Journal of Porphyrins and Phthalocyanines

A set of 45 acids or acid combinations was examined in the condensation of pyrrole + benzaldehyde (10 mM each in CH₂Cl₂) leading to meso-tetraphenylporphyrin (TPP). Initial screening experiments identified suitable acid catalysts and the optimal concentration (in terms of TPP formation) for each acid. Subsequent experiments followed the time course of reactions using the best conditions identified in the screening experiments. The reaction course was followed by monitoring reactions from 1 min to 24 h for the yield of TPP (by UV-vis and HPLC), the yields of N-confused tetraphenylporphyrin and tetraphenylsapphyrin (by HPLC), the quantity of unreacted benzaldehyde (by TLC), and the oligomer composition (by laser desorption mass spectrometry). Diverse acids (Brønsted or Lewis; soluble or insoluble) were found to provide yields of TPP ranging up to ~50%. Only 10 acids gave no TPP. In addition, N-confused tetraphenylporphyrin was found to be a ubiquitous byproduct, whereas tetraphenylsapphyrin was not widely observed. MgBr₂-etherate and CuCl₂ each catalyzed porphyrinogen formation and resulted in porphyrin metalation following oxidation and neutralization of the reaction mixture, thereby providing direct, one-flask syntheses of magnesium and copper porphyrins. Observations concerning the reaction course obtained from prior studies of TFA or BF₃-etherate catalysis have been found to be quite general across a broad range of acid catalysts. Collectively, these results show that many acids have potential utility in porphyrin syntheses.

The stability towards acidolysis of intermediates used in the preparation of core-modified porphyrinic macrocycles was examined. Experiments were performed using analogs of 5-phenyldipyrromethane in which one of the two pyrrole rings was modified (XPM). The XPMs utilized in these studies had X = 2-furyl (OPM), 2-thienyl (SPM), 2-selenyl (SePM), or 3-pyrrolyl (^NCPPM). The XPMs possess a potentially labile linkage to a heteroatom-modified ring as well as a potentially labile linkage to a pyrrole ring. The stability of the two types of linkages was examined under acid catalysis conditions commonly used in the preparation of porphyrinic macrocycles. The methodology employed enabled characterization of the stability of the XPM (GC analysis), the yield of porphyrin (UV-vis absorption), and the composition of porphyrinic species (laser-desorption mass spectrometry, LD-MS) formed upon acidolysis of the XPM. These experiments showed that (1) the linkage to the heteroatom-modified ring is fairly stable in the presence of a linkage to pyrrole, and (2) the linkage to pyrrole is much more stable in the XPMs than is the case in 5-phenyldipyrromethane. Additional experiments with analogs of 5-phenyldipyrromethane having both rings modified (X₂M), where X = 2-furyl (O₂M), 2-thienyl (S₂M), or 2-selenyl (Se₂M) confirmed that linkages to furyl, thienyl, and selenyl rings are stable towards acidolysis. Taken together, the observations concerning stability of various linkages provide a foundation for understanding the types and yields of core-modified porphyrinic macrocycles formed in reactions with reactants that are not direct precursors to the isolated products.

The synthesis and spectroscopic characterization of a new soluble silicon(IV) phthalocyanine complex is presented. The compound shows an increased solubility compared to its SiPcCl₂ precursor and this allowed solution ¹H NMR characterization. The assignment of the ¹H NMR signals for the axial ligands is greatly facilitated due to the anisotropic high ring current effects from the macrocycle. In addition, good quality crystals were grown from this more soluble material for molecular structure determination by single-crystal X-ray diffraction analysis. The molecular structure determination shows that the complex crystallizes in a non-centrosymmetric space group due to the inherent chirality of the naproxene ligands. Bond lengths and angles fit well to other analogous compounds previously reported.

The interaction between iron(II) tetrasulfophthalocyanine ([Fe^IITSPc]⁴⁺) and histamine results in the oxidation of the central metal by oxygen in the former, with the formation of a complex denoted as [(His)Fe^IIITSPc]³⁻ (where His = histamine). The rate constant for the formation of the complex is k_f = 2.41 × 10⁻²dm³.mol⁻¹.s⁻¹ and an equilibrium constant of 6.3 dm³.mol^-1 was obtained. The oxidation state of the central metal of [Fe^IITSPc]⁴⁻ before and after the coordination of histamine is confirmed by spectroelectrochemistry. Further electrochemical oxidation of this [(His)Fe^IIITsPc]³⁻ derivative results in a metal-based process proposed to involve an Fe^IV phthalocyanine species.

