Unraveling factors leading to efficient norbornadiene-quadricyclane molecular solar-thermal energy storage systems

Article abstract of DOI:10.1039/c7ta04259k

Developing norbornadiene-quadricyclane (NBD-QC) systems for molecular solar-thermal (MOST) energy storage is often a process of trial and error. By studying a series of norbornadienes (NBD-R₂) doubly substituted at the C7-position with R = H, M

Full text of DOI:10.1039/c7ta04259k

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Moth-Poulsen and H. Ottosson, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA04259K.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Unraveling Factors Leading to Efficient Norbornadiene-
Quadricyclane Molecular Solar-Thermal Energy Storage Systems
Kjell Jorner,ab Ambra Dreos,c Rikard Emanuelsson,ad Ouissam El Bakouri,e Ignacio Fdez.
Galván,bf Karl Börjesson,cg Ferran Feixas,*e Roland Lindh,*bf Burkhard Zietz,*b Kasper Moth-
Poulsen*c and Henrik Ottosson*ab
a Department of Chemistry – BMC, Uppsala University, Box 576, SE-75123 Uppsala, Sweden. b
Department of Chemistry – Ångström Laboratory, Uppsala University, Box 523, SE-75120
Uppsala, Sweden. c Department of Chemistry and Chemical Engineering, Chalmers University of
d
Technology, Kemigården 4, SE-41296 Gothenburg, Sweden.
Department of Engineering
e
Sciences, Uppsala University, Box 534, SE-75121 Uppsala, Sweden. Institut de Química
Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus
Montilivi, 17003 Girona, Spain. f Uppsala Center of Computational Chemistry – UC3, Uppsala
University, Box 523, SE-75120 Uppsala, Sweden.g Department of Chemistry & Molecular
Biology, University of Gothenburg, Kemigården 4, SE-41296 Gothenburg, Sweden.
Electronic supplementary information (ESI) available: Experimental procedures, spectral data
for new compounds, computational details and Cartesian coordinates for calculated structures.
1

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Abstract: Developing norbornadiene-quadricyclane (NBD-QC) systems for molecular solar-
thermal (MOST) energy storage is often a process of trial and error. By studying a series of
norbornadienes (NBD-R2) doubly substituted at the C7-position with R = H, Me, and iPr, we
untangle the interrelated factors affecting MOST performance through a combination of
experiment and theory. Increasing the steric bulk along the NBD-R2 series gave higher quantum
yields, slightly red-shifted absorptions, and longer thermal lifetimes of the energy-rich QC isomer.
However, these advantages are counterbalanced by lower energy storage capacities, and overall R
= Me appears most promising for short-term MOST applications. Computationally we find that it
is the destabilization of the NBD isomer over the QC isomer with increasing steric bulk that is
responsible for most of the observed trends and we can also predict the relative quantum yields by
characterizing the S1/S0 conical intersections. The significantly increased thermal half-life of NBD-
iPr2 is caused by a higher activation entropy, highlighting a novel strategy to improve thermal half-
lives of MOST compounds and other photo-switchable molecules without affecting their electronic
properties. The potential of the NBD-R2 compounds in devices is also explored, demonstrating a
solar energy storage efficiency of up to 0.2%. Finally, we show how the insights gained in this
study can be used to identify strategies to improve already existing NBD-QC systems.
Introduction
Widespread implementation of solar energy technologies is essential for the future development of
a sustainable energy supply (or conversion) based entirely on renewable energy sources. Due to
daily and yearly variations in both solar influx and energy demand, technologies for solar energy
storage are particularly relevant.1 Solar energy can be converted directly into latent chemical
energy via several methods such as natural or artificial photosynthesis, 2 , 3 or photo-induced
isomerization in chemical compounds – so-called molecular solar thermal (MOST) systems.4 Since
2

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the initial idea to use photoinduced isomerizations for solar energy storage was conceived more
than a century ago, 5 several molecular systems have been presented, including anthracene
dimerization,5, 6 azobenzenes,7 ruthenium fulvalene,8 , 9 dihydroazulene-vinylheptafulvene10 and
norbornadiene-quadricyclane (NBD-QC).11 - 18 From an applications perspective, many design
challenges need to be addressed for the system to function efficiently.13,19-21 Notably, a large
fraction of the solar spectrum should be absorbed, the photoconversion must proceed with near
unity quantum efficiency and the energy should be stored at a high energy density. Moreover, the
photoisomer should be stable over time, the chemical compounds switchable for multiple cycles
and the back-reaction from photoisomer to parent compound triggered externally by, e.g., a solid
state catalyst.
