Phenylazoindole dyes 3: Determination of azo-hydrazone tautomers of new phenylazoindole dyes in solution and solid state

Article abstract of DOI:10.1016/j.molstruc.2014.09.082

A new two series of phenylazo indole dyes was synthesized and the structures of the dyes were confirmed by UV-vis, FT-IR, HRMS and ¹H/¹³C NMR spectroscopic techniques. Five of these dyes (I, I′, II′, III and III′) were also characterized in solid state by using single crystal X-ray diffraction studies besides other spectroscopic techniques. The geometries of the azo and hydrazone tautomeric forms of the dyes were optimized by using Density Functional Theory (DFT). In addition, the effects of the donor and acceptor groups on the azo and hydrazone forms of the dyes were evaluated experimentally and theoretically. The results indicate that the phenylazoindole dyes derived from 2-phenyl indole as coupling component exist as azo form in solution, gas phase and solid state.

Full text of DOI:10.1016/j.molstruc.2014.09.082

Journal of Molecular Structure 1081 (2015) 175–181
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Journal of Molecular Structure
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Phenylazoindole dyes 3: Determination of azo-hydrazone tautomers
of new phenylazoindole dyes in solution and solid state
a
c
d
a,
Banu Babür a, Nurgül Seferog˘lu b, , Ebru Aktan , Tuncer Hökelek , Ertan Sßahin , Zeynel Seferog˘lu
⇑
⇑
a Gazi University, Department of Chemistry, 06500 Ankara, Turkey
b Gazi University, Advanced Technologies, 06500 Ankara, Turkey
c Hacettepe University, Department of Physics, 06800 Ankara, Turkey
d Atatürk University, Department of Chemistry, Erzurum, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Two series of new phenylazo indole
dyes were efficiently synthesized and
characterized.
ꢀ
The most stable tautomeric form of
the dyes was determined as
experimental and theoretical in
solution, gas phase and solid state.
The experimental results are
compatible with the theoretical
results.
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
A new two series of phenylazo indole dyes was synthesized and the structures of the dyes were con-
firmed by UV–vis, FT-IR, HRMS and 1H/13C NMR spectroscopic techniques. Five of these dyes (I, I0, II0,
III and III0) were also characterized in solid state by using single crystal X-ray diffraction studies besides
other spectroscopic techniques. The geometries of the azo and hydrazone tautomeric forms of the dyes
were optimized by using Density Functional Theory (DFT). In addition, the effects of the donor and accep-
tor groups on the azo and hydrazone forms of the dyes were evaluated experimentally and theoretically.
The results indicate that the phenylazoindole dyes derived from 2-phenyl indole as coupling component
exist as azo form in solution, gas phase and solid state.
Received 16 July 2014
Received in revised form 24 September
2014
Accepted 27 September 2014
Available online 16 October 2014
Keywords:
Azo dyes
Phenylazoindole dyes
Azo-hydrazone tautomers
Crystal structures
DFT calculations
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
medicinal studies, non-linear optics (NLO) and optical data storage
[1,4–7]. However, the some of dyes include azo chromophoric sys-
Azo colorants are the most widely used class of dyes because of
their synthetic facilities, good stabilities/fastnesses and large color
ranges [1–3]. They have been used as dyes owing to their versatil-
ities in various fields and high technologies which include textile
fibers, chemosensors, coloring different materials, biological-
tem release of carcinogenic amines after reductive cleavage to
environment [8]. Their optical, technical, environmental and appli-
cation performances of azo dyes are strongly related to their tauto-
meric equilibriums, molecular structures and spectroscopic
properties in solution and solid state [9–12]. Hence, the study of
azo-hydrazone tautomerism is one of the most interesting research
field in dye chemistry [13–15].
In our earlier studies, we have synthesized and determined the
tautomeric behaviors of a number of aryl/heteroarylazo indole
dyes in solution and solid state [16–22] Fig. 1.
⇑
Corresponding authors.
E-mail addresses: nurguls@gazi.edu.tr (N. Seferog˘lu), znseferoglu@gazi.edu.tr (Z.
Seferog˘lu).
http://dx.doi.org/10.1016/j.molstruc.2014.09.082
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

176
B. Babür et al. / Journal of Molecular Structure 1081 (2015) 175–181
Fig. 1. Tautomeric forms of previous synthesized hetarylazoindole dye [22].
Scheme 1. The synthetic pathway of the dyes (I–VI and I0–VI0).
