pH-sensitive 4,(4-Nitrophenoxy)benzeneamine) derived azo dye: X-ray crystallographic, DFT and electrochemical studies

Article abstract of DOI:10.1016/j.molstruc.2020.128667

A novel nitro terminated azo dye, (E)-3-((4-(4-nitrophenoxy)phenyl)diazenyl)-[1,1′-biphenyl]-4-ol (1), was produced by coupling biphenyl-4-ol with 4,(4-Nitrophenoxy)benzeneamine) and was characterized by various spectroscopic and XRD technique. The chemical structure of the compound (1) has a nitro terminal as an electron-withdrawing moiety (A), a biphenyl ring of coupling part as a proton-donating unit (D), and both groups are linked by an azo group connector. The stimulating pH-controlled absorption behavior of chemosensor was investigated using UV/VIS spectroscopic technique. It was observed that synthesized dye (1) has high sensitivity in pH range from 7.9 to 10.46 and could be applied as a selective ‘naked-eye’ colorimetric sensor for H⁺ ion concentration, as it exhibited a very strong hyperchromic shift in the absorption band from 399 nm to 521 nm with a significant color change from yellow to red. The sensitivity of chemosensor was observed to be fully reversible and rapid which enhance its chances of practical implicitly. Further, DFT simulations were performed using ADF-Modeling suite 2019, to get an idea of relevance with experiments. Additionally, redox behavior of the (1) dye was also investigated by employing cyclic voltammetry using 0.1M TBAP as supporting electrolyte. Cis–Trans conformational changes were analyzed to investigate the photoisomerization ability of the synthesized dye.

Full text of DOI:10.1016/j.molstruc.2020.128667

Journal Pre-proof
pH-sensitive 4,(4-Nitrophenoxy)benzeneamine) derived azo dye: X-ray
crystallographic, DFT and electrochemical studies
Samina Qamar, Zareen Akhter, Sammer Yousuf, Fouzia Perveen
PII:
S0022-2860(20)30992-3
DOI:
https://doi.org/10.1016/j.molstruc.2020.128667
MOLSTR 128667
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 March 2020
Revised Date: 22 May 2020
Accepted Date: 9 June 2020
Please cite this article as: S. Qamar, Z. Akhter, S. Yousuf, F. Perveen, pH-sensitive 4,(4-
Nitrophenoxy)benzeneamine) derived azo dye: X-ray crystallographic, DFT and electrochemical studies,
Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.128667.
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Credit author statement
Samina Qamar: Conceptualization, Methodology, writing, Original draft preparation, Zareen Akhter:
Supervision, Conceptualization, investigation. Sammer Yousuf: Validation, Formal analysis, writing.
Fouzia Perveen: Formal Analysis, Writing.

pH-Sensitive 4,(4-Nitrophenoxy)benzeneamine) derived azo dye:
Crystallographic, DFT and electrochemical Studies
X-Ray
Samina Qamara, Zareen Akhtera*, Sammer Yousuf b Fouzia Perveenc
,
a Department of Chemistry, Quaid-I-Azam University Islamabad Tele.No.92-5190642111(Off), Pakistan
b H. E. J. Research Institute of Chemistry, International Center for Chemical and Biological Sciences, University of Karachi, Karachi-75270, Pakistan.
c Research Center for Modeling and Simulations, National University of Sciences and Technology, Islamabad 44000, Pakistan.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A novel nitro terminated azo dye, (E)-3-((4-(4-nitrophenoxy)phenyl)diazenyl)-[1,1'-biphenyl]-4-ol
(1), was produced by coupling biphenyl-4-ol with 4,(4-Nitrophenoxy)benzeneamine) and was
characterized by various spectroscopic and XRD technique. The chemical structure of the
compound (1) has a nitro terminal as an electron-withdrawing moiety (A), a biphenyl ring of
coupling part as a proton-donating unit (D), and both groups are linked by an azo group connector.
The stimulating pH-controlled absorption behavior of chemosensor was investigated using UV/VIS
spectroscopic technique. It was observed that synthesized dye (1) has high sensitivity in pH range
from 7.9-10.46 and could be applied as a selective ‘naked-eye’ colorimetric sensor for H+ ion
concentration , as it exhibited a very strong hyperchromic shift in the absorption band from 399
nm to 521 nm with a significant color change from yellow to red. The sensitivity of chemosensor
was observed to be fully reversible and rapid which enhance its chances of practical implicitly.
