Photoswitching of Dihydroazulene Derivatives in Liquid-Crystalline Host Systems

Article abstract of DOI:10.1002/chem.201700055

Photoswitches and dyes in the liquid-crystalline nematic phase have the potential for use in a wide range of applications. A large order parameter is desirable to maximize the change in properties induced by an external stimulus. A set of photochromic and nonphotochromic dyes were investigated for these applications. It was found that a bent-shaped 7-substituted dihydroazulene (DHA) photoswitch exhibited liquid-crystalline properties. Further investigation demonstrated that this material actually followed two distinct reaction pathways on heating, to a deactivated form by a 1,5-sigmatropic shift and to a linear 6-substituted DHA. In addition, elimination of hydrogen cyanide from the photoactive DHA gave both bent and linear azulene dyes. In a nematic host that has no absorbance around 350 nm, it was found that only the linear DHA derivative has nematic properties; however, both 6- and 7-substituted DHAs were found to have large order parameters. In the nematic host, ring opening of either DHA to the corresponding vinylheptafulvene resulted in a decrease in dichroic order parameter and an unusually fast back-reaction to a mixture of both DHAs. Likewise, only the linear azulene derivative showed mesomorphic properties. In the same nematic host, large order parameters were also observed for these dyes.

Full text of DOI:10.1002/chem.201700055

A Journal of
Accepted Article
Title: Photoswitching of Dihydroazulene Derivatives in Liquid
Crystalline Host Systems
Authors: Anne Ugleholdt Petersen, Martyn Jevric, Richard J. Mandle,
Mark T. Sims, John N. Moore, Stephen J. Cowling, John W.
Goodby, and Mogens Broendsted Nielsen
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201700055
Link to VoR: http://dx.doi.org/10.1002/chem.201700055
Supported by

10.1002/chem.201700055
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Photoswitching of Dihydroazulene Derivatives in Liquid
Crystalline Host Systems
Anne Ugleholdt Petersen,[a] Martyn Jevric,[a] Richard J. Mandle,[b] Mark T. Sims,[b] John N. Moore,[b]
Stephen J. Cowling,[b] John W. Goodby,[b] and Mogens Brøndsted Nielsen*[a]
Abstract: Photoswitches and dyes in the liquid crystalline nematic
phase have the potential for use within a wide range of applications.
A high order parameter is desirable in order to maximize the change
in properties induced by an external stimulus. A set of photochromic
and non-photochromic dyes were investigated for these applications.
It was found that a bent-shaped 7-substituted dihydroazulene (DHA)
photoswitch exhibited liquid crystalline properties. Further
investigation demonstrated that this material actually underwent two
distinctive reaction pathways upon heating to a deactivated form via
a 1,5-sigmatropic shift and to a linear 6-substituted DHA. In addition,
elimination of hydrogen cyanide from these photoactive DHAs gave
both bent and linear azulene dyes. In a nematic host, which has no
absorbance around 350 nm, it was found that only the linearly
shaped DHA derivative possessed nematic properties; however,
both 6- and 7-substituted DHAs were found to have high order
parameters. In the nematic host, ring opening of either DHA to the
corresponding vinylheptafulvene (VHF) resulted in a decrease in
dichroic order parameter and an unusually fast back reaction to a
mixture of both DHAs. Likewise, only the linearly shaped azulene
derivative showed mesomorphic properties. In the same nematic
host, high order parameters were also observed for these dyes.
crystal technology is liquid crystal host systems doped with
guest dyes,[4] where the absorbance of polarized light is different
when parallel and perpendicular to the host director. Such
devices have a wide range of potential applications including
possible applications in displays,[5] high performance thin-film
polarizers,[6] optically controlled diffraction gratings,[7] security
devices,[8] switchable solar windows,[9] and 3D imaging for
biological applications.[10] The degree to which alignment occurs
is described by the dichroic order parameter, which is a measure
of how well the dye aligns with the director for the liquid crystal,
and how well the transition dipole moment is aligned with the
long axis of the dye molecule.[4, 11]
The dihydroazulene (DHA) 1 presents an example of a T-
type photochrome, which upon irradiation opens to
a
vinylheptafulvene (VHF) 2.[12] As the VHF is not affected by
visible light, full conversion to VHF in solution is possible and is
not complicated by photostationary states. VHF converts back to
DHA thermally, with a rate depending on the solvent polarity, the
temperature, and the DHA substituent pattern.[13, 14] Of particular
interest in this system are the large structural and electronic
differences between the DHA and VHF, where VHF is very polar
on account of strong resonance contributors of a zwitterion
having tropylium coupled with “malonitrile anion character” as
shown in Scheme 1. DHA and VHF differ significantly in their
absorption spectra. In solution the DHA form appears yellow,
while the VHF is intensely red in color. Lastly, there is a
structural reorganization in the molecule going from DHA to VHF,
and VHF is most stable as its s-trans conformer 2b (Scheme 1).
Many structural modifications have been made to DHA altering
the rate of the back reaction,[14-16] but to date there are no DHA
derivatives whose VHF form is long-lived enough for potential
use in holographic storage, while at the same time showing high
stability.
Introduction
Molecular photoswitches have been classified as being either P-
type, where inter-conversion between two states can be
facilitated in both directions using light, or as T-type, in which
thermal energy can be used to achieve a back reaction.[1]
Further, some photoswitches can be both P- and T-type, such
as spiropyrans and azobenzenes.[1] Use of photoswitches in
liquid crystal technology has shown potential for controlling the
macrostructure of liquid crystals, which has amongst others
shown applications in holographic image storage, for which
photoswitches that undergo extremely slow back reactions in a
liquid crystal host are desirable.[2] The properties of the
photoswitch may also be used to modulate the amount of light
passing through a sample on account of a change in the
chromophoric properties, which is of relevance in eye wear
technologies.[3] Another topic of interest within the field of liquid
NC
8a
8
CN
Ph
7
NC
CN
Ph
h!
6
"
1 DHA
2a VHF (s-cis)
Ph
Ph
CN
CN
CN
[a]
[b]
Dr. A. U. Petersen, Dr. M. Jevric, Prof. Dr. M. B. Nielsen
Department of Chemistry
University of Copenhagen
Universitetsparken 5, DK-2100 Copenhagen Ø
E-mail: mbn@chem.ku.dk
Dr. R. J. Mandle, Dr. M. T. Sims, Dr. J. N. Moore, Dr. S. J. Cowling,
Prof. Dr. J. W. Goodby
CN
2b VHF (s-trans)
Scheme 1. DHA/VHF photo-/thermoswitch showing the ring-opening to s-cis
VHF, and the more stable s-trans conformer. Relevant positions on the DHA
are numbered.
Department of Chemistry
University of York
Heslington, York, YO10 5DD, UK
Supporting information for this article is given via a link at the end of
the document
In a previous study, we have reported that DHAs 3 and 4,
which are both substituted with biaryl units (Figure 1), exhibited
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mesophases not far above room temperature.[17] The core DHA
structure is rod-shaped, which is a desirable quality for forming
the nematic phase. However in the nematic phase, full
conversion to the VHF had not been possible, but photolysis of 4
did, however, result in annihilation of the defects and established
a higher degree of order within the resulting mesophase when
the VHF underwent ring-closure back to 4.[17] Synthetic
developments in the chemistry of DHA have allowed for the
Here we present studies of the properties of guest-host
materials of DHA 5 in a bicyclohexyl host, which has a nematic
phase at room temperature, and we report the behavior of the
components of this dynamic system throughout a photo-thermal
cycle. In addition, azulene derivative
6 was conveniently
prepared from 5, and the dichroic order parameters for the
molecular components of 5 and 6 were determined in aligned
cells.
19]
introduction of aromatic substituents at the 7-position.[18,
However, placing a substituent at this position on the seven-
membered ring becomes complicated as when performing
switching experiments in solution, an isomeric scrambling giving
DHAs with the substituents in either the 6- or 7-positions can
occur after one ring-opening/closure cycle.[19] This is possible as
the VHF has zwitterionic character, allowing rotation around the
exocyclic olefin to form an isomeric E/Z-VHF mixture.[19]
Nevertheless, attaching a substituent at the 7-position allows for
the possibility of extending the results found for 3 by having alkyl
chains on both ends of this rod-shaped motif. Thus the aim of
this study was to extend our initial findings by introducing a
second octyloxyphenyl unit to the 7-position and assessing its
properties both neat and incorporated into a guest-host system
and studying how it is affected by a photo-thermal cycle and
subsequent ring closure in regard to the order parameter.
