Non-photoisomerizable butterfly shaped tetrasubstituted azobenzenes: Synthesis and photophysical studies

Article abstract of DOI:10.1039/d0nj01092h

Thetrans-azobenzene (AB) substituted derivatives with bulky carbazole (1), phenothiazine (2), and phenyl (3) have been synthesized to understand the effect of steric crowding around the -NN- linkage on the photoisomerization process. The substituents maintain orthogonality to the azobenzene plane and the electronic properties exhibit a combination of individual chromophores. Irradiation of1and2with various light sources including a 365 nm pen-ray lamp, and a 150 W Xe-Hg lamp, and even exposure to sunlight for a week does not yield thecisisomer revealing the exceptional photostability of the synthesized azobenzene derivatives. These studies suggest that possessing steric crowding around the azo linkage would be a potential strategy to develop photostable azobenzene-based functional materials.

Full text of DOI:10.1039/d0nj01092h

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Non-photoisomerizable butterfly shaped
tetrasubstituted azobenzenes: synthesis and
Cite this:DOI: 10.1039/d0nj01092h
photophysical studies†‡
bc
Arunachalam Raman,ab Afeefah U. Neelambra,c Venugopal Karunakaran
and
ab
Shanmugam Easwaramoorthi
*
The trans-azobenzene (AB) substituted derivatives with bulky carbazole (1), phenothiazine (2), and phenyl
(3) have been synthesized to understand the effect of steric crowding around the –NQN– linkage on
the photoisomerization process. The substituents maintain orthogonality to the azobenzene plane and the
electronic properties exhibit a combination of individual chromophores. Irradiation of 1 and 2 with various
light sources including a 365 nm pen-ray lamp, and a 150 W Xe–Hg lamp, and even exposure to sunlight for
a week does not yield the cis isomer revealing the exceptional photostability of the synthesized azobenzene
derivatives. These studies suggest that possessing steric crowding around the azo linkage would be a
potential strategy to develop photostable azobenzene-based functional materials.
Received 3rd March 2020,
Accepted 5th May 2020
DOI: 10.1039/d0nj01092h
rsc.li/njc
Nevertheless, certain cyclotriazobenzenes are known to exhibit
resistance to photoisomerization by a strong ring strain on the
Introduction
Azobenzene (AB) is
a most fascinating and well-known azobenzene, but their photophysical properties are not discussed
chromophore due to its reversible trans 2 cis isomerization in detail.15 Few bridged azobenzenes, i.e., diazocine and hetero-
induced by light and heat energy. It has been used for applica- diazocine derivatives yielded a more stable cis isomer than trans
tions including optical memory, logic devices, molecular isomers due to the ring strain.16–18 Imposing steric constraints
motors, and machines.1 On the other hand, repeated isomeri- around the NQN bond by bulky substitutions would probably
zation cycles would also lead to the gradual degradation of azo arrest the rotation of the phenyl ring and thereby inhibit the cis
compounds with the formation of toxic by-products that are isomer formation. It has been reported that the bulkiness of the
prohibited by the international regulating authorities such as alkyl group at the ortho phenylene positions has a significant
REACH.2 Hence, their application in textile and leather indus- role in controlling the photoisomerization.19 Furthermore,
tries is not permitted. Thus, developing non-photoisomerizable Schweighauser et al. observed that thermal cis to trans isomeriza-
azobenzenes is essential for their use in photostable azo-dyes,3 tion reaction rates decreased with the increasing bulkiness of the
photorefractivity,4 molecular detection5 and light-emitting substituent at the meta position with the methyl to phenyl group
materials.6 The chemistry of the azo linkage is controlled by due to London dispersion interactions.20 Therefore, bulkier
ortho-cyclopalladation,7 boron complexation,8 and the for- substituents at the meta position would offer a promising way
mation of self-assembly9 approaches to inhibit the photoiso- to arrest the rotation of the NQN bond and is therefore expected
merization process. Metal-binding and hydrogen bonding with to inhibit the formation of the cis isomer. Carbazole and pheno-
the NQN bond leads to complex formation and tautomerization thiazine are the best choices for bulky substituents towards the
to hydrazine, respectively, exhibiting diverse photochemistry.10–14 steric approach. Apart from the bulkiness, suppressing the inter-
molecular interaction with enhanced thermal stability and better
electronic properties having higher triplet energy levels would
a Inorganic and Physical Chemistry Laboratory, CSIR-Central Leather Research
make them a better choice to be used as a host material in the
Institute, Adyar, Chennai 600 020, India. E-mail: moorthi@clri.res.in
b Academy of Scientific and Innovative Research (AcSIR), CSIR-CLRI Campus,
phosphorescent organic light-emitting diodes (PhOLEDs).21,22
Chennai 600 020, India
c Photosciences & Photonics Section, Chemical Sciences and Technology Division,
CSIR – National Institute for Interdisciplinary Science and Technology,
Thiruvananthapuram, India
Results and discussion
† This paper is dedicated to late Professor P. Natarajan on his 80th Birth
Anniversary.
