A comprehensive spectroscopic, solvatochromic and photochemical analysis of 5-hydroxyquinoline and 8-hydroxyquinoline mono-azo dyes

Article abstract of DOI:10.1016/j.molstruc.2020.129323

A series of novel substituted-azo dyes 8-(aryldiazenyl)quinolin-5-ol (5a-i) were synthesized by the coupling reaction of 5-hydroxyquinoline with diazotized aniline derivatives in the presence of NaNO₂ in HCl/H₂O mixture. The study of

Full text of DOI:10.1016/j.molstruc.2020.129323

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A comprehensive spectroscopic, solvatochromic and photochemical
analysis of 5-hydroxyquinoline and 8-hydroxyquinoline mono-azo
dyes
Hamid Rashidnejad , Mohamad Ramezanitaghartapeh ,
Nader Noroozi Pesyan , Peter J. Mahon , M. Manuela M. Raposo ,
Paulo J. Coelho , Andrew Ng Kay Lup , Alireza Soltani
PII:
S0022-2860(20)31641-0
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129323
MOLSTR 129323
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15 September 2020
21 September 2020
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Hamid Rashidnejad ,
Mohamad Ramezanitaghartapeh ,
Paulo J. Coelho ,
Peter J. Mahon ,
M. Manuela M. Raposo ,
Andrew Ng Kay Lup , Alireza Soltani , A comprehensive spectroscopic, solvatochromic and photo-
chemical analysis of 5-hydroxyquinoline and 8-hydroxyquinoline mono-azo dyes, Journal of Molecular
Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.129323
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Highlights




Novel azo dyes based on 5-hydroxyquinoline and their well-known 8HQ analogues were
synthesized.
Partially deuteration technique in 13C NMR spectroscopy was used in order to investigate the
tautomerization in these azo dyes.
The study of solvatochromism in azo dyes strengthens explanation of tautomerism disclosing
the effect of solvent used on these phenomena.
Photochromism phenomena justifies E/Z isomerization in the studied azo dyes.
1

A comprehensive spectroscopic, solvatochromic and photochemical analysis of
5-hydroxyquinoline and 8-hydroxyquinoline mono-azo dyes
Hamid Rashidnejad a, Mohamad Ramezanitaghartapeh b,**, Nader Noroozi Pesyan a, Peter J.
Mahon b, M. Manuela M. Raposo c, Paulo J. Coelho d, Andrew Ng Kay Lup e, Alireza Soltani f,*
a Department of Organic & Inorganic Chemistry, Faculty of Chemistry, Urmia University, Urmia, 57159, Iran
b Department of Chemistry and Biotechnology, Swinburne University of Technology, Hawthorn, VIC 3122,
Australia
c Centro de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
d Centro de-Química- Vila Real, Universidade de Tràs-os-Montes e Alto Douro, 5001-801, Vila Real, Portugal
e School of Energy and Chemical Engineering, Xiamen University Malaysia, Jalan Sunsuria, Bandar Sunsuria,
43900 Sepang, Selangor Darul Ehsan, Malaysia
f Golestan Rheumatology Research Center, Golestan University of Medical Science, Gorgan, Iran
Abstract
A series of novel substituted-azo dyes 8-(aryldiazenyl)quinolin-5-ol (5a-i) were synthesized by
the coupling reaction of 5-hydroxyquinoline with diazotized aniline derivatives in the presence
of NaNO2 in HCl/H2O mixture. The study of the spectroscopic and solvatochromic properties
were performed by FT-IR, 1H and 13C-NMR and UV-Visible spectroscopies. The tautomerism of
these dyes was studied using the deuteration technique and solvatochromic measurements.
Photochromic properties of these 5-hydroxyquinoline azo dyes were also examined via E/Z and
Z/E photochemical isomerization reactions and compared with the existing 8-hydroxyquinoline
analogous. The novel substituted-azo dyes exhibited higher Z/E thermal isomerization rates and
have larger absorbance wavelength range than their 8-hydroxyquinoline analogous, making them
potential molecular switches.
Keywords: 8-hydroxyquinoline; 5-hydroxyquinoline; heterocyclic azo dyes; tautomerism;
solvatochromism; photochromism
Corresponding Author:
Alireza Soltani: alireza.soltani46@yahoo.com, alireza.soltani@goums.ac.ir
Mohammad Ramezanitaghartapeh: mramezanitaghartapeh@swin.edu.au
2

1. Introduction
Organic dye chemistry is facing a very interesting development as a result of their
application in different research fields including electronic [1], linear and nonlinear optics [2],
reprography [3], sensors and biological technologies [4]. Mono-azo dyes are the most significant
type of azo dyes [5-8] and have found applications in memory and recording devices [9,10],
molecular switches [11,12], thermochromic [13], photovoltaic fluorescent devices [14,15],
supramolecular systems [16,17], halographic data storage materials [18], and metal sensors [19].
Their synthesis route [20], absorptivity [21,22], solubility [23,24] and stability [25,26] have been
deeply studied.
The biological properties, coordination and metal extraction ability have made quinoline
and its derivatives very important compounds [27]. Several quinoline derivatives show medicinal
properties such as antimalarial, anti-allergy, antivirus, and anti-infection [28,29].
8-hydroxyquinoline, 8HQ (Figure 1A) is one of the well-known derivatives bearing the
quinoline core which is widely used in inorganic chemistry due to its chelating properties for a
wide range of metal ions [30-32]. Its derivatives show also antibacterial activities [33].
Furthermore, some efficient chromophoric and metallochromic indicators have 8HQ in their
structure [34]. The presence of a stable chromophore azo group (-N=N-) conjugated with an
aromatic ring creates a molecule with intense and highly stable color in different solvents
throughout the visible region of light [35-38]. Although numerous papers have described the
synthesis and properties of azo-8-hydroxyquinoline [39-42], there is no report on the synthesis of
similar 5-hydroxyquinoline (5HQ) azo analogous (Figure 1B). Therefore, the study of the
photophysical properties of 5HQ may provide further insights on its potential as molecular
switches compared to the 8HQ analogs. Recently, the effect of substitution, temperature,
concentration, solvent, acid and base addition on the visible absorption spectra of several
heteroarylazo dyes was performed by Seferoğlu and coworkers [43-47], and Ertan and coworkers
studied the solvatochromism of some heteroarylazo-8HQ dyes. They showed that the tautomeric
equilibrium was deeply solvent dependent and that the azo form was predominant in polar-
aprotic solvents such as dimethylsulfoxide (DMSO) and dimethylformamide (DMF) [48,49].
3

