New heterocyclic green, blue and orange dyes from indazole: Synthesis, tautomerism, alkylation studies, spectroscopic characterization and DFT/TD-DFT calculations

Article abstract of DOI:10.1016/j.molstruc.2016.04.078

Tautomerism and alkylation studies on the green intermediate 2-(5-hydroxyimino-1-methyl-4,5-dihydro-1H-4-indazolyliden)-2-phenylacetonitrile led to the synthesis of new heterocyclic green, blue and orange dyes in high yields. The structures of all newly s

Full text of DOI:10.1016/j.molstruc.2016.04.078

Accepted Manuscript
New heterocyclic green, blue and orange dyes from indazole: Synthesis,
tautomerism, alkylation studies, spectroscopic characterization and DFT/TD-DFT
calculations
Soodabeh Poorhaji, Mehdi Pordel, Shirin Ramezani
PII:
S0022-2860(16)30414-8
10.1016/j.molstruc.2016.04.078
MOLSTR 22492
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 November 2015
Revised Date: 1 April 2016
Accepted Date: 25 April 2016
Please cite this article as: S. Poorhaji, M. Pordel, S. Ramezani, New heterocyclic green, blue
and orange dyes from indazole: Synthesis, tautomerism, alkylation studies, spectroscopic
characterization and DFT/TD-DFT calculations, Journal of Molecular Structure (2016), doi: 10.1016/
j.molstruc.2016.04.078.
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Graphical Abstract

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New heterocyclic green, blue and orange dyes from indazole: Synthesis,
tautomerism, alkylation studies,–spectroscopic characterization and DFT/TD-
DFT calculations
Soodabeh Poorhaji, Mehdi Pordel* and Shirin Ramezani
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
*Corresponding author. Tel.: +98 0511 8414182; fax: +98 0511 8424020. E-mail:
mehdipordel58@mshdiau.ac.ir
Abstract— Tautomerism and alkylation studies on the green intermediate 2-(5-hydroxyimino-1-
methyl-4,5-dihydro-1H-4-indazolyliden)-2-phenylacetonitrile led to the synthesis of new
heterocyclic green, blue and orange dyes in high yields. The structures of all newly synthesized
compounds were confirmed by spectral and analytical data. The optical properties of the dyes
were spectrally characterized by using a UV-vis spectrophotometer and results show that they
exhibited interesting photophysical properties. Solvent effects on the absorption spectra of these
dyes have been studied and the absorption band in polar solvents undergoes a red shift. Density
functional theory calculations of the dyes were performed to provide the optimized geometries
and relevant frontier orbitals. Calculated electronic absorption spectra were also obtained by
time-dependent density functional theory method.
Keywords: 5-Nitro-1H-indazole; Heterocyclic dyes; Optical properties; Density function theory
calculations; Tautomerism
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1. Introduction
Donor–acceptor (D–A) heterocyclic dyes are among the most important conjugated organic
materials, and have attracted much academic and technological research interest. In these
compounds the electron-donating and electron-accepting groups are connected through a π-
conjugated linker. Tuning different donor moiety or acceptor moiety in a D–A molecule would
modify its physical and chemical properties. They have been evaluated and employed in the new
areas of information-recording materials [1], information-display media [2], or optoelectronic
devices [3]. These dyes can also be applied to organic photoconductors [4], solar-energy
utilizations [5], sensitizers [6], biomedical probe [7], photo-catalysts [8], and so on. Furthermore,
donor–acceptor heterocyclic dyes have been used extensively in the preparation of disperse dyes
with outstanding dischargeability on cellulose acetate. These dyes are characterized also by
having generally excellent brightness and high extinction coefficients. These new trends of dye
chemistry have been recently developed and these classes of dyes are of significant importance
in high-technology industries.
On the other hand, indazole scaffolds show interesting biological properties, such as anti-
depressant [9], anti-inflammatory [10], anti-tumor [11] and anti-HIV activities [12]. Recently
indazole nucleus become of interest as a key moiety of dyes [13] and fluorescent compounds
[14–17].
In continuation of our previous study on the synthesis of new green dyes from indazole [13], in
current work, we have synthesized some new green, blue and orange dyes via alkylation and
oxidation studies on the green intermediate 2-(5-hydroxyimino-1-methyl-4,5-dihydro-1H-4-
indazolyliden)-2-phenylacetonitrile in high yields. In addition, tautomerism, optical properties
and DFT calculations of the dyes have also been examined.
