Tuning of Bistability, Thermal Stability of the Metastable States, and Application Prospects in the C3-Symmetric Designs of Multiple Azo(hetero)arenes Systems

Article abstract of DOI:10.1002/chem.202004620

Light-responsive molecular systems with multiple photoswitches in C₃-symmetric designs have enormous application potential. The design part of such molecular systems is critical due to its influence in several properties associated with the photoswitches. In order to tune, and in the evaluation of the design–property relationship, we synthesized 18 tripodal systems with variations in the core, linkers, connectivity, and azo(hetero)arene photoswitches. Through extensive spectroscopic and computational studies, we envisaged the factors controlling near-quantitative photoisomerization in both the directions (bistability) and the thermal stability of the metastable states. Furthermore, we also evaluated the impact of designs in obtaining reversible photo-responsive sol-gel phase transitions, solvatochromism, photo- and thermochromism.

Full text of DOI:10.1002/chem.202004620

Accepted Article
Accepted Article
Title: Tuning of Bistability, Thermal Stability of the Metastable States
and Application Prospects in the C3 Symmetric Designs of
Multiple Azo(hetero)arenes Systems
Authors: Sugumar Venkataramani, Debapriya Gupta, Ankit Kumar
Gaur, Pravesh Kumar, Himanshu Kumar, Anjali Mahadevan,
Sudha Devi, and Saonli Roy
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004620
Link to VoR: https://doi.org/10.1002/chem.202004620
01/2020

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
Tuning of Bistability, Thermal Stability of the Metastable States
and Application Prospects in the C3 Symmetric Designs of
Multiple Azo(hetero)arenes Systems
Debapriya Gupta,a† Ankit Kumar Gaur,a† Pravesh Kumar,a† Himanshu Kumar,a† Anjali Mahadevan,a†
Sudha Devi,a Saonli Roy,a and Sugumar Venkataramania*
Dedication ((optional))
unsymmetrical azoarenes that can isomerize at different
wavelengths of light leads to multistate (or orthogonal)
photoswitching.[11] Such systems in principle, exhibit
photoswitching over 2n states, where n = 2, 3, 4, etc., is the
number of unsymmetrical azoarenes. Alternatively, macrocyclic
ring strain can also dictate such phenomenon.[4e] The concepts
of multistate photoswitching are quite advantageous in designing
smart materials, optomechanics, and optoelectronic devices.[12]
Conversely, certain applications like energy storage devices and
catalysis necessitate quantitative end-to-end photoswitching
without much involvement of the intermediate states, that is,
bistability.[13] On demand, a C3 symmetric system with three
identical azoarenes can be expected to yield high percentage
ZZZ- and EEE-isomers (among the four possible EEE-, EEZ-,
EZZ- and ZZZ-photoisomers) in forward and reverse
isomerization steps, respectively. However, the existence of
photostationary states (PSS) limits such quantitative
conversions.[3d,6b]
Abstract: Light-responsive molecular systems with multiple
photoswitches in C3 symmetric designs have enormous application
potential. The design part of such molecular systems is critical due
to its influence in several properties associated with the
photoswitches. In order to tune, and in the evaluation of the design-
property relationship, we synthesized 18 tripodal systems with
variations in the core, linkers, connectivity, and azo(hetero)arene
photoswitches. Through extensive spectroscopic and computational
studies, we envisaged the factors controlling near-quantitative
photoisomerization in both the directions (bistability) and the thermal
stability of the metastable states. Furthermore, we also evaluated
the impact of designs in obtaining reversible photo-responsive sol-
gel phase transitions, solvatochromism, photo- and thermochromism.
Introduction
Although factors controlling the multistate photoswitching are
known to a reasonable extent, the corresponding knowledge in
achieving bistability in the systems with two or more azoarenes
is highly desirable. Also, unraveling additional characteristics
related to the azoarenes such as solubility, concentration
dependency, the thermal stability of the photoswitched states,
photoswitching stability, wavelength of light for irradiation, etc.
are equally pivotal in improving the design. However, systematic
studies on such a plethora of factors towards achieving the
bistability, and the impact of overall design towards applications
are unknown.
One of the recent advancements in the chemistry of azoarenes
is the huge impact of systems with multiple photoswitchable
units.[1] Variations in the linkers and connections of two or more
azoarenes in linear[2], branched[3], and cyclic[4] fashion were
mainstays in this regard. Despite the possibility of several
designs utilizing multiple azoarenes, C3 symmetric systems have
been profoundly explored. Indeed, the resulting tripodal designs
afforded a wide range of applications such as holographic
grating element[5], liquid crystals (LCs)[6], rewritable actuators[7]
hierarchical supramolecular assembly[8], photoswitchable
polymers[9], rewritable image printing and erasing[10], etc. They
differ in characteristic features such as molecular size,
rigidity/flexibility, groups enabling supramolecular interactions,
etc. Ironically, such characteristics influence the properties
associated with the azoarenes attached, which has not been
fully understood.
It is intriguing to investigate the photoisomerization abilities of
individual units to switch between E- and Z-isomeric states in
multiple
azoarenes
connected
systems.
Indeed,
in
bis(azobenzene) systems, the relative position of the azo groups
(meta or para) and the extended -conjugation can influence the
electronic coupling between them in the forward isomerization
step.[1a] However, such electronic conjugation can be decoupled
by steric factors.[2a] On the other hand, connection of
[a]
Ms. D. Gupta, Mr. A. K. Gaur, Mr. P. Kumar, Mr. H. Kumar, Ms. A.
Mahadevan, Dr. S. Devi, Dr. S. Roy, Dr. S. Venkataramani
Department of Chemical Sciences
Scheme 1. C3 symmetric designs of tripodal targets. (C: (Het)aryl core; Y:
Indian Institute of Science Education and Research (IISER) Mohali
Sector 81, SAS Nagar, Knowledge City, Manauli – 140306, Punjab,
INDIA E-mail: sugumarv@iisermohali.ac.in
Equally contributed
Linker; R: Azo(hetero)arene photoswitches)
In this regard, we have chosen 18 different C3 symmetric
photoswitchable systems (Scheme 1). All those target
molecules have been designed in such a way that they have the
following features: (a) supramolecular assembly through
†
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
hydrogen bonding and/or stacking;[3c,8] (b) steric factors for
disassembly through methyl groups at the core, linker or
peripheral positions; (c) rigid or flexible connections of
azoarenes through different linkers {-C(O)N(H)-, -C(O)N(CH3)-, -
N(H)-, -C(H2)-, -C(H2)N(H)-}; (d) position of the azo group
relative to the linker (ortho, meta, and para) in tuning the
electronic coupling; (e) difference in the core units (benzene vs
triazine); (f) introduction of azoheteroarene photoswitches owing
to their improved photoswitching efficiency, the remarkable
thermal stability of the Z-isomer, and also increasing application
potential,[14] and (g) the orientation of the heterocycle moieties.
