Molecular Solar Thermal Energy Storage Systems with Long Discharge Times Based on the Dihydroazulene/Vinylheptafulvene Couple

Article abstract of DOI:10.1002/ejoc.201801776

Molecular solar thermal energy storage (MOST) systems based on photochromic molecules that undergo photoisomerization to high-energy isomers are attractive for storage of solar energy in a closed-energy cycle. One challenge is to control the discharge time of the high-energy isomer. Here we show that incorporation of a strong acceptor substituent in the seven-membered ring of the dihydroazulene/vinylheptafulvene (DHA/VHF) couple increases the half-life of the energy-releasing VHF-to-DHA back-reaction from hours to more than a day in a polar solvent. For some derivatives, the absorption maximum of the photo-active DHA is also significantly redshifted, thereby better matching the solar spectrum. Synthetic protocols and kinetics studies are presented together with a computational study of the energy densities of the systems and excitation spectra. The computations show that the increased lifetime of the high-energy isomer is counter-balanced by a lower energy storage capacity in vacuo than for the parent system, but a slightly higher energy density than for the parent system in a polar solvent.

Full text of DOI:10.1002/ejoc.201801776

A Journal of
Accepted Article
Title: Molecular Solar Thermal Energy Storage Systems with
Long Discharge Times Based on the Dihydroazulene/
Vinylheptafulvene Couple
Authors: Josefine Mogensen, Oliver Christensen, Martin Drøhse Kilde,
Martin Abildgaard, Lotte Metz, Anders Kadziola, Martyn
Jevric, Kurt V. Mikkelsen, and Mogens Broendsted Nielsen
This manuscript has been accepted after peer review and appears as an
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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
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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: Eur. J. Org. Chem. 10.1002/ejoc.201801776
Link to VoR: http://dx.doi.org/10.1002/ejoc.201801776
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10.1002/ejoc.201801776
European Journal of Organic Chemistry
FULL PAPER
Molecular Solar Thermal Energy Storage Systems with Long
Discharge Times Based on the Dihydroazulene/Vinylheptafulvene
Couple
Josefine Mogensen,[a] Oliver Christensen,[a] Martin Drøhse Kilde,[a] Martin Abildgaard,[a] Lotte Metz,[a]
Anders Kadziola,[a] Martyn Jevric,*[a],[b] Kurt V. Mikkelsen,*[a] and Mogens Brøndsted Nielsen*[a]
Abstract: Molecular solar thermal energy storage (MOST) systems
based on photochromic molecules that undergo photoisomerization
to high-energy isomers are attractive for storage of solar energy in a
closed-energy cycle. One challenge is to control the discharge time
of the high-energy isomer. Here we show that incorporation of a
strong acceptor substituent in the seven-membered ring of the
dihydroazulene/vinylheptafulvene (DHA/VHF) couple increases the
half-life of the energy-releasing VHF-to-DHA back-reaction from
hours to more than a day in a polar solvent. For some derivatives,
the absorption maximum of the photo-active DHA is also significantly
redshifted, thereby better matching the solar spectrum. Synthetic
mainly present in solution, while the s-cis conformer is the
reactive one for the ring closure reaction. Photochromism has
also been observed in polymeric films[5] and in crystals by
reversible DHA – s-cis-VHF conversions.[6]
Removal of one of the two cyano groups at position C1
(DHA 2; Figure 1) has a remarkable effect on the VHF half-life
as the back-reaction is basically completely set on hold.[3b] This
infinitely long energy storage time is in principle convenient if the
back-reaction could in some way be triggered. We have found
that some metal ions do promote the back-reaction of simple
VHFs,[7] but in the case of the mono-cyano substituted VHF,
decomposition occurred in time instead of the desired ring
closure reaction.
Another approach is to attach substituent groups in the
seven-membered ring of DHA. Indeed, structure-property
relationships have shown that introduction of an aryl group with
an electron-withdrawing substituent at position C7 of DHA
causes a longer lifetime of the VHF.[8] Thus, the VHF of the p-
cyanophenyl-substituted DHA 3 (Figure 1) has a half-life of 641
min in acetonitrile at 25 oC. We imagine, however, a more
significant rate decrease of the thermal back-reaction by placing
an electron-withdrawing cyano group directly at the DHA/VHF
system without a bridging phenylene unit. Moreover, a strong
dicyanomethylene acceptor could be advantageous for this
purpose as well as for possibly inducing a redshift of the DHA
absorption maximum, depending on its position on the seven-
membered ring. For these reasons, we decided to target DHA
derivatives 4-8 with an acceptor at either C6 or C7 of DHA. Here
we present synthesis, optical and switching studies, as well as
theoretical studies of these acceptor-functionalized DHAs/VHFs.
protocols and kinetics studies are presented together with
a
computational study of the energy densities of the systems and
excitation spectra. The computations show that the increased
lifetime of the high-energy isomer is counter-balanced by a lower
energy storage capacity in vacuum than for the parent system, but a
slightly higher energy density than for the parent system in a polar
solvent.
Introduction
Photochromic molecules that upon irradiation undergo
a
conversion from a low-energy isomer to a high-energy isomer
have attracted considerable interest in recent years as a
potential way of storing solar energy.[1] One of several
challenges is to achieve high-energy isomers that do not quickly
return to the original isomer by a thermal relaxation. One
candidate, when properly substituted, for molecular solar
thermal
energy
storage
(MOST)
systems
is
the
dihydroazulene/vinylheptafulvene (DHA/VHF) couple (Scheme
1) that was first reported by Daub and co-workers[2] and recently
explored by us in the context of solar energy storage.[3] DHA 1
(R = Ph; see Figure 1) undergoes a photoisomerization to its
corresponding VHF that in time returns to the DHA; in
acetonitrile at 25 oC with a half-life of 218 min.[4] The s-trans
conformer of VHF is usually the most stable and therefore
NC
1
light
heat
8
4
R
CN
NC
CN
R
7
CN
CN
s-trans VHF
2
6
R
5
3
DHA
s-cis VHF
Scheme 1. Dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermoswitch.
