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

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 a 429 nm Soret band, dominates at low concentrations (10−7 M, 300 K) and higher temperatures (10−5M, 320 K) in toluene solution. Decreasing temperatures (320 to 235 K for a 10−5 M solution) and increasing concentrations (10−7 M to 10−4M, 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 λmax at 439 nm. At the highest concentrations (10−3 M) and lowest temperatures (225–190K) used, a dramatic blue shift (439 to 422 nm) 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.