1 research outputs found
μ‑Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO<sub>2</sub> Reduction Catalyst
Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin
(Fe-p-TMA) is a water-soluble catalyst capable of
electrochemical
and photochemical CO2 reduction. Although its catalytic
ability has been thoroughly investigated, the mechanism and associated
intermediates are largely unknown. Previous studies proposed that
Fe-p-TMA enters catalytic cycles as a monomeric species.
However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer
that exists in equilibrium with its monomeric form. The propensity
for μ-oxo formation is highly dependent on the solution pH and
ionic strength. Indeed, the μ-oxo form is stabilized in the
presence of electrolytes that are key components of catalytically
relevant conditions. By leveraging the ability to chemically control
and spectrally address both species, we characterize their ground-state
electronic structures and excited-state photodynamics. Global fitting
of ultrafast transient absorption data reveals two distinct excited-state
relaxation pathways: a three-component sequential model consistent
with monomeric relaxation and a two-component sequential model for
the μ-oxo species. Relaxation of the monomeric species is best
described as a ligand-to-metal charge transfer (τ1 = ∼500 fs), an ionic strength-dependent metal-to-ligand charge
transfer (τ2 = 2–4 ps), and finally relaxation
of a ligand field excited state to the ground state (τ3 = 5 ps). Conversely, excited-state relaxation of the μ-oxo
species proceeds via cleavage of an FeIII–O bond
to generate transient FeIVO and FeII porphyrin species (Ï„1 = 2 ps) that recombine to
the ground-state μ-oxo species (τ2 = ∼1
ns). This latter lifetime extends to timescales relevant for chemical
reactivity. It is therefore emphasized that further consideration
of catalyst speciation and chemical microenvironments is necessary
for elucidating the mechanisms of catalytic CO2 reduction
reactions