Considering the practical applications of the photostable
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj01092h azobenzenes in optoelectronic devices, we have synthesized
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butterfly-shaped m-tetra carbazole and phenothiazine substituted
azobenzene derivatives.23 Tetrasubstituted azobenzene derivatives
have rarely been reported due to difficulties associated with
their synthesis arising from steric factors. However, recently
Schweighauser et al. reported that the overall yield of the two
meta-substituted azobenzene reduced while going from isopro-
pyl to phenyl substitution as the substituent size increases.20
Here, we synthesized azobenzene derivatives 1, 2, and 3 in four
steps, as shown in Scheme 1. The first step involves bromina-
tion of 4-nitroaniline (4) with n-bromosuccinimide (NBS) in
acetonitrile (ACN) yielding 5 (95% yield) as a yellow solid. The
next step, deamination of 5 by treatment with concentrated
H2SO4 and NaNO2 in ethanol medium, formed 6 (white solid,
75% yield). Afterwards, the intermediates 7 and 8 were obtained
by the Buchwald coupling of 6 with carbazole and phenothia-
zine respectively in the presence of CuI, K2CO3, 1,10 phenan-
throline in DMF by refluxing for 48 hours to give a yield of
B72%. Compound 9 was obtained by the Suzuki coupling
reaction of 7 with Pd(OAc)2, P(o-tol)3 and phenylboronic acid
Fig. 1 Expanded 1H NMR spectra of target molecules 1, 2, and 3 in CDCl3.
(yield of 74%). The final butterfly-shaped azobenzene deriva- carbazole/phenothiazine rings to para protons (H1c,d and H2c,d) on
tives 1, 2 and 3 were synthesized by the reaction of 7, 8, and 9 the azobenzene ring which makes it magnetically anisotropic for
with dried Zn and NaOH in ethanol as a solvent (yield of B4%). compound 1 and 2 when compared to the phenyl moiety of
All the target compounds were obtained as yellow color solids compound 3 (H3c,d). On the other hand, for compound 1, both
and have been characterized by 1H, 13C NMR, and MALDI mass ortho protons are shifted upfield by a magnitude of 0.5 ppm with
spectrometry (see the ESI,‡ Fig. S10–S26).
respect to 3 (H3a,b = 8.20 and 7.99 ppm) whereas that of 2 does not
1
A close examination of the H NMR spectra of 1, shown in show any significant shift in the value of the chemical shift. The
Fig. 1, shows two singlets at 8.72 and 8.51 ppm characteristic of above observations suggest that N-amination using carbazole
the ortho protons (H1a and H1b), and that of para protons are and phenothiazine causes more perturbation to ortho and para
observed at 8.1 and 7.9 ppm (H1c and H1d). A similar observa- protons in the azobenzene ring. Also, the azobenzene moiety
tion is also made for compound 2 wherein two singlets at 8.05 possesses centrosymmetric geometry with the rotational axis
and 7.99 ppm correspond to ortho protons (H2a and H2b) and passing along the NQN bond, and hence it is expected to show
that of para protons observed at 7.29 and 7.10 ppm (H2c and H2d) only one each resonance signal for ortho and para protons.
of the azobenzene moiety. Notable changes in the chemical shifts Nevertheless, the observed different chemical shift values for
were observed for para protons (H1c,d and H2c,d) becoming shifted ortho and para protons are attributed to the twisting of the
downfield by the magnitude of 0.5 ppm and 1.3 ppm for azobenzene core caused by the steric congestion originating from
compound 1 and 2, respectively, and for compound 3 (H3c,d
=
the closely spaced bulky substituents.
