Journal of Porphyrins and Phthalocyanines

This paper describes the preparation of a bis free-base porphyrin and a bis-zinc porphyrin system in which the two 5,10,15-tris(3,5-ditertbutylphenyl)porphyrinyl units are connected in meso position by a tetraenic chain. The preparation of the dyad relies on the Wittig-Horner-Emmons reaction between diethyl 3-[5-[10,15,20-tris(3,5-ditert-butyl-phenyl)-porphyrinyl]] prop-2-enyl phosphonate and the aldehyde-1,3-diene[5-[10,15,20-tris(3,5-ditert-butyl-phenyl)-porphyrinyl]]. The two latter porphyrin derivatives were obtained via a Stille cross-coupling reaction between the corresponding tributyltin derivatives and the 5-iodo-10,15,20-tris(3,5-ditert-butyl-phenyl)-porphyrin. The UV-visible absorption and fluorescence spectra of the bis-porphyrin systems are measured and indicate that both the energy level and the lifetime of the porphyrin singlet excited state are reduced upon the attachment of the polyene chain to the porphyrin unit.

The reaction of the open-chain tetrapyrrolic ligand 2,2'-bidipyrrin with molecular dioxygen in the presence of a catalytic amount of manganese(II)-acetate was found to produce 10,10'-diformyl-2,2'-bidipyrrin with a remarkable selectivity. To identify the active oxidant in this transformation, 2,2'-bidipyrrin was treated with potassium superoxide, sodium peroxide and manganese(III) acetate, respectively, in dry DMF. While superoxide produces the dialdehyde in high yield, the treatment with peroxide or manganese(III) acetate leads to decomposition only. Nickel(II)-, palladium(II)- and copper(II)-2,2'-bidipyrrin were found to undergo macrocyclization reactions when treated with potassium superoxide in dry DMF at room temperature. While the respective 10-oxocorrolates are formed from the nickel(II)- and palladium(II)-2,2'-bidipyrrins, the copper(II) chelate reacts very slowly and yields copper(III) corrole in low yield. The results strongly point to superoxide as the active oxidant in a recently found new synthesis of corroles.

Electrochemical investigations on divalent transition metal complexes with a conjugated linear tripyrrole ligand, namely 3,4,8,9,13,14-hexaethyl-2,15-dimethyltripyrrin (HTrpy) are reported. This tripyrrin ligand behaves as a tridentate monoanionic ligand and forms a series of neutral metal complexes of the type TrpyMX, where M = Zn(II), Cu(II), Ni(II) Co(II) or Pd(II) and X is a chloride anion (Cl^-). The studied nickel, cobalt and zinc complexes undergo respectively three and two ligand-centered reversible one-electron reductions and a reversible ligand-centered one-electron oxidation. Only irreversible electron transfers are observed for TrpyPdCl, due to the instability of the reduced complex. TrpyCoCl does not undergo any metal-centered electron transfer which is unexpected compared to related complexes. The copper(II) complex shows an additional reduction step occurring on the metal, generating a stable copper(I) complex. A comparison of the redox behavior of tripyrrin complexes with porphyrins and linear tetrapyrrolic complexes, namely bidipyrrins, clearly indicates that the observed redox behavior of tripyrrin complexes is mainly metal dependent. The observed HOMO-LUMO gap in the studied series of tripyrrins is rather small (about 1.70 V) compared to porphyrins (2.25 V), but it is close to bidipyrrins (1.60 V).

The double-decker lutetium(III) phthalocyanine [(C₆H₁₃S)₈Pc]₂Lu was investigated by electrochemical and spectroelectrochemical methods and comparisons made to previously investigated [(C₁₂H₂₅S)₄Pc]₂Lu and (Pc)₂Lu under the some experimental conditions. All three compounds undergo a single reversible one-electron oxidation and up to four reversible one-electron reductions in CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The octa- and tetra substituted phthalocyanine derivatives exhibit one oxidation and three or four reductions in solution while five reductions can be detected for the two compounds in a gel-like cast film membrane of tetraoctylphosphonium bromide (4C₈P⁺Br⁻) at a basal plane pyrolytic graphite electrode which was immersed in aqueous 0.5 M KCl. The half-wave potentials of these SR substituted complexes in CH₂Cl₂ are negatively shifted by 210-490 mV from E_1/2 values for the same compounds in the aqueous 4C₈P⁺Br⁻ matrix and five reductions of these compounds are observed under the latter experimental conditions. The UV-visible spectra of the compounds were measured in five different oxidation states and ESR spectra were also characterized for the neutral and doubly reduced complexes. As was expected, [(Pc)₂Lu]⁻,{[(C₆H₁₃S)₈Pc]₂Lu}⁻ and {[(C₁₂H₂₅S)₄Pc]₂Lu}⁻ are ESR silent while electrogenerated [(Pc)₂Lu]²⁻, {[(C₆H₁₃S)₈Pc]₂Lu}²⁻ and {[(C₁₂H₂₅S)₄Pc]₂Lu}²⁻ show broad strong signals at g = 2.0046, 2.0041 and 2.0034 with linewidths of 13.9, 16.1 and 12.9, respectively. These signals are indicative of organic free radicals where the unpaired electrons of the dianion are delocalized over two macrocycles.

