Photomechanical response of sulfonylhydrazone molecular crystals

Article abstract of DOI:10.1039/d1ce00478f

Sulfonylhydrazones are a novel hydrazone-based organic molecular photoswitch. The four derivatives with unsubstituted ando-,m- andp-nitro substituted sulfonylhydrazone (SH-1,SH-2,SH-3, andSH-4, respectively) derived fromp-toluenesulfonyl hydrazide and corresponding benzaldehydes were examined with respect to their solution state photoswitching and photomechanical response in the crystalline state. All the compounds displayed UV-inducedE→Zisomerization in the solution state, whereas the back conversion was slow. Single crystals ofSH-3displayed rapid and large photomechanical bending, whereasSH-1andSH-4crystals underwent slow bending to a comparatively lesser extent. On the contrary,SH-2crystals did not show any photomechanical effects. The photomechanical deflection of the crystal tip ofSH-1increased linearly; forSH-3, the deflection increased in a sub-linear manner for few seconds, after which it started to revert towards the light source. ForSH-4crystals, the photo-induced tip deflection initially increased in a sub-linear fashion, after which no motion was observed even after continuous exposure to UV light irradiation. The photomechanical behavior of the crystals was also inspected with the aid of velocity-time and acceleration-time plots that unravelled the instantaneous photomechanical motion at various time intervals.

Full text of DOI:10.1039/d1ce00478f

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Photomechanical response of sulfonylhydrazone
molecular crystals†
Cite this: CrystEngComm, 2021, 23,
4910
a
a
b
a
Pragyan J. Hazarika,
Poonam Gupta,
Anilkumar Gunnam,
Suryanarayana Allu, b Ashwini K. Nangia b and Naba K. Nath
*
Sulfonylhydrazones are a novel hydrazone-based organic molecular photoswitch. The four derivatives with
unsubstituted and o-, m- and p-nitro substituted sulfonylhydrazone (SH-1, SH-2, SH-3, and SH-4,
respectively) derived from p-toluenesulfonyl hydrazide and corresponding benzaldehydes were examined
with respect to their solution state photoswitching and photomechanical response in the crystalline state.
All the compounds displayed UV-induced E → Z isomerization in the solution state, whereas the back
conversion was slow. Single crystals of SH-3 displayed rapid and large photomechanical bending, whereas
SH-1 and SH-4 crystals underwent slow bending to a comparatively lesser extent. On the contrary, SH-2
crystals did not show any photomechanical effects. The photomechanical deflection of the crystal tip of
SH-1 increased linearly; for SH-3, the deflection increased in a sub-linear manner for few seconds, after
which it started to revert towards the light source. For SH-4 crystals, the photo-induced tip deflection
initially increased in a sub-linear fashion, after which no motion was observed even after continuous
exposure to UV light irradiation. The photomechanical behavior of the crystals was also inspected with the
aid of velocity-time and acceleration-time plots that unravelled the instantaneous photomechanical motion
at various time intervals.
Received 10th April 2021,
Accepted 20th June 2021
DOI: 10.1039/d1ce00478f
rsc.li/crystengcomm
Akin to the azobenzene (–NN–) functional group, the
compounds with –CN– units, viz., hydrazones,28–31 also
undergo photo-induced E–Z isomerization. Unlike the major
classes of molecular photoswitches, hydrazones, despite
being popular as organic molecular photoswitch, is
comparatively less explored in their solid-state response to
Introduction
Amidst the emergence of mechanically responsive crystals,1–7
the exploration and development of photo-responsive single
crystals1–3,7–10 have captivated considerable recognition due
to their potential implications in photo-controllable devices
and sensors, actuators, etc. Interaction with light radiations
can bring about various changes within
a molecule,
summarized into two major categories: pericyclic reaction
and isomerization. Pericyclic reactions involve the formation
and dissociation of bonds, whereas isomerization involves a
change in the molecular configuration. Based on this light-
triggered behavior, molecular photoswitches such as
azobenzene, fulgide, spiropyran, anthracene, diarylethene,
etc., are developed and later implemented in light-powered
smart materials.11–16 With the progress of time, single crystals
of these classes of molecules were demonstrated to display
photomechanical responsiveness17–25 similar to liquid
crystalline and polymeric materials.26,27
a Department of Chemistry, National Institute of Technology, Meghalaya 793003,
India. E-mail: nabakamal.nath@gmail.com, nabakamal.nath@nitm.ac.in
b School of Chemistry, University of Hyderabad, Central University P.O., Prof. C. R.
