Strain, switching and fluorescence behavior of a nine-membered cyclic azobenzene

Article abstract of DOI:10.1039/c8nj01643g

Cyclic azobenzenes with ring strain possess improved photochromic properties in comparison to their acyclic analogs. In this study, we report a nine-membered cyclic azobenzene. This cyclic azobenzene follows the stability of the acyclic systems with the t

Full text of DOI:10.1039/c8nj01643g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE
Strain, Switching and Fluorescence Behavior of a Nine-Membered
Cyclic Azobenzene
Received 00th January 20xx,
Accepted 00th January 20xx
Monochura Saha, Sanjib Ghosh, Subhajit Bandyopadhyay*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cyclic azobenzenes with ring strain possess improved photochromicproperties in comparisonto the acyclic analogs. In this
study, we report a nine-membered cyclic azobenzene. This cyclicazobenzene follows the stability of the acyclic systems
with the trans isomer as the thermally stable form indicating its flexible structure, yet at the same time, it displays the
fluorescence properties thatis unusual for azobenzenes and requires structural rigidity. The properties of the molecule are
thus a result of a balance between the flexibility and rigidity of its structure.
Introduction
light,
electrocatalytic cis
demonstrated with azobenzene systems recently.
large number of azobenzene based photochromic systems
or
thermally.10
trans isomerization has also been
Besides
photoisomerization,

11
Although,
Photochromic compounds switch reversibly between two
isomeric forms having different colors, structures, and
functional properties.1 They are promising candidates for
optical memory and logic devices, molecular motors and
machines2-3 and are currently been used in photochromic
a
are reported - the majorities of them have acyclic structures
that often suffer from several drawbacks including their non-
quantitative interconversion leading to photostationary states,
12
low quantum yields and thermal instability of the cis isomers.
4
glasses and smart windows. Azobenzenes are one of the most
In contrast, Herges and coworkers have recently demonstrated
widely used photochromic systems that undergo isomerization
with light of different wavelengths.5 Despite its discovery
almost two centuries ago by Mitscherlich, excitement with
the superior switching properties of the cyclic azobenzene
photochromes13-14 where the boat-like cis form isomerizes to
the trans isomer with >90% conversion with soft UV light (385
nm), and with 520 nm visible light, the reverse photoreaction
is almost quantitative. The faster switching of the cyclic
azobenzene systems primarily occurs due to the molecular
strain in the cyclic azobenzene molecule. The superior
photophysical properties of the cyclic azobenzene compounds
6
azobenzenes never seem to decline. Usually, for azobenzenes,
the trans form are the thermally more stable form.7-9 When
the thermodynamically stable trans form is exposed to UV
light, it is converted to the less stable cis form. The cis form
reverts to the trans isomer either upon irradiation with visible
make
them realistic candidates
for applications
in
photoswitchable drugs and functional materials15 However,
certain cyclic azobenzene systems having enhanced reactivity
undergo undesired rearrangement and subsequent cleavage.16
Cyclic azobenzene molecule containing sulfur with various
a.
Department of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Kolkata, Mohanpur, Nadia WB 741246, India.
Footnotes relating to the title and/or authors should appear here.
†
Electronic Supplementary Information (ESI) available: [Procedures for the synthesis
,
characterization, photo and thermal isomerization studies and crystallographic
2 n
lengths of methylene chains [(CH ) , n = 3
data CIF file (CCDC 157005). See DOI: 10.1039/x0xx00000x
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Journal Name
structurally rigid enough, it may display fluorescence
properties.
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DOI: 10.1039/C8NJ01643G
N
N
O
S
O
N
N
S
O
O
O
Results and discussion
N
N
O
Synthesis
and
characterization.
