Metal-assisted
salphen organic frameworks (MaSOFs) are known to possess high affinities
to CO2 due to Lewis acidic metal sites and are therefore
able to selectively adsorb CO2 over CH4 or N2. By aligning two metal centers in a carefully designed geometry,
a “single molecular trap” (SMT) effect is generated,
resulting in an interaction of two metal centers with one molecule
CO2 by synergic effects. A condensation of a rigid triptycene
based trissalicylaldehyde with tetrammino benzene is used to realize
these metal alignments into MaSOFs. Characterization of the discrete
trinuclear complexes proves that the chosen geometry is nearly optimal
for synergic CO2 adsorption. The corresponding MaSOFs show
high selectivities of CO2 against CH4 with a
selectivity S
IAST (according to the Ideal
Adsorbed Solution Theory) of up to 13 and a selectivity of S
IAST up to 70 against N2, which are
also reflected by isosteric heat of adsorptions (Q
st) of up to 35 kJ/mol. Density functional theory (DFT)
calculations support the hypothesis by geometry optimized models and
furthermore show a positive cooperative effect by an energy gain of
∼14 kJ/mol during the adsorption of CO2 in the second
binding pocket of the trinuclear metal–salphen compared to
a monomolecular adsorption.
Molecular solar thermal (MOST) systems combine solar energy conversion, storage, and release in simple one‐photon one‐molecule processes. Here, we address the electrochemically triggered energy release from an azothiophene‐based MOST system by photoelectrochemical infrared reflection absorption spectroscopy (PEC‐IRRAS) and density functional theory (DFT). Specifically, the electrochemically triggered back‐reaction from the energy rich (Z)‐3‐cyanophenylazothiophene to its energy lean (E)‐isomer using highly oriented pyrolytic graphite (HOPG) as the working electrode was studied. Theory predicts that two reaction channels are accessible, an oxidative one (hole‐catalyzed) and a reductive one (electron‐catalyzed). Experimentally it was found that the photo‐isomer decomposes during hole‐catalyzed energy release. Electrochemically triggered back‐conversion was possible, however, through the electron‐catalyzed reaction channel. The reaction rate could be tuned by the electrode potential within two orders of magnitude. It was shown that the MOST system withstands 100 conversion cycles without detectable decomposition of the photoswitch. After 100 cycles, the photochemical conversion was still quantitative and the electrochemically triggered back‐reaction reached 94 % of the original conversion level.
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