title: Spectroelectrochemical Investigation of the One-Electron Reduction of Nonplanar Nickel(II) Porphyrins.
abstract: The electrochemical reduction of a series of nickel porphyrins with an increasing number of substituents was investigated in acetonitrile. A one-electron reduction of [5,15-bis(1-ethylpropyl)porphyrinato]nickel(II) leads to π-anion radicals and to efficient formation of phlorin anions, presumably by disproportionation and subsequent protonation of the doubly reduced species. The phlorin anion was identified by using cyclic voltammetry and UV/Vis and resonance Raman spectroelectrochemistry, complemented by quantum-chemical calculations to assign the spectral signatures. The theoretical analysis of the potential-energy landscape of the singly reduced species suggests a thermally activated intersystem crossing that populates the quartet state and thus lowers the energy barrier towards disproportionation channels. Structure-reactivity correlations are investigated by considering different substitution patterns of the investigated nickel(II) porphyrin cores, that is, for the porphyrin with additional β-aryl ([5,15-bis(1-ethylpropyl)-2,8,12,18-tetra(p-tolyl)porphyrinato]nickel(II)) and meso-alkyl substitution ([5,10,15,20-tetrakis(1-ethylpropyl)porphyrinato]nickel(II)), no phlorin anion formation was observed under electrochemical conditions. This observation is correlated either to kinetic inhibition of the disproportionation reaction or to lower reactivity of the subsequently formed doubly reduced species towards protonation.
authors.affiliation: Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743, Jena, Germany.|Leibniz Institute of Photonic Technology Jena (IPHT), Albert-Einstein-Straße 9, 07745, Jena, Germany.
authors.forename: Julian
authors.initials: J
authors.lastname: Schindler
authors.affiliation: Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743, Jena, Germany.
authors.forename: Stephan
authors.initials: S
authors.lastname: Kupfer
authors.affiliation: Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743, Jena, Germany.|Leibniz Institute of Photonic Technology Jena (IPHT), Albert-Einstein-Straße 9, 07745, Jena, Germany.
authors.forename: Linda
authors.initials: L
authors.lastname: Zedler
authors.affiliation: Leibniz Institute of Photonic Technology Jena (IPHT), Albert-Einstein-Straße 9, 07745, Jena, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Wächtler
authors.affiliation: Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743, Jena, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Gräfe
authors.affiliation: School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity Biomedical Science Institute, 152-160 Pearse Street, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland.
authors.forename: Aoife A
authors.initials: AA
authors.lastname: Ryan
authors.affiliation: School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity Biomedical Science Institute, 152-160 Pearse Street, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland.
authors.forename: Mathias O
authors.initials: MO
authors.lastname: Senge
authors.affiliation: Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743, Jena, Germany.|Leibniz Institute of Photonic Technology Jena (IPHT), Albert-Einstein-Straße 9, 07745, Jena, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Dietzek
keywords: electrochemistry; porphyrinoids; quantum chemistry; reaction mechanisms; spectroscopic analysis