Norbornadiene-Based Photoswitches with Exceptional Combination of Solar Spectrum Match and Long-Term Energy Storage

Article abstract of DOI:10.1002/chem.201802932

Norbornadiene-quadricyclane (NBD-QC) photoswitches are candidates for applications in solar thermal energy storage. Functionally, they rely on an intramolecular [2+2] cycloaddition reaction, which couples the S₀ landscape on the NBD side to the

Full text of DOI:10.1002/chem.201802932

A Journal of
Accepted Article
Title: Norbornadiene-based photoswitches with exceptional
combination of solar spectrum match and long term energy
storage
Authors: Martyn Jevric, Anne U Petersen, Mads Mansø, Sandeep
Kumar Sing, Zhihang Wang, Ambra Dreos, Christopher
Sumby, Mogens Brøndsted Nielsen, Karl Börjesson, Paul
Erhart, and Kasper Moth-Poulsen
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To be cited as: Chem. Eur. J. 10.1002/chem.201802932
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Norbornadiene-based photoswitches with exceptional
combination of solar spectrum match and long term energy
storage
Martyn Jevric, Anne U. Petersen, Mads Mansø, Sandeep Kumar Singh, Zhihang Wang, Ambra Dreos,
Christopher Sumby, Mogens Brøndsted Nielsen, Karl Börjesson, Paul Erhart,
and Kasper Moth-Poulsen*
Abstract: Norbornadiene-quadricyclane (NBD-QC) photo-
switches are candidates for applications in solar thermal energy
storage. Functionally they rely on an intramolecular [2+2]
cycloaddition reaction, which couples the S0 landscape on the
NBD side to the S1 landscape on the QC side of the reaction and
vice-versa. This commonly results in an unfavourable correlation
between the first absorption maximum and the barrier for thermal
back-conversion. Here, we demonstrate that this correlation can
be counteracted by using steric repulsion to hamper the rotational
motion of the side groups along the back-conversion path. It is
shown that this modification reduces the correlation between the
effective back-conversion barrier and the first absorption
maximum and also increases the back-conversion entropy. The
resulting molecules exhibit exceptionally long half-lives for their
metastable forms without significantly affecting other properties,
most notably solar spectrum match and storage density.
In this context, photo-induced energy storage via isomerization
reactions in molecules has been proposed as viable
a
alternative.[1] This process has been referred to as molecular solar
thermal (MOST) storage systems[2] or known as solar thermal
fuels,[3] and are receiving increasing interest with new molecular
designs and device architectures being developed. [4] In these
systems, photochromic molecules are converted by irradiation
into their respective metastable photoisomers. The energy stored
in this fashion can later be released on demand either via thermal
or catalytic activation.
Several photochromic motifs, such as tetracarbonyl-
fulvalene-diruthenium
compounds,[5]
azobenzenes,[3,6]
norbornadienes[7] and dihydroazulenes[8] have been investigated,
both in solution and neat forms,[6d,7l] as well as appended to
polymers or integrated onto solid supports.[3,6e] The focus of our
group has been on assessing the potential of the norbornadiene
(NBD)/ quadricyclane (QC) couple, where light can be used to
facilitate a [2+2p] intramolecular cyclization to form the high
energy QC1 from NBD1 (Figure 1). It has also been found that
the stored energy, which has been measured to be up to 1000 kJ
Renewable and clean energy sources are a very active
research area. As an increasing part of our energy demand is
provided from renewables, technologies for storage and load
levelling become ever more important in order to compensate for
daily and seasonal variations in energy production and demand.
kg-1,[7c] can be released by means of
a
catalyst[7c] or
electrochemically.[7j] A set of selection criteria for an optimal
MOST system has been formulated, which includes high
absorbance of NBD around 500 nm to match the most intense
band in the solar spectrum (hνabs in Figure 2); high quantum yield
(Φ) and a non-conflicting absorbance for the corresponding QC
(hνabsQC in Figure 2); low molecular weight; high energy difference
between NBD and QC isomers; and a high activation energy with
high-energy heat release (ΔH‡ in Figure 2), i.e. a stable QC form
at ambient temperature implying a long half-life (t1/2).
