Reversibly photoswitchable alkoxy azobenzenes connected benzenetricarboxamide discotic liquid crystals with perpetual long range columnar assembly

Article abstract of DOI:10.1039/c8ob01579a

Liquid crystals (LCs) with photoswitchable groups are very interesting owing to their dual applications. In this regard, we report the synthesis of long chain alkoxy azobenzene incorporated benzenetricarboxamides 7a-c based room temperature columnar LCs. Apart from the light induced isomerization in the solution phase, the salient feature of these systems is the reversible photoisomerization even in the bulk state with perpetual columnar self-assembly at room temperature. Based on the observation of mesomorphic textures under polarised optical microscopy (POM) and grazing incidence small/wide angle X-ray scattering (GISAXS/GIWAXS) studies, the columnar assembly was found to be stable upon photoisomerization. However, subtle changes in height profile have been observed in AFM measurements after photoswitching. Interestingly, a temperature dependent change between rectangular and hexagonal mesophases in 7a has been observed. Upon extending the alkoxy chain length, only the hexagonal mesophase was observed. For comparison, the corresponding N-methylated derivative of 7a has also been synthesized. Despite the better photoswitching behaviour, due to the lack of planarity and H-bonding, 8a did not show any columnar mesophase.

Full text of DOI:10.1039/c8ob01579a

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Reversibly photoswitchable alkoxy azobenzenes
connected benzenetricarboxamide discotic liquid
crystals with perpetual long range columnar
assembly†
Cite this: DOI: 10.1039/c8ob01579a
Sudha Devi,‡a Indu Bala,‡a Santosh Prasad Gupta,b Pravesh Kumar,a
a
Santanu Kumar Pal *a and Sugumar Venkataramani
*
Liquid crystals (LCs) with photoswitchable groups are very interesting owing to their dual applications. In
this regard, we report the synthesis of long chain alkoxy azobenzene incorporated benzenetricarbox-
amides 7a–c based room temperature columnar LCs. Apart from the light induced isomerization in the
solution phase, the salient feature of these systems is the reversible photoisomerization even in the bulk
state with perpetual columnar self-assembly at room temperature. Based on the observation of meso-
morphic textures under polarised optical microscopy (POM) and grazing incidence small/wide angle
X-ray scattering (GISAXS/GIWAXS) studies, the columnar assembly was found to be stable upon photo-
isomerization. However, subtle changes in height profile have been observed in AFM measurements after
photoswitching. Interestingly, a temperature dependent change between rectangular and hexagonal
mesophases in 7a has been observed. Upon extending the alkoxy chain length, only the hexagonal meso-
phase was observed. For comparison, the corresponding N-methylated derivative of 7a has also been syn-
thesized. Despite the better photoswitching behaviour, due to the lack of planarity and H-bonding, 8a did
not show any columnar mesophase.
Received 5th July 2018,
Accepted 8th October 2018
DOI: 10.1039/c8ob01579a
rsc.li/obc
Besides this, azobenzene based compounds can be very
useful in making light modulated switches.8–10 Tuning of the
Introduction
Recently, there has been increasing interest in discotic liquid light induced E–Z isomerization channels and the thermal
crystalline (DLC) materials1–3 endowed with photoresponsive stability of the Z-isomer is crucial in making azobenzene
groups.4a These materials have emerged as fascinating func- based photoswitches.11–15 Primarily, the structural changes
tional materials as they combine the optical properties of accompanying isomerization provide many opportunities in
photochromic groups and self-assembling and dynamic fea- the design and incorporation of azobenzenes in the molecular
tures of DLCs.4b,c Photochromic liquid crystals (LCs) are very systems. Usually, the LC phases of azobenzene incorporated
useful as they can change their mesomorphic state upon compounds changed to the isotropic phase on irradiation
irradiation.5–7 Among the various photochromic groups like because of the destabilization of the LC phases by
spirooxazines, diarylethenes, fulgides etc., azobenzene Z-azobenzene.16 Particularly, when multiple azobenzenes are
systems4b,5–7 are widely studied owing to the rich photochemis- introduced through amide linkages, the resulting system can
try of the azo chromophore and the ease of functionalization form an interesting supramolecular assembly. The H-bonding
at aryl groups.