For the NBD-QC system (Fig. 1), several of these requirements are fulfilled, but unfortunately
it only absorbs light below 300 nm.13,21 NBD undergoes a photochemical [2+2]-cycloaddition
reaction to form the strained high-energy QC isomer. Thermally, this reaction is forbidden
according to the Woodward-Hoffmann rules22 and the energy-releasing back-reaction from QC to
NBD proceeds via a high-energy transition state (TS) of diradical character (experimental gas-
phase activation energy of 33.5 kcal/mol).18,23,24
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Fig. 1 Schematic figure of the thermal and photochemical isomerization between NBD and QC.
Shown are the excitation energy, Eabs, storage energy, Estorage, and activation energy for thermal
back-reaction, E‡.
However, a challenge with the system is that all reported photoisomers of compounds with red-
shifted absorption into the visible part of the spectrum have too short lifetimes to be practically
useful for long-term energy storage applications,4,12,14,20,25 and the quantum yields are often well
below unity.15 It is therefore desirable to develop a detailed understanding of how the NBD-QC
system can be chemically engineered with respect to absorption spectrum, quantum yield, energy
storage and barrier for the thermal back-reaction. Herein we report how to disentangle and balance
the different factors influencing the efficiency of the NBD-QC system. We do this by a case study
of three NBD-R2 molecules which carry the R = H, Me and iPr substituents on the C7 atom, giving
different degrees of steric pressure on the NBD isomer (Fig. 2).
4

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Fig. 2 NBD-QC systems investigated in this study. DA-R2, P-R2 and NBD-tBu2 are studied only
computationally.
We observe that increased steric bulk leads to higher quantum yield for formation and an increased
lifetime of the QC isomer, although these gains are balanced by a decrease in energy storage
capacity. Importantly, we can rationalize the increase in quantum yields computationally by
characterization of the conical intersections through which the photochemical reaction occurs. One
of us has previously shown that design and screening of NBDs with respect to spectral and energy
storage characteristics can be performed computationally.26 Now, for the first time also relative
quantum yields can be predicted computationally. Furthermore, a simple DFT scheme is used to
compute accurate enthalpy barriers for the thermal back-reactions. We expect that the insight and
tools of this study will become important in the search for improved NBD/QC systems for MOST
applications. At the end, the NBD-R2 molecules are evaluated in a solar energy conversion
demonstration device, highlighting the possibility for future applications. The present results will
also be important for NBD-QC systems considered as photo-switches, e.g., in molecular
electronics27 and conjugated materials.28
Results and discussion
We reasoned that increasing the steric bulk going from R = H to Me and finally iPr would push the
double bonds increasingly closer to each other, thereby increasing their interaction in a controlled
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way. Our initial hypothesis was that this mainly electronic effect would have a gradual influence
on the properties of importance for MOST performance such as absorption spectrum and quantum
yield. However, as we show below, this is only true for the absorption characteristics, and in fact
the relative energies of the NBD and QC isomers, as affected by the steric pressure, is much more
important.
In this section we first present the experimental characterization of the NBD-R2 series and then
go on to untangle the reasons for the experimental trends by both experimental and computational
investigations. After showing the use of NBD-iPr2 in a model MOST device, we finally
demonstrate how the insights and computational methodology developed in this study can be used
to identify strategies for improving known NBD compounds.