[a]
Dr. M. Jevric, Dr. A. U. Petersen, M. Sc. M. Mansø, M. Sc. Z. Wang,
M. Sc. A. Dreos, Prof. K. Moth-Poulsen.
Department of Chemistry and Chemical Engineering, Chalmers
University of Technology, SE-41296 Gothenburg, Sweden.
E-mail: kasper.moth-poulsen@chalmers.se
M. Mansø, Prof. M. B. Nielsen.
Department of Chemistry, University of Copenhagen,
Universitetsparken 5, 2100 Copenhagen Ø, Denmark.
Dr. S. Kumar, Assoc. Prof. P. Erhart.
Department of Physics, Chalmers University of Technology,
SE-41296 Gothenburg, Sweden.
Prof. C. Sumby.
[b]
[c]
[d]
[e]
Department of Chemistry, School of Physical Sciences, The
University of Adelaide, SA 5005, Australia.
Ass. Prof. K. Börjesson.
Department of Chemistry and Molecular Biology, University of
Gothenburg, SE-41296 Sweden.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201802932
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spectrum match or energy storage. Based on experimental
evidence and quantum chemical calculations, the decoupling of
back-conversion barrier and absorption spectrum is attributed to
the effect of steric repulsion on the rotational energy landscape
between QC and the saddle point of the transition state (TS). The
present approach thus promises to provide a general means for
optimizing storage lifetimes without compromising solar spectrum
match.
σ*+σ*
Figure 1. NBD-QC couple 1 and previously synthesised NBD derivatives
NBD2, NBD3 and NBD4 with promising MOST properties.
π*+π*
S1
QC
Φ
hν
LUMO
abs
In practice, MOST designs must be adaptable to different
environments. Target storage times, for example, typically vary
from a few hours to many months depending on targeted
application. It is common when developing MOST systems that
one property is improved at the expense of another.[7g] Importantly,
in NBD-QC systems the storage time is typically correlated with
the first absorption maximum, such that longer storage times
come at the expense of a deterioration of the solar spectrum
match.[7d,7f,7g,7i] This behaviour can be understood from the
coupling of the S0 and S1 energy landscapes (Figure 2). While
HOMO and LUMO have p-p and p*+p* character on the NBD side
of the reaction, they have s-s and s*+s* character in the case of
QC. The conversion from NBD to QC corresponds to an
intramolecular cycloaddition reaction, which involves an avoided
crossing of HOMO and LUMO levels. As a result, reducing the S0-
S1 spacing for NBD commonly implies a lowering of the barrier for
thermal back-conversion (DHǂ). This detrimental correlation is
general and has been observed in the original works by Yoshida,
where donor-acceptor molecules red-shifted absorption reduced
the energy storage times from months to minutes.[7c,7h] In the work
of our group, the introduction of increased donor strength and
extended conjugation (molecules NBD2 vs NBD3 in Figure 1)[7g]
lead to energy storage half-lives of a few hours at room
temperature. This trend was also observed for NBD4 series, using
an electronic donor in conjugation with an acceptor (X = donor, Y
= acceptor).[7i]
hνabs
σ-σ
ΔH‡
HOMO
QC
S0
π-π
NBD
NBD
TS
QC
Figure 2. (a) Reaction dynamics for interconversion between NBD and QC.
Absorption of a photon leads to activation of NBD from the singlet ground state
(S0) to the first excited state (S1). During deexcitation the molecule transforms
into its QC form with quantum efficiency Φ. The QC-NBD back-conversion is
associated with an energy barrier ΔH‡. (b) The HOMO and LUMO levels on
the NBD and QC side of the reaction are related to each other, which implies
in general coupling between absorption spectrum and the barrier for back-
conversion.