We
have,
therefore,
Vogtle 1981
synthesized a cyclic azobenzene system 1 containing a nine-
R
Shinkai, Manabe 1980
membered ring having a sulfur and methylene-bridges (Figure
O
1
). However, as aptly pointed out by Herges,45 synthesis of
O
these
compounds
are
notoriously difficult and lack
R
O
reproducibility. In contrast, the optimized two-step synthesis
of 1 containing the nine membered cyclic azobenzene system
is reproducible, albeit the low yield (typically <20%). The
azobenzene compound 1 was synthesized upon the reduction
of the bis(nitrobenzyl)sulfane precursor 3 with glucose/NaOH
O
N
N
N
N
Herges, Temps 2009
Tamaoki 2012
S
X
as the reducing agent.46-47
The product was purified by
preparative TLC and was characterized thoroughly by the 1H
and 13C NMR, IR spectroscopy and mass spectrometry.
N
N
N
N
1
X = S and O, Herges 2016
This work
In addition, red crystals of compound 1 were grown by
recrystallization of the chromatographed product upon slow
evaporation from an acetonitrile solution at room temperature.
Figure 1: Reported cyclic azobenzenes and our compound 1.
and 4] have been reported three decades ago.17 Voegtle’s
Single crystal XRD afforded the structure of compound
Figure 5). The precursor was synthesized from the
corresponding o-nitrobromobenzyl compound 2 upon reaction
1
(
3
group has
reported an intra-annularly azo-substituted
48 49
with sodium sulfide in MeOH at room temperature. - This
synthesis in milligrams to gram scale was repeated twelve
times to check the reproducibility. The best yields (22%) were
obtained when the reaction was performed in 100 mg scale
[
3.2.3](1,2,3)phane although the photochromic properties of
18
the compound was not discussed by the authors (Figure 1).
Azobenzene-bridged crown ethers and azacrown ethers were
synthesized by Shinkai and Manabe’s group (Figure 1).1
Tamaoki and coworkers have reported the crystal structure of
9-20
(with respect to the starting material 3). In the grams scale, the
yields are typically lower (<20%).
a
macrocyclic cis azobenzene and systematically studied its
thermodynamic parameters for the switching.21 The same
group has reported several cyclic azobenzene molecules with
large ring size (>20) and has demonstrated their functions as
O2N
Br
S
22-
chiroptical switches, chirality sensors, and molecular brakes.
S
N
NO2
Na2S
NO2 NaOH, glucose,
H2O, EtOH, 80 oC, 72h
27
The superior switching properties of the cyclic azobenzenes
with eight-membered ring systems have sparked tremendous
N
MeOH, 30 oC, 12h
interest
for
mechanistic
investigations
primarily
by
computational methods.28-33 It was observed that in contrast
2
3 (60%)
1 (18-22%)
to Herge’s eight membered ring systems, the large ring-size in
these molecules were flexible enough to allow the trans form
to be the thermally stable form, similar to what is observed in
case of the acyclic azobenzene switches.
Scheme 1. Synthesis of the cyclic azobenzene 1
In the first step of for the preparation of the precursor
compound of 2 from 4-tert butyl toluene (see Scheme S1), the
presence of the t-butyl groups forces the nitration exclusively
to the ortho position with respect to the methyl groups. In
addition, it should be noted that obtaining the cyclic
azobenzene product requires the presence of the two nitro
This prompted us to explore the limit of the ring size for
obtaining the cis form as the thermally stable form over its
trans isomer. Moreover, the azobenzene compounds are non-
fluorescent. Certain structural features such donor–acceptor
substitution,34
bulky ortho substitution,
hydroxyl
boron-chelated
substitution,
35
groups of the thioether
3 in a close proximity in the
36
N=N
imparts
intramolecular cyclization step. Thus, it is conceivable that the
presence of the two t-butyl groups in the thioether
intermediate 3 ensures that the two bulky t-butyl groups are
fluorescence properties to these photochromes. Alternatively,
self-assembled azobenzene can display
aggregates37-38
aggregation-induced emission properties.39-44 In general, the
fluorescence properties of strained cyclic azobenzene systems
that do not have any of the above features have not been
studied. We envisaged that if our 9-membered azobenzene is
far apart and thereby forces the two –NO
2
groups close to
each other that probably helps in obtaining a consistent yield
of the target cyclic azobenzene 1. This also minimizes the
intermolecular polymeric azobenzene formation.