Synthesis and characterization: We synthesized NBD-H2, NBD-Me2 and NBD-iPr2 and
characterized their photophysical and photochemical properties (for experimental details, see the
ESI). NBD-Me2 has previously been reported.12 We also considered synthesizing NBD-tBu2 but
discarded it for two reasons: (1) the QC-tBu2 isomer is calculated to be more stable than NBD-
tBu2, making it uninteresting for MOST applications, and, (2) the synthesis is considerably more
challenging (see the ESI for further discussion). However, we also include computational data for
NBD-tBu2 for comparison. The absorption spectra show peaks at 240 nm, 260 nm and 340 nm
(Fig. 3) which are slightly red-shifted by 3-5 nm through substitution in the order NBD-H2 < NBD-
Me2 < NBD-iPr2, showing the effect of increased through-space interaction of the double bonds
enforced by the steric bulk in line with our initial hypothesis. This observation is consistent with
the calculated TD-DFT peaks (LC-HCTH/6-311+G(2d,p), Table 1).29-31
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Table 1. Experimental and calculated properties of NBD-R2 compounds.
a
Hstorage (kcal/mol)b t1/2 (hours)c H‡ (kcal/mol)c

pos (nm)d
341 (330)
346 (339)
349 (343)
- (375)
Aonset (nm)e


0.73 ± 0.03
0.86 ± 0.10
0.88 ± 0.04
-
20.4 (19.7)
16.3 (16.7)
11.1 (13.9)
- (-2.9)
6.3
7.7
56
-
21.4 (19.6)
21.5 (19.0)
21.7 (19.8)
- (30.6)
391
403
414
-
NBD-H2
NBD-Me2
NBD-iPr2
NBD-tBu2
a
Quantum yield in acetonitrile after excitation at 334 nm b Measured by differential scanning calorimetry (DSC).
c
Calculated CAM-B3LYP-D3(BJ)/6-311+G(d,p)//PBE-D3(BJ)/6-31+G(d,p) energies in parenthesis Derived from
Eyring plot in toluene. Calculated CAM-B3LYP-D3(BJ)/6-311+G(d,p)/SMD=toluene//PBE-D3(BJ)/6-31+G(d,p)
values in parenthesis. d Absorption maximum, given for the lowest-energy absorption peak and obtained by spectral
deconvolution using four Gaussian peaks. Calculated LC-HCTH/6-311+G(2d,p)/SMD=acetonitrile//PBE-D3(BJ)/6-
31+G(d,p) values in parenthesis. e Absorption onset defined as log() = 2.
20000
15000
10000
5000
0
334 nm
NBD-H2
NBD-Me2
NBD-iPr2
200
250
300
350
400
450
Wavelength (nm)
Fig. 3 Absorption spectra of NBD-H2, NBD-Me2 and NBD-iPr2 in acetonitrile. Vertical bar
shows wavelength for irradiation for the quantum yield measurements (334 nm, FWHM 2 nm).
Quantum yields for the NBD→QC isomerization after excitation with monochromatic light of 334
nm (FWHM 2 nm, Fig. 3) show an increase in the order NBD-H2 (0.73 ± 0.03) < NBD-Me2 (0.86
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± 0.10) < NBD-iPr2 (0.88 ± 0.04), although the difference between NBD-Me2 and NBD-iPr2 is
not statistically significant (Table 1). The presence of isosbestic points testifies to clean
photoreactions (Fig. S7 to Fig. S9). At the same time, the energy storage enthalpies as measured
by differential scanning calorimetry (DSC) also decrease: 20.4 kcal/mol for NBD-H2, 16.3
kcal/mol for NBD-Me2 and 11.1 kcal/mol for NBD-iPr2, in fair agreement with the calculated DFT
values (CAM-B3LYP-D3(BJ)/6-311+G(d,p)//PBE-D3(BJ)/6-31+G(d,p), Table 1).32-35 There is
also an increase in thermal half-lives of the QC isomers at 25 °C: 6.3 hours for NBD-H2, 7.7 hours
for NBD-Me2 and 56 hours for NBD-iPr2. Although the experimental activation enthalpies (H‡)
are approximately constant, in accordance with our calculations, a higher activation entropy (S‡)
is affecting the half-life of QC-iPr2, which is significantly longer than the other two (Table 1 and
Table S1, vide infra). As the back-reaction from the QC to NBD for structurally very similar NBDs
has been shown to be biradical in nature,18 we do not expect any significant influence of the solvent
on the kinetic parameters. In support of this the calculated activation enthalpies are invariant within
0.2 kcal/mol between the gas-phase, toluene and acetonitrile, consistent with a biradical mechanism
(Table S8).