Two approaches were employed in the synthesis of NBD
derivatives (full experimental procedures can be found in the SI),
one involving a [4+2p] Diels-Alder route, in which by using a
literature method suitable dienophiles could be made on a large
scale from a host of acetophenones furnishing the set of NBDs
shown in Figure 3.[9] In the cases where the initial method did not
work, this was supplemented by the reported procedure
employing a palladium catalysed Suzuki reaction upon 2-chloro-
3-cyanonorbornadiene.[7g] A recent theoretical study showed that
certain aromatic donor units could enhance the MOST properties,
including dimethylaniline and thiophene, among other electron
rich substituents, which were accordingly included in this
series.[10] Figure 4 shows the X-ray crystal structure for NBD2h,
which during the course of this investigation was shown to
possess excellent absorption properties (additionally, the
structure of NBD2b can be found in SI). Large quantities of QCs,
which were required for differential scanning calorimetry (DSC) to
determine heat release capabilities of these materials, were
generated in either chloroform or toluene by irradiation using
either a 310 nm or 365 nm. With the exception of NBD2a and 2b,
which suffered from inner filter effects, all photoconversions went
Here, we demonstrate a strategy to effectively counteract
this correlation, which involves introducing substituents in the
ortho position of 2-cyano-3-aryl substituted norbornadienes. We
present a series of 17 new norbornadiene and quadricyclane
compounds. Four of these bear substituents in the ortho position
and exhibit an unprecedented increase of a factor of up to two
orders of magnitude of the storage lifetime (t1/2) as compared to
their non-substituted analogues without a deterioration of solar
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to completion in either toluene or chloroform. Pure samples of
QC2a and QC2b could instead be obtained by column
chromatography. Furthermore, in one instance, concentration of
a toluene solution of QC2n resulted in a rapid partial back
conversion thereby furnishing a mixture of NBD and QC forms
upon drying. The NBD2n/QC2n ratio did not seem to change if
the mixture was stored at –18 °C for a prolonged period of time.
toluene (Table 1). This allowed for a direct comparison between
the new molecules and previously reported NBD2 and NBD3.
Corresponding QC2a-q were generated in situ using a selection
of wavelengths, 310 nm, 340 nm, or 365 nm. Here, the spectral
window is defined as the offset between the absorption onsets of
the respective NBD and QC species. This quantity provides a
measure for the efficiency of the absorption during operation and
is of fundamental importance when designing practical MOST
systems. It was found that the quantum yields of
photoisomerisation (Φ) for the nitro-substituted derivatives
NBD2a and NBD2b are rather low compared to all the other
derivatives. Compounds with the methoxy donating group in
different combinations and even in multiple substitution patterns,
show only small variations in absorbance maxima and quantum
yields. In many instances, there are, however, large differences
between the onsets of absorption for NBDs relative to the
corresponding QCs. The highest measured quantum yield of
82% was recorded for the methylenedioxyaryl substituted
derivative NBD2k, which otherwise has very similar properties
as NBD2h. The use of naphthalene substituents, NBD2l and
NBD2m, did not seem to enhance the absorption properties for
the NBDs. Compound NBD2n exhibits the largest red-shift in the
UV-vis spectrum, along with a very large extinction coefficient
but the optical window of operation was small. While the only
structural difference between NBD2n and NBD3 is the absence
of the acetylenic linker between the donor and NBD motif, an
increase in the quantum yield by more than a factor of two was
observed, accompanied by a lowering of the absorbance
maximum by 20 nm with a slightly lower extinction coefficient.
This difference has previously been shown to be due a better
spatial alignment of HOMO and LUMO orbitals in the case of
NBD3, which leads to a larger transition dipole moment.[7g] The
kinetic stability of QC2a-q was determined by monitoring the
concentration of NBD2a-q at three different temperatures (Table
1). For most compounds room temperature half-lives range
between hours and at most several weeks and are thus
comparable to those of substituted norbornadiene compounds
synthesized previously. By contrast unusually high kinetic
stabilities were measured for QC2d, QC2f, QC2i, QC2l and
QC2m, which feature room temperature half-lives well on the
order of years and/or back-conversion barriers exceeding 130
kJ/mol (ΔHǂ, Table 1). Even more strikingly, they feature strongly
red-shifted absorption spectra with good solar spectrum match
and thus break the correlation between the first absorption
maximum and the back-conversion barrier (Figure 5a).
Figure 3. NBDs made in this study with yields using the Diels-Alder reaction or
via palladium catalysed Suzuki Coupling (highlighted B).