In the current communication, we synthesized two series of
Computational study
phenylazoindole dyes by using traditional azo coupling between
2- or 1,2-disubstituted indoles and diazonium salts of para
substituted aniline derivatives. The first series of compounds
include 2-phenyl indole (I–VI) as coupling component can exist
in two possible tautomeric forms namely azo and hydrazone
forms. The second series include 1-methyl-2-phenyl indole (I–VI)
as coupling component dyes (I0–VI0) exist only in azo form
(Scheme 1). Thus, the dyes derived from 1-methyl-2-phenyl indole
will be used as model compound for the first series of compounds
to determine their tautomeric behaviors in solution and solid
state. This manuscript mainly includes the exploration of the
azo-hydrazone tautomerism of the dyes experimentally and
theoretically in solution and solid state. The UV–vis absorption
spectra of these dyes in different solvents were studied and the
effects of the solvents and the substituents on the azo/hydrazone
tautomeric equilibrium were also evaluated.
All quantum chemical calculations were performed using the
GAUSSIAN 09 software [23]. The ground state geometries of azo-
hydrazone tautomers of the dyes I–IV and their related model com-
pounds I0–IV0 were optimized using the Density Functional Theory
(DFT) with the B3LYP/6311+g(d,p) [24]. The frequency analyses
indicate that the optimized geometries were in a minimum of the
potential surface with no imaginary frequencies. Time Dependent
Density Functional Theory (TD-DFT) computations [25] were used
to obtain the vertical excitation energies and oscillator strengths
at the optimized ground state equilibrium geometries and at the
same hybrid functional and basis sets. All the computations in sol-
vents of different polarities were carried out using the Self-Consis-
tent Reaction Field (SCRF) under the Polarizable Continuum Model
(PCM) [26].
Crystallography analyses
Experimental section
For the crystal structure determinations, the single-crystals of
the compounds I, I0, II0, III and III0 were used for data collection
on a four-circle Rigaku R-AXIS RAPID-S diffractometer (equipped
with a two-dimensional area IP detector). Graphite-monochromat-
Materials and instrumentation
The chemical used in the syntheses of all dyes were obtained
from Aldrich Chemical Company (USA) and were used without fur-
ther purification. FT-IR spectra were recorded on a Mattson 1000
FT-IR spectrophotometer (Gazi University Department of Chemis-
ed Mo K
with w = 5° for one image were used for data collection. The lat-
tice parameters were determined by the least-squares methods on
the basis of all reflections with F2 > 2 (F2). Integration of the inten-
a radiation (k = 0.71073 Å) and oscillation scans technique
D
try, Turkey) in KBr (m
, in cmꢁ1). 1H NMR spectra were recorded
r
sities, corrections for Lorentz and polarization effects and cell
refinement were performed using CrystalClear (Rigaku/MSC Inc.,
2005) software [27]. The structures were solved by direct methods
using SHELXS-97 [27] and refined by a full-matrix least-squares
procedure using the program SHELXL-97 [28]. H atoms were posi-
tioned geometrically and refined using a riding model. The final
difference Fourier maps showed no peaks of chemical significance.
on a Bruker-Spectrospin Avance DPX 400 MHz Ultra-Shield (Hacet-
tepe University, Department of Chemistry, Turkey) in DMSO-d6
with TMS as the internal reference. Chemical shifts were expressed
in d units (ppm). Ultraviolet–visible (UV–vis) absorption spectra
were recorded on an Analytikjena Specord 200 Spectrophotometer
(Gazi University, Department of Chemistry, Turkey) at the wave-
length of maximum absorption (kmax, in nm) in range of solvents,
i.e. dimethylsulfoxide (DMSO,
e, 46.45), methanol (
e, 32.66), acetic
acid ( , 6.17) and chloroform (
e
e
, 4.81). All melting points were
Synthesis of the dyes
uncorrected and in °C (Electrothermal 9200 melting point appara-
tus). Mass analyses were done on a Waters LCT Premier XE (TOF
MS) (Gazi University Laboratories, Department of Pharmacological
Sciences).
Diazotization of various aniline derivatives was affected with
HCl and NaNO2. A typical procedure is that which is described in
below; all compounds (I–IV and I0–IV0) were prepared in a similar

B. Babür et al. / Journal of Molecular Structure 1081 (2015) 175–181
177
Scheme 2. Possible tautomeric forms of the phenylazo indole dyes in different solvents and pH.
manner (Scheme 2). The compounds were purified by recrystalliza-
tion from ethanol. Characterization data, melting points and yields
of the dyes are given in below.
154.23, 142.91, 136.11, 132.11, 131.10, 129.97, 129.72, 129.59,
129.22, 129.16, 124.74, 123.42, 123.28, 121.25, 119.45, 112.44;
HRMS (m/z): found 298.1333 (calculated: 298.1344) for
C
20H16N+3, [MꢁH]+.