basic condition using phenylazo-1H-3,5-dimethylpyrazole 15
with
1,3,5-tris(bromomethyl)benzene
17a,
and
1,3,5-
tris(bromomethyl)mesitylene 17b, respectively.[17a] Similarly, the
target 8 has been synthesized using 4-aminoazobenzene 11c
and 1,3,5-tris(bromomethyl)benzene 17a using K2CO3.[17b]
Electronic spectral data and photochemical aspects:
Foremost, we carried out UV-Vis spectroscopic studies for all the
18 tripodal targets 1-8. Due to the azo chromophores, all the
targets in their native state (EEE) exhibited a strong *
absorption bands (Figure 1 and supporting Section S3). Also,
the spectral data revealed that n* absorption feature was
silent except in the cases of 1b, 2a, 2c, 5a, 6, and 7. Apart from
the contributions from the linkers through electronic effects, the
shifts in the max corresponding to the * band signify the
extent of -conjugation. The targets with an azo group at a para
position relative to the linker units have the maximum
bathochromic shift, followed by ortho and then meta. For targets
with flexible connections, such as 6, 7, and 8 the *
absorption features were observed at 338, 342, and 413 nm,
respectively. Compared to these max values, the corresponding
simple photoswitches, N-methyl phenylazo-3,5-dimethyl-
pyrazole, and 4-(N-methylamino)azobenzene absorb at 339 and
416 nm, respectively (see supporting Tables S3-1, and S3-2).
This indicates that the flexible linkers have a weak influence on
the photoswitches. Accordingly, those targets behave like three
independent photoswitches despite connected together. The
electronic effect arising due to the donating power of the amine
linker in 8 leads to the overlapping of * and n* bands
Through these investigations, we synthesized and characterized
the 18 tripodal targets successfully and subjected them to
extensive spectroscopic and computational studies. Except 1c
the remaining 17 targets are reported for the first time. Herein,
we summarise the outcomes of those studies, in particular,
tuning of photoswitching behavior, understanding the thermal
stability of the metastable states, and potential application
prospects through the factors associated with the designs.
Based on these studies, we have also identified the best tripodal
C3 symmetric system exhibiting bistability.
(aminoazobenzene
type).[18]
Compared
to
benzenetricarboxamide (BTA) derivatives, the corresponding N-
methylated derivatives render a strong blue shift in the *
absorption. This is particularly prominent in para derivatives 2c
and 4c. On contrast, their ortho and meta analogs 2a,b and 4a,b
showed a slight red shift in the max relative to the corresponding
carboxamides 1a,b, and 3a,b, respectively.
Interestingly, all the targets exhibited high molar absorptivity
values indicating very intense * absorption features. Primarily
this can be due to the presence of three azo(hetero)arenes.
Moreover, this trend is attributable to the substantial localization
of the transitions at the azo groups. For the BTA derivatives 1a-c,
and 3a-c, and their N-methylated analogs 2a-c, and 4a-c, the
values have no particular dependency on the position of the
azo group relative to the linker. However, in the ortho derivatives,
the values were found to be lower.
All the derivatives showed good to excellent photoswitching
upon irradiation at 365 nm. However, in the cases of 5c and 8,
effective isomerization took place at 405 nm owing to red shifted
* band. The photoisomerization of EEE-isomers exhibited
strong spectral changes accompanying the formation of the
three potential photoisomers, EEZ, EZZ, and ZZZ. Since these
isomeric species are indistinguishable based on the absorption
features, the exact composition is impossible to quantify by
using UV-vis spectroscopy. Apart from that, we observed an
overlap of the * absorption bands of the azo group with that
of aromatic moieties upon photoisomerization. This is prominent
in the cases of 1a, 2a-c, 4a,b and 5a,b. However, in all those
cases, enhancement in the intensity of n* band confirmed the
effective photoswitching towards ZZZ-isomer. The max
corresponding to the n* absorption bands appears in the
range between 401 (for 5a) and 446 nm (for 2a). For all the
targets, we screened different wavelengths of light to induce the
reverse isomerization step towards the native state. Based on
the observed spectral changes at their respective PSS, we have
also estimated the fraction of molecules undergoing
Scheme 2. Synthesis of the target molecules 1-8: (i) SOCl2, cat.
DMF, reflux, 4-5 h or PCl5, toluene, reflux, 4-5 h; (ii) 10, 11a-c,
Et3N, DCE, rt, overnight, 1a-60%; 1b-75%; 1c-80%; (iii) 10,
12a-c, Et3N, DCE, rt, overnight, 2a- 62%, 2b-70%, 2c-72%; (iv)
10, 13a-c, pyridine, toluene, reflux, 4-5 h, 3a-67%, 3b-75%, 3c-
85%; (v) 10, 14a-c, Et3N, DCE, rt, overnight, 4a-33%, 4b-68%,
4c-20%; (vi) 16, 11a-c, glacial CH3COOH, 60 oC, 30 min, 5a-
55%, 5b-62%, 5c-65%; (vii) 17a, 15, NaH, CH3CN, reflux,
overnight, 6-63%; (vii) 17b, 15, Et3N, DCE, rt, overnight, 7-70%;
(ix) 17a, 11c, K2CO3, CH3CN, reflux, overnight, 8-62%.
Results and Discussion
Synthesis: We have synthesized all the 18 tripodal targets
using the procedures adopted from the literature (Scheme 2).
For accessing the amide derivatives 1a-c and 2a-c, their
corresponding amino azobenzenes (11a-c) or the N-methylated
aminoazobenzene analogs (12a-c) have been treated with the in
situ generated trimesoylchloride 10, derived from trimesic acid
9.[3c,10,15] In a similar procedure, 3a-c and 4a-c have been
synthesized using the corresponding amine derivatives or their
N-methylated analogs of phenylazo-3,5-dimethylisoxazoles
(13a-c and 14a-c).[6b,14c] For the synthesis of triazine derivatives
5a-c, aromatic nucleophilic substitution has been exploited using
the corresponding aminoazobenzene derivatives 11a-c and
cyanuric chloride 16.[16] The targets 6 and 7 have been
synthesized through a nucleophilic substitution strategy under
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
photoisomerization in the forward and reverse directions (Table
S3-1 in the supporting information). Interestingly, in all the three
triazine derivatives 5a-c, we observed an isosbestic point close
to the n* absorption max. This significantly influence the
reverse isomerization step causing poor conversion. For
selected derivatives 1c, 2c, 3c, 4c, 5c, 6, 7, and 8, the fatigue
resistance has also been tested up to five cycles. All of them
showed excellent reproducibility in the forward and reverse
isomerization steps. In order to gain further insights into the
electronic spectral data, computations have been performed at
DFT level and discussed in a separate section.