8.57, 8.45 ppm). This is due to the closest interaction of two bulky
Fig. 2 displays the UV-visible absorption spectra of com-
pounds 1, 2, and 3 and a parent compound AB in chloroform.
The absorption spectra of 1 having a vibrational feature with
Fig. 2 UV-vis spectra of target molecules 1, 2, 3 and AB in CHCl3.
Scheme 1 Synthetic route to the target derivatives of 1, 2 and 3.
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the maximum at 336 nm resembles that of carbazole p–p*
absorption at a shorter wavelength and a weaker n–p* band
centred around 410 nm relates to the azobenzene NQN
transition.24 For 2, the prominent absorption band centered
at 313 nm originates from a p–p* band of phenothiazine,
whereas the lower energy band corresponds to the n–p* transi-
tion of the azobenzene unit.25
Interestingly, a well-resolved, n–p* transition generally
appearing at around 448 nm for AB becomes smeared, blue-
shifted and exhibits as a shoulder for 1 and 2. This reveals that
the planarity of the azobenzene core is significantly distorted
owing to carbazole and phenothiazine substitution and is
1
consistent with the H NMR observations.
Considering the bulkiness of the substituent and the
absence of new absorption features, we believe that the carba-
zole and phenothiazine lie in a plane perpendicular to the
azobenzene plane and hence are electronically decoupled.
On the contrary, meta-phenyl substitution in compound 3
shows a slightly red-shifted p–p* transition with a maximum
Fig. 3 UV-visible absorption spectral changes of (a) 1, (b) 2 and (c) 3 after
photoirradiation with 365 nm and (d) 3 after being stored for 77 h in the
dark at room temperature in CHCl3.
at l = 335 nm due to the extended p-conjugation. However, no isomer was achieved after 77 h which is slower than AB due to
measurable changes in the n–p* transitions (448 nm) were the dispersion effect of phenyl rings as shown in Fig. 3d.
observed. Interestingly, the absorption spectrum remains con- The photoisomerization was also monitored by 1H NMR
stant for 1 and 2 even after a prolonged irradiation time using spectroscopy in CDCl3 and PSS was calculated to be 45 : 55 by
365 nm light or after keeping the samples under sunlight for a integrating the peaks corresponding to trans and cis isomers
day, reflecting the absence of trans - cis isomerization. This (ESI,‡ Fig. S2).
feature could occur due to the sterically crowded environment
To further understand the excited state relaxation dynamics
around NQN that prevents cis isomer formation as it leads to a of 1 and 2 derivatives, the femtosecond time-resolved transient
much more sterically crowded environment having all the bulky absorption spectra (TAS) were measured at different time
substituents in close proximity; the second possibility would be intervals by exciting the compound at 325 nm in chloroform
that a significant proportion of the light may be absorbed using femtosecond pump–probe spectroscopy. The p–p*
by the phenothiazine/carbazole, hence no sufficient light is (S2 ’ S0) transition of the azobenzene was selectively excited
available for azobenzene to absorb and subsequently isomerize and the spectra are shown in Fig. 4.