General synthetic procedure
3-(4-Methoxyphenyldiazenyl)-2-phenyl-1H-indole (II)
The proper aniline or derivative (2.0 mmol) was dissolved in
hydrochloric acid (1.5 mL conc. HCl in 4 mL water). The solution
was then cooled to 0–5 °C with stirring. Sodium nitrite (0.15 g,
2.0 mmol) in water (3 mL) was gradually added to this solution
over 15 min period at 0–5 °C with stirring. The mixture was stirred
for an additional 1 h at 0–5 °C. Excess nitrous acid was destroyed
by addition of urea. Indole derivatives which is used as coupling
component (2.0 mmol) was dissolved in acetic acid/propionic acid
(7 mL, ratio 5:2) and cooled to 0–5 °C in a salt/ice bath. The cold
diazonium salt solution was added to this cooled solution over
15 min with vigorous stirring in a dropwise manner, while main-
taining the pH at 4–6 by addition of diluted NaOH solution. The
mixture was further stirred for 1 h at 0–5 °C and resulting solid
was filtered, washed with cold water, dried, and recrystallized
from ethanol.
This dye was obtained from 4-methoxyaniline and 2-phenyl-
1H-indole. Yield: 81%, m.p.: 180–182, FT-IR (KBr) mmax (cmꢁ1):
3406 (NAH), 3191 (aromatic CAH), 2920 (aliphatic CAH), 1599
(C@C), 1370 (N@N), 1232 (CAO); 1H NMR (DMSO-d6): d 12.40
(brs, NAH), 8.49 (d, 1H, J = 7.79 Hz), 8.17 (d, 2H, J = 8.41 Hz), 7.80
(d, 2H, J = 8.94 Hz), 7.60 (m, 2H), 7.51 (m, 2H), 7.30–7.20 (m, 2H),
7.11 (d, 2H); 13C NMR (DMSO-d6): 160.28, 148.49, 131.89,
129.81, 129.34, 129.18, 129.04, 125.45, 124.30, 123.35, 123.20,
122.88, 119.69, 114.92, 112.66, 55.91; HRMS (m/z): found
328.1450 (calculated: 328.1450) for C21H18N3O+, [MꢁH]+.
3-(4-Methylphenyldiazenyl)-2-phenyl-1H-indole (III)
This dye was obtained from 4-methylaniline and 2-phenyl-1H-
indole. Yield: 74%, m.p.: 152–153, FT-IR (KBr) mmax (cmꢁ1): 3395
(NAH), 3069 (aromatic CAH), 2971–2925 (aliphatic CAH), 1598
(C@C), 1385 (N@N); 1H NMR (DMSO-d6): d 12.33 (brs, NAH),
8.49 (d, 1H, J = 7.79 Hz), 8.16 (d, 2H, J = 7.23 Hz), 7.73 (d, 2H,
J = 7.03 Hz), 7.62 (t, 2H, J = 7.43 Hz), 7.54–7.50 (m, 2H), 7.38 (d,
2H, J = 7.36 Hz), 7.34–7.23 (m, 2H); 13C NMR (DMSO-d6): 152.25,
142.25, 139.03, 136.44, 131.94, 131.16, 130.25, 129.87, 129.49,
129.34, 129.14, 125.44, 124.65, 123.27, 121.90, 119.48, 112.35;
HRMS (m/z): found 312.1502 (calculated: 312.1501) for C21H18N+3,
[MꢁH]+.
3-(Phenyldiazenyl)-2-phenyl-1H-indole (I)
This dye was obtained from aniline and 2-phenyl-1H-indole.
Yield: 78%, m.p.: 159–160, FT-IR (KBr) mmax (cmꢁ1): 3406 (NAH),
3055 (aromatic CAH), 2925 (aliphatic CAH), 1490 (C@C), 1389
(N@N); 1H NMR (DMSO-d6): d 12.40 (brs, NAH), 8.51 (d, 1H,
J = 7.79 Hz), 8.17 (d, 2H, J = 7.91 Hz), 7.82 (d, 2H, J = 8.08 Hz), 7.62
(m, 2H), 7.55 (m, 4H), 7.43–7.25 (m, 3H); 13C NMR (DMSO-d6):

178
B. Babür et al. / Journal of Molecular Structure 1081 (2015) 175–181
3-(4-Chlorophenyldiazenyl)-2-phenyl-1H-indole (IV)
1382 (N@N); 1H NMR (DMSO-d6): d 8.52 (d, 1H, J = 7.76 Hz), 7.75
(d, 2H, J = 7.52 Hz), 7.69–7.57 (m, 6H), 7.40 (t, 1H, J = 7.17 Hz)
7.35–7.27 (m, 3H), 3.81 (s, 3H, ANACH3), 2.32 (s, 3H, ACH3); 13C
NMR (DMSO-d6): 152.08, 146.39, 138.88, 137.55, 132.00, 130.10,
129.61, 128.60, 124.55, 123.65, 122.90, 121.74, 118.39, 111.18,
95.69, 32.04, 21.34; HRMS (m/z): found 326.1652 (calculated:
326.1657) for C22H20N+3, [MꢁH]+.