Table 1. Estimation of PSS composition for
forward and reverse isomerization steps of the
selected targets using 1H-NMR spectroscopy
%Composition of individual isomers
at PSSa ([D6]DMSO)
Conc.
(mM)

S.
No.
Compound
(nm)
EEE
EEZ
27
EZZ
ZZZ
365
-
28
12
72
7
1.
2.
3.
4.
5.
6.
7.
1bb
1cb
2cc
3ad,e
3bb
3cb
4bf
4.16
4.16
4.16
1.98
2.48
2.26
7.79
435
54
365
-
-
14
7
86
-
450
59
34
365
-
-
18
10
82
-
435
33
57
365
-
-
26
c
74
c
490
c
c
365
-
-
18
16
82
-
490
41
43
365
-
3
12
-
85
-
490
76*
24
365
-
-
25
13
75
2
490
49
36
365
-
-
10
8
90
1
8.
4cf
4.16
490
57
34
365
12
46
38
40
38
14
12
-
9
5ag
5cg
6h
5.62
6.94
4.16
4.16
490
405
7
16
42
33
17
44
-
10.
11.
12.
490
41
365
-
-
9
91
2
490
55
33
10
365
-
3
6
91
15
7h
490
24
33
28
aDetermined using the normalized integral values of the signals corresponding
to the four photoisomers based on the following protons: bamide N-H; camide
N-CH3; dsolubility increased after irradiation at 365 nm; ebenzene core-H;
fisoxazole-CH3; glinker amine -NH-; hlinker -CH2-; Wavelength of irradiation:
normal – forward; bold – reverse isomerization step (*poor solubility causes
lower %EEE-isomer in the reverse isomerization step than actual).
Quantification of photoisomers: For understanding bistability,
the estimation of the individual photoisomers (PSS composition)
for the forward and reverse isomerization steps is necessary.
Towards this, we have conducted the photoswitching studies of
all the 18 targets in [D6]DMSO (at mM concentration). The
analysis of the PSS compositions has been carried out using 1H-
NMR spectroscopy (Table 1 and table S4-1 in the supporting
information). For the majority of these targets, we identified a
non-overlapping proton signal that can exhibit sequential shifts
upon isomerization. For each system, the choice of the signal
varies between core protons, the methyl protons, the N-H
protons depending on the spectral overlap. The broadness and
overlapping signals in 2a, 2b, 4a, and 8 hindered quantification
of the photoisomers, whereas the solubility issues in the reverse
isomerization step did not allow us to estimate the PSS
compositions in 1a and 3a. Indeed, 4a exhibited rotamers with
many overlapping signals.[19]
Figure 1. Representative data on the analysis of photoswitching in 3c: (a) UV-
Vis data depicting the wavelength-dependent photoisomerization in the
forward and reverse isomerization sequence (in DMSO, 9.0 M); (b)
Photoswitching stability of 3c over five cycles of forward and reverse
isomerization indicating fatigue resistance; (c) Monitoring the thermal reverse
isomerization kinetics using UV-Vis spectroscopy (in DMSO, 90 ± 2 oC); (d)
The corresponding exponential growth curves of the native state; 1H-NMR
spectral analysis for the estimation of PSS compositions in forward and
reverse isomerization (region-wise split, in [D6]DMSO, 2.26 mM): (e) before
irradiation (EEE-isomer); (f) after irradiation at 365 nm/forward isomerization;
(g) after irradiation at 490 nm/reverse isomerization (The individual
photoisomers of the 3c are indicated; poor solubility causes low signal to noise
ratio in “g”).
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
Among the systems for which we have obtained the PSS
composition in the forward direction, 1c, 2c, 3c, 4c, 6, and 7
showed a maximum conversion of ZZZ isomers in the range of
82-91%. Particularly, the N-methylated derivatives or systems
having methyl groups (in the core or the heterocyclic units: 6,
and 7, 3a-c, and 4a-c) showed a maximum isomerization. On
the other hand, all triazine derivatives 5a-c, and BTA derivatives
1a, and 1b showed a poor isomerization conversion. Although
1c showed a good conversion of about 86% ZZZ-isomer, it
displayed a concentration dependency (vide infra). Concerning
the position of the azo group, the systems with para connections
exhibited excellent isomerization conversions followed by meta
connections. Comparatively the systems with ortho connections
showed only a moderate isomerization. Presumably the longer
wavelength * absorption and extended conjugation play a
vital role in effective forward isomerization of para derivatives.
This argument is supported by the computed HOMO-LUMO
gaps (supporting section S9). The converse holds true for the
meta derivates, and so exhibit slightly lower conversions.
Whereas, the tautomerism or steric effects are attributed for the
poor isomerization conversions in the cases of ortho derivatives.
groups, the other derivatives such as 2c, 3c, 4c, and 6 did not
show any effect of concentration on the forward isomerization
conversions (Section S5 in the supporting information). Notably,
3c exhibited no concentration-dependent photoswitching, albeit
possessing
a BTA core, the moiety capable of forming
supramolecular assembly through hydrogen bonding. Probably,
the presence of two peripheral methyl groups in the isoxazole
moiety either hinders the stacking or induces a weak assembly.
Thermal stability: Apart from the photoswitching characteristics,
the stability of the metastable states under ambient conditions is
quite significant in the application point of view. In general, a Z-
isomer of the azoarene can undergo Z-E reverse thermal
isomerization through inversion or rotational or concerted
inversion or inversion-assisted rotational pathways that depend
on substitution, type of azoarene, etc.[18] Thermal barriers and
the half-lives associated with the individual metastable states
can be tuned by changing the design. Since multiple
photoswitchable units containing systems lead to different
photoisomers, unraveling the thermal stability of the metastable
states involves certain difficulties. This situation is due to the
involvement of individual reverse thermal isomerization steps. All
our 18 targets contain three photoswitchable groups each, and
so the reverse isomerization reactions happen over three
consecutive steps, namely, ZZZ-EZZ, EZZ-EEZ, and EEZ-EEE
with rate constants k1, k2 and k3, respectively. Indeed, we have
investigated the thermal reverse isomerization of the derivatives
1b and 1c in [D6]DMSO using 1H-NMR spectroscopy. For
understanding the rate-limiting steps, and also to obtain further
insights, we performed the reverse thermal isomerization
kinetics experiments at variable temperature.
