Room Temperature Reversible Z→E Photoisomerization of Azobenzene Appended to Anthraquinone-Benzimidazole Based Photoswitches with Resolved n→π* Absorption Band

Article abstract of DOI:10.1002/ejoc.201900930

Light-triggered substituted azobenzene-based molecular switches are well known for their potential applications as molecular machines. The Z-isomer stability affected by heteroaromatic groups, extended aromaticity, different substituents, and varying the solvents based on their polarity. In this regards, five different anthraquinone/imidazole-based azobenzene derivatives have been synthesized with different electronic push/pull substituents. The detailed study of photoswitching processes with kinetic data was appraised by using UV/Vis and ¹H-NMR spectroscopic techniques. The investigation of light-induced hydroxyl group conversion to quinone-hydrazone tautomerization has been observed during the isomerization. Most importantly, intermolecular hydrogen bonding interaction between tautomeric form and a higher polar solvent (DMSO) block the isomerization, but in a less polar solvent (DCM), light-induced trans ? cis isomerization takes place by tautomerization pathway with well-separated π→π* and n→π* transitions. Further, the detailed mechanistic study of solvent-assisted light-induced tautomerization with the role of hydrogen bonding and the following WRITE-READ-ERASE-READ system have been derived by using molecular logic gates.

Full text of DOI:10.1002/ejoc.201900930

A Journal of
Accepted Article
Title: Room Temperature Reversible Z → E Photoisomerization of
Azobenzene Appended to Anthraquinone-Benzimidazole Based
Photoswitches with Resolved n → π* Absorption Band
Authors: Daulat Phapale, Archana Kushwaha, and Dipanwita Das
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10.1002/ejoc.201900930
European Journal of Organic Chemistry
Room Temperature Reversible Z → E Photoisomerization of Azobenzene
Appended to Anthraquinone-Benzimidazole Based Photoswitches with
Resolved n → π* Absorption Band
Daulat Phapale,a Archana Kushwaha,a Dr. Dipanwita Das*a
aDepartment of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019 (India)
Fax: (91)022-3361-1020
E-mail: dr.das@ictmumbai.edu.in
Web: www.ictmumbai.edu.in
Abstract
The exploration of light-triggered substituted azobenzene-based molecular switches are famous
for their potential applications as molecular machines. The Z-isomer stability affected by
heteroaromatic groups, extended aromaticity, different substituents and varying the solvents based
on their polarity. In this regards, five different anthraquinone/imidazole-based azobenzene
derivatives have been synthesized with different electronic push/pull substituents. The detailed
study of photoswitching processes with kinetic data was appraised by using UV‒vis and 1H-NMR
spectroscopic techniques. The investigation of light-induced hydroxyl group conversion to
quinone-hydrazone tautomerization has been observed during the isomerization. Most
importantly, intermolecular hydrogen bonding interaction between tautomeric form and a higher
polar solvent (DMSO) block the isomerization, but in a less polar solvent (DCM), light-induced
1
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European Journal of Organic Chemistry
trans ↔ cis isomerization takes place by tautomerization pathway with well-separated π → π* and
n → π* transitions. Further, the detailed mechanistic study of solvent-assisted light-induced
tautomerization with the role of hydrogen bonding and the following WRITE-READ-ERASE-
READ system have been derived by using molecular logic gates.
Keywords: Photochromism, azobenzene, tautomerization, light-triggered, molecular logic gate,
heteroaromatic, feedback loop
Graphical Abstract
Key Topic: Photoisomerization
TOC Text:
Photochromic behavior of modified azobenzene based organic compounds have been synthesized
and studied by using UV-vis absorption study. The investigation of light-induced hydroxyl group
conversion to quinone-hydrazone tautomerization has been observed during the isomerization in
higher polar solvent which prooved by 1H-NMR spectroscopy.