A new method was devised for the synthesis of 1,9-diacyldipyrromethanes - crucial intermediates in the synthesis of meso-substituted corroles and porphyrins with different substituents. The diacylodipyrromethanes formation involves acylation of dipyrromethanes with salts made in situ from POCl₃ and tertiary amides. This modified Vilsmeier approach gives higher yields and no concomitant formation of monoacyldipyrromethanes as compared with the Grignard route. Moreover, compounds possessing groups previously inaccessible (CN, NO₂ etc.) can be synthesized. During optimization of the transformation of diacyldipyrromethanes into meso-substituted corroles it was found that if macrocyclization reaction mediated by DDQ is performed in the presence of large excess of pyrrole, meso-substituted [22]pentaphyrins(1.1.1.0.0) can be obtained in moderate yield. The currently described procedure constitutes a new method for the synthesis of these valuable porphyrinoids. Corroles possessing interesting, easy to transform, functional groups were obtained in 3-40% yield.

The construction of a porphyrin array by a stepwise, hierarchical self-assembly process is described. Four molecular clip host molecules are complexed, in a solid state self-assembly process, to one porphyrin guest molecule. When dissolved in chloroform, the 4:1 host-guest complexes spontaneously self-assemble into an array in which the porphyrins are organized in a cofacial stack. The ensemble is stabilized by a combination of π-π interactions between the porphyrins and between the aromatic surfaces of the host molecules.

Molecular systems capable of photoinduced vectorial electron transfer (ET) were assembled using the Langmuir-Blodgett technique. The active part of the systems consisted of phytochlorin-fullerene donor-acceptor layers and polythiophene secondary donor layers. Such structures can promote multi-step inter-layer ET in a direction determined by the film architecture. Transient photovoltage and photocurrent were studied and ET rates and efficiencies were determined. The primary intramolecular charge separated state was formed upon photoexcitation in less than 1 ns and it recombined in approximately 50 ns. Addition of the polythiophene layer at the side of the phytochlorin moieties increased the distance of charge transfer and the lifetime of the charge separated state by promoting spontaneous interlayer electron transfer from the polymer to the phytochlorin cation.

A variety of directly-linked diporphyrins can be prepared by an oxidative coupling reaction of 5,15-diaryl metalloporphyrins. Regioselectivity of the coupling reaction depends on the metal in the porphyrin core; Zn^II-porphyrins favor meso-meso coupling and Pd^II-porphyrins favor meso-β coupling. β-β Linked diporphyrins were isolated for the first time from the oxidation of 5,15-diaryl Pd^II-porphyrin with DDQ and Sc(OTf)₃.

In addition to controlling the structure of multi-chromophoric arrays, monitoring the spatial orientation of the chromophores in artificial light harvesting devices is a challenge of growing interest. We report in this article our recent advances in this field. It is expected that a better understanding of the physicochemical properties of rigid cofacial porphyrinic tweezers and an identification of the factors governing them will be crucial for the design and the elaboration of new nano-molecules endowed with original properties. Extended multi-porphyrinic architectures, polypeptides bearing pendant porphyrins have been synthesized as linear devices, and a star-like pentaporphyrin as an arborescent array. The structure and the original conformation of the latter confer to this system an unusual duality in its physicochemical properties.

The efficiency of different series of tris-(2-aminoethyl)amine TREN-capped porphyrins as catalysts for the electroreduction of dioxygen has been studied after adsorption of the molecules at the surface of a graphite electrode. The influence of two structural features have been explored: the relative position of the two metals in the iron-copper bimetallic complexes and the nature of the benzyl groups of the tripod itself. These compounds have also been compared with picket porphyrins bearing aromatic amino functions, e.g. quinoline. The conclusions obtained in the case of TREN-capped catalysts cannot be extended to those bearing aromatic pickets.

Electric Field Induced Second-Harmonic (EFISH) Generation and Hyper Raleigh Scattering (HRS) measurements were performed on a representative series of axially substituted titanium(IV) and gallium(III) tetra-tert-butylphthalocyanines. The comparison of γ_EFISH and β_HRS values within the series and in particular in the case of the dimeric species shows the influence of both the reduction of symmetry of the phthalocyanine ring and the intramolecular stacking on the second order nonlinear optical properties.