Figure 4. Single X-ray crystal structure of 2h. Crystals obtained from diethyl
ether and n-heptane. Ellipsoids are depicted at 50% occupancy. CCDC-
1841959 contain the supplementary crystallographic data. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
The absorbance properties for all NBD-QC couples, including
the kinetics for the thermal back-conversion were acquired in
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Table 1: Absorbance properties and kinetic data for the thermal back-conversions for NBD-QC couples 2a-q at room temperature (298K). Absorbance parameters
λmax, λonset and spectral window are reported in nm. Thermodynamic parameters for thermal back-conversion were determined from the Eyring equation including
the Gibbs free energy (DGǂ), enthalpy (DHǂ), entropy (DSǂ) of activation and half-lives (t 25 °C).
½
Photophysical properties
Kinetic data
ǂ
ǂ
ǂ
NBD
λonset
QC
λonset
314
344
397
D
D
D
G
t
½ 25 °C
(days)
54
0.204
7.74
H
S
spectral
window
44
112
4
λmax (ε x 103)
309 (7.7)
398 (30)
Φ (%)
58
(kJ mol-1) (J mol-1K-1)
(kJ mol-1)
112.4
25g
35g
2a
358
456
401
112.0
92.5
100.9
-1.31
-19.1
-20.9
28
20
98.2
337 (11)
107.1
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
302 (7.1)
308 (8.3)
301 (6.9)
325 (13)
322 (6.5)
315 (8.3)
340 (14)
330 (11)
331 (13)
338 (12)
321 (7.6)
326 (16)
374 (23)
324 (12)
336 (11)
312 (9.4)
382
359
350
377
368
365
389
385
384
383
373
384
427
380
390
367
19
49
50
40
70
60
68
73
68
82
59
60
73
55
61
56
380
307
310
341
339
312
312
306
305
318
325
343
401
300
353
312
2
117.0
116.3
179.9
111.1
134.4
116.4
116.2
139.4
109.0
106.4
142.4
165.8
110.5
114.1
95.3
11.1
10.2
195.5
0.92
44.1
11.8
19.4
58.7
-3.1
113.7
113.3
121.6
110.8
121.3
112.9
110.4
121.9
109.9
103.8
124.0
115.4
106.9
110.4
98.4
108.7
89.8
2680
33.3
2273
78
28.7
2971
24.2
1.95
6729
212
52
40
36
29
53
77
79
79
65
48
41
26
80
37
55
8.8
61.7
169.0
12.0
12.3
-10.4
-6.7
7.04
28.5
0.22
22.7
107.8
109.8
behaviour are computationally very demanding if not
impractical for the molecules studied here. Here, we therefore
adopt the approach from Jorner et al. and approximate the
biradical character using unrestricted open-shell DFT.[7b] This
These encouraging features do not compromise the energy
storage properties of the compounds as measured using
differential scanning calorimetry (SI section 3).
approach yields ΔH‡
values in good agreement with
All high-performing derivatives feature a substitution in
the ortho position of the aromatic substituent, which, as will be
explored below, can sterically interfere with the back-
conversion reaction. The importance of this steric effect is
emphasized by the observation that the compounds (2c vs 2d,
2g vs 2f, 2h vs 2i, and 2l vs 2m) with corresponding or similar
para substitutions exhibit comparable absorption spectra but
much shorter half-lives and smaller back-conversion barriers
(QC2c, QC2g, QC2j, QC2k and QC2m); shaded blue triangles
in Figure 5). Comparison of the Eyring plots (SI section 3) as
well as the back-conversion enthalpy ΔH‡ and entropy ΔS‡
shows a qualitative difference between high-performing and
“regular” compounds. In fact, the half-lives of QCs with
substituents in the ortho position were found to be up to two
orders of magnitude greater than the aforementioned para
regioisomers (see SI Figure 5.1 and SI Table 5.2).
experiment and reference CASPT2 calculations for a series of
diaryl-substituted NBD compounds investigated earlier (Figure
5b).[7i,11] While it also yields reasonable agreement for the
“regular”, i.e., non-high-performing, molecules studied here, for
several of the high-performing compounds (QC2d, QC2i,
QC2m) the calculated and measured barriers are, however,
barely correlated (Figure 5b). The difficulties in treating the
complexity of the electronic structure of the transition state are
a natural source of uncertainty. It is nonetheless possible to
extract valuable information pertaining to the mechanisms that
give rise to the very long half-lives of the high performers.
