Journal of Porphyrins and Phthalocyanines

Dithallium phthalocyanine (Tl₂Pc) has been synthesized from pure Tl metal and o-phthalodinitrile. The isolated compound was characterized by elemental analysis, X-ray powder diffraction, electron diffraction and EDX spectroscopy. Based on these results, the compound synthesized by the present method was concluded to be Tl₂Pc. The electrical conductivity of this powdery Tl₂Pc was measured and it exhibited the semiconductive property, in contrast with a previous report.

Coupling of 4,5-diiodophthalonitrile with tert-butyldimethylsilylacetylene and a palladium catalyst gave 4,5-bis(tert-butyldimethylsilylethynyl)phthalonitrile. Cleavage of the silyl moiety with tetrabutylammonium fluoride gave 4,5-diethynylphthalonitrile, while condensation in 2-N,N-dimethylaminoethanol without or with Zn(OAc)₂ gave metal-free 2,3,8,9,16,17,23,24-octakis(tert-butyldimethylsilylethynyl)phthalocyanine or its zinc derivative. Cleavage of the silyl groups of the zinc derivative gave an insoluble product whose ¹H NMR spectrum was indicative of 2,3,9,10,16,17,23,24-octaethynylphthalocyaninato zinc(II). Bromination of phthalonitrile with N,N-dibromoisocyanuric acid gave a separable mixture of 3,6-, 3,4- and 4,5-dibromophthalonitrile along with the monobromophthalonitriles. Coupling of 3,4-dibromophalonitrile with tert-butylacetylene gave 3,4-bis(tert-butylethynyl)phthalonitrile, which on condensation with lithium 1-pentanolate gave 1,2,8,9,15,16,22,23-octakis(3,3-dimethyl-1-butynyl)phthalocyanine as a single isomer, the first known 1,2,8,9,15,16,22,23-octasubstituted phthalocyanine.

Nickel(II) octa(4-morpholinyl)- (11a) and copper(II) octa(1-pyrrolidinyl)- (11b) azaphthalocyanines have been prepared from the corresponding pyrazine diiminoimides (10). The precursor pyrazine dicarbonitriles (2) gave 6-cyanopyrazine-5-alkyl carboximidates (3-9) as stable intermediates when reacted with ammonia and catalytic amounts of sodium alkoxide in alcohols. Compounds 3-9 were converted to the diiminoimides 10 upon reflux in propanol or butanol for several hours. This unusual reaction pattern was observed for pyrazine-2,3-dicarbonitriles 2 substituted in the 5- and 6-positions with morpholine, thiomorpholine, piperidine or pyrrolidine.

The application of phthalocyanine derivatives in medicine as photosensitizers for photodynamic therapy of cancer diseases is reviewed. The emphasis is on the work of Russian authors, which is less covered in the scientific literature.

A new concept widely applicable in organometallic chemistry has been introduced for the preparation of cofacial macrocyclic dimers or oligomers. The most notable feature is the use of dimetal or oligometal salts/compounds as templates. In order to exemplify the advantage of this method, a cofacial diphthalocyanine linked by an Si—Si bond has been synthesized in 30% yield in a one-step reaction using hexachlorodisilane as a template and characterized. The cofacial dimer structures are supported by various spectroscopic techniques, including electronic absorption, magnetic circular dichroism, nuclear magnetic resonance and differential pulse voltammetry. However, the unusual phenomenon of a red shift of the main band in the Soret band region was observed.

The UV-vis absorption, MCD and fluorescence spectra of phthalocyaninatosilicon (SiPc) covalently linked to one or two TEMPO radicals have been studied in comparison with those of (dihydroxy)SiPc. The electronic absorption and MCD spectra show little change on substitution of TEMPO radicals, indicating that the electronic interaction between excited singlet SiPc (¹SiPc*) and doublet TEMPO (²TEMPO) is very weak or non-existent. In contrast, fluorescence quantum yields decrease markedly with increasing number of TEMPO radicals. This result is interpreted reasonably by assuming that the decay from ¹SiPc* to excited triplet SiPc (³SiPc*) partially becomes a spin-allowed transition owing to the interaction between ³SiPc* and ²TEMPO.