Rao Road, Hyderabad 500046, India
Scheme
acylhydrazone,
1
(a) and (b) Molecular structures of hydrazone, and
respectively. (c) Molecular structure of
† Electronic supplementary information (ESI) available. CCDC 2076194–2076197.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1ce00478f
sulfonylhydrazone derivatives under study. (d) E ↔ Z isomerization of
sulfonylhydrazone.
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light.32–35 The E–Z isomerization around the –CN– bond
leads to a change in its configuration, which further results
in the change in the electronic and stearic relation of the
substituents on each fringe of the hydrazone core. The
observed (Fig. 1a), and the rest of the crystal structure is
completed by weak hydrogen bonds involving C–H donors
from the aromatic rings. SH-2 crystallized in the triclinic
¯
system with space group P1, with one molecule of E-SH-2 in
photomechanical
response
exhibited
by
hydrazone
the asymmetric unit (Fig. S2, Table S2†). In the crystal
structure, the anticipated N–H⋯OS hydrogen-bonded
catemer synthon is not observed. Instead, the synthon here is
composed of N–H⋯O hydrogen bond (d = 2.27 Å, D =
3.028(2) Å, Θ = 170°) involving oxygen from the nitro donor
and N⋯O interactions (3.060(2)) (Fig. 1b). The SO group is
involved with weak hydrogen bonding with methyl and
aromatic C–H donors. SH-3 crystallized in the orthorhombic
system with space group Pna21 and one molecule of E-SH-3
in the asymmetric unit (Fig. S2, Table S2†). N–H⋯OS
hydrogen-bonded (d = 1.84 Å, D = 2.938(7) Å, Θ = 168°)
catemer synthon (Fig. 1c) is present in the crystal structure
along with several C–H⋯O hydrogen bonds involving
aromatic and methyl C–H donor and acceptor from nitro and
SO groups. SH-4 crystallized in the monoclinic system with
space group P21/c with two molecules of E-SH-4 in the
asymmetric unit (Fig. S2, Table S2†). N–H⋯OS hydrogen-
bonded catemer synthon (d = 2.11 Å, D = 2.912(3) Å, Θ =
160°; d = 2.10 Å, D = 2.922(3) Å, Θ = 169°) again prevailed in
the crystal structure (Fig. 1d). The crystal structure is
completed by several weak C–H⋯O hydrogen bonds involving
aromatic and methyl C–H groups with oxygen as acceptors
from SO and nitro groups.
compounds (Scheme 1a) in the solid-state led us to explore
the UV responsive acylhydrazone molecular crystals35
(Scheme 1b). This generated and augmented our interest and
curiosity in the hydrazone functional group, which further
inspired us to investigate its properties by implementing a
molecular engineering approach and designing molecules
with sulfonylhydrazone functional groups (Scheme 1c).
Sulfonylhydrazones are well known for their pharmacological
activity,36 and earlier, their crystal polymorphism was
explored.37,38 However, the photomechanical response of the
single crystals of this class of photoswitch (Scheme 1d) has
not been examined yet. Here, we have embarked on a
systemized investigation of the photoswitching and
photomechanical response in
sulfonylhydrazones derived from p-toluenesulfonyl hydrazide
and unsubstituted along with nitro substituted
a
series of crystalline
benzaldehydes (SH-1, SH-2, SH-3, and SH-4) (Scheme 1).