This dye was obtained from 4-chloroaniline and 2-phenyl-1H-
indole. Yield: 73%, m.p.: 153–155, IR (KBr) mmax (cmꢁ1): 3413
(NAH), 3061 (aromatic CAH), 1476 (C@C), 1372 (N@N); 1H NMR
(DMSO-d6): d 11.65 (brs, NAH), 8.37 (d, 1H, J = 7.54 Hz), 7.94 (d,
2H, J = 7.39 Hz), 7.60 (d, 2H, J = 8.47 Hz), 7.38–7.20 (m, 6H), 7.12–
7.04 (m, 2H); 13C NMR (DMSO-d6): 152.84, 143.47, 136.45,
133.37, 132.14, 130.85, 130.02, 129.79, 129.21, 124.94, 123.62,
123.51, 123.29, 119.33, 112.45; HRMS (m/z): found 332.0949 (cal-
culated: 332.0955) for C20H15ClN+3, [MꢁH]+.
3-(4-Chlorophenyldiazenyl)-1-methyl-2-phenyl-1H-indole (IV0)
This dye was obtained from 4-chloroaniline and 1-methyl-2-
phenyl-1H-indole. Yield: 72%, m.p.: 169–170, FT-IR (KBr) mmax
(cmꢁ1): 3012 (aromatic CAH), 2987 (aliphatic CAH), 1466 (C@C),
1375 (N@N); 1H NMR (DMSO-d6): d 8.52 (d, 1H, J = 7.65 Hz), 7.63
(m, 5H), 7.52 (m, 2H), 7.43 (d, 1H, J = 7.90 Hz), 7.35–7.24 (m, 4H),
3.80 (s, 3H, ANACH3); 13C NMR (DMSO-d6): 152.21, 146.97,
137.16, 132.72, 131.80, 131.50, 129.28, 129.16, 128.87, 128.15,
124.30, 123.50, 122.81, 122.42, 117.78, 110.82, 31.62; HRMS (m/
z): found 346.1111 (calculated: 346.1111) for C21H17ClN+3, [MꢁH]+.
3-(4-Cyanophenyldiazenyl)-2-phenyl-1H-indole (V)
This dye was obtained from 4-cyanoaniline and 2-phenyl-1H-
indole. Yield: 71%, m.p.: 293–294, FT-IR (KBr) mmax (cmꢁ1): 3293
(NAH), 3052 (aromatic CAH), 2227 (C„N), 1595 (C@C), 1364
(N@N); 1H NMR (DMSO-d6): d 12.60 (brs, NAH), 8.50 (d, 1H,
J = 7.03 Hz), 8.16 (d, 2H, J = 7.44 Hz), 7.98 (d, 2H, J = 8.46 Hz), 7.92
(d, 2H, J = 8.56 Hz), 7.63 (t, 2H, J = 7.30 Hz), 7.56 (t, 2H,
J = 7.97 Hz), 7.33 (m, 2H); 13C NMR (DMSO-d6): 156.65, 145.26,
136.62, 134.09, 132.96, 130.52, 130.24, 130.15, 129.27, 125.33,
124.12, 123.42, 122.56, 119.46, 119.17, 112.64, 110.58; HRMS
3-(4-Cyanophenyldiazenyl)-1-methyl-2-phenyl-1H-indole (V0)
This dye was obtained from 4-cyanoaniline and 1-methyl-2-
phenyl-1H-indole. Yield: 74%, m.p.: 257–258, FT-IR (KBr) mmax
(cmꢁ1): 3022 (aromatic CAH), 2928 (aliphatic CAH), 2222
(C„N), 1469 (C@C), 1386 (N@N); 1H NMR (DMSO-d6): d 8.55 (d,
1H, J = 7.68 Hz), 7.93 (d, 2H, J = 8.40 Hz), 7.78 (d, 2H, J = 8.30 Hz),
7.73 (d, 2H, J = 8.04 Hz) 7.67–7.60 (m, 4H), 7.46 (t, 1H,
J = 7.18 Hz), 7.40 (t, 1H, J = 7.21 Hz), 3.85 (s, 3H, ANACH3); 13C
NMR (DMSO-d6): 156.53, 149.10, 137.87, 133.97, 133.12, 132.03,
130.04, 129.06, 128.69, 125.18, 124.52, 123.04, 122.34, 119.44,
118.13, 111.55, 110.44, 32.29; HRMS (m/z): found 337.1458 (calcu-
lated: 337.1453) for C22H17N+4, [MꢁH]+.