Numbering of the DHA core is shown.
[a]
J. Mogensen, O. Christensen, M. D. Kilde, M. Abildgaard, Dr. M.
Jevric, L. Metz, Prof. Dr. A. Kadziola, Dr. M. Jevric, Prof. Dr. K. V.
Mikkelsen, Prof. Dr. M. B. Nielsen
Results and Discussion
Synthesis and X-Ray Crystallography. A cyano group was
introduced at position C7 via the protocol outlined in Scheme 2.
First, DHA 1 was subjected to a well-established bromination-
elimination protocol, providing the bromo-functionalized
intermediate 9.[9] We have previously shown that DHA
derivatives can be cyanated by a Stille coupling[3d] and therefore
9 was treated with Bu3SnCN and Pd(PPh3)4, which provided the
target product 4. The yield of 4 was rather low, and the toxicity of
stannanes makes alternative protocols attractive. We attempted
Department of Chemistry
University of Copenhagen
Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
E-mail: jevric.m@gmail.com, kmi@chem.ku.dk, mbn@chem.ku.dk
Homepage: https://chem.ku.dk/research_sections/solarenergy/
Dr. M. Jevric
Current address:
Flinders Centre for Nanoscale Science and Technology
Flinders University
Sturt Road, Bedford Park, Adelaide 5042, Australia
[b]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201801776
European Journal of Organic Chemistry
FULL PAPER
a Rosenmund-von Braun[10] reaction by treating the bromination-
elimination product formed from 1 with CuCN in DMF at 120 oC,
but after several attempts this reaction only gave an unidentified
cyano-substituted DHA (different from the 7-substituted one) in a
poor yield of 3%. Fully unsaturated azulenes were formed as
side-products in the reaction due to elimination of HCN.
Attempts of using Zn(CN)2 and a Pd catalyst turned out
unsuccessful, providing unidentified products. Cyanation of the
DHA at position C7 is thus a rather challenging task, and the
method using Bu3SnCN and Pd(PPh3)4 is at present the most
reliable.
Compound 4 could be crystallized from hot ethanol, and
the structure was confirmed by X-ray crystallography (Figure 2).
The VHF was obtained by irradiation of a solution of 4 in CD3CN
until complete conversion occurred using a long-wave UV lamp
followed by removal of the solvent under a stream of N2. The
residue was then crystallized from CH2Cl2/heptane, and the
structure is shown in Figure 2. The structure obtained from the
investigated single crystal corresponds to the E-VHF isomer (in
its s-trans conformation) where also Z-to-E isomerization at the
exocyclic fulvene double bond has occurred (Figure 3). Such
isomerization is known to occur thermally for VHF in polar
solvents,[9] and it can also be induced by light.[11] Both isomers
were indeed present in solution according to NMR spectroscopy.
NC
H
CN
Ph
CN
Ph
DHA 1
DHA 2
years
VHF half-life:
(MeCN, 25 oC)
218 min
NC
NC
New targets
CN
Ph
NC
NC
CN
Ph
DHA 3
641 min
DHA 4
O
NC
CN
Ph
NC
CN
Ph
O
DHA 5
DHA 6
NC
CN
NC
NC
CN
CN
Ph
NC
Figure 2. Molecular structures of DHA 4 (top; CCDC 1877316) and the
corresponding E-VHF in the s-trans conformation (bottom; CCDC 1877315).
CN
Ph
NC
DHA 7
DHA 8
Ph
Ph
NC
CN
CN
Figure 1. Selection of known and new acceptor-functionalized DHAs as well
as DHA with only one cyano group at C1. For previously studied DHAs, the
corresponding VHF half-lives in acetonitrile are listed.
CN
CN
Z-VHF of 4
E-VHF of 4
1) Br2
CH2Cl2, -78 oC
Figure 3. Structure of Z-VHF (s-trans conformer) that can be generated by
ring-opening of 4 and its E-isomer (s-trans conformer) that is formed by
rotation around the exocyclic fulvene bond.
NC
Br
CN
Ph
1
2) LiHMDS
THF, 0 oC
9
Our next objective was to introduce acetyl groups at the
seven-membered ring. We have previously found that tert-butyl
thioethers can conveniently be converted into thioacetates under
mild conditions by the action of acetyl chloride and bismuth(III)
triflate.[12, 13] This reaction was found to be somewhat sensitive to
the presence of a terminal alkyne that could be converted into a
vinyl triflate. We imagined that related conditions could possibly
be employed to convert the known[9b] 7-ethynyl-substituted DHA
10 into the product 5. Moreover, Lui et al.[14] have found that
terminal alkynes can be converted to acetyl groups by triflic acid
Bu3SnCN
NC
CN
NC
Pd(PPh3)4
Ph
PhH, reflux
4 (10%)
Scheme 2. Synthesis of 7-cyano-substituted DHA 4. LiHMDS
= lithium
hexamethyldisilazide.
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10.1002/ejoc.201801776
European Journal of Organic Chemistry
FULL PAPER
in trifluoroethanol. Treating 10 with bismuth(III) triflate (0.2
equiv.) in trifluoroethanol (and a few drops of water) at elevated
temperature provided an almost equal mixture of the 7- and 6-
substituted isomers 5 and 6 in an overall good yield (Scheme 3).