To understand this behaviour better we analysed the
back-conversion reaction using quantum chemical calculations
at the level of density functional theory (DFT); technical details
are provided in the SI section 4. While the biradical transition
state has
a
pronounced multi-reference character,[11]
computational techniques that can accurately account for this
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freedom. To address the former, we computed the vibrational
spectrum ωi in the QC and TS points of the potential energy
landscape (PES), from which we obtained the effective
vibrational pre-factor ω‡= Πi ωiQC / Πi ωiTS. It is evident from the
data in Table 5.2 in the SI that for the high performers QC2d,
QC2i and QC2m, ω‡ is at least one order of magnitude larger
than for any of the other compounds and, in particular, that ω‡
is substantially increased relative to the respective variants
(e.g., QC2c, QC2j, QC2k).
While a comprehensive treatment of the configurational
degrees of freedom is computationally very demanding and
beyond the scope of this work, further insight can be gained
from inspection of the PES associated with rotations of the
donor side group with respect to its bond to the parent
molecule. Here, comparison of this PES for the pair
QC2c/QC2d (Figure 6) and the series QC2i/QC2j/QC2k
reveals that substitution at the ortho position has a very
pronounced effect on the QC PES, strongly limiting the number
of accessible states, whereas its impact at the TS is only
modest. The pre-factor (and thus the entropy of back-
conversion determined in Table 5.2 in the SI) of the ortho-
substituted compounds will therefore be enhanced also for
configurational reasons relative to their non-ortho substituted
counterparts.
Figure 5. a) Plot of absorption onset for previously reported NBDs and
new derivatives against corresponding QC half-lives; b) Plot of theoretical
vs. experimental back-conversion barrier; c) molecular structures.
Figure 6. Calculated energy landscapes of the donor side group rotational
angle in the QC form (orange squares) and the TS (blue circles) for 2c
(left) and 2d (right). Substitution in the ortho position causes a dramatic
change in the QC landscape, but not in the landscape of the TS. The black
arrow indicates the minimum energy on the QC landscape.
At the level of transition state theory, the rate of QC-NBD
conversion is related to the ratio of the partition functions at the
initial (QC) and transition states (TS), respectively, where the
latter can be approximately decomposed into contributions
from vibrational, configurational, and electronic degrees of
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In summary the above analysis demonstrates the main
factors that contribute to an enhanced half-life. The high-
performers developed in this work either feature side groups
with substitution on the ortho position (QC2d, QC2f, QC2i) or
large asymmetric side groups (QC2l, QC2m). These designs
affect the rotational motion of the side group relative to the
parent compound, both in terms of the vibrational and
configurational degrees of freedom, which modifies the normal
correlation between the first excitation energy (i.e., the S0-S1
spacing on the NBD side) and the back-conversion barrier ΔH‡,
and gives rise to a larger entropy of back-conversion ΔS‡. This
thus provides a rationale that is transferable to other
compounds and possibly even other chemical reactions and
photoswitch systems. While the present analysis is primarily
qualitative in nature, more detailed investigations are
warranted to comprehensively explore the back-conversion
reaction and to establish the role of the underlying
computational techniques, including both the treatment of the
electronic structure and the evaluation of the transition rates.
the absorption properties of the NBD beyond 500 nm to better
match the solar spectrum.
Acknowledgements
M.J., A.U.P, M.M., Z.W., A.D. and K.M.P. acknowledges funding
from the K&A Wallenberg foundation as well as funding from the
Swedish Strategic Research council.
Keywords: molecular photoswitch • energy storage • energy
storage • norbornadiene • quadricyclane • solar energy
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Martyn Jevric, Anne U. Petersen, Mads
Mansø, Sandeep K. Singh, Zhihang
Wang, Ambra Dreos, Christopher
Sumby, Mogens Brøndsted Nielsen, Karl
Börjesson, Paul Erhart, and Kasper
Moth-Poulsen
A series of 2-cyano-3-aryl
norbornadienes were synthesized and
the properties studied for Molecular
Solar Thermal (MOST) energy
storage. The quadricyclane isomers
with substituents in the ortho-position
exhibited exceptionally high thermal
stability without compromising the
other properties. Calculations show a
greatly enhanced barrier of rotation in
the quadricyclane form coming about
from this steric congestion interfering
with the transition state.
Page No. – Page No.
Norbornadiene-based photoswitches
with exceptional combination of solar
spectrum match and long term
energy storage
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