[18]
The N-methylation in the BTA derivatives influences the
forward photoswitching only to a marginal extent in the cases of
2a-c and 4a-c. Flexibility enables better forward photoswitching
in the case of pyrazole derivatives 6 and 7. Likewise, the
estimation of PSS compositions in the reverse isomerization
step revealed 1c, 2c, 3c, 4c and 6 as the best photoswitchable
systems, however, they exhibit
a moderate isomerization
conversion towards the native EEE-isomers. Overall, 2c, 3c, 4c,
and 6 can be the best tripodal photoswitches in both forward and
reverse directions, if EEZ-isomer is also taken into account for
the quantification. The weak n-* absorption features of the
native state limit the complete reverse isomerization under
irradiation conditions. The details of the PSS estimation and the
data are given (Section S4 in the supporting information).
Interestingly, the target 1b showed a slower rate compared to 1c,
and so the reaction kinetics was followed at a reasonably higher
temperature. Using the composition of the individual isomers at
each time interval, we have obtained the time evaluation profiles
for both the tripodal systems (Figure 2a,b). By fitting such
profiles of each isomerization step, we have estimated the
individual rate constants, and the results are tabulated (see
supporting Section S6).[20] For each step, we have also
calculated the activation parameters using Eyring and Arrhenius
plots (Table 2). For comparison, transition states have been
calculated at B3LYP/6-311G(d,p) and M06-2X/6-311G(d,p)
levels of theory through an inversion mechanism for the
individual steps (see supporting Section S9). Using these, the
corresponding energy barriers associated with all the
isomerization steps have also been computed for 1a-c (Table 2,
Figure 2c and supporting section S9). Although the trends are
quite similar at both the levels of theory, the results obtained at
B3LYP/6-311G(d,p) are closer to the estimated activation
barriers through experiments.
Solubility and concentration dependency in forward
photoisomerization step: Solubility is a crucial parameter for
the solution processing of the molecules for various applications
and device fabrication. Majority of the targets were found to
have a reasonable solubility in different solvents. However, we
observed the solubility issues in the cases of BTA or triazine
derivatives with the azo groups attached at the ortho position
relative to the linkers. In contrast, we observed better solubility
for those systems with azo groups connected at meta or para
positions. In general, the N-methylation of the carboxamide
groups or incorporation of the methylene connections to
azoheteroarenes further enhances the solubility. Interestingly, in
the cases of 1a, 3a, 3c, and 5a, irradiation at 365 nm (forward
photoisomerization) improved the solubility, and on irradiating
the solution enriched with the photoisomers at 470 (for 1a) or
490 nm induced precipitation.
The comparison of the experimental free energy of activation
G‡ in both the molecules revealed that the first step (ZZZ-EZZ)
is the fastest. Also, the final step (EEZ-EEE) is found to be the
rate-limiting step. Interestingly, the computed enthalpy of
activation H‡ of all the three steps in 1a, 1b, and 1c is nearly
the same, whereas they differ only by entropy S‡ contribution.
On the other hand, the experimental data revealed that each
step has a difference in enthalpy as well as the entropy of
activation values. The difference could be arising due to the
consideration of gas-phase reaction in the computations,
whereas the experiments have been performed in solution
phase. Possibly, the dynamics of the photoswitches, steric
crowding, and the involvement of solvent interactions can be
responsible for it. However, the rate constants k1, k2 and k3 for
Selected targets (those with azo groups connected at para-
position) were tested for concentration dependency in the
forward photoisomerization step. Except for the BTA derivative
1c, and the triazine derivative 5c, other derivatives showed no or
negligible dependency upon varying the concentration up to 10
times. Indeed, Thiele’s and Kim’s group reported the importance
of the concentrations in photoswitching efficiency of 1c. The
possible supramolecular assembly through hydrogen bonding,
and  stacking have been accounted for limiting the
isomerization conversions in 1c and similar systems.[3c,8b,d,e]
A
similar hydrogen bonding between the electron-deficient triazine
and N-H linker might be responsible for the concentration
dependency in the case of 5c. Since the supramolecular
interactions and assemblies can be hindered through the methyl
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
Table 3. Kinetics dataa
both 1b and 1c exhibited a ratio of almost 3:2:1 confirming the
independent nature of the three azobenzene units.[3c] Apart from
the direct estimation of the activation parameters, we have also
estimated the same by including the statistical consideration and
the normalized rate constant as described in the literature (see
supporting Table S6-2).[2a,3d] Indeed, the normalized rate
constants for all the three steps were found to be quite similar.
To compare the thermal stability of the metastable states in all
the 18 targets, we have deduced the first order formation
constants of the native EEE-isomer from their respective
photoswitched states. In this regard, we have utilized the UV-Vis
spectroscopy and monitored the changes in the absorption
corresponding to the max of the * band. Using the
exponential growth of EEE-isomer, we estimated the rate
constants, and the half-lives (Table 3). Indeed, all the derivatives
exhibit higher thermal stability for the photoswitched states at
room temperature, and so the isomerization rates were very
slow. Hence, we performed the kinetics experiments at elevated
temperatures. Interestingly, the rate constants measured by UV-
Conc.
[M]
S.
No.
Temp.
(oC)
Rate constant
(min-1)
Half- life
(min)
1
2
1a
60 ± 2
1.1x10-2 ± 3.9x10-4
62
NA
60 ± 2
90 ± 2
5.5x10-3 ± 1.4x10-4
126
12
15.3
11.8
1bb
5.8x10-2 ± 1.4x10-4
3
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6
60 ± 2
60 ± 2
60 ± 2
60 ± 2
90 ± 2
1.9x10-2 ± 5.8x10-4
1.0x10-2 ± 4.8x10-4
4.1x10-3 ± 1.0x10-4
1.3x10-2 ±5 .6x10-4
6.1x10-2 ± 2.7x10-3
2.7x10-2 ± 6.8x10-4
9.4x10-2 ± 2.9x10-3
6.6x10-2 ± 4.6x10-3
2.9x10-2 ± 8.1x10-4
4.8x10-2 ± 1.1x10-3
4.2x10-2 ± 1.1x10-5
6.4x10-3 ± 3.1x10-4
4.9x10-2 ± 4.1x10-3
3.9x10-2 ± 4.8x10-4
4.1x10-2 ± 1.0x10-3
1.7x10-3 ± 4.4x10-5
36
66
168
53
11
26
7
10.6
18.7
12.8
14.1
15.7
13.0
9.0
4
5
6
7
8
90 ± 2
90 ± 2
1
Vis (overall formation rate constant) and H-NMR spectroscopy
(the average of normalized rate constants[2a,3d] of the individual
steps) for 1b (5.8 x 10-2 vs 8.3 x 10-2 min-1) and 1c (1.9 x 10-2 vs
2.6 x 10-2 min-1) are quite comparable and exhibited a narrow
difference. This makes the comparison of the overall formation
rates of the other targets quite useful in understanding the
trends and also the influence of design part in the thermal
stability of the metastable states.