2
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reversal of the cis to the trans form, attained with blue light,
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DOI: 10.1039/C8NJ01643G
also followed a first order kinetics with a rate constant 4.3 ×
1
0-2 min-1 at 298 K. These values of the rate constants are
comparable to the acyclic azobenzene switches reported in the
literature.51
0
0
0
0
0
0
0
0
.45
.40
.35
.30
.25
.20
.15
.10
7
0 min
0
min
Figure 2: Trans-cis isomerization of the cycic azobenzene 1 (top); UV–vis spectral
-
5
changes upon exposure of trans-1 (3 × 10 M) in acetonitrile with 366 nm UV
light (irradiation time: 0 – 100 min); Inset: The zoomed-in version of the 450 nm
bands of the two isomers.
0.05
.00
0
2
50
300
350
400
450
500
550
600
Wavelength(nm)
The trans isomer displayed absorption bands at λmax = 238, 316, 360
Figure 3: Cis-trans isomerization of the cyclic azobenzene 1 (top); UV–vis spectral
-
5
-1
-1
changes upon exposure of cis-1 (3 × 10 M) in acetonitrile with 466 nm UV light
irradiation time: 0 - 70 min).
(
sh.) and 444 nm. The band at λmax = 316 nm (ε = 9500 M cm )
(
corresponds to a π–π* transition and the band at 444 nm is a
symmetryforbidden n–π* band (ε= 286 M−1 cm ). Irradiation with
UV-light of366 nm converts the trans to the cis isomer. It might be
noted that exposure to 254 nm UV light also converted the trans
form to the cis form. However, when multiple cycles were carried
out with the 254 nm light source, partial decomposition of the
sample was observed. Hence the 366 nm softer UV was chosen as
the light source. The photoisomerization between trans-1 and cis-1
forms was monitored with UV/Vis spectroscopy in acetonitrile.
Upon exposure of a solution (30 M) of trans-1 in acetonitrile to UV
light (λ =366 nm, 8 watts), the thermally stable trans 1 form
graduallyisomerized to cis-1 and reached a saturation in 80 min,
which upon exposure to the irradiation for another 20 min did not
change the spectra to any significant extent. Upon
photoisomerization (Figure 2) from trans-1 to cis-1, the absorption
maxima at 316 and 450 nm gradually decreased and a new peak
appeared at 406 nm (ε = 370 M-1 cm-1).
−1
3
The photoswitchingbehavior of the compound 1 in CD CN was also
studied bythe NMR spectroscopy. In the presence of 366 nm light,
the signals at 7.83, 7.34 and 7.20 corresponding to the aromatic
protons of the azobenzene unit graduallydiminished and growth of
three new peaks in the upfield region at 7.08, 7.02, 6.60 for the
same set of protons were observed (Figure 4). This was consistent
with what is normallyobserved for the trans to the cis conversion of
the acyclic azobenzene systems52 and distinctlydissimilar to eight-
membered azobenzene systems are thermally stable in the cis
conformation.53 Therefore, at this point it was apparent that the
effect of ring strain responsible for the stability of cis form for the
smaller ringsystems does not extend well to the nine -membered
azobenzene ring system. This was further confirmed when the
single crystal XRD structure of compound 1 obtained at room
temperature was indeed in the trans form. The sample did not
undergo switching to the cis form even upon heating at 50 oC.
The reverse reaction of the thermally less stable cis-1 to the
trans-1 form can be achieved by exciting the n-π* band of the
cis form in the 450-470 nm range. This was achieved upon the
exposure of the sample with blue LED light (λ = 466 nm)
(Figure 3). Under continuous irradiation with the blue light, a
photostationary state (PSS) consisting of ~70:30 of the
a
cis:trans isomers were obtained. This relative ratio of the two
forms was calculated from the UV-vis spectra of the pure trans
form and the PSS mixture using Fischer’s method at 316 nm. 5
The spectra of the cis-rich sample of 1 have been mentioned as
the cis-form. It might also be noteworthy here to mention that
the reversal of the cis to the trans form were attempted with a
0
-
2
4
89 nm laser source, although the results were discouraging Figure 4: NMR traces of compound 1 in CD
3
CN (10 M) displaying the clear shift
in the aromatic protons. (A) after radiation of the trans with 366 nm light for 240
because of slower and poor conversion to the trans form. The min; (B) trans form (bottom trace).