Figure 1. Molecular structures of 8-hydroxyquinoline (A) and 5-hydroxyquinoline (B)
The photochemical E-Z isomerization of aromatic azo dyes (Scheme S1), in solution or
incorporated in polymeric matrices, is a well-established phenomenon that has been the subject
of intense research [50]. Under the UV light irradiation, the thermally stable E-isomer is partially
converted to the meta-stable Z-isomer resulting in a photostationary equilibrium between the two
isomers. The Z-isomers is thermally unstable and returns spontaneously back to the initial E-
form. However, in some cases, the Z-isomer is thermally stable and then the back reaction can be
promoted using the light of a different wavelength (Figure S1). Despite numerous studies, the
mechanism for this interconversion is not completely clear. Two major competitive mechanisms
can be considered: the in-plane inversion mechanism involving the sp hybridization of one
nitrogen atom [51] and the rotational mechanism [52]. The dynamics of these dyes are also very
dependent on their substitution pattern and environment [53].
These azo-aromatic chromophores enable a fully reversible and clean photoreaction
creating a valuable photochemical system. The spatial arrangement of E-isomer molecule allows
an effective extension of the π-conjugation while the same phenomenon is not possible for the Z-
isomer since the two aromatic rings are not in the same plane. Consequently, the thermally stable
E-isomer exhibits a much more intense optical absorption than the Z-isomer [54]. The azo
isomerization reaction requires a large geometrical change within the molecular configuration
and is therefore sensitive to the steric effects. Moreover, these molecules show a great tendency
to aggregate, especially if they are not soluble in a solvent, which leads to self-assembly
4

resulting in the inhibition of the photo-isomerization process [55,56]. As the reversible structural
changes occur at a single-molecule level, these molecules are very interesting for fully photon-
controlled applications such as the photo-switchers technologies [57] and also for their
physicochemical properties (e.g. fluorescence, electrical conductivity, magnetism, permeability,
and reactivity) [58,59].
In this work, it was studied the solvatochromism of new arylazo-5HQ and compare them
to their 8HQ analogs. The different substitution of the basic hydroxyl group relative to the basic
quinoline core is predicted to have an impact on its azo/hydrazone equilibrium. Nevertheless,
this study attempts to investigate and relate its effects qualitatively and quantitatively with the
spectrokinetics performance of 5HQ. Besides, the dyes tautomerism was evaluated by 13C NMR
using the deuterium isotope effect [60,61] which showed that the position of the
carbonyl(hydroxyl) on the quinoline ring has a significant effect on the spectrokinetic of these
dyes. The photoswitching properties of the 5HQ azo derivatives were also investigated to
determine their potential application as molecular switches.
2. Experimental
2.1. General
1
For the measurement of H NMR and 13C NMR spectra of azo dyes, 4(a-i) and 5(a-i)
were dissolved in CDCl3 and/or (CD3)2SO (99.8%) and partial deuteration was achieved by the
addition of two drops of D2O and/or CD3OD. The 1H NMR and 13C NMR spectra were recorded
1
on a Bruker 300 FT-NMR spectrometer (300 MHz for H, 75 MHz for 13C) in CDCl3 and/or
(CD3)2SO, at room temperature (25 °C) using 5 mm direct detection broad band probes and
deuterium lock (Urmia University, Urmia, Iran). The center of the solvent of (CD3)2SO signal
was used as an internal standard which was related to tetramethylsilane with δ = 2.49 ppm (1H)
and a septet at δ = 39.5 ppm (13C) and for the solvent of CDCl3 signal was δ = 7.27 ppm (1H,
corresponds to the trace CHCl3 as an impurity) and a triplet at δ = 77.06 ppm (13C). Also, the
1
center of the added solvent of CD3OD signal appeared as septet at δ = 48.04 ppm (13C). The H
signal of HDO appeared at δ = 3.70 ppm (variable). The recording conditions were as follows:
1H NMR: pulse width (90°) = 8.2 µs, acquisition time = 2.65 s, digital resolution = 0.19 Hz/data
5