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European Journal of Organic Chemistry
Introduction
The metamorphosis between two stable isomeric interconvertible forms by applying an
external stimulus has attracted much attention.[1] In recent years, azobenzene-based
photoswitchable compounds have tremendous interest because of light-triggered isomerization of
N=N double bonds, from one state to another state with an acute change in geometry of the
molecule, electronic properties, dipole moment, refractive index, and electrochemical
properties.[2,3] Due to photolabile and photo-induced peculiar changes of azobenzene derivatives
have been employed as photochromic compounds owing to excellent candidates.[4–6] Derivatives
of azobenzene exist in two conformations either in trans or cis, the trans form energetically more
stable than cis. The trans → cis (E → Z) isomerization takes place via UV‒vis light irradiation
and cis → trans (Z → E) traveling through thermally, or visible light irradiation.[7] The thermal
reversibility of Z isomers is very capricious and varies from milliseconds to several days which
depends upon the electron-donating/withdrawing group present on the azobenzene, solvent used
for photoswitching study, temperature, and pH. Such interchangeable azobenzene isomers used an
organic chromophore for many applications in diverse fields like biochemistry,[8] material
science,[9–15] organic, and supramolecular chemistry,[16–23] and technical applications like optical
memories,[24,25] photo-responsive nanomachines,[26,27] electro-optical modulators,[28,29], etc.
The curiosity and applicability of light-triggered (E → Z) and thermal isomerization (Z →
E) mechanism of azobenzene derivatives have been expanding the area of research.[30–33] X-ray
crystallography and computational studies proved that trans azobenzene has a planar structure with
C2h symmetry, and non-planer cis azobenzene possesses C2 symmetry.[34–37] The intense absorption
bands appear in UV spectral region due to π → π* symmetry allowed transition and low energy
band arise in the visible part which corresponds to n → π* symmetry forbidden transition (weaker
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than π → π* transition). For light-triggered isomerization of azobenzene- inversion, rotation,
concerted inversion, and inversion assisted rotation mechanism have been proposed.[38–41]
However, only two mechanisms are considered, an inversion of the N=N double bond, the non-
bonding electrons present on the nitrogen atoms may initiate n → π* transition (S1 ← S0) and
rotation through the –C–N=N–C– dihedral angle implicate a π → π* electronic transition (S2 ←
S0),[42,43] but still the mechanism is debatable. To apprehend the mechanism of light-triggered
isomerization of trans-azobenzene has been studied by using various methods like DFT, Ultrafast
photoisomerization dynamics, Resonance Raman analysis, etc.[30–33,38–41,44–47] Structural
modification and appropriate substitution design of azobenzene derivatives affect on the relaxation
of azo dye. By considering the substitution design, the electronic environment around N=N
changed and isomerized quickly from Z → E configuration.[48,49] It is also possible that lifetime of
the Z isomer can be prolonged to hours, days or even years by substituting the bulky groups to
inhibit the thermal Z → E isomerization and this slow relaxation of Z isomer is precious material
for memory storage.[50–53] Previously, G. A. Wooley and his coworkers and S. Hecht and his
coworkers discover ortho/para substituted azobenzene derivatives exhibit complete separation of
the n → π* transition.[54–58] Accordingly, it is essential to bear such system display in two states
which slowly revert from Z to thermodynamically stable E isomer in the absence of an external
stimulus. Although light-triggered and solvent dependance structural variations can be susceptible
for various applications with the development of a new class of substituted azobenzene. To
elaborate new substituted azo derivatives display low isomerization rates at room temperature
which is an imperative task of research. The hydroxy-substituted azo derivatives exhibit
surprisingly different properties than azobenzene. In the presence of UV light, p-hydroxy azo
derivatives get tautomerized into phenyl-hydrazone like structure which prevents the cis-trans
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isomerization that can be restored by substitution/alkylation of –OH group. The N=N bond
converted to N–N–H bond and a hydroxyl group into quinone, which showed strong interaction
with the solvent molecule through intermolecular hydrogen bonding.[41,59–61]
Recently, S. Venkataramani and coworkers have been studied the electronic as well as
steric effect on phenylazopyrazole derivatives and the pivotal role of solvent-assisted
tautomerization with hydrogen bonding to control the stability of Z-isomer.[62] Similarly, König
and coworkers reported that the Z-isomer lifetime in phenylazoindoles have been tuned from
nanosecond to days in protic solvent due to hydrazone tautomerization.[63] Herein, we illustrate the
photochemical exercise and the electronic activity of p-hydroxyazo benzimidazole (p-hAbz)
compounds with unsubstituted (R = H, 1a), electron withdrawing (R = NO2, 1b, and 2b), and
electron donating (R = CH3, 1c, and 2c) groups at para position with respect to N=N bond (Scheme
1). After irradiation of ligands, incorporation of extending aromaticity and electronic push/pull
effect showed a very dramatic change in their absorption spectra with the huge bathochromic shift
in the visible region. We also calculate the E → Z, and Z → E isomerization rates of five NO2/CH3
substituted azo ligands by using UV‒vis spectroscopy. However, the light-triggered isomerization
from trans → cis have been carried out with well-separated π → π* and n → π* transitions but for
the cis-trans isomerization, no external stimuli are used. Upon irradiation, p-hAbz compounds get
tautomerized due to solvent assisted proton exchange and form thermally unstable
phenylhydrazone which showed intermolecular hydrogen bonding with solvent (DMSO) without
participating in trans ↔ cis isomerization, but in a less polar solvent like dichloromethane (DCM),
tautomerization followed by trans ↔ cis isomerization takes place. We elucidated the plausible
mechanism of isomerization as well as solvent assisted tautomerization in different solvents, and
the ON-OFF gated photochromic property has been also constructed.