Further, DFT simulations were performed using ADF-Modeling suite 2019, to get an idea of
relevance with experiments. Additionally, redox behavior of the (1) dye was also investigated by
employing cyclic voltammetry using 0.1M TBAP as supporting electrolyte. Cis–Trans
conformational changes were analyzed to investigate the photoisomerization ability of the
synthesized dye.
Available online
Keywords:
Characterization
Chemosensor
UV-VIS studies
DFT Simualtions
Cyclic voltammetry studies
2020 Elsevier Ltd. All
1. Introduction
halochromic (change in color upon change in pH) properties in
the molecule.
The significance of pH cannot be denied in any field of life, like
agriculture, pharmaceutical or cosmetic industry, and its most
imperative role is in human body [1]. A slight fluctuation in pH
can result in large scale physiological and pathological
consequence in the whole system. As a result, from many years,
designing of new investigation tools for determination of
hydrogen ion concentration has captured the attention of several
researchers. These studies have led to the development of more
innovative, more sensitive but less expensive pH sensors.
Though until now many fluorescent probe detection methods [2]
have been reported for accurate determination of hydrogen ion
concentration, but visible colorimetric chemosensors are always
broadly welcome relying on its feasibility to determine the
chemical changes with naked eyes [2-3].
Computational approaches employing different quantum
parameters, cyclic voltammetry and photo-isomerization are now
widely being used as a major tool for better understanding of
chemical reactivity of the synthesized azo compounds.
In this work a visible colorimetric pH sensor has been
synthesized taking an idea from D–π–A azo chromophore
system, however, in our chemical structure conjugation breaks at
the ethereal oxygen between the two-phenyl rings. Here in the
chemical structure, biphenyl moiety of the coupling component
is acting as proton donor group and NO2 is incorporated as
electro withdrawing unit (As proved by the XRD data). The
structure of the synthesized dye (1) having a D–π–A system is
shown Figure 1
Class of compounds containing (-N=N) moiety in conjugation
with aromatic rings are generally referred as azo dyes [4-6]. The
literature shows that azo dyes have been immensely used in
various fields and applications based on its cheaper cost and ease
of synthesis [7-9]. The most common commercial application of
these compounds is in food coloring [10], nonlinear optics [11],
textiles [12,13], electro-optic, liquid crystal display [14,15] and
photovoltaic industry.
Figure 1: Designed molecular structure for chemosensor
Azo dyes having D–π–A chromosphere have wider chances of
acting as pH-sensitive chemosensors, where D is electron
donating group and A is an acceptor group; bridged with π bond
of azo group. Often D–π–A types of chromosphere show
extended conjugation because of the delocalization of electron
on donating and withdrawing group [3]. This electron push pull
effect in molecule results in high color fastness and induces
We have done the characterization of synthesized dye using
various spectroscopic techniques together with the detail
exploration of π-π stacking interaction in the crystal lattice using
XRD technique. The pH sensing properties was investigated by
UV-VIS spectroscopic technique. Further computational study
was done by DFT for finding brief relevance with spectral
analysis. We also report this dye to be electro-active and

2
Journal of Molecular Structure
photoisomerization active using cyclic voltammetry and UV-
salt was used as supporting electrolyte in the system. Three
VIS techniques respectively.
electrodes, Pt wire (0.1 cm thickness), glassy carbon (GC 0.071
cm2) and Ag/AgCl electrode were employed as counter, working
and reference electrode respectively. Working electrode was
polished and washed each time before using.
2. Experimental:
2.1 Materials
2.6 Photoisomerization:
Nitroaniline (99% purity), Fluro-nitrobenzene, Potassium
carbonate, Bipenyl-4-ol, Sodium nitrite, TBAP and all other
reagents were purchased from Sigma Aldrich company used as
received. N,N-Dimethylformamide (DMF) used in first reaction
was pre-distilled and dried by anhydrous magnesium sulfate
MgSO4.
The sample solution (0.1mmol) of the synthesized compound was
prepared in DMF solvent at room temperature. The solution was
then irradiated with short wavelength UV light (365 nm) for
specific interval of time. UV-VIS absorption spectra were
recorded after each interval of time using Perkin Elmer LAMDA
235 UV/VIS spectrophotometer.
2.2 Instrumentations and procedures
2.7 Synthesis
The melting temperature of both initial precursor and synthesized
dye was determined and reported by employing an open capillary
tube in Mel-Temp apparatus of Mitamura Riken Kogyo
company. Bruker spectrophotometer accompanying ATR
sampling accessory, was used for FT-IR data collection in the
range of 4000-400 cm-1.