point, d1 = 1 s, td = 32768, ns = 16, experimental time = 64 s, spectral width = 6172.8 Hz, rg =
13
4597.6, dw = 81 µs, de = 6 µs; C NMR: pulse width (90°) = 6.3 µs, acquisition time = 1.82 s,
digital resolution = 0.27 Hz/data point, d1 = 2 s , td = 65536, ns = 1000, experimental time =
3922 s, spectra width = 17985.6 Hz, rg = 5792.6, dw = 27.8 µs, de = 6 µs.
For the characterization of synthesized azo dyes, IR spectra were performed in the range
of 4000-400 cm-1 on a NEXUS 670 FT IR spectrometer by preparing KBr pellets (Urmia
University, Urmia, Iran). Melting points were determined with a digital melting point apparatus
(Electrotermal) and were uncorrected. 5-Hydroxy quinoline (5HQ), 8-hydroxy quinoline (8HQ),
anilines and its derivatives and deuterated solvents ((CD3)2SO, CDCl3, CD3OD and D2O) were
purchased from Merck and Sigma-Aldrich.
2.2. Synthesis
The 8-HQ and 5-HQ mono-azo dyes (4 and 5) were synthesized using a similar method
described before [62]. A mixture of aniline and/or its derivatives (1.0 mmol), water (10 ml), and
o
conc. HCl (3.0 mmol) was cooled to 0 C. Aqueous NaNO2 (1.0 mmol) was added with stirring
at such a rate as to prevent a temperature rise. The mixture was kept at 0 oC for 10 min and added
in portions to a solution of 8HQ and/or 5HQ (1.1 mmol) in THF (25 ml) and acetic acid or their
mixture with a saturated aqueous solution of sodium acetate (0.01 gr per every 10 mmol of
aniline). The reaction mixture was continuously stirred for several hours. The precipitated dye
was filtered off, washed with water and cold isopropyl alcohol, and dried at 20-30 oC.
3. Result and discussion
3.1. Synthesis and spectroscopic characterizations
5-(Aryldiazenyl)quinolin-8-ols (4a-i) were prepared by the coupling reaction of 8-
hydroxyquinoline (8HQ) with diazotized aniline derivatives with NaNO2 in a HCl/H2O mixture
(Scheme 1,a) [62]. The 8-(Aryldiazenyl)quinolin-5-ols (5a-i) were prepared from 5-
hydroxyquinoline (5HQ) using the same method of synthesis (Scheme 1,b). Some organic azo
dyes derived from β-naphthol and its derivatives show an intramolecular H-bond between OH
6

group and azo nitrogen (Figure 2) [63]. The corresponding proton chemical shift in the 1H NMR
spectra of these compounds deshields up to δ 14 ppm which confirms the coupling from the OH
vicinal position [63]. In the present work, the absence of the highly deshielded proton (around 14
ppm) confirms that no intramolecular H-bonds are present, which was expected if the reaction
had occurred in the OH vicinal position (7-position for 8HQ dyes and 6-position for 5HQ dyes)
1
(Scheme 1). Detailed data of H and 13C NMR and FT-IR spectroscopies are available in the
experimental section.
Scheme 1. General reaction for the synthesis of azo dyes 4a-i and 5a-i.
Figure 2. Intramolecular H-bond in azo dyes based on β-naphthol [63].
The general azo/hydrazone tautomerism in (4a-i) and (5a-i) azo dyes is depicted in
scheme 2. The azo/hydrazone tautomerism of 8HQ dyes substituted with benzothiazolo [49,42]
and 4-cyanophenylazo [64] groups are quite influenced by the solvent nature. Electron-donating
substituents shift the tautomeric equilibrium towards the azo form, while an electron-
withdrawing group favors the hydrazone form [42]. The azo/hydrazone ratio of some
(aryldiazenyl)quinoline-8-ol dyes [64], evaluated by NMR, was largely influenced by the
7

position of electron-withdrawing groups. The azo form predominates in polar solvents such as
DMF [65] and these dyes exist entirely in the azo form in some anisotropic media as these
solvents provide relatively rigid, packed and polarizable media which prevent the azo/hydrazone
tautomerism [65]. Besides that, the hydroxyl group of azo tautomer and the protonated nitrogen
group of hydrazone tautomer are both able to form hydrogen bonds with certain solvents. Both
groups will form hydrogen bonds with different extensions as hydroxyl is a stronger hydrogen
bond donor. Thus, hydrogen bond acceptor (HBA) solvents such as dichloromethane (DCM) and
DMF (dimethylformamide) will tend to stabilize the azo form through their hydrogen bonding
with the hydroxyl group of azo tautomer whereas hydrogen bond donor (HBD) solvents such as
methanol will tend to stabilize the hydrazone form through their hydrogen bonding with the
protonated nitrogen of hydrazone tautomer.
Scheme 2. General tautomerization for azo dyes 4a-i (A), and 5a-i (B).
The 5HQ dyes (5) can exist in two tautomeric forms: the azo-form (5[I]) and hydrazone-
form (5[II]) (Scheme 2-B). Curiously the hydrazone form (5[II]) has an intramolecular hydrogen
bond with the nitrogen atom of the quinoline ring moiety (Scheme 2-B). For dye 5e, with an
ortho-nitro function, a bifurcated intramolecular hydrogen bond with one oxygen of the nitro
group and the nitrogen atom of the quinoline heterocycle can be expected (Scheme 3) [60,61,66].
8

