This study probes a series of linkers
and anchoring groups for direct interfacial electron transfer (IET)
from high-potential porphyrins into semiconductor surfaces. Eight
different linker–anchor combinations of CF3-substituted,
high-potential porphyrins were designed, synthesized, and characterized.
Specifically, a series of four anchors was examined (carboxylate,
hydroxamate, phosphonate, and silatrane), along with two different
linkers (phenylene and benzanilidylene), which differ in terms of
their electronic conjugation and overall length. The electrochemical
and photophysical properties of the porphyrins were evaluated by steady-state
and transient spectroscopies in solution and on mesoporous SnO2 substrates for use as dye photosensitizers in aqueous photoelectrochemical
cells. IET dynamics were measured using time-resolved terahertz (TRTS)
and transient absorption spectroscopies. From TRTS measurements, injection
yields were determined relative to a commonly used phosphonated ruthenium(II)
polypyridyl complex, which is reported to have near quantitative injection
yield. We find that IET occurs through space rather than through the
linkers, due to the tilted orientation of the adsorbed porphyrins
in direct contact with the metal oxide surface. As a result, the anchoring
groups have a less significant effect on IET dynamics than for adsorbates
exhibiting through-linker injection. Experiments are supported by
DFT calculations, including the analysis of different electron-injection
pathways. Direct IET offers the advantage of the selection of anchoring
groups based solely on chemical/photoelectrochemical stability and
synthetic viability, irrespective of the electronic coupling of the
anchoring group to the metal oxide surface.
Photosynthetic
CO2 fixation is mediated by the enzyme
RuBisCo, which employs a nonredox-active metal (Mg2+) to
bind CO2 adjacent to an organic ligand that provides reducing
equivalents for CO2 fixation. Attempts to use porphyrins
as ligands in reductive catalysis have typically encountered severe
stability issues owing to ligand reduction. Here, a synthetic zinc–bacteriochlorin
is reported as an effective and robust electrocatalyst for CO2 reduction to CO with an overpotential of 330 mV, without
undergoing porphyrin-like ligand degradation (or demetalation) even
after prolonged bulk electrolysis. The reaction has a CO Faradaic
efficiency of 92% and sustains a total current density of 2.3 mA/cm2 at −1.9 V vs Ag/AgCl. DFT calculations highlight the
molecular origin of the observed stability and provide insights into
catalytic steps. This bioinspired study opens avenues for the application
of bacteriochlorin compounds for reductive electrocatalysis with extended
life beyond that seen with porphyrin counterparts.
Here, we report the quantitative electroreduction of CO2 to CO by a PNP-pincer iridium(I) complex bearing amino linkers in DMF/water. The electrocatalytic properties greatly depend on the choice of linker...
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