Azobenzene-Based Organic Salts with Ionic Liquid and Liquid Crystalline Properties

Article abstract of DOI:10.1021/acs.cgd.5b01024

Two sets of new azobenzene-based bromide salts are synthesized, and their thermal photochromic properties are studied. Both sets are based on the imidazolium cation. The first set (1) features a symmetric biscation where two imidazolium head groups (Im) w

Full text of DOI:10.1021/acs.cgd.5b01024

Article
pubs.acs.org/crystal
Azobenzene-Based Organic Salts with Ionic Liquid and Liquid
Crystalline Properties
Kathrin Stappert, Johanna Muthmann, Eike T. Spielberg, and Anja-Verena Mudring*,†,‡,§,∥
†
†
†,‡
†
̈
̈ ̈
Anorganische Chemie III − Materials Engineering and Characterization, Fakultat fur Chemie and Biochemie, Ruhr-Universitat
Bochum, 44780, Bochum, Germany
‡
§
∥
Universitat
Critical Materials Institute, Ames Laboratory, 333 Spedding Hall, Ames, Iowa 50011, United States
Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, Iowa 50011, United States
S Supporting Information
̈ ̈ ̈
sbibliothek, Universitat Duisburg Essen, Universitatsstrasse 9, 45141 Essen, Germany
*
ABSTRACT: Two sets of new azobenzene-based bromide salts are synthesized, and
their thermal photochromic properties are studied. Both sets are based on the
imidazolium cation. The first set (1) features a symmetric biscation where two
imidazolium head groups (Im) with different alkyl chains (Cn) are connected to a
central azobenzene unit (Azo): [Azo(C1-Im-Cn) ]; n = 6, 8, 10, 12, 14. The other one
2
contains an n-alkyl-imidazolium cation (Cn-Im) bearing a terminal azobenzene unit
(
C1-Azo) substituted with an alkoxy chain (O-Cm) of either two (2) or six (3) carbon
atoms: [C1-Azo-O-Cm-Im-Cn]; m = 2, n = 8, 10, 12 and m = 6, n = 8, 10, 12, 14, 16. For
both cation classes, the influence of alkyl chains of varying length on the thermal phase
behavior was investigated by differential scanning calorimetry (DSC) and polarizing
optical microscopy (POM). For five compounds (Azo(-C1-Im-C12) (1d), Azo(-C1-
2
Im-C12) (1e), C1-Azo-O-C2-Im-C10 (2b), C1-Azo-O-C2-Im-C12 (2c), and C1-Azo-O-C6-Im-C16 (3e)), the formation of a
2
liquid crystalline phase was observed. The biscationic salts (1) are all comparatively high melting organic salts (180−240 °C),
and only the two representatives with long alkylchains (C12 and C14) exhibit liquid crystallinity. The monocationic salts with an
O−C2 bridge (2) melt between 140 and 170 °C depending on the alkyl chain length, but from an alkyl chain of 10 and more
carbon atoms on they form a smectic A liquid crystalline phase. The representatives of the third set with a O−C6 bridge qualify
as ionic liquids with melting points less than 100 °C. However, only the representative with a hexadecyl chain forms a liquid
crystalline phase. Representative single crystals for all sets of cations could be grown that allowed for single crystal structure
analysis. Together with small-angle X-ray scattering experiments they allow for a more detailed understanding of the thermal
properties. Through irradiation with UV-light (320−366 nm) all compounds undergo trans−cis isomerization, which reverses
under visible light (440 nm).
INTRODUCTION
molecular LCs, the characteristic properties of an ILC depend on
the direction in which they are measured and can be influenced
by the orientation of the ions. ILCs have been proposed for use in
optical, catalytic, electronic, and separation technologies. They
■
1
Ionic liquids (ILs) are salts with a melting point below 100 °C.
Their unique properties such as low vapor pressure, wide liquid
range, good thermal stability, considerable electric conductivity,
and large electrochemical window can be tuned by variation of
8
can be used as electrolytes with anisotropic conductivity or as
9
,10
2
“ILC-SILP” (SILP: supported IL phases). ILs with calamitic
(rod-like) ions usually exhibit lamellar mesophases such as
smectic A or smectic C; sometimes also higher ordered phases
get stabilized by the electrostatic interactions. Besides calamitic
cations and anions. Hence ILs are sometimes called “designer
3
solvents”. This makes them very interesting for severa4l
applications: among others, they are used as alternative solvents,
5
as electrolytes in dye-sensitized solar cells, and for the
electrodeposition of metals but also have been used as a new
class of active pharmaceutical ingredient (API). In recent years,
1
1,12
6
monocationic 1-alkyl-3-methylimidazolium salts
and related
13
7
triazolium salts, bicationic imidazolium cations also have been
14−17
investigated.
Two types of cation structures that lead to the
a number of cation types for ILs have been developed, but still by
far the most commonly used are imidazolium cations. As a
consequence, most of the ionic liquid crystals (ILCs) studied so
far are based on imidazolium cations. Endowing the imidazolium
cation with a long alkyl chain or another mesogenic unit reliably
leads to the formation of a mesophase, and these compounds are
addressed as ILCs. Thus, ILCs are a subclass of ILs which
combine the properties of liquid crystals (LCs) and ILs. As in
18
formation of mesophases can be distinguished: Either the
imidazolium cation is endowed with a long alkyl chain and it is
itself the mesogenic unit, or the imidazolium acts as the ionic
headgroup and is linked to a conventional mesogenic unit.
Received: July 19, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 1. General structure of the synthesized compounds.
1
9
20
Commonly used mesogenic groups are benzyl, biphenyl, or
temperature the orange precipitate was filtered off, washed with
chloroform and water ,and dried under a vacuum overnight at room
2
1
22
triphenyl or other rigid aromatic systems such as azobenzene
with long alkoxy chains.
temperature.
1
Yield: 2.42 g (6.58 mmol, 67%), H NMR (200 MHz, CDCl ) δ: 7.89
3
Azobenzene, with its two phenyl rings separated by an NN
(
d, J = 8.5 Hz, 4H), 7.54 (d, J = 8.5 Hz, 4H), 4.56 (s, 4H).
Synthesis of 4,4′-Bis(N-hexyl imidazole-methyl)azobenzene Di-
double bond, has attracted considerable attention due to its high
23
optical sensitivity. Upon irradiation with light of appropriate
energy (wavelength), it is possible to switch between the two
possible isomeric states. In general, the trans conformation is
thermodynamically preferred and can be converted to the cis
state by irradiation with UV-light. The reverse transition from
the cis to the trans conformation can either be induced thermally
or by visible light. This reversible cis-trans photoisomerization
process offers a wide range of applications such as optical data
storage, optical switching, polarization holography, and the
use of azobenzene functionalized molecules as a photolabile
bromide (1a). A total of 0.33 g (0.96 mmol) of compound 6 and 0.59 g
(3.86 mmol) of hexyl imidazole were dissolved in acetonitrile and heated
under reflux for 20 h. After reaching room temperature the solution was
poured onto cold ethyl acetate and stored at −40 °C for several hours.
The precipitated orange product was filtered off and dried under a
vacuum at room temperature overnight.