The dye-sensitized solar cell using visible and near-infrared sensitization of nanocrystalline TiO₂ films by aluminium phthalocyanine, aluminium 2,9,16,23-tetraphenoxy-29H, 31H-phthalocyanine hydroxide (AlTPPc), was developed and its photoelectrochemical properties were investigated. The short-circuit photocurrent (I_SC) was 0.026 mA cm^-2, the open-circuit photovoltage (V_OC) was 186 mV, and the fill factor (FF) of solar cell using AlTPPc adsorbed on nanocrystalline TiO₂ film electrode was estimated to be 40.4%, respectively. By using AlTPPc adsorbed on nanocrystalline TiO₂ film electrode, photovoltage and photocurrent were higher compared with that of only nanocrystalline TiO₂ film electrode and were maintained under near-infrared irradiation. Thus, the solar cell using the near-infrared sensitization by AlTPPc adsorbed on nanocrystalline TiO₂ film electrode was developed.

The synthesis and characterization of phthalocyaninato-ruthenium (PcRu) complexes with potential functional axial ligands are described. The solubility of these PcRu complexes was much improved compared to their parent phthalocyanines without Ru, enabling purification by normal flash column chromatography and also NMR measurements in common solvents (e. g. DMSO-d₆ and CDCl₃). Adsorption of these phthalocyanine dyes onto the surface of a semiconductor through the carboxyl group(s) in the axial ligands prevents to some extent formation of H-aggregates, which is typical for phthalocyanines. It also prevents stacking of the dye molecules on the surface. The photovoltaic behavior of sandwich solar cells based on nanostructured TiO₂ films sensitized by these PcRu complexes was studied under irradiation with visible light. For a solar cell based on bis(4-carboxypyridine)-phthalocyaninato ruthenium(II) (1) sensitized nanoporous-nanocrystalline TiO₂, a monochromatic incident photon-to-current conversion efficiency (IPCE) of 21% was obtained at 640 nm. The overall conversion efficiency (η) was 0.61%, which is one of the best results for a solar cell based on a phthalocyanine dye. For a cell based on (4-carboxypyridine)-(4-(2-ethoxy)ethyloxycarbo-nylpyridine)-2,3,9,10,16,17,23,24-octa(n-pentyloxy)-phthalocyaninato ruthenium(II) (5) sensitized TiO₂, a IPCE of 6.6% at 640 nm and η of 0.58% were obtained.

The association behavior of 5,10,15,20-tetrakis(4-N-benzyl-pyridyl)porphyrin (TBzPyP) was investigated in aqueous solution at 27 °C and various ionic strengths by optical absorption, fluorescence and resonance light scattering (RLS) spectroscopies. The results show that TBzPyP exists as monomer at low ionic strength and as ill-defined aggregates at high ionic strength and does not show concentration dependent aggregation over an extended concentration range between 5 × 10⁻⁷ to 1 × 10⁻⁴M in water. The interaction of the TBzPyP with human serum albumin (HSA) in 5 mM phosphate buffer, pH = 7.00, and at 27 °C, has been also studied by optical absorption, fluorescence and RLS spectroscopies. The formation of two types of complexes (HSA: TBzPyP and (HSA)₂: TBzPyP) has been demonstrated by analysis of optical absorption spectral patterns of TBzPyP at various concentrations of HSA. Based on absorption data, a binding model has been proposed and used to analyze the binding process. The calculated binding constant for formation of HSA: TBzPyP and (HSA)₂: TBzPyP are 3.79 × 10⁶ M⁻¹ and 1.46 × 10⁵ M⁻¹, respectively. The RLS spectra of TBzPyP at various concentration of HSA do not show any aggregation of TBzPyP in the presence of HSA. Due to the fact that binding of TBzPyP quenches the HSA fluorescence, a step-by-step aggregation model, which has been developed by Borissevich, I. E. et al, was used to determine the average aggregation number of HSA, < J >, from the fluorescence quenching. The < J > value is 1.4, which further confirms the formation of the two mentioned complexes at this concentration range of HSA.

We present our experimental results on the measurements of excited state dynamics in phthalocyanines (Pc, CuPc, FePc, and ZnPc) using degenerate four wave mixing with incoherent light (DFWM-IL). Three different timescales were observed, for all samples, in the phase conjugate signal recorded as a function of delay in probe beam in the standard backward geometry. The fastest component (< 170 fs) is associated to the phase relaxation in the S_n states and the slowest components (∼3-5 ps and 35-55 ps) to the vibration and population relaxation times from the S₁ state. The present results are compared with those obtained with shorter pulses and various techniques for similar molecules. The consequences of such short lifetimes are also discussed in view of the strong optical limiting properties of these molecules.