The starting precursor (4,(4-nitrophenoxy)benzeneamine) was
synthesized according to the previous reports and recrystallized
using ethanol-methanol (1:1) system [4]. Diazonium salt of 4,4-
nitrophenoxyaniline was synthesized by dissolving amine (0.5 g,
0.217 mmol) in 3 mL HCl-H2O (1:1) mixture and temperature
maintained to -5-0 C. Next, in ice cooled mixture equimolar 5
mL sodium nitrite NaNO2 solution (0.149 g, 0.217 mmol) was
◦
An advanced, Bruker Digital NMR spectrophotometer (300
MHz, Switzerland) was used to collect the Proton (1H) and
carbon (13C) NMR spectra of the synthesized product. In NMR
results, signals (chemical shift) are reported in ppm unit
downfield to the Tetramethyl silane (Me4Si) as an internal
reference. All the halochromic properties of the synthesized dye
added drop-wise with thorough stirring for about ten minutes at 0
◦C.
In the next step, synthesized solution was then dispensed drop-
wise to the coupling component i.e biphenyl-4-ol (0.36g, 0.217
mmol) in 5 ml solution of 10% of NaOH below 5 C and
continued stirring for about two hours (120 mins) to ensure the
completion of reaction. Finally, dilute solution of HCl was used
to complete precipitation (scheme1). The precipitates was
◦
was measured on
Spectrophotometer.
a
Perkin Elmer lambda (235)
For X-ray Diffractional studies, single crystals of synthesized dye
were grown at room temperature, by gentle evaporation in
chloroform solvent. The respective data was collected on D8
Venture diffractometer (Bruker APEXI) I) built in with
PHOTON 100 (2) detector of CMOS technology. Employed with
Cu kα (λ= 1.54178 Å] radiations source rapped in a fine focused
sealed hose. Multi-scan mode was used for absorption correction
[16,17]. Crystal structure refinement and details of data
collection and are shown in Table 1. Diffraction data of single
crystal was solved by Bruker SAINT Program as implemented in
WinGX program, adapted with the SHELXTL program method.
Molecular structure is presented in ORTEP view employed using
ORTEP-3 program [18], while the molecular interactions were
interpreted with the help of PLATON and projected by using
MERCURY (3.6) software [19]. Hydrogen bond geometry, bond
lengths and bond angles are presented in Table 2 and Table S1
respectively.
◦
filtered and dried in oven at 70 C. Ethanol-water (1:1) system
was used for recrystallization. All the salts and acid were
removed while washing with distilled water and remaining
organic residues were discarded by rinsing with small famount of
ether.
2.3 UV-VIS spectral investigation:
A solution (DMSO–water 9:1v/v) of (1) was used to explore
the light absorption properties of the synthesized dye.
To further investigate the influence of hydrogen ion
concentration on absorption spectrum of the chemosensor, a
UV/VIS titration of chemosensor was done with triethylamine
(TEA), using (5 10-5 mol/L) concentration of chemosensor in
9:1 DMSO-H2O system.
2.4 Computational studies
Computational studies are one of the most significant and
versatile strategy, used to develop the theoretical understanding
of a chemical construction, in correlation with the experimental
facts, using various quantum approaches [20]. DFT calculation
were done using ADF-Modeling suite 2019 [21, 22] employing
the generalized gradient approximation (GGA) [23] in
combination with the Perdew Wang (PW91) [24] density
functional.