Substantial effort has been made to tune thermal half-lives of the photo-stable isomers in MOST
systems such as azoheteroarenes36 and dihydroazulene-vinylheptafulvene.37 This has been done
also for NBD-QC by, e.g., methyl substitution at bridgehead positions or placing trifluoromethyl
groups on the bridges.18,38 Based on the current results, we conclude that C7-substitution is a novel
type of approach to achieve this kind of tuning in the NBD-QC system. Unfortunately, the
beneficial increase in quantum yield and thermal lifetime with steric bulk is accompanied by a loss
in the energy storage capacity. If shorter storage times are needed, NBD-Me2 can be considered
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the best candidate for MOST applications, otherwise the longer lifetime of the NBD-iPr2 system
is needed.
Origin of the long half-life of QC-iPr2: Why is the half-life of QC-iPr2 so long? We decided to
investigate this further, aiming to find new ways of improving the NBD/QC system for MOST
applications. The experimental H‡ values over the QC-R2 series are similar with 21.4 kcal/mol
for QC-H2, 21.5 kcal/mol for QC-Me2 and 21.7 kcal/mol for QC-iPr2. The striking difference lies
in S‡, which is approximately constant going from QC-H2 (-31.1 e.u.) to QC-Me2 (-32.2 e.u.),
while that for QC-iPr2 is much more negative (-45.3 e.u.). At room temperature, this contributes a
difference of ca 1 kcal/mol in G‡. It turns out that the underlying reason is due to increased steric
crowding in the NBD isomer that hinders the rotation of the iPr groups while they are free to rotate
in the QC isomer. As the TS lies between the two isomers, it will experience some of the hindered
rotation and this results in the more negative S‡ for R = iPr.
Indeed, in support of our hypothesis the 1H NMR signals of NBD-iPr2 at room temperature are
much broader than those of QC-iPr2, indicating the presence of hindered rotation.39 Therefore, we
carried out variable temperature 1H NMR experiments in CDCl3 to confirm this. The spectra of a
50:50 mixture of NBD and QC isomers (Fig. 4a) clearly show several signals for the NBD isomer
at -32°C (slow rotation on NMR timescale) which are broadened with increasing temperature and
have coalesced at 45°C (fast rotation on NMR timescale). This is especially clear for the methyl
and methine protons of the iPr groups as seen in the temperature-dependent 1H NMR spectrum of
pure NBD-iPr2 (Fig. 4b). The presence of several conformers at the lower temperature is further
confirmed by chemical exchange peaks in the NOESY spectrum at -32°C that disappear at 45°C
(Fig. S1 and Fig. S2). Also, we see weak NOE peaks between the bridgehead and iPr methyl
protons and the phenyl protons for NBD but not for QC supporting the increased steric congestion
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in the NBD isomer (Fig. S3 to Fig. S6). The hindered rotation in NBD-iPr2 described here is similar
to that previously seen in 7,7-aryl-substituted norbornanes which have analogous temperature-
dependent 1H NMR spectra.40 In contrast, the 1H peaks of the QC isomer are not affected over this
1
temperature range (Fig. 4a), indicating free rotation of the iPr groups. The room-temperature H
NMR spectra of NBD-iPr2 and QC-iPr2 display the same behavior in toluene-d8 as in CDCl3,
showing that the hindered rotation is not sensitive to the solvent (Fig. S66).
The increased activation entropy of QC-iPr2 → NBD-iPr2 as compared to R = H and Me points
to a new strategy in the design of MOST materials with longer thermal half-lives, where the
activation enthalpy and entropy can potentially be tuned independently of each other. This strategy
seems particularly promising for extending the thermal stability of photo-switchable materials
without making large changes to otherwise advantageous electronic properties.