24
1
Yield: 0.38 g (59%), H NMR (200 MHz, DMSO) δ: 9.48 (s, 2H),
25
7.97−7.85 (m, 8H), 7.66 (d, J = 8.4 Hz, 4H), 5.59 (s,4H), 4.21 (t, J = 7.2
13
Hz, 4H), 1.84−1.76 (m, 4H), 1.25 (s, 12H), 0.84 (t, J = 6.3 Hz, 6H). C
2
6
27
28
NMR (50 MHz, DMSO) δ 151.8, 138.3, 136.3, 129.5, 123.0, 122.9,
1
22.6, 51.4, 48.9, 40.6, 39.9, 39.7, 39.5−39.3 (m), 39.1, 38.7, 38.3, 30.4,
2
9
surfactants. For this, azobenzene-based polymers and liquid
29.1, 25.1, 21.8, 13.7. MS (FAB): m/z = 153 (32%, hexylimidazole), 208
30
+
+2
+2
crystalline polymers have been scrutinized intensively. The
study of azobenzene based ILCs shows that the ionic character
has a great influence on the mesomorphic behavior; for example,
as often encountered for ionic LCs, layered, smectic phases are
stabilized. In addition to ILCs, ILs with photochromic
functionalities have been developed based on the azobenzene,
(98%, 4,4′-dimethylazobenzene ), 256 (100%, K /2), 360 (67%, K -
+2 +2 +
hexylimidazole), 427 (10%, K -C
H17), 511 (100% K -H ). Elemental
8
analysis: calcd % for C H Br N (M = 672 g/mol): C 57.2, H 6.6, N
32 44
2
6
1
2.5 found: C 56.84, H 6.73, N 12.38.
Synthesis of 4,4′-Bis(N-imidazole-methyl)azobenzene (9). A total
3
1
32
of 1.2 g (13.16 mmol) of sodium imidazolide was solved in THF and
2
.42 g (6.58 mmol) of compound 6 was added. The mixture was heated
33
and their photoresponsive conductivity was investigated.
under reflux for 20 h and then cooled to room temperature. The excess
On the basis of these ideas we developed new azobenzene-
based salts in order to create novel photoswitchable ILCs. To
achieve this, we have chosen a symmetrically imidazolium-
substituted bicationic azobenzene system with alkyl chains of
various lengths on both sides (1) as well as an azobenzene system
with just one cationic n-alkyl-imidazolium group (2, 3) (Figure
NaH was filtered off and the solvent was removed under a vacuum. The
orange product was dried overnight under vacuum at room temperature.
1
Yield: 1.62 g (72%), H NMR (200 MHz, CDCl ) δ: 7.90 (d, J = 8.2
3
Hz, 4H), 7.66 (s, 2H), 7.30 (d, J = 8.3 Hz, 4H), 7.14 (s, 2H), 6.95 (s,
2
H), 5.22 (s, 4H).
Synthesis of 4,4′-Bis(N-alkylimidazole-methyl)azobenzene Dibro-
mide (1b−e). A solution of compound 9 and 4 equiv of the respective
alkyl bromide in acetonitrile was heated under reflux for 6 days, cooled
to room temperature, and poured into cold ethyl acetate. The
precipitated orange product was dried under a vacuum overnight at
room temperature.
1
) where two differently long alkoxy chains are used to connect
the azobenzene unit and the imidazolium ring. Bromide was
chosen as the counteranion in all cases. The influence of the
chain length, symmetry, and crystal structure on the mesophase
was investigated with X-ray diffraction (XRD), differential
scanning calorimetry (DSC), polarizing optical microscopy
4,4′-Bis(N-octylimidazole-methyl)azobenzene Dibromide (1b).
1
Yield: 52%, H NMR (200 MHz, DMSO) δ: 9.36 (s, 2H), 7.97−7.84
m, 8H), 7.63 (d, J = 8.4 Hz, 4H), 5.55 (s, 4H), 4.19 (t, J = 7.2 Hz, 4H),
.88−1.70 (m, 4H), 1.23 (s, 20H), 0.84 (t, J = 6.5 Hz, 6H). C NMR
(
POM), and small-angle X-ray scattering (SAXS). The photo-
1
3
chromic properties of all compounds were analyzed with UV−vis
spectroscopy.
5
3.6, 51.1, 32.8, 31.0, 30.2, 29.9, 27.2, 23.6, 14.4. MS (FAB): m/z = 208
+
+
(
40%, 4,4′-dimethylazobenzene ), 293 (100%, dioctylimidazole ), 388
EXPERIMENTAL SECTION
+2 +2 +2 +
■
5
(30%, K -octylimidazole), 455 (12%, K -C
H17), 567 (29% K -H ).
8
Synthesis. Synthesis of 4,4′-Dimethylazobenzene (5). A total of
Elemental analysis: calcd % for C H Br N (M = 728 g/mol): C 59.3,
36
52
2
6
.79 g (0.24 mol) of magnesium turnings was washed with water,
H 7.2, N 11.5 found: C 57.61, H 7.24, N 11.63.
ethanol, and diethyl ether and then added to a solution of 6.52 g (0.048
mol) of nitrotoluene in 150 mL of methanol. The mixture was stirred at
room temperature for 2 h. After addition of water the solution was
acidified until just acidic with hydrochloric acid and afterward
4,4′-Bis(N-decylimidazole-methyl)azobenzene Dibromide (1c).
1
Yield: 55%, H NMR (200 MHz, DMSO) δ: 9.40 (s, 2H), 7.91 (dd, J
= 9.5, 5.1 Hz, 8H), 7.64 (d, J = 8.4 Hz, 4H), 5.56 (s, 4H), 4.19 (t, J = 7.1
1
3
Hz, 4H), 1.79 (s, 4H), 1.22 (s, 28H), 0.83 (t, J = 6.2 Hz, 6H). C NMR
(50 MHz, DMSO) δ: 151.8, 138.3, 136.3, 129.4, 123.1, 122.9, 122.7,
51.4, 48.9, 31.2, 29.2, 28.8, 28.6, 28.3, 25.4, 22.0, 13.9. MS (FAB): m/z =
neutralized with aqueous NaHCO . The product was purified by
3
column chromatography (cyclohexane/diethyl ether 40:1) and dried
+
+2
overnight at room temperature under a vacuum.
208 (100%, 4,4′-dimethylazobenzene ), 312 (30%, K /2), 416 (72%,
1
+2 +2 + +2 −
Yield: 2.05 g (9.76 mmol, 40%), H NMR (200 MHz, CDCl ) δ: 7.82
K -decylimidazole), 624 (73% K -H ), 706 (25%, K +Br ).
3
(
d, J = 8.3 Hz, 4H), 7.31 (d, J = 8.1 Hz, 4H), 2.44 (s, 6H).
4,4′-Bis(N-dodecylimidazole-methyl)azobenzene Dibromide (1d).
1
Synthesis of 4,4′-Bis(bromomethyl)azobenzene (6). A mixture of
Yield: 43%, H NMR (200 MHz, DMSO) δ: 9.34 (s, 2H), 7.93 (d, J = 8.2
2
.05 g (9.76 mmol) of 4,4′-dimethylazobenzene (5), 5.21 g (29.29
Hz, 4H), 7.85 (s, 4H), 7.62 (d, J = 8.4 Hz, 4H), 5.54 (s, 4H), 4.19 (t, J =
1
3
mmol) of N-bromosuccinimide, and 0.16 g (0.53 mmol) of benzoyl
peroxide in chloroform was refluxed for 6 h. After being cooled to room
7.0 Hz, 4H), 1.90−1.68 (m, 4H), 1.21 (s, 34H), 0.83 (s, 6H). C NMR
(50 MHz, DMSO) δ: 151.8, 138.3, 136.3, 129.4, 123.1, 122.9, 122.6,
B
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
5
1.4, 49.0, 31.2, 29.2, 28.9, 28.9, 28.8, 28.8, 28.6, 28.3, 25.4, 22.0, 13.9.