Results and discussion
Synthesis, crystallization, and characterization
Sulfonylhydrazones were easily prepared in good yields by
condensation reaction between the corresponding aldehyde
and p-toluenesulfonyl hydrazide followed by characterization
(see Experimental section for details). Suitable quality single
crystals were obtained by slow evaporation from solvents
where SH-1, SH-3, and SH-4 crystallized as long needles
whereas SH-2 crystallized as thick plates (Table S1, Fig. S1†)
Photoswitching
Solution state UV-vis spectroscopy was carried out to examine
the photoswitching property of the four compounds in
acetonitrile solvent with concentrations of SH-1: 9.1 × 10−6 M,
SH-2: 1.56 × 10−5 M, SH-3: 1.56 × 10−5 M, and SH-4: 1.96 ×
10−5 M. The E isomer of SH-1 exhibited two UV absorption
bands at λmax = 214 nm and 272 nm. Upon UV irradiation for
10 min, the absorbance of the peaks decreased, and λmax
shifted to 224 nm and 270 nm, thus indicating E → Z
isomerization. Heating the Z isomer solution for 120 min at
50 °C initiated the regeneration of the E isomer (Fig. 2a). The
E → Z isomerization of SH-2 was carried out by irradiating
the solution with UV light for 15 min. This resulted in the
shift in the value of λmax from 227 nm to 219 nm, 261 nm to
260 nm, and 324 nm to 305 nm. This was accompanied by a
decrease in the absorbance at λmax = 324 nm and 261 nm and
an increase in the absorbance at 227 nm. Only a slight
change in the spectra of the Z-isomer was observed even after
heating the solution for 180 min at 60 °C (Fig. 2b). Upon
irradiating the E-isomer of SH-3 with UV light for 12 min, it
resulted in the shift and increase in the intensity of the
absorption band from λmax = 228 nm to 226 nm and a shift
and decrease in the intensity of the absorption band from
λmax = 264 nm to λmax = 262 nm indicating its conversion to
the Z-isomer. The back conversion was initiated by heating
the solution for 240 min at 50 °C (Fig. 2c). Similarly, in the
case of SH-4, E → Z conversion was indicated by the decrease
Crystal structure analysis
SH-1 crystallized in monoclinic space group P21/c with one
molecule of E-SH-1 in the asymmetric unit (Fig. S2, Table
S2†). In the crystal structure, N–H⋯OS hydrogen-bonded
(d = 2.27 Å, D = 3.047(3) Å, Θ = 165°) catemer synthon is
Fig. 1 Hydrogen bonding synthon present in the crystal structures of
(a) SH-1, (b) SH-2, (c) SH-3, and (d) SH-4.
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Fig. 2 Photoswitching of (a) SH-1, (b) SH-2, (c) SH-3 and (d) SH-4 in
acetonitrile solution. Colour codes: black solid line signifies the spectra
recorded before UV irradiation. The solid blue line signifies the spectra
obtained after UV irradiation, and the broken red line signifies the
spectra obtained after heating the solution.
in the intensity of the absorption band at λmax = 226 nm, the
diminishing of the peak at λmax
= 316 nm, and the
Fig. 4 (a)–(c) Display the plot of maximum angular deflection against
the crystal length, width, and thickness, respectively, for the molecular
crystals of SH-3. (d) Snapshots of the photomechanical bending of the
15 crystals of SH-3 exposed to UV light from the left direction. Note: in
(a)–(d), the corresponding crystals are designated with the same color
codes.
appearance of a new peak at λmax = 266 nm on UV exposure
for 9 min. The Z → E back thermal conversion was obtained
by heating the solution of Z-isomer for 240 min at 50 °C
(Fig. 2d). Although UV-induced E → Z isomerization is
comparatively faster in all the cases, the Z → E thermal
conversion was slow and only partially achieved.
faces of the crystals from a distance of 2.5 cm with
unfocussed UV light from a mercury–xenon lamp (power
density 361 mW cm−2, λ = 365 nm) attached with a heat filter.
To our amazement, SH-1, SH-3, and SH-4 molecular crystals
displayed photomechanical motion. In contrast, molecular
crystals of SH-2 were unaffected by UV light. SH-2 crystallized
as thick plate-shaped crystals, due to which no significant
photomechanical response was observed. Fifteen crystals
Light-induced mechanical response
The photomechanical response of the crystals of SH-1, SH-2,
SH-3, and SH-4 was investigated by irradiating the wider
each of SH-1, SH-3, and SH-4 with
a wide range of
dimensions (Table S1†) were randomly hand-picked from
their respective crystallization vessels for the kinematic
analysis of photo-induced mechanical response. In all the
cases, the crystals displayed bending type photomechanical
motion away from the UV light source. The photomechanical
movement is due to the E ↔ Z isomerization that causes
heterogeneous photoconversion with reference to its depth,
resulting in the expansion of the crystal's photo-reacted layer
along its longest axis. Each of the 15 crystals of SH-1, SH-3,
and SH-4 were glued and mounted by one of its terminals to
a metallic needle keeping the longest axis of the crystal and
the needle identical.
Fig. 3 (a)–(c) Display the plot of maximum angular deflection against
the crystal length, width, and thickness, respectively, for the molecular
crystals of SH-1. (d) Snapshots of the photomechanical bending of the
15 crystals of SH-1 exposed to UV light from the left direction. Note: in
(a)–(d), the corresponding crystals are assigned the same color codes.