Scheme1.Synthesis of (E)-3-((4-(4-nitrophenoxy)phenyl)diazenyl)-[1,1'-
biphenyl]-4-ol
2.7.1 Spectroscopic data of dye (1)
Compound (1) is named as (E)-3-((4-(4-nitrophenoxy)phenyl)diazenyl)-
[1,1'-biphenyl]-4-ol on the basis IUPAC rule
2.5 Cyclic voltammetry studies
The highly sensitive Eco Chemie Auto Lab.PGSTAT 12
potentiostat was used to collect electrochemical data of the
synthesized dye (1). Tetra butyl ammonium perchlorate (TBAP)
Yield= 91% Bright yellow dye; m.p. 411.4 ◦C; FT-IR absorption
peaks (ATR, ʋ , cm-1): = 3416.36, 3037.52, 1507.08, 1346.89,

1
1580.25, 1485.59, 1244.21; H NMR (300 MHz, Acetone): δ
indicated the presence of hydroxyl OH group in the structure. In
case, para product is forming all the proton signals should be
doublet, but the presence of one more singlet peak at 7.85 ppm
(due to proton adjacent to carbon containing OH group) is also
supporting the formation of dye with hydroxyl group ortho to the
azo moiety. Other signals (δ) for the aromatic protons were
present from 6.8 to 8.15 ppm which were found to be complete
consistence with the previous reports and further integration of
peaks also correlate with the proposed chemical structure.
In 13C NMR presence of 18 carbon peaks instead of 16 peaks
also confirmed the formation of ortho product instead of para as
we know that para products are usually more symmetric showing
a smaller number of carbon peaks as compare to the ortho
products.
[ppm]=11.37 (s, 1H, -OH), 8.13 (d, = 9.3 Hz, 2H), 7.85 (s, Ar-H),
7.74 (d, J= 8.7 Hz 2H), 7.321 (t, 3H), 7.14 (d, J=7.2 Hz 2H,),
6.98 (d, J=9.3 Hz, 2H), 6.90 (d, J=8.7 Hz, 2H), 6.83 (d, J=8.7 Hz,
2H), 13C NMR (75 MHz, Acetone): δ [ppm]= 115.58, 117.21,
120.72, 123.49, 123.70, 125.31, 125.85, 126.53, 128.46, 132.12,
142.74, 143.85, 151.57, 154.51, 160.82, 162.93, 168.82, 171.93
C24H17N3
3. Results and Discussion:
Azo dye (1) was synthesized by following classical diazotization-
coupling
reaction
between
biphenyl-4-ol
and
4,4-
nitrophenoxyaniline. Effective yield of yellow colored dye was
obtained, as nitro moiety in the chemical structure of precursor
make diazonium salt more electrophilic in nature and increase its
susceptibility towards coupling component. It was expected that
the biphenyl-4-ol will couple from para position to the azo group
as shown in previous report [25], but unexpectedly it resulted
different product by coupling hydroxyl group ortho to the azo
moiety as shown by the single crystal XRD results. The most
evident reason behind the formation of this product is
intramolecular hydrogen bonding present between hydroxyl
group ortho to the azo (N=N) moiety as depicted by the XRD
studies which increases the stability of the E configuration.
3.2 X-ray Diffraction Measurements (1).
The title compound (1) crystallizes in triclinic crystal system
ꢀ
having P1 space group. Structural studies of compound (1)
showed an asymmetric unit comprises of four planer aromatic
rings (A-D) and a central azo bridge (Figure 3). A hydroxy-
substituted phenyl ring B (C-7—C-12), and ring C (C-13—C-18)
connected through azo linkage (N-1=N-2) with torsion angle of
176.90ᵒ. The bond length of 1.270 (1) Å between N=N indicating
the trans configuration of azo bond [26]. Ring B found to linked
with ring A (C-1—C-6) via C-2 while ring C show an attachment
with 4-nitrophenoxy group i.e. ring D (O-2—O-4/N-3/C-19—C-
24) through C-16 atom.
Interestingly during experimentation, it was accidently observed
that with a slight increase in basic condition (more than 10%
NaOH) of coupler component, the reaction gives the red colored
precipitate that after washing with distill water gives Yellow
colored dye as shown in Figure 2 This finding successfully
indicated the pH sensitivity of the synthesized compound which
is further evaluated by the spectral investigation as discussed
later.
The planes of aromatic ring A—B, and C—D form dihedral
angles of 27.87ᵒ and 65.51ᵒ, respectively. The dihedral angle of
6.24ᵒ between rings B—C indicates that both rings are nearly
coplanar to each other.
In the crystal structure of compound (1), the three-dimensional
pattern is consolidated via conventional and non-conventional
hydrogen bonds. C—H…O interactions contributed to this
network by C-14—H-14…O-1, and C-17—H-17…O-4
intermolecular contacts having H…A distance of 2.5724 Å, and
2.544 Å respectively (Figure 4).