1-Dodecyl-12-(2-(4-((4-methylphenyl)diazenyl)phenoxy)ethyl)-
+
1
MS (FAB): m/z = 208 (100%, 4,4′-dimethylazobenzene ), 340 (15%,
K /2), 444 (50%, K -dodecylimidazole), 679 (54% K -H ), 761
imidazolium Bromide 2c. Yield: 63%, H NMR (200 MHz, CDCl
3
) δ:
+
2
+2
+2
+
10.88 (s, 1H), 7.88 (d, J = 9.0 Hz, 2H), 7.78 (d, J = 8.3 Hz, 2H), 7.55 (s,
1H), 7.29 (d, J = 8.6 Hz, 2H), 7.20−7.14 (m, 1H), 7.00 (d, J = 9.0 Hz,
2H), 5.07−4.98 (m, 2H), 4.58−4.48 (m, 2H), 4.25 (t, J = 7.5 Hz, 2H),
+
2
−
(
15%, K +Br ). Elemental analysis: calcd % for C H Br N (M = 840
44 68 2 6
g/mol): C 62.85, H 8.15, N 9.99 found: C 58.26, H 7.97, N 9.98.
,4′-Bis(N-tetradecylimidazole-methyl)azobenzene Dibromide
2
.42 (s, 3H), 1.91 (d, J = 6.5 Hz, 2H), 1.36−1.19 (m, 18H), 0.87 (t, J =
4
1
13
(1e). Yield: 40%, H NMR (200 MHz, DMSO) δ 9.33 (s, 2H), 7.92
6.4 Hz, 3H). C NMR (50 MHz, CDCl ) δ: 159.6, 150.7, 147.7, 141.3,
3
(
d, J = 8.3 Hz, 4H), 7.85 (s, 2H), 7.62 (d, J = 8.6 Hz, 4H), 5.54 (s, 4H),
137.4, 129.8, 124.7, 123.5, 122.7, 121.5, 114.8, 66.7, 50.4, 49.5, 31.9,
4
3
1
2
.19 (t, J = 6.8 Hz, 4H), 1.86−1.70 (m, 4H), 1.21 (s, 45H), 0.84 (t, J =
.3 Hz, 6H). 13C NMR (50 MHz, DMSO) δ: 151.8, 138.3, 136.3, 129.4,
23.1, 122.9, 122.7, 51.5, 49.0, 35.8, 31.2, 29.2, 28.9, 28.9, 28.8, 28.8,
8.6, 28.3, 25.4, 22.0, 13.9. MS (FAB): m/z = 208 (83%, 4,4′-
30.3, 29.6, 29.6, 29.4, 29.4, 29.0, 26.3, 22.7, 21.5, 14.2. MS (FAB): m/z =
+
+
405 (60%, Didodecylimidazol ), 475 (100% K ). Elemental analysis:
calcd % for C30 O (M = 555 g/mol): C 64.85, H 7.80, N 10.08
H43BrN
4
found: C 65.1, H 8.06, N 9.95.
+
+2
dimethylazobenzene ), 368 (37%, K /2), 461 (100%, ditetradecylimi-
Synthesis of 1-(4-((6-Bromohexyl)oxy)phenyl)-2-(4-
methylphenyl)diazene 15b. This compound was synthesized by the
same procedure described for the synthesis of 15a. From 14-
methylphenyldiazenylphenol (13) (3 g, 14.15 mmol), dibromohexane
+
2
+2
dazolium), 472 (79%, K -tetradecylimidazole), 539 (19%, K -
C H ), 735 (69% K -H ), 817 (23%, K +Br ). Elemental analysis:
calcd % for C H Br N (M = 897 g/mol): C 64.3, H 8.5, N 9.4 found:
C 60.1, H 8.15, N 9.73.
Synthesis of 4-Methylphenyldiazenylphenol 13. Ten grams of p-
toluidine (93.45 mmol) and 25 mL of concentrated hydrochloric acid
were dissolved in 111 mL of water and cooled to 0 °C. A total of 6.45 g of
NaNO (93.45 mmol) in 15 mL of water were added slowly. A solution
of phenol (8.77g, 93.45 mmol), Na CO (10.18 g, 96 mmol), and
NaOH (3.84 g, 96 mmol) was added dropwise, and the mixture was
stirred for another 4 h. After the pH was adjusted to 7 with hydrochloric
acid, the precipitated product was filtered off, washed with water, and
dried under a vacuum at room temperature for 24 h.
+
2
+
+2
−
14
29
48
76
2
6
(
10.36 g, 42.45 mmol) and 7.22 g of potassium carbonate, 15b was
obtained as an orange solid.
1
H NMR (200 MHz, acetone) δ: 7.82−7.72 (m, 2H), 7.66 (d, J = 8.3
Hz, 2H), 7.23 (d, J = 8.1 Hz, 2H), 7.03−6.93 (m, 2H), 4.00 (t, J = 6.4 Hz,
H), 3.40 (t, J = 6.7 Hz, 2H), 2.29 (s, 3H), 1.79−1.71 (m, 6H), 1.64−
.57 (m, 2H), 1.42 (dd, J = 9.0, 5.5 Hz, 4H).
2
1
2
2
3
Synthesis of 1-Alkyl-(2-(4-((4-methylphenyl)diazenyl)phenoxy)-
hexyl)imidazolium Bromide 3a−e. This compound was synthesized
by the same procedure described for the synthesis of 2a−c. A solution of
1
5b and 1 equiv of the respective alkylimidazole was heated under reflux
in acetonitrile for 3 days. After being cooled to room temperature the
solution was poured into cold ethyl acetate and kept at −40 °C for 12 h.
The orange precipitate was filtered off and dried under a vacuum at
room temperature overnight.
1
Yield: 11.9 g (60%), H NMR (200 MHz, acetone) δ: 7.88−7.70 (m,
4H), 7.35 (d, J = 8.0 Hz, 2H), 7.06−6.95 (m, 2H), 2.41 (s, 3H).
Synthesis of 1-(4-((2-Bromoethyl)oxy)phenyl)-2-(4-
methylphenyl)diazene 15a. A mixture of 3 g (14.15 mmol) of 13,
1
-Octyl-8-(2-(4-((4-methylphenyl)diazenyl)phenoxy)hexyl)-
3.64 mL (42.45 mmol) of 1,2-dibromoethane, and 7.22 g (52.36 mmol)
1
imidazolium Bromide 3a. Yield: 70%, H NMR (200 MHz, CDCl ) δ:
3
of K CO in 100 mL of acetonitrile was heated under reflux for 20 h.
2
3
1
(
2
4
0.76 (s, 1H), 7.76 (dd, J = 20.0, 8.6 Hz, 4H), 7.24 (s, 2H), 7.16−7.11
After being cooled to room temperature the suspension was poured into
water and extracted with dichloromethane. The combined organic
phases were dried with sodium sulfate and filtered, and the solvent was
removed under a vacuum. The product was dried under a vacuum at
m, 1H), 6.91 (d, J = 8.9 Hz, 2H), 4.40−4.18 (m, 4H), 3.97 (t, J = 6.1 Hz,
H), 2.35 (s, 3H), 1.85 (ddd, J = 24.4, 10.8, 5.3 Hz, 6H), 1.56−1.33 (m,
13
H), 1.29−1.13 (m, 10H), 0.80 (t, J = 6.2 Hz, 3H). C NMR (50 MHz,
CDCl ) δ: 161.4, 150.9, 147.0, 140.9, 137.7, 129.8, 124.7, 122.6, 121.8,
3
room temperature for 20 h to give an orange powder.
1
2
2
21.7, 114.8, 67.9, 50.3, 50.1, 31.8, 30.4, 29.1, 29.0, 28.9, 26.4, 25.9, 25.6,
1
Yield: 1,7 g (38%), H NMR (200 MHz, CDCl ) δ: 7.96−7.86 (m,
+
3
2.7, 21.6, 14.2. MS (FAB): m/z = 475 (100%, K ), 1031 (2%
2
4
H), 7.84−7.74 (m, 2H), 7.30 (d, J = 8.0 Hz, 2H), 7.07−6.97 (m, 2H),
.38 (t, J = 6.3 Hz, 2H), 3.68 (t, J = 6.3 Hz, 2H), 2.43 (s, 3H).