trans-1 to cis-1 isomerization with 366 nm light followed a first
order kinetics with a rate constant 2.1 × 10-2 min-1 at 273 K. The
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Single crystal XRD data confirmed the trans-1 structure. The
crystals of were obtained upon slow evaporation of
View Article Online
DOI: 10.1039/C8NJ01643G
1
a
solution of 1 in acetonitrile at room temperature and were
subsequently analyzed by X-ray diffraction crystallography. The
solved structure is shown in Figure 5 and Figure S17 clearly
shows that the trans geometry of the molecule.
crystallographic analysis data have been summarized in Table
and the details are listed in the Supporting Information
The X-ray
1
(
Tables S1). The crystal system is monoclinic and space group is
3
P21/c, Z = 4, and the unit cell volume is 2020.6(5) Å . The C(9)-
N(1)-N(2)-C(14) dihedral angle is 167.015°. This deviation from
the ideal dihedral angle of 180o, expected in an acyclic
structure, indicates the ring strain. The N=N bond distance of
1.257 Å is close to the ones reported for acyclic azobenzene
systems. [CCDC 157005 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.]
It was interesting to note that the (Ar)C-N=N-C(Ar) interplanar
o
angle of 19 between the two phenyl rings in 1 is slightlylower than
that of the 21 angle that exists for the calculated trans structure of
Herge’s S-containing an eight-membered ring. The strain energy
o
Figure 5: Single crystal X-ray diffraction crystals of compound 1: (A) top view, (B) side
view. The hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 50%
probability.
(
∆Hstrain) estimated byPople’s computational isodesmic reaction
Despite the certain angle strain, akin to the acyclic system, the
trans form is more stable for compound 1. We still wondered if
there was, at all, any effect of the nine-membered ring on the
cis to trans thermal reversal of the switching of molecule 1.
Thus, the thermal reversibility of compound 1 was studied in
six different solvents with varying polarity (see Supporting
Information). It was found that the switching rate, measured
by the first order kinetics, was slowest in cyclohexane (polarity
index of 0.2), and was fastest in DMSO (polarity index of 7.2).
54
approach indicate that the strain is considerablylower in the case
of the trans form of compound 1 compared to its cis isomer (Figure
S7). The combination of a sulfur and two methylene units offers a
flexibilitythat allows the molecule 1 to reside comfortably in the
trans form under the ambient temperature. This flexibility of our
nine-membered ring system is not possible in the eight-membered
ring system developed by Herges. There the strain confers it an
unusualthermal stabilityin its cis form compared to the trans. In
our case, compared to the trans form, the strain of the cis form is
12 kcal/mol higher which makes it thermodynamically less stable.
Table 2. Activation energy and half-life of cis-1 in various solvents at 333K.
Table 1: Bond angles and bond lengths of compound-1 observed in crystal structure in
acetonitrile solvent.
Solvent
Solvent
polarity
index
7.2
5.8
5.1
4.4
3
0.2
E‡
(kcal/mol)
t1/2(min)
Bond length (Å)
Bond Angles (˚)
C1-C4
C2-C4
C3-C4
1.446(16)
1.508(19)
1.556(19)
1.539(11)
1.3900
C1 C4 C2
108.6(12)
110.0(12)
117.7(10)
99.3(12)
110.5(10)
118.7
DMSO
Acetonitrile
MeOH
Ethyl acetate
Benzene
Cyclohexane
17.8
22
32
49
60
78
366
C1 C4 C3
C1 C4 C5
C2 C4 C3
C2 C4 C5
C9 C8 C11
C6 C5 C4
C6 C5 C10
C10 C5 C4
C5 C6 C7
C8 C7 C6
C7 C8 C9
C7 C8 C11
18.0
18.3
18.5
18.8
19.5
C4 -C5
C5 -C6
C5 -C10
C6 -C7
C7 -C8
C8 -C9
C8 -11
C9 -C10
C9 -N1
C11- S1
C12 -C13
1.3900
1.3900
119.1(6)
120.0
1.3900
1.3900
120.9
120.0
1.520(8)
1.3900
120.0
120.0
1.358(6)
1.827(9)
1.554
The correlation of the rate constant for the cis to trans reversal
under the room temperature (25 oC) conditions with the
solvent polarity index has been shown in the Figure S9. The
activation energy (E ) of the reaction changed significantly as
121.2
‡
the solvent was altered. This is reflected in the half-life of the
cis form under the thermal conditions .