9
90 ± 2
10
11
12
13
14
15
16
17
18
11
24
14
17
108
14
18
17
6.9
17.9
14.5
13.7
10.1
27.6
11.7
22.0
8.1
90 ± 2
90 ± 2
60 ± 2
60 ± 2
60 ± 2
90 ± 2
90 ± 2
60 ± 2
Table 2. Experimental activation parametersa,b for
the thermal reverse isomerization steps and
computational datac
Activation parametersc,d
7
8
3.5
Ea
G‡
H‡
S‡
S.
No.
aRate constants and half-lives corresponding to the overall formation of EEE-
isomer in each case, which was followed by UV-Vis spectroscopy in DMSO at
the specified temperature; bAdditional measurement has been performed at 90
± 2 oC.
(cal mol-1 K-
Step
(kcal
(kcal
(kcal
mol-1)
mol-1)
mol-1)
)
1
1
2
3
ZZZ-EZZ
EZZ-EEZ
EEZ-EEE
[22.9]
[22.9]
[23.2]
21.1
[20.2]
[21.2]
[21.9]
24.2
[23.0]
[23.0]
[23.2]
21.1
[2.8]
[1.1]
[0.4]
1a
1b
Based on the data, we observed that the systems with meta
connections of the azo groups relative to the linker units have
longer half-lives and higher thermal stability of the
photoswitched states. In contrast, those with ortho and para
connections have lower half-lives. As expected, the introduction
of azoheteroarenes based photoswitches tremendously
improved the thermal stability of the photoswitched states.
Overall, the thermal stability of the photoswitched states follows
the order: meta > para > ortho.[21] Significantly, the incorporation
of N-methylation at the amide nitrogen increases the half-lives in
all the cases, whereas the flexibility in the linker or steric factor
reduces it.
±
±
±
±
±
±
±
±
±
±
±
±
±
-12.4 ± 5.5
[1.8]
(-6.0)
0.4
2.4
1.8
4
5
6
7
8
9
ZZZ-EZZ
EZZ-EEZ
EEZ-EEE
ZZZ-EZZ
EZZ-EEZ
EEZ-EEE
[23.1]
(21.7)
22.4 ±
1.1
[23.1]
(22.6)
22.4
[20.8]
(22.8)
24.2
[21.9]
(21.0)
21.1
-12.4 ± 1.2
[2.3]
(-3.5)
0.5
0.4
[20.5]
(22.9)
25.5
[21.9]
(21.9)
22.4
±
±
±
±
-10.4 ± 3.3
[-1.7]
(-2.5)
1.1
1.5
1.1
[23.2]
(23.0)
19.5
[23.0]
(23.0)
23.0
[21.9]
(22.3)
19.5
-11.6 ± 2.4
[2.6]
(-5.0)
1.9
1.1
0.8
Computational studies[22]: To compare the experimental data
and in understanding the properties, we subjected all the 18
targets to DFT computations at B3LYP/6-311G(d,p) level of
theory. This includes the optimization of each tripodal system in
EEE, EEZ, EZZ, and ZZZ-isomeric states. For gaining further
insights, we inspected the HOMO-1, HOMO, and LUMO orbitals
for each one of the optimized structures. The molecular orbital
analysis revealed that HOMO-1 and HOMO orbitals of the native
(EEE) isomers of 1a-c, 2a, 3a-c, 4a, 6, 7, and 8 are degenerate,
whereas in the cases of 2b, 2c, 4b, and 4c, they are slightly
non-degenerate. But in the case of the triazine-based system
5a-c, the HOMO-1 and HOMO orbitals are non-degenerate. We
observed -natured HOMO orbitals for all EEE- isomers, except
in the cases of 2a and 4a, for which the orbitals are /n mixed.
[22.6]
(21.0)
21.4
[19.9]
(21.9)
23.2
[21.4]
(20.4)
20.5
-9.3 ±
0.6
[1.5]
(-0.5)
0.8
0.3
0.2
1c
[22.6]
(22.4)
21.4
0.8
[22.7]
(23.0)
[20.6]
(21.7)
23.6
0.3
[21.9]
(22.8)
[21.5]
(21.7)
20.5
0.2
[21.5]
(22.4)
-10.7 ± 0.6
[-0.6]
(1.4)
aEstimated by using Arrhenius and Eyring equation through variable
temperature kinetics measurements by 1H-NMR spectroscopy bat 298 K.
cComputed data at B3LYP/6-311G(d,p) level of theory are indicated in the
square brackets. All the energies are zero-point energy corrected. dThe
activation parameters deduced using normalized rate constants in kcalmol-1
(Ea, G‡, and H‡) and calmol-1K-1 (S‡) are indicated in bold (parentheses)
(All the relevant data are available in the supporting information)
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
Figure 2. Representative time evaluation profiles of the thermal reverse isomerization kinetics experiments of (a) 1b (at 80 oC, in [D6]DMSO) and (b) 1c (at 50 oC,
in [D6]DMSO) monitored by 1H-NMR spectroscopy. (c) Computational data (B3LYP/6-311G(d,p) level of theory) on the thermal barriers for the reverse
isomerization steps of 1b,c in terms of G‡ (in kcal/mol) through inversion mechanism; The relative energies of the photoisomers are also indicated in kcal/mol);
(d) HOMO-LUMO gaps (in eV) in the photoisomers of 1a-c, and (e) Computed electronic absorption spectral data (TD-DFT/6-311G(d,p) level of theory) depicting
the changes in the spectral shifts and intensities of the photoisomers in 1a-c.