or both can contribute to the prevention of isomerization. Since
In Fig. 4, the TAS is dominated by the broad excited state
both the compounds are electronically de-coupled, it would be absorption (ESA) having a maximum at around 470 and 500 nm
appropriate to compare molar extinction coefficient (e) values for 1 and 2, respectively. The negative change in absorbance,
of the individual moieties with that of compounds 1 and 2. the bleach observed around 340 nm for 2, is due to the ground
As can be seen in the absorption spectrum shown in the ESI,‡ state absorption of the derivative. The intensity of the transient
Fig. S1, it can be inferred that the four times e values for the absorbance is decreased with increased delay time. It is
N-phenylcarbazole and N-phenylphenothiazine is less by a observed that the transient dynamics of derivative 1 is longer
factor of two when compared to that of 1 and 2, suggesting compared to that of 2 and attained equilibrium within 500 and
that the predominant absorption in that spectral region is due 100 ps for 1 and 2, respectively. The femtosecond transient
to the azobenzene unit. Thus, the overall spectrum between 250 absorption spectra of 1 and 2 derivatives were analyzed with the
to 400 nm is the combination of all the chromophores in 1 and global analysis program GLOTARAN.26 Three and two exponen-
2, however, with the major contribution from the azobenzene tial components were optimally obtained to fully describe the
moiety. Furthermore, the absorption spectral features and relaxation dynamics of 1 and 2 in chloroform, respectively.
e values suggest that all the moieties in chromophore 1 and 2 Those were t1 = 683 Æ 110 fs, t2 = 13.4 Æ 0.8 ps, and t3 = 125 Æ 5 ps
contribute to the electronic transition and hence the option of for 1 and t1 = 317 Æ 110 fs and t2 = 32 Æ 1 ps for 2.
non-availability of light for the azobenzene to isomerize can be
The excitation of the donor–acceptor p-conjugated systems
ruled out as shown in Fig. 3a and b. Nevertheless, 3 with less undergoes various relaxation pathways based on the inherent
steric congestion around the NQN bond undergoes trans - cis electronic nature of the chromophores.27 Normally, after photo-
photoisomerization upon 365 nm light irradiation as evident excitation to the Franck–Condon state, the relaxation process
from the decreased absorbance of the p–p* transition (335 nm) by solvation leads to the local excited state and is further
with a concomitantly increased absorbance of the n–p* band relaxed to a (twisted) charge transfer state decaying to the
(436 nm) as shown in Fig. 3c. The photostationary state ground state through the triplet state by charge recombination.
calculated from the changes of absorption spectra was found Due to the bulky and steric nature as well as the results of
to be 45 : 55.19 The rate of thermal conversion of cis-3 to trans-3 theoretical optimization, the carbazole and phenothiazine
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have been used extensively as host materials for phosphorescent
OLEDs due to their high triplet energy and hole-transporting
functionality due to their bulkiness. Therefore, the phosphores-
cence spectra of 1, 2 and 3 were measured at a low temperature in
2-methyl THF and could be calculated from the first phosphor-
escent emission peak as presented in the ESI,‡ Fig. S5. The
experimentally calculated triplet energies of 1, 2 and 3 are 2.5,
2.4, and 2.4 eV, respectively. These values are compatible with the
well-known green emitter iridium-(III) fac-tris(2-phenylpyridine)
and red dopant tris(1-phenylisoquinoline)iridium having triplet
energies of ET = 2.42 eV and ET = 2.0 eV, respectively.30,31 There-
fore, an effective energy transfer is expected from 1 and 2 to the
host green Ir(ppy)3 and red Ir(piq)3 dopant. Indeed, even 3 also
has a better compatibility with the triplet energy of the emitters.
Density functional theory (DFT) calculations further support
the photophysical properties of 1, 2, and 3 at the molecular
level in a gas phase using Gaussian 03 at the B3LYP/6-31G
level.32 The optimized structures of the trans/cis azobenzene
derivatives are shown in the ESI,‡ Fig. S6. The dihedral angle
of the m-phenylene-carbazole/phenothiazine ring (C–N–CQC)
of 1 and 2 are calculated to be 1281, 941 respectively which is
relatively high when compared with
3 (m-phenyl–phenyl
(C–C–CQC angle = 1421)) resulting in the highly twisted nature
of two phenyl rings due to its bulky substituents. The reduced
dihedral angle of 3 is due to the reduced steric hindrance and is
similar to the biphenyl configuration. The bond distance
Fig. 4 Femtosecond time resolved transient absorption spectra of (a) 1
and (b) 2 in chloroform obtained by exciting at 325 nm.
