Aggregation of a symmetrical metalloporphyrin. Concentration and temperature dependence of the absorption and magnetic circular dichroism spectra of dilute zinc octamethyltetrabenzporphyrin solutions

Abstract

Absorption and magnetic circular dichroism (mcd) spectra of toluene solutions of zinc octamethyltetrabenzporphyrin, Zn(OMBP), exhibit marked concentration and temperature dependence. Monomeric Zn(OMBP), 1, characterized by a429 nm Soret band, dominates at low concentrations M, 300 K) and higher temperatures M , 320K) in toluene solution. Decreasing temperatures (320 to 235 K for a M solution) and increasing concentrations M to M , 300 K) result in 8-10 nm red shifts in this band. These spectral changes are attributed to the concurrent formation of a species 2 with a low extinction coefficient and another compound, 3, with h,,, at 439nm. At the highest concentrations M) and lowest temperatures (225-190K) used, a dramatic blue shift (439 to 422nm) due to a fourth species (4) is observed. All the spectral changes observed are completely reversible. In addition, and consistent with self-association, species 2, 3, and 4 are formed at higher temperatures as the concentration is increased. Possible aggregate structures are proposed and evidence is presented for a novel Zn-N (pyrrole) intermolecular bond. [Traduit par le journal]

Occlusion of Foreign Molecules in Primary Asphaltene Aggregates from Near-UV–Visible Absorption Studies

“…In the absence of other molecular species, porphyrins demonstrate the ability to self-assemble spontaneously into aggregates through noncovalent interactions. , One of the well-proven consequences of self-aggregation is notable hypochromism (intensity decrease) of the Soret peak in near-UV–visible absorption spectra. − In support of the interpretation of our data for more concentrated solutions in Figures and is a strong hypochromism of porphyrins incorporated into extended structures of various macromolecules (e.g., DNA) and into complex supramolecular assemblies. − On the other hand, hypochromism is observed in noncovalent aggregates of porphyrins with individual molecules and nanoparticles (quantum dots, fullerenes, and carbon nanotubes). − Of the immediate relevance to our results in most dilute solutions (with the predominance of nonaggregated one to three ring basic asphaltene molecules) are experimental and theoretical results reported in a recent series of publications by the research group of Prof. M. R. Gray. ,,, One of the problems investigated in these publications was an apparent “invisibility” of metalloporphyrins in crude oils and asphaltenesi.e., low intensity of porphyrin characteristic absorption peaks. The authors concluded that petroporphyrins that do not contribute a strong Soret band in near-UV–vis spectroscopy are bound to other asphaltene molecules/aggregates.…”