Interestingly, it has been found that DHA can be encouraged to
undergo the elimination of hydrogen cyanide to form the
corresponding 1-cyanoazulene.[20] Azulenes are strong
Results and Discussion
Synthesis and Physical Properties. DHA 5 was synthesized
according to Scheme 2, and for comparative purposes the
analogue 7 with no alkoxy substituents on the phenyl groups
was also made. Both these DHAs could be synthesized from
iodo-substituted DHA 8 via a previously reported bromination-
elimination protocol,[18] which allowed for the regioselective
introduction of
a reactive vinyl bromide to the 7-position
(compound 9). After a two-fold palladium catalyzed Suzuki
coupling, purification of products 5 and 7 involved flash column
chromatography followed by crystallization. The low yield
attributed to 7 included the isolation of significant amounts of
azulene byproducts. Azulene formation by elimination of HCN is
known to occur under cross-coupling conditions,[18] and in this
case the product mixture was complicated also by
1
debromination of 9. In the H NMR spectrum, a DHA without a
chromophores, which is the result of
a high degree of
substituent upon the seven-membered ring exhibits an apparent
doublet of triplets for the 8a proton at ca. δ 3.7 ppm, whereas for
5 and 7, now having a substituent in the 7-position, the
multiplicity of this signal appears as a distinctive doublet of
doublets.
polarization of the bicyclic motif, and they have therefore
attracted attention as dyes for guest-host systems, but to date
their potential for use has been limited because of low order
parameters.[21] The current work has therefore also focused
upon the order parameter properties arising from having a nitrile
in the 1-position.
NC
CN
I
NC
CN
8
OC8H17
1) Br2, CH2Cl2, -78 oC
2) LiHMDS, THF, 0 oC
3
NC
F
F
NC
CN
Br
CN
OC8H17
I
4
9
C8H17
O
O
X
B(OH)2
NC
CN
Pd(OAc)2, RuPhos, K3PO4
toluene/H2O
X
OC8H17
C8H17
5
NC
CN
X
CN
OC8H17
5 (X = OC8H17; 45%)
7 (X = H; 20%)
6
Scheme 2. Synthesis of DHAs 5 and 7 via a bromination-elimination protocol
followed by a Suzuki cross coupling.
Figure 1. Chemical structures of DHAs 3 and 4 previously shown to exhibit
liquid crystalline properties, and the chemical structures of 5 and 6 used in the
present work.
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A preliminary investigation of 5 using polarized optical
microscopy and differential scanning calorimetry (DSC) showed
that events such as the melting point and appearance of the
mesophase changed when cooling and heating the sample
throughout several cycles in the absence of light (see SI). Using
DSC in conjunction with NMR at different temperatures showed
5 to undergo conversion when heated to temperatures above
100 °C. Two distinctive reaction pathways seemed to occur;
isomerization to the 6-substituted isomer 10 and reaction via a
1,5-sigmatropic shift furnishing compound 11 (Scheme 3).
However, decomposition to unidentified products was evident
when heating 5 above 180 °C. It is conceivable that the origin of
10 could arise from a ring-opening/closure cycle via a VHF
intermediate. Product 11 is not photoactive as one requirement
for the DHA to be photoactive is that the sp3 carbon of the
cycloheptatriene is in the 8a position. In order to probe whether
these processes are specific to compound 5, or more general for
DHA, base system 7 was subjected to the same experiments.
As DHA 7, having no long alkyl substituents, presumably should
not exhibit any liquid crystal properties, it can be shown that the
formation of any possible liquid crystalline phases do not
contribute to these isomerization processes. In this case,
analogous products 12 and 13 were obtained, however in
differing product ratios, as discussed below.
DHA 7 at 110 °C for one week gave a clean conversion to 13
(Entry 1). However, when 7 was subjected to 140 °C for several
hours (Entry 2), a 5.6 mixture of 12 and 13 was obtained, whilst
heating to 180 °C resulted in DHA 12 being the major product
(Entry 3). Compound 13 exhibited very good thermal stability
and was found to be stable above 200 °C. Conversely, heating
12 (see SI) showed evidence of conversion back to 7 in addition
to 13. These results suggest that an equilibrium exists between
photoactive DHAs 7 and 12; however, the 1,5-sigmatropic shift
experienced by 7 is irreversible. No evidence was found
pertaining to a 1,5-sigmatropic shift of 12.
Interestingly, the opposite trend was observed for DHA 5,
where heating at a lower temperature favored DHA 10 (Entry 4),
which could thereby be purified by careful chromatography. On
the other hand, considerably more heat energy was required for
the 1,5-sigmatropic shift to occur to form 11 and significant
decomposition accompanied its formation (Entries 5 and 6). As
observed with 13, its analogue 11 was found to also be
thermally stable though small amounts of decomposition could
be observed by 1H NMR spectroscopy, but in this case no
evidence of formation of either 5 or 10 could be found. It
appears that the electron-donating octyloxyphenyl contributes to
the formation of the thermodynamic 6-substituted DHA 10 at
lower temperatures, presumably by better stabilizing the VHF,
while the sigmatropic shift has the lowest activation barrier for
DHA 7.
X
NC
CN
Table 1. Products obtained when heating either DHA 5 or 7 neat at given
temperatures and times.[a]
X
5 (X = OC8H17) or 7 (X = H)
!, neat
Temperature
(°C)
Product
ratio[b]
Isolated
yield
Entry
DHA
Time (h)
0:1
(12:13)
100%
(13)
1
2
3
4
5
7
7
7
5
5
110
140
180
110
140
168
4.7
1
NC
CN
5:6
(12:13)
30% (12),
30% (13)
X
X
10 (X = OC8H17) or 12 (X = H)
4:1
(12:13)
28% (12)
86% (10)
X
+
5:1
(10:11)
40
24
NC
CN
H
H
[c]
X
16% (11),
24%
-
(5/10)[d]
11 (X = OC8H17) or 13 (X = H)
[c]
Scheme 3. Reactions of DHAs 5 and 7 when heated neat. For product ratios
6
5
175
1
-
7% (11)
and yields, see Table 1.
[a] The heating was performed in a sealed vessel under argon in the dark. [b]
Product ratio determined by 1H NMR spectroscopy. [c] Not determined due to
significant decomposition of the sample. [d] Ratio of 3:7 (5:10).
Interest remained in the possibility of heating DHAs 5 and 7
on a preparative scale to selectively form single reaction
products. The results of different heating experiments are
summarized in Table 1. One strong motivation included the
isolation of photoactive DHA 10, which has the ideal properties
for a liquid crystalline material and it would be desirable to
determine its contribution in a device as it is expected to be
present after a photo-thermal cycle. It was found that heating
Azulenes usually form in small quantities when attempting
to subject DHA to further functionalization under reaction
conditions such as Suzuki and Sonogashira cross couplings.
The most effective base to intentionally effect the formation of
azulenes from DHA has been found to be 1,8-
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diazabicyclo[5.4.0]undec-7-ene (DBU). It was thought that by
eliminating hydrogen cyanide from both 5 and 10, it may be
possible to access dyes with mesomorphic properties. DHA 5
could be transformed into the corresponding azulene 6 in a low
yield of 32% by treatment with an excess of DBU at room
temperature (Scheme 4).
Liquid-crystalline and Photoswitching Properties of DHAs
5 and 10. Since it was not possible to investigate the liquid
crystalline properties of neat samples of 5 and 10, they were
both doped into a liquid crystalline host. To be compatible with
the DHA/VHF couple, the host should have low absorbance
around 350 nm (the region where DHA absorbs) and possess
a nematic phase at ambient temperature in addition to the
host being miscible with the dopants. Many commonly used
hosts incorporate phenyl rings (e.g. E7; CAS# 63748-28-7)
and so are not suitable due to absorbance in the near-UV. For
this reason host 15 (for which the transition temperatures and
composition are given in Figure 2) was employed as it has
practically no absorbance above 300 nm
C8H17
O
NC
CN
OC8H17
5
DBU, toluene
C8H17
O
CN
C3H7
CN
C5H11
CN
OC8H17
Host 15 (1:1:1 ratio)
C7H15
CN
6 (32%)
Cr 3.9 Bcryst 20.0 N 76.7 Iso
Iso 77.6 N 21.0 Bcryst -18.4 Y
Scheme 4. Synthesis of azulene 6 by elimination of hydrogen cyanide from
DHA 5.
Figure 2. Composition and transition temperatures (oC) for liquid crystal
host 15.