moieties in these derivatives repelled each other through the between the two closest hydrogen atoms of the substituent
p–p interaction leading to a twisted conformation. carbazole/phenothiazine on the azobenzene ring of 1 and 2 is
The shortest time constant (t1) obtained for both derivatives increased from a trans to cis isomer due to the bulkiness, and
is attributed to the formation of the charge-separated state with the steric effect mainly occurs on the cis isomer as shown in the
a conformational change after decaying from the local excited ESI,‡ Fig. S7. In the case of 3 and 4, the small substituent
state, based on the electronic and steric nature of the chromo- (phenyl/H) reduced the bond distance between the closest
phores. Based on the literature, the transient absorption hydrogens atoms which stabilize the cis isomer. Furthermore,
maximum that appeared around 470 nm is assigned to the triplet the computed DH and DG values for the formation of the trans
state absorption of the carbazole moiety.28 Thus, in the case of 1, to cis isomer are summarized in the ESI,‡ where Table S1 reveals the
the charge carriers in the charge-separated state would recombine steric effect on the stabilization of the trans isomer. The calculated
and lead to the formation of a triplet state having a time constant values for the carbazole and phenothiazine derivatives of 1 and 2
of t2, 13.4 Æ 0.8 ps. The third time constant, 125 Æ 5 ps (t3) is represent a decrease of DHtrans-cis and increase of DGtrans-cis
attributed to the triplet state lifetime of the carbazole moiety. It is values compared to 3 and 4 due to the destabilization of the cis
found that the formation of a charge-separated state is more isomer and may be reflected in a higher activation barrier.
favorable in case 2 when compared to 1, which is reflected in the However, computed oscillator strengths of 1 and 2 dominate via
formation of a radical cation at around 520 nm having a time azobenzene p–p* origin than the carbazole and phenothiazine
constant of 13.4 Æ 0.8 ps. This is consistent with the report that unit which is in good agreement with the experimental UV-vis
the phenothiazine radical cation absorbs at around 520 nm.29 The absorption spectra as shown in the ESI,‡ Fig. S8. The theoretically
transient features of azobenzene which is the center core of the calculated bandgap between the HOMO and LUMO of 1 and 2 are
derivatives have not been observed and it needs further investiga- 2.67 and 2.09 eV, respectively. In the case of 3, the HOMO and
tion. Furthermore, the transient spectral features corresponding LUMO were localized only on the azobenzene unit and the
to the cis isomer has not been observed, otherwise submerged by calculated energy gap between the HOMO and LUMO was
the dominant ESA. This is consistent with the steady-state absorp- 3.7 eV, which correlates to the unsubstituted azobenzene and
tion spectra where the formation of the cis isomer has not been could cause photoisomerization as shown in the ESI,‡ Fig. S9.
detected during irradiation as shown in the ESI,‡ Fig. S3.
The photoisomerization inhibition of 1 and 2 is due to the
conformational rigidity of the azobenzene ring by the tetra-
substitutions improving the morphological stability of the
Conclusions
azobenzene ring when compared to AB as shown in the ESI,‡ In summary, two novel butterfly-shaped m-tetra carbazole and
Fig. S4 (DSC curve). Carbazole and phenothiazine based derivatives phenothiazine-substituted azobenzene derivatives 1 and 2 were
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12 H. Y. Lee, J. Jo, H. Park and D. Lee, Chem. Commun., 2011,
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lent photostablility under the broad spectrum of UV and visible
light due to strong steric effects. A relatively smaller phenyl
substitution 3 slows down the trans - cis photoisomerization
process to a significant extent. The experimentally measured
triplet energy of compounds 1 and 2 indicates that these
molecules can also be used as host materials for solution
´
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There are no conflicts to declare.
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Acknowledgements
The financial support for this work through MLP project funds
of CSIR-CLRI and SERB (EMR/2017/000149) is greatfully
acknowledged. RA thanks DST for the INSPIRE fellowship.
This is a CSIR-CLRI communication number A/2020/INO/
CLRI-SERB/1368.
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