reinforced with π–π stacking provides the necessary driving
force in this regard. Such systems have found applications in
remote-controllable actuators,6c photoresponsive surfactants,17
light induced mass transport18 etc. Lee et al. explored the
stimulus responsive supramolecular assemblies such as fibers,
gels, and spheres using these photoswitchable triamides.19
Recently, Thiele and co-workers reported the Z–E isomerisa-
tion processes of multi-state photochromic benzenetricarboxa-
mide systems using in situ irradiation NMR spectroscopy.20
This revealed that the photoisomerization and the reverse
aDepartment of Chemical Sciences, Indian Institute of Science Education and
Research (IISER), Mohali, Sector-81, Knowledge city, Manauli-140306, India.
E-mail: skpal@iisermohali.ac.in, sugumarv@iisermohali.ac.in
bDepartment of Physics, Patna University, Patna- 800005, India
†Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedure, characterization details, optical microscopy studies, X-ray diffraction
studies and photophysical studies. See DOI: 10.1039/c8ob01579a
‡These authors contributed equally.
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thermal isomerization processes are not cooperative, and also thesized using 4a-NMe. The detailed synthetic procedures for
depend on the concentration. all the compounds are described in the ESI.† All the intermedi-
Furthermore, the utilization of these benzenetricarboxa- ates and the target molecules have been characterized by
mide cores can create DLCs, due to the well-known behaviour 1H, 13C-NMR, IR, UV-Vis and HRMS spectral techniques
for the formation of columnar mesophases. Hence, in order to (Fig. S12–S32, see the ESI†).
find a reliable class of systems, which exhibits LC behaviour at
Analysis of photoswitching behaviour of 7a–c/8a by using
UV-Vis and NMR (in solution state)
ambient temperature as well as photochromism, we incorpor-
ated multiple photochromic groups in our design with alkoxy
groups on its periphery. The photochromic moiety is com-
posed of three photoactive azobenzene side groups that are
directly linked to a central tricarboxamide benzene core
(Scheme 1). Herein, we reported the investigations on the LC
properties of these systems and also their photoswitching
behaviour in solution as well as in the LC form.
The photoswitching behaviour of all the target molecules 7a–c
and the N-methylated derivative 8a has been studied in the
solution phase (CHCl3 and DMSO) and analysed by UV-Vis
spectroscopy (Fig. 1, S1–S5, see the ESI†). The photoswitching
in the solid state for 7a (vide infra) and 8a (Fig. S6, see the
ESI†) has been carried out at room temperature. Additionally,
the analysis of photoswitching behaviour has also been carried
out using an NMR spectroscopic technique. Upon irradiation
at 365 nm light, the molecules isomerize to all-cis configur-
ation, inferred from the spectral changes in the π–π* and n–π*
absorptions. All the three target compounds showed almost
identical absorption spectra for both before and after switch-
ing. The absence of spectral changes clearly indicated that the
chain length did not perturb the absorption spectra of all the
three molecules at all. To envisage the thermal stability of the
resulting isomer, kinetics has been followed for all the three
isomers (Fig. S9–S11, see the ESI†). The net rate of formation
of 7a–c-EEE is found to decrease upon increasing the alkyl
chain length (Fig. S9–S11,† Table 1). Similarly the
N-methylated 8a has also been subjected to photoswitching.
Under comparable concentration in CHCl3, 8a showed a fast
photoswitching leading to all-cis isomer; however, the reverse
isomerization did not happen even after prolonged irradiation
at different wavelengths. In contrast, the DMSO solution of 8a
showed almost quantitative reverse photoswitching at 505 nm.
Since UV-Vis spectroscopy is unable to differentiate among the
Synthesis of target molecules 7a–c/8a
The target molecules 7a–c/8a have been synthesized using
4-aminoacetanilide 1 as the starting material (Scheme 1).