When the crystals of SH-1 with dimensions in the range of
l = 1.70–6.39 mm, w = 0.02–0.11 mm, and t = 0.02–0.07 mm
were irradiated with UV light (power density of 580.0 mW
cm−2) for 60 s, they underwent deflection away from the UV
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Fig. 5 (a) Crystal of SH-3 displaying photosalient effect on exposure
to UV light. (b) Crystal of SH-3 undergoing photomechanical bending
and displaying wave-like motion.
Fig. 6 (a)–(c) Display the plot of maximum angular deflection against
the crystal length, width, and thickness, respectively, for the molecular
crystals of SH-4. (d) Snapshots of the photomechanical bending of the
15 crystals of SH-4 exposed to UV light from the left direction. Note: in
(a)–(d), the corresponding crystals are assigned the same color codes.
light with an average angular speed in the range of 0.0064–
0.104° s−1 covering a maximum angular deflection of 0.39–
6.16° within a time frame of 58.9–60 s (Fig. 3 and Video S1†).
Similarly, crystals of SH-3 with dimensions in the range l =
1.68–5.51 mm, w = 0.03–0.11 mm, and t = 0.01–0.11 mm
upon irradiation with UV light for 2 s underwent large and
rapid deflection away from the light source achieving
maximum angular deflection of 10.89–76.73° with an average
angular speed of 5.53–39.48° s−1 in 1.9–2.0 s (Fig. 4 and Video
S2†). Interestingly, after bending to a certain extent, some of
the crystals shattered, thus displaying photosalient effect
(Fig. 5a and Video S3†).39 On many occasions, the crystals
revert with a wave-like motion after reaching maximum
deflection (Fig. 5b and Video S4†). Such behavior may be
attributed to the exposure of the concave face of the crystal
after they bend away more than ∼45° from the initial
position. The crystals of SH-4 within dimension range of l =
1.26–4.65 mm, w = 0.05–0.17, t = 0.02–0.07 mm bent away
when exposed to UV light for 60 s (Fig. 6 and Video S5) with
an average angular speed of 0.0059–0.051° s−1, attaining
maximum angular deflection of 0.35–3.03° in 58.96–59.69 s.
To attain insight into the relation of photomechanical
bending variance with extrinsic factors such as crystal
dimensions, we correlated the maximum angular deflection
displayed by the molecular crystals of SH-1, SH-3, and SH-4
with their respective crystal length, width, and thickness
(Fig. 3, 4 and 6). Due to marginal stiffness, thinner and
longer molecular crystals are expected to undergo
photomechanical bending to larger extent as compared to
the thicker and shorter crystals. Also, crystals having a wider
surface area can absorb a significant number of photons on
the irradiated surface. Here, in each case long and wide
crystals have higher thickness and hence they are not the
ones with maximum angular deflection. Instead the thinner
crystals are displaying high deflection.
intervals. It was observed that on UV exposure, crystals of SH-
1, SH-3, and SH-4 followed three different types of
trajectories. Fig. 7b, d and f show the representative and
illustrative plots of the traced trajectories of the crystal of SH-
1, SH-3, and SH-4.
The plot of the angular deflection of the crystal of SH-1,
SH-3, and SH-4 against time interval revealed that SH-1
Fig. 7 (a), (c), and (e) Show the overlapped images of a sequence of
snapshots of frames at regular interval of time displaying the bending
motion of the molecular crystals of SH-1, SH-3, and SH-4 when
exposed to UV light for 58, 85 and 60 seconds respectively from left
direction. (b), (d) and (f) Represents the plot of maximum angular
deflection of the crystals of SH-1, SH-3, and SH-4, respectively, against
time intervals.
The kinematic analysis of one crystal each of SH-1, SH-3,
and SH-4 was carried out by tracing the motion of the crystal
tip from the recorded videos and calculating the relative
angular deflection from its initial position at different times
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continued bending away from the UV light source almost
linearly as long as it was exposed to it (Fig. 7b). SH-3, on the
other hand, underwent rapid photomechanical bending
during the first 2 seconds. Then the bending increased
linearly to the next 40 seconds, after which the crystal started
moving back slowly towards the UV light source (Fig. 7d).
The plot of SH-4 signifies that the crystal underwent
photomechanical bending upon UV light exposure for the
initial 5 seconds. After that, the crystal moved at a slower
pace for the next 3 seconds attaining maximum angular
deflection. No distinguishable motion was observed for the
next 52 seconds (Fig. 7f).