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2. Experimental
2.1. Materials
Methanol, ethyl acetate (EtOAc), tetrahydrofuran (THF), acetonitrile, dichloromethane (DCM),
N,N-dimethylformamide (DMF), methyl iodide, dimethyl sulfate (DMS), potassium tert-
butoxide, tert-butanol and benzyl cyanide were purchased from Merck. Potassium hydroxide was
purchased from Sigma-Aldrich. All solvents were dried according to standard procedures.
Compounds 1 and 3 were synthesized as in literature [18, 13].
2.2. Equipment
Absorption spectra were recorded on a Varian Cary 50-bio UV–visible spectrophotometer. UV–
vis scans were recorded from 200 to 800 nm. Melting points were measured on an
Electrothermaltype-9100 melting-point apparatus. The IR (as KBr discs) spectra were obtained
on a Tensor 27 spectrometer and only noteworthy absorptions are listed. The 13C NMR (100
MHz) and the 1H NMR (400 MHz) spectra were recorded on a Bruker Avance DRX-400
Fourier-transformer spectrometer in DMSO-d6 and CDCl3. Chemical shifts are reported in parts
per million downfield from TMS as the internal standard; coupling constant J is given in hertz.
The mass spectra were recorded on a Varian Mat, CH-7 at 70 eV. Elemental analysis was
performed on a Thermo Finnigan Flash EA microanalyzer. All measurements were carried out at
room temperature.
2.3. Computational methods
DFT calculations have been performed with the Gaussian 98 software package [19] by using the
B3LYP hybrid functional [20] and the 6-311++G (d,p) basis set. Firstly, geometry of the
compounds 3, 4 and 5 was fully optimized in the MeOH solution.
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Here, one of self-consistent reaction field methods, the sophisticated Polarized Continuum
Model (PCM) [21] has been used for investigation of the solvent effects. The PCM calculations
have been performed in the MeOH solution and the zero-point corrections were considered to
obtain energies. Based on the optimized geometries and using time-dependent density functional
theory (TD-DFT) [22–24] methods, the electronic spectra of the compounds 3, 4 and 5 were
predicted.
2.4. Synthesis of 2-(1-methyl-5-nitroso-1H-indazol-4-yl)-2-phenylacetonitrile (4).
Compound 3 (5.0 g, 18 mmol) was heated 4 hr under reflux in EtOAC or other solvents such as
CHCl3, MeCN and MeOH (80 mL). Also, dye 3 converts to 4 under the influence of visible light
after 2 days in above mentioned solvents at rt. Furthermore, the process can catalyze in acidic
media (pH 2-6, H2O-MeOH, 95:5) and takes place in less than 4 hours.
After concentration the blue solution at reduced pressure, the precipitate was collected by
filtration, washed with water and then air dried to give dark blue powder 4. More purification
was achieved by recrystallization from acetone.
Yield (90%), mp 135-137 °C; 1H NMR (DMSO-d6) δ 4.21 (s, 3H, N-CH3), 6.54 (s, 1H, benzylic
CH), 7.24–7.30 (m, 3H, Ar H), 7.33–7.37 (m, 2H, Ar H), 7.48 (d, J=9.5 Hz, 1H, Ar H), 8.53 (d,
J=9.5 Hz, 1H, Ar H), 8.65 (s, 1H, Ar H);13C NMR (DMSO-d6): δ 35.6, 36.7, 107.3, 112.3,
125.8, 127.9, 131.0, 133.2, 133.8, 134.6, 143.5, 149.4, 151.0, 167.2; IR (KBr): 2340 cm-1 (CN),
1535 cm-1 (N=O). MS (m/z) 276 (M+). Anal. Calcd for C16H12N4O (276.3): C, 69.55; H, 4.38; N,