(m/z): found 323.1294 (calculated: 323.1297) for
C
21H15N+,
[MꢁH]+.
3-(4-Nitrophenyldiazenyl)-2-phenyl-1H-indole (VI)
This dye was obtained from 4-nitroaniline and 2-phenyl-1H-
indole. Yield: 70%, m.p.: 290–291, IR (KBr) mmax (cmꢁ1): 3415
(NAH), 3039 (aromatic CAH), 1600–1530 (C@C), 1376 (N@N);
1H NMR (DMSO-d6):
d
12.70 (brs, NAH), 8.53 (d, 1H,
J = 7.64 Hz), 8.37 (d, 2H, J = 8.91 Hz), 8.17 (d, 2H, J = 7.47 Hz),
7.97 (d, 2H, J = 8.91 Hz), 7.67–7.52 (m, 4H), 7.38 (t, 1H,
J = 6.96 Hz), 7.33 (t, 1H, J = 7.17 Hz); 13C NMR (DMSO-d6):
158.16, 146.67, 145.99, 136.84, 130.48, 130.31, 130.25, 129.28,
125.51, 125.46, 124.28, 123.48, 122.51, 119.17, 112.77; HRMS
(m/z): found 343.1198 (calculated: 343.1195) for C20H15N4O+,
[MꢁH]+.
3-(4-Nitrophenyldiazenyl)-1-methyl-2-phenyl-1H-indole (VI0)
This dye was obtained from 4-nitroaniline and 1-methyl-2-phe-
nyl-1H-indole. Yield: 70%, m.p.: 235–237, FT-IR (KBr) m
max (cmꢁ1):
3045 (aromatic CAH), 2940 (aliphatic CAH), 1512 (C@C), 1380
(N@N); 1H NMR (DMSO-d6): d 8.55 (d, 1H, J = 7.55 Hz), 8.31 (d,
2H, J = 8.83 Hz), 7.85 (d, 2H, J = 8.84 Hz), 7.84–7.75 (m, 2H), 7.73
(d, 1H, J = 8.01 Hz), 7.70–7.63 (m, 3H), 7.45 (t, 1H, J = 7.29 Hz),
7.42 (t, 1H, J = 7.17 Hz), 3.90 (s, 3H, ANACH3); 13C NMR (DMSO-
d6): 158.01, 149.70, 146.69, 137.96, 133.59, 132.07, 130.16,
128.95, 128.74, 125.48, 125.36, 124.75, 123.09, 122.35, 118.11,
111.66, 32.39; HRMS (m/z): found 357.1352 (calculated:
357.1352) for C21H17N4O+2, [MꢁH]+.
3-(Phenyldiazenyl)-1-methyl-2-phenyl-1H-indole (I0)
This dye was obtained from aniline and 1-methyl-2-phenyl-
1H-indole. Yield: 81%, m.p.: 136–137, FT-IR (KBr) mmax (cmꢁ1):
3052 (aromatic CAH), 2928 (aliphatic CAH), 1472 (C@C), 1383
(N@N); 1H NMR (DMSO-d6): d 8.53 (d, 1H, J = 7.43 Hz), 7.76 (d,
2H, J = 6.69 Hz), 7.55–7.68 (m, 6H), 7.30–7.50 (m, 5H), 3.85 (s,
3H, ANACH3); 13C NMR (DMSO-d6): 154.04, 146.94, 137.59,
132.17, 132.01, 129.67, 129.58, 129.51, 129.13, 128.61,
124.65, 123.81, 122.89, 121.78, 118.33, 111.22, 32.05; HRMS
(m/z): found 312.1490 (calculated: 312.1501) for
C
21H18N+3,
Results and discussions
[MꢁH]+.
Spectral characterizations
3-(4-Methoxyphenyldiazenyl)-1-methyl-2-phenyl-1H-indole (II0)
This dye was obtained from 4-methoxyaniline and 1-methyl-2-
phenyl-1H-indole. Yield: 84%, m.p.: 165–167, FT-IR (KBr) mmax
(cmꢁ1): 3063 (aromatic CAH), 2975 (aliphatic CAH), 1478 (C@C),
1378 (N@N), 1252 (CAO); 1H NMR (DMSO-d6): d 8.52 (d, 1H,
J = 7.88 Hz), 7.75 (dd, 2H, J1 = 8.19 Hz, J2 = 1.20 Hz), 7.68–7.58 (m,
6H), 7.40 (td, 1H J1 = 6.96 Hz, J2 = 1.25 Hz), 7.32 (t, 1H,
J = 8.32 Hz), 7.05 (d, 2H, J = 7.08 Hz), 3.84 (s, 3H, AOCH3), 3.83 (s,
3H, ANACH3); 13C NMR (DMSO-d6): 160.36, 148.24, 145.71,
137.51, 131.98, 129.74, 129.48, 128.58, 124.43, 123.44, 123.29,
122.91, 118.47, 114.80, 111.09, 55.88, 31.98; HRMS (m/z): found
342.1595 (calculated: 342.1606) for C22H20N3O+, [MꢁH]+.