Scheme 3. Bifurcated intramolecular H-bond in 5e in hydrazone-form 5e[II].
The FT-IR spectra of dyes 4 and 5 showed four distinct transmittance peaks at 3200-3280
cm-1, 1500-1515 cm-1, 1550-1600 cm-1 and 1220-1250 cm-1 which correspond to the O-H, C=O,
N=N and C-O functional groups, respectively. The presence of C=O and C-O groups indicated
that the azo dyes exist in azo/hydrazone tautomeric equilibrium. For nitro-substituted azo dyes,
the C-O peaks were significantly shifted to 1345-1355 cm-1 indicating the destabilization of azo
tautomers as bifurcated intramolecular hydrogen bonds are formed in the hydrazone tautomers.
1
The comprehensive and detailed spectra and interpretations of FT-IR and H and 13C NMR
characterizations were provided as supplementary material (Figures S2-S93).
3.2. Isotopic splitting pattern in the 13C-NMR spectra
The 13C NMR spectrum of dyes 5a and 5b shows two distinct signals for the methoxy
group (5a) at δ 56.38 and 54.86 ppm and methyl group (5b) at δ 21.50 and 20.47 ppm,
respectively, which confirms the existence of two tautomeric forms in equilibrium (Figure 3A).
See supporting information for all 13C NMR spectra of the corresponding dyes.
9

Figure 3. Expanded 13C NMR spectra of the aliphatic region of dyes 5a (A-a) and 5b (A-b) (CDCl3), (A) and
expanded 13C NMR spectra of 5c in the aromatic region before (B-a) and after adding two drops of CD3OD (B-b),
(B).
Deuterium isotope effects on carbon-13 chemical shifts are of great potential use for
spectral assignments and structure determination [60,61,67]. These effects unambiguously reveal
the chemical shifts of the carbon atoms in the neighborhood of the deuterium atom [68,69]. The
isotope exchange process between water or any other protic solvent and easily exchangeable
groups (e.g. O-H, N-H and S-H) are of interest for the study of reaction mechanism and
elucidation of the structural dynamics in biological macromolecules [70]. The process takes
place in two steps: (1) recording a normal 13C NMR spectrum from the sample, and (2) adding a
few drops of the second deuterated solvent (e.g. D2O, CD3OD, etc. depending this fact that the
second solvent does not cause the compound precipitate in the tube) to the same NMR tube and
re-recording the spectrum. This action leads to isotopic shifts and splitting of the resonance of
the carbon atoms bearing exchangeable hydrogen [71,72]. This investigation needs a delicate
attention since the changes are sometimes within ppb [60,61,67]. In the present work, the
addition of a few drops of deuterated methanol (CD3OD) caused the splitting of the carbon
signals. As an example, Figure 3B presents the isotopic splitting of the 13C NMR signals of
carbons C5 and C1′ of 5c (in DMSO) before and after the addition of two drops of CD3OD. The
C1′ and C5 atoms appear as singlets at 152.337 and 158.461 in DMSO. After the addition of a
10

few drops of CD3OD, carbon C5 (α to OH) shows a doublet with signals at δ = 158.438 and
158.379 ppm (Δδ = 59 ppb), and the C1′ carbon atom resonates at δ = 152.346 and 152.266 ppm
(Δδ = 80 ppb). These observations indicate that the two carbon atoms (C5 and C1′) show a β-
isotope effect demonstrating the existence of an equilibrium between two isomeric forms [72,73]
(Scheme 4).
Scheme 4. Possible and impossible form of tautomerism in 5c and 5h.
However, for dye 5h, C1′ carbon atom shows nearly two singlets at δ = 149.477 and
149.437 ppm (Δδ = 40 ppb) while the C5 shows one singlet at 159.469 ppm. These observations
denote the presence of the hydrazone form (5h[II]) in the dye 5h. As it is shown in the Table 1,
the C2 and C8a carbons in 5c and 5h do not show β-isotope effect. Therefore, it could be
concluded that the generation of 5[III] in its salt form is not favored (Scheme 4, Table 1).
A similar investigation was carried out for dyes 4b and 4d as representative examples for
series 4. The β-isotope effect in C1′ for 4d (two singlet at δ =152.997 and 152.945 with Δδ = 52
ppb) denotes the hydrazone form in this compound. However, β-isotope effect for 4b occurs just
in C8 (δ = 155.012 and 154.947 with Δδ = 65 ppb) demonstrating the azo form in this dye (Table
1). The solvatochromic studies also strengthens these observations (see section 3.3).
11

Table 1. Selected 13C NMR data for azo dyes derived from 5-HQ (5a-h) and 8-HQ (4a-h)a before and after adding
two drops of D2O and/or CD3OD (in case we had problem obtaining a clear and accurate spectra using D2O due to
the solubility issue of the samples in D2O)
Entry Dyes
C2
139.453
139.479
139.362
C5
158.461
158.438, 158.379
C8
138.328
138.308
138.225
138.223
C8a
135.334
135.320
133.554
C1'
152.337
152.346, 152.266
149.444
149.477, 149.437
148.81
148.83
152.973
152.997, 152.945
1
2
3
4
5cb,c
5hb,c
4bd,e
4db,c
159.474
159.469
139.350
133.537
-
-
-
-
-
-
-
-
155.060
-
-
-
-
155.012, 154.974
157.989
157.960
a The peaks intensity of 4c, 4e and 4f were low
b before adding two drops of CD3OD
c after adding CD3OD
d before adding D2O
e after adding D2O
3.3. UV-spectroscopy, solvent effect on absorption spectra of compounds in various solvents
For these dyes, two possible transitions are expected (the π→π* and n→π*), which are
however dependent on the nature of the functional group as well as the dipole moment of the
solvent used for the UV-Vis spectroscopy measurement (Figures S94-S99) [74]. Since the
tautomeric equilibrium of a dye molecule is strongly dependent on the nature of its environment
[75,76], the solvatochromic behavior of the dyes in various solvents was studied. For this
purpose, the absorption spectra of 4a-i and 5a-i dyes were measured in various solvents at a
concentration of 10-3 M (Table 2). All absorptions are depicted in Figure 4.
Table 2. Maximum wavelength of absorption of dyes 4a-i and 5a-i in various solvents
Entry
1
2
3
4
5
6
7
8
Dye label
4a
G
p-OMe
p-Me
3-Cl-2-Me
H
o-NO2
m-NO2
p-NO2
2,5-di-Cl
5-Cl-2-Me
p-OMe
p-Me
DCM
386
382
387
381
395, 477s
390
411
397, 498s
390
DMF
Methanol
388
398, 491s
389, 508s
398, 511
411s, 478
408,539
406, 534
431, 634
413, 542
405, 514
395, 478s
389, 514
395, 510s
384, 481s
402, 543
398, 536
386, 466s
392, 466s
382, 453s
395, 480s
453
4b
4c
4d
4e
4f
4g
4h
4i
5a
5b
5c
5d
5e
472
399, 484s
397, 467s
391
9
10
11
12
13
14
15
386
384
386
375
385
3-Cl-2-Me
H
o-NO2
m-NO2
390, 468s
372, 455s
397, 472s
383, 474s
395, 470s
390
5f
12

