Porphyrins aggregation

The intriguing process of induction (and subsequent transfer and amplification) of chirality from organic or inorganic templates to achiral porphyrin aggregates has been in the focus of our interests for more than a decade. Our studies concern exploitation of the electrostatic interactions as tools to build non-covalent aggregates and have been carried out in aqueous solution. 

Scheme 2 Picture of the pH-induced disassembly of the chiral porphyrin aggregate and of the two possible reassembling paths. Route a the chiral aggregate is completely disassembled after the pH-jump. Route b some undetectable chiral seed survive the pH-jump. Modified from [45]...

The non-covalent synthesis of the chiral porphyrin aggregate has been performed as discussed in the previous paragraph. Addition of an equimolar amount of H6TPyP4+ and CuTPPS (Figs. 14 and 1, respectively) to a solution at pH 2.3 (by HC1) of L-Phe or D-Phe leads to mirror images induced CD signal in the... 

In order to demonstrate that the chiral porphyrin reassembly is due to the presence of the (spectroscopically silent) chiral seeds of the porphyrin aggregate, various solutions of the chiral aggregates were kept at pH 9.0 (that is in the disassembled state) for different time intervals before reassembling them (lowering the pH at 2.3) [49]. In fact, if the chiral reassembly is driven by the presence of inert chiral seeds, then (at pH 9.0) there must be a time interval after which the chiral seeds will disassemble. Then chirality would not be reversible anymore and the system will reassemble in a non-chiral fashion (see route a in Scheme 2). Indeed, after about 24 h at pH 9, the CD at pH 2.3 is not restored anymore because the chiral seeds also disassembled. 

The formation of the J aggregates is hierarchically controlled. The only way to obtain these species is by allowing porphyrin aggregation with A— or A—[Ru(phen)3]2+ at a pH value of around 6 and then decreasing the pH to 2.5. 

DPPS or trans-DPPS (not removable) added chirality in porphyrin aggregate)... 

Bellacchio E, Lauceri R, Gurrieri S et al (1998) Template-imprinted chiral porphyrin aggregates. J Am Chem Soc 120 12353-12354... 

Purrello R, Raudino A, Monsu Scolaro L et al (2000) Ternary porphyrin aggregates and their chiral memory. J Phys Chem B 104 10900-10908... 

Lauceri R, Purrello R (2005) Transfer, memory and amplification of chirality in porphyrin aggregates. Supramol Chem 17 61-66... 

The presence of water disrupts the absorption spectrum of protoporphyrin IX as it can be seen in figure 9.8a and b where absorption spectra are displayed in pure dioxane and in a solvent of 70% dioxane-30% water. The spectrum distortion is the result of porphyrin aggregation by water. 

Dimerization of protoporphyrin IX in aqueous solution has been studied by fluorinietric techniques. Fluorescence intensity increase linearly with the concentration of protoporphyin in solution up to 0.05 pM. Beyond this concentration, the intensity is no more linear and goes downward as the result of the porphyrin aggregation. In an organic solution, the fluorescence intensity of porphyrin increases linearly with the increase of the porphyrin concentration (Margalit et al. 1983). 

Recently, ESR spectroscopy has been used to study srlver(II) porphyrins. Aggregation in metalloporphyrins, in tetrachloroethane (TCE) and 2-methyltetrahydrofuran (MTHF) has been studied at 77 K. In MTHF a well-resolved signal due to monomeric Ag mesoporphyrm(IX) dimethyl ester (MPD) was observed. In TCE, however, additional signals on the monomer spectrum and a g = 4 signal with a partially resolved structure were found when the gain was increased. The g = 4 signal was ascribed to the AM = 2 transition of the dimeric [Ag"MPD]2. In an equimolar molar mixture of Cu MPD and Ag MPD a heterodinuclear complex was formed. 

The diverse chemical and photophysical prop>erties of porphyrins are in many cases due to their different aggregation mode and, as a result of interchromophoric interactions, perturbations in the electronic absorption spectra of dyes occur. Deviations from Beer s law are often used to investigate the porphyrin aggregation in solution. 

The kinetics of the formation of the porphyrin aggregate and its structure are sensitive of experimental conditions (Giovannetti et al., 2010). The monomer - aggregated sp>ecies is a system of multiple equilibria. Spectrophotometric monitoring in the time of the Uv- Vis absorbance permit to obtain information of intermediate species, of typ>e of the aggregate, and of their transformation. For this, for evaluated the polymerization kinetic constants, the concentrations of monomeric [M] and dimeric form [D], can be calculated from the relative absorption maxima at each time. If kpoi is the polymerisation kinetic constant. Cm and Co denoted the initial monomer and dimer concentrations, the reaction rate can be expressed as in equation (2) ... 

T. Hatano, M. Takeuchi, A. Ikeda, and S. Shinkai, New morphology-controlled poly(aniline) synthesis using anionic porphyrin aggregate as a template, Chem. Lett., 32, 314-315 (2003). 

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