(a)
(b)
50:50
Ar Ar
Hb
Meb Meb
Hb Ha
Ha Ha
Ha
Mea Mea
Ha
Ha
45°C
45°C
35°C
25°C
15°C
5°C
-5°C
-15°C
-25°C
-32°C
-32°C
Mea
Mea
45°C
-32°C
Fig. 4 (a) 1H NMR spectra of a 50:50 mixture of NBD and QC isomers of NBD-iPr2 at temperatures
from -32°C to 45°C in CDCl3. Peaks originating from the NBD isomer are shown in orange and
1
those from the QC isomer in blue. Axis has been broken to show the regions of interest. (b) H
NMR spectra at temperatures from -32°C to 45°C in CDCl3 for methyl (red) and methine (orange)
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protons of a sample of pure NBD isomer of NBD-iPr2. “Ar” means aryl protons, “Me” means
methyl protons and “H” means tertiary protons as shown in the structures. For experimental details
and full spectra, see the ESI, section 3.
Quantum chemical calculations: To decipher the factors responsible for the experimental trends
we turned to quantum-chemical calculations. To evaluate the effect of steric bulk on the relative
energies of the NBD and QC isomers, we calculated the reaction NBD-R2/QC-R2 + 2 H2 → NBD-
H2/QC-H2 + 2 R-H. The energy release, Erel, increases by 2.9 kcal/mol from NBD-Me2 (-32.9
kcal/mol) to NBD-iPr2 (-35.8 kcal/mol), while it is nearly constant from QC-Me2 (-29.6 kcal/mol)
to QC-iPr2 (-29.9 kcal/mol), demonstrating that the decrease in energy storage capacity (calculated
value of 2.6 kcal/mol) is almost entirely due to increased steric repulsion in the NBD isomer while
the QC isomer is mostly unaffected. Consistent with this finding, the decrease in storage can also
be related to the Taft steric constant ES, although the relation is non-linear (Fig. 5).41
H
20
15
10
5
Me
iPr
Experimental
Calculated
0
tBu
-5
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Taft parameter
Fig. 5 Relation between the Taft steric parameter Es and Hstorage for NBD-R2. Experimental values
are given in black and computed values in red.
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To rationalize the trend in quantum yields we located the minimum energy conical intersections
(CIs) between the S1 and S0 states for NBD-H2 and NBD-Me2 using 2-state-average (2-SA-)
CASSCF(2in2)/6-31G(d) calculations (see ESI, section 9. for full details).42-44 It is known that
photoisomerization for P-H2 occurs efficiently after triplet sensitization with quantum yields of up
to 0.91, and only ca. 0.05 for direct irradiation.20 Donor-acceptor NBDs (DA-NBDs) with electron-
donating and electron-accepting substituents show reverse reactivity with one study giving a
quantum yield of 0.68 under direct irradiation and only 0.06-0.15 with triplet sensitization.45 As
the NBD-R2 compounds are also DA-NBDs, and as the photoreactions are performed under
oxygen-containing atmosphere with high quantum yields, it is reasonable to assume that the present
photoreactions also proceed through the singlet state. Furthermore, Goodson and co-workers
recently reported that the photoconversion of a derivative of NBD-H2 with -COOEt instead of -
COOMe substituents was not affected by the triplet quencher 1,3-cyclohexadiene.18
So what is the mechanism of the singlet state isomerization? A rhombically distorted S1/S0 CI
for P-H2 was previously located by Antol using MR-CIS.46 This CI preferentially leads back to the
NBD isomer, in agreement with experiment. However, no similar studies of DA-NBDs have been
reported yet. Yamazaki hypothesized that the switch from S1 to T1 photoreactivity from P-H2 to
DA-NBDs is due to a larger ionic character in the S1 state of the latter.45 Indeed, we find that while
the P-H2 exhibits a symmetrical, rhombically distorted CI structure with C---C distances of 1.948
Å (compared to 1.987 Å with MR-CIS)43, that for NBD-H2 has one shorter bond which is almost
fully formed (1.730 Å) while the other remains long (2.210 Å) (Fig. 6). The C---C lengths for
NBD-Me2 are 1.737 Å and 2.196 Å, while NBD-iPr2 could not be calculated due to its large size.