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Scheme 1. Schematic representation of cis-trans isomerization of 1(a-c) and 2(b, c).
Scheme 2. The synthetic route for substituted azobenzene derivatives. Conditions, and reagents:
(i) NaNO2/HCl, Diazotization (ii) aq. KOH, Salicylaldehyde (iii) 1,2-diaminoanthraquinone,
reflux at 130 ºC in nitrobenzene up to 12-14 h (iv) o-phenylenediamine, reflux at 90 ºC in
ethanol/acetic acid (9/1) for 24 h.
Results and Discussion
Synthesis and Characterization
The diazonium salt of R-aniline coupled with the Salicylaldehyde to give the R-azosal (R
= H, NO2, and CH3) as starting material. In the next step, condensation of R-azosal with 1,2-
diaminoantraquinone and o-phenylenediamine yielding p-hydroxyazo benzimidazole (p-hAbz)
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photoswitches which are outlined in Scheme 2. Characterization of these photoswitches have been
1
done by using elemental analysis, H and 13C-NMR (Supporting Information Figures S1‒S9),
HRMS (Supporting Information Figures S10‒S14) and UV‒Vis spectroscopy which is supported
by the conventional structure.
Photochemistry
E ↔ Z Photoisomerization
Photoisomerization exercise of p-hAbz compounds were studied in dichloromethane upon
irradiation with 254 nm UV light. Due to the substitution of NO2/CH3 groups at para position
correspondence with N=N bond, electronic absorption spectra of p-hAbz showed a significant
variation. In the trans form, two characteristic absorption band appeared at 350 nm and 418 nm in
ligand 1a which is related to azobenzene π → π* and anthraquinone n → π* transitions respectively
(Figure 1(a)). Upon irradiation, the intensity of 350 nm band was increased gradually, at the same
time a concomitant increase in the low energy absorption band at 545 nm was observed which
corresponds to n → π* transition from azobenzene which indicates that isomerization from trans
→ cis with two clear isobestic points (452 and 409 nm). The photostationary state (PSS) was
achieved after 48 min irradiation time with 3 min interval time to record the absorption spectra.
The similar trend in absorption band has been observed in the trans form of ligand 1c as shown in
Figure 1(c) but due to the substitution of ‒CH3 on the azobenzene, the photostationary state was
achieved after 110 min irradiation time with two isobestic points (475 and 385 nm) which indicates
two absorbing species are involved during isomerization (E and Z). After irradiation of 1c ligand,
the new absorption band generated with slight redshift than 1a ligand at 553 nm with a concomitant
change in 417 and 353 nm band (Figure 1(c)). This unusual behavior observed due to the
tautomerization of the light-induced hydroxy group and the formation of quinone hydrazone
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(Scheme 3). Electron pushing effect of ‒CH3 has been played a role in blocking the quinone
hydrazone formation and the charge is delocalized towards the anthraquinone part (Scheme 3 (ii)),
therefore, the absorption intensity at 417 nm band was affected in the 1c ligand. This phenomenon
may not observe in 1a; thus a minimal change was dictated at 418 nm as compared to 1c. In ligand
1b, only one broad absorption band is found at 386 nm due to the presence of ‒NO2 group. The
azobenzene π → π* transition was shifted to higher wavelength and mixed with the anthraquinone
n → π* transition. Therefore, the absorption band was getting broad in the range of 350‒450 nm.