Synthesis of 1-Alkyl-(2-(4-((4-methylphenyl)diazenyl)phenoxy)-
+
−
*K +Br ). Elemental analysis: calcd % for C H BrN O (M = 555 g/
30
43
4
mol): C 64.85, H 7.80, N 10.08 found: C 63.51, H 8.13, N 9.79.
-Decyl-10-(2-(4-((4-methylphenyl)diazenyl)phenoxy)hexyl)-
1
ethyl)imidazolium Bromide 2 a−c. A solution of 15a and 1 equiv of
the respective alkylimidazolide was heated under reflux in acetonitrile
for 3 days. After being cooled to room temperature the solution was
poured into cold ethyl acetate and kept at −40 °C for 12 h. The orange
precipitate was filtered off and dried under a vacuum at room
temperature overnight.
1
imidazolium Bromide 3b. Yield: 51%, H NMR (200 MHz, CDCl ) δ:
3
1
7
4
=
(
1
2
(
0.81 (s, 1H), 7.82 (dd, J = 19.9, 8.6 Hz, 4H), 7.29 (d, J = 5.5 Hz, 2H),
.23−7.19 (m, 1H), 6.97 (d, J = 8.9 Hz, 2H), 4.35 (dt, J = 17.9, 7.5 Hz,
H), 4.03 (t, J = 6.1 Hz, 2H), 2.41 (s, 3H), 1.97−1.77 (m, 6H), 1.47 (d, J
13
10.7 Hz, 4H), 1.34−1.20 (m, 14H), 0.86 (t, J = 6.4 Hz, 3H). C NMR
50 MHz, CDCl ) δ: 161.4, 150.9, 147.0, 140.9, 137.9, 129.8, 124.7,
3
1
-Octyl-8-(2-(4-((4-methylphenyl)diazenyl)phenoxy)ethyl)-
22.6, 121.7, 121.6, 114.8, 67.9, 50.3, 50.1, 31.9, 30.4, 29.6, 29.5, 29.4,
1
imidazolium Bromide 2a. Yield: 54%, H NMR (200 MHz, CDCl ) δ:
3
9.1, 28.9, 26.4, 26.0, 25.6, 22.8, 21.6, 14.2. MS (FAB): m/z = 503
1
7
4
0.59 (s, 2H), 7.75 (dd, J = 19.1, 8.6 Hz, 4H), 7.58 (s, 1H), 7.22 (d, J =
.3 Hz, 2H), 7.19 (s, 1H), 6.94 (d, J = 9.0 Hz, 2H), 5.02−4.88 (m, 2H),
.52−4.39 (m, 2H), 4.28−4.12 (m, 2H), 2.35 (s, 3H), 1.83 (d, J = 6.7
+
+
−
100%, K ), 1087 (1.2%, 2*K +Br ). Elemental analysis: calcd % for
C H BrN O (M = 583 g/mol): C 65.85, H 8.12, N 9.60 found: C 63.9,
3
2
47
4
H 8.14, N 9.23.
-Dodecyl-12-(2-(4-((4-methylphenyl)diazenyl)phenoxy)hexyl)-
13
Hz, 2H), 1.29−1.11 (m, 10H), 0.78 (t, J = 5.8 Hz, 3H). C NMR (50
MHz, CDCl ) δ: 159.6, 150.7, 147.7, 141.3, 137.4, 129.8, 124.7, 123.5,
1
1
3
imidazolium Bromide 3c. Yield: 66%, H NMR (200 MHz, Acetone) δ:
1
2
22.7, 121.5, 114.8, 66.7, 50.4, 49.5, 31.7, 30.3, 29.1, 28.9, 26.3, 22.6,
1.5, 14.1. MS (FAB): m/z = 419 (100%, K ). Elemental analysis: calcd
1
9
0.29 (s, 1H), 7.94−7.73 (m, 6H), 7.36 (d, J = 8.1 Hz, 2H), 7.11 (d, J =
.1 Hz, 2H), 4.45 (dt, J = 11.3, 7.3 Hz, 4H), 4.13 (t, J = 6.4 Hz, 2H), 2.42
+
%
for C H BrN O (M = 499 g/mol): C 62.52, H 7.06, N 11.22 found:
(s, 3H), 1.82 (dd, J = 16.9, 9.2 Hz, 6H), 1.61−1.43 (m, 4H), 1.30 (d, J =
26
35
4
17.1 Hz, 18H), 0.86 (t, J = 6.4 Hz, 3H). 13C NMR (50 MHz, CDCl ) δ:
C 62.27, H 7.34, N 11.35.
-Decyl-10-(2-(4-((4-methylphenyl)diazenyl)phenoxy)ethyl)-
imidazolium Bromide 2b. Yield: 49%, H NMR (200 MHz, CDCl ) δ:
3
1
161.4, 150.8, 146.9, 140.9, 137.6, 129.8, 124.7, 122.6, 121.9, 121.7, 114.8,
1
3
67.9, 50.3, 50.0, 31.9, 30.4, 30.3, 29.7, 29.6, 29.5, 29.4, 29.1, 28.9, 26.4,
10.99 (s, 1H), 7.88 (d, J = 8.9 Hz, 2H), 7.78 (d, J = 8.3 Hz, 2H), 7.55 (s,
+
2
(
6
5.9, 25.5, 22.8, 21.6, 14.2. MS (FAB): m/z = 531 (100%, K ), 1143
+
−
1H), 7.31 (s, 2H), 7.18 (s, 1H), 6.99 (d, J = 9.0 Hz, 2H), 5.03 (d, J = 4.7
1.7%, 2*K +Br ). Elemental analysis: calcd % for C H BrN O (M =
34 51 4
Hz, 2H), 4.53 (d, J = 4.5 Hz, 2H), 4.30−4.17 (m, 2H), 2.42 (s, 3H),
11 g/mol): C 66.76, H 8.4, N 9.16 found: C 66.54, H 9.44, N 9.07.
13
2
.03−1.82 (m, 2H), 1.42−1.14 (m, 18H), 0.86 (t, J = 6.4 Hz, 3H).
C
1
-Tetradecyl-12-(2-(4-((4-methylphenyl)diazenyl)phenoxy)hexyl)-
1
NMR (50 MHz, CDCl ) δ: 159.6, 150.8, 147.9, 141.4, 138.3, 129.9,
imidazolium Bromide 3d. Yield: 48%, H NMR (200 MHz, CD CN)
3
3
1
2
4
24.8, 123.5, 122.8, 121.0, 114.8, 67.0, 49.6, 31.9, 30.3, 29.5, 29.3, 29.1,
δ: 8.68 (s, 1H), 8.04−7.81 (m, 4H), 7.49 (dd, J = 6.0, 4.8 Hz, 4H), 7.16
+
6.4, 22.8, 21.6, 14.2. MS (FAB): m/z = 349 (100%, Didecylimidazol ),
(dt, J = 4.3, 3.1 Hz, 2H), 4.33−4.10 (m, 6H), 2.53 (s, 3H), 1.91−1.81
+
13
47 (97% K ). Elemental analysis: calcd % for C H BrN O (M = 527
(m, 6H), 1.75−1.56 (m, 4H), 1.36 (s, 22H), 0.98 (t, J = 6.5 Hz, 3H). C
28
39
4
g/mol): C 63.75, H 7.45, N 10.62 found: C 65.03, H 8.75, N 10.71.