As the reaction mixture was shielded from light, the DHA must
have opened thermally to the Z-VHF, which can then return to
the original DHA or to the 6-substituted isomer via an E/Z-
isomerization at the fulvene double bond.
NC
CN
CN
Ph
11
Figure 4. Azulene product isolated after treatment of 5 with malononitrile in
AcOH/HMDS at 75 oC.
To incorporate the dicyanovinyl group, a Knoevenagel
condensation between 5 and malononitrile in toluene with
ammonium acetate and glacial acetic acid was first attempted.
Yet, after reflux for 24 hours, there was no sign of product
formation. Another set of conditions with a buffer solution of
glacial acetic acid and hexamethyldisilazane (HMDS) as solvent
UV-Vis Absorption and Switching Studies. The new acceptor-
substituted DHAs were all photo-active and underwent
photoisomerization (by irradiation at 365 nm) to their
corresponding VHFs; yet, full conversion of 8 into its VHF was
not accomplished (hence being less photo-active than 7).
Longest-wavelength absorption maxima in MeCN of the DHA
and VHF forms are listed in Table 1 (ignoring some long-
wavelength absorptions of the VHF of 7), while the absorption
spectra are shown in Figure 5. It transpires that the 6-substituted
DHAs 6 and 8 both exhibit a redshifted absorption relative to
their corresponding 7-substituted isomers (5 and 7). The 6-
substituted isomers have the substituent group in linear
conjugation with the largest part of the DHA core (C2-C6), while
the substituent is only in linear conjugation with the C7-C8
double bond for the 7-substituted isomers. For the VHFs we
cannot exclude some E/Z isomerization under the conditions as
this may occur both thermally and photochemically as
mentioned above and previously established for VHF
o
was then attempted, with stirring at 75 C for 1-2 days. Under
these conditions, the Knoevenagel reaction worked, but the
conditions also resulted in elimination of HCN and formation of
compound 11 with a fully unsaturated azulene core (Figure 4);
this compound was isolated in a yield of 72%. Although not the
purpose of this work, the synthetic route presents an interesting
azulene functionalization methodology.[15] Next, we turned to a
TiCl4-catalyzed Knoevenagel condensation in chloroform at 55-
60 oC with pyridine as base, and, gratifyingly, product 7 could
now be isolated, albeit in low yield (9%) (together with unreacted
starting material). The reaction conditions worked, however,
significantly better on 6 as substrate that was successfully
converted into product 8 in a yield of 62%. A shorter reaction
time had to be used in the synthesis of 7 as decomposition of
the product seemed to occur according to TLC analysis.
1
derivatives.[9,11] Indeed, H-NMR spectroscopic studies revealed
formation of a pure VHF isomer upon irradiation in CDCl3, but a
mixture of VHF isomers in CD3CN (see SI, Figures S5-S7).
NC
CN
Ph
Table 1. UV-Vis absorption maxima (λmax), molar absorptivities (ε) in
brackets, rate constants (k) and half-lives (t1/2) for the VHF ring closure
reactions at 25 oC in MeCN.
10
CF3CH2OH
80 oC
Bi(OTf)3, H2O
k / 10-4 min-1
t1/2 / 103 min
DHA
VHF
O
λmax / nm (ε /
103 M-1 cm-1)
λmax / nm (ε /
103 M-1 cm-1)
NC
NC
CN
Ph
CN
Ph
O
+
4[a]
5[b]
6[b]
7[a]
8[a]
360 (14.6)
358 (12.3)
372 (18.1)
337 (13.8)
394 (19.4)
433 (26.9)
454 (24.9)
451 (24.2)
451 (25.6)
427-442[c]
5.4
6.4
6.5
4.4
4.2
1.3
1.1
1.1
1.6
1.7
5 (21%)
6 (23%)
TiCl4, Py
CHCl3, 58 oC
TiCl4, Py
NC
NC
CN
NC
CN
CHCl3, 55 oC
CN
NC
CN
CN
NC
NC
CN
Ph
[a] Rate constant determined by extrapolation to 25 oC from measurements
at various temperatures. [b] Rate constant determined as an average of two
measurements at 25 oC. [c] Mixture of DHA and VHF.
Ph
7 (9%)
8 (62%)
Scheme 3. Synthesis of acceptor-functionalized DHAs. Py = pyridine.
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10.1002/ejoc.201801776
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FULL PAPER
Computations. Energy densities. For 1 an energy density of
0.11 MJ kg-1 was previously reported in vacuum; the number
corresponds to the Gibbs free energy difference between DHA
and VHF in its most stable s-trans conformation.[3e] The energy
densities in MJ kg-1 for the new couples were calculated in
vacuum and in solvents of different polarities using the DFT
functional M06-2X and the basis set 6-311+G(d), and the results
are listed in Table 2. Solvent effects were included using the
integral equation formalism variant of the polarizable continuum
model (IEFPCM). The unit MJ kg-1 was chosen to balance any
increase in Gibbs free energy difference by an undesirable
increase in molecular weight. In Figure 6, energy densities are
plotted relative to that of the DHA/VHF couple 1 in the various
media. First, as previously noted,[3a,e] we see from Table 2 that
the energy density of 1 decreases when increasing the polarity
of the solvent. This decrease is also observed for the new
compounds, but to a less significant degree than for the parent
system. In actual fact, while the energy densities of compounds
4-8 are all lower than that of 1 in vacuum, the energy densities
of 4, 5, 6, and 8 are all higher in dichloromethane, ethanol, and
acetonitrile than that of 1, albeit all these compounds have a
higher molecular weight than 1. The VHF of 1 has significant
zwitterionic character on account of the electron-withdrawing
cyano substituents at C1, and this form is stabilized in a polar
solvent, thereby decreasing the DHA-VHF energy difference. By
placing an electron-withdrawing substituent at the seven-
membered ring, this electronic push-pull effect is to some extent
counter-balanced, which would explain why the new derivatives
do not experience the same unfavorable decrease in energy
density when proceeding from vacuum to a polar medium. It
should be emphasized that the energy differences are all very
small, but at least the energy density in polar solvents is not
compromised by the acceptor substitution.
