Synthesis, absorption and fluorescence of hydrazone colorants based on pyrrolinone esters

Article abstract of DOI:10.1016/j.dyepig.2011.03.013

Ten azo dyes were prepared by diazotization of a series of electronically different para substituted anilines and subsequent azo coupling of these diazonium salts with ethyl 4,5-dihydro-5-oxo-2-aryl(1H)pyrrole-3-carboxylate as a the coupling component. Al

Full text of DOI:10.1016/j.dyepig.2011.03.013

Dyes and Pigments 91 (2011) 170e176
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, absorption and fluorescence of hydrazone colorants
based on pyrrolinone esters
,
b
c,
Tarek Aysha a , Stanislav Lunák Jr. , Antonín Lycka d, Radim Hrdina a
*
ꢀ
ꢀ
a Institute of Organic Chemistry and Technology, University of Pardubice, Studentská 95, CZ-53210 Pardubice, Czech Republic
b Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-53210 Pardubice, Czech Republic
c Research Institute for Organic Syntheses (VUOS), Rybitví 296, CZ-533 54 Pardubice 20, Czech Republic
d University of Hradec Králové, Rokitanského 62, CZ 500 03 Hradec Králové 3, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Ten azo dyes were prepared by diazotization of a series of electronically different para substituted
anilines and subsequent azo coupling of these diazonium salts with ethyl 4,5-dihydro-5-oxo-2-aryl(1H)
pyrrole-3-carboxylate as a the coupling component. All of the dyes were confirmed as keto-hydrazone
tautomers and were found as a mixtures of E- and Z-isomers with respect to the exocyclic C]N bond by
1H-NMR spectroscopy. The absorption spectra are all similar irrespective of substituent and solvent. By
comparison the fluorescence is strongly dependent on the electronic character of the substituents. All
compounds fluoresce in a low temperature solvent glass and in the solid state and only the 4-cyano-
phenyl and 4-nitrophenyl derivatives show fluorescence in solution at room temperature. The
spectroscopic behavior is explained in terms of competition between E/Z isomerization and fluorescence
after excitation.
Received 2 August 2010
Received in revised form
6 March 2011
Accepted 8 March 2011
Available online 16 March 2011
Keywords:
Azo
Hydrazone
Tautomerism
Pyrrolinone ester
Fluorescence
Isomerization
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Azo tautomers arise from a reaction usually with an E-configu-
ration of an exocyclic N]N bond and Z isomers can be obtained by
Azo compounds are by far the largest group of commercial
organic dyes and pigments [1,2]. Their synthesis is economically
attractive; usually a two-step sequence of diazonium salt formation
from either an aromatic or heteroaromatic primary amine which
reacts with a wide choice of coupling components, usually amino-
and hydroxy-substituted (hetero)aromatics or enolizable (di)
ketones. The products of coupling components with the latter two
types of passive components can be formally called hydroxy-azo
compounds, which may immediately undergo a proton transfer
from oxygen to more distant nitrogen, followed by reordering of
single and double bonds forming a keto-hydrazone tautomer. An
equilibrium between both tautomers in solution depends mainly
on the type of coupling component, on the character of substituents
on both parts of the molecule and sometimes even on either the
environment or the temperature. Azo dyes derived from frequently
used enols e.g. 3-substituted 1-phenylpyrazol-5-ones are found
exclusively as the keto-hydrazone tautomers [3,4].
a photochemical E/Z isomerization [5]. Hydrazone tautomers are
usually arranged in a form enabling intramolecular hydrogen
bonding between oxygen and nitrogen atoms taking part in the
formal tautomeric proton transfer, thus forming a Z-arrangement of
the exocyclic C]N bond. Such a configuration in the solid state was
experimentally confirmed by X-ray diffractometry for b-naphthol
based hydrazones [6] or a pyrazoline hydrazone [7], which is
probably the structurally closest compound with respect to the
pyrrolinone hydrazones investigated in this presented study.
The absorption spectra of hydrazones are generally bath-
ochromatically shifted with respect to corresponding azo tauto-
mers; the effect of polar substituents on absorption maxima is
mutually opposite [8,9]. Hydrazone tautomers quite often fluo-
resce, e.g. the derivatives of pyrazolone hydrazones [10]. Only
hydrazone fluorescence was observed in the case, where both
tautomers were present, and thus excited state intramolecular
proton transfer process (from oxygen to nitrogen) was detected by
comparison of absorption and fluorescence excitation spectra [11].
Ethyl 4,5-dihydro-5-oxo-2-phenyl(1H)pyrrole-3-carboxylate
(phenyl pyrrolinone ester 1) has become an interesting inter-
mediate in the colorant chemistry in recent years. The
* Corresponding author. Tel.: þ420 773240675.
E-mail address: tarekaysha@hotmail.com (T. Aysha).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.013