16
17
18
p-NO2
2,5-di-Cl
5-Cl-2-Me
412
398, 492s
388
429, 633
408, 541
408, 513
474
414, 468s
392, 472s
5g
5h
5i
s: Shoulder
Figure 4. Absorption spectra of 4a-i (A) and 5a-i (B) in: a) DCM, b) methanol, c) DMF
A similar study was carried out for some derivatives of 4 by Saylam et.al [49]. It was
found that the absorption maxima of these dyes are strongly dependent on the solvent polarity.
The dyes show generally a bathochromic shift as the polarity of solvent increases from DCM (P
= 3.1), methanol (P = 5.1) to DMF (P = 6.4) where P is the solvent polarity index [48].
Dyes 5a-i show nearly one absorption maximum in dichloromethane with exception for
5e and 5h (Table 2) in their corresponding UV-visible spectrum indicating a dominant single
tautomeric form of these dyes in dichloromethane solvent. The absorption spectra of dyes 5e and
5h display one absorption maximum with a shoulder in DCM and methanol and two absorption
maxima in DMF (Table 2, Figure 5A). The existence of either two absorption maxima in DMF
or a shoulder indicates for two forms of tautomers (as an equilibrium mixture of two tautomers)
[49]. It is then proposed that dyes 5e and 5h exist as a mixture of tautomeric forms in all used
solvents. However, having asymmetric and unbalanced absorbance shows that one tautomeric
form dominates the other. It is well-known that increase in the polarity of a solvent the shifts the
tautomerization equilibria to hydrazone form [77], therefore a different mixture ratio of
tautomers were observed due to the change in the solvent used in order of dichloromethane →
13

methanol → DMF where the shoulders in dichloromethane and methanol gradually become a
distinct peak in DMF. The partial appearance of hydrazone-form in dye 5h in dichloromethane is
well-agreed with the obtained result of the isotopic splitting pattern in the previous section.
Figure 5. Absorption spectra of 5e and 5h in DCM, in methanol and in DMF (A), and hydrazone-form tautomer of
5e and 5h (B).
Considering the tautomerization of dyes 5e and 5h, the existence of nitro (NO2) or
Chlorine (Cl) on the ortho-position of phenyl ring makes them different from other derivatives.
Interaction between these groups [61,66,67] and the hydrogen atom of the hydrazone form brings
a relative stability to the hydrazone form compared to other dyes in this series (Figure 5B).
Similar results were obtained for dyes 4e and 4h.
In order to investigate the bifurcated intramolecular H-bond and its effect on the stability
of hydrazone form, dyes 5e, 5f and 5g with nitro group on different positions of the phenyl ring
could be further analyzed. It was observed that the maximum of the absorption bond of
hydrazone form appears at a longer wavelength compared to the azo form [49,78,79]. Although
the azo form is highly dominant in all obtained spectra of these dyes, a relative stability of
hydrazone form in dye 5e in dichloromethane is more obvious than hydrazone form in dyes 5f
and 5g. See Figure 6A-C for more details, in which it could be observed the appearance of a
shoulder in right hand side of the peak maxima of 5e (Figure 6A). With the increase of the
polarity of solvent (dichloromethane → methanol), the azo form is the main tautomer for dyes 5e
and 5f, but the stability of hydrazone form also appears in both compounds. This is despite the
fact that dye 5g does not show a shoulder and it has a maximum peak in the middle region
(Figure 6B). In comparison with the next solvent (DMF), it can be concluded that 5g in methanol
14

exists in a balanced tautomerization between azo / hydrazone forms. For further study on the
effect of polarity of solvent (DMF), a more polar solvent was selected and the spectra were
measured in it. Herein, the stability of hydrazone form in dyes 5e and 5f was increased due to
high basicity and high relative permittivity of this solvent [80]. Therefore, shoulders have
changed to distinct peaks (Figure 6C). Although the azo form in dyes 5e and 5f in DMF is still
the dominant form, the hydrazone form could become more abundant to reach an equilibrium
with the azo form where it is hardly possible through utilization of methanol and
dichloromethane solvents. However, the main peak of 5g has shifted to a higher wavelength
(λmax= 633 nm) and a shoulder protruded lower wavelength where the azo form of 5e and 5f
exist. Therefore, the hydrazone form is the dominant form of 5g exists in DMF (Figure 6C). A
similar result also obtained for 4e, 4f and 4g, however for 4e, the stability of hydrazone form is
lower than 5e due to the lack of nitrogen atom of the quinoline group responsible of
strengthening of H-bond. The stabilization of hydrazone form in dyes 4 through intramolecular
H-bond has already been observed when –COOH was recruited as substituent at the ortho- and
para- position of the phenyl ring [49]. These results for para-nitro are consistent with those for
para-COOH azo dyes, however the absorption spectra of ortho-nitro shows two maximums in
DMF evidencing the existence of both tautomeric forms while it just exhibit one maximum peak
for the ortho-COOH derivative. Therefore, it could be concluded that the bifurcated H-bond and
the stability of hydrazone form depends on the nature of the group in ortho-position.
15