After exposure to UV light, the 386 nm band decreases gradually, while the low energy new band
is generating at 558 nm with time, corresponding to azobenzene n → π* transition (Figure 1(b)).
Photostationary state of cis isomer in 1b was achieved after 95 min irradiation time. Due to the
presence of strong electron pulling effect of ‒NO2 in the 1b ligand, charge is delocalized towards
the nitro group, not to the anthraquinone part. Ligands 1a, 1b, and 1c display typical behavior in
their first-order rate constants (kforward, E → Z) shown in Table 1 in dichloromethane.
hυ
Scheme 3. The plausible photochromic exercise of p-hAbz compounds in dichloromethane.
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Figure 1. UV‒vis absorption spectral changes of (a) 1a, (b) 1b and (c) 1c in DCM (1 × 10–5 M)
after UV irradiation. The inset shows first order photoisomerization kinetic plot (kforward, E → Z).
After removal of the anthraquinone part in p-hAbz ligands (2b and 2c), the photochromic
system could be more labile for the isomerization. The ligand 2b shows two distinct absorption
peaks at 320 and 387 nm as shown in Figure 2(a). After irradiation of 2b ligand, the 387 and 320
nm peak intensity decrease but 387 nm absorption band was utterly diminished, and the blue shift
was observed (310‒360 nm broad peak), simultaneously the 524 nm absorption band increases.
The photostationary state was observed after 22 min and the irradiation time interval is 2 min. The
three isobestic points at 452, 356, and 343 nm were found during the process and color was also
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changed from light yellow to pink (Figure 2(a)). In the trans form of ligand 2c, mixing of
transitions in the absorption band were observed due to the CH3 substitution at 4-position
corresponding with the N=N bond, therefore, a broad absorption band detected around 300‒370
nm (Figure 2(b)). After irradiation of 2c ligand, the low energy absorption band at 547 nm is
increased with 445 nm isobestic point suggesting that isomerization from trans → cis form and
color changes from light yellow to purple (Figure 2(b)). Photostationary state was achieved after
26 min irradiation. The forward rates (kforward, E → Z) for 2b and 2c are given in Table 1 which
are much higher than 1a, 1b, and 1c ligands. We observed that the electronic substitution and
aromatic groups or heteroatom connected to the azobenzene affects the rate of isomerization of
azobenzene.
Figure 2. UV‒vis absorption spectral changes of (a) 2b and (b) 2c in DCM (1 × 10–5 M) after UV
irradiation. The inset shows first order photoisomerization kinetic plot (kforward, E → Z).
The irradiation time to acquire photostationary state of all ligands differs due to the effect
of substitution. In 1b and 1c ligands, more time is required to achieve photostationary state since
during the isomerization in 1b, steric hindrance takes place between NO2 group and lone pair
present on the azo group N atom and due to the bulkiness of NO2 group, planarity is also disturbed.
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However, in 1c CH3 group prevents the tautomerization and azo double bond character was
unaffected. So, for the isomerization, rotation around NN double bond needed more energy than
rotation around an NN single bond character. Due to tautomerization in 1a, double bond character
of the azo group converts to a single bond, so it required less energy as well as less time for
isomerization and to achieve photostationary state.[64] The similar observation has been found in
the case of 2b and 2c. The cis → trans conversion takes place in the dark at room temperature with
the retention of isobestic points (Supporting Information Figure S15). The observation of kinetic
data implies that 1b has a higher rate of cis → trans isomerization than 1a and 1c. Similarly, 2b
and 2c ligands, cis → trans rates were found to be 1.65 × 10‒3, and 7.46 × 10‒4 min‒1 (Supporting
Information Figure S16) in dark at room temperature. The spectroscopic, kinetic and half-life (τ)
data of p-hAbz compounds in DCM are given in Table 1.