Compound 1 has been diazotized and treated with phenol to
obtain the hydroxyazobenzene derivative 2 as per the literature
procedure.21 In order to obtain the ethers 3a–c, O-alkylation
has been performed with different alkyl bromides (n-hexyl,
n-octyl and n-decyl bromides) using potassium carbonate as a
base in the presence of a catalytic quantity of potassium iodide.
Without purification of the acetanilide derivatives 3a–c,
they were subjected to acid hydrolysis, which led to their
corresponding amine products 4a–c in good yields. The
N-methylated 4a-NMe derivative was synthesized by reductive
methylation using 3a, formaldehyde and sodium cyanoboro-
hydride at 0 °C. Finally, the in situ generated trimesoyl chloride 6
has been used to react with the amine compounds 4a–c to
obtain the corresponding triamide target molecules 7a–c. In a
similar way, the N-methylated derivative 8a has been syn-
Scheme 1 Synthesis of target compounds 7a–c/8a and their photoi-
somerization channel. (i) Conc. HCl, NaNO2, phenol, aq. NaOAc, 0 °C,
4 h, 91%; (ii) K2CO3, R-Br, cat. KI, dry DMF, 100 °C, 6–8 h; (iii) conc. HCl,
EtOH, reflux, 5–6 h, 4a-82%, 4b-84%, 4c-84%; (iiia) for 4a-NMe: formal-
dehyde, NaCNBH3, MeOH, 0 °C, 2 h, 32%; (iv) trimesic acid (5), PCl5,
toluene, reflux; (v) pyridine, toluene, RT, 4–5 h, 7a-76%, 7b-72%,
7c-75%, 8a-40%.
Fig. 1 Analysis of photoswitching behaviour using UV-Vis spectroscopy
for the target molecules: (a) 7a (9.6 μM); (b) 7b (9.5 μM); (c) 7c (8.2 μM).
All the spectra are recorded in CHCl3; spectral colour scheme: blue –
spectra of 7a–c-EEE; red – spectra recorded after irradiation at 365 nm;
green – spectra recorded after reverse switching using white light (32 W
CFL lamp).
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Table 1 Absorption spectral properties and solution phase formation
rate constants of 7a–c-EEE using UV-Vis spectroscopy
centration. Again, the photoswitching studies on the
N-methylated 8a have also been carried out in DMSO-d6. The
forward switching at 365 nm led to almost quantitative conver-
sion into all-cis isomer; however, even after continuous
irradiation, the reverse isomerization was found to be incom-
plete. Due to the overlapping signals, the identification of
composition of various isomers was difficult (Fig. S8, see the
ESI†).
a,b
TM
λmax/(ε)a (EEE)
λmax
k (x10−5 c
)
Half lifec
7ad
369 (75 058 152)
375
369 (83 663 312)
375
369 (84 613 42)
375
323, 446
320, 448
323, 446
320, 448
323, 446
320, 448
2.82
4.00
4.32
410
286
267
7b
7c
a λmax values (nm) and ε values in parentheses (L mol−1 cm−1) of
7a–c-EEE; bold – in CHCl3 and normal font – in DMSO. b After 365 nm
irradiation. c Rate constants (min−1) and the half-lives (min) have been
measured in DMSO at 25 1 °C; abbreviation: TM – target molecules.
d For 7a, the absorption maxima in the solid state and LC state were
observed at 377 nm and 372 nm, respectively.22
Thermal behaviour of 7a–c/8a
The thermal behaviour of the newly synthesized tris-azobenze-
netricarboxamide series was first studied by polarized optical
microscopy. All the compounds were found to exhibit LC
behaviour at room temperature. Under POM, compound 7a
exhibited LC behaviour at room temperature and melted into
an isotropic liquid at 238.0 °C. On cooling from its isotropic
melt, it exhibited a needle like texture consisting of rectilinear
defects and homeotropic domains (Fig. 3b) indicating the Colh
mesophase. Then it transformed into a four brush texture
(Fig. 3a) at 160.4 °C, a representative of the Colr mesophase,
which then stayed up to room temperature. Compound 7b
exhibited mostly homeotropically aligned textures which on
pressing (top of the glass slide) gives isogyre type textures
(Fig. 3c), and further phase confirmation led to the existence
of the Colh phase as predicted by the X-ray diffraction (XRD)
technique (vide infra). And also, on shearing in one direction,
the compound formed homogeneous domains, a characteristic
of LCs (Fig. 3d).