Figure 6. Absorption spectra of azo dyes 5e, 5f and 5g in DCM (A), in methanol (B) and in DMF (C).
The UV-visible spectra of dyes 5d and 4d and their para-nitro analogous (5g and 4g)
indicate for the domination of the azo form in dichloromethane solvent. The influence of nitro
group in both 5g and 4g creates a bathochromic shift about Δλ = 30±3 nm than unsubstituted
analogous (5d and 4d) (Figure 7A). In DMF, dye 5g is dominantly present in the hydrazone form
16

with a bathochromic shift (Δλ3 = 249 nm) in comparison with 5d that dominantly available in the
azo form (peak 4 in Figure 7B). Dye 4g also in DMF shows a similar spectrum with hydrazone
form more present with a bathochromic shift (Δλ2 = 156 nm) compared to 4d. Here the point is
that 4d also shows a hydrazone form being present unlike 5d (4d shows hydrazone form as
dominant tautomer, while the 5d shows dominantly azo form with Δλ1=94 nm). The smaller
peaks 1, 2 and also shoulder 3 attributes to the azo form of 5g, 4g and 4d (Figure 7B).
Figure 7. Comparison of the absorption spectra of dyes 5d and 5g with 4d and 4g in DCM (A) and DMF (B).
Compound 5i is the only dye that has similar behavior of 5g in DMF exhibiting a mixture
of both tautomers with hydrazone-form being dominantly available. However, the
tautomerization behavior in its 8HQ analogous (4i) is completely reversed where the azo-form is
the major form. Their spectra are almost symmetric to each other (Figure 8).
Introduction of an electron donor group like methoxyl (4a and 5a) and methyl (4b and
5b) in the para-position of the phenyl ring (unlike in ortho-position [81]) does not have a
specific effect on the UV-visible spectrum in these dyes, compared to the other substituents,
whereas the addition of the stronger electron donor methoxyl group in dyes 4a and 5a results in a
less stabilization of the hydrazone-form than the methyl group in dyes 4b and 5b. All the UV-
visible spectra of the synthesized compounds studied in three different solvents of
dichloromethane, methanol and DMF were presented in the Figures S94-S99.
17

Figure 8. Comparison of the absorption spectra of 4i and 5i in DMF.
3.4. Substituents effect on the absorption spectra of the dyes
The extended conjugated system within these azo dyes allows the absorption of visible
light. Therefore, the solutions of these compounds in different solvents exhibit yellow, orange
and red colors, which indicates their absorption of the light in the violet, blue and green part of
the spectrum, respectively. Moreover, these results indicate that the azo dyes tend to absorb in
the near-UV region of the visible right rather that in the near-IR region of light.
The role of electronic effects of substituents on the λmax is a well-known phenomenon in
azo dye compounds [49,75,76]. A bathochromic shift is expected when azo dyes are
functionalized
with stronger electron-acceptor substituents (Table 2). The introduction of
the nitro group as an electron-acceptor substituent in various position (ortho-, meta-, para-) of
the phenyl ring generates bathochromic shifts compared to azo derivatives which are not
functionalized with the nitro group (4d or 5d). Therefore, a minimum bathochromic shift was
observed for 4f in dichloromethane (Δλ= 9 nm) whereas a maximum bathochromic shift value
was noticed for 5g in DMF (Δλ= 249 nm). Furthermore, the bathochromic shift increases with
the existence of a nitro group at ortho- and para-position (compared to meta-) due to the
resonance effect. Similarly, introduction of an electron-donating group (for example the
methoxyl group in 4a or 5a, and the methyl group in 4b or 5b) at the para-position of phenyl
ring causes a bathochromic shift compared to unsubstituted analogous (4d or 5d): a minimum
18

bathochromic shift was observed for 4b in dichloromethane (Δλ= 1 nm) whereas a maximum
bathochromic shift value was noticed for 5b (Δλ= 19 nm). Compounds 4a and 4b show also a
hypsochromic shift of about Δλ= -80 nm and Δλ= -89 nm, respectively. These results are fairly
matching with those reported for 5-phenylazo-8-hydroxyquinoline dyes [75,76]. It is well known
that aryl azo dyes substituted with electron-acceptor groups will exhibit higher bathochromic
shifts compared to the corresponding azo dyes functionalized with electron-donor groups. The
introduction of halogen substituents to the phenyl ring (4c, 5c, 4h, 5h, 4i and 5i) results in the
appearance of an intermediate bathochromic shift (more than electron-donating groups (EDG)
and less than electron-accepting groups (EAG) following the order EAG > halogen > EDG). The
existence of two chlorine atoms on the phenyl ring in dye 5h causes a bathochromic shift
compared to 5c and 5i which are not functionalized with halogen groups. Furthermore, among
dyes 4c, 4h and 4i, which have hypsochromic shift relative to 4d, compound 4h shows a small
shift. A similar hypsochromic shifts were noticed in azo dyes with chlorine in meta-position [82],
nitro-substituted (ortho- or para-) [83] and crowding substituents at the phenyl ring [84].
However, it could be concluded that bathochromic or hypsochromic effects caused by the
addition of a substituent is highly dependent on the applied solvent [85].
In this section, the colors of solutions for the studied dyes under room temperature are
depicted in the Figure 9A-C. Almost all dyes in dichloromethane exhibit in the same color with
an exception for nitro-substituted derivatives, which show a shade of orange (Figure 9A).
Obvious color changes were observed by naked eye upon changing the solvent to methanol
(Figure 9B), but the most significant color change was noticed using as solvent DMF. Figure 9C
shows that all dyes exhibit a bathochromic shift in the UV-visible using an aprotic polar solvent
such as DMF. Indeed, Figure 9A-C provides a straightforward evidence of the solvatochromic
properties of the synthesized dyes.
19

