Full Paper
doi.org/10.1002/chem.202005427
Chemistry—A European Journal
solar thermal collectors and photovoltaic heating are under in-
tensive investigation and are already in application.[4] To bridge
the gap between energy demand and the availability of solar
energy, thermal energy storage is required. MOST systems are
ideal candidates to meet the high and continuous demand of
industrial process heat and domestic heating owing to their
lack of transformation losses, long storage time, and fast
energy release.
In this paper we provide the tools that allow this enormous
potential to be harnessed. On the one hand, we have designed
new and much more efficient magnetic catalysts. On the other
hand, we have synthesized a variety of novel NBD–QC inter-
conversion couples and performed detailed investigations re-
garding their MOST potential. By studying the kinetics of these
magnetic NP-based catalysts, for the first time we were able to
show that the second generation, based on cobalt porphyrins
as the active catalyst, is far more reactive than the previously
reported cobalt salphen [Fe3O4-Cat1], which is impressively
shown by a 22.6-fold increase in reactivity. This new catalyst
generation in combination with the outstanding photophysical
properties of the novel NBD–QC couples (lonset up to 450 nm
and Fꢀ70%) paves the way toward an efficient MOST system
utilizing the sunlight-driven isomerization of NBD to QC and a
magnetic NP-anchored quasi-homogenous catalyst. Finally, we
have investigated the heat release capacity of the nanoparti-
cle-catalyzed back-conversion of QC1 to NBD1 in a 1m solu-
tion, yielding a temperature increase to 39.58C in a glass vial
without insulation. This demonstrates that the generation of
low to medium heat for industrial processes as well as domes-
tic heating is a field of application for this MOST concept.
Several photoswitches have been investigated with regard
to their MOST potential.[5] Despite the diversity of potential
candidates, the key requirements for a MOST system are
always the same: 1) good absorption properties of the parent
molecule across the solar spectrum; 2) little competing absorp-
tion of the metastable photoisomer; 3) high quantum yield of
the photoisomerization; 4) long half-life of the metastable
energy-rich isomer; 5) high energy storage density; and
6) good cyclability.[6]
The best studied[7] and arguably the most promising among
MOST candidates is the interconversion system of norborna-
diene (NBD) and quadricyclane (QC), shown in Scheme 1. The
parent system already combines a huge energy density with
the long half-life of QC.[8] Moreover, it has been shown that by
introducing substituents into the carbon framework of NBD, it
is possible to redshift the absorption into the visible region of
the spectrum, and at the same time, increase the quantum
yield.[9] However, these beneficial photophysical properties
come at the cost of an increased molecular weight and there-
fore lower energy storage capacities.[10] Optimization of one
characteristic of any NBD–QC couple always affects the remain-
ing characteristics as well, rendering the design of the ideal
NBD–QC interconversion system a challenging and yet un-
solved task. Nevertheless, by performing detailed and system-
atic investigations, the group of Moth-Poulsen in particular has
made considerable progress recently.[11] Next to a tailor-made
NBD–QC couple, an efficient and well-designed catalyst is of
paramount importance for a practical MOST system.[6a,12] The
catalyst must enable on-demand energy release with a suffi-
cient turnover number (TON) and turnover frequency (TOF).
Furthermore, the formation of side products must be ruled
out, and the catalyst should be readily separable from the
solar fuel to allow easy cyclization of the process. The latter re-
quirement is best met by heterogeneous catalysts; however, in
many cases they are inferior to homogeneous catalysts in
terms of reactivity and selectivity.[13] By using high-surface-area
materials such as magnetic iron oxide nanoparticles (NPs) as
the support for a molecular catalyst, the best of both worlds
can be achieved. Through the deposition of self-assembled
monolayers, the material’s properties can be modified to facili-
tate the encapsulation of apolar molecules,[14] interchangeable
dispersibility,[15] drug delivery,[16] or the design of quasi-homo-
geneous catalysts.[17] Recently, we showed that by anchoring a
CoSalphen catalyst onto magnetic Fe3O4 NPs, it is possible to
adapt this concept for the design of an easily cyclable MOST
system.[18] Despite this very successful proof of concept, the
huge potential of combining magnetic NP catalysts with the
NBD/QC interconversion in a MOST system has, so far, not
nearly been exhausted.
Results and Discussion
We opted to utilize commercially available Fe3O4 nanoparticles
with a specific surface area of 100 m2 gÀ1 (derived from BET
measurements) as the metal oxide core for the immobilization
of catalytically active functional molecules. Previously, we re-
ported that Fe3O4 nanoparticles, functionalized with a cobalt
(II) salphen moiety carrying a carboxylic acid (Cat1) as anchor
group, act as an effective magnetic catalyst for the exothermic
back-conversion of QC to NBD.[18] On the basis of these results,
we improved the nanoparticle functionalization process and
expanded it to cobalt tetraphenylporphyrin-based catalysts,
which are known to facilitate the QC to NBD interconversion
efficiently.[19] Cat2 is an asymmetric A3B porphyrin with a single
carboxylic acid as anchoring group, whereas Cat3 is a symmet-
ric A4 porphyrin carrying four carboxylic acid groups for an-
choring on metal oxide surfaces (Scheme 2). For the prepara-
tion of the magnetic catalyst, Fe3O4 nanoparticles were treated
with 10 wt% of the respective cobalt complex (Cat1, Cat2, or
Cat3) in isopropanol (Scheme 2). The resulting dispersions
were subsequently irradiated by tip sonication for 30 min and
then stirred under ambient conditions for 48 h. After centrifu-
gation, the reduced color in the supernatant indicated immobi-
lization of the cobalt complexes on the NPs. The resulting NP
hybrids [Fe3O4-Cat1–3] were submitted to multiple washing
steps consisting of redispersion in isopropanol and centrifuga-
tion to remove any unbound ligands. Finally, a washing step
with separation induced by an external magnet instead of cen-
trifugation was conducted. After oven drying, the catalyst load-
ing was determined by elemental analysis. Loadings of
7.9 wt% catalyst for [Fe3O4-Cat1], 3.9 wt% for [Fe3O4-Cat2],
and 7.4 wt% for [Fe3O4-Cat3] were determined (Supporting In-
formation).
Chem. Eur. J. 2021, 27, 4993 –5002
4994 ꢀ 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH