Intermolecular London Dispersion Interactions of Azobenzene Switches for Tuning Molecular Solar Thermal Energy Storage Systems

Article abstract of DOI:10.1002/cplu.201900330

The performance of molecular solar thermal energy storage systems (MOST) depends amongst others on the amount of energy stored. Azobenzenes have been investigated as high-potential materials for MOST applications. In the present study it could be shown that intermolecular attractive London dispersion interactions stabilize the (E)-isomer in bisazobenzene that is linked by different alkyl bridges. Differential scanning calorimetry (DSC) measurements revealed, that this interaction leads to an increased storage energy per azo-unit of more than 3 kcal/mol compared to the parent azobenzene. The origin of this effect has been supported by computation as well as X-ray analysis. In the solid state structure attractive London dispersion interactions between the C?H of the alkyl bridge and the π-system of the azobenzene could be clearly assigned. This concept will be highly useful in designing more effective MOST systems in the future.

Full text of DOI:10.1002/cplu.201900330

Accepted Article
Title: Intermolecular London Dispersion Interactions of Azobenzene
Switches for Tuning Molecular Solar Thermal Energy Storage
Systems
Authors: Anne Kunz, Andreas H Heindl, Ambra Dreos, Zhihang Wang,
Kasper Moth-Poulsen, Jonathan Becker, and Hermann
Andreas Wegner
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.201900330
Link to VoR: http://dx.doi.org/10.1002/cplu.201900330
A Journal of
www.chempluschem.org

10.1002/cplu.201900330
ChemPlusChem
COMMUNICATION
Intermolecular London Dispersion Interactions of Azobenzene
Switches for Tuning Molecular Solar ThermalEnergy Storage
Systems
Anne Kunz,[a] Andreas H. Heindl,[a] Ambra Dreos,[b] Zhihang Wang,[b] Kasper Moth-Poulsen[b], Jonathan
Becker[c] and Hermann A. Wegner*[a]
Abstract: The performance of molecular solar thermal energy
storage systems (MOST) depends i.a. on the amount of energy
stored. Azobenzenes have been investigated as high-potential
materials for MOST applications. In the present study it could be
shown that intermolecular attractive London dispersion interactions
stabilize the (E)-isomer in bisazobenzene linked by different alkyl
bridges. Differential scanning calorimetry (DSC) measurements
revealed, that this interaction leads to an increased storage energy
per azo-unit of more than 3 kcal/mol compared to the parent
azobenzene. The origin of this effect has been supported by
computation as well as X-ray analysis. In the solid state structure
attractive London dispersion interactions between the C–H of the
alkyl bridge and the π-system of the azobenzene could be clearly
assigned. This concept will be highly useful in designing more
effective MOST systems in the future.
proposed.[3] Herein, the azobenzene represents a highly potent
candidate as there is longstanding experience in studying this
system as a dye as well as a molecular switch.[7,8] The storage
energy of a given MOST system is based on the energy differ-
rence between two states. In case of the azobenzene, this is the
(E)-isomer as the lower energy state and the (Z)-isomer as the
metastable high-energy state (Figure 1). Critical parameters for
such MOST systems represent on one hand the actual storage
energy: In case of the azobenzene the energy difference bet-
ween the (E)- and the (Z)-isomer. Additionally, the barrier for the
thermal back-conversion should be high enough to ensure long
storage times. Besides these variables, the molecules should
absorb as much light of the spectrum of the sun as possible in
the lower energy form [(E)-azobenzene] and ideally not at all in
the higher energy form [(Z)-azobenzene]. Finally, the quantum
yield should be as high as possible for the conversion from (E)-
to (Z)-azobenzene. The parent azobenzene has a storage
energy of ~9-10 kcal/mol, which is in a medium range compared
to other molecular entities discussed for MOST applications.[9]
Another downside of azobenzenes is, that the (E)-azobenzene
absorbs in the UV-light, while the storage form absorbs in the
visible range. Fortunately, the properties of azobenzene can
readily be modified by changing the substitution pattern.[10] For
example the introduction of fluorine atoms[11] as well as methoxy
groups[12] shift the absorption to longer wavelength. There have
been different strategies to increase the storage energy.[7,14-16]
Stabilizing groups can influence the (E)-(Z)-energy difference
even by subtle interactions such as London dispersion.[13]
Feng[17-19] and also later Grossmann[20] used nanotemplating as
a tool. Another approach is based on incorporating the azoben-
zene into macrocyclic structures to utilize strain energy in the
(Z)-isomer.[21,22] Alternatively, azobenzenes can be connected to
carbon nanotubes to improve the storage energy.[23]
The development of energy storage solutions is one of the most
urgent tasks for modern societies. While there is tremendous
effort in storing electrical energy, the ability of harvesting the
energy of the sun and release it at a given point of time as heat
is also of importance since heating and cooling constitutes
nearly half of the final energy use in the EU.[1,2] In this respect
molecular solar thermal energy storage (MOST) systems,
sometimes also referred as solar fuels, represent an attractive
solution for both short (hours) and long term (seasonal) solar
energy storage.[3,4] The general principle is based on converting
a molecule from one state into another metastable state with
higher energy. The concept has been proposed already by
Weigert over 100 years ago.[5] In recent years, functional
prototypes for MOST applications have been demonstrated.[6]
While these ones are mostly based on the norbornadiene-
quadricyclane system, other molecular entities have also been
In contrast to altering the azobenzene moiety itself the
interaction of the individual molecules with each other can also
contribute to increase the overall storage energy (Figure 1).[24,25]
The connection between the molecules should be loose enough,
that they can be easily broken upon isomerization and rebuild
during the back reaction. Therefore, weak interactions, such as
London dispersion as part of the van-der-Waals interaction
might be a good choice adding beneficial stabilizing effects to
the system.[13,26] Herein, we envisioned probing these interac-
tions by connecting two azobenzenes with different sized linkers.
Although the stored energy/weight is a key parameter to quantify
the performance of a MOST system, the fundamental insights
might promote the design of more efficient MOST systems.
[a]
Anne Kunz, Andreas H. Heindl, Prof. Dr. Hermann A. Wegner
Institute of Organic Chemistry, Justus Liebig University
Heinrich-Buff-Ring 17, 35392 Giessen,
Germany and Center for Materials Research (LaMa), Justus Liebig
University
Heinrich-Buff-Ring 16, 35392 Giessen, Germany
Hermann.a.wegner@org.chemie.uni-giessen.de
Dr. Ambra Dreos, Prof. Dr. Kasper Moth-Poulsen
Department of Chemistry and Chemical Engineering
Chalmers University of Technology
SE-412 96 Gothenburg, Sweden
Dr. Jonathan Becker
Institute of Inorganic and Analytical Chemistry
Justus Liebig University
Heinrich-Buff-Ring 17, 35392 Giessen, Germany
[b]
[b]
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900330
ChemPlusChem
COMMUNICATION
was determined in solution by 1H NMR analysis
(dichloromethane-d2). For compounds 4b and 4c,
a
photostationary state of all-(Z), (E,Z), all-(E) of ~65%, 25%, 10%
was observed. Compounds 4a and 4d showed higher all-(Z)
ratios of up to 75%. As the thermal all-(Z) to all-(E) isomerization
is crucial for the possible storage time the process has been
studied exemplary for compound 4a by 1H NMR spectroscopy
revealing a t1/2 for the all-(Z) of 2.5 h (SI, Table S7).
Figure 1. Increasing the storage energy in azobenzene molecular solar
thermal storage (MOST) systems by intermolecular interactions.
As linkers between the azobenzene units, a methylene bridge
was chosen. Such a methylene connector will keep the azoben-
zenes sufficiently apart due to the low flexibility, but also allows
for the introduction of substituents to induce intermolecular
interactions. To harvest the London dispersion energies, alkyl
substituents with different sizes were introduced. Specifically,
the interaction of the (E)-azobenzenes with the surface of alkyl
groups was targeted. Hence, besides dimethyl, cyclic alkanes
such as cyclopentyl, cyclohexyl and cycloheptyl were selected.
Figure 2. Switching process of 4c analyzed by UV-Vis spectroscopy (2*10-5
mol/L in acetonitrile).
To study the thermodynamic properties, differential scanning ca-
lorimetry (DSC) has been performed on all compounds. (Table 1
& SI). The storage enthalpies of molecules 4a-d as well as for
unsubstituted azobenzene were measured by DSC on neat
samples that have been irradiated at 365 nm in solution
(10 mg / 3 mL in dichloromethane-d2, SI, Figure S8, S10, S12,
S14, S16). The solvent was removed under continuous
irradiation at 365 nm and exclusion of ambient light by blowing a
N2-stream over the sample. Then, 1H NMR spectroscopy was
conducted to determine the degree of conversion (SI, Figure S7,
S9, S11, S13, S15). The molar storage enthalpy of a single (Z)-
azo group in the mixture of the (E,E)-, (E,Z)- and (Z,Z)-isomers
was calculated based on the measured storage energy density
and the degree of conversion. Although the electronic and steric
properties of all investigated individual azobenzene units are the
same, there is a significant deviation in the energy difference
between the (E)- and the (Z)-isomer of the studied compounds.
The highest storage enthalpy was determined for the
cyclohexane substituted derivative 4c, followed by compound 4b
with the cyclopentyl bridge. The smallest enthalpies were
determined for the dimethyl 4a and the cycloheptane 4d
azobenzenes. After the DSC measurements again, 1H NMR
spectroscopy was utilized to ensure the integrity of the
compounds. All compounds 4a-d showed larger storage
energies per azounit compared to the parent azobenzene, which
has been measured by the same method (Table 1, entry 5). The
value compares well with the literature.[28]
Scheme 2. Synthesis of bis-azobenzenes 4 with different connectors.
The synthesis commences with the addition of aniline 2 to the
corresponding ketone 1 (Scheme 2). While the bisanilines 3a
and 3c could be obtained in good yields, the results for the other
ketones were rather low. This discrepancy can be related to
difficulties in the purification for the five and the seven-
membered rings. While the dimethyl as well as the cyclohexyl
compound can be conveniently purified by crystallization, the
other two derivatives had to be isolated by column chroma-
tography, which was hampered by polarity and solubility issues.
The bisanilines 3 were then condensed by a Baeyer-Mills
reaction to the target bisazobenzenes 4a-d using the procedure
optimized in our group for bisazobenzene compounds.[27]
All compounds 4 can be isomerized by irradiation in
solution at 365 nm and back with 450 nm. The switching process
can be conveniently followed by UV-Vis spectroscopy (Figure 2,
and SI Figure S2-S5). There is only marginal optical difference
between the four derivatives 4a-d. The photostationary state
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900330
ChemPlusChem
COMMUNICATION
between the cyclohexyl part in 4c with the aryl ring of a second
molecules can be seen. The C–H---π distance is below 3.5 Å,
which are typical for this kind of interactions. In compound 4a
also C–H---π contacts between two molecules can be observed.
However, the increased dispersion donor strength of
cyclohexane compared to a methyl group can be attributed to
the stronger interaction. Although crystal-packing effects cannot
be excluded to influence the arrangement of the molecules, this
stabilizing effect on the (E)-isomer could be assigned to the
increased energy gap between the (E)- and the (Z)-isomer
determined in the DSC measurements.
Table 1. Storage enthalpies per azobenzene unit of compounds 4a-d by
computations (PBE0-D3(BJ)/def2-TZVP) and by DSC measurements
(including parent azobenzene as comparison).
ΔHcomp
kcal/mol
/
ΔHDSC
/
Entry
X
kcal/mol]
1
2
3
4
5
H, H
12.4
10.6
(CH2)2
12.4
11.7
(CH2)3
12.2
13.1
(CH2)4
12.2
10.8
In conclusion, it has been demonstrated that even subtle
intermolecular interactions, such as C–H---π London-dispersion
interactions can have a considerable effect on the stabilization of
photoisomers in molecular switches. In the presented study
different alkyl-substituted bis-azobenzenes have been prepared,
their photochemical properties investigated as well as their
thermodynamic parameters. It could be revealed, that although
all azobenzenes are electronically equal, that increasing London
dispersion strength lead to an increased energy difference
between (E)- and (Z)-state. Such interactions can be used to
design efficient compounds for MOST systems in the future.
Azobenzene
12.4
9.3
In order to investigate the effects further, a computational
modeling study of the storage energy was performed. All
structures were optimized at the PBE0 density functional theory
level[29] with D3(BJ) dispersion correction[30] using the def2-TZVP
basis set[31] (Figure 3 and SI). There are negligible deviations in
the overall structure of all compounds for the (all-E)- as well as
for the (all-Z)-isomer. Furthermore, there was no interaction of
the bridging unit with the azobenzene parts detectable.
Figure 3. Optimized geometry of the all-(E) and the all-(Z)-isomer of
compound 4c (PBE0-D3(BJ)/def2-TZVP).
In general, an increased storage enthalpy can be rationa-
lized by a destabilization of the (Z)-isomer or a stabilizing effect
on the (E)-isomer. While the bridging units are more or less
shielded in the all-(Z)-isomer, the star-shaped arrangement in
the all-(E)-isomers allows close intermolecular interaction bet-
ween the alkyl-bridge and the aromatic parts of the azo-units.
There have been intensive studies on aromatic-aromatic inter-
action showing that the T-shaped arrangement in the benzene
dimer is most favorable.[32] Also, the interactions of cycloalkanes
have been addressed. Thereby, it could be shown that σ/σ and
π/π dispersion are equally important.[33] Furthermore, the inter-
actions of cycloalkanes with benzene was investigated by mea-
suring the enthalpies and volumes of mixing.[34] In these results
the strongest interaction has been determined for cyclohexane
and to a less extend for cycloheptane and cyclopentane mirro-
ring our results on the (E)-(Z)-energy difference in compounds 4.
Combining the experimental and computational data we propose,
that in the all-(E) isomer there is an intermolecular interaction
composed of the contact of the π-face of the azobenzene with
the cycloalkane bridge by London dispersion.
Figure 4. Part of the solid-state structure showing two molecules of 4c with
proposed C–H---π and London dispersion interactions (top) and of 4a (bottom).
CCDC 1916020 (4c) and 1916021 (4a) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
Thankfully, a single crystal could be obtained of compound
4c (Figure 4) as well as for 4a. The proposed interaction
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900330
ChemPlusChem
COMMUNICATION
[14] K. Masutani, M.-A. Morikawa, N. Kimizuka, Chem. Commun. 2014, 50,
15803–15806.
Acknowledgements
[15] G. D. Han, S. S. Park, Y. Liu, D. Zhitomirsky, E. Cho, M. Dincă, J. C.
Grossman, J. Mater. Chem. A 2016, 4, 16157–16165.
[16] A. K. Saydjari, P. Weis, S. Wu, Adv. Energy Mater. 2016, 7, 1601622.
[17] W. Feng, W. Luo, Y. Feng, Nanoscale 2012, 4, 6118.
[18] Y. Feng, H. Liu, W. Luo, E. Liu, N. Zhao, K. Yoshino, W. Feng, Nature
2013, 3, 52.
Financial support by the DFG (grant WE 5601/6-1 and the SPP
1807) is acknowledged. We thank Dr. Jonathan Becker, Institute
of Inorganic Chemistry of the Justus Liebig University, for X-ray
analysis.
[19] X. Zhao, Y. Feng, C. Qin, W. Yang, Q. Si, W. Feng, ChemSusChem
2017, 10, 1395–1404.
Keywords: azobenzene • London dispersions • molecular solar
thermal energy storage • molecular switches • noncovalent
interactions
[20] D. Zhitomirsky, J. C. Grossman, ACS Appl. Mater. Interfaces 2016, 8,
26319–26325.
[21] E. Durgun, J. C. Grossman, J. Phys. Chem. Lett. 2013, 4, 854–860.
[22] R. Reuter, N. Hostettler, M. Neuburger, H. A. Wegner, Chimia 2010, 64,
180–183.
[1]
[2]
Ioan Sarbu, Calin Sebarchievici, Sustainability 2018, 10, 191.
Secretariat of the European Technology: Common Vision for the
Renewable Heating& Cooling sector in Europe, 2011.
[23] T. J. Kucharski, N. Ferralis, A. M. Kolpak, J. O. Zheng, D. G. Nocera, J.
C. Grossman, Nature Chem. 2014, 6, 441–447.
[3]
a) A. Lennartson, K. Moth-Poulsen, in Molecular Devices for Solar
Energy Conversion and Storage (Eds.: H. Tian, G. Boschloo, A.
Hagfeldt), Springer, Singapore, 2018, 327–352; b) K. Moth-Poulsen, D.
Ćoso, K. Börjesson, N. Vinokurov, S. K. Meier, A. Majumdar, K. P. C.
Vollhardt, R. A. Segalman, Energy Environ. Sci. 2012, 5, 8534–8537.
C. L. Sun, C. Wang, R. Boulatov, ChemPhotoChem 2019, 82, 894.
G. Jones, S. H. Chiang, P. T. Xuan, J. Photochem. 1979, 10, 1–18.
Z. Wang, A. Roffey, R. Losantos, A. Lennartson, M. Jevric, A. U.
Petersen, M. Quant, A. Dreos, X. Wen, D. Sampedro, K. Börjesson, K.
Moth-Poulsen, Energy Environ. Sci. 2019, 12, 187–193.
E. N. Cho, D. Zhitomirsky, G. G. D. Han, Y. Liu, J. C. Grossman, ACS
Appl. Mater. Interfaces 2017, 9, 8679–8687.
[24] A. M. Kolpak, J. C. Grossman, Nano Lett. 2011, 11, 3156–3162.
[25] W. Feng, S. Li, M. Li, C. Qin, Y. Feng, J. Mater. Chem. A 2016, 4,
8020–8028.
[26] J. P. Wagner, P. R. Schreiner, Angew. Chem. Int. Ed. 2015, 54, 12274
– 12296.
[4]
[5]
[6]
[27] S. Bellotto, R. Reuter, C. Heinis, H. A. Wegner, J. Org. Chem. 2011,
76, 9826–9834.
[28] R. J. Corruccini, E. C. Gilbert, J. Am. Chem. Soc. 1939, 61, 2925–2927.
[29] a) C. Adamo, V. Barone, J. Chem. Phys. 1999, 110, 6158; b)J. P.
Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865.
[30] a) S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010,
132, 154104; b) S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem.
2011, 32, 1456.
[7]
[8]
[9]
L. Dong, Y. Feng, L. Wang, W. Feng, Chem. Soc. Rev. 2018, 47,
7339–7368.
[31] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297.
[32] W. L. Jorgensen, D. L. Severance, J. Am. Chem. Soc. 1990, 112,
4768–4774.
A. Lennartson, A. Roffey, K. Moth-Poulsen, Tetrahedron Lett. 2015, 56,
1457–1465.
[10] E. Merino, Chem. Soc. Rev. 2011, 40, 3835–3853.
[11] D. Bléger, J. Schwarz, A. M. Brouwer, S. Hecht, J. Am. Chem. Soc.
2012, 134, 20597–20600.
[33] A. A. Fokin, D. Gerbig, P. R. Schreiner, J. Am. Chem. Soc. 2011, 133,
20036–20039.
[34] A. E. P. Watson, I. A. McLure, J. E. Bennett, G. C. Benson, J. Phys.
Chem. 1965, 69, 2753–2758.
[12] S. Samanta, A. A. Beharry, O. Sadovski, T. M. McCormick, A.
Babalhavaeji, V. Tropepe, G. A. Woolley, J. Am. Chem. Soc. 2013,
135, 9777–9784.
[13] L. Schweighauser, M. A. Strauss, S. Bellotto, H. A. Wegner, Angew.
Chem. Int. Ed. 2015, 54, 13436–13439.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900330
ChemPlusChem
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
The performance of molecular solar
thermal energy storage system
(MOST) depends i.a. on the amount
of energy stored. Herein it could be
shown that intermolecular attractive
London dispersion interactions
stabilize the (E)-isomer in
bisazobenzene linked by different
alkyl bridges. This leads to an
increased storage energy per azo-
unit of more than 3 kcal/mol
Anne Kunz, Andreas H. Heindl, Ambra
Dreos, Kasper Moth-Poulsen, Jonathan
Becker and Hermann A. Wegner*
Page No. – Page No.
Intermolecular London Dispersion
Interactions of Azobenzene
Switches for Tuning Molecular Solar
Thermal Storage Systems
compared to the parent azobenzene.
Layout 2:
COMMUNICATION
Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
Page No. – Page No.
Title
Text for Table of Contents
This article is protected by copyright. All rights reserved.