Figure 9. Colors of the solutions for dyes (4a-i) and (5a-i) under room temperature: A) in DCM, B) in methanol, C)
in DMF (full images were provided in Figures S100-S105)
3.5. Photochromic properties
The photochromic behavior of the azo dyes 4a-i, 5a-i were studied in acetone (10-5 M)
solutions. The UV-Visible spectra of the samples were measured while irradiating with UV-vis
o
light (250-700 nm) from a 150 W ozone free xenon lamp, equipped with a water filter at 20 C,
(Table 3). The change in the visible spectra of dye 5d is depicted in Figure 10A, as an example.
The coloration rates were effectively independent on the nature of substituents present on the
benzene ring. When the irradiation stopped the absorption of the azo E-isomer increased again.
This procedure was repeated several times, and this behavior was fully reproducible indicating
that there is no degradation observed under these limited experimental conditions. The kinetics
of the coloration and discoloration process of dye 5d are shown in Figure 10B.
The irradiation of azo dyes 4a-i and 5a-i let to a fast and significant drop in the
absorption at λmax (around 380-400 nm) due to the partial conversion of the E-isomer into Z-
isomer, which absorbs below 350 nm.
20

Table 3. Spectrokinetic properties under continuous visible irradiation: maximum wavelength of absorption (λmax),
Z/E isomerization rate constant (kΔ) and half-life (t1/2) of azo dyes (4a-i) and (5a-i)
Entry
Dye
G
kΔ(min-1)
t1/2 (s)
λmax
387
383
391
382
395
386
415
399
388
393
381
392
382
395
393
420
400
387
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
p-OMe
p-Me
5-Cl-2-Me
H
o-NO2
m-NO2
p-NO2
2,5-di-Cl
3-Cl-2-Me
p-OMe
p-Me
5-Cl-2-Me
H
o-NO2
m-NO2
p-NO2
0.33
0.242
5.26
0.286
*
126
171
7.3
145
*
*
*
13.1
66
18.7
10.7
48.9
8.4
*
5.4
*
17.7
9.24
4a
4b
4c
4d
4e
4f
4g
4h
4i
5a
5b
5c
5d
5e
5f
5g
5h
5i
*
*
3.17
0.63
2.22
3.89
0.85
4.97
*
7.69
*
2.35
4.50
2,5-di-Cl
3-Cl-2-Me
* No any spectral change is noticeable upon irradiation for several minutes which suggests a very fast switching
between the E and Z forms.
The novel azo dyes 8-(aryldiazenyl)quinolin-5-ol (5a-i) were reported to have shorter
half-life values (5.4 – 48.9 s) and larger absorption wavelength range (381-420 nm) as compared
with the azo dyes 5-(aryldiazentl)quinolin-8-ol (4a-i), showing the better prospect of the former
in the application for fast molecular photoswitches. The kinetics of the thermal Z / E
isomerization (coloration, in dark condition and at 20 oC) was slower than the E / Z isomerization
but also mono-exponential which can be considered as pseudo first order reactions. The derived
pseudo first order kinetics which were presented by Arya et al [86]. The photochemical
isomerization of azo dyes involves three reactions which occur at the same time upon the
irradiation of UV-Vis-light: i.) E/Z photoisomerization, ii.) Z/E photoisomerization and iii.) Z/E
thermal isomerization (Figure 10B) [87]. A continuous irradiation will result in an E/Z
photostationary equilibrium state of azo dye. Upon the removal of irradiation (dark cycle), Z/E
thermal isomerization will occur to form the thermodynamically more stable E isomer. Though
the kinetics of E/Z and Z/E isomerization had mono-exponential trend, various studies were done
to show that they do not proceed in the manner of an elementary reaction step. Spectroscopic
studies have reported of the formation of singlet-triplet transition state during azobenzene
21

photochemical isomerization. Such formation involved the excitation of ground states to the S1
(푛 → 휋∗) and the S2 (휋 → 휋∗) states via inversion and rotation mechanisms respectively [88].
The relaxation mechanism of these excited states also displayed wavelength dependence and do
not obey Kasha’s rule, causing a change in quantum yield without any detectable side reactions
during isomerization.
Figure 10. Absorption spectra of dye 5d under visible irradiation and in a dark condition (A) and visible
irradiation/dark cycles for dye 5d (B).
The UV/Vis absorption spectra of all dyes except 4e, 4g, 5e and 5g derivatives showed
bathochromic shifts during E/Z isomerization and hypsochromic shifts during Z/E isomerization.
The nitro substituted dyes do not show any spectral change upon irradiation for several minutes
suggesting a very fast switching between the Z and E forms. The change in Z/E isomerization
rate constants of dye 4 and 5 were also noted when the type and the position of substituent are
changed. Here, the comparisons of rate constants were made with reference to the rate constant
of the unsubstituted dye (G = H). With the use of electron donating or withdrawing substituent,
the rate constants were slightly altered which indicate the polar (inductive and resonance) effects
on the Z/E isomerization. The rate constant was the most affected when a substituent of high
22