Figure 5. UV-Vis absorption spectra of DHAs 4 (green curve), 5 (blue solid
curve), 6 (blue broken curve), 7 (red solid curve), and 8 (red broken curve;
not full conversion) on top, and their corresponding VHFs at bottom (colors
correspond to the DHA precursors). Solvent: MeCN.
The thermal VHF-to-DHA back-reactions were followed by
UV-Vis absorption spectroscopy (first-order decay of VHF
absorption) at various temperatures in MeCN (see SI), and the
rate constants at 25 oC were determined by extrapolation for 4, 7,
and 8. For 5 and 6, the rate constants were determined as an
o
average of two measurements at 25 C. The data are listed in
Table 2. Absolute energy densities (MJ kg-1) of the DHA/s-trans VHF couples
(difference in Gibbs free energy of the two isomers) in various media.[a]
Calculated using M06-2X/6-311+G(d) and IEFPCM.
Table 1. Only small differences are seen between the two VHF
isomers generated from each DHA isomer. As mentioned above,
we cannot exclude that both isomers are present due to
isomerization by rotation around the exocyclic fulvene bond, but
as the decays were well fitted by a single exponential function,
the two VHF isomers undergo ring closures with similar rates.
For all derivatives, a significant increase in the VHF half-life has
been achieved in comparison to the VHFs of 1 and 3. We also
observe a clear trend in regard to the rate of ring closure and the
character of the substituent: the half-life increases as the
electron-withdrawing character increases in the progression
C(O)Me, CN, CMe(C(CN)2); the electron-withdrawing character
can be represented by Hammett substituent constants: σp = 0.50
for C(O)Me, σp = 0.66 for CN, σp = 0.84 for CH(C(CN)2).[16]
The redshifted absorption of 8 in combination with the
increased VHF lifetime is particularly important in the context of
MOST systems, while its small photoactivity is a drawback. One
question remaining is how the functionalizations influence the
energy densities. To answer this question, we subjected the
compounds to a computational study.
vacuum
0.108
0.097
0.098
0.102
0.085
0.096
CH
PhMe
0.085
0.087
0.080
0.104
0.075
0.084
CH2Cl2
0.068
0.076
0.076
0.081
0.059
0.076
EtOH
0.062
0.072
0.071
0.078
0.055
0.073
MeCN
0.061
0.071
0.072
0.077
0.055
0.074
1
4
5
6
7
8
0.088
0.088
0.083
0.091
0.074
0.085
[a] CH = cyclohexane.
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Relative Energy Storage Density - M06-2X
Vacuum
Cyclohexane
Toluene
Dichloromethane
Ethanol
Acetonitrile
Conclusions
0.02
In conclusion, a selection of acceptor-substituted DHAs was
prepared using DHA 1 as starting material. Conveniently, the
acetyl functionality could be introduced by hydrolysis of a
terminal alkyne substituent. Knoevenagel condensation with
malononitrile was more challenging, but ultimately we found
good conditions that allowed synthesis of the 6-substituted
derivative in good yield, while the 7-substituted derivative was
obtained in rather low yield. Introduction of a cyano group
directly at C7 was also rather challenging, but achieved by
cyanation of a bromo-substituted precursor.
0.01
0.00
−0.01
−0.02
4
5
6
7
8
Compound
Figure 6. Calculated energy storage densities (Gibbs free energies) relative to
the DHA/VHF system 1 in vacuum (black) and various solvents (shown in
colors) (M06-2X/6-311+G(d); IEFPCM); from left to right, compounds 4, 5, 6, 7,
and 8.
By attaching the acceptor substituent directly to the seven-
membered ring rather than via a phenylene unit, we find that the
rate of the VHF-to-DHA back-reaction drops significantly, which
is of particular importance in the context of photochromic
molecules for solar energy storage. The VHF-to-DHA conversion
is in general fastest in polar solvents,[4] and the increase of the
half-life to be on the timescale of a day in acetonitrile is therefore
an important step towards MOST systems based on DHA/VHF.
For derivatives with the substituent at position C6 of DHA, we
find at the same time a convenient redshift in the characteristic
DHA absorption. This redshift was also found from computations,
which show that the CAM-B3LYP/6-311+G(d) level of theory
provides excellent agreement with experimental DHA and VHF
absorptions, and it is thus a convenient method for predicting
absorptions of this couple.
