Mechanism for Copper(II)-Mediated Disaggregation of a Porphyrin J-Aggregate

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Abstract
J-aggregates of anionic meso-tetrakis(4-sulfonatophenyl)porphyrin form at intermediate pH (2.3–3.1) in the presence of NiSO4 or ZnSO4 (ionic strength, I.S. = 3.2 M). These aggregates convert to monomeric porphyrin units via metallation with copper(II) ions. The kinetics for the disassembly process, as monitored by UV/vis spectroscopy, exhibits zeroth-order behavior. The observed zeroth-order rate constants show a two-term dependence on copper(II) ion concentrations: linear and second order. Also observed is an inverse dependence on hydrogen ion concentration. Activation parameters have been determined for the disassembly process leading to ΔH≠ = (+163 ± 15) kJ·mol–1 and ΔS≠ = (+136 ± 11) J·K–1. A mechanism is proposed in which copper(II) cation is in pre-equilibrium with a reactive site at the rim of the J-aggregate. An intermediate copper species is thus formed that eventually leads to the final metallated porphyrin either through an assisted attack of a second metal ion or through a direct insertion of the metal cation into the macrocycle core.
Funding Information
  • Ministero dell’Istruzione, dell’Università e della Ricerca (PRIN 2015XBZ5YA)
  • Università degli Studi di Messina (RES_AND_MOB_2015_DE_LUCA)
  • Consiglio Nazionale delle Ricerche (CUP G78B14000100006, CUP G77B17000180009)

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