Table 1. Spectroscopic and kinetic data of p-hAbz compounds in DCM.
kZ→E
λmax (nm)
(Z isomer)
(n → π*)
kE→Z
(min‒1)
(In dark at
RT)
λmax (nm)
Time (min)
(PSS)
τ (h)
Compounds
(sec‒1)
(Irradiation)
(E isomer)
(Z isomer)
1a
1b
1c
2b
2c
418a, 350b
386c
545
558
553
524
547
48
95
70
22
7.25 × 10-4 5.85 × 10-4
3.64 × 10-4 1.18 × 10-3
5.28 × 10-4 4.71 × 10-4
1.61 × 10-3 1.65 × 10-3
50
25
62
18
39
417a, 353b
387b
340b
26
2.0 × 10-3
7.46 × 10-4
b
an → π* transition wavelength of anthraquinone part, π → π* transition wavelength of azobenzene part,
cmixed transition state.
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Figure 3. UV‒vis absorption spectral changes of (a) 1a, (b) 1b, (c) 1c, (d) 2b and (e) 2c in DMSO
(1 × 10–4 M) after UV irradiation.
Effect of Solvents on Photoisomerization
The remarkably different absorption spectra of p-hAbz compounds have been observed in
a higher polar solvent like DMSO. As shown in Figure 3, the new absorption maxima found in the
visible region (540‒620 nm), but after irradiation, the similar band (around 580 nm) was
completely diminished and there is no generation of a new peak. The possibility of such behavior
is due to a robust intermolecular hydrogen bonding between p-hAbz compounds with solvent
molecules and the light-induced tautomerization,[59] as shown in Scheme 4. However, in case of
1c and 2c, the intensity of absorption band ~ 580 nm is very short or negligible, and after
irradiation, no effective change was observed in DMSO (Figure 3 (c and e)). Due to aggregation
of solvent molecules with p-hAbz, the possibility of intramolecular charge transfer from hydroxyl
to the azo group (Scheme 4a(i)) or benzimidazole/anthraquinone part (Scheme 4a(ii)). The electron
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hυ
Scheme 4. Plausible schematic representation of (a) solvent induced charge transfer with
intermolecular hydrogen bonding with solvent molecules and (b) light-induced tautomeric
polymer formation via intermolecular hydrogen bonding in p-hAbz compounds.
pushing effect of ‒CH3 substitution may restrict the charge transfer as well as hydrogen transfer to
azo group, therefore, charge was delocalized towards the imidazole or anthraquinone part and
stabilized by hydrogen bonding (Scheme 4a (C3)). However, in the case of 1a, 1b, and 2b
compounds, the charge readily delocalized towards the azo group and the tautomerized products
have been generated in DMSO. After exposing to UV light, the p-hAbz compounds undergo
solvent-assisted tautomerization through intramolecular hydrogen transfer from hydroxyl to the
azo nitrogen which further stabilizes and restricts the trans ↔ cis isomerization[61] (Scheme 4a
(C2)). The tautomerized trans isomer was stabilized due to the aggregation of DMSO solvent
molecules through the intermolecular hydrogen bonding, or there may be the possibility of
hydrogen-bonded complex structure (Scheme 4b). These observations are suggesting that the
ground state of p-hAbz compounds is getting more stability in higher polar solvents as compare to
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the nonpolar solvents. The thermal stability of the Z-isomer of p-hAbz compounds is totally
dependant upon the concentration of Z species, geometry, bulkiness of the substituents and solvent
used for photoswitching study. The solvent assisted and light-induced tautomeric form of p-hAbz
compounds in DMSO is more facile towards the isomerization because N=N azo group converted
to N‒N single bond but this observation does not found due to the higher stability of hydrogen-
bonded solvent adduct formation. The involvement of imidazole N‒H proton in tautomerization is
also possible but the availability of anthraquinone, light-induced imidazole hydrogen transfer
possible towards the anthraquinone part.
Figure 4. 1H-NMR spectral titration of (a) 1c and (b) 2c in DMSO-d6 before and after irradiation.
We also tried to characterize the thermally unstable tautomeric form of p-hAbz compounds
1
after irradiation by using H-NMR spectroscopy in DMSO-d6 solvent (Figure 4). However, the
equilibrium nature of the intramolecular hydrogen transfer, we could not get any new tautomerized
N‒N‒H proton signal as well as no change was observed in the aromatic proton region (7.0 to 9.0
ppm) after irradiation and only, the disappearance of hydroxyl proton has been found which
suggests that the intramolecular hydroxyl proton transfer takes place over the system (Figure 4a).