In differential scanning calorimetry (DSC), it displayed a
mesophase to isotropic transition at 190 °C, while on cooling
it showed no observable transition, which may be due to low
enthalpy changes. Similar to 7b, compound 7c also exhibited a
birefringent texture along with rectilinear defects in some
areas, a characteristic of the Colh phase (Fig. 3e). And on
heating, it showed the mesophase to isotropic transition at
175 °C. The phase transition temperatures for all the com-
three individual isomers (EEZ, EZZ and ZZZ-isomers), we
shifted our attention to NMR spectroscopy for understanding
their presence (Fig. 2, Fig. S7, see the ESI†). The NMR spectra
of target compounds recorded in DMSO-d6 showed well-
resolved and sharp signals. Besides that, we also observed the
formation of a photostationary state (PSS) upon irradiation at
365 nm, where all the four species coexist (Fig. 1). Based on
the spectral changes, we observed that upon E to Z isomeriza-
tion, the signals were found to be shifted to the upfield region.
Due to their non-overlapping nature, the aromatic benzene
core protons (appearing as a singlet in EEE-isomer) have been
used for identifying the individual isomers. Also, the signals
clearly showed a progressive upfield shift as a result of gaining
more Z-configuration. At the PSS corresponding to 365 nm, the
major species was found to be ZZZ-isomer, which showed a
maximum upfield shift of the benzene core protons. The
appearance of signals with 1 : 2 and 2 : 1 integral ratios has
been used to differentiate between the EEZ and ZZE-isomers.
Based on the photoswitching studies, it is clear that maximum
ZZZ-isomer conversion (64%) can be achieved at 14.1 mM con-
Fig. 2 1H NMR spectra corresponding to: (a) 7a-EEE as a 14.1 mM concentration solution in DMSO-d6; (b) the same solution after being subjected
to irradiation at 365 nm for 130 min; (c) the expanded region depicts the benzene core protons in “b” with the indication of normalized integral
values corresponding to EEE, EEZ, EZZ and ZZZ isomers of 7a.
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Fig. 3 Photomicrographs: (a) compound 7a at 120 °C (Colr, ×500); (b)
at 220 °C (Colh, ×500); (c) 7b at 68.6 °C (Colh, ×100); (d) 7b on shearing
at room temperature (Colh, ×100); (e) 7c at 69.2 °C; (Colh, ×500); (f) 7c
at 35.2 °C (×200), observed on cooling from isotropic liquid with a scan
rate of 5 °C min−1
.
Table 2 Thermal behavioura of the target compounds 7a–c
TM
Heating
Cooling
b
7a
7b
7c
Colr 165 Colh 238 Isob
Colh 190 Isoc
Iso 230 Colh b160.4 Colr
Iso 185 Colh b
Colh 175 Isoc
Iso 165 Colh
a Phase transition temperature (°C). b Transition temperature from
POM. c Transition temperature from DSC. Abbreviations: TM = target
molecules, Colr = columnar rectangular, Colh = columnar hexagonal,
Iso = isotropic liquid.
pounds are listed in Table 2. So, all the compounds exhibited
enantiotropic mesomorphic behaviour as very well observed by
POM and X-ray studies, however, some of the transitions were
not observed in DSC probably due to the low enthalpy change
associated with the transitions as reported earlier.23a,24,25 As
expected, the N-methylated derivative 8a did not show any
specific texture corresponding to the columnar mesophase
(data not shown) plausibly due to the absence of inter-
molecular H-bonding and lack of planarity.26 Furthermore, the
XRD measurement of 8a showed a weak but broad peak at a
Fig. 4 X-ray diffraction pattern: (a) compound 7a in Colr at 140 °C; (b)
compound 7a in Colh at 190 °C; (c) compound 7b in Colh at 25 °C; (d)
compound 7c in Colh at 115 °C, obtained on cooling from the isotropic
liquid, ha and hc are the spacings appearing due to alkyl chain–chain
and core–core correlation, respectively.
small angle, which is contrary to the 7a–c derivatives (exhibit- sition in compound 8a upon melting (mp 185 °C) prior to
ing sharp peaks in the small angle regions). This indicates the filling it in the capillary for XRD measurements. The NMR
absence of columnar order in 8a (i.e. the structure is more spectral data for the heated sample of 8a confirmed no
random). We have also checked the possibility of decompo- decomposition upon melting.