4
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Generally, acyclic azobenzenes devoid of anyfluorophores are non-
fluorescent in either the cis or the trans forms. This happens as they
undergo non-radiative decaythrough the rotations of the phenyl
rings or through photoisomerisation in the photo excited state.5
Interestingly, studies ofacyclic azobenzenes have also shown that
the cis form can display a strong fluorescence on prolonged
standing because of the formation of the head-to-tail
intermolecular J-aggregatesresulting in an aggregation-induced
emission where the non-radiative mechanisms are impeded.
Kunitake and his coworkers reported azobenzene containing
compounds which show strong fluorescence enhancement in the
aqueous bilayer system.57-58 In contrary, compound 1 displays
an emission in both the cis-rich state (apparent quantum yield
of the PSS70/30, Φapp = 0.060) and the trans (Φ = 0.043) forms
View Article Online
DOI: 10.1039/C8NJ01643G
5-56
Figure 7: Absorption changes observed of 1 (3 × 10-5 M) at 316 nm after alternating
upon excitation at 309 nm (excitation spectra, Figure S5) using irradiations at 366 nm (half cycles) and 466 nm (full cycles) over eight complete cycles.
anthracene as the reference. This unusual emission is as high
as that reported for the aggregated cis form of the acyclic
azobenzenes. It is conceivable that the locked structure of the
cyclic compound 1 with restricted degrees of freedom slows
The photostability of compound 1 was checked using UV-vis
spectroscopy over eight switching cycles in an acetonitrile
down the non-radiative fluorescence deactivation processes.
solution purged with dry nitrogen (Figure 7). Upon alternating
For the reported acyclic azobenzenes which are non-
irradiation of 366 and 466 nm light and repeating the
sequence of the irradiation in multiple cycles, the
corresponding absorbance of 1, peak at 316 nm was used to
monitor the switching in cycles. Even after eight cycles, no
significant decomposition of the sample was observed.
fluorescent, formation of large J-aggregates in the cis form
results
in an aggregation induced enhancement
of
fluorescence. However, here the cis-1 does not form any
aggregate even on standing for two days. Molecular modeling
studies of the cis form reveal a bowl-like structure. Lack of new
signals in the larger hydrodynamic volume regime in the
dynamic light scattering studies also supports the lack of the Table 3. Brief comparison of Herge’s compound with compound 1.
formation of the larger aggregates upon standing. It is likely
that the t-butyl group of cis-1 causes steric repulsion that
hinders close approach of the “cis-bowls” (Figure S10).
S
S
1
Properties
N
N
N
N
Ring size
Eight
Nine
5
5
5
5
Thermally stable
form
Cis
Trans
4x10
3x10
2x10
1x10
trans 1
cis 1
t
1/2 at 298 oK
3.5 d
-
Strained
Colorless to red
30 min
Obtained
Flexible and less strain
Colorless to faint yellow
Crystal structure
Ring flexibility
Color change (Z to E)
Switching rate
Synthesis
Fast
Slower
Difficult
Easy to prepare
0
350
400
450
500
550
Conclusion
Wavelength (nm)
To sum up, compound 1 clearly defines the limit of the ring
size of the cyclic azobenzenes in terms of its relative stability
and photoswitching behavior. The strained 8-membered
azobenzene ring systems reported earlier by Herges are
thermally stable in the cis form. Increasing the chain length
merely by one methylene unit shifts the stability towards the
trans form. The salient different between compound 1with its
eight membered analog is presented in Table 3. In addition, it
was also revealed in this study that although azobenzenes are
Figure 6: Emission spectra (λex= 309 nm, slit 5/5) of trans-1 and cis-1.