In the case of a 2c compound, both N‒H and O‒H proton signals were disappeared after irradiation
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due to the involvement of the hydrogen transfer over the imidazole or azo part (Figure 4b). The
photoswitching recyclability of 2b and 2c compounds have been checked by UV‒vis absorption
study up to 6 and 8 cycles respectively and no degradation was observed (Supporting Information
Figure S17). Further irradiation of samples, the intensity of bands was decreased and other new
peaks have been generating which suggesting that degradation of compounds takes place in
solution.
In 2
(UV light
irradiation)
In 1
Output 1
(trans → cis)
(p-hAbz
compounds)
0
1
0
1
0
0
1
1
0
0
0
1
Figure 5. Molecular logic gate (AND) diagram and truth table of p-hAbz compounds (In 1) trans
→ cis isomerization using light (In 2) in DCM as a solvent.
Gated Photochromism
According to UV‒vis absorption data and reversibility nature of the p-hAbz compounds,
the construction of photochromic logic gate has been done. The change in the peak of UV‒vis
spectra, i.e. trans → cis and again cis → trans depicts an ON-OFF state in the system.[65] There
are two photochromic cycles (light-induced and dark), the Output 1 generated in the former system
is ON (i.e., 1or high or λmax of cis species) as the first cycle depicts a AND gate as shown in Figure
5. According to Boolean arithmetic calculation, both the inputs are in ON state (i.e. In 1 (p-hAbz
compounds) = 1, In 2 (light) = 1). The second cycle of the logic gate having an Output 2, which
follow an INHIBIT logic gate as shown in Figure 6. The INHIBIT logic gate comprises three logic
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gates, one being NOT gate which shows a reversible effect, OR gate and the third one is a AND
gate. When the In 2 is in ON state, then the Output 2 will be in OFF state (i.e., 0 or low). The
Output 2 is high when all the two inputs are in ON state (i.e. In 3 (Output 1) = 1, In 4 (dark) = 1)
and In 2 is in OFF state (i.e. In 2 = 0 or low). The observed UV‒vis absorption peak result as
Output 2 which is λmax of original trans-species of p-hAbz compounds.
The photo-degradability of the receptor is tested for many cycles and shows remarkable
reversibility with WRITE-READ-ERASE-READ sequence (Figure 7). In 1 and In 2 work as
WRITE when both are in ON state and READ as output trans → cis, which further ERASE when
In 3 is ON state and In 2 is in ON state, these sequential steps depict feedback loop where the
Output 2 is an input for the OR gate (Figure 6).
In 2
(UV light
irradiation)
Output 1 as
In 3
(cis species)
Output 2
(cis → trans)
trans-p-hAbz species
In 4
(Dark)
0
1
0
0
1
0
1
1
0
0
0
1
0
1
1
1
0
0
1
0
1
1
0
1
0
0
0
0
0
0
0
1
Figure 6. Sequential logic gate circuit diagram and truth table based on the reversible trans ↔ cis
isomerization using light and dark conditions in DCM as a solvent.
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Figure 7. Proposed reversible logic circuit demonstration of “WRITE-READ-ERASE-READ”
function.
Conclusion
In conclusion, the detail investigation of solvent dependant E – Z photoisomerization with
highly tunable and excellent band separation (π – π* and n – π*) of five photoswitches has been
found. Thermal stability of Z-isomer was controlled by an electronic push/pull substituents,
extended aromatic and heteroaromatic groups without inhibiting the photochromic function with
extending the half-life to days. Without using light or any external stimuli, cis → trans
isomerization was restored. In DMSO, the formation of intermolecular hydrogen bonding with p-
hAbz compounds was observed and the aggregated complex or polymeric hydrogen-bonded
complex formed without subjected to the trans ↔ cis isomerization. In 1c and 2c compounds,
1
solvent and photo-induced tautomerization have been proved by using H NMR study. The
molecular logic gate construction study has been done by using light and reversibility nature of
the p-hAbz compounds. The photochromic compounds 2b and 2c show excellent photoswitching
recyclability which opens the door for a myriad of applications.