Table 1. Crystallographic details for the dye (1)
Compound
1A
Empirical formula
Formula weight
C24H17N3O4
411.40
(a)
(b)
Temperature
100(2) K
Wavelength
1.54178 Å
Crystal system
Space group
Unit cell dimensions
Triclinic
P-1
Figure 2: (a) Product without washing, with slight excess of
NaOH in coupling component (b) After washing with distill
water for complete neutralization
a = 7.3640 (2) α = 100.4340 (10)°.
b = 7.7256 (2) Å β = 90.0020 (10)°.
c = 17.8444 (4) Å γ = 100.4480 (10)°.
981.28(4) Å3
2
Volume
3.1 Spectroscopic results (FT-IR and NMR)
Z
1.392 Mg/m3
Density (calculated)
FT-IR spectrum of the synthesized dye (1) is shown in
supplementary information Figure S1. The broad band of OH
peak at 3416.36 cm-1 predicted the predomination of azo-enol
over hydrazo-keto tautomer. Additionally, absence of any C=O
stretch in the spectrum further confirmed the absence of hydrazo-
keto tautomer as shown below.
Absorption coefficient
-1
0.796 mm
428
F(000)
Crystal size
0.210 x 0.160 x 0.060 mm3
5.043 to 68.376°.
Theta range for data collection
Index ranges
-8<=h<=8, -9<=k<=9, -21<=l<=21
28019
3578 [Rint = 0.0260]
99.6 %
Reflections collected
Independent reflections
Completeness to theta = 67.679°
Refinement method
2
Full-matrix least-squares on F
3578 / 0 / 284
1.037
Peaks at 1507.08 cm-1 belong to azo moiety similarly signals at
1346.89 and 1580.25 cm-1 predicting the presence of two
symmetric peaks of nitro group.
Data / restraints / parameters
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0346, wR2 = 0.0925
Further structure of synthesized yellow dye (1) was also
confirmed by NMR spectroscopic results shown in Figure S2.
NMR results were obtained in deuterated Acetone solvent using
300 and 75 MHz frequency for proton and 13C NMR
R1 = 0.0384, wR2 = 0.0956
0.231 and -0.237 e.Å-3
Largest diff. peak and hole
1
respectively. In the H NMR spectra, the signal at 11.37 ppm

4
Journal of Molecular Structure
Some of selected hydrogen bonding in dye (1) are shown in
(C-13—C-18) with the distance of 3.3622 (2) Å. Hence, the
Table 2.
neighboring molecules connected in a way to form an infinite
chain running along the c-axis.
Table 2. Hydrogen bonds geometry (A°) of dye 1A.
3.3 Proposed pH-sensitivity mechanism for the chemosensor
dye:
D—H...A
D—H/Å
H...A/Å
D...A/Å
D—H...A/ᵒ
For spectral investigation of pH-sensitivity of the dye, a solution
(DMSO–water 9:1v/v) of (1) was used. In a simple UV spectrum
of dye (1) show a peak about 335 nm due to π-π* transition in
aromatic groups and shoulder at 399 nm due to n-π* transition of
azo group.
O-1—H-1A...N-2
C-14—H-14...O-1i
C-17—H-17...O-4ii
0.950(19)
0.95
1.704(19)
2.57
2.5663(12)
3.2456(15)
149.2(16)
128.1
0.95
2.54
3.4647(15)
163.3
Symmetry Code: (i) x+1, y, z, (ii) -x+2, -y+2, -z
The effect of pH towards the dye was investigated after
observing a reversible and rapid color change from yellow to red
at pH 8 and 11. Figure 5(c) is representing the vivid change in
color of the solution at different pH. For further evaluation
UV/Vis titration of chemosensor was done with triethylamine
(TEA), using (5 10-5 mol/L) concentration of chemosensor in
9:1 DMSO-H2O system. The dye underwent vivid color change
from yellow (399 nm) to red (521 nm), representing an obvious
bathochromic shift of 122 nm. With increasing pH, a decrease in
absorption intensity at shoulder of 399 nm and increase at 521
nm observed as shown in Figure 5(a-b). Equilibrium between
the two species was indicated by the presence of isosbestic point
observed at 460 nm Figure 5(e).
Figure 3: Molecular structure of dye (1) with atomic numbering scheme.
The thermal ellipsoids are drawn with 50% probability level. Dye (1)
form the S6 motif via intramolecular O-1—H-1…N-2 hydrogen bonds
represented as dotted line.