The NPA charges47 of S1 and S0 in the vicinity of the CI geometries support Yamazaki’s hypothesis
of larger ionic character as NBD-H2 has a larger charge polarization than P-H2 with positive charge
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on the donor side and negative charge on the acceptor side (Fig. 6).48 There are also shallow minima
on the S1 surface in between the Franck-Condon (FC) geometry and the CIs, but the depth (0.4
kcal/mol for P-H2 and 3.2 kcal/mol for NBD-H2 with 3-SA-CASSCF(2in2)) is small compared to
the excess energy from the FC point (29.0 kcal/mol for P-H2 and 30.1 kcal/mol for NBD-H2 at the
same level) and they are not likely to be populated significantly (for further discussion, including
the effect of dynamical correlation, see the ESI). An overview of the PES for NBD-H2 is given in
Fig. 7.
Fig. 6 Conical intersection geometries for P-H2 (left) and NBD-H2 (right) at the 2-SA-
CASSCF(2in2)/6-31G(d) level. NPA charges in the vicinity of the CIs are given as a sum for the
carbon atom in question and its substituents (the distinction between the S0 and S1 states is not
possible at the exact CI position).
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Fig. 7 S0 and S1 energies for NBD-H2 along the path from the optimized S0 minimum to the
optimized S1 minimum to the CI at the 3-SA-CASSCF/6-31G(d) level. The points between the
optimized structures have been obtained by linear interpolation in internal coordinates.
Characterization of the CIs reveals a dramatic difference in terms of relaxation pathways (Fig.
8a,b).49 The CI for P-H2 has one dominant relaxation direction towards the NBD isomer, which is
consistent with the very low quantum yield for direct irradiation. On the contrary, the CIs for NBD-
H2 and NBD-Me2 have plausible relaxation pathways to both the NBD and QC isomers. For P-R2
the difference in gradient of relaxation to NBD and QC (GradientCI) decreases with increasing
steric bulk, but remains large even with R = tBu and the effect on the quantum yield should be
small. For NBD-R2 on the other hand the relaxation pathways are almost equally viable.
Consequently, going from NBD-H2 to NBD-Me2 should favor relaxation to the QC isomer,
consistent with the observed increase in quantum yield. These trends are also seen for DA-R2 with
the full range of R = H, Me, iPr and tBu (Fig. S20).
Interestingly, there is an almost perfect linear correlation between Estorage and GradientCI for
both the P-R2 and DA-R2 systems with R2 values of 0.985 and 0.993, respectively (Fig. 8c),
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meaning that a lower energy storage capacity is associated with a higher quantum yield for both
systems. For NBD-R2 there are only two points but they also follow this trend. We rationalize this
finding by applying the Hammond postulate.50 By decreasing the energy difference between the
NBD and QC isomers, the TS is moved towards the NBD side of the reaction coordinate (Fig. 8d).
If the excited state potential energy surface is not affected to an equal extent, the CI empties on the
S0 surface closer to the QC side with decreasing energy difference, which leads to a more efficient
photoreaction. This explanation is corroborated by excellent correlations between calculated extent
of bond formation in the S0 TS and Estorage for both P-R2, DA-R2 and NBD-R2 (Fig. S40). It is
therefore not the increased electronic interaction between the double bonds due to steric bulk but
rather the decreased energy difference between the NBD and QC isomers which is responsible for
the increase in quantum yield.
Although dynamics should be considered to get a rigorous understanding of how the quantum
yields are affected by substitution, the present calculations show that steric bulk at the C7 position
can perturb the potential energy surfaces around the CI in a manner consistent with more favorable
relaxation to the QC isomer. It is likely that other ways of changing the relative energy of the NBD
and QC isomers, besides substitution at C7, will have similar effects on the quantum yields that
can now be predicted with the computational model given above. It would also be illuminating to
explore the effects of dynamical correlation on the CI characterization, but this was not feasible at
present.
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Fig. 8. (a) CI analysis for P-R2 (b) CI analysis for NBD-H2 and NBD-Me2 (c) Correlation between
energy difference between NBD and QC isomers and gradient difference between the relaxation
pathways to NBD and QC. (d) Effect of NBD destabilization on TS position and relaxation
pathways from the CI.  is the angular coordinate in the branching plane that lifts the degeneracy
of the CI.