The phenylazoindole dyes were synthesized by coupling
2-phenylindole and 1-methyl-2-phenyl indole with aniline and
4-substituted diazotized aniline derivatives (Scheme 1). The
structure of the synthesized dyes (I–VI and I0–VI0) was confirmed
by FT-IR, 1H/13C NMR and HRMS. Five of these dyes (I, I0, II0, III
and III0) were also characterized by using single crystal X-ray
diffraction techniques. Molecular structures of the synthesized
dyes were given in Table 1.
The determination of azo-hydrazone tautomerism is the most
interesting research field because both tautomeric structures have
different technical, biological and photophysical properties [3].
Thus, it is necessary to determine the most tautomeric structure
for all newly synthesized azo dyes. The stability of tautomers
depends on environmental conditions such as solvent, acid/base
and heat. The quantitative and qualitative evaluation of the azo-
hydrazone tautomeric equilibria can be done by UV–vis and NMR
3-(4-Methylphenyldiazenyl)-1-methyl-2-phenyl-1H-indole (III0)
This dye was obtained from 4-methylaniline and 1-methyl-2-
phenyl-1H-indole. Yield: 82%, m.p.: 122–124, FT-IR (KBr) mmax
(cmꢁ1): 3040 (aromatic CAH), 2920 (aliphatic CAH), 1467 (C@C),

B. Babür et al. / Journal of Molecular Structure 1081 (2015) 175–181
179
13C NMR spectra for dyes (I–VI). Detailed spectral data are given
in the Experimental section.
Table 1
Structures of dyes.
In addition to NMR results, to determination of the most tauto-
meric form of dyes (I–VI), the UV–vis absorption spectra of synthe-
sized dyes in various solvents with different dielectric constants
are evaluated and results are reported in Table 2. The absorption
maxima (kmax) values can be directly proportional to polarity of
solvents and the electronic effect of the substituents in diazo part
[38]. To investigate of solvatochromic behaviors of the dyes, the
absorption spectra of phenylazoindole dyes (I–VI) were measured
Compound number
R1
R2
I
II
III
IV
V
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
in various solvents at a concentration of approximately 2 ꢃ 10ꢁ5
,
OCH3
CH3
Cl
CN
NO2
H
OCH3
CH3
Cl
CN
NO2
and were compared with those of the corresponding model
compounds (I0–VI0) containing the 1-methyl-2-phenylindole
(Table 1).
VI
I0
The synthesized phenylazoindole dyes (I–VI) may be theoreti-
cally involved in azo-hydrazone tautomers (Scheme 2). Thus, the
role of the azo-hydrazone tautomerism in relation to the visible
absorption spectra and solvatochromic behavior of the dyes have
been investigated. It is obtained that the absorption maxima of
the dyes have correlations with the solvent polarities and small
bathochromic shifts were observed with the increasing polarity
of the solvents used. The visible absorption spectra of the prepared
dyes showed one absorption maximum in all solvents used with
the exception of the dyes V and VI in DMSO. It is known that
two maxima are observed for compounds exhibiting azo-hydra-
zone tautomeric equilibria or anion.
In addition, when a comparison between kmax values of the dyes
(I–VI) and their corresponding model compounds (I0–VI0), the dyes
are in favor of the predominantly azo form except V and VI in all
solvents used. The small hypsochromic and bathochromic shifts
in kmax of the dyes in these solvents are due to solute–solvent
interactions.
The effects of the substituents on the absorption maxima can
also be found from Table 2. The introduction of the cyano and nitro
groups in para position into the diazo component results in large
bathochromic shifts in all solvents used (for dye VI kmax = 65 nm
in DMSO, kmax = 51 nm in methanol, kmax = 48 nm in acetic acid,
kmax = 52 nm in chloroform relative to kmax of dye I in the same sol-
vents). This bathochromic shift is attributed to the stronger elec-
tron-withdrawing character of nitro and cyano groups. The both
groups enhance electron delocalization from indole to phen-
yldiazenyl part in the dye molecule.