NMR (50 MHz, CDCl ) δ: 161.5, 150.8, 146.9, 141.0, 137.8, 129.8,
3
C
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
a
Scheme 1. Synthesis of Compound 1a−e
a
(
i) MeOH, Mg, rt; (ii) NBS, BPO, CHCl , reflux, 6 h; (iii) CH CN, reflux, 20 h; (iv) THF, reflux, 20 h; (v) CH CN, reflux, 20 h.
3 3 3
1
2
24.8, 122.6, 121.8, 121.6, 114.8, 67.9, 50.3, 50.1, 32.0, 30.4, 29.8, 29.7,
9.6, 29.5, 29.1, 28.9, 26.4, 26.0, 25.6, 22.8, 21.6, 14.2, 7.9. MS (FAB):
with silver behenate and the sample−detector position was fixed
at 455 mm. For measurements, the samples were placed between
aluminum foil in a copper sample holder, the sample chamber
was evacuated, and the temperature was controlled via a JUMO
IMAGO 500 multichannel process and program controller. The
program “a2tool” (Hasylab) was used for data reduction,
analysis, and correction.
UV−visible absorption spectra were measured at room
temperature on solutions in 1 cm cuvettes using an Agilent
Cary 50 spectrometer. Photoisomerization experiments were
performed in the sample chamber of a Horiba Jobin Yvon
Fluorolog-3 spectrometer using the instrument’s continuous
xenon lamp (450 W) and double grating to achieve
monochromatic light. Absorption spectra were recorded within
the sample chamber using fiber optics coupled to the Agilent
Cary 50 spectrometer via the fiber optic accessory. During all
measurements the samples were well stirred and the temperature
controlled.
+
+
m/z = 461 (100%, ditetradecylimidazol ), 559 (42%, K ). Elemental
analysis: calcd % for C H BrN O (M = 639 g/mol): C 67.59, H 8.67,
N 8.76 found: C 67.06, H 8.57, N 8.58.
-Hexadecyl-12-(2-(4-((4-methylphenyl)diazenyl)phenoxy)hexyl)-
imidazolium Bromide 3e. Yield: 60%, H NMR (200 MHz, CDCl ) δ:
36
55
4
1
1
3
1
0.81 (s, 1H), 7.82 (dd, J = 20.0, 8.5 Hz, 4H), 7.30 (s, 2H), 7.26−7.17
m, 2H), 6.97 (d, J = 9.0 Hz, 2H), 4.35 (dt, J = 18.5, 7.3 Hz, 4H), 4.03 (t,
J = 6.1 Hz, 2H), 2.41 (s, 3H), 2.08−1.74 (m, 6H), 1.50 (s, 4H), 1.26 (d, J
11.8 Hz, 24H), 0.86 (t, J = 6.4 Hz, 3H). 13C NMR (50 MHz, CDCl )
(
=
3
δ: 161.4, 150.9, 147.1, 140.9, 138.0, 129.8, 124.7, 122.7, 121.6, 121.5,
1
2
5
14.8, 67.9, 50.1, 32.1, 30.4, 29.8, 29.6, 29.5, 29.1, 28.9, 26.4, 26.0, 25.6,
+
2.8, 21.6, 14.3. MS (FAB): m/z = 517 (100%, dihexadecylimidazol ),
+
87 (20%, K ). Elemental analysis: calcd % for C H BrN O (M = 667
38
59
4
g/mol): C 68.34, H 8.91, N 8.39 found: C 67.28, H 8.55, N 8.39.
INSTRUMENTATION
■
Differential scanning calorimetry (DSC) was performed with a
computer-controlled PhoenixDSC 204 F1 thermal analyzer
Single crystal X-ray measurements were carried out on a Stoe
IPDS-I single-crystal X-ray diffractometer with graphite mono-
chromated Mo Kα radiation (λ = 0.71073 Å at 100 K). Crystal
(Netzsch, Selb, Germany). Measurements were carried out at a
heating rate of 5 °C/min in sealed aluminum crucible with an
argon flow rate of 20 mL/min. The samples were placed in
aluminum pans which were cold-sealed. Given temperatures
correspond to the onset of the respective thermal process.
Optical analyses were made by heated-stage polarized optical
microscopy (POM) with an Axio Imager A1 microscope (Carl
34
structure solution by direct methods using SIR 92 yielded the
35
heavy atom positions. Refinement with SHELXL-97 allowed
for the localization of the remaining atom positions. Hydrogen
atoms were added and treated with the riding atom mode. Data
36
reduction was performed with the program package X-Red and
numerical absorption correction was carried out with the
̈
Zeiss MicroImaging GmbH,Gottingen, D) equipped with a hot
37
stage, THMS600 (Linkam ScientificInstruments Ltd., Surrey,
UK), and Linkam TMS 94 temperature controller (Linkam
Scientific Instruments Ltd., Surrey, UK). Images were recorded
at a magnification of 100× as a video with a digital camera during
heating and cooling the sample which was places between two
program X-Shape. To illustrate the crystal structures, the
38
program Diamond was used.
RESULTS AND DISCUSSION
■
−1
coverslips. Heating and cooling rates were 5 K/min .
Synthesis. Two different synthetic routes were chosen for the
symmetric (1) and the asymmetric azo-compounds (2 and 3),
both starting with an azobenzene coupling. The first step in the
synthesis of the symmetric type 1 cations was the reductive
coupling of p-nitrotoluene with magnesium (Scheme 1).
Temperature-dependent SAXS experiments were carried out
at the BW4 Beamline of DORIS III, Hasylab (DESY, Hamburg,
Germany) at a fixed wavelength of 1.38 Å. The data were
collected with a MarCCD detector. The detector was calibrated
D
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
a
Scheme 2. Synthesis of Compounds 2a−c and 3a−e
a
(
i) NaNO , NaOH, Na CO , HCl, H O, rt, 4 h; (ii) K CO , CH CN, reflux, 18 h.
2 2 3 2 2 3 3
Figure 2. Crystal structure of compound 1a.
Figure 3. Crystal packing of compound 1a (left) S-conformation of the cation; (right) structural segregation of polar (red) and apolar (green) domains.
Bromination of the two methyl groups was achieved with
bromosuccinimide (NBS) in the presence of benzoyl peroxide
imidazolide and then alkylated with the respective alkyl bromide
(1b−e).
(
BPO). Addition of hexyl-imidazole results in the formation of
the ionic compound 1a. For the salts with longer alkyl chains the
,4′-bis(bromomethyl)azobenzene was reacted with sodium
The azobenzene core for the asymmetric compounds was
formed via base catalyzed azo-coupling of p-methylaniline and
4
phenol (Scheme 2). After formation of the bromo-alkyl-ether the
E
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
corresponding alkyl-imidazole was alkylated with this to form the
compounds 2a−c and 3a−e.
Structural Analysis. Representative single crystals for all sets
of cations could be grown that allowed for single crystal structure
analysis. Single crystals of compound 1a (Azo(-C1-Im-C6)2)
could be grown from methanol solution. 1a crystallizes in the
alternating with bromide counteranions. Within the bilayer the
opposing cations are tilted with an angle of about 60° (Figure 6,
left).
In 2b the cations also adopt a U-shape, but single layers are
formed by head-to-tail arranged cationic entities (Figure 6,
right). Thus, within one layer (parallel to the a,c plane), all
cations have the same orientation. Along the b-axis the
orientation of the cations alternates, and the open sides of the
“U” are facing in opposite directions.
monoclinic space group P2 /c. The planar azobenzene unit
1
adopts a trans conformation, and the imidazolium rings point in
opposite directions from the azobenzene with an angle of 107°
between the azobenzene plane and a plane through the
imidazolium ring (Figure 2). The alkyl chains form an angle of
The crystal structure of 2c is distinctly different to the other
two structures, because the two sides of the “U” units are forced
apart and the ion is rather resembling a L (Figure 7, right).