Most of the new acceptor-substituted systems exhibit a
slightly higher energy density (Gibbs free energy difference
between DHA and s-trans VHF forms) than the parent system in
polar solvents despite the increased molecular weights of these
compounds. Thus, the acceptor substitution is not in disfavor of
the energy storage capacity. It is clear, however, that the energy
density of each of these systems is very small. It may be slightly
increased by allowing for example two DHA units to share the
same dicyanovinyl unit introduced in the seven-membered ring –
thereby reducing the overall molecular weight per DHA unit. Yet,
the structural modifications presented here should be combined
with others that can more strongly increase the energy densities,
such as for example our previous use of loss of aromaticity of
benzo-fused DHAs upon DHA-to-VHF conversion.[3d,f]
Excitation spectra. The excitation spectra of DHAs and VHFs of
7 and 8 in MeCN were calculated at the CAM-B3LYP/6-
311+G(d) level of theory using linear-response Time-Dependent
Density Functional Theory (TD-DFT). Solvent effects were
included using IEFPCM. The calculated absorption spectra are
shown in Figure 7. The calculations reveal a significant redshift
(41 nm) in the DHA absorption when proceeding from 7 (348
nm) to 8 (389 nm), in agreement with the experimental findings
(where a redshift of 57 nm was observed). For the VHF of 7, we
find absorptions at 445 nm (s-cis conformer) and 450 nm (s-
trans conformer), and for the VHF of 8, absorptions at 441 nm
(s-cis conformer) and 435 nm (s-trans conformer). The
calculations thus support the small blueshift of the VHF of 8
relative to that of 7 that was inferred from the experimental
studies, but with some ambiguity due to the lack of full DHA-to-
VHF conversion.
×105
DHA
DHA 7
s-cis-VHF
s-trans-VHF
0.8
0.6
0.4
0.2
0.0
×105
Experimental Section
DHA 8
0.8
0.6
0.4
0.2
0.0
General Methods. All solvents and reagents were purchased from
chemical suppliers and used as received. DHAs 1[4] and 10[9b] were
prepared according to literature methods. All photochromic compounds
were handled in the dark at all times except during light-induced ring
opening. All glassware containing photochromic compounds was
wrapped in tin foil. Thin layer chromatography (TLC) was performed in
the dark, and carried out on commercially available pre-coated aluminum
plates (silica 60) with fluorescence indicator. A color change from yellow
to red or orange upon irradiation by UV light (365 nm) indicates the
presence of DHA. Purification by flash column chromatography was
performed on silica gel (40-63 µm). CDCl3 for NMR spectoscopic studies
was passed through activated aluminum oxide before use. NMR spectra
were acquired on a Bruker 500 MHz instrument with cryoprobe; the 1H
250
300
350
400
450
500
Wavelength (nm)
Figure 7. Calculated (TD-DFT) excitation spectra in MeCN for DHAs 7 (top)
and 8 (bottom) and their corresponding s-cis and s-trans VHFs. Method: CAM-
B3LYP/6-311+G(d). The spectra were calculated assuming Gaussian band
shapes of the absorption peaks with a standard deviation of 0.4 eV.
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10.1002/ejoc.201801776
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NMR and 13C NMR spectra are referenced to the residual solvent peak
(CDCl3 δH = 7.26 ppm, δC = 77.16 ppm). Infrared spectroscopy (IR) data
were obtained on a Bruker Alpha FT-IR spectrometer equipped with a
Platinum ATR single reflection diamond module (ATR = attenuated total
reflectance); samples were loaded by evaporation from a suitable solvent.
The most characteristic absorption due to functionalization in the seven-
membered ring is listed for compounds 4-8, while full spectra are listed in
the SI. High Resolution Mass Spectrometry (HRMS) was performed on a
MALDI-FT-IR instrument equipped with a 7 T magnet, using dithranol as
matrix. Melting points are uncorrected. Elemental Analysis was
performed at London Metropolitan University.
The phases were separated and the aqueous phase was extracted with
Et2O (50 mL). The combined organics were dried over MgSO4, filtered
and the solvent removed by rotary evaporation. The crude material was
dissolved in benzene (150 mL) and the solution sparged with argon. To
this degassed solution was added Bu3SnCN (354 mg, 1.12 mmol),
Pd(PPh3)4 (122 mg, 0.11 mmol) and the resulting solution heated to
reflux point for 16 h. The solvent was removed in vacuo and the residue
subjected to flash column chromatography (EtOAc/toluene 1:49) to afford
the title compound (29 mg, 10%) as an off-yellow colored solid. Rf = 0.46
(EtOAc/toluene 1:19). M.p. = 173 °C. 1H NMR (500 MHz, CDCl3): δ =
7.76-7.74 (m, 2H), 7.53-7.48 (m, 3H), 6.92 (s, 1H), 6.82 (ddd, J = 11.4,
6.4, 0.9 Hz, 1H), 6.52 (d, J = 11.4 Hz, 1H), 6.49 (d, J = 4.9 Hz, 1H), 6.43
(dd, J = 6.4, 1.8 Hz, 1H), 3.83 (dd, J = 4.9, 1.8 Hz, 1H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 142.83, 140.06, 134.63, 131.25, 131.06, 130.91,
129.78, 129.61, 127.41, 126.62, 120.75, 117.18, 114.28, 114.06, 112.20,
50.94, 44.75 ppm. IR: ν = 2223 cm-1 (C≡N – 7-membered ring). Analysis
calcd (%) for C19H11N3: C 81.12, H 3.94, N 14.94; found: C 80.85, H 3.69,
N 14.87. X-ray crystals were obtained from an evaporating ethanol
solution.
UV-Vis Absorption Spectroscopy and Switching Studies. All the UV-
Vis experiments were performed in MeCN. The recorded absorption
spectra of the pure DHAs and the absorption spectra resulting from the
light-induced ring openings, using an UV lamp (365 nm), were performed
in a 1-cm quartz cuvette at 25 oC until full conversion of the DHA was
achieved (yet, complete conversion of DHA 8 was not achieved). The
back-reactions for the VHFs of 5 and 6 were followed at 25 oC in 1-cm
quartz cuvettes. After the ring openings of DHAs 4, 7 and 8, the solutions
were transferred to glass ampules, which were then sealed, and the back
reactions were followed at various elevated temperatures (see SI). All the
absorption spectra were recorded in the wavelength range from 800 to
200 nm on a Varian Cary 50 UV-Vis Spectrophotometer.