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
On the other hand, the LUMO of all the EEE-isomers
corresponds to * orbitals. On comparison of the molecular
orbitals of ZZZ-isomers of all the targets revealed a mixed /n
nature in both HOMO and LUMO orbitals. Indeed, in all the
cases, the non-bonding orbitals corresponding to the azo-
nitrogen are well stabilized and are low-lying. The HOMO-LUMO
gap among the ZZZ-isomers revealed that the maximum was for
the meta, followed by ortho and then para- isomers (Figure 2d
and supporting Figure S9-1). Interestingly, the compounds 5a,
and 5b exhibit higher HOMO-LUMO gaps for the photoswitched
states rather than the native state with
a
significant
destabilization of the LUMO orbitals. This is corroborating with
the observed changes in UV-Vis spectral data. In all the
derivatives, the isomerization renders a blue shift along with
decrease in the absorption of the * band, whereas in 5a and
5b, only intensity changes have been observed. To delineate the
electronic transitions corresponding to their UV-Vis spectral data,
and to shed light into the progressive changes upon
photoisomerization, we performed TD-DFT computations for 1a-
c. We observed a blue shift in the max corresponding to *
transition involving HOMO and LUMO upon isomerization, which
is in line with the experimental data. The TD-DFT computations
predicted a blue shift for each progressive Z-isomerization step
from EEE-isomers of * transitions involving the azo group
(Figure 2e). Furthermore, the molar absorptivity () increases in
the order: ortho < meta < para. The -value decreases drastically
as the three-fold symmetry is broken upon isomerization from
EEE-EEZ, and drops further in the successive steps. As a result,
the -value corresponding to ZZZ-isomers were the lowest.
Advantages of the design: The variations in the design
provided substantial scope for tuning the photoswitching
behaviour and the stability of the metastable states. Also,
modifications in the core, linker, photoswitches, the relative
orientation of azo groups offered advantages in application
perspectives. In this regard, we carefully scrutinized the salient
features of each design. The extended conjugation arising due
to BTA core results in solubility issues mainly due to the
tautomerism in 1a or supramolecular assembly through the
combination of hydrogen bonding, and a possible stacking
in 1b,c. On the introduction of methyl groups at the amide
nitrogen, the resulting systems 2a-c exhibited improved solubility
and photoswitching. However, the light induced reverse
isomerization did not improve. The connection of phenylazo-3,5-
dimethylisoxazole to BTA core in 3a-c led to an interesting
situation, where the supramolecular assembly at the core
through hydrogen bonding competes with the steric factor
arising due to peripheral methyl groups towards disassembly. As
a result, all these systems can be able to exhibit stimuli-
responsive sol-gel phase transitions. The most striking aspect is
the reversibility in the sol-gel formation in both directions by light.
The gel-form in its native state collapsed to sol upon irradiation
at 365 nm, whereas it reversed to a partial gel-form by irradiating
at 490 nm light (Figure 3a,b and supporting Section S7). Such
photoresponsive reversible sol-gel phase transitions have been
monitored by polarized optical microscopy (POM). The gel-form
of the sample exhibited a birefringence pattern under POM, and
upon irradiation at 365 nm light resulted in the sol-form. This can
be monitored by the disappearance of birefringence due to the
attainment of isotropic phase. The process can be reversed by
irradiation at 490 nm light that causes the reappearance of the
birefringence. On the other hand, the systems with N-
methylation at BTA cores 4a-c did not exhibit such reversible
sol-gel properties at all.
Figure 3. Evaluation of potential application prospects of various designs of C3
symmetric tripodal multiple azo(hetero)arenes: Reversible light responsive sol-
gel behaviour in 3c. (a) Gel form in 3c (Formed by using 4.3 mM (0.4 wt%)
solution of 3c in a mixture of DMF-H2O (3:1) at room temperature) can be
converted into sol form by irradiation at 365 nm; this can be reversed to a
partial gel by 490 nm irradiation; (b) The corresponding polarized optical
microscopic (POM) images for gel and sol forms are depicted (at 10x zoom) (i)
Before irradiation; (ii) After 365 nm irradiation (21 s), and (iii) after 490 nm
irradiation (523 s); Solvatochromism in 5c: (c) UV-Vis spectral data showing
normalized absorptions of 5c in different solvents; (d) Linear relation between
the max corresponding to the * absorption feature of 5c in different
solvents and the hydrogen bond acceptor ability,  (R2 = 0.84); Photo- and
thermochromism in 6: (e) Yellow coloured solid-state and solution phase (in
CH3CN) samples of 6 in native state; (f) After exposure to a light of wavelength
365 nm exhibiting solid-state and solution phase photochromism; (g) After
heating at 90 oC for 30 min (for the solid sample) and 70 oC for 1 h (for solution
phase sample) to obtain thermochromism in reversing to the native state; (h)
Overall summary depicting the effect of variation in the design of C3 symmetric
tripodal multiple azo(hetero)arene systems and evaluation of key properties
(Evaluation scale: ++ = More effective; + = Less effective; 0 = ineffective; x =
unknown).
The replacement of benzene core by electron-deficient triazines,
and having an amine group as the linker for connecting
azobenzenes resulted in the designs 5a-c. Such systems
exhibited longer wavelength absorptions indicating planarity,
and/or extended -conjugation. Apart from that, a potential
supramolecular assembly might be responsible for poor solubility,
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
and concentration-dependent photoswitching. Interestingly, 5c
with a maximum possibility of -conjugation and the lowest
HOMO-LUMO gap exhibited solvatochromism. The max
corresponding to the * absorption feature of 5c exhibited a
maximum value of 390 nm in DMSO, whereas it shifted up to
364 nm in chloroform. This trend has a linear correlation with the
hydrogen bond acceptor ability () of the various solvents
(Figure 3c,d and supporting Section S7).
Acknowledgements
We are grateful to the Science and Engineering Research Board
(SERB), New Delhi (CRG/2019/005744) for financial support.
We are also thankful to IISER Mohali for the financial support,
the departmental and central research facilities, and also the
NMR, HRMS, and other instrumentation support. DG, AKG, and
HK thank MHRD, PK thanks CSIR (09/947(0077)/2016-EMR-1),
and AM thanks DST INSPIRE for their Research Fellowships,
respectively. We are thankful to the Dr. Santanu Kumar Pal and
Ms. Indu Bala for helping in POM studies. We are thankful to Dr.
P. Anbarasan, IIT Madras for helping in the recording of solid-
state NMR. We thank Mr. Mayank Saraswat for helping in the
use of Root program.