20.28. Found: C, 69.27; H, 4.36; N, 20.07.
2.5. Synthesis of 2-(1-methyl-5-nitro-1H-indazol-4-yl)-2-phenylacetonitrile (5).
To a stirred solution of dye 4 (2.76 g, 10 mmol) in MeOH (10.0 mL) were added 30% H2O2 (5.5
mL, 21 mmol) at 60 °C. After the 4 h, the orange solution was diluted with H2O (50 mL), chilled
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and filtered, and then the solid was washed with H2O and air dried to give crude 5. Dye 5 was
recrystallized from MeOH. Yield (85%), mp 156-159 °C; 1H NMR (DMSO-d6) δ 4.23 (s, 3H, N-
CH3), 6.61 (s, 1H, benzylic CH), 7.26–7.34 (m, 3H, Ar H), 7.37–7.40 (m, 2H, Ar H), 7.43 (d,
J=9.5 Hz, 1H, Ar H), 8.61 (s, 1H, Ar H), 8.69 (d, J=9.5 Hz, 1H, Ar H);13C NMR (DMSO-d6): δ
37.1, 39.6, 105.6, 110.1, 125.7, 125.9, 131.6, 133.5, 133.9, 134.2, 142.4, 149.4, 151.3, 167.6; IR
(KBr): 2345 cm-1 (CN), 1335, 1545 cm-1 (NO2). MS (m/z) 292 (M+). Anal. Calcd for C16H12N4O2
(292.3): C, 69.75; H, 4.14; N, 19.17. Found: C, 69.52; H, 4.11; N, 18.95.
2.6. Synthesis of 5-(methoxyimino)-1-methyl-1H-indazol-4(5H)-ylidene)-2-
phenylacetonitrile (6).
To a green solution of dye 3 (1.4 g, 5 mmol) in acetonitrile (50 mL), dimethyl sulfate (DMS) (0.9
g, 7 mmol) and K2CO3 (5.5 g, 40 mmol) were added. The mixture was stirred for 12 h at rt and
then poured into water. The product was extracted with CH2Cl2 (2×50 mL). The extract was
dried (MgSO4), treated with charcoal and evaporated to give crude 6. More purification was
achieved by recrystallization from MeOH-H2O (1:1).
Yield (75%), mp 125-127 °C; 1H NMR (CDCl3) δ 4.21 (s, 3H, N-CH3), 4.31 (s, 3H, O-CH3),
7.33–7.37 (m, 2H, Ar H), 7.48 (d, J=9.5 Hz, 1H, Ar H), 7.55 (d, J=9.5 Hz, 1H, Ar H), 7.59–7.61
(m, 2H, Ar H), 7.79 (t, J=8.0 Hz, 1H, Ar H), 8.21 (s, 1H, Ar H); 13C NMR (DMSO-d6): δ 36.3,
65.7, 108.8, 114.2, 121.6, 122.6, 125.0, 128.1,129.7, 130.8, 131.2, 135. 4, 145. 2, 153.5, 162.0;
IR (KBr): 2220 cm-1 (CN). MS (m/z) 290 (M+). Anal. Calcd for C17H14N4O (290.3): C, 70.33; H,
4.86; N, 19.30. Found: C, 70.19; H, 4.83; N, 19.11.
2.7. Synthesis of 2-(1-methyl-5-nitroso-1H-indazol-4-yl)-2-phenylpropanenitrile (7).
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Methyl iodide (1.4 g, 10 mmol) and potassium tert-butoxide (1.12 g, 10 mmol) were added to a
solution of compound 4 (2.76 g, 10 mmol) in tert-butanol (50 mL). After the mixture was stirred
for 4 h at rt, it poured into water. The product was extracted with CH2Cl2 (2×40 mL). The extract
was dried (MgSO4), treated with charcoal and evaporated to give crude 7. More purification was
performed by recrystallization from n-hexane-EtOAc (1:1).
Yield (60%), mp 131-134 °C; 1H NMR (CDCl3) δ 3.15 (s, 3H, CH3), 4.23 (s, 3H, N-CH3), 7.21–
7.27 (m, 3H, Ar H), 7.34–7.38 (m, 2H, Ar H), 7.51 (d, J=9.5 Hz, 1H, Ar H), 8.49 (d, J=9.5 Hz,
1H, Ar H), 8.62 (s, 1H, Ar H);13C NMR (CDCl3): δ 30.4, 35.5, 44.2, 107.9, 111.6, 126.5, 128.3,
131.3 133.4, 133.9, 134.6, 143.2, 149.7, 151.2, 167.8; IR (KBr): 2345 cm-1 (CN), 1545 cm-1
(N=O). MS (m/z) 290 (M+). Anal. Calcd for C17H14N4O (290.3): C, 70.33; H, 4.86; N, 19.30.