7-Acetyl-2-phenyl-1,1(8aH)-dicarbonitrile (5) and 6-Acetyl-2-phenyl-
1,1(8aH)-dicarbonitrile (6). Compound 10 (48.5 mg, 0.17 mmol) was
suspended in 2,2,2-trifluoroethanol (5 mL), after which 4 drops of H2O
and Bi(OTf)3 (23 mg, 0.035 mmol) were added. The suspension was then
stirred for 48 h at 80 oC. The reaction mixture was diluted with CH2Cl2
(100 mL), washed with H2O (2 x 50 mL) and brine (50 mL), dried over
MgSO4, filtered, and the solvent was removed in vacuo. The residue was
subjected to flash column chromatography (SiO2, EtOAc/toluene 1:19),
which gave 5 (11 mg, 21%) as an orange solid and 6 (12 mg, 23%) as a
yellow solid. Compound 5: Rf = 0.50 (EtOAc/toluene 1:9). M.p. = 51-53
oC. 1H NMR (500 MHz, CDCl3): δ = 7.78-7.74 (m, 2H), 7.52-7.46 (m, 3H),
7.11 (d, J = 11.7 Hz, 1H), 6.92 (s, 1H), 6.76 (dd, J = 11.7, 6.2 Hz, 1H),
6.70 (d, J = 4.7 Hz, 1H), 6.38 (dd, J = 6.2, 1.6 Hz, 1H), 3.80 (dd, J = 4.7,
1.6 Hz, 1H), 2.46 (s, 3H) ppm. 13C NMR (126 MHz, CDCl3): δ = 196.91,
141.38, 139.44, 138.77, 132.58, 131.70, 130.59, 130.22, 129.51, 127.92,
126.63, 126.47, 121.23, 114.74, 112.81, 50.62, 45.03, 26.54 ppm. IR: ν =
1678 cm-1 (C=O). HRMS (MALDI+, FT-ICR, dithranol): m/z = 321.09976
[M+Na+], calcd for (C20H14N2ONa+): 321.09983. Compound 6: Rf = 0.43
Calculations. Density Functional Theory (DFT) calculations were
performed using Gaussian09.[17] The DFT functionals used were M06-
2X[18] and CAM-B3LYP[19], and solvents were modeled using the Integral
Equation Formalism Polarizable Continuum Model (IEFPCM).[20] The
DHA and VHF structures show a significant degree of conformational
freedom. For this reason, a conformer search utilizing a genetic algorithm
was performed to find the lowest energy conformers. The program
OpenBabel version 2.4.1 was used to generate the conformers.[21] For
each DHA/VHF structure, up to 500 conformers were generated and
geometry-optimized using CAM-B3LYP/D95V level of theory.[22] Unique
conformers were then selected based on having a unique geometry,
(evaluated using the Kabsch algorithm with a threshold of 0.5 Å) and a
unique dipole moment (requiring a difference of at least 0.1D between
unique conformers).[23] Finally, for each group of conformers, the one
with the lowest energy E0 is given a Boltzmann factor of 1. The
Boltzmann factor F = exp(-(E0- En)/kBT)) for every conformer in this group
compared to the lowest-energy conformer was then calculated. If F <
0.001, the conformer was eliminated, as it has a low probability of
appearing. The remaining unique, significant conformers were then
optimized using the chosen functionals and the 6-311+G(d) basis set,
both in vacuum and in the chosen solvents.
o
(EtOAc/toluene 1:9). M.p. = 165-167 C. 1H NMR (500 MHz, CDCl3): δ =
7.80-7.76 (m, 2H), 7.53-7.47 (m, 4H), 6.97 (s, 1H), 6.89 (dd, J = 10.5, 1.9
Hz, 1H), 6.50 (dd, J = 6.8, 1.9 Hz, 1H), 5.97 (dd, J = 10.5, 3.9 Hz, 1H),
3.74 (dt, J = 3.9, 1.9 Hz, 1H), 2.47 (s, 3H) ppm. 13C NMR (126 MHz,
CDCl3): δ = 197.76, 144.30, 143.81, 139.72, 136.39, 131.71, 130.98,
130.11, 129.57, 126.76, 125.84, 120.20, 119.60, 114.77, 112.44, 51.03,
45.19, 26.39 ppm. IR: ν = 1666 cm-1 (C=O). HRMS (MALDI+, FT-ICR,
dithranol): m/z
=
321.09980 [M+Na+], calcd for (C20H14N2ONa+):
321.09983.
The excited states and oscillator strengths of the structures were
obtained using linear-response Time-Dependent Density Functional
Theory (TD-DFT). 15 electronic excitations were calculated for each
structure, and the UV-VIS spectra were calculated assuming Gaussian
band shapes of the absorption peaks with a standard deviation of σ = 0.4
eV.
7-(1,1-Dicyanoprop-1-en-2-yl)-2-phenylazulene-1,1(8aH)-
dicarbonitrile (7). Compound 5 (91 mg, 0.30 mmol) was heated to 58 oC
in CHCl3 (50 mL), and malononitrile (233 mg, 3.53 mmol), pyridine (0.07
mL, 0.9 mmol), and titanium(IV)chloride (0.5 mL, 4.6 mmol) were added.
