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

Mariachiara Trapani; Ilaria G Occhiuto; Roberto Zagami; Giovanna De Luca; Maria A Castriciano; Andrea Romeo; Luigi Monsù Scolaro; Robert F Pasternack

doi:10.1021/acsomega.8b02913

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

ACS Omega. 2018 Dec 31;3(12):18843-18848. doi: 10.1021/acsomega.8b02913.

Authors

Mariachiara Trapani¹, Ilaria G Occhiuto², Roberto Zagami¹, Giovanna De Luca^{2

3}, Maria A Castriciano¹, Andrea Romeo^{1

2}, Luigi Monsù Scolaro^{1

2}, Robert F Pasternack⁴

Affiliations

¹ CNR-ISMN, Istituto per lo Studio dei Materiali Nanostrutturati, c/o Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ad Ambientali, 98166 V.le F. Stagno D'Alcontres 31, Messina, Italy.
² Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ad Ambientali, and C.I.R.C.M.S.B., University of Messina, V.le F. Stagno D'Alcontres 31, Vill. S. Agata, 98166 Messina, Italy.
³ CNR-IPCB, Istituto dei Polimeri, Compositi e Biomateriali, P.le Enrico Fermi 1, 80055 Portici, Naples, Italy.
⁴ Department of Chemistry & Biochemistry, Swarthmore College, 500 College Avenue, Swarthmore, Pennsylvania 19081, United States.

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.