Azulene 14 was also synthesized from DHA 5, using a two-
step procedure (Scheme 5). As described above, heating a neat
sample of 5 at 110 °C for 40 hours gave 10 in high yield. This
was followed by elimination of hydrogen cyanide, made possible
by addition of catalytic DBU and excess of hexamethyldisilazane
(HMDS) to a toluene solution of 10. HMDS was added to
maintain the catalytic cycle of DBU and was also introduced as a
potential cyanide scavenger. This protocol facilitated the
formation of azulene 14, which could be conveniently isolated by
recrystallization following column chromatography. While
azulene 6 was isolated as a blue-green solid, azulene 14 was a
brown solid and appeared maroon in solution (UV-Vis absorption
spectra are included in SI).
Both DHAs 5 and 10 were individually doped into host 15
at five different concentrations and examined by polarized
optical microscopy (POM) and DSC. The partial phase
diagrams are shown in Figure 3. Assuming
a linear
relationship between TN-Iso and concentration, extrapolation of
the clearing points for the nematic phase for 5 in 15 leads to a
virtual nematic to isotropic (N-I) transition temperature of
17 °C for 5, whereas using this method for compound 10 in
host 15 gave a virtual N-I transition temperature of 84 °C. The
increased thermal stability of the virtual nematic phase of 10
relative to 5 is likely to be a consequence of the linear
molecular shape of the former, the latter being bent.
It is known that the VHF formed upon irradiation of a 7-aryl
substituted DHA can undergo ring closure to a mixture of both
6- and 7-substituted isomers, usually in approximately the
same ratio, although this ratio is specific to the substituent
pattern of the DHA.[15d] As the reverse reaction for the
DHA/VHF couple is thermally driven, this approach could not
be used to determine the liquid-crystalline properties of the
VHF. However, it was still important to construct a phase
diagram of the DHA mixture after a successive photo-thermal
cycle. Separate mixtures containing pure 5 in matrix 15 and
another consisting of pure 10 also in matrix 15 were both
irradiated to form VHFs (E/Z mixture), and then allowed to
close back to a mixture of 5/10. Upon irradiation, both 5 and
10 underwent ring-opening to VHFs having identical UV-Vis
absorption profiles. Further inspection by UV-Vis absorption
spectroscopy (described in the following section) showed that
ring closure in the dark at room temperature from both
mixtures gave the same ratio of 5 to 10. Both sets of mixtures
formed after irradiation of 5 or 10 were measured by DSC, and
5
110 °C, neat
NC
CN
C8H17
O
OC8H17
10
HMDS, DBU
toluene, rt, 4 days
CN
C8H17
O
OC8H17
14 (42%)
Scheme 5. Synthesis of azulene 14 from DHA 5 via formation of 10 as an
intermediate.
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the partial phase diagrams are shown in Figure 4. By
extrapolation, this mixture of 5/10 exhibited a nematic phase
with a clearing point at 33 °C. This was a substantially lower
clearing point than for neat 10 in matrix 15 and higher than
found for 5. Knowing the individual virtual nematic-isotropic
transition temperatures of 5 and 10 it is possible, assuming a
linear relationship between concentration and these transition
temperatures, to obtain a ratio for the mixture of 5:10. By this
method a value of 3:1 is obtained for the ratio of 5:10,
matching that found from the absorbance spectra, thus a
clearing point of 33.5 °C for the pure mixture results from a
ratio of 3:1 of 5:10.
Figure 4. Partial phase diagrams a) for 5:10 in a ratio of 3:1 in weight
percentages in host 15, arising from irradiation of samples containing either
5 or 10, giving the same ratio upon back reaction to the DHA, b) for 5:10
mixture extrapolated to weight percentage of 100%.
The dielectric properties of independent mixtures of 5 and
10 in host 15, as well as the corresponding E/Z mixture of
VHFs, and the 5/10 mixtures (obtained from ring closure of the
corresponding VHFs), were also studied. These showed only
small or no change in the dielectric anisotropy upon
irradiation, going from the DHA to VHF (for more information,
see SI).
Photochemical Properties. All photochromic DHAs made in
this study were examined by UV-Vis absorption spectroscopy
in acetonitrile, which included measuring the kinetics of the
back reaction for the conversion of the VHFs to their
respective DHAs. When irradiated with light, the DHAs opened
to their corresponding VHFs (Figure 5), which ultimately
closed to afford mixtures of the 7-substituted and 6-substituted
DHA isomers (Scheme 6). This being said, they opened to the
same mixtures of VHFs, as shown by the matching absorption
spectra and the same resultant mixtures of DHAs, shown in
Scheme 6. Thus, DHAs 5 and 10 opened to VHF 16 (E/Z) and
closed to 5/10 in the same ratio, irrespective of whether one
started the measurement with either 5 or 10. The same was
the case for the system 7/12.
Figure 3. Partial phase diagrams for a) 5 in host 15 at different weight
percentages, b) 5 extrapolated to weight percentage of 100%, c) 10 in host
15 at different weight percentages, d) 10 extrapolated to weight percentage
of 100%.
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[a] Rate constant for the thermal back reaction measured at 25 °C unless
otherwise stated. [b] Measured in a homogenously aligned cell at ambient
temperature. [c] Measured in an unaligned cell at ambient temperature.
X
NC
CN
X
DHA 5 (X = OC8H17
)
DHA 7 (X = H)
or
NC
CN
X
X
DHA 10 (X = OC8H17
DHA 12 (X = H)
)
X
h!
NC
CN
CN
Z-VHF
X
NC
X
E-VHF
Z/E-VHF 16 (X = OC8H17
Z/E-VHF 17 (X = H)
)
X
"
DHAs 5 + 10 or DHAs 7 + 12
Scheme 6. Ring-opening/closure cycles of DHA couples 5/10 and 7/12.
Table 2. Absorption maxima (λmax) for DHAs and VHFs in acetonitrile and in
host 15. Rate constants (k) and associated half-lives (t1/2) at 25 °C of the back
reactions (VHF to DHA mixture) are also listed.
λmax
λmax
λmax
Starting
compd
k [a]
t1/2
(min)
Medium
(DHA)
(VHF)
(DHA mixture)
(10-5 s-1
)
Figure 5. UV-Vis absorption spectra of a) DHAs 7, 12, 7/12 and VHF 17 in
acetonitrile, b) DHAs 5, 10, 5/10 and VHF 16 in acetonitrile, c) DHAs 5, 10,
5/10 and VHF 16 in host 15; as the density is not known for this blend, the
extinction coefficient is reported with the unit of g mol-1 cm-1. For ease of
comparison, extinction coefficients are plotted in the figures for both pure
(nm)
(nm)
(nm)
7
12
5
CH3CN
CH3CN
5.52
5.51
9.20
9.24
65.7
66.1
62.2
209
210
126
125
18
367
389
374
402
379
407
378
492
492
508
508
505
505
505
375
375
383
383
382
382
382
compounds and mixtures. d) Decay of absorption of 16 in host 15 at λmax
=
505 nm at ambient temperature (including a first-order kinetics fit).
CH3CN
10
5
CH3CN
The absorption spectra for DHAs 5 and 10 and kinetics
data for the VHF to DHA back reactions were measured
individually in acetonitrile. In both cases, under irradiation they
opened to the VHF 16, giving the same spectrum (Figure 5b
and Table 2). In the dark, VHF 16 closed to a mixture of 5/10
in an estimated ratio of 7:3, with a rate constant of 9.2 x 10-5 s-
Host 15 [b]
Host 15 [b]
Host 15 [c]
10
5
18
19
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1 following 1st order kinetics. The ratio of 5/10 was determined
from the contributions of known absorption spectra of pure 5
and 10. Having an electron-donating group at either the 6- or
7-position indeed speeds up the rate of the back reaction,
which may be attributed to it stabilizing the zwitterionic
transition state. Although there is the same electron-donating
group on the 2-position, the substituent at this position also
contains a bridging phenyl group diminishing its influence.
This is evidenced by the parent system, which has a slower
rate of ring-closure and itself gives a mixture of 6- and 7-
substituted isomers.
responsive to the application of an electric field. Incorporation
of such a composite material into a device gives the possibility
of regulating the passage of light through a cell containing the
DHA form in host 15 by turning on and off an electric field;
however, this possibility is not available in the VHF form.