Mechanism for this bathochromic shift is closely related to the
structural features of chemosensor. Since chemosensor has
biphenyl 4-ol moiety in its chemical structure, having that
aptitude to interchange between phenol and phenolate form in
acidic and basic media respectively as shown in Figure 6;
leading to significant change in optical and physiological
properties of the compound. Upon increasing pH, deprotonation
of biphenyl -4 ol occurs converting in to active phenoxide, this
deprotonation affects the electronic properties of chromospheres
because of increased conjugation and induced some new charge
transfer intramolecular interaction leading to visible color change
and shifting λmax in UV/Vis spectrum.
Further the shift can be explained on the basis of increased
stabilization of the excited state comparative to the ground state
because of increased polarity of the solution system, which
minimizes the energy gap between excited and ground state
leading to positive slovatochomism (bathochromic shift).Thus
Figure 4: Molecule of (1a) form the herringbone pattern via
intermolecular C-14—H-14…O-1 and C-17—H-17…O-4 hydrogen
bonds represented by dotted lines along c-axis.
The intra-molecular O-1—H-1…N-2 interaction stabilizes the
molecule by forming an S6 ring of graph set motif (Figure 3).
The association between molecules further enhanced by ꢁ − ꢁ
stacking between centroids of ring B (C-7—C-12), and ring C
intramolecular
charge
transfer,
rearrangement
versus
isomerization can be the reasons of this spectral changes .
(a)
(b)
(c)
(d)
(e)

Figure 5: (a) pH-dependent value of absorbance at (399 nm); (b) pH-dependent value of absorbance at 521 nm; (c) Photograph of
colorimetric compound (1a) at different pH; Yellow (acidic medium) Red (Basic medium) d) UV/Vis absorption spectra of dye ( 5 10-
5mol/L) at varying pH in DMSO solution containing 10% water. e) Extended form of the isosbestic point
+
Figure 6. Rearrangement in chemical structure at different pH responsible for color change
3.4 Reversibility Test:
evident that dye exhibited greater peak intensities at 337nm, 399
nm and 523 nm (Figure 8b). First excitation was found to exhibit
highest oscillator strength > 0.9. Comparing Figure 7 and 8b
revealed that computationally calculated spectrum complements
regions of electronic excitations.
Investigation about the reversibility of the chemosensor was done
by changing the pH between 8 to 11 and then reverses to the 8 for
over
4 rounds, using saturated sodium hydroxide and
Hydrochloric acid as shown in Figure 7. Accordingly, to the
expectation different colors of medium reappeared at repetition
of cycle and response time was less than 1 second. This study
indicates that the change in color of chemosensor was fast
responsive and fully reversible over a narrow pH range.
Figure 8: Optimized geometry of titled compound (a) and
UV- spectrum displaying peak absorption calculated at LDA-
GGA level of theory. Intensity is given by the molar
extinction coefficient
3.6 Electrochemical studies:
Cyclic voltammetry is one of the multi-mode analytical
technique and used to study redox behavior of the compound (1)
in DMF (0.1mM) sample solution containing 1M TBAP as
supporting electrolyte in the system. Voltammetry behavior was
evaluated thoroughly both in positive and negative range, but dye
found to be active only in negative potential range from -1.4 to -
0.7 V. In the cyclic voltammograms of (1) as shown in Figure 9
an asymmetric peak in the reduction scan is attributed to the one
step reduction of azo (N=N) group to amine, as reported
literature shows that strongly alkaline medium is required for
prior reduction of nitro group [28-29]. As there is no
corresponding oxidation peak to the azo group reduction, in the
reverse direction (-1.4 to -0.7), shows that it is an irreversible
process. However, a corresponding small anodic peak -0.8-0.9 V
region is attributed to the oxidation of biphenyl-4-ol hydroxyl
group in the synthesized compound. As literature shows that the
presence of hydroxyl group ortho to the azo (N=N) group favors
the electrochemical oxidation of the (-OH) substituted dyes. This
-OH oxidation peak is quasi irreversible in nature as observed
after increasing scan rate.[30]
Figure 7: Reversibility of color change (measured at 521nm)
3.5 Density functional theory simulations:
In order to calculate spectral properties, we performed DFT using
ADF-Modeling suite 2019 employing the generalized gradient
approximation (GGA) due to Becke and Perdew, in combination
with the Perdew Wang (PW91) density functional. The basis sets
representing the electron density consist of both Hermane
Skillman numerical atomic orbitals (NAOs) and Slater-type
orbitals (STOs) with a frozen core. Scalar relativistic corrections
were also included through the zeroth order regular
approximation [27]. A three-dimensional translational symmetry
was used for the calculations of compound structure and density
of state. In all results overall convergence was well within 0.1eV,
with the GGA. UV spectrum was evaluated at singlet and triplet
excitation states at numerical quality taken as good.