Nature of the S1 state minimum: As the shallow S1 minimum (Fig. 7) could potentially affect the
quantum yield negatively if it gets too deep, we decided to investigate its nature further to see how
it can be avoided. Based on our previous interest in excited state aromaticity51 we hypothesized
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that the minimum could be stabilized by homoaromaticity52 in the S1 state with four electrons in
the cycle (aromatic according to Baird’s rule for the excited state)51. To this end, we calculated the
multicenter indices (MCIs) of the CASSCF wave functions at the S1 minima of P-H2, DA-H2 and
NBD-H2.53 The resulting values (0.032, 0.024 and 0.021) are large and approach that of the S2 state
cyclobutadiene at its D2h ground state geometry (0.049) which was previously assessed as clearly
aromatic.54 The homoaromaticity is also corroborated by qualitative S1 NICS scans which show
the characteristic minima of aromatic compounds (see the ESI).55,56 While the MCI values decrease
from P-H2 to DA-H2 and NBD-H2 the depth of the minima at the same time increases along with
,
the increasing donor-acceptor character (0.4 kcal/mol for P-H2, 2.1 kcal/mol for DA-H2 and 3.2
kcal/mol for NBD-H2 at the CASSCF level). Therefore, the less S1-homoaromatic compounds have
the deeper minima. Fortunately, the depth of the minimum is decreased going from NBD-H2 (3.2
kcal/mol) to NBD-Me2 (2.3 kcal/mol), so steric pressure is effective in suppressing this potentially
detrimental minimum. Nevertheless, it appears that stronger donor-acceptor character of the NBD
than that seen here could be detrimental to the MOST performance as it leads to a deeper minimum
from where decay would lead to the NBD isomer which would reduce the quantum yield.
MOST application: After having untangled the different factors affecting the properties along the
NBD-R2 series, we went on to evaluate their performance for MOST applications. For this purpose,
we chose NBD-iPr2 as it shows a useful half-life of 56 hours, enabling day-to-day storage. We
used a custom-made microfluidic device17,57 with an irradiated area of 4 cm2 and a channel depth
of 60 m (Fig. 9). A 50 mM solution of NBD-iPr2 in toluene was circulated through the flow cell,
and the conversion of NBD-iPr2 to QC-iPr2 was monitored by 1H NMR spectroscopy. The back-
conversion of QC-iPr2 to NBD-iPr2 releases 16.3 kcal/mol, according to DSC (vide supra). The
solar energy storage rate at the working conditions of the device is 2.4 Js-1m-2, with a solar energy
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storage efficiency MOST = 0.2% (see the ESI, section 8, for details). This efficiency could be
increased in different ways, for example by increasing the solubility (higher concentration),
increasing the path length of the collector, or decreasing the residence time (as shown in ref 9).
Design of NBD systems with more red-shifted onset of absorption should lead to higher energy
storage efficiencies,17 while the solar harvesting efficiency observed here is two orders of
magnitude higher than a previous ruthenium fulvalene based MOST system, due to a much higher
quantum yield of the NBD/QC system.9,54 Hence, it is comparable to the 0.1% and 1.1% achieved
recently by some of us with a different pair of NBDs.17 Although we use a flow device here, it has
been shown previously that NBDs can be incorporated in solid polymers films which show photo-
switching with negligible degradation after 1000 cycles.12 There is therefore potential to construct
solid-state MOST devices with NBDs that could be used when solvent is impractical.
Fig. 9 Schematic representation of the microfluidic device used to test the performance of NBD-
iPr2 for MOST applications. A solution of NBD-iPr2 in toluene-d8 was circulated through the
device at a constant flow rate, while irradiated with a lamp simulating the solar spectrum.
Experimental predictions based on the presented methodology: Can the insights gained from
the NBD-R2 series be used to design new or improved NBDs for MOST applications? In a recent
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series of studies one of us designed and synthesized MQ-H2 (Table 2) and showed its potency in a
hybrid device combining solar heating and molecular thermal storage.15,17 Using the device
described above it displayed MOST = 1.1%, clearly superior to the 0.2% of NBD-iPr2. Could
increasing the steric bulk from MQ-H2 to MQ-Me2 improve this performance even further, e.g.,
by improving the low quantum yield of 0.28? Our calculated storage enthalpies and activation
enthalpies for MQ-H2 are in good agreement with experiment, while the peak absorption calculated
by TD-DFT is too high by ca. 0.2 eV (also seen in the original article17). Upon Me-substitution, the
calculated energy storage enthalpy is lowered from 24.8 to 21.7 kcal/mol (ca 13%) while the
activation enthalpy increases only slightly (21.4 vs. 21.9 kcal/mol). The excitation energy is
lowered somewhat from 3.30 to 3.26 eV (374 vs. 380 nm). These trends are the same as seen for
NBD-R2. We located the CIs for the NBD → QC interconversion and find a steeper gradient for
relaxation to NBD than QC, in line with the quantum yield of only 0.28 (Fig. S44). For MQ-Me2,
the gradient difference becomes smaller, indicating that the quantum yield should be increased as
compared to MQ-H2. We therefore predict that MQ-Me2 will show a higher quantum yield and a
lower energy storage than MQ-H2, and future experiments will be needed to reveal which one
would overall be best for MOST applications.