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known to be non-fluorescent, the molecule
Journal Name
o
1
displays recorded at 25 C using a Fluoromax-3, Xe-150 W, 250−900
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DOI: 10.1039/C8NJ01643G
interesting fluorescence behavior in both the cis and the trans nm.
forms with modest quantum yields. This is due to the lack of
flexibility,
and
consequently,
restricted
non-radiative Synthesis of compound 1. To a solution of
NaOH (1.484 g,
2
relaxation of the ring structure which is absent in the acyclic 37.10 mmol) in H O (5 mL), compound 3 (0.245 g, 0.588 mmol)
azobenzenes.
in EtOH (40 mL) was added. A solution of glucose (1.05 g) in
hot water was added to the reaction mixture. The reaction
mixture was stirred at 70 C for 72h. The solution turned red in

Experimental section
color. Upon completion of the reaction,
concentration in
Instruments and Reagents: All reactants and reagents were vacuum, the residue was extracted in DCM, washed with brine,
commercially available and were used without further dried over Na SO filtered, concentrated in vacuo, and
2
4
,
purification unless otherwise indicated. Solvents used were purified by preparative TLC (100-200 silica gel) using 5% EtOAc
purified and dried by standard methods. Reactions were in hexane as eluent. The expected compound was obtained as
monitored by thin-layer chromatography (TLC) using Merck a red solid (41 mg, 19 %;* Varied between 18-22%), m. p. 170
plates (TLC Silica Gel 60 F254). Developed TLC plates were oC; 1H NMR (500 MHz, CDCl
visualized with UV light (254 nm). Silica gel (100−200 mesh, = 8 and 2 Hz, 2H), 7.20 (d, J = 8 Hz, 2H), 4.13 (s, 4H), 1.40 (s,
Merck) was used for column chromatography. Yields refer to 18H); 2.1 (Acetone), 1.5 (CDCl Water), 0.88 and 1.256
C NMR (125 MHz, CDCl ) δ 152.1, 148.5,
) δ 7.83 (d, J = 2 Hz, 2H), 7.34 (dd,
3
J
3
-
13
the
chromatographically
and
spectroscopically
pure (Hexane impurity)
3
compounds. The structures of the compounds were 131.5, 130.4, 130.1, 122.1, 33.3, 32.9, 30.9; IR (KBr) 2965 (w),
determined by 1D and 2D NMR spectrometry and other 2908 (w), 1615 (m), 1421 (s), 1362 (s), 1025 (s), 907 (m), 739
spectroscopic techniques. The 1H and 13C NMR spectra were (m), 617 (w), 558 (w) cm ; ESI-MS m/z (calc.) 375.2 [M+Na] ,
−1
+
recorded with 400 MHz Jeol and 500 MHz Bruker instruments. (obtained) 375.2; HRMS (ESI/Q-TOF) m/z: [M + Na]+ Calcd for
Chemical shifts are reported as δ values relative to an internal
22 28 2
C H N NaS 375.1871; Found 375.1870.