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XRD experiments were performed to deduce the exact struc- supported by POM studies. In contrast, compounds 7b and 7c
ture of the assembly of the mesophases. Compound 7a at showed only Colh phase (Fig. 4c & d). Hence, the Colh to Colr
lower temperature, in the range of 25 to 160 °C, exhibited phase transition occurs on increasing the temperature as in 7a
many peaks in the small angle region. In addition, there were (Fig. 5) and also on increasing the peripheral alkoxy chain
ha and hc peaks in the wide-angle region. The ha peak mainly length on going from 7a to 7c. The lattice parameters are
originated due to the liquid like order of the fluid chain and found to be a = 43.35 Å and 49.74 Å (Table 3), respectively,
the hc peak appears due to the core–core separation and it is which reflect the increasing trend. This trend is obvious
indicative of π–π interaction, reflecting the columnar nature of because the periphery chain length (size of the molecule) is
the mesophase (Fig. 4a). The peaks of the small angle could be increasing from compound 7a to 7c.
indexed on the rectangular lattice. The calculated 2D lattice
parameters are a = 57.20 Å and b = 44.84 Å (at 140 °C). The hc
Reconstructed electron density maps (EDM)
peak sets the value of the other lattice parameter i.e. c = 3.89 Å Electron density maps for the Colr and Colh phase of com-
(Table 3). In brief, the observed phase is Colr. Furthermore, pound 7a were reconstructed (Fig. 6a and b). The procedure to
this phase transformed into the Colh phase at a temperature of reconstruct the electron density map has been described in
160.4 °C, and persisted in the temperature range of 160.4 to the earlier literature.23,25 The peak intensities and their phase
238 °C. The phase is confirmed by the observation of three values were obtained to reconstruct the maps. The details
pffiffiffi
pffiffiffi
peaks in the ratio of 1 : 1= 3 : 1= 7 which correspond to the (multiplicity and phase values) are given in Table 3.
reflections from the (10), (11) and (21) planes, respectively, of
Photoswitching studies in solid state
the hexagonal lattice (Fig. 4b and Table 3). Furthermore, ha
and hc peaks were also observed in the wide-angle region After confirming the mesomorphic properties of all the three
where the hc peak confirmed its columnar nature. The calcu- molecules, we focused on the photoswitching behaviour in the
lated lattice parameter was found to be a = 39.24 Å and the solid state. In this regard, the solid-state photoswitching of 7a
other lattice parameter was set by the value of the spacing of has been studied in KBr using UV-Vis spectroscopy (Fig. 7a).
the hc peak (c = 3.62 Å).
The spectral changes upon irradiation at 365 nm clearly indi-
In summary, compound 7a exhibits the Colr phase at lower cated the isomerization process. Molecule 7a also exhibited
temperature (25–160 °C) and the Colh phase at higher tempera- the photoswitching in thin film, however, with a slight blue
ture (160.4–238 °C). This phase behaviour has also been well shift in the π–π* absorption bands (Fig. S33, see the ESI†).