Table 2. Experimental and calculated properties of MQ-R2 compounds.
Hstorage (kcal/mol) t1/2 (hours) H‡ (kcal/mol) pos (nm) Aonset (nm)
24.6 5.05 22.1 398 456

MQ-H2 exp.
0.28
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MQ-H2 calc.
-
-
24.8
21.7
-
-
21.4
21.9
374
380
-
-
MQ-Me2 calc.
Conclusions and outlook
In summary, we have shown that the quantum yields and thermal lifetimes of a series of DA-NBDs
are increased with larger steric bulk at the C7 carbon. However, the energy storage capacity
simultaneously decreases and on balance NBD-Me2 with a quantum yield of (0.86 ± 0.10), an
energy storage capacity of 16.3 kcal/mol and a thermal half-life of 7.6 hours at 25 °C is the best
alternative for short-term MOST applications. It was found that the significantly longer thermal
half-life of 56 hours for NBD-iPr2 is mainly caused by a more negative activation entropy due to
hindered rotation of the iPr groups in the TS. This points to a new strategy in the design of thermally
stable MOST compounds and other photo-switchable molecules where the activation enthalpy and
entropy could be tuned independently. The potential of the NBD-R2 compounds for MOST
applications was demonstrated by incorporation of NBD-iPr2 in a microfluidic flow reactor with a
solar energy storage efficiency MOST = 0.2%, two orders of magnitude better than a previous
ruthenium fulvalene system. We explain the increased thermal lifetime and decreased storage
capacity of NBD-Me2 and NBD-iPr2 as compared to NBD-H2 due to higher steric repulsion in the
NBD isomer than in the QC isomer. The increased quantum yields were rationalized by quantum-
chemical characterizations of the S0/S1 conical intersections. We believe that the present study
paves the way for rational design of new NBD-QC systems using computational tools, where
quantum yields, energy storage capacities and thermal lifetimes can be finely balanced for optimal
performance. Along these lines, we presented calculations showing how the low quantum yields
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of one of the currently most promising NBDs, MQ-H2, should be improved by methyl substitution
in MQ-Me2.
Acknowledgments
Prof. Adolf Gogoll is acknowledged for fruitful discussions on the NMR investigations. H.O.
acknowledges the Swedish Research Council (grants 2015-04538 and 2011-04177) for financial
support. The Swedish NMR centre at the University of Gothenburg is acknowledged for support.
K.M.P. and A.D. acknowledges the K. & A. Wallenberg foundation and the Swedish Foundation
for Strategic Research for financial support. K.B. acknowledge the Swedish Foundation for
Strategic Research and the Ragnar Söderberg foundation for financial support. O.E.B. and F. F.
acknowledge the Ministerio de Economía y Competitividad of Spain (projects CTQ2014-54306-P
and CTQ2014-59212-P) and the Generalitat de Catalunya (project 2014SGR931, Xarxa de
Referència en Química Teòrica i Computacional, and grant No. 2014FI_B 00429 to O.E.B.). R.L.
and I.F.G. acknowledge the Swedish Research Council (grants 2012-3910 and 2016-03398) for
financial support. We also thank NSC, Linköping, within the Swedish National Infrastructure for
Computing (SNIC), Sweden, for the allotment of computer time.
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TOC image
Increased steric bulk in norbornadienes with application in solar energy storage leads to
higher quantum yields, red-shifted absorption and higher thermal stability.