redox reactions of the TPyTAPZn thin film from thermodynamic and kinetic viewpoints.

Non-ionic amphiphilic zinc(II) phthalocyanine 4 with eight oxyethylene side chains exhibits excellent solubility in water and organic solvents such as benzene, chloroform, methanol, THF and DMF. The monomer–aggregate equilibrium of 4 was affected by the solvent polarity. Photoreduction of methylviologen (MV²⁺) in the presence of triethanolamine was also studied with the use of non-ionic complex 4 under light irradiation. The yield for the radical formation of MV²⁺ was greatly increased by the addition of dimethylsulfoxide (DMSO) and cetyltrimethylammonium chloride (CTAC) in aqueous solution.

The fluorescence behavior of porphyrins and phthalocyanines allows one to discriminate between monomeric and associated states of the dyes. These criteria are necessary prerequisites to decide about the inclusion behaviour of the dyes in oligomeric cyclodextrins. Masking the hydrophobic portion of the dyes by cyclodextrins allows one to transport these substances as inert through the body of an animal. The story is depicted here of how to proceed from functionalized porphyrinoids to tumor therapeutic drugs.

Improvement of electrical conductivity was attempted by polymerizing octacyanometallophthalocyanine (MPcOC; M≡Fe, Co) units into one-dimensional molecular complexes. The synthetic route to MPcOC was established by reacting tetracyanobenzene with a metal(II) acetate in sulfolane. Poly(μ-pyrazine)octacyanophthalocyaninatoiron(II) and -cobalt(II) were synthesized and their doping with lithium was carried out. The Fe(II) and Co(II) octacyanophthalocyanine complexes showed good electrical conductivities of the order of 10^-8-10^-4S cm^-1. The electrical conductivities of the lithium-doped complexes were improved by two or three orders of magnitude in comparison with the corresponding monomeric and polymeric complexes.