Table 3 The indices, observed and calculated d-spacings and planes of the diffraction peaks of the lattices
TM
Mesophase
Lattice constants (Å)
dobs a (Å)
dcal b (Å)
MI
RI (M)
7a
Colh at 140 °C
a = 57.20
b = 44.84
c = 3.89
44.78
35.16
28.91
23.90
20.83
17.93
15.28
14.14
11.46
10.59
9.93
9.54
4.81
3.80
33.99
19.57
13.15
4.95
44.84
35.29
2.60
01
11
20
21
12
22
03
40
50
24
34
60
ha
hc
10
11
21
ha
hc
10
20
21
ha
hc
10
ha
22.91(2)
100.00(4)
78.84(2)
7.34(2)
10.64(4)
10.98(4)
5.72(2)
5.65(2)
4.33(2)
4.26(4)
4.16(4)
4.16(2)
24.11
20.87
17.65
14.95
14.30
11.44
10.44
9.66
9.53
Colh at 190 °C
Colh at 25 °C
Colh at 115 °C
a = 39.24
c = 3.62
33.17
19.15
8.93
4.89
3.88
39.38
19.69
14.88
3.62
7b
7c
a = 45.47
c = 3.88
39.38
19.89
15.14
4.79
a = 49.76
c = 3.62
43.09
4.94
43.09
ꢀ
ꢁ
ꢀ ꢂ
ꢃ
ꢁ
1
h2 k2
a2 b2
1
4
3
h2 þ hk þ k2
l2
c2
a dobs: Experimental d-spacing. dcal: Calculated d-spacing for Colr by using the relation:
¼
þ
; for Colh:
¼
þ
;
b
d2
d2
a2
h, k, and l are the indices of the reflections corresponding to the columnar hexagonal (Colh) phase; a & c are the unit cell parameters, ha & hc
spacing appear due to alkyl chain–chain and core–core correlation, respectively. Abbreviation: TM – target molecules, MI – Miller indices, RI (M)
– relative intensity (multiplicity).
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365 nm light, the sample retained the texture without exhibit-
ing any appreciable change in the mesophase (Fig. S34a and b,
see the ESI†). This indicates that the columnar self-assembly
was unperturbed upon photoisomerisation (Fig. 8c).
Presumably, the attachment of azobenzene moieties to a rigid
benzene tricarboxamide core provides too tight packing to
change the molecular self-assembly upon isomerization. The
structure in which the rigid core is attached to azobenzene
units via flexible alkyl spacers is more effective in changing the
columnar self-assembly as reported earlier.27 To further see
the change in the columnar self-assembly upon UV irradiation,
GISAXS/GIWAXS studies were carried out on thin films of 7a
(Fig. S34c–f, see the ESI†). Similarly, the GISAXS/GIWAXS pat-
terns were unaltered before and after irradiation as shown in
Fig. S34 (see the ESI†). All this experimental evidence clearly
envisage the possibility of photoswitching in these target
molecules without affecting their supramolecular assembly.
Furthermore, the atomic force microscopy (AFM) measure-
ments of the dropcast sample of 7a in HPLC grade dichloro-
methane were performed on a glass substrate (Fig. 8a & b,
Fig. S35, see the ESI†). We monitored the changes in the mor-
phology of the thin film before and after irradiating the
sample for 30 minutes with 365 nm UV light. As suggested by
an earlier report, the disturbance or changes in the columnar
assembly can alter the height profile.27 However, we observed
only subtle changes in the height profile that indicate the
Fig. 5 Model representing temperature induced Colr to Colh transition
in 7a.
Fig. 6 (a) Reconstructed electron density map of columnar rectangular
(Colr) and (b) columnar hexagonal (Colh) phases. Red represents the
highest electron density and deep blue is the lowest.
Fig. 7 Photoswitching behaviour in the solid state for 7a: (a) in KBr; (b)
in the thin film LC state. The blue coloured spectrum is due to 7a-EEE
(before irradiation), whereas the red one indicates the changes after
irradiation at 365 nm. The spectra appearing in green in “b” was
observed after reverse switching on heating.
Furthermore, we also considered the photoswitching studies
in its LC state (Fig. 7b). In this regard, compound 7a was first
heated to its isotropic temperature and then cooled back to
room temperature LC state as observed through POM. Then
the same sample has been subjected to photoswitching and
monitored by UV-Vis spectroscopic studies. On irradiating the
sample with 365 nm UV light, the compound exhibited charac-
teristic changes accompanying the E–Z isomerization.