steric factor such as chlorine was introduced. The placement of these substituents at ortho (2, 6)
and para (3, 5) positions had the most significant steric effect in affect the isomerization rate
constant. The polar and steric effects of substituents on the Z/E isomerization reactivity were
analyzed using the Taft equation:
푘∆,퐺
log (
) = 휌∗휎∗ + 훿퐸푆
푘∆,퐻
where 푘∆,퐺 is the Z/E isomerization constant for G-substituted dye, 푘∆,퐻 is the Z/E isomerization
constant for unsubstituted dye, 휌∗ is the sensitivity factor of Z/E isomerization to polar effect, 휎∗
is the polar substituent constant, 훿 is the sensitivity factor of Z/E isomerization to steric effect
and 퐸푆 is the steric substituent constant. The values of 휎∗ and 퐸푆 can be obtained from the work
by Hansch et. al [89]. Based on regression analysis, the values of 휌∗ and 훿 for Z/E isomerization
were -0.462 and -0.286 for dye 4 and 1.446 and -0.286 for dye 5.
The negative 훿 values for the Z/E isomerization of both dyes indicate that the increase in
steric bulk of substituent will accelerate the Z/E isomerization rate and favor the formation of the
more stable E isomer as E isomer experiences lesser steric hindrance. As dye molecule
undergoes Z/E isomerization with the attempt of lessening steric hindrance to achieve better
stability, one would also expect that the Z/E isomerization to have a negative entropy of
activation as it forms a transition state of lower steric effect. The 휌∗ sensitivity factors were very
much different for the Z/E isomerization of both dyes. For dye 4, a negative 휌∗ denotes that its
Z/E isomerization is accelerated by electron donating effect. This effect was noted in the
aforementioned discussion to further stabilize the extended π-conjugation system in E isomer,
thus favoring the formation of E isomer. However, this polar effect is not as significant as
compared with the polar effect in dye 5 as the former also strengthened the π-bond character of
azo group in Z isomer. For dye 5, a positive 휌∗ denotes that its Z/E isomerization is accelerated
by electron withdrawing effect. In dye 5, the nitrogen position of another pyridine group is
facing the conjugated azo group (Scheme 1) to form an azo-hydrazone tautomeric equilibrium. In
hydrazone-form, the π-bond character of azo group is lesser and could be further weakened using
an electron donating substituent, thus enabling easier Z/E isomerization. The position of electron
donating substituent at position 2 (ortho-) and 3 (meta-) also further stabilizes the E isomer by
enabling the formation of bifurcated intramolecular hydrogen bonds (Scheme 3).
23

4. Conclusion
Novel substituted 8-(aryldiazenyl)quinolin-5-ol azo dyes were synthesized and
investigated in this work. Spectroscopic studies confirmed the tautomerism of these dyes in the
form of azo/hydrazone equilibrium. For nitro substituted azo dyes, bifurcated intramolecular
hydrogen bonds were formed which subsequently stabilize the hydrazone tautomer. The effect of
the substituents of different electronic nature and hydrogen bond acceptor strengths on the
azo/hydrazone and the E/Z equilibria were also investigated. These equilibrium were reported to
be mainly affected by steric effect of substituent and field effects due to hydrogen bonding and
the contribution of substituent to the extended π-conjugation azo system. Photochemical
isomerization kinetics also revealed that the novel substituted-azo dyes have higher Z/E thermal
isomerization rates and have larger absorbance wavelength range than their 8-hydroxyquinoline
analogs, making them potential molecular switches. These insights would serve as the bases for
further studies on the photochromic performance of other substituted 5-hydroxyquinoline dyes in
near future.
Acknowledgements
Hamid Rashidnejad and Dr. Nader Noroozi Pesyan want to thank Urmia University’s
higher education department for provided facilities. We thank the clinical Research Development
Unit (CRDU), Sayad Shirazi Hospital, Golestan University of Medical Sciences, Gorgan, Iran.
Mohammad Ramezani Taghartapeh acknowledges the SUPRA scholarship provided by
Swinburne University of Technology. M.M.M. Raposo would like to gratefully acknowledge the
financial support from Foundation for Science and Technology (FCT), to Centro de Química,
CQUM, under the project UID/QUI/00686/2020.
Author contributions
Hamid Rashidnejad, Mohamad Ramezanitaghartapeh, Alireza Soltani: Methodology,
Investigation, Writing-original draft. M. Manuela M. Raposo, Paulo J. Coelho, Andrew Ng Kay:
Writing - review & editing. Nader Noroozi Pesyan and Peter J. Mahon: Data curation.
24

Conflicts of Interest Statement
The authors whose names are listed immediately below certify that they have NO
affiliations with or involvement in any organization or entity with any financial
interest (such as honoraria; educational grants; participation in speakers’ bureaus;
membership, employment, consultancies, stock ownership, or other equity interest;
and expert testimony or patent-licensing arrangements), or non-financial interest
(such as personal or professional relationships, affiliations, knowledge or beliefs)
in the subject matter or materials discussed in this manuscript.
Graphical Abstract
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