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Experimental Section
General Methods. The synthesis of starting materials, R-Azosal (R = H, NO2, and CH3) have
been reported previously.66 The reagents o-phenylenediamine, salicylaldehyde and para-
substituted aniline derivatives were purchased from Alfa Aesar. 1,2-diaminoanthraquinone was
purchased from Sigma Aldrich. For the synthesis of ligands, AR grade solvents like ethanol,
nitrobenzene, acetic acid, methanol, chloroform, and diethyl ether were purchased from SD Fine
Chemicals and used without further purification. For the absorption study, UV-spectroscopic grade
dichloromethane and dimethyl sulfoxide were purchased from Spectrochem and used without
further purification. The absorption spectra were recorded on Shimadzu UV-1800
spectrophotometer at room temperature in 1 cm path length quartz cuvette and throughout the
study 2 cm3 solution were maintained. 1H-NMR recorded on 400 MHz NMR spectrometer DMSO-
d6 as a solvent. The high-resolution mass spectra (HRMS) of all ligands were recorded on Q-TOFs
mass analyzer with electrospray ionization (ESI) by using positive and negative modes in 1290
Infinity UHPLC system from Agilant Technologies. Elemental analysis data recorded on Vario
Micro Cube instrument. The photoirradiation study was performed on a 150 W Xe lamp with a
254 nm optical filter or UV chamber (short UV, λ = 254 nm).
General synthesis procedure for scheme 2:
For 1(a-c). In a 50 ml round bottom flask (1.5 mmol) 1,2-diaminoanthraquinone and (1.51 mmol)
of substituted azo-salicylaldehyde was dissolved in 7-8 ml of nitrobenzene. The mixture was then
stirred for 12-14 h at 130 ºC, the formation of product was monitored by thin layer chromatography
(CHCl3: MeOH, 9:1). After completion of the reaction, 50 mL diethyl ether was poured into the
reaction mixture then product gets precipitated as dark brown solid, filtered and washed with
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diethyl ether and dried. Further purification was done by column chromatography (CHCl3:MeOH,
9:1).
For 2b, and 2c. In 100 mL round bottom flask o-phenylenediamine (1.5 mmol) and substituted
azo-salicylaldehyde (1.51 mmol) were refluxed about 24 h in ethanol (10-15 mL) and acetic acid
(4-5 drops) (9:1) mixture then kept in ice bath for an hour and product get precipitated. The vacuum
filtration has been done and washed with cold ethanol and dried in vacuum. Further purification
was done by column chromatography (CHCl3:MeOH, 9:1).
(E)-2-(2-hydroxy-5-(phenyldiazenyl)phenyl)-1H-anthra[1,2-d]imidazole-6,11-dione
(1a).
Yield 85%; Anal. calcd for C27H16N4O3: C 72.97, H 3.63, N 12.61. Anal. found: C 72.63, H 3.78,
1
N 12.56; H NMR (400 MHz, DMSO-d6) δ 12.60 (s, 1H, –OH), 8.73 (s, 1H, –NH), 8.09 – 7.82
(m, 5H), 7.81 – 7.71 (m, 5H), 7.56 – 7.47 (m, 3H), 7.07 (d, J = 8.6 Hz, 1H); 13C NMR (126 MHz,
DMSO-d6) δ 206.26, 183.49, 182.30, 156.08, 152.46, 145.39, 134.55, 134.50, 134.23, 133.46,
133.31, 130.98, 129.53, 128.43, 127.09, 126.50, 126.07, 122.66, 121.75, 118.46; HR-MS (ESI+)
calcd for C27H16N4O3 [M + H]+ 445.1256, found 445.1300.
(E)-2-(2-hydroxy-5-((4-nitrophenyl)diazenyl)phenyl)-1H-anthra[1,2-d]imidazole-6,11-dione
(1b). Yield 79%; Anal. calcd for C27H15N5O5: C 66.26, H 3.09, N 14.31. Anal. found: C 66.73, H
3.12, N 14.56; 1H NMR (400 MHz, DMSO-d6) δ 13.11 (s, 1H, –OH), 8.54 (s, 1H, –NH), 8.38 (s,
1H), 8.25 – 8.08 (m, 4H), 8.01 – 7.79 (m, 5H), 7.27 (s, 1H), 6.92 (s, 1H), 6.65 (s, 1H); HR-MS
(ESI–) calcd for C27H15N5O5 [M – H]+ 488.1007, found 488.0974.
