Role of Cobalt(III) Cationic Complexes in the Self-Assembling Process of a Water Soluble Porphyrin

• Main Authors: Nadia Manganaro, Roberto Zagami, Mariachiara Trapani, Maria Angela Castriciano, Andrea Romeo, Luigi Monsù Scolaro
• Format: Article
• Language: English
• Published: MDPI AG 2021-12-01
• Series: International Journal of Molecular Sciences
• Subjects: porphyrins; J-aggregates; aggregation kinetics; symmetry breaking; chiral supramolecular assemblies
• Online Access: https://www.mdpi.com/1422-0067/22/1/39

Under moderate acidic conditions, the cationic (+3) complexes ions tris(1,10-phenanthroline)cobalt(III), [Co(<i>phen</i>)₃]³⁺, and hexamminecobalt(III), [Co(NH₃)₆]³⁺, efficiently promote the self-assembling process of the diacid 5,10,15,20-<i>tetrakis</i>(4-sulfonatophenyl)porphyrin (H₂TPPS₄) into J-aggregates. The growth kinetics have been analyzed according to a well-established autocatalytic model, in which the rate determining step is the initial formation of a nucleus containing <i>m</i> porphyrin units (in the range 2–3), followed by a stage whose rate constant <i>k_c</i> evolves as a power of time. The observed catalytic rate constants and the extent of J-aggregation increase on increasing the metal complex concentration, with the <i>phen</i> complex being the less active. The UV/Vis extinction spectra display quite broad envelops at the J-band, especially for the amino-complex, suggesting that electronic dipolar coupling between chromophores is operative in these species. The occurrence of spontaneous symmetry breaking has been revealed by circular dichroism and the measured dissymmetry <i>g</i>-factor decreases on increasing the aggregation rates. The role of these metal complexes on the growth and stabilization of porphyrin nano-assemblies is discussed in terms of the different degree of hydrophilicity and hydrogen bonding ability of the ligands present in the coordination sphere around the metal center.