Found: C, 70.17; H, 4.82; N, 19.19.
3. Results and discussion
3.1. Synthesis and Structure of the new dyes 4–7
As depicted in Scheme 1, 1-methyl-5-nitro-1H-indazole (1) was prepared by reaction of 5-nitro-
1H-indazole with methyl iodide in DMF and KOH using literature method [18]. The reaction of
1-methyl-5-nitro-1H-indazole with benzyl cyanide (2) led to the formation of key intermediate 2-
(5-hydroxyimino-1-methyl-4,5-dihydro-1H-4-indazolyliden)-2 phenylacetonitriles (3) via the
nucleophilic substitution of hydrogen [25–27] in basic methanol solution in excellent yield
(Scheme 1). The structure of dye 3 has been established by DFT calculations previously [13].
<Scheme 1>
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When compound 3 was heated under reflux in EtOAC or other solvents such as CHCl3 and
MeCN, new blue dye 4 was obtained in excellent yield (Scheme 2).
The structure of the new synthesized compound 4 was deduced from their spectral and
microanalytical data. For example, the IR spectrum of 4 showed a stretching vibration band at
1535 cm-1 indicating for N=O group. The 1H NMR spectrum of 4 showed three distinguished
singlet signals at δ 4.21, 6.54 and 8.65 ppm for protons of methyl, benzyl and pyrazole ring
respectively, a multiplet in the range of 7.24–7.35 ppm attributed to five aromatic protons of
phenyl ring and two doublet signals at δ 7.48 and 8.53 ppm assignable to two aromatic protons
H-4 and H-5 while the 13C NMR spectrum displayed a signal at δ 35.6 ppm confirming the
presence of benzylic carbon. The mass spectrum of 4 showed the molecular ion peak at m/z 276
(M+) corresponding to the molecular formula C16H12N4O.
<Scheme 2>
Oxidation of blue dye 4 with 30% H2O2 in MeOH led to the formation of new orange dye 5 in
high yield (Scheme 3). The structural assignments of compound 5 were based on the analytical
and spectral data. For example, in the IR spectrum of 5, two strong absorption bands at 1335 and
1545 cm-1 are assignable to nitro group. Furthermore, the mass spectrum of 5 showed the
molecular ion peak at m/z 292 (M+) corresponding to the molecular formula C16H12N4O2.
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<Scheme 3>
Conversion of dye 4 to dye 3 was also examined. The color of blue solution of dye 4 in
KOH/MeOH was smoothly changed to green which indicates the formation of 3 anion (Dye 3
was precipitated after neutralization the solution with dilute HCl). However, these evidences
prove that dyes 3 and 4 are not well enough stable and they can convert to each other in
appropriate conditions.
Alkylation of active protons of dyes 3 and 4 prevents from tautomerization of these dyes.
Compound 3 was methylated by dimethyl sulfate (DMS) in K2CO3 and MeCN to give new green
dye 5-(methoxyimino)-1-methyl-1H-indazol-4(5H)-ylidene)-2 phenylacetonitrile (6) at rt
(Scheme 4). New blue dye 2-(1-methyl-5-nitroso-1H-indazol-4-yl)-2-phenylpropanenitrile (7)
was also obtained from the reaction of dye 4 with methyl iodide in potassium tert-butoxide and
tert-butanol in good yield (Scheme 4). All the newly synthesized dyes have been characterized
by elemental analysis and spectroscopic data. The spectral details of all these are given in
experimental section.
<Scheme 4>
3.2. Optical properties
The new dyes 4–7 were spectrally characterized by using a UV-vis spectrophotometer and the
wavelength range of spectrophotometer was 200–800 nm. Fig. 1 shows the visible absorption
spectrum of compounds 3 and 4–7 in dilute (2 × 10-6 M) methanol solution. Characteristics of
absorption spectra for 3 and 4–7 in methanol are presented in Table 1. Values of extinction
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coefficient (ε) were calculated as the slope of the plot of absorbance vs concentration.
Absorbance intensity and extinction coefficient (ε) in blue dye 4 were the biggest values. It
should be emphasized that nitroso group is the strongest chromophore, being able to shift
bathochromically absorption of given aromatic compound more than any other two-atom
chromophore.