After 2 h, additionally malononitrile (232 mg, 3.51 mmol), pyridine (0.07
mL, 0.9 mmol), and titanium(IV)chloride (0.5 mL, 4.6 mmol) were added,
and the reaction mixture was heated for 2 more h. The reaction mixture
was diluted with CH2Cl2 (100 mL) and poured on ice/water (150 mL). The
aqueous phase was extracted with CH2Cl2 (4 × 100 mL), and the organic
phases were combined, dried over MgSO4, filtered, and the solvent was
removed in vacuo. The residue was subjected to flash column
chromatography (SiO2, EtOAc/ THF/heptane 7:30:63 -> 10:35:55), which
gave 7 (10 mg, 9%) as an orange solid in addition to unreacted starting
material (15 mg). Rf = 0.17 (EtOAc/THF/heptane 7:30:63). M.p. = 146-
148 oC. 1H NMR (500 MHz, CDCl3): δ = 7.77-7.74 (m, 2H), 7.53-7.47 (m,
3H), 6.93 (s, 1H), 6.88 (dd, J = 11.4, 6.2 Hz, 1H), 6.46 (d, J = 11.4 Hz,
2-Phenylazulene-1,1,7(8aH)-tricarbonitrile (4). To a stirring solution of
DHA 1 (256 mg, 1.00 mmol) in CH2Cl2 (25 mL) at -78 °C was added
dropwise a solution of Br2 (1.28 mL, 0.78 M in CH2Cl2, 1.00 mmol) and
resulting reaction mixture stirred for a further 30 min. The solvent was
removed under reduced pressure and the residue dissolved in dry THF
(30 mL) and the vessel cooled in an ice bath. To this solution was added
dropwise a solution of LiHMDS (1.1 mL, 1.0 M in toluene, 1.1 mmol),
generating the bromide 9, and the contents allowed to stir 2 h, where the
ice bath was removed after the first hour. Saturated aqueous NH4Cl (10
mL) was added followed by dilution with H2O (50 mL) and Et2O (50 mL).
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801776
European Journal of Organic Chemistry
FULL PAPER
1H), 6.42 (dd, J = 6.2, 1.4 Hz, 1H), 6.06 (d, J = 4.8 Hz, 1H), 3.79 (dd, J =
4.8, 1.4 Hz, 1H), 2.49 (s, 3H) ppm. 13C NMR (126 MHz, CDCl3): δ =
174.43, 142.64, 141.20, 136.54, 134.98, 131.24, 130.89, 129.98, 129.57,
126.74, 126.60, 122.54, 120.60, 114.54, 112.49, 112.06, 111.82, 87.11,
50.78, 44.78, 24.06 ppm. IR: ν = 2230 cm-1 (C=C(C≡N)2)). HRMS
Energy Environ. Sci. 2012, 5, 8534-8537; d) T. J. Kucharski, N. Ferralis,
A. M. Kolpak, J. O. Zheng, D. G. Nocera, J. C. Grossman, Nat. Chem.
2014, 6, 441-447; e) K. Moth-Poulsen, in Organic Synthesis and
Molecular Engineering (Ed. M. B. Nielsen), Wiley, Hoboken, USA, 2014,
pp. 179-196; f) A. Lennartson, A. Roffey, K. Moth-Poulsen, Tetrahedron
Lett. 2015, 56, 1457-1465; g) D. Zhitomirsky, E. Cho, J. C. Grossman,
Adv. Energy Mater. 2015, 1502006; h) E. N. Cho, D. Zhitomirsky, G. G.
D. Han, Y. Liu, J. C. Grossman, ACS Appl. Mater. Interfaces 2017, 9,
8679-8687; i) J. Gurke, M. Quick, N. P. Ernsting, S. Hecht, Chem.
Commun. 2017, 53, 2150-2153; j) M. Mansø, A. U. Petersen, Z. Wang,
P. Erhart, M. B. Nielsen, K. Moth-Poulsen, Nat. Commun. 2018, 9:
1945.
(MALDI+, FT-ICR, dithranol): m/z
(C23H14N4Na+): 369.11107.
=
369.11125 [M+Na+], calcd for
6-(1,1-Dicyanoprop-1-en-2-yl)-2-phenylazulene-1,1(8aH)-
dicarbonitrile (8). Compound 6 (51 mg, 0.17 mmol) was heated to 55 oC
in CHCl3 (25 mL), and malononitrile (126 mg, 1.91 mmol), pyridine (0.04
mL, 0.5 mmol), and titanium(IV)chloride (0.03 mL, 0.3 mmol) were added.
After 4.5 h, additionally malononitrile (123 mg, 1.86 mmol), pyridine (0.04
mL, 0.5 mmol), and titanium(IV)chloride (0.03 mL, 0.3 mmol) were added,
and the reaction mixture was heated overnight. The reaction mixture was
diluted with CH2Cl2 (75 mL) and poured on ice/water (150 mL). The
aqueous phase was extracted with CH2Cl2 (3 × 75 mL). The organic
phases were combined, dried over MgSO4, filtered, and the solvent was
removed in vacuo. The residue was subjected to flash column
chromatography (SiO2, EtOAc/toluene 1:9), which gave 8 (36 mg, 62%)
as an orange solid. Rf = 0.43 (EtOAc/toluene 1:9). M.p. = 169-170 oC. 1H
NMR (500 MHz, CDCl3): δ = 7.80-7.77 (m, 2H), 7.54-7.49 (m, 3H), 7.02
(d, J = 7.0 Hz, 1H), 6.97 (s, 1H), 6.49 (dd, J = 7.0, 1.7 Hz, 1H), 6.38 (d, J
= 10.4 Hz, 1H), 6.03 (dd, J = 10.4, 4.0 Hz, 1H), 3.82 (dt, J = 4.0, 1.7 Hz,
1H), 2.53 (s, 3H) ppm. 13C NMR (126 MHz, CDCl3): δ = 174.63, 144.51,
144.08, 138.14, 134.26, 131.43, 131.22, 129.91, 129.62, 126.83, 124.89,
121.52, 119.26, 114.46, 112.68, 112.52, 112.35, 85.32, 50.88, 45.13,
22.89 ppm. IR: ν = 2227 cm-1 (C=C(C≡N)2)). HRMS (MALDI+, FT-ICR,
dithranol): m/z = 369.11104 [M+Na+], calcd for (C23H14N4Na+): 369.11107.