(E)-2-(2-hydroxy-5-(p-tolyldiazenyl)phenyl)-1H-anthra[1,2-d]imidazole-6,11-dione
(1c).
Yield 76%; Anal. calcd for C28H18N4O3: C 73.35, H 3.96, N 12.22. Anal. found: C 73.40, H 3.50,
1
N 12.28; H NMR (400 MHz, DMSO-d6) δ 12.75 (s, 1H, –OH), 8.86 (s, 1H, –NH), 8.19 – 7.91
(m, 5H), 7.83 (s, 3H), 7.71 (d, J = 7.7 Hz, 2H), 7.33 (d, J = 7.9 Hz, 2H), 7.15 (d, J = 8.7 Hz, 1H),
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13
2.36 (s, 3H, –CH3); C NMR (126 MHz, DMSO-d6) δ 206.26, 196.80, 187.20, 156.46, 150.54,
149.66, 141.99, 141.20, 134.34, 133.45, 130.10, 128.48, 127.17, 126.61, 126.58, 126.49, 126.12,
125.19, 124.31, 122.68, 31.00; HR-MS (ESI+) calcd for C28H18N4O3 [M + H]+ 459.1412, found
459.1435.
(E)-2-(1H-benzo[d]imidazol-2-yl)-4-((4-nitrophenyl)diazenyl)phenol (2b). Yield 78%; Anal.
calcd for C19H13N5O3: C 63.51, H 3.65, N 19.49. Anal. found: C 63.89, H 3.92, N 19.89; 1H NMR
(400 MHz, DMSO-d6) δ 13.93 – 13.60 (broad s, 2H, –NH and –OH), 8.80 (s, 1H), 8.41 (d, J = 8.6
Hz, 2H), 8.07 – 7.95 (m, 3H), 7.67 (s, 2H), 7.29 (s, 2H), 7.20 (d, J = 8.5 Hz, 1H); 13C NMR (126
MHz, DMSO-d6) δ 184.05, 184.03, 182.05, 174.07, 166.85, 148.24, 144.05, 140.40, 125.57,
123.42, 117.08; HR-MS (ESI+) calcd for C19H13N5O3 [M + H]+ 360.1052, found 360.1071.
(E)-2-(1H-benzo[d]imidazol-2-yl)-4-(p-tolyldiazenyl)phenol (2c). Yield 80%; Anal. calcd for
C20H16N4O: C 73.15, H 4.91, N 17.06. Anal. found: C 73.89, H 4.99, N 17.69; 1H NMR (400 MHz,
DMSO-d6) δ 13.85 – 13.40 (broad s, 2H, –NH and –OH), 8.69 (d, J = 2.0 Hz, 1H), 7.92 (d, J = 6.6
Hz, 1H), 7.81 – 7.60 (m, 4H), 7.38 (d, J = 7.8 Hz, 2H), 7.29 (s, 2H), 7.19 (d, J = 8.8 Hz, 1H), 2.38
(m, 3H); 13C NMR (126 MHz, DMSO-d6) δ 161.20, 161.06, 151.46, 150.58, 145.44, 141.49,
130.40, 125.64, 122.67, 118.62, 113.42, 21.43; HR-MS (ESI+) calcd for C20H16N4O [M + H]+
329.1358, found 329.1388.
Acknowledgements
We thanks to Department of Science and Technology (DST) (INSPIRE IFA-13 CH-94 and
SERB/YSS//000241) for the financial support. DP and AK acknowledge to DST-INSPIRE project
for fellowship. The Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of
Technology Bombay (IITB) and Department of Pharmaceutical Science and Technology, Institute
20
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10.1002/ejoc.201900930
European Journal of Organic Chemistry
of Chemical Technology (ICT) Mumbai, are greatly acknowledge for providing HR-MS and 1H-
NMR facilities. A special thanks to ICT Mumbai for providing the research facilities.
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