UV-Vis absorption studies were also carried out on 5 and
10 in the nematic host medium 15 at ambient temperature, for
which the absorption spectra can be seen in Figure 5c. These
were measured in a homogenously aligned cell at values of λ
> 340 nm, owing to the indium tin oxide (ITO) coating on the
glass that the cell was made from. Again ring opening of the
independent mixtures of DHAs 5 and 10 gave VHF 16, and
upon ring closure the same absorption maximum was
observed for the resulting mixture of 5/10, which was found to
be formed in ratio of 3:1 (determined in the same fashion as
explained earlier) which was in agreement with the phase
diagrams. It was found that the rate of the ring closure of 16
was significantly faster in the nematic phase of host 15 than in
acetonitrile, with a rate constant of 6.6 x 10-4 s-1. This large
increase in reaction rate could be explained by the effect of
the nematic material impacting the VHF 16, driving ring
closure to DHA 5/10 to form a structure that is less disruptive
to the orientational order of the nematic host. The nematic
host could as well be affecting the s-trans and s-cis conformer
equilibrium of VHF. As the VHF is less rod-like in the core
structure, it seems possible that the VHF could be destabilized
in the nematic host, relative to in solution, contributing to an
enhanced rate of ring closure. Surprisingly, DFT calculations
(B3LYP/6-31G(d)) comparing the dipole moments of host 15
(4.56 Debye) to acetonitrile (3.81 Debye) showed that 15 was
more polar, and this should thus also be considered as one
factor to the faster ring closure of VHF 16 in the nematic host
(since ring closure is promoted in polar media[13]). The
measurement was performed in homogeneously aligned cells,
and it was thought that the long range order would not have
any effect on the kinetics. However, in order to verify this
assumption, the measurement was repeated in an unaligned
cell, which gave similar results.
Figure 6. Absorption spectra in liquid crystal host 15, with the light polarized
parallel and perpendicular to alignment, of a) DHA 5 and the corresponding
VHF 16 and the mixture of 5/10 formed upon closing of 16, b) DHA 10 and
the corresponding VHF 16 and the mixture of 5/10 formed upon closing of
16.
Table 3. Order parameters (S) for DHAs 5 and 10 in host 15 before, during
and after irradiation.
Starting compound
5
10
Before irradiation
During irradiation
After irradiation
0.62
0.07
0.66
0.72
0.06
0.69
Studies of DHAs 5 and 10 in the aligned cells were also
used to examine how well both the DHAs and VHFs align with
host 15. In order to obtain the experimental order parameter
for the DHAs and VHFs in host 15, the absorption spectra
were measured with the incident light polarized parallel and
perpendicular to the alignment of the cell (Figure 6). From
these spectra, it is clear that the DHAs 5 and 10 are more
aligned within the host than E/Z-VHF 16. The dichroic order
parameters determined from the spectra in Figure 6 are given
in Table 3. The molecule with the greatest degree of linearity,
10, has a higher order parameter than the bent molecule 5,
while it is clear that the VHF is poorly aligned. This implies that
DHAs 5 and 10, and the mixture 5/10 in host 15 should be
Density functional theory (DFT) optimizations and time-
dependent DFT calculations were carried out for the ethoxy-
substituted analogs of DHAs 5 and 10 and VHF 16, hereon
denoted by 5ʹ, 10ʹ and 16ʹ, in order to determine the angle, β,
between the minimum moment of inertia (MOI) axis and the
electronic transition dipole moment (TDM) vector associated
with the longest wavelength transition of significant oscillator
strength. These calculations (see Table 4) in essence can give
an indication of how well the TDM is aligned with the long axis
for each molecule.[22] For 5ʹ, this angle was calculated to be
9.4° and for 10ʹ, a value of 2.8° was obtained, agreeing with
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the trend of the experimental results, in that 10 exhibits a
higher order parameter than 5. The rod-like natures of 5ʹ and
10ʹ, quantified by their calculated aspect ratios listed in Table
4, are consistent with the positive and relatively high
experimental dichroic order parameters of 5 and 10. As for
VHF structure 16ʹ, the results of the calculations were found to
structure 6 did not exhibit a liquid crystalline phase, nor did
its corresponding DHA precursor 5. On the other hand, linear
azulene 14 exhibited liquid crystalline phases with transition
temperatures, enthalpy and entropy values summarized in
Figure 8, and the DSC thermogram is shown in Figure 9.
This material melted into a smectic A phase at 138 °C. This
phase has a large temperature range of 185 °C before the
transition into a nematic phase occurred at 323 oC. These
transitions occurred at very high temperatures, and a little
decomposition was observed when analyzing the material
from the DSC pan by 1H NMR spectroscopy after three
cycles of heating to as high as 360 °C.
be
more
complicated
because
four
major
conformers/configurations of 16ʹ were possible (Figure 7),
dependent on the position of the alkyloxyphenyl substituent on
the seven-membered ring and the s-cis/s-trans conformers of
the exocyclic “butadiene” parts. The calculated energies
indicate that the s-trans conformers are more stable than the
s-cis conformers, and that the s-trans E-VHF 16ʹc is the most
stable of the VHF structures.
The calculated properties for 5ʹ, 10ʹ and the four structures
of VHF 16ʹ are given in Table 4. The relatively rod-like natures
of 16ʹa-b which are quantified by aspect ratios of 2.2-2.5,
combined with calculated angles of ca. 21-25° between the
TDM vectors and the minimum MOI axes, are indicative of
compounds that may be expected to exhibit positive and
relatively high dichroic order parameters in an aligned liquid
crystal host. In contrast, the aspect ratios of 16ʹc-d are both
close to 1, suggesting that the molecular alignment, and
therefore the dichroic order parameters, of these structures
would be low. The lower calculated energies of 16ʹc-d than
16ʹa-b (Figure 7) suggest 16ʹc-d would be the dominant
structures at room temperature, which is consistent with the
low experimental order parameters measured after irradiation
of 5 and 10 (Table 3). This result was not unexpected, as
when the s-cis conformer is formed it can close rapidly to the
DHA, and therefore s-cis conformers are merely transient and
do not accumulate when VHF 16 has been generated during
the irradiation process. This is reflected by the extremely fast
rate of back reaction when the VHF had been locked in this
conformation.[16]
Table 4. Calculated transition wavelengths (λ), oscillator strengths (f),
angles (β) between the TDM and minimum MOI axes, and aspect ratios
for DHAs 5ʹ and 10ʹ and VHF structures 16ʹa-dʹ. The longest wavelength
transitions with f > 0.2 are listed.
Aspect
ratio
f
Structure
λ / nm
β / o
5ʹ
358
378
432
417
405
400
0.85
1.29
0.31
0.82
1.00
0.85
9.4
2.12
2.89
2.49
2.21
1.39
0.88
10ʹ
2.8
16ʹa
16ʹb
16ʹc
16ʹd
20.9
24.5
39.3
16.9
Figure 7. DFT optimised structures of 5ʹ, 10ʹ and 16ʹa-d (ethoxy-substituted
analogues of 5, 10, and 16a-d) showing the TDM vectors associated with the
calculated transitions listed in Table 4 (red arrows) and the minimum MOI axes
(blue arrows). Calculated relative energies of 16ʹa-d are also shown.
Liquid Crystal Properties of Azulenes 6 and 14. Unlike the
DHAs employed in this study, their corresponding azulenes
were thermally stable and their liquid-crystalline properties
could be measured in their neat forms. Yet, the bent
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CN
C8H17
O
OC8H17
14
Cr 138.4 SmA 323.3 N 333.0 Iso
Iso 332.0 N 322.1 SmA116.0 Cr
Figure 8. Transition temperatures (°C) for azulene 14.
Figure 10. POM images of 14 showing a) the smectic A phase at 340 °C, b)
the nematic phase at 349 °C, c) the smectic C phase at super cooling.
Figure 9. a) DSC thermograms with heating-cooling rate of 10 °C min-1 for
compound 14. b) Zoom of the DSC.
Photophysical Properties of Azulenes. Azulenes 6 and 14
have two different conjugation patterns, which affected the color
of these materials. While bent molecule 6 appeared blue,
linearly shaped azulene 14 appeared maroon in color. In the
absorbance spectra in 1,2-dichloroethane, the difference
appeared as a peak at 440 nm for 14, where 6 clearly exhibited
a shoulder around 420 nm (Figure 11a). As both azulenes are
highly colored, the obvious use was as dopants in a guest-host
system. Therefore azulenes 6 and 14 were doped into host 15,
and the absorption spectra were measured with the light parallel
and perpendicular to the alignment of the cell (Figure 11b).
Azulene 6 had an order parameter of S = 0.71, while the order
parameter for azulene 14 was determined to be S = 0.75. These
are much higher order parameters than the formerly reported
results for azulenes, where the order parameter is around S =
0.3,[21] which shows that with the right substitution pattern
azulenes are promising candidates for guest-host systems.
When examining 14 under POM around 340 °C (measured
from the heating mantle), a focal conic texture was observed.