reference of tetramethylsilane (TMS) for 1H NMR or the
13
2
solvent peak. In the case of C NMR the solvent peak was Synthesis of compound 3. To a solution of Na S (0.043 g, 0.551
used for calibration. The spectroscopic-grade solvents for the mmol) in MeOH, 4-t-butyl-o-nitro benzyl bromide (0.1 g, 0.367
spectroscopic experiments were distilled and checked prior to mmol) in MeOH (2 mL) was added drop wise under argon
o
their use to ensure that they were free from any fluorescent atmosphere at room temp. (30 C). The reaction mixture was
impurity. UV−vis spectra were recorded with a Cary 60 UV−vis stirred at the ambient temperature for 24h. The solution
spectrophotometer. Fluorescence measurements were carried turned yellow in color. Upon completion of the reaction as
out with a Horiba Jobin Yvon fluorimeter (Fluoromax-3, Xe-150 indicated by the TLC,
W, 250−900 nm). Mass spectra were recorded with Melting concentration, extraction with EtOAc ,washing with water,
point was recorded on a Mettler Toledo DSC1 spectrometer. drying over Na SO concentration, and purification by flash
The mass spectrometric data were obtained from an column chromatography (silica gel) using n-hexane as eluent,
AcquityTM Ultra Performance LC-ESI/quadrupole-TOF MS. the expected compound was obtained as yellow viscous
Melting point was measured with a Secor, India digital melting liquid (0.025 g, 80 %),
= 0.4 (5% EtOAc in hexane). 1H NMR
point apparatus in melting point tube. (500 MHz, CDCl ) δ 7.93 (d, J = 2 Hz, 2H), 7.53 (dd, J = 8 and
Photoisomerisation studies: Trans to cis photoisomerisation 2 Hz, 2H), 7.34 (d, J = 8 Hz, 2H), 3.92 (s, 4H), 1.33 (s, 18H); 13C
reaction of compound 1 have been carried out under the UV NMR (125 MHz, CDCl ) δ 152.1, 148.5, 131.5, 130.4, 130.1,
radiation chamber equipped with cooling fans. mL UV 122.1, 33.3, 32.9, 30.9
the reaction mixture upon
2
4
,
a
R
f
3
3
A
3
.
ESI-MS m/z calculated 455.1401,
+
cuvette was kept under ice cold conditions. A 366 nm UV light obtained: 455.1700. HRMS (ESI/ Q-TOF) ) m/z: [M + Na] Calcd
-2
(
irradiation power 2.3 mW cm ) was used for the radiation of for C22
the sample. The UV-vis absorbance data were recorded
subsequently with Cary 60 UV–vis spectrophotometer at Synthesis of Compound 2: Compound
ambient temperature conditions. The cis to trans isomerization following the method reported by Rose et al.
2 4
H28KN O S 455.1401; Found 455.1402.
a
2
was prepared
59
The 1H NMR
were conducted both thermally and photochemically. For the data matches well with the reported literature. R
f
= 0.6 (5%
photochemical reaction, 466 nm LED (irradiation power of 0.5 EtOAc in hexane). 1H NMR (500 MHz, CDCl
3
) δ 8.01 (s, 1H),
−
2
mW cm ) light source was used. The cis to trans isomerization 7.61(d, J = 8 Hz, 1H), 7.48 (d, J = 8 Hz, 1H), 4.80 (s, 2H), 1.34 (s,
under the thermal conditions were carried out in the 9H); 13C NMR (125MHz, CDCl3) δ 153.7, 147.7, 132.2, 130.7,
spectrophotometer itself using
a
Peltier heating system 129.7, 122.4, 34.9, 30.8, 28.9.
o
accessory with an accuracy of ± 1 C.
The photoisomerization followed
spectroscopy before and after the isomerization was studied in
quartz fluorescence cuvettes (Starna). The
by
fluorescence
Conflicts of interest
a
There are no conflicts to declare.
photoisomerization conditions are the same as described for
the absorption studies. The fluorescence spectra were
Acknowledgements
6
| J. Name., 2012, 00, 1-3
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New Journal of Chemistry
Journal Name
ARTICLE
The authors thank DST-UKIERI for financial support (Grant DST- 31 C. W. Jiang, R. H. Xie, F. L. Li and R. E Allen. Chem. Phys.
View Article Online
DOI: 10.1039/C8NJ01643G
Lett. 2012, 521, 107–112.
2
014-15-040). MS is supported by an INSPIRE doctoral
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2 C. Gao, B. Li, P. Zhang and K. Han, J. Chem.
fellowship by DST-India.
Research Fellowship.
SG is funded by a CSIR Senior
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New Journal of Chemistry
Page 8 of 8
View Article Online
DOI: 10.1039/C8NJ01643G
Strain, Switching and Fluorescence Behavior of a Nine-Membered Cyclic
Azobenzene
Monochura Saha, Sanjib Ghosh, Subhajit Bandyopadhyay*
The work defines the smallest ring size for obtaining the trans form of cyclic azobenzene as the thermally
stable form.