II0
III0
IV0
V0
VI0
spectroscopy in solution. However, these methods have some dis-
advantages. In the case of UV–vis spectroscopy, equilibrium con-
stant (KT) cannot generally be obtained directly because the
molar extinction coefficient of the individual tautomers cannot
be determined, due to overlapping peaks of the two forms
[29,30]. Also, 1H NMR spectroscopy is not suitable for determina-
tion of azo-hydrazone tautomeric equilibria and equilibrium con-
stant (KT). Because the equilibrium between azo and hydrazone
tautomers is rapidly established on the NMR time scale [30,31].
However, 15N, 14N and 13C NMR chemical shift data can readily
be employed to study quantitatively the azo-hydrazone tautomeric
equilibrium [32–35]. Furthermore these methods are not
employed for all azo dyes. Among the different methods, UV–vis
and NMR spectroscopy are still the best methods for qualitative
studies of tautomeric behavior in solutions. Also, the useful system
for determination of the azo-hydrazone tautomerism is to use
model compounds [16–22,36].
Hydrazide protons in hydrazone tautomer can easily be identi-
fied by the presence of one broad singlet in the 1H NMR spectrum.
Generally they are observed at ꢂ14–15 ppm [35,37]. In this study,
the 1H NMR spectra of dyes (I–VI) showed the ANH peaks in the
range of 11.65–12.70 ppm in DMSO-d6. Thus the observed values
for ANH peaks clearly showed that the dyes I–VI can exist as azo
form in DMSO. In addition, imine carbons could not observed in
The kmax values of the dyes V and VI peaks at 431, 522 nm and
465, 600 nm showed respectively, which indicate the existence of
both azo as well as hydrazone forms in DMSO. The peaks at 431
and 465 nm were attributed to the azo form because for their
Table 2
Influences of solvents and pH on kmax (nm) of dyes (I–IV) and (I0–VI0).
Compound number
DMSO
Methanol
Methanol + HCl
Methanol + KOH
Acetic acid
Chloroform
kmax (nm)
log eb
kmax (nm)
log eb
kmax (nm)
kmax (nm)
kmax (nm)
log eb
kmax (nm)
log eb
I
II
III
IV
V
400
405
401
412
4.76
4.75
4.73
4.75
4.76
4.78
4.70
4.73
4.74
4.78
4.78
4.70
394
396
393
400
421
445
386
390
393
395
414
436
4.61
4.70
4.76
4.59
4.79
4.76
4.64
4.74
4.71
4.72
4.65
4.78
461
393
396
393
400
393
396
394
400
419
441
386
391
387
396
414
434
4.69
4.78
4.72
4.74
4.73
4.79
4.70
4.67
4.72
4.79
4.67
4.75
390
394
391
398
419
442
387
393
392
395
415
441
4.66
4.72
4.76
4.75
4.75
4.78
4.78
4.72
4.73
4.75
4.76
4.75
401,493(s)a
400,472(s)a
410,451(s)a
427
431,522(s)a
465,600
392
422
VI
I0
451
456
493
473
449,552(s)a
384
II0
III0
IV0
V0
VI0
397
401
402
422
390
395
394
414
397,451(s)a
432
454
441
435
a
s: shoulder.
e
b
in cmꢁ1 Mꢁ1
.

180
B. Babür et al. / Journal of Molecular Structure 1081 (2015) 175–181
model compounds (V0 and VI0) was observed similar kmax values
422 and 454 nm which exist only azo form.
Theoretical study
To provide conclusive evidence for the tautomeric structure of
the compounds, the effects of acid and base on their absorption
spectra were also investigated. For this purpose, the spectra of
the dyes were recorded in acidic and basic media. The protonation
and deprotonation of the dyes can lead to the common cation and
anion (Scheme 2). It was observed that the absorption curves of the
dyes were not sensitive to base, except dye VI when various
amounts of 0.1 M KOH was added to their methanolic solutions
(Table 2). This result indicates that dyes I–V exist in one tautomeric
form in alkaline media and there are no dissociated species. As
shown in Table 2, when introduction of strong electron withdraw-
ing group such as nitro in diazocomponent, a new shoulder was
appeared at longer wavelength (552 nm) with the addition of
0.1 M KOH to methanolic solution of the dye VI. This indicated that
this dye exists in a partly dissociated state in methanolic KOH
solutions.
The most relevant geometrical parameters of azo and hydra-
zone forms of the dyes I–VI and model compounds (dyes I0–VI0),
which can be only in the azo form are given in Table S1 (Supporting
Information, page S2). For the dyes I–IV, the NAN bond lengths are
between 1.265 and 1.270 Å, in accordance with their model com-
pounds I0–IV0 in the azo form and between 1.316 and 1.329 Å in
the hydrazone form. However, the X-ray data show that they are
double bonds which characterize the azo form.