The influence of electron-withdrawing substituents on the photoelectrochemical properties of phthalocyanines is shown in a comparison between hexadecafluorophthalocyaninatozinc(II) (F₁₆PcZn) and the unsubstituted phthalocyaninatozinc(II) (PcZn). The role of surface states in the photoelectrochemistry of both materials has been investigated by time-resolved photocurrent measurements in the millisecond range. The charging and discharging of surface states could clearly be seen as spikes at the beginning and the end of illumination. Surface states were filled with photogenerated electrons at PcZn and with photogenerated holes at F₁₆PcZn. In the steady state under illumination only cathodic photocurrents were detected at PcZn, while at F₁₆PcZn both cathodic and anodic photocurrents were observed. An adsorption step of electroactive species prior to the charge transfer was derived from the dependence of the steady state photocurrents on the electrolyte concentration for both materials. The concentration dependence of the charging and discharging currents, however, showed that charge transfer from surface states to the electrolyte occurs at PcZn, while at F₁₆PcZn the surface states only represent recombination centres.

The synthesis of and spectroscopic data on subazaporphyrins, subphthalocyanines, subnaphthalocyanines and their related compounds reported to date are summarized, together with the results of molecular orbital (MO) calculations from our laboratory. Thus the problems and key points in the synthesis of these subazamacrocycles and their electronic absorption and magnetic circular dichroism (MCD) spectra are discussed, together with those of the monosubstituted-type low-symmetrical phthalocyanines obtained by their ring expansion reaction. The Q bands of these subazamacrocycles are orbitally doubly degenerate, and in the Soret band region, three transitions to the orbitally degenerate state are found. The MO calculations suggest that the HOMO levels destabilize considerably while the LUMO levels change slightly with increasing size of the π system. In addition, the electronic adsorption data are reproduced by the calculation only when the two pyrrole hydrogens in the monosubstituted-type phthalocyanines are assumed to be linked to the two pyrrole nitrogens along the short axis.

Serendipity has been a major player in most dye discoveries, and phthalocyanines are no exception. The true account of their discovery in 1928 is given using information provided by one of the individuals involved (Ron Greig). One of the inventors, Drescher, after making a key observation as to the nature of the insoluble blue impurity formed in the routine manufacture of phthalimide from phthalic anhydride, was killed weeks later when, on his beloved Sunbeam motorcycle, he had a head-on collision with a steamroller on his way to work. The unique properties of phthalocyanine dyes and pigments make them the colorant of choice for most blue and green colours. Thus most blue and green cars, including sports cars, are coloured by phthalocyanine pigments. In addition to these traditional uses, phthalocyanines are also finding extensive use in modern hi-tech areas. They are used for their colour as, for example, cyan dyes in ink jet printing, and in colourless applications such as infrared absorbers in security. The discovery, traditional and hitech applications of phthalocyanines are described in detail in this paper.

Cyclic voltammetry and spectroelectrochemistry studies are reported for the tetramethyl- tetrapyridinoporphyrazine complexes N,N′,N″,N‴-tetramethyltetra-2,3-pyridinoporphyrazine palladium(II) (1a), N,N′,N″,N‴-tetramethyltetra-3,4-pyridinoporphyrazine palladium(II) (1b), N,N′,N″,N‴-tetramethyltetra-2,3-pyridinoporphyrazine platinum(II) (2a) and N,N′,N″,N‴-tetramethyltetra-3,4-pyridinoporphyrazine platinum(II) (2b). Cyclic voltammetry on Pt disc electrodes showed two reductions. The first reduction was assigned to one-electron transfer to the ring with the formation of a monoanion species. All the tetramethyltetrapyridinoporphyrazine complexes are readily reduced to the monoanion species in the presence of histidine or cysteine. The rate constants for the interaction of complexes 1a, 1b, 2a and 2b with histidine and cysteine range from ~2 × 10⁻³ to 0.26 dm³mol⁻¹s⁻¹.

The sensor applications of MPc derivatives on carbon-type electrodes are briefly reviewed. The electrochemical properties of metallophthalocyanines in solution and on a surface are reviewed with emphasis on predicting redox potentials using Hammett substituent constants and using ligand electrochemical parameters. Design requirements for a two-electron redox process at a metallophthalocyanine center are discussed. A two-dimensional approach using both Hammett parameters and ligand electrochemical parameters is introduced. Some preliminary test examples are presented.