Viewed along the a-axis the alkyl chain and the azobenzene unit
are perpendicular to the imidazolium ring and parallel to each
other (Figure 7, left). The cations make up a bilayer with a head-
to-head arrangement of the imidazolium cores. When viewed
along the b-axis, it becomes obvious that the cations no longer
resemble a U but more an L (Figure 7, right). The angle between
the arms of one cation is about 90°. (Measured as the angle
between the first carbon of the alkyl chain, the C2 of the
imidazolium ring and the methyl group at the para position of the
azobenzene.) Layers with alternating charged, polar and apolar
domains are visible in all three structures (2a, 2b, and 2c). The
compounds with the shortest (C H ) and with the longest
112° with the imidazolium plane and point back in the direction
of the azobenzene unit (Figure 2, left). Thus, a single cation
adopts an S-shape configuration with all moieties (azobenzene,
imidazole, alkyl chains) in one plane. The two counter bromide
anions are located at the top and at the bottom of the S (Figure 3,
left) featuring very weak Br···H-interactions (Figure 2, right).
Within the bc-plane the bications arrange in layers with
interdigitated alkyl chains (Figure 2, left). The azobenzene-
planes of neighboring cations are perpendicular to each other;
thus, the two alkyl chains of one cation interdigitate with those of
two other cations.
A structural segregation of polar (red) and apolar (green)
domains can be made out within the layers (Figure 3, right). The
polar region is composed of aromatic azobenzene units and
cationic imidazolium groups. The hydrophobic alkyl chains form
the apolar domains. The aromatic parts of different cations form
chains through the layer supported by interionic interactions
8
17
(C H ) alkyl chain feature bilayers with head-to-head arranged
12
25
cations. In contrast, the salt with an intermediate chain length of
10 carbon atoms (2b) forms a single layered structure. From the
imidazolium ring to the methyl group of the azobenzene a
distance of about 14 Å is measured in all three crystal structures.
The C H chain has approximately the same length as this
(Figure 3, right). While no π−π interactions can be detected for
the azobenzene units, π−π interactions can be assumed between
the imidazolium rings, although positively charged, as the
distance between imidazolium cations belonging to two different
layers is rather small (3.5 Å).
12
25
entity, while being shorter for 2b (11.5 Å) and 2a (8.9 Å),
respectively. Thus, with varying the alkyl chain length the relative
influence of the different interactions (π−π vs van-der-Waals vs
ionic) is shifted, and as a consequence, the ion packing and the
crystal structure changes.
Single crystals of compounds 2a−c could be grown either from
a dichloromethane solution or a mixture of dichloromethane and
acetonitrile. Compound 2a (C1-Azo-O-C2-Im-C8) crystallizes
in the acentric triclinic space group P1, compound 2b (C1-Azo-
O-C2-Im-C10), and 2c (C1-Azo-O-C2-Im-C12) crystallize in
The compounds 3c and 3e crystallize in the monoclinic space
group P2 /c and are isostructural; therefore only the crystal
1
structure of compound 3e will be discussed here in detail. As
expected, the planar azobenzene unit has a trans conformation.
The two alkyl chains stretch out in opposite directions from the
imidazolium head (Figure 8). The terminal hexadecyl chain has
an all trans conformation except for the bond from the carbon
atom connected to the imidazolium core and the second carbon
of the alkyl chain. The middle hexyl chain starts in trans
conformation from the imidazolium but is bent in the middle.
The bonds from the fourth to the fifth and from the fifth to the
sixth carbon atom in this row are gauche. Thus, the cations adopt
a zigzag structure with parallel arrangement (Figure 9). Within
the layers two cations form a pair and cross each other in the
middle (Figure 8). The orientation of these ion pairs is
alternating from layer to layer, so that always the alkyl tail of
one cation is facing the azobenzene unit of the cation in the next
layer. The cationic imidazolium head lies in the middle of the
layers surrounded by the bromide counteranions.
̅
the triclinic space group P1. The azobenzene unit adopts the
generally thermodynamically favored trans conformation in all
three structures. In 2a the imidazolium ring is situated
perpendicular to the planar azobenzene group as well as to the
alkyl chain which is parallel to the azobenzene leading to an U-
shape of the cation (Figure 6, left). The bromide anions in 2a are
involved in hydrogen bonding with the first carbon-H of the
chain (Figure 4, Table 1), whereas in 2b and 2c one of the two
CH-groups of the triazolium ring forms C−H···Br hydrogen
bonds to the bromide anion (Figure 5).
The crystal structure of 2a shows that the cations form a
bilayer structure with head-to-head arranged triazolium groups
Thermal Behavior. The thermal properties of all com-
pounds were examined by POM and DSC. The transition
temperatures, enthalpies, and phase transition assignments are
listed in Table 2. Discrepancies to various degrees both in the
transition temperatures as well as enthalpies between heating and
cooling cycles are observed. The reason for this lies in the slow
transition kinetics which prevent that thermodynamic equili-
brium is reached. Thus, supercooling and incomplete phase
Figure 4. Hydrogen bonding interactions in detail for 2a.
F
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 1. Hydrogen Bond Details for 2a, 2b, and 2c
C1-Azo-O-C2-Im-C8
C1-Azo-O-C2-Im-C10
C1-Azo-O-C2-Im-C12
C12−H12B···Br
2.9457(6)
C17−H17···Br
2.6544(4)
C13−H13···Br
2.8247(3)
H···A (Å)
H···A (deg)
150.328(2)
170.863(2)
160.496(1)
results in a complete crystallization at 66 °C followed by melting
into the isotropic liquid at 138 °C, as observed in a previous
thermal cycle. In contrast, 2b exhibits more complex phase
behavior with one endothermic and three exothermic events in
the heating curve (Figure 11). The exothermic event at 7 °C and
the two endothermic events at −13 and 92 °C belong to solid−
solid phase transitions as evidenced by POM. At 114 °C a
smectic mesophase is formed. This phase can be assigned on the
basis of its optical defect textures: oily streak textures were
observed by POM (Figure 13). At 149 °C 2b melts into an
isotropic liquid. Three exothermic peaks were observed during
cooling, which were attributed to Liso → SmA (144 °C), SmA →
Cr″ (85 °C), and Cr″ → Cr′ (76 °C) transitions. 2c exhibits a
smectic A phase at 164 °C upon cooling the isotropic liquid. No
crystallization occurs during further cooling, but at 56 °C upon
reheating. The smectic A phase is then formed (126 °C) before
the clearing point is reached at 167 °C. Within this group of
azobenzene salts the clearing points increase with increasing
chain length, and the compound with the longest alkyl chain (2c)
has the largest mesophase temperature window. The thermal
behavior of these three compounds differs significantly, reflected
by the distinctly different crystals structures.
The compounds 3a−d do not show any liquid crystalline
behavior (Figure 12). Peaks in addition to the melting peaks can
be attributed to solid−solid phase transitions (Table 1). No
trend for the melting temperatures of these ILs can be
recognized: 3a and 3b melt around 75 °C, melting of 3c starts
at 98 °C, and 3d with an even longer alkyl chain melts at 86 °C.
However, the salt with the longest alkyl chain (C16, 3e) exhibits
a smectic A phase in a very narrow temperature window (97−86
°C) when cooling the molten salt (Table 2). Such a mesophase
formation was not observed upon heating the other compounds
like 3d. 3d directly melts from the crystalline into the liquid phase
(Figure 12). As 3c and 3e are isostructural, it seems to be valid to
assume that the structures of all other compounds in this series
are similar, too. This allows us to conclude that the length of the
alkyl chain length essentially determines the ability to form a
mesophase. With 16 carbon atoms the alkyl chain of 3e is long
Figure 5. Hydrogen bonding interactions in detail for 2b (top) and 2c
bottom).