The photomechanical motion of the molecular crystals of
SH-1, SH-3, and SH-4 was interpreted with the help of a
representative velocity-time and acceleration–time plot
(Fig. 8). The instantaneous angular velocity of one of the SH-
1 crystals was plotted at the different time intervals of UV
light exposure. The plot revealed that the crystal initially
moved with an instantaneous angular velocity of 0.046° s−1 in
0.29 s. Then its angular velocity fluctuated and attained a
maximum value of 0.06° s−1 at 2.33 s; after that, the crystal
slowed down. Its instantaneous angular velocity decreased in
a fluctuating manner till the termination of UV light. The
crystal attained a maximum angular velocity of 18.65° s−1 and
1.34° s−1 at 0.3 s and 0.29 s respectively. Then the crystal
slowed down, implying that its angular velocity decreased
exponentially as long as it was exposed to UV light. The
instantaneous angular acceleration–time plot revealed the
molecular crystals of SH-3 and SH-4 gained maximum
momentum at 0.3 and 0.29 s with an angular acceleration of
62.32° s−2 and 4.55° s−2, respectively. After that, it started to
retardate uniformly.
The origin of the largely different photomechanical
behavior of the molecular crystals of the four compounds
cannot be derived solely from their crystal structures. All four
compounds have very different molecular packing and
intermolecular interaction energies in their crystal lattices.
This difference is mainly due to the flexibility around the
sulfonyl hydrazone functional group and the difference in
substituents at different positions of the aromatic rings. A
combination of various analytical techniques such as photo-
crystallography and nanoindentation, coupled with
computational calculations, may provide a better overview of
the differences in the photomechanical properties of these
crystals.
instantaneous angular acceleration vs. time graph illustrates Experimental
that the SH-1 crystal accelerated at 0.16° s−2 in 0.29 s, after
Materials
which it started retarding in
a uniform manner. The
p-Toluenesulfonyl chloride, aldehydes (purchased from
Spectrochem), solvents (from Emplura), hydrazine hydrate
(Spectrochem), and glacial acetic acid (from Himedia) were
used as obtained.
instantaneous angular velocity-time graph for the molecular
crystal of SH-3 and SH-4 showed a similar trend where the
Synthesis and characterization
The compounds were synthesized using a reported procedure
(Scheme 2).37,38
In the first step, hydrazine hydrate (1.1 equivalent, 5.76 ×
10−3 mol, 0.28 mL) was dropwise added to a THF solution of
p-toluenesulfonyl chloride (1.0 equivalent, 5.24 × 10−3 mol, 1
g) at 0 °C. The mixture was stirred continuously for 2 hours,
resulting in approximately 0.9
g
of p-toluenesulfonyl
hydrazide. In the second step, the aldehyde (1 equivalent,
1.07 × 10−3 mol, 0.11 g for SH-1 and 1.07 × 10−3 mol, 0.16 g
for SH-2, SH-3, SH-4) was added to an ethanolic solution of
the p-toluenesulfonyl hydrazide (1 equivalent, 1.07 × 10−3
mol, 0.2 g) followed by the addition of a catalytic amount of
glacial acetic acid. The resulting reaction mixture was refluxed for
3
hours, upon which the product sulfonylhydrazone
precipitated out as white crystalline solid, which was then
filtered and purified by recrystallization. The crystals thus
Fig. 8 (a), (c), and (e) Display the representative instantaneous angular
velocity of the crystal of SH-1, SH-3, and SH-4, respectively,
undergoing photomechanical motion. (b), (d) and (f) Display the
representative instantaneous angular acceleration of the crystal of SH-
1, SH-3, and SH-4, respectively undergoing photomechanical motion.
Onsets represent a closer view.
Scheme 2 Synthetic scheme of sulfonylhydrazones.
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obtained were then characterized by determining melting
point (Fig. S3†), FT-IR spectroscopy (Fig. S4†), 1H NMR (Fig.
S5†), and single-crystal X-ray diffraction (Fig. S2, Table S2†).
SH-1. Melting point: 122 °C. FT-IR: 3218 cm−1, 1604 cm−1,
1372 cm−1. 1H NMR: (500 MHz, DMSO-d6): δ 11.43 (s, 1H),
7.90 (s, 1H), 7.76 (d, J = 8.3 Hz, 2H), 7.56–7.54 (m, 2H), 7.41–
7.38 (m, 5H), 2.35 (s, 3H).
Mechanical effects
The photo-induced mechanical effects of the obtained
crystals was recorded and studied using microscope
(Leica M80) attached with a camera (MC170HD), and the
videos were analyzed in LAS software (ver. 4.9.0). The
detail of the calculations are explained in the ESI† (Fig.