Molecules of azaporphyrins combine two acidic pyrrole-type NH groups and several basic centres—two internal pyrrolenine nitrogen atoms and from one to four meso-nitrogen atoms—in one highly conjugated aromatic system. The acid–base properties of such unique multicentre ampholites have been the subject of numerous studies and discussions. Along with a short critical historical review the paper presents the latest results of a systematic investigation of the influence of successive aza substitution on the spectral properties and thermodynamic stability of the acid–base forms of azaporphyrins.

Absorption and MCD spectra of the two-electron oxidized lutetium phthalocyanine trimer [(Pc)Lu(CRPc)Lu(Pc)]²⁺ (Pc ≡ phthalocyanine;CRPc ≡ 15-crown-5-substituted Pc) as well as the MCD spectrum of the oxidized lutetium Pc dimer [Lu(Pc)₂]⁺ are reported. These ‘two-electron-deficient’ Pc stacks show absorption bands in the NIR region whose intensities are comparable with those of the Q bands. The band of [Lu(Pc)₂]⁺ showed only an MCD B term with no A term contribution, from which we assign the band to a non-degenerate transition whose transition moments are perpendicular to the Pc planes. In both trimer and dimer cases the excitation energy is higher than that of the corresponding NIR band in the ‘one-electron-deficient’ Pc stack [(Pc)Lu(CRPc)Lu(Pc)]⁺ or [Lu(Pc)₂], but the magnitudes of the shift were significantly different (1500 cm⁻¹ in the trimer and 4500 cm⁻¹ in the dimer). The energy shifts are shown to be represented by a pair of two-electron repulsion integrals in a localized orbital basis. The model reproduced well the direction and magnitude of the shifts using numerical values of the integrals.

Substituted amido copper phthalocyanine (CuPc) derivatives are being investigated as additives in pigment systems. Propylamido, dodecylamido and octadecylamido copper phthalocyanine have been prepared by reaction of the corresponding amine hydrochloride and the mono (carboxamide) of copper phthalocyanine in a melt reaction. The carboxamide of CuPc was prepared by reaction of phthalic acid and trimellitic acid in a urea melt; reagents were selected to maximize the mono-substituted product. The mono (carboxamide) of CuPc as prepared therefore contained CuPc and a statistical mixture of other CuPc carboxamides; however, the formation of the alkyl derivatives allowed purification by high-temperature thin layer chromatography, which gave good separations, and characterization with MALDI-TOF mass spectrometry. Initial investigations show that these alkyl amido CuPcS, when used in CuPc pigment compositions for paints improve the dispersion rheology and dispersibility/colouring strength.

An azadibenzoporphyrin (13,17-diethyl-12,18-dimethyl-5-azadibenzo[b,g]porphyrin, 6) is prepared by a 2 × 2 synthesis from a 1-bromobenzopyrromethene precursor by treatment with sodium azide at 140 °C in solvent. Yields depend on solvent, 35% being obtained in quinoline and 84% in DMF. Treatment of 1-bromo-8-ethyl-7,9-dimethylbenzopyrromethene hydrobromide with sodium azide at 60 °C in DMF gives the fused tetrazolo system 8 (8-ethyl-7,9-dimethyltetrazolo[1,5-i]benzopyrromethene) (X-ray structure), which appears to be, or to lead to, an intermediate in the macrocyclization at the higher temperature.

Rapid reactions of tetra(tert-butyl)phthalocyanine, H₂(tbpc), and its zinc complex, [Zn(tbpc)], with tellurium tetrachloride (TeCl₄) in non-aqueous solutions have been investigated. Upon mixing respective solutions containing the reactants, drastic spectral changes occurred in chloroform and toluene even at room temperature. The electronic spectra of the products indicated that the reactions involved an acid-base equilibrium where the phthalocyanines and tellurium tetrachloride acted as a Lewis base and a Lewis acid respectively. By monitoring the spectral changes based on various initial molar ratios of the reactants, the compositions of the products for each system have been determined. [Zn(tbpc)] formed a 1:1 (tbpc:TeCl₄) adduct in both chloroform and toluene, showing the same spectra close to those of the known monoprotonated phthalocyanines. On the other hand, H₂(tbpc) formed a 1:1 adduct in chloroform but a 1:3 adduct in toluene even though both products showed essentially the same spectra close to those of the known diprotonated phthalocyanines. Equilibrium constants have been calculated for the four systems. Magnetic circular dichroism (MCD) spectra of conjugated acids of phthalocyanines have been studied for the first time.