Journal of Porphyrins and Phthalocyanines

As part of an investigation aimed at developing artificial models for mimicking haemoprotein activity, four C-aminoacyl protohaemin IX derivatives with one or two -L-Leu-L-HisOMe (complexes 3 and 1) or -β-Ala-L-HisOMe (complexes 4 and 2) peptide fragments have been studied by means of electronic and ESR spectroscopy. The bis-His complexes 1 and 2 are predominantly low-spin (S = 1/2) species, while the mono-His complexes 3 and 4 are predominantly high-spin (S = 5/2) species. Products of the treatment of starting complexes 1–4 with trichloroacetic acid, acetic acid, methylimidazole, sodium cyanide and sodium hydroxide have been analysed by electronic spectroscopy. Weak-field ligands lead to predominantly high-spin hexacoordinated complexes, while strong-field ligands result in predominantly low-spin ones.

Soluble metal-free and Cu-, Ni- and Co-substituted 3,4-tetrapyridinoporphyrazines (Tpps)5–8 have been synthesized. Poly(methyl methacrylate) (PMMA) spin coated and Langmuir–Blodgett (LB) films containing these compounds have been prepared. Third-harmonic generation (THG) was used to measure the third-order-non-linear optical susceptibility of both types of films at 1.064 μm. For the spin-coated samples a strong enhancement of the THG response was found on increasing the Tpp concentration of the chlorobenzene solution used to prepare the samples. The enhancement was associated with the formation of molecular aggregates in the solution which were ‘frozen’ into the films. In order to facilitate comparison, the final Tpp concentration in the PMMA films was kept approximately constant in all cases. Metal incorporation (particularly Co) in the macrocycle strongly enhances the THG susceptibility of the films prepared using concentrated solutions, whereas the effect is efficiently inhibited in those obtained from less concentrated solutions. We conclude that the enhancement in the THG response occurs via the influence of the metal on the Tpp aggregation process in the solution. For the LB films the THG results are more comparable with those of the spin-coated samples prepared using low-concentration solutions. This indicates that dimers, previously identified in the LB films, and possibly other small-size oligomers are formed in the low-concentration Tpp solutions. On the other hand, larger aggregates should be formed in the spin-coated samples prepared from high-concentration solutions.

The redox behaviour of a new covalently linked benzoporphyrin–anthraquinone compound (ZnBPAQ) was investigated by cyclic voltammetry and in situ UV-vis spectroelectrochemistry in N,N-dimethylformamide solution. Three redox waves of this compound were found in the available potential range. The first oxidation process occurs on the benzoporphyrin ring at 0.570 V and is a quasi-revisible diffusion-controlled one-electron electrode reaction. The first reduction process is also a quasi-reversible diffusion-controlled one-electron electrode reaction and occurs at –0.950 V on the anthraquinone moiety. However, the second reduction process occurs at –1.410 V and involves an overlap of the first reduction of the benzoporphyrin ring and the second reduction of the anthraquinone moiety. The in situ UV-vis spectroelectrochemical results affirm the above conclusions.

Electrically conducting neutral radical crystals can be obtained by electrochemical oxidation of dicyano(phthalocyaninato)cobalt(III) anion, [Co(Pc)(CN)₂]^-. The electrolysis of the potassium salt in DMSO gives Co(Pc)(CN)₂·2DMSO. The solvent-free crystal can be obtained by electrolysis of K[Co(Pc)(CN)₂] in acetonitrile in the presence of various second components or using salts with other cations. The intermolecular π–π overlap is efficient in the solvent-containing crystal, while that in the solvent-free crystal is much less. The electrical resistivity at room temperature of the latter has been found to be 4.5 × 10³ Ω cm, which is nearly three orders higher than that of Co(Pc)(CN)²·2DMSO.

The synthesis is described of fixed distance triads consisting of zinc porphyrin (ZP), metal-free porphyrin (HP) and pyromellitimide (I) in which the ZP and HP moieties are bridged by a rigid 4,4'-biphenylene spacer and related spacers, being held with dihedral angles defined by the torsional angle (θ) of the 4,4'-biphenylene spacer varying from nearly coplanar (θ = 0°, phenanthrene spacer) to nearly orthogonal (θ = 90°, 2,2',6,6'-tetramethylbiphenylene spacer). Using these models, the effects of the dihedral angle of the two porphyrins have been examined for singlet–singlet excitation energy transfer from ZP to HP, hole transfer from HP⁺ to ZP and charge recombination of ZP⁺–HP–I^– to the ground state. In the three processes examined, the observed rate constants are roughly in the order of 1d (coplanar)≥1c (nearly coplanar) >1a (tilted) >1b (orthogonal).