Some areas had this texture, which can be seen in Figure 10a,
and other areas were homogenously aligned giving black areas.
From this, it was possible to conclude that it was a smectic A
phase. If the sample was heated further, a nematic phase
appeared, giving a Schlieren texture, which is shown in Figure
10b. If the glass slide was removed from the heat, and quickly
examined under the microscope, a third phase could be
observed, shown in Figure 10c. The third phase was identified
as a Smectic C phase due to the observation of both a Schlieren
texture and focal-conic defects The reason we do not see this in
the DSC is that the phase transition occurred under the melting
point of 14, which also meant that shortly after the slide was
removed from the heat, the material crystallized.
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Table 5. Calculated transition wavelengths (λ), oscillator strengths (f), angles
(β) between the TDM and minimum MOI axes, and aspect ratios for
azulenes 6ʹ, 14ʹ, and 18-21. The longest wavelength transitions with f > 0.2
are listed.
Compound
λ / nm
313
383
321
388
294
379
f
β / o
4.1
3.1
4.5
0.2
2.6
0.2
Aspect ratio
1.96
6ʹ
14ʹ
18
19
20
21
1.55
0.62
1.65
0.66
1.78
0.65
2.36
1.99
2.31
1.99
3.29
An alternative approach could be to incorporate an
additional cyano group into the 3-position, giving a higher
amount of symmetry to the molecule. Such structures are
shown in Figure 13, along with the calculated visible TDM
vectors (see data in Table 5). Azulene 18 has an angle of β =
4.5°, which is not so different from that of 6 and 14. By
contrast, 19 has a significantly smaller angle of β = 0.2°, which
Figure 11. a) UV-Vis absorption spectra of azulenes 6 and 14 in 1,2-
dichloroethane. b) UV-Vis absorption spectra in liquid crystal host 15 with the
light polarized parallel and perpendicular to alignment of 6 and 14.
suggests that this analogue should give
a high order
parameter. Another possibility could be to exchange the cyano
group at the 2-position with a hydrogen atom to give structures
20 and 21 (Figure 13). These exhibit the same trend as 18 and
19. For the 7-substituted system 20, the angle, β, was
calculated to be 2.6°, whilst for the 6-substituted system 21,
the angle was 0.2°. Further, the aspect ratios of compounds 19
and 21 are higher than those of 18 and 20. These results give
DFT calculations were done in the same manner for 6 and
14 as for DHA 5 and 10, in order to find the angles, β, between
the TDM and minimum MOI axes and their aspect ratios (Figure
12; Table 5). For 6ʹ (the ethoxy analogue of 6) the angle was
found to be 4.1° and the aspect ratio found to be 1.96 which is
consistent with the high experimental order parameter. For 14ʹ
(the ethoxy analogue of 14), the angle was calculated to be 3.1°
and the aspect ratio calculated as 2.36 which again is consistent
with the experimental order parameters, showing a slightly
higher order parameter for 14 compared to 6. In order to
incorporate these systems into devices, the aim is a higher order
parameter, ideally approaching 1. To obtain this, the angle
between TDM and MOI should be as small as possible.
a
clear indication that the 6-substituted azulenes are
interesting molecules for future work.
Figure 12. DFT optimized structures of 6ʹ and 14ʹ (ethoxy-substituted
analogues of 6 and 14) showing the TDM vectors associated with the
calculated transitions (red arrows) and the minimum MOI axes (blue arrows).
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using Gaussian 09.[23] Calculated energies referred to in the text
correspond to the Gibbs energies calculated for the optimized structures.
Minimum moment of inertia (MOI) axes were calculated as the
eigenvectors associated with the minimum eigenvalues of the
diagonalised moment of inertia tensors of the geometries studied. Each
molecular length was determined as the length of the van der Waals
molecular surface along the minimum MOI axis, and each molecular
width was calculated as twice the maximum perpendicular distance from
the minimum MOI axis to the van der Waals molecular surface. Each
molecular aspect ratio was calculated as the ratio of the molecular length
to the molecular width.
Figure 13. DFT optimized structures of azulenes 18, 19, 20, and 21 (from top
to bottom) showing the TDM vectors associated with the calculated TDMs (red
arrows) and minimum MOI axes (blue arrows).
Conclusions
In conclusion, we have shown that the substitution pattern on
the seven-membered ring of DHA has significant consequences
for its liquid crystalline properties. We have managed to isolate
both 6- and 7-substituted DHA derivatives on a preparative scale,
rather than just as a mixture of isomers, which has allowed for
thorough investigations. Thus, whereas substitution on position
7 provides a bent structure that did not result in liquid crystallinity,
substitution on position 6 provides a more rod-like structure with
liquid crystalline properties. When measuring the DHAs in a
nematic host system, they showed good alignment, giving high
dichroic order parameters. Interestingly, the order parameters
were lowered significantly upon irradiation resulting in
conversion to the VHF forms. The ability of the system to
overwrite by irradiation means that it is promising for future
devices. In addition a highly coloured and relatively stable
azulene derivative could be formed directly from the DHA, and it
also showed promising results as a dye for guest-host systems,
giving a higher order parameter than those previously reported
for azulenes. The DFT calculations reported here provide a
rationale for the observed order parameters of the dyes and
additionally enabled identification of suitable molecules for future
synthesis and study.
Synthetic Protocols
2-(4'-(Octyloxy)-[1,1'-biphenyl]-4-yl)-7-(4-(octyloxy)phenyl)azulene-
1,1(8aH)-dicarbonitrile (5): To a stirring solution of 8 (1.07 g, 2.80
mmol) in CH2Cl2 (25 mL), under an argon atmosphere at -78 °C was
added dropwise over 5 min a solution of Br2 (3.9 mL, 0.78M in CH2Cl2,
3.1 mmol), and the contents of the vessel were allowed to stir for 4 h.
The vessel was directly subjected to rotary evaporation, without the use
of a water bath, and when dry, the contents were dissolved in THF (50
mL). The vessel was cooled in an ice bath and to this stirring solution
was added a solution of LiHMDS (3.1 mL, 1M in toluene, 3.1 mmol), and
the mixture was allowed to stir for 1 h in the ice bath and then a further
hour after the bath had been removed. The reaction was quenched using
saturated aqueous NH4Cl (20 mL) and diluted with water (20 mL), and
the mixture was extracted with Et2O (3 x 50 mL). The combined organic
phase was dried over MgSO4, filtered and the solvent removed in vacuo.
The residue was taken up in toluene (60 mL) and water (10 mL) and the
mixture degassed. To the stirring biphasic mixture were added (4-
(octyloxy)phenyl)boronic acid (2.56 g, 10.2 mmol), K3PO4 (4.19 g, 19.7
mmol), Pd(OAc)2 (32 mg, 0.14 mmol) and RuPhos (140 mg, 0.30 mmol),
and the resulting mixture was allowed to stir at ambient temperature for
20 h. The contents of the flask were diluted with saturated aqueous
NH4Cl (40 mL), water (40 mL), and toluene (50 mL). The phases were
separated and the aqueous phase was extracted with toluene (50 mL).
The combined organics were dried over MgSO4, filtered and the solvent
removed in vacuo. The crude residue was purified by flash column
chromatography (gradient elution of 50% toluene/heptane to 70%
toluene/heptane) to give an orange oil which was crystalized from
CH2Cl2/EtOH to afford 5 (940 mg, 45%) as a fluffy yellow solid. Rf=0.31
(60% toluene/heptane). M.p. = 118-137 °C. 1H NMR (500 MHz, CDCl3): δ
= 7.80 (d, J = 8.6 Hz, 2H), 7.66 (d, J = 8.6 Hz, 2H), 7.56 (d, J = 8.8 Hz,
2H), 7.33 (d, J = 8.8 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 6.91 (s, 1H), 6.88
(d, J = 8.8 Hz, 2H), 6.80 (dd, J = 11.5, 5.8 Hz, 1H), 6.75 (d, J = 11.5 Hz,
1H), 6.35 (br d, J = 5.8 Hz, 1H), 5.94 (d, J = 4.6 Hz, 1H), 4.01 (t, J = 6.6
Hz, 2H), 3.97 (t, J = 6.6 Hz, 2H), 3.82 (dd, J = 4.6, 1.6 Hz, 1H), 1.83-1.76
(m, 4H), 1.51–1.43 (m, 4H), 1.37–1.29 (m, 16H), 0.90 (t, J = 7.0 Hz, 3H),
0.88 (t, J = 7.0 Hz, 3H) ppm. 13C NMR (125 MHz, CDCl3): δ = 159.50,
159.36, 142.64, 141.01, 140.96, 139.37, 132.61, 132.49, 132.33, 132.13,
131.08, 129.03, 128.72, 128.22, 127.39, 126.86, 120.08, 115.47, 115.13,
114.75, 114.61, 113.25, 68.31, 68.26, 51.09, 45.10, 31.98, 29.52, 29.52,
29.42, 29.40, 26.22, 26.19, 22.82, 22.82, 14.26 ppm, 4Cs masked.