<Fig. 1>
<Table 1>
The absorption spectra of dyes 4–7 were measured in different solvents. As shown in Table 2,
the absorption spectra of 4–7 in polar solvents undergo a red shift. Increasing the solvent polarity
stabilizes the excited state molecule comparative to the ground-state molecule with the observed
red shift of the absorption maximum as the experimentally observed result (Tables 2). For
example, in the absorption spectra of blue dye 4, λabs shifts from 505 to 605 nm, as the solvent
changes from cyclohexane to methanol (Figure 2 and Table 2).
<Fig. 2>
<Table 2>
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3.4. DFT and TD-DFT calculations
The color intensity of dyes 3–7 indicates efficient intramolecular charge transfer (ICT) states
[13] from the donor site (endocyclic N-3 or OH group) to the acceptor moiety (CN or N=O
group). To gain a deeper insight into the optical properties and the UV-visible absorption spectra
of the dyes, we performed DFT and TD-DFT calculations at the B3LYP/6-311++G(d,p) level
and obtained the optimized geometries, Sum of electronic and zero-point Energies (kJ.mol-1),
HOMO and LUMO frontier orbitals and electronic spectra of dyes 3–5.
The optimized geometries of the compounds 3–5 are shown in Fig. 3. In the optimized
geometries of the dyes 3–5, all of indazole rings, N=O and cyano groups are essentially planar
and the C=C bond lengths (1.38-1.44 Ångstrom) of the aromatic rings are in the expected range
[28] (Tables S1–S3; see Supplementary data).
According to Table 3, dye 4 has lower Sum of electronic and zero-point Energies in the solution
phase (PCM). Thus, dye 4 is the lowest energy isomeric form. It is in good agreement with the
experimental results, because the green solution of dye 3 converts to the blue solution of dye 4
after 2 days at rt (Experimental section).
The energy difference between the HOMO and LUMO frontier orbitals is one of the important
characteristics of molecules, which has a determining role in such cases as electric properties,
electronic spectra and photochemical reactions. The HOMO and LUMO maps of 3–5 are shown
in Figure 4. Separation energies between the HOMO and LUMO (ꢀε = εLUMO – εHOMO) in dyes
3–5 are 3.11, 3.25 and 3.83 eV, respectively. It can be seen from Figure 4 that dye 3 has more π-
system overlap in its HOMO and LUMO frontier orbitals which led to the lower separation
energy between the HOMO and LUMO compared to dyes 4 and 5. In the Supporting information
(Scheme S1), neutral and some charge-separated mesomeric structures of dyes 3–5 are presented.
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Calculated electronic absorption spectra were also obtained by time-dependent density functional
theory (TD-DFT) method. The TD-DFT electronic spectra calculations on 3 show two electronic
–
transition bands. There is a relatively sharp peak at 466 nm (oscillator strength: 0.0789), which
can be attributed to π – π* transitions (donor endocyclic N-3 to the acceptor CN group), and a
relatively broad band in the range of 450 to 750 nm with an oscillator strength of 0.1575, which
can be linked to n – π* transitions from the donor OH group to the acceptor CN group, compared
with the experimental values of 500–800 nm. The TD-DFT electronic spectra calculations of 4
reveal a relatively sharp peak in the range of 500–650 nm which correspond to the experimental
data (550ꢀ770 nm) with oscillator strength of 0.0923. These bands can be linked to π – π*
–
transitions from the donor endocyclic N-3 to the acceptor N=O group. Also, The TD-DFT
electronic spectra calculations on dye 5 show that there are a relatively sharp peak at 445 nm
(oscillator strength: 0.0799), which can be attributed to π – π* transitions from donor endocyclic
N-3 to the acceptor NO2 group. This electronic transition band can be compared with the
experimental values of 485 nm.
The calculated electronic absorption spectra of compounds 3–5 are shown in the Supporting
information (Figures S1–S3).