[2]
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1984, 23, 960-961; b) H. Görner, C. Fischer, S. Gierisch, J. Daub, J.
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a) S. T. Olsen, J. Elm, F. E. Storm, A. N. Gejl, A. S. Hansen, M. H.
Hansen, J. R. Nikolajsen, M. B. Nielsen, H. G. Kjaergaard, K. V.
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Elm, S. T. Olsen, A. N. Gejl, F. E. Storm, B. N. Frandsen, A. B. Skov, M.
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120, 9782-9793; f) Z. Wang, J. Udmark, K. Börjesson, R. Rodrigues, A.
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ChemSusChem 2017, 10, 3049-3055; g) A. B. Skov, J. F. Petersen, J.
Elm, B. N. Frandsen, M. Santella, M. D. Kilde, H. G. Kjaergaard, K. V.
Mikkelsen, M. B. Nielsen, ChemPhotoChem 2017, 1, 206-212; h) M. D.
Kilde, P. G. Arroyo, A. S. Gertsen, K. V. Mikkelsen, M. B. Nielsen, RSC
Adv. 2018, 8, 6356-6364; i) N. Ree, M. H. Hansen, A. S. Gertsen, K. V.
Mikkelsen, J. Phys. Chem. A. 2017, 121, 8856-8865.
2-(1-(3-Cyano-2-phenylazulen-5-yl)ethylidene)malononitrile
(11).
Compound 5 (53 mg, 0.18 mmol) and malononitrile (106 mg, 1.60 mmol)
were dissolved in glacial acetic acid (7 mL), after which a solution of
HMDS (5 mL) and glacial acetic acid (13 mL) was added. The reaction
mixture was then heated to 75 oC for 48 h. The reaction mixture was
diluted with toluene (100 mL) and H2O (100 mL), and the aqueous phase
was further extracted with toluene (2 x 100 mL). The organic phases
were combined, dried over MgSO4, filtered, and the solvent removed in
vacuo. The residue was subjected to flash column chromatography (SiO2,
EtOAc/toluene 2:5), which gave 11 (41 mg, 72%) as a dark green solid.
Rf = 0.16 (EtOAc/toluene 2:5). M.p. = 208 oC. 1H NMR (500 MHz, CDCl3):
δ = 8.52 (broad d, J = 1.9 Hz, 1H), 8.47 (dt, J = 9.7, 0.8 Hz, 1H), 8.09-
8.06 (m, 2H), 7.76 (ddd, J = 10.4, 1.8, 0.9 Hz, 1H), 7.71 (s, 1H), 7.58-
7.49 (m, 4H), 2.82 (s, 3H) ppm. 13C NMR (126 MHz, CDCl3): δ = 177.45,
154.40, 143.92, 142.14, 139.56, 136.84, 135.06, 133.55, 133.00, 130.42,
129.51, 128.92, 127.27, 119.74, 117.12, 112.10, 112.07, 98.44, 88.04,
[4]
[5]
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25.42 ppm. HRMS (MALDI+, FT-ICR, dithranol) m/z
[M+Na+], calcd for (C22H13N3Na+): 342.10072.
= 342.10037
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Acknowledgments
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a) M. Å. Petersen, S. L. Broman, K. Kilså, A. Kadziola, M. B. Nielsen,
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University of Copenhagen is acknowledged for financial support.
a) M. Å. Petersen, S. L. Broman, A. Kadziola, K. Kilså, M. B. Nielsen,
Eur. J. Org. Chem. 2009, 2733-2736; b) S. L. Broman, M. Å. Petersen,
C. Tortzen, A. Kadziola, K. Kilså, M. B. Nielsen, J. Am. Chem. Soc.
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Keywords: Conjugation • Cyanides • Electrocyclic reactions •
Fused-ring systems • Photochromism
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10.1002/ejoc.201801776
European Journal of Organic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Energy release*
Half-life
NC
NC
Me
CN
CN
Ph
NC
CN
Ph
O
Josefine Mogensen, Oliver Christensen,
Martin Drøhse Kilde, Martin Abildgaard,
Lotte Metz, Anders Kadziola, Martyn
Jevric*, Kurt V. Mikkelsen*, Mogens
Brøndsted Nielsen*
X
X
<
<
<
NC
<
H
NC
Me
hours
day
VHF
DHA
A selection of dihydroazulene (DHA) derivatives with an electron-withdrawing group
in the seven-membered ring was prepared using a preformed DHA as precursor.
The DHAs all underwent photoisomerizations to the corresponding
vinylheptafulvenes (VHFs). The VHFs thermally returned to DHAs with half-lives
that strongly depended on the electron-withdrawing character of the substituent.
Page No. – Page No.
Molecular Solar Thermal Energy
Storage Systems with Long
Discharge Times Based on the
Dihydroazulene/Vinylheptafulvene
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