HRMS (MALDI +ve) calcd for C46H52N2O2 [(M)·+]: m/z = 664.4023; exp
664.4029.
Experimental Section
General Methods. Chemicals were used as purchased from commercial
sources. Purification of products was carried out by flash
chromatography on silica gel (40−63 µm, 60 Å). Thin-layer
chromatography (TLC) was carried out using aluminum sheets precoated
with silica gel. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were
recorded on a Bruker 500 MHz instrument with a non-inverse cryoprobe
using the residual solvent as the internal standard (CDCl3, 1H 7.26 ppm
and 13C 77.16 ppm). All chemical shifts are quoted on the δ scale (ppm),
and all coupling constants (J) are expressed in Hz. In APT spectra, CH
and CH3 correspond to negative signals and C and CH2 correspond to
positive signals. High resolution mass spectra (HRMS) were acquired
using matrix assisted laser desorption ionisation (MALDI). Melting points
are uncorrected. All solution based spectroscopic measurements were
performed in a 1-cm path length cuvette. Photoswitching experiments
were performed using a Vilber Lourmet TLC lamp with a wavelength at
365 nm at 610 µw/cm3. The thermal back reaction was measured by
heating the sample (cuvette) by
a Peltier unit in the UV-Vis
spectrophotometer. UV-Vis absorption measurements on liquid crystals
were performed scanning the region 900-190 nm, and the thermal back
reaction was followed at ambient temperature. Dielectric measurements
were done on an INSTEC ALCT Property Tester. Compound 8[18] was
made by its literature method. Density functional theory (DFT)
optimisations and time-dependent DFT (TD-DFT) calculations were
carried out on isolated molecules at the CAM-B3LYP/cc-pVDZ level
2-([1,1'-Biphenyl]-4-yl)-7-phenylazulene-1,1(8aH)-dicarbonitrile (7):
To a stirring solution of 8 (1.07 g, 2.80 mmol) in CH2Cl2 (25 mL) at –
78 °C was added dropwise a solution of Br2 in CH2Cl2 (3.9 mL, 0.78M,
3.1 mmol), and the resulting solution was stirred for 4 h. The cold bath
was removed, and the solvent was immediately removed under reduced
pressure. The crude mixture was dissolved in THF (50 mL) and cooled in
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an ice bath. To this stirring solution was added a solution of LiHMDS (3.1
mL, 1M in toluene, 3.1 mmol), and the contents of the vessel were
allowed to stir 1 h. The reaction was quenched by the addition of
saturated aqueous NH4Cl (20 mL), followed by water (20 mL), and the
mixture was extracted with Et2O (3 x 50 mL). The combined ethereal
extracts were dried over MgSO4, filtered and the solvent removed in
vacuo. The crude residue was taken up in toluene (60 mL) and water (10
mL) and degassed with argon before adding phenyl boronic acid (1.03 g,
4.26 mmol), K3PO4 (2.73 g, 12.9 mmol), Pd(OAc)2 (64 mg, 0.29 mmol)
and RuPhos (264 mg, 0.57 mmol) to the vessel. The biphasic mixture
was stirred for 24 h at 80 °C. The contents of the vessel were diluted with
water (100 mL) and the phases were separated. The aqueous phase was
extracted with toluene (2 x 50 mL) and the combined organics dried over
MgSO4, filtered and the solvent removed by rotary evaporation. The
crude residue was subjected to flash column chromatography (gradient
elution of 40% toluene/heptane to toluene) affording the DHA 7, which
was then crystalized from CH2Cl2/heptane to give 7 (234 mg, 20%) as a
yellow crystalline solid. In addition, 2-([1,1'-biphenyl]-4-yl)azulene-1-
carbonitrile (55 mg, 6%) as a dark blue solid and 2-([1,1'-biphenyl]-4-yl)-
7-phenylazulene-1-carbonitrile (72 mg, 7%) as a dark green solid could
be isolated during the course of the chromatography. (7): Rf=0.21 (40%
toluene/heptane). M.p. = 181-211 °C (decomposes). 1H NMR (500 MHz,
CDCl3): δ = 7.87 (d, J = 8.4 Hz, 2H), 7.74 (d, J = 8.4 Hz, 2H), 7.68–7.64
(m, 2H), 7.54–7.47 (m, 2H), 7.46–7.36 (m, 6H), 6.97 (s, 1H), 6.88–6.78
(m, 2H), 6.41 (d, J = 5.9 Hz, 1H), 6.05 (d, J = 4.7 Hz, 1H), 3.89 (dd, J =
4.7, 1.6 Hz, 1H) ppm. 13C NMR (125 MHz, CDCl3): δ = 143.00, 140.93,
140.75, 140.18, 140.00, 139.93, 132.55, 132.20, 131.50, 129.40, 129.12,
128.71, 128.20, 128.18, 128.00, 127.84, 127.19, 126.89, 120.37, 116.30,
115.37, 113.15, 51.10, 45.13 ppm. HRMS (MALDI +ve) calcd for
C30H20N2 [(M)·+]: m/z = 408.1621; exp 408.1628. Analysis calcd (%) for
C30H20N2: C 88.21, H 4.94 N 6.86; found: C 88.40, H 4.66, N 6.82. 2-
([1,1'-Biphenyl]-4-yl)azulene-1-carbonitrile: Rf=0.33 (toluene). 1H NMR
(500 MHz, CDCl3): δ = 8.63 (d, J = 9.7 Hz, 1H), 8.40 (d, J = 9.5 Hz, 1H),
8.16 (d, J = 8.3 Hz, 2H), 7.79–7.75 (m, 3H), 7.03–7.67 (m, 2H), 7.58 (s,
1H), 7.56–7.45 (m, 4H), 7.43–7.37 (m, 1H) ppm. HRMS (MALDI +ve)
calcd for C30H20N2 [(M)·+]: m/z = 305.1199; exp 305.1208. Data in
accordance with literature.[24] 2-([1,1'-Biphenyl]-4-yl)-7-phenylazulene-
1-carbonitrile: Rf=0.49 (toluene). M.p. > 230 °C. 1H NMR (500 MHz,
CDCl3): δ = 8.90 (d, J = 1.8 Hz, 1H), 8.39 (d, J = 9.8 Hz, 1H), 8.19 (d, J =
8.2 Hz, 2H), 8.01 (ddd, J = 10.3, 1.8, 0.8 Hz, 1H), 7.78 (d, J = 8.3 Hz, 2H),
7.71–7.68 (m, 4H), 7.58 (s, 1H), 7.58–7.44 (m, 6H), 7.43–7.37 (m, 1H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 152.30, 145.24, 143.51, 142.55,
142.41, 141.80, 140.44, 139.07, 136.80, 136.46, 133.38, 129.24, 129.21,
129.07, 128.61, 128.24, 127.96, 127.93, 127.66, 127.26, 118.44, 116.27,
sealed and placed under an argon atmosphere before being heated in an
oil bath at 180 °C for 1 h. 1H NMR analysis indicated the formation of 12
and 13 in a 4:1 ratio. The material was then subjected to flash column
chromatography (13% EtOAc/heptane) followed by recrystallization from
hot CCl4 to give pure 12 (16 mg, 28%).
2-([1,1'-Biphenyl]-4-yl)-6-phenylazulene-1,1(8aH)-dicarbonitrile (12):
Rf=0.34 (13% EtOAc/heptane). M.p. = 221-223 °C. 1H NMR (500 MHz,
CDCl3): δ = 7.85 (d, J = 8.3 Hz, 2H), 7.72 (d, J = 8.3 Hz, 2H), 7.65–7.63
(m, 2H), 7.50–7.47 (m, 4H), 7.43–7.38 (m, 3H), 7.37–7.34 (m, 1H), 6.99
(s, 1H), 6.96 (d, J = 6.9 Hz, 1H), 6.56 (d, J = 10.4 Hz, 1H), 6.48 (dd, J =
6.9, 1.8 Hz, 1H), 6.04 (dd, J = 10.4, 4.2 Hz, 1H), 3.86 (ddd, J = 4.2, 1.8,
1.8 Hz, 1H) ppm. 13C NMR (125 MHz, CDCl3): δ = 143.09, 142.87,
141.33, 139.97, 139.94, 139.13, 131.92, 129.46, 129.12, 128.86, 128.83,
128.24, 128.20, 128.16, 128.00, 127.18, 126.83, 120.97, 120.22, 115.32,
112.98, 51.10, 45.06 ppm, 1C masked. HRMS (MALDI +ve) calcd for
C30H21N2 [(M+H)+]: m/z = 409.1699; exp 409.1711.