<Fig. 3>
<Table 3>
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<Fig. 4>
4. Conclusion
Tautomerism, oxidation and alkylation studies on the 2-(5-hydroxyimino-1-methyl-4,5-dihydro-
1H-4-indazolyliden)-2-phenylacetonitrile led to the synthesis of some new donor–acceptor green,
blue and orange dyes. The interesting optical properties of these dyes were examined and the
solvent effects on the absorption spectra of the dyes were studied. DFT and TD-DFT calculations
of the dyes 3–5 were performed to gain a deeper insight into the charge transfer properties,
optimized geometries, HOMO and LUMO frontier orbitals and electronic spectra by using the
B3LYP hybrid functional and the 6-311++G(d,p) basis set. The results showed that all of
indazole rings, N=O and cyano groups are essentially planar and the separation energies between
the HOMO and LUMO in dyes 3–5 were 3.11, 3.25 and 3.83 eV, respectively. Also, electronic
spectra of dyes 3–5 were in relatively good agreement with visible absorption spectra.
Further investigation into the scope and application of these new dyes is in progress and will be
reported soon.
Acknowledgment
We would like to express our sincere gratitude to Research Office, Mashhad Branch, Islamic
Azad University, Mashhad-Iran, for financial support of this work.
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11:1929
[28] Dal H, Süzen Y, Sahin E. Synthesis, spectral studies of salicylidine-pyridines: Crystal and
molecular structure of 2-[(1E)-2-aza-2-(5-methyl(2-pyridyl)ethenyl)]-4-bromobenzen-1-ol..
Spectrochim Acta Part A. 2007; 67:808.
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Schemes and Figures:
Scheme 1. Synthesis of green dye 3.
Scheme 2. Tautomerism in green dye 3 and new blue dye 4
Scheme 3. Oxidation of blue dye 4 to new orange dye 5
Scheme 4. Methylation of dyes 3 and 4 to new dyes 6 and 7
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1.
0.95
0.75
0.55
0.35
0.15
3
4
5
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Visible absorption spectra of compounds 3–7 in methanol solution (2 × 10-6 mol L-1)
2.
0.9
0.75
0.6
0.45
0.3
0.15
400
450
500
550
Wavelength (nm)
THF EtOAC
600
650
700
750
800
Cyclohexane
MeCN
DMF
Fig. 2. Visible absorption spectra of compound 4 in different solvents (2 × 10-6 mol L-1)
3.
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Fig. 3. Optimized geometries of the compounds 3–5
Fig. 4. The HOMO (down) and LUMO (up) frontier orbitals of the dyes 3–5.
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Tables:
Table 1. Spectroscopic properties of dyes 3–7 in MeOH solvent
Dye
3
4
5
6
7
λmax (nm)a
ε × 10 -3 (M-1 cm-1)b
679 605 485 665 600
395 415 265 290 295
a Wavelengths of maximum absorbance (λmax
)
b Extinction coefficient
Table 2. Spectroscopic data for 4–7 at 298 K in dependence of the solvent
λabs (nm)/4 λabs (nm)/5 λabs (nm)/6 λabs (nm)/7
Solvent
Cyclohexane
THF
505
535
545
590
595
605
435
455
465
480
480
485
595
620
630
655
660
665
495
530
535
580
590
600
EtOAc
MeCN
DMF
MeOH
Table 3: Sum of electronic and zero-point Energies (kJ.mol-1) of dyes 3 and 4 in the solution
phase (PCM).
Dye
ꢀE
3
4
E (CHCl3) -2389643.5370 -2389676.1356 32.5986
E (MeCN) -2389656.0770 -2389687.8525 31.7755
E (MeOH) -2389660.0484 -2389687.6585 27.6101
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Figure Captions:
4.
Scheme 1. Synthesis of green dye 3.
Scheme 2. Tautomerism in green dye 3 and new blue dye 4
Scheme 3. Oxidation of blue dye 4 to new orange dye 5
Scheme 4. Methylation of dyes 3 and 4 to new dyes 6 and 7
Fig. 1. Visible absorption spectra of compounds 3–7 in methanol solution (2 × 10-6 mol L-1)
Fig. 2. Visible absorption spectra of compound 4 in different solvents (2 × 10-6 mol L-1)
Fig. 3. Optimized geometries of the compounds 3–5
Fig. 4. The HOMO (down) and LUMO (up) frontier orbitals of the dyes 3–5.
5.
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Highlights



Some new green, blue and orange dyes were synthesized from indazole.
The optical and solvatochromic properties of the dyes were investigated.
Tautomerism, oxidation and alkylation reactions were examined in these
compounds.

DFT/TD-DFT calculations of the dyes are performed at the B3LYP/6-
311++G(d,p) level.