Interestingly, the sample reverted to its native configuration
upon heating that can be rationalized from the changes in the
n–π* absorption. This clearly demonstrates the reversible
photoswitching of molecule 7a in the LC form. Nevertheless,
POM studies of the samples in the LC form showed that both
the samples (before and after irradiation at 365 nm) exhibited
Fig. 8 AFM images of dropcast film of 7a on a glass substrate: (a)
before irradiation; (b) after irradiation. (c) Schematic showing columnar
assembly retains upon reversible photoisomerization. Note: All the
identical images. Despite
a
prolonged irradiation using photoswitching studies were carried out at room temperature.
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retention
photoisomerization.
of
columnar
self-assembly
during
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K.-U. Jeong, ACS Appl. Mater. Interfaces, 2016, 8, 9490;
(d) S. Pan, M. Ni, B. Mu, Q. Li, X.-Y. Hu, C. Lin, D. Chen
and L. Wang, Adv. Funct. Mater., 2015, 25, 3571.
Conclusions
In conclusion, we have synthesized three long chain alkoxy
azobenzene connected benzenetricarboxamide derivatives. All
the three molecules are found to behave as discotic liquid crys-
tals. Interestingly, the molecule with a peripheral C6 alkyl
chain exhibits temperature dependent changes in the meso-
phases. A Colr phase was observed at low temperature,
whereas, a Colh at higher temperature was seen. The other two
higher homologues revealed only Colh phases at all tempera-
ture ranges. All the three molecules showed solution state
photoswitching. At the μM concentration range, all the three
molecules showed nearly complete photoswitching, however,
at a higher concentration (mM range), NMR spectroscopy pre-
dicted a partial isomerization with the coexistence of all four
possible isomers at the photostationary state. Significant
photoswitching was also observed in the solid state and LC
phase however, without changing its supramolecular assembly,
which was also supported by AFM and POM studies. For com-
parison, an N-methylated analogue 8a has been synthesized
and studied for the columnar mesomorphic behaviour.
However, due to the lack of H-bonding and planarity, it did
not exhibit any mesophases despite having better photoswitch-
ing behaviour. Thus, the tris-azobenzenetricarboxamide based
systems are found to be versatile DLCs exhibiting reversible
photoswitching behavior without losing their LC properties
upon isomerization.
7 (a) J. H. Sung, S. Hirano, O. Tsutsumi, A. Kanazawa,
T. Shiono and T. Ikeda, Chem. Mater., 2002, 14, 385;
(b) H. Adachi, Y. Hirai, T. Ikeda, M. Maeda, R. Hori,
S. Kutsumizu and T. Haino, Org. Lett., 2016, 18, 924;
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Conflicts of interest
There are no conflicts to declare.
8 (a) C. Dugave and L. Demange, Chem. Rev., 2003, 103,
2475–2532; (b) J. Garcia-Amoros and D. Velasco, Beilstein
J. Org. Chem., 2012, 8, 1003.
Acknowledgements
SV is grateful to the SERB, New Delhi for the financial support
(EMR/2014/000780). We are thankful to the IISER Mohali for
the NMR, HRMS, SAXS/WAXS and other instrumentation facili-
ties and also for providing the start-up grant. We are thankful
9 (a) S. Kobatake, S. Takami, H. Muto, T. Ishikawa and
M. Irie, Nature, 2007, 446, 778; (b) S. Guo, K. Matsukawa,
T. Miyata, T. Okubo, K. Kuroda and A. Shimojima, J. Am.
Chem. Soc., 2015, 137, 15434.
to C. Sai Srinivas and Dr Sabyasachi Rakshit for AFM studies. 10 (a) J. Henzl, M. Mehlhorn, H. Gawronski, K.-H. Rieder and
SD thanks the UGC for Junior and Senior Research
Fellowships. IB and PK acknowledge the CSIR (09/947(0061/
2015-EMR-1 and 09/947(0077/2016-EMR-1, respectively) for fel-
lowships. Dr SK Pal acknowledges funding from CSIR
(02(0311)/17/EMR-II).
K. Morgenstern, Angew. Chem., Int. Ed., 2006, 45, 603;
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