2-([1,1'-Biphenyl]-4-yl)-7-phenylazulene-1,1(6H)-dicarbonitrile (13):
Rf=0.28 (13% EtOAc/heptane). M.p. > 230 °C. 1H NMR (500 MHz,
CDCl3): δ = 7.84 (d, J = 8.5 Hz, 2H), 7.72 (d, J = 8.5 Hz, 2H), 7.67–7.59
(m, 4H), 7.48 (t, J = 7.6 Hz, 2H), 7.46–7.34 (m, 4H), 7.22 (s, 1H), 7.08 (s,
1H), 6.61 (d, J = 9.2 Hz, 1H), 5.69 (dt, J = 9.2, 7.3 Hz, 1H), 3.08 (d, J =
7.3 Hz, 2H) ppm. 13C NMR (125 MHz, CDCl3): δ = 144.94, 142.32,
140.07, 139.73, 138.30, 137.82, 134.02, 133.71, 129.10, 129.07, 128.91,
128.75, 128.11, 128.05, 127.95, 127.16, 126.19, 123.83, 123.73, 117.31,
113.00, 44.59, 32.02 ppm. HRMS (MALDI +ve) calcd for C30H21N2
[(M+H)+]: m/z
= 409.1699; exp 409.1698. Analysis calcd (%) for
C30H20N2: C 88.21, H 4.94, N 6.86; found: C 88.14, H 4.77, N 6.85.
Entry 4: Under an argon atmosphere, compound 5 (99 mg, 0.15 mmol)
was heated conventionally to 110 °C for 40 h in a sealed microwave tube.
This resulted in the formation of a mixture of 10 and 11 in the ratio 5:1.
The material was then subjected to column chromatography (gradient
elution of 60% toluene/heptane to toluene) to give 10 (85 mg, 86%).
Entry 5: Under an argon atmosphere, 5 (80 mg, 0.12 mmol) was heated
conventionally to 140 °C for 24 h in a sealed microwave tube. The
mixture was subjected to column chromatography (75% toluene/heptane)
to give 11, a mixture of 5 and 10, and a mixture containing all three
components. The fractions containing all three components were
subjected to a second flash column (60% toluene/heptane), where
compound 11 was separated from photoactive DHAs 5 and 10. All
combined fractions containing 11 were recrystallized to give this product
(13 mg, 16%) as a bright yellow solid. The combined components
containing photoactive DHAs 5 and 10 were also recrystallized to give a
mixture of 5/10 in a ratio of 3:7 (19 mg, 24%).
94.92 ppm. HRMS (MALDI +ve) calcd for C29H19
382.1590; exp 382.1600. Analysis calcd (%) for C29H19N: C 91.31, H 5.02
N 3.67; found: C 91.18, H 4.84, N 3.56.
N =
[(M)·+]: m/z
Heating Experiments of DHAs 5 and 7 (Scheme 3 and Table 1)
Entry 6: Compound 5 (18 mg, 0.03 mmol) was conventionally heated to
175 °C for 1 h under an argon atmosphere in a vial suitable for
microwave reactions. The resulting material was subjected to flash
column chromatography (gradient elution of 60% toluene/heptane to
toluene). The first yellow component was collected and subsequently
recrystallized from CH2Cl2/EtOH giving 11 (1.2 mg, 7%) as a yellow solid.
Entry 1: Under an argon atmosphere, in a sealed microwave tube,
compound 7 (53 mg, 0.13 mmol) was heated conventionally to 110 °C for
1 week. This resulted in the exclusive transformation to 13 (53 mg,
100%).
Entry 2: DHA 7 (20 mg, 0.49 mmol) was heated conventionally under an
argon atmosphere to 140 °C for 4 h and 40 min in a sealed microwave
vial. This resulted in the formation of a mixture of 12 and 13 in the ratio
5:6. The material was subjected to flash column chromatography (13%
EtOAc/heptane), and it was possible to obtain pure samples of 12 (6 mg,
30%) as a yellow solid and 13 (6 mg, 30%) also a yellow solid.
2-(4'-(Octyloxy)-[1,1'-biphenyl]-4-yl)-6-(4-(octyloxy)phenyl)azulene-
1,1(8aH)-dicarbonitrile (10): Rf = 0.49 (60% toluene/heptane). M.p. =
154-159 °C. 1H NMR (500 MHz, CDCl3): δ = 7.80 (d, J = 8.3 Hz, 2H),
7.66 (d, J = 8.3 Hz, 2H), 7.56 (d, J = 8.5 Hz, 2H), 7.40 (d, J = 8.5 Hz, 2H),
6.99 (d, J = 8.5 Hz, 2H), 6.95 (s, 1H), 6.92 (d, J = 8.5 Hz, 2H), 6.89 (d, J
= 7.0 Hz, 1H), 6.51 (d, J = 10.3 Hz, 1H), 6.44 (dd, J = 7.0, 1.7 Hz, 1H),
6.01 (dd, J = 10.3, 4.3 Hz, 1H), 4.01 (t, J = 6.5 Hz, 2H), 3.99 (t, J = 6.5
Hz, 2H), 3.83 (ddd, J = 4.3, 1.7, 1.7 Hz, 1H), 1.84 – 1.77 (m, 4H), 1.51–
Entry 3: A CH2Cl2 (5 mL) solution of DHA 7 (57 mg, 0.14 mmol) was
dried inside a microwave vial using a stream of nitrogen. The vial was
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1.44 (m, 4H), 1.42–1.24 (m, 16H), 0.91–0.88 (m, 6H) ppm. 13C NMR (125
MHz, CDCl3): δ = 159.33, 159.28, 142.37, 139.46, 138.52, 133.95,
133.47, 132.01, 131.44, 128.68, 128.66, 128.05, 127.23, 126.60, 120.76,
119.91, 119.34, 115.27, 115.17, 114.98, 114.64, 112.93, 68.43, 68.16,
50.98, 44.86, 31.83, 31.78, 29.38, 29.37, 29.28, 29.26, 29.20, 28.98,
26.07, 26.05, 25.94, 22.67, 22.65, 14.11 ppm. HRMS (MALDI +ve) calcd
for C46H53N2O2 [(M+H)+]: m/z = 665.4102; exp 665.4101. Analysis calcd
(%) for C46H52N2O2 (664.40): C 83.09, H 7.88, N 4.21; found: C 82.96, H
7.89, N 4.24.
then recrystallized from CH2Cl2/EtOH to give pure 14 (43 mg, 42%) as a
brown powder. Rf=0.29 (75% toluene/heptane). M.p. = Cr 138.4 SmA
323.3 N 333.0 Iso; Iso 332.0 N 322.1 SmA 116.0 Cr °C. 1H NMR (500
MHz, CDCl3): δ = 8.62 (d, J = 10.3 Hz, 1H), 8.39 (d, J = 10.4 Hz, 1H),
8.15 (d, J = 8.3 Hz, 2H), 7.74–7.72 (m, 3H), 7.68 (dd, J = 10.4, 1.4 Hz,
1H), 7.62 (d, J = 8.7 Hz, 2H), 7.61 (d, J = 8.7 Hz, 2H), 7.55 (s, 1H), 7.04
(d, J = 8.7 Hz, 2H), 7.01 (d, J = 8.7 Hz, 2H), 4.05 (t, J = 6.6 Hz, 2H), 4.02
(t, J = 6.6 Hz, 2H), 1.86–1.79 (m, 4H), 1.52–1.46 (m, 4H), 1.44–1.21 (m,
16H), 0.95–0.86 (m, 6H) ppm. 13C NMR (125 MHz, CDCl3): δ = 160.24,
159.29, 152.61, 150.99, 144.40, 141.81, 141.05, 137.01, 136.19, 134.83,
132.91, 132.71, 130.05, 129.01, 128.26, 128.24, 127.86, 127.36, 118.57,
116.48, 115.24, 115.08, 94.06, 68.43, 68.30, 31.99, 29.54, 29.53, 29.52,
29.45, 29.41, 29.41, 29.39, 14.27 ppm, 5Cs masked. HRMS (MALDI +ve)
calcd for C45H52NO2 [(M+H)+]: m/z = 638.3993; exp 638.3999. Analysis
calcd (%) for C45H51NO2 (637.91): C 84.73, H 8.06 N 2.20; found: C
84.80, H 7.99, N 2.14.