The results generated by simulations were investigated to identify
compounds with strong UV/Vis absorption characteristics. It is

6
Journal of Molecular Structure
transition and a weaker band in visible region due to n-π*
transition. In actual n-π* transition is forbidden in trans
configuration according to the symmetry rules but allowed in less
stable cis configuration [33].
After irradiating the compounds solution with 350 nm UV light
for controlled varying time scale, a decrease in intensity of
absorption in 300-400 nm region, and increase in absorption in
visible region was observed, which confirmed the conversion of
trans configuration to cis one. Spectra of dyes is shown in Figure
11.
In Spectra decrease in absorption intensity at 399 cm-1 (π to π*)
and increase a peak at 410 cm-1 (n to π*) confirms the trans to cis
isomerization. Photoisomerization in this compound is reversible
and it was confirmed by placing these solutions in dark and
recording spectra, which revealed the back conversion of cis
form to trans form. According to the literature both out of plane
rotation of N=N and in plane inversion mechanistic pathway is
possible for photoisomerization [34].
Figure 9 Cyclic voltammogram of the synthesized dye (1) on
different scan rate (15-200 mV/s)
3.6.1 Effect of scan Rate
0
Further examination was carried out to study the effect of scan
rate on the redox properties as shown in Figure 9. For this 1.1
×10 -3 M solution of dye (1) was placed in electrochemical cell
with 0.1 M TBAP and scan rate was varied from 15-200 mV/s.
Result of different scan rate illustrated that oxidative and
reductive current increases with increasing scan rate. The slightly
negative shift in peak potential with increasing scan rate shows
the irreversible nature of reduction processes of azo group.[31]
Figure 9. Further cathodic peak current when plotted against
square root of scan rate (15-200 mV/s) showed a linear
relationship indicating diffusion-controlled process as shown in
Figure 10.[32]
Figure 11: Photo-induced isomerization in (1) azo-compound
recorded as a time sequence.
Conclusion
This work explains the synthesis of a pH sensitive azo dye (1)
which was characterized by various spectroscopic (FT-IR, NMR)
and single crystal XRD technique. The dye found to be pH
sensitive showing yellow and red color in acid and basic medium
respectively. Which was further investigated by UV-VIS spectral
changes and a bathochromic shift of 122 nm was observed with a
sensitivity range of 7.99-10.46. Computational chemistry was
also applied using DFT simulation performed showed a good
relevance with the UV spectral data. Cyclic voltammetry applied
to illustrate redox behavior and compound fond to be redox
active with an irreversible cathodic peak of azo group and anodic
peak due to oxidation of -OH group. Linear relationship of the
square root of the scan rate and cathodic current proof the
reaction to be diffusion controlled. Positive results of
photoisomerization of compound justified it as potential
candidate to be use in molecular switches, photosensitive
devices.
Figure 10: Cathodic peak current when plotted against square
root of scan rate
Table 3: Results of cyclic voltammetric studies
Scan rate
(mV/s)
ipc (µA)
Ipa µA)
Epc (V)
Epa (V)
15
50
100
150
200
2.139
-6.988
-1.147
-1.179
-1.203
-1.217
-1.23
-0.9254
-0.8975
-0.8735
-0.8454
-0.829.5
2.780
3.660
4.320
4.690
-9.048
-10.21
-11.01
-12.06
3.7 Photoisomerization
Acknowledgements:
On the basis of azo moiety, photoisomerization phenomenon was
investigated in the synthesized compounds, and positive results
were obtained. Trans configuration of azo containing compounds
showed high intensity of absorption in UV region due to π to π*
Authors are appreciative to the Higher education commission
Pakistan for supporting this work under the project grant number
6883.

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Highlights
Nitro terminated azo dye was synthesized by classical diazotization-coupling strategy
Spectral studies revealed the compound to be pH sensitive in the range 7.9 -10.46
Computed DFT simulation showed good relevance with UV spectra.
Redox activity was shown by Voltammetry

The authors declare no competing financial interest.