S6, Table S3).
a
SH-2. Melting point: 153 °C. FT-IR: 3190 cm−1, 1603 cm−1,
1345 cm−1. 1H NMR (500 MHz, DMSO-d6): δ 11.90 (s, 1H),
8.28 (s, 1H), 8.02 (dd, J = 1, 8.2 Hz, 1H), 7.82 (dd, J = 1, 7.8
Hz, 1H) 7.78–7.74 (m, 3H), 7.65–7.62 (m, 1H), 7.43 (d, J = 8.0
Hz, 2H) 2.37 (s, 3H).
Conclusions
Tuning the molecular level motion is one of the most
fundamental goals in the subject scope of molecular
switches. With the progress of time, there arises a need that
requires the fabrication of intricated functions into such
systems which can be controlled externally. This entails the
expansion of the prevailing accessible toolbox in the field to
reinvigorate the existing research area. Here, we have
synthesized and characterized a series of unsubstituted and
SH-3. Melting point: 167 °C. FT-IR: 3223 cm−1, 1601 cm−1,
1353 cm−1. 1H NMR (500 MHz, DMSO-d6): δ 11.80 (s, 1H),
8.36 (t, J = 1.83 Hz, 1H), 8.21 (qd, J = 0.78, 5.0, 3.1 Hz, 1H),
8.05 (s, 1H), 8.0 (d, J = 7.8 Hz, 1H), 7.77 (d, J = 8.31 Hz, 2H),
7.68 (t, J = 8.2 Hz, 1H), 7.42 (d, J = 8.0 Hz, 2H) 2.36 (s, 3H).
SH-4. Melting point: 156 °C. FT-IR: 3190 cm−1, 1602 cm−1,
1
1344 cm−1. H NMR (500 MHz, DMSO-d6) δ 11.90 (s, 1H), 8.23
nitro-substituted
sulfonylhydrazone
based
molecular
(d, J = 8.85 Hz, 2H), 8.01 (s, 1H), 7.82 (d, J = 8.9 Hz, 2H), 7.78
(d, J = 8.3 Hz, 2H), 7.42 (d, J = 8.0 Hz, 2H), 2.36 (s, 3H).
photoswitches and explored their photomechanical response
in the crystalline state. The photomechanical performance of
the molecular crystals of SH-1, SH-2, SH-3, and SH-4 was
found to be significantly different in terms of crystal tip
trajectory, the extent of deflection, acceleration, and velocity.
Addressing the photomechanical characteristics of the
crystals of sulfonylhydrazone photoswitch obtained by
intense investigation makes them a potential candidate in
the evolution of photo-controllable systems.
FT-IR spectroscopy
FT-IR spectrometer from Perkin Elmer was used to record the
FT-IR spectra of the synthesized compounds in the UATR
mode.
Thermal microscopy
Melting point determination of the synthesized compounds
Author contributions
was carried out by placing the samples on
a
thermal stage (Linkam) fitted with a microscope (Leica DM
2700P) and camera (Leica MC170HD).
The manuscript is written with contributions from all
authors.
UV-vis spectra
Conflicts of interest
The UV-vis spectrometer (Lambda 35) from Perkin Elmer was
used to record the solution state UV-vis spectra of the
synthesized compounds. UV light emerging from a mercury–
xenon lamp (power density 361 mW cm−2, wavelength 365
nm) was used to irradiate the solutions to study the
photoswitching property.
There are no conflicts to declare.
Acknowledgements
PJH and PG thank NIT Meghalaya for a fellowship. AG and SA
thank CSIR for a fellowship. NKN acknowledge SERB-ECR
(file no: ECR/2016/000331), and SERB-CRG (file no: CRG/2019/
004607) and AKN acknowledge SERB-DST for funding (JC
Bose fellowship, SR/S2/JCB06/2009 and Pharmaceutical
materials and continuous process, CRG/2019/001388). We
thank NIT Meghalaya and the University of Hyderabad for
instrument facilities.
1H NMR spectroscopy
Bruker Advance 500 MHz spectrometer (Bruker-Biospin,
1
Karlsruhe, Germany) was used to record the H NMR spectra
of the synthesized compounds in DMSO-D6 solution.
Single crystal X-ray diffraction
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Diffraction frames were recorded in the Apex II40 program,
integrated with Bruker SAINT,41 and absorption correction
was carried out using SADABS.42 Crystal structure solution
and refinement were done with the SHELX package.43–45
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