The values of N1AC8 and adjacent N2AC7 single bonds for the
azo form are compatible with the X-ray single-crystal analyses
results, which show that the dyes I, I0, II0, III and III0 are in the
azo form Fig. 2. However, the results show that the optimized
structures of the molecules are all nearly planar. There is no X-
ray structures for the dyes II, IV–IV0, V–V0, VI–VI0 to compare, we
shall consider the stabilities of the azo and hydrazone forms of
the dyes. On the other hand, it is seen that there is no effects of
the electron-withdrawing (IV–IV0, V–V0 and VI–VI0) and electron-
donor (III–III0 and II–II0) substituents at para position of the phenyl
rings on the geometrical structures with respect to dyes I–I0.
To compare the stabilities of the azo and hydrazone forms of the
dyes, each form of the dyes are optimized in gas-phase and in dif-
The kmax values of some dyes showed large bathochromic shifts
when various amounts of 0.1 M HCl were added to their methano-
lic solutions except dyes V, VI, V0 and VI0 containing the electron-
withdrawing substituents (Table 2). It showed that the dyes
observed at longer wavelengths exist in azonium tautomer, which
contains a delocalized positive charge (Scheme 2).
ferent solvents having different dielectric constants (e) such as
Dye I
Dye I’
Dye III
Dye III’
Dye II’
Fig. 2. Molecular structures of the dyes I, I0, II0, III and III0. Thermal ellipsoids are drawn at the 40% probability level.

B. Babür et al. / Journal of Molecular Structure 1081 (2015) 175–181
181
DMSO (
and chloroform (
Et) and Gibbs free energies (
I–VI in their azo and hydrazone forms (they are given in
Table S2, Supporting Information, page S3) for gas phase and for
different solvents. On the basis of these results, the azo tautomer
is more stable than hydrazone tautomer for the dyes I–VI in gas
phase and in solution. However, the stability of the azo tautomer
slightly increases from DMSO to chloroform with decreasing
dielectric constant.
The energy analyses for stabilities of the dyes show that the azo
form is more stable than the hydrazone form both in gas phase and
solution [17]. In our continuation study, the electronic spectra of
the compounds in azo and hydrazone forms were also obtained
in the framework of TD-DFT calculations with B3LYP using 6-
311+g(d,p) basis set to support the assignment of absorption bands
for azo and hydrazone forms. In these calculations, different sol-
vents were used to evaluate the solvent effects on maximum
absorption wavelengths for each molecule. For azo and hydrazone
forms of dyes I–VI and their related model compounds I0–VI0, the
maximum wavelengths (kmax), oscillator strengths (f) and major
contributions of the calculated transitions in different solutions
are summarized in Table S3 (Supporting Information, pages S4
and S5).
The obtained results indicate that the calculated kmax values are
higher than the corresponding values in the experimental data. The
differences between calculated and experimental values change
from 5 nm for the azo tautomeric form of dye I to 65 nm for dye
VI0 in methanol in relative to experimental data. However, the
absorption bands for dyes I–VI (azo form) and dyes I0–VI0 arise
excitations from HOMO to LUMO in solvents used.
On the other hand, it is obtained that the calculated kmax values
and the corresponding oscillator strengths values did not change
significantly in all the employed solvents and did not correlate
with the polarity of the solvent for each dye. As the polarity
increases from chloroform to DMSO, the biggest differences are
23 nm for azo form of dye VI and 13 nm for dye VI0 according to
the experimental results (Table 2).
The effects of the substituents on the absorption maxima could
also be found in Table 2. It is seen that the kmax values of the azo
tautomeric form showed a shift to longer wavelengths for the dyes
V, VI, V0, VI0 including the electron-withdrawing substituents and
no significant changes for dyes II, II0, III, III0 as compared to the
dyes I and I0. The obtained results are compatible with the experi-
mental and theoretical data.
e
= 46.45 D), methanol (
= 4.81 D). The calculated relative total energies
G) and enthalpies ( H) of dyes
e = 33.1 D), acetic acid (e = 6.17 D)
Appendix A. Supplementary material
e
The detailed spectroscopic data (copies of 1H/13C NMR and UV–
vis spectra for all new compounds) including crystallographic data
(CIF) of all new compounds are available. Crystallographic data
(excluding structure factors) have been deposited with the Cam-
bridge Crystallographic data Centre as the supplementary publica-
tion Nos. CCDC-988708 (for I), CCDC-990250 (for I0), CCDC-990618
(for II0), CCDC-990613 (for III), CCDC-990904 (for III0). These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.molstruc.2014.09.082.
(D
D
D
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Acknowledgement
The theoretical part of this work is supported by Gazi University
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