(
transitions are favored, a phenomenon quite often observed for
compounds that belong to the class of ILs.
Figure 10 shows the heating curves of the symmetric
azobenzene salts 1a−e. For all five compounds decomposition
begins when the substances are about to melt. Therefore, the
cooling curves are featureless (not shown here). 1a and 1b with
the shortest alkyl chains melt into an isotropic liquid at 212 and
2
1
25 °C without showing signs for a preceding phase transitions.
c melts in the same temperature region (222 °C) but features an
additional thermal effect around 140 °C. As no phase change
could be observed by POM, we attribute this thermal event to a
solid−solid phase transition. For the salts with the longer alkyl
chains the melting points and decomposition temperatures are
slightly lower. 1d exhibits the formation of a mesophase around
150 °C and 1e around 100 °C. The thermal event at lower
temperature (75 °C) is due to a solid−solid phase transition.
Figure 11 shows the second heating cycle of the asymmetric
azobenzene compounds with the short alkyl chains in the middle
(
2a−c). During cooling of 2a from the isotropic liquid, a partial
crystallization occurs around 95 °C. Reheating of the sample
Figure 6. Crystal structure of 2a (left), 2b (right). For clarity the H atoms are omitted and some ions are displayed transparent.
G
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Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 7. Crystal structure of 2c.
Figure 8. Crystal structure of 3e viewed along the a-axis.
The layer spacing increases when the mesophase is adopted, as
commonly observed for smectic A phases (Figure 14).
Photochromic Properties. The UV−vis spectra 1b, 2a, and
3a dissolved in methanol are shown in Figure 15. (UV−vis
spectra of all 13 compounds can be found in the Supporting
Information.) All compounds show similar absorption spectra
with an absorption band maximum around 340 nm, a weaker
absorption band around 230 nm, and a very weak band around
4
40 nm. The band at 340 nm can be assigned to the π−π*
transition of the trans isomer and the one at 440 to the n−π*
transition, as observed in similar compounds.
17,25
The π−π*
band is clearly blue-shifted for the symmetrical azobenzene
compounds (1a−c): 321 nm compared to 343 or 348 nm for the
asymmetric azobenzene salts (2a−c and 3a−e). This is due to
the different substitution of the azobenzene moiety: alkyl for 1a−
c and alkoxy for compounds 2a−c and 3a−e. The red shift of 5
nm from 343 nm (2a) to 348 nm (3a) might be due to the
electron-donating ability of the alkyl groups. The same shifts can
be observed for the weaker absorption bands at 222, 236, and 239
nm, respectively. However, no shift for the n−π* band is found.
For all compounds this weak band lies at 440 nm.
Figure 9. Crystal structure of 3e viewed along the b-axis.
enough to separate the polar domains in the ion packing
(compare Figure 8) and to allow the ions to form a smectic A
phase before melting to the isotropic liquid. The identification of
transition temperatures and the proof of mesophase formation
were undertaken by POM. Figure 13 shows some representative
POM textures for observed mesophases obtained by heating and
cooling of the compounds.
Azobenzene compounds undergo cis−trans isomerization
when irradiated with the appropriate wavelength. After synthesis
the compounds are mainly in the thermodynamically stable trans
conformation which then can be switched to the cis conformation
by irradiation with UV-light (Figure 16). When irradiating 1a
with UV-light (366 nm) a decrease in the strong π−π*
absorption band (321 nm) and a simultaneous increase in the
weak absorption band at 440 nm are observed (Figure 16).
Additionally a new absorption band at 249 nm grows, while the
weak band at 222 nm disappears. This change can be detected in
transmission measurements on methanolic solutions as well as in
reflectance measurements on the pure sample (Figure 16). It was
not possible to observe the isomerization in the liquid crystalline
Small Angle X-ray Scattering. To identify the mesophases
of the symmetric compounds 1a−e and to comprehend the
molecular arrangement of the azobenzene salts temperature-
dependent SAXS measurements were carried out. The layer
distances, as calculated from Bragg’s law, are listed in Table 3.
The distance of 14 Å observed from SAXS for 1a can be
identified as the layer spacing in the crystal structure. As
compounds 1b and 1c show a similar layer distance the same
molecular arrangement can be assumed. Upon further extension
of the alkyl chain (C12 and C14) the crystal structure changes
and a significantly larger layer distance was measured (Table 3).
For that reason, an interdigitated rod-shaped structure is
supposed for compounds 1d and 1e in the crystalline phase.
H
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 2. Transition Temperatures and Enthalpies Obtained from DSC Thermograms
a
a
heating
cooling
b
c
no.
formula
transition
T [°C]
ΔH [kJ/mol]
T [°C]
ΔH [kJ/mol]
Td [°C]
1
1
1
a
b
c
Azo(-C1-Im-C6)2
Azo(-C1-Im-C8)2
Azo(-C1-Im-C10)2
Cr → LISO
Cr → LISO
Cr→?
212.6
242.1
144.2
222.2
151.9
181.4
75.8
60.3
59.5
9.5
277
238
253
?
→LISO
55.4
2.1
1
1
d
e
Azo(-C1-Im-C12)2
Azo(-C1-Im-C14)2
Cr→SmA
SmA→LISO
Cr‘→Cr
262
266
10.3
10.6
9.5
Cr→SmA
SmA →LISO
Cr‘→Cr
101.7
184.6
66.3
9.9
2
2
a
C1-Azo−O−C2-Im-C8
C1-Azo−O−C2-Im-C10
−20.2
29.9
0.5
272
267
Cr→Liso
137.6
−13.2
7.8
94.4
−2.4
b
Cr′→Cr
Cr→Cr′
−2.1
8.4
Cr′→Cr″
Cr″→SmA
SmA →LISO
SmA→Cr
Cr→SmA
SmA →LISO
Cr→Cr′
92.3
75.8
85.2
−1.0
−20.4
−4.3
114.4
146.9
56.1
22.9
3.8
144.3
2
3
c
a
C1-Azo−O−C2-Im-C12
C1-Azo−O−C6-Im-C8
−16.4
18.3
2.7
264
268
126.5
167.0
24.2
164.2
−2.9
−20.9
1.6
Cr′→Cr
62.4
Cr′ →LISO
Cr′→Cr
Cr′ →LISO
Cr′→Cr
Cr′ →LISO
Cr′→Cr
Cr′ →LISO
Cr →LISO
S →Cr
75.1
12.5
−10.4
40.4
40.8
3
3
3
3
b
C1-Azo−O−C6-Im-C10
C1-Azo−O−C6-Im-C12
C1-Azo−O−C6-Im-C14
C1-Azo−O−C6-Im-C16
20.9
267
270
273
280
74.7
25.1
45.1
77.4
23.8
79.3
−11.0
−13.1
−7.0
−1.4
−15.3
c
92.9
98.0
d
e
24.7
2.2
28.3
31.7
85.8
99.6
84.6
96.9
−2.6
−31.6
LISO → SmA
a
b
Phase-transition temperatures (°C) and enthalpies (kJ/mol, in parentheses) are measured during heating and cooling at 5 K/min. The phase
transitions were identified by POM and SAXS measurements. Thermal decomposition temperature.
c
Figure 11. DSC heating traces of compounds 2a−c.
Figure 10. DSC heating traces of compounds 1a−e.
phase, because the temperatures are too high and the thermal
back reaction takes place, so that no cis form is accumulated.
This photoisomerization is reversible and can be monitored
through the change in absorbance at a certain wavelength (Figure
effective conversion constant k can be calculated for the trans
to cis conversion and for the cis to trans isomerization (Table 3).