The flexible connections between photoswitches and the core
units make the designs 6, 7, and 8 more advantageous. Due to
the absence of -conjugation, the photoswitches are
independent. Also, the availability of more room due to the
conformational flexibility imparts solid-state photoswitching
feasible. We illustrated such solid-state photoswitching using the
Keywords: Azo(hetero)arenes
Bistability • Sol-gel phase transitions • Photochromism
• Multiple photoswitches •
tripodal system 6 that exhibited
a fast switching with a
[1] a) F. Cisnetti, R. Ballardini, A. Credi, M. T. Gandolfi, S. Masiero, F. Negri,
S. Pieraccini, G. P. Spada, Chem.–Eur. J. 2004, 10, 2011–2021; b) A.
Galanti, V. Diez-Cabanes, J. Santoro, M. Valášek, A. Minoia, M. Mayor, J.
Cornil, P. Samorì, J. Am. Chem. Soc. 2018, 140, 16062–16070; c) C.
Slavov, C. Yang, L. Schweighauser, C. Boumrifak, A. Dreuw, H. A.
Wegner, J. Wachtveitl, Phys. Chem. Chem. Phys. 2016, 18, 14795–
14804.
contrasting color change upon irradiation at 365 nm. More
importantly, the forward switching can be light-induced
(photochromism), whereas the reverse step needs temperature
(thermochromism) (Figure 3e-g). Although other designs such as
2a-c, 3a-c, and 4a-c are also capable of such solid-state
photoswitching, the color changes and fast switching are
characteristics of 6, and 7, which make them useful in the
potential applications such as energy storage devices and
rewritable image printing. The overall impact of the variations in
the design in terms of changes associated with the selected
properties of the photoswitches is summarised (Figure 3h).
[2] a) D. Bléger, J. Dokić, M. V. Peters, L. Grubert, P. Saalfrank, S. Hecht, J.
Phys. Chem. B. 2011, 115, 9930–9940; b) Á. Moneo, G. C. Justino, M. F.
N. N. Carvalho, M. C. Oliveira, A. M. M. Antunes, D. Bléger, S. Hecht, J.
P. Telo, J. Phys. Chem. A. 2013, 117, 14056−14064.
[3] a) J. Robertus, S. F. Reker, T. C. Pijper, A. Deuzeman, W. R. Browne, B.
L. Feringa, Phys. Chem. Chem. Phys. 2012, 14, 4374–4382; b) J.
Bahrenburg, C. M. Sievers, J. B. Schönborn, B. Hartke, F. Renth, F.
Temps, C. Näther, D. Sönnichsen, Photochem. Photobiol. Sci. 2013, 12,
511–518; c) J. Kind, L. Kaltschnee, M. Leyendecker, C. M. Thiele, Chem.
Commun. 2016, 52, 12506-12509; d) A. Galanti, J. Santoro, R.
Mannancherry, Q. Duez, V. Diez-Cabanes, M. Valášek, J. D. Winter, J.
Cornil, P. Gerbaux, M. Mayor, P. Samorì, J. Am. Chem. Soc. 2019, 141,
9273−9283. e) I. C–Y. Hou, V. Diez-Cabanes, A. Galanti, M. Valášek, M.
Mayor, J. Cornil, A. Narita, P. Samorì, K. Müllen, Chem. Mater. 2019, 31,
6979–6985.
Conclusions
Through this investigation, we have systematically designed and
synthesized 18 C3 symmetric tripodal multiple azo(hetero)arenes
containing photoswitchable systems. By varying the core, linker,
the relative position of the azo group, and the photoswitches, the
photoswitching characteristics, and thermal stability of the
metastable states have been analysed. Based on the studies,
the following key outcomes have been envisaged: (a) Albeit
moderate to excellent photoswitching in the majority of the cases,
the efficiency in the forward and reverse isomerization lies in the
order (based on the orientation of azo group): para > meta >
ortho; Among the target systems, 3c followed by 4c showed the
best photoswitching in both directions. (b) Flexibility allows
higher % forward photoswitching, and the systems to behave
like three independent photoswitches, however, limits the
reverse isomerization; Also, it enables the solid-state
photoswitching. (c) The thermal stability of the metastable states
strongly depends on the relative position of the azo group and
follows the trend: meta > ortho > para. Indeed, the first step is
the fastest among the three consecutive isomerization steps,
whereas the third step is the rate determining step; (d) The N-
methylation of the amides or the incorporation of
azoheteroarenes with methyl groups in the heterocycles
improves the solubility, photoswitching efficiency, and also the
thermal stability of the metastable states. Through these studies,
we also demonstrated that subtle changes in the C3 symmetric
tripodal design can cause a variety of outcomes such as photo-
and thermochromism, solvatochromism, and reversible photo-
responsive sol-gel phase transitions. These important
[4] a) R. Reuter, H. A. Wegner, Chem.–Eur. J. 2011, 17, 2987–2995; b) R.
Reuter, H. A. Wegner, Org. Lett. 2011, 13, 5908–5911; c) C. Slavov, C.
Yang, L. Schweighauser, H. A. Wegner, A. Dreuw, J. Wachtveitl,
ChemPhysChem. 2017, 18, 2137-2141; d) A. Vlasceanu, M. Koerstz, A.
B. Skov, K. V. Mikkelsen, M. B. Nielsen, Angew. Chem. Int. Ed. 2018, 57,
6069 –6072; e) A. H. Heindl, J. Becker, H. A. Wegner, Chem. Sci. 2019,
10, 7418–7425; (f) A. H. Heindl, L. Schweighauser, C. Logemannb, H. A.
Wegner, Synthesis. 2017, 49, 2632–2639.
[5] a) K. Kreger, P. Wolfer, H. Audorff, L. Kador, N. Stingelin-Stutzmann, P.
Smith, H-W. Schmidt, J. Am. Chem. Soc. 2010, 132, 509–516; b) P.
Wolfer, H. Audorff, K. Kreger, L. Kador, H-W. Schmidt, N. Stingelinaef, P.
Smith, J. Mater. Chem. 2011, 21, 4339–4345; c) L. M. Goldenberg, L.
Kulikovsky, O. Kulikovska, J. Tomczyk, J. Stumpe, Langmuir. 2010, 26,
2214–2217.
[6] a) M. Pfletscher, C. Wölper, J. S. Gutmann, M. Mezger, M. Giese, Chem.
Commun. 2016, 52, 8549-8552; b) S. Devi, I. Bala, S. P. Gupta, P.
Kumar, S. K. Pal, S. Venkataramani, Org. Biomol. Chem. 2019, 17, 1947–
1954.
[7] J. Lee, S. Oh, J. Pyo, J-M. Kim, J. H. Je, Nanoscale. 2015, 7, 6457–6461.