2-(4'-(Octyloxy)-[1,1'-biphenyl]-4-yl)-7-(4-(octyloxy)phenyl)azulene-
1,1(6H)-dicarbonitrile (11): Rf=0.51 (60% toluene/heptane). M.p. = 174-
175.5 °C. 1H NMR (500 MHz, CDCl3): δ = 7.79 (d, J = 8.4 Hz, 2H), 7.67
(d, J = 8.4 Hz, 2H), 7.57 (d, J = 8.8 Hz, 2H), 7.56 (d, J = 8.8 Hz, 2H), 7.18
(s, 1H), 7.01 (s, 1H), 7.00 (d, J = 8.8 Hz, 2H), 6.92 (d, J = 8.8 Hz, 2H),
6.58 (d, J = 9.2 Hz, 1H), 5.62 (dt, J = 9.2, 7.2 Hz, 1H), 4.02 (t, J = 6.2 Hz,
2H), 4.00 (t, J = 6.2 Hz, 2H), 3.04 (d, J = 7.2 Hz, 2H), 1.91–1.73 (m, 4H),
1.50–1.44 (m, 4H), 1.42–1.23 (m, 16H), 0.93–0.84 (m, 6H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 159.85, 159.38, 143.99, 141.85, 138.00,
137.85, 133.76, 132.87, 132.33, 131.93, 129.38, 128.49, 128.16, 127.51,
126.08, 123.78, 123.14, 115.76, 115.11, 114.82, 113.20, 68.36, 68.31,
44.57, 31.98, 31.97, 31.90, 29.86, 29.53, 29.51, 29.43, 29.41, 29.40,
29.37, 26.22, 26.18, 22.82, 14.26 ppm, 1C masked. HRMS (MALDI +ve)
calcd for C46H52N2O2 [(M)·+]: m/z = 664.4023; exp 664.4029.
Absorption maxima (λmax) and extinction coefficients (ε) for selected
compounds are summarized in Table 6.
Table 6. Absorption maxima (λmax) and extinction coefficients (ε); sh =
shoulder. For the blends, the densities are not known (therefore ε is reported
in units of g mol-1 cm-1).
Compound
Medium
λmax
ε
2-(4'-(Octyloxy)-[1,1'-biphenyl]-4-yl)-7-(4-(octyloxy)phenyl)azulene-1-
carbonitrile (6): To a solution of 5 (78 mg, 0.117 mmol) in toluene (3
mL) was added DBU (0.1 mL, 6.7 mmol), and the resulting solution was
stirred at ambient temperature for 2 h. The reaction was quenched with
aqueous HCl (2M, 10 mL), and the mixture was extracted with CH2Cl2 (4
x 20 mL). The combined organics were dried through a pad of cotton and
the solvent was removed under reduced pressure. The residue was
subjected to flash column chromatography (gradient elution of 13%
toluene/heptane to 70% toluene/heptane) and recrystallized from
CH2Cl2/EtOH to give pure 6 (24 mg, 32%) as a blue-green solid. Rf=0.38
(60% toluene/heptane). M.p. = 165.5-167.5 °C. 1H NMR (500 MHz,
CDCl3): δ = 8.87 (d, J = 1.8 Hz, 1H), 8.34 (d, J = 9.8 Hz, 1H), 8.16 (d, J =
8.3 Hz, 2H), 7.98 (ddd, J = 10.4, 1.8, 0.7 Hz, 1H), 7.73 (d, J = 8.3 Hz, 2H),
7.63 (d, J = 8.8 Hz, 2H), 7.61 (d, J = 8.8 Hz, 2H), 7.55–7.46 (m, 2H), 7.05
(d, J = 8.8 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H), 4.05 (t, J = 6.5 Hz, 2H), 4.02
(t, J = 6.6 Hz, 2H), 1.88–1.78 (m, 4H), 1.53–1.46 (m, 4H), 1.42–1.31 (m,
16H), 0.90 (t, J = 7.0 Hz, 3H), 0.90 (t, J = 6.9 Hz, 3H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 159.41, 159.18, 152.22, 145.22, 142.50, 141.91,
141.43, 138.35, 136.05, 135.91, 135.52, 132.61, 132.53, 129.61, 129.01,
128.11, 127.47, 127.25, 118.50, 115.68, 115.14, 114.95, 94.21, 68.27,
68.16, 31.84, 29.39, 29.30, 29.27, 26.08, 26.07, 22.68, 14.12 ppm, 6Cs
CH3CN
host 15
374 nm
378 nm
30 x 103 M-1 cm-1
DHA 5
4.38 x 109 g mol-1 cm-
1
DCE
host 15
361 nm
367 nm
70 x 103 M-1 cm-1
Azulene 6
DHA 7
CH3CN
369 nm
33 x 103 M-1 cm-1
38 x 103 M-1 cm-1
CH3CN
host 15
402 nm
407 nm
DHA 10
3.96 x 109 g mol-1 cm-
1
11
CH3CN
CH3CN
CH3CN
416 nm
389 nm
399 nm
28 x 103 M-1 cm-1
44 x 103 M-1 cm-1
27 x 103 M-1 cm-1
57 x 103 M-1 cm-1
DHA 12
13
DCE
host 15
346 nm (431 nm, sh)
347 nm (434 nm, sh)
Azulene 14
33 x 103 M-1 cm-1
masked. HRMS (MALDI +ve) calcd for C45H51NO2 [(M)·+]: m/z
=
CH3CN
host 15
508 nm
508 nm
VHF 16
VHF 17
3.90 x 109 g mol-1 cm-
637.3914; exp 637.3920. Analysis calcd (%) for C45H51NO2 (637.91): C
84.73, H 8.06 N 2.20; found: C 84.86, H 7.94, N 2.14.
1
CH3CN
492 nm
43 x 103 M-1 cm-1
2-(4'-(Octyloxy)-[1,1'-biphenyl]-4-yl)-6-(4-(octyloxy)phenyl)azulene-1-
carbonitrile (14): Under an argon atmosphere, 5 (106 mg, 0.27 mmol)
was heated conventionally in
a sealed vial suitable for microwave
reactions to 110 °C for 40 h. This resulted in a mixture of two compounds
10 and 11 in a ratio of 5:1. The mixture was dissolved in CH2Cl2 (50 mL).
To this solution was added HMDS (0.5 mL, 2.4 mmol) and DBU (0.04 mL,
0.27 mmol), and the mixture was left to stir for 3 days, after which time,
more DBU (0.04 mL, 0.27 mmol) was added, and the mixture was left to
stir for a further day. Then the last portion of DBU (0.04 mL, 0.27 mmol)
was added, and the mixture was left to stir for 5 h, after which it was
quenched with 2 M HCl (20 mL) and extracted with CH2Cl2 (4 x 20 mL).
The combined organics were filtered through cotton and the volatiles
removed in vacuo. The residue was subjected to column chromatography
(gradient elution of 75% toluene/heptane to toluene). The material was
Acknowledgements
The Danish Council for Independent Research | Technology and
Production Sciences (#12-126668) and University of
Copenhagen are acknowledged for financial support. EPSRC
grant EP/M020584/1 for the development of dyes for liquid
crystal applications is acknowledged.
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10.1002/chem.201700055
Chemistry - A European Journal
FULL PAPER
Petersen, M. Jevric, J. Elm, S. T. Olsen, C. G. Tortzen, A. Kadziola, K.
V. Mikkelsen, M. B. Nielsen, Org. Biomol. Chem. 2016, 14, 2403-2412;
d) M. Cacciarini, M. Jevric, J. Elm, A. U. Petersen, K. V. Mikkelsen, M.
B. Nielsen, RSC Adv. 2016, 6, 49003-49010.
Keywords: Dyes/Pigments • Electrocyclic reactions • Fused-ring
systems • Liquid crystals • Photochromism
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Entry for the Table of Contents
FULL PAPER
Derivatives of the
A. U. Petersen, M. Jevric, R. J. Mandle,
M. T. Sims, J. N. Moore, S. J. Cowling,
J. W. Goodby, M. B. Nielsen*
dihydroazulene/vinylheptafulvene
photo-/thermoswitch were prepared
and studied for their liquid crystalline
and switching properties in liquid
crystalline host systems. The
properties were found to be strongly
dependent on the position of a
substituent group in the seven-
membered ring of the dihydroazulene.
Page No. – Page No.
Photoswitching of Dihydroazulene
Derivatives in Liquid Crystalline Host
Systems
heat
light
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