The effective conversion constants of the two reactions cannot be
compared directly, because the intensities of the absorbance and
of the irradiation light are different for the two species. However,
1
7A). With these measurements the photoisomerization
I
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 3. Layer Spacing (d, Å) of Compounds 1a−e
no.
formula
25 °C
150 °C
180 °C
1
1
1
1
a
b
c
Azo(-C1-Im-C6)2
Azo(-C1-Im-C8)2
Azo(-C1-Im-C10)2
Azo(-C1-Im-C12)2
14
13.4
14.4
38.6
d
39.7
19.7
13.1
56.6
42.9
21.5
14.1
10.6
1
1
9.2
2.7
1
e
Azo(-C1-Im-C14)2
54.4
3
1
1
8.6
9.2
2.8
42.9
14.2
10.7
9
.6
Figure 12. DSC heating traces of compounds 3a−e.
a comparison within the series of synthesized compounds is
possible. For the symmetric compounds 1a−e a significant
decrease in effective conversion constant with increasing chain
length is observed for both conversions. The trans−cis
isomerization is slowed down when the alkyl chains become
longer. For the asymmetric compounds 2a−c and 3a−e with the
azobenzene group at one end, no significant influence of the alkyl
chain length on the isomerization rate was found. The influence
of the solvent on the effective conversion constant was
investigated for compound 2b (Figure 17B). To this avail,
effective conversion constants were determined for solutions of
Figure 14. Schematic model of the molecular arrangement in the
smectic A phase of compounds 1d and 1e.
2
b in solvents of different polarity (Table 3). The solvents can be
ordered with decreasing effective conversion constant: MeOH >
EtOH > PrOH > BuOH > THF. The isomerization is slowed
down with decreasing polarity.
CONCLUSION
■
A set of 13 new azobenzene-based bromide salts were
synthesized, and their thermal and photochromic properties
were investigated. Two different systems based on imidazolium
cations were studied: a monocation, featuring a n-alkylimidazo-
lium group connected via an alkoxy chain (ethyl or hexyl) to a
terminal azobenzene unit, and a symmetrical bication with a
azobenzene group linking to two imidazolium rings which are
substituted with alkyl chains. The alkyl chain length was
systematically varied to see the influence on the thermal and
photophysical behavior. All bicationic compounds decompose
during the melting process. The salts with the longest alkyl chains
(
1d and 1e) exhibit a liquid crystalline phase from 150/102 °C
until 181/184 °C. The compounds with a terminal azobenzene
Figure 15. Normalized UV−vis spectra of compounds 1b, 2a, and 3a.
group and a long alkoxy chain (−OC H −, 3a−3e) qualify as
6
12
ILs as they melt below 100 °C. In the solid state, when the
Figure 13. Representative textures as seen between crossed polarizers: (left) LC phase of 1b at 137.6 °C, (middle) SmA phase of 2b at 100.3 °C, (right)
SmA phase of 3e at 82.7 °C.
J
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 16. Left: change of the extinction coefficient of compound 1a in methanol when irradiated with UV-light (366 nm); right: change of the
absorption of a solid sample upon irradiation.
Figure 17. (A) Time evolution of absorbance of 1a at 320 nm upon UV and visible light irradiation. (B) Effective conversion constant k of 2b in
dependency of the solvent.
Table 4. Calculated Effective Conversion Constants of All
Compounds
tail single layer (2b), or head-to-head linear bilayer with an
opened U (L) shape (2c). A smectic A phase is formed by the
cations with a short alkoxy chain and a long alkyl chain (2b and
compound
solvent
ktrans−cis (1/min)
kcis−trans (1/min)
2c) and by the cation with the longer alkoxy chain and the
1
1
1
1
1
2
2
2
3
3
3
3
3
2
2
2
2
a
b
c
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
0.60978
0.12996
0.10718
a
0.47528
0.10283
0.08438
a
longest alkyl chain (3e). The synthesis affords all compounds in
the thermodynamically stable trans conformation. By irradiation
with UV-light (320−366 nm) they undergo a trans−cis
isomerization which reverses under visible light (440 nm). The
effective conversion constants of these reactions were calculated
and show that the chain length has no influence on the
photoisomerization of compound with terminal azobenzene
units, but in the bicationic compounds the long alkyl chains
hinder the isomerization process. The influence of the
photoisomerization on the mesophase could not be investigated,
because the liquid crystalline phases of these cations are formed
at high temperatures (>100 °C), where the thermal reverse
reaction is faster than the UV-induced trans−cis transformation.
Thus, to develop photoswitchable ILCs the phase transition
temperatures need to be further lowered.
d
e
a
b
c
a
a
0.40176
0.44124
0.43997
0.47929
0.43037
0.44149
0.42897
0.52874
0.38644
0.36214
0.35508
0.34428
0.1094
0.11186
0.11055
0.11418
0.12394
0.1225
0.12205
0.12056
0.09238
0.10991
0.09671
0.07728
a
b
c
d
e
b
b
b
b
PrOH
BuOH
THF
ASSOCIATED CONTENT
Supporting Information
a
■
Not measured.
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.5b01024.
connecting carbon chain is longer (−OC H −) an elongated
6
12
rod-shape is adopted by the cation, in contrast to the cations with
Crystal structure of compound 3c, table of crystallographic
and refinement details, normalized UV-Vis spectra (PDF)
a short alkoxy-bridge (−OC H −), where the cations form a U-
2
4
shape. For the U-shaped cations bearing an (−OC H −) bridge,
2
4
Crystallographic information file for C H Br N (CIF)
the terminal alkyl chain influences the ion packing. The U-shaped
cations either form a head-to-head tilted bilayer (2a), a head-to-
32 44
2
6
Crystallographic information file for C26
H35BrN
O (CIF)
4
K
DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Crystallographic information file for C H BrN O (CIF)
(26) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules
2
8
39
4
1
(
(
992, 25, 2268−2273.
Crystallographic information file for C H BrN O (CIF)
3
0
43
4
27) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873−1875.
Crystallographic information file C H BrN O (CIF)
3
4
51
4
28) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995,
Crystallographic information file C H BrN O (CIF)
3
8
59
4
6
6, 1166−1168.
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2000, 231, 255−264.
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(
AUTHOR INFORMATION
Corresponding Author
E-mail: anja.mudring@ruhr-uni-bochum.de, mudring@iastate.
■
(
3
(
*
edu. Tel: +49-234-32-27408, +1 515 5095616.
Cryst. 2008, 11, 1299−1305.
(32) Yam, V. W. W.; Lee, J. K. W.; Ko, C. C.; Zhu, N. Y. J. Am. Chem.
Soc. 2009, 131, 912.
Notes
The authors declare no competing financial interest.
(
33) Zhang, S.; Liu, S.; Zhang, Q.; Deng, Y. Chem. Commun. 2011, 47,
641−6643.
34) SIR-92: Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi,
A. J. Appl. Crystallogr. 1993, 26, 343.
35) Sheldrick, G. M. SHELXL-97, Programs for Crystal Structure
Analysis; University of Gottingen: Germany, 1997.
6
(
ACKNOWLEDGMENTS
■
This work was supported in part by the German Science
Foundation DFG, Iowa State University, and the Critical
Materials Institute, an Energy Innovation Hub funded by the
U.S. Department of Energy, Office of Energy Efficiency and
Renewable Energy, Advanced Manufacturing Office.
(
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(36) X-RED, v. 1.22, Stoe Data Reduction Program (C); Stoe & Cie
GmbH: Darmstadt, Germany, 2001.
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37) X-Shape, v. 1.06, Crystal Optimisation for Numerical Absorption
Correction (C); Stoe & Cie GmbH: Darmstadt, 1999.
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DOI: 10.1021/acs.cgd.5b01024
Cryst. Growth Des. XXXX, XXX, XXX−XXX