[8] (a) Y-J. Choi, D-Y. Kim, M. Park, W-J. Yoon, Y. Lee, J-K. Hwang, Y-W.
Chiang, S-W. Kuo, C-H. Hsu, K-U. Jeong, ACS Appl. Mater. Interfaces.
2016, 8, 9490−9498; b) S. Lee, S. Oh, J. Lee, Y. Malpani, Y-S. Jung, B.
Kang, J.Y. Lee, K. Ozasa, T. Isoshima, S. Y. Lee, M. Hara, D.
Hashizume, J. M. Kim, Langmuir. 2013, 29, 5869−5877; c) Y. Huang, D-
H. Kim, Nanoscale, 2012, 4, 6312-6317; d) R. Reuter, H. A. Wegner,
Chem. Commun., 2013, 49, 146-148; e) M. Yamauchi, K. Yokoyama, N.
Aratani, H. Yamada, S. Masuo, Angew. Chem. Intl. Ed. 2019, 58, 14173 –
14178; Angew. Chem. 2019, 131, 14311 –14316; f) I. Abe, M. Hara, T.
Seki, S. J. Cho, M. Shimizu, K. Matsuura, H-K. Cheong, J. Y. Kim, J. Oh,
J. Jung, M. Han, J. Mater. Chem. C. 2019, 7, 2276-2282.
conclusions
will
be
helpful
in
designing
multiple
azo(hetero)arenes incorporated systems for
applications.
a
variety of
[9] (a) C. Probst, C. Meichner, H. Audorff, R. Walker, K. Kreger, L. Kador, H-
W. Schmidt, J. Polym. Sci., Part B: Polym. Phys. 2016, 54 , 2110–2117;
b) X. Liu, X-S. Luo, H-L. Deng, W. Fan, S. Wang, C. Yang, X-Y. Sun, S-L.
Chen, M-H. Huang, Chem. Mater. 2019, 31, 5421−5430.
Conflicts of interest
There are no conflicts to declare.
[10] S. Devi, A. K. Gaur, D. Gupta, M. Saraswat, S. Venkataramani,
ChemPhotoChem. 2018, 2, 806−810.
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
[11] a) F. Zhao, L. Grubert, S. Hecht, D. Bleger, Chem. Commun. 2017, 53,
3323–3326; b) C. Yang, C. Slavov, H. A. Wegner, J. Wachtveitl, A. Dreuw,
Chem. Sci. 2018, 9, 8665–8672; c) A. H. Heindl, H. A. Wegner, Beilstein J.
Org. Chem. 2020, 16, 22-31.
have synthesized the isomeric benzoyl derivatives of aminoazobenzenes
(Bz-11a-c). Using these derivatives, photoswitching and reverse thermal
isomerization kinetics have been performed. The trends are quite similar
to the tripodal designs, which confirms that the position of the azo groups
is quite critical for tuning the properties. All the data related to these
studies are available in the ESI.
[12] a) C. Weber, L. Pithan, A. Zykov, S. Bommel, F. Carla, R. Felici, C. Knie,
D. Bléger, S. Kowarik, J. Phys. Condens. Matter. 2017, 29, 434001/1-
434001/8; b) D. Bléger, Z. Yu, S. Hecht, Chem. Commun. 2011, 47,
12260–12266; c) J. Andréasson, U. Pischel, S. D. Straight, T.A. Moore, A.
L. Moore, D. Gust, J. Am. Chem. Soc. 2011, 133, 11641– 11648.
[22] Frisch, M. J. et al. Gaussian Inc., Wallingford CT, 2009.
[13] a) L. Dong, Y. Feng, L. Wang, W. Feng, Chem. Soc. Rev. 2018, 47,
7339—7368; b) V. Blanco, D. A. Leigh, V. Marcos, Chem. Soc. Rev. 2015,
44, 5341−5370; c) S. Grewal, S. Roy, H. Kumar, M. Saraswat, N. K. Bari,
S. Sinha and S. Venkataramani, Catal. Sci. Technol., 2020, DOI:
10.1039/D0CY01090A.
[14] a) S. Crespi, N. A. Simeth, B. König, Nat. Rev. Chem. 2019, 3, 133-146;
b) S. Grewal, D. Gupta, A. K. Gaur, M. Saraswat, S. Venkataramani in
Photoisomerization: Causes, Behavior and Effects (Ed.: D. Sampedro,
Nova Publishers, USA, 2019; c) P. Kumar, A. Srivastava, C. Sah, S. Devi,
S. Venkataramani, Chem. Eur. J. 2019, 25, 11924 –11932.
[15] K. Yano, Y. Itoh, F. Araoka, G. Watanabe, T. Hikima, T. Aida, Science,
2019, 363, 161-165.
[16] K. A. Kolmakov, J. Heterocyclic Chem. 2008, 45, 533-539.
[17] a) S. N. L. Andree, A. S. Sinha, C. B. Aakeröy, Molecules. 2018, 23, 163;
b) H-C. Yu, L. Li, J. Gao, J. Tong, W. Zheng, M. Cametti, A. Famulari, S.
V. Meille, F. Guo, J. Marti-Rujas, Dalton Trans. 2015, 44, 15960-15965.
[18] H. M. D. Bandara, S. C. Burdette, Chem. Soc. Rev. 2012, 41, 1809-1825.
[19] Apart from 4a, the other N-methylated amide derivatives 2a-c, and 4b,c
also exhibited broadening in the N-methyl proton signals. On the other
hand, the quantification has been made using the isoxazole methyl proton
signals in 4b,c. The data are available as Section S4 in the ESI.
[20] The kinetics data have been fitted using a program ROOT (version 6.08)
available in https://root.cern.ch. The rate constants have been deduced by
numerically solving the rate expressions as indicated in the reference 10.
The data are available in the ESI.
[21] In order to understand whether the trend is influenced by the tripodal
design or by the position of the azo group relative to the linker alone, we
This article is protected by copyright. All rights reserved.

10.1002/chem.202004620
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
What matters in the
Debapriya Gupta, Ankit Kumar
photoswitching and thermal
stability and application
prospects in tripodal
Gaur, Pravesh Kumar, Himanshu
Kumar, Anjali Mahadevan, Sudha
Devi, Saonli Roy, and Sugumar
Venkataramani*
photoswitchable systems?
Page No. – Page No.
Tuning of Bistability, Thermal
Stability of the Metastable States
and Application Prospects in the
C3 Symmetric Designs of
Multiple Azo(hetero)arenes
Systems
This article is protected by copyright. All rights reserved.