T. Aysha et al. / Dyes and Pigments 91 (2011) 170e176
171
concentrations 1 ꢀ10ꢁ5 mol/L and the measurements of absorbance
were recorded before and after irradiation by daylight for 10 h.
A Perkin-Elmer (P.-E.) LS55 was used for measuring fluorescence
spectra at room and low temperatures. Methyltetrahydrofuran
(MTHF) was used as a solvent to create an organic frozen glass to
measure low temperature fluorescence spectra using a commercial
low temperature accessory also from P.-E. This accessory also from
P.-E. contains an isolated box filled by liquid nitrogen and the
measurements are carried out in a round cuvette (diameter about
1 mm). The solid-state luminescence spectra were recorded also on
the same instrument equipped with a P.-E. accessory for solid-state
measurements. Polycrystalline samples were placed under quartz
plate and the emission spectra were recorded using front face
geometry. The fluorescence quantum yields in solution (4F) were
determined using 4-dicyanomethylene-2-methyl-6-[p(dimethyla-
mino)styryl]-4H-pyran (DCM) (4F ¼ 0.57, lmax ¼ 520 nm) in 1-
propanol as the standard [20].
Table 1
Structure of prepared dyes.
Dye
Ar
R
Dye
Ar
R
1a
1b
1c
1d
1e
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
H
2a
2b
2c
2d
2e
2-naphthyl
2-naphthyl
2-naphthyl
2-naphthyl
2-naphthyl
H
4-CN
4-NO2
4-OH
4-MeO
4-CN
4-NO2
4-OH
4-MeO
methylene carbon in position 4 is activated by carbonyl and
ester groups and a condensation with aldehydes [12,13], esters
[14] and a reaction with aromatic nitriles [15e17] can be carried
out. The products of an azo coupling reaction between pyrro-
linone ester
1 and diazotized aromatic amines were first
described in a patent in 2001 [18]. Since this patent only
contains a few examples of the derived dyes without detailed
spectral data and as we are generally interested in the dyes
from pyrrolinone esters, it was decided to investigate these
compounds in detail. First of all the unsubstituted 15N-labeled
ethyl 5-oxo-2-phenyl-4-(2-phenylhydrazono)-4,5-dihydro-1H-
pyrrole-3-carboxylate (compound 1a in Table 1) was synthe-
sized according to Scheme 1 [19]. The product was formed as
a hydrazone tautomer as confirmed by 13C and 15N chemical
shifts, and was obtained as a mixture of E and Z isomers
according to nJ(15N, 13C) coupling constants. A comparison of
the 1H-NMR chemical shifts with GIAO DFT calculations enabled
determination of a distinct configuration of the carboxy ester
group in both isomers.
Here we present the syntheses of a representative set of ten
derivatives (Table 1) from the azo coupling of diazotized para
substituted anilines with phenyl and 2-naphthyl pyrrolinone ester
1, 2 (Scheme 1). Their tautomeric form was estimated by NMR
spectroscopy. The dependence of their absorption spectra on the
character of the substituents was studied together with their
fluorescence ability in various environments.
An EA 1108 FISONS instrument was used for elemental analysis.
Melting points of compounds were checked on a Büchi 510 melting
point apparatus. Thin-layer chromatography (TLC) was performed
by a Kieselgel 60 F254 (Merck, Darmstadt, Germany), for observa-
tion of reaction progress and the purity of the prepared interme-
diates and dyes.
Positive-ion and negative-ion atmospheric pressure chemical
ionization (APCI) mass spectra were measured on an ion trap
analyzer Esquire 3000 (Bruker Daltonics, Bremen, Germany) in the
range m/z 50e1000. The samples were dissolved in acetonitrile and
analyzed by direct infusion at the flow rate 100
m
L minꢁ1. The
selected precursor ions were further analyzed by MS/MS analyses
under the following conditions: the isolation width m/z ¼ 4, the
collision amplitude in the range 0.7e1.0 V depending on the
precursor ion stability, the temperature of drying gas was 330 ꢂC,
the APCI temperature was 400 ꢂC, the tuning parameter compound
stability was 100%, the flow rate and the pressure of nitrogen were
4 L minꢁ1 and 45 psi, respectively.
Chromatographic apparatus consisted of an LC 1100 Series
(Agilent Technologies, USA) and the ion trap mass spectrometer
MSD TRAP XCT Plus system (Agilent Technologies, USA) equipped
with ESI and APCI probes was used. Negative-ion ESI mass spectra
were recorded in mass range 50e1500 Da in all experiments. The
ion trap analyzer was tuned to obtain an optimal response in the
range of expected m/z values (target mass was set to m/z ¼ 500).
Another ESI ion source parameters were as follows: drying gas flow
8 L minꢁ1, nebulizer gas pressure 40 psi, drying gas. Samples were
dissolved in methanol in appropriate concentrations for MS
2. Experimental
2.1. Instrumental equipment
N-Methylpyrrolidinone (NMP), dioxane, dimethyl sulfoxide
(DMSO), acetonitrile and acetone (all of spectroscopic grade) were
purchased from Fluka and used for room temperature spectro-
scopic measurements.
The absorption spectra were recorded using a PerkineElmer
Lambda 35 spectrophotometer in 1 cm pathlength quartz cuvettes.
Photochemical isomerization was also monitored by absorption
spectroscopy. The dye solutions in NMP were prepared in the dark at
detection. Injection volumes of 20 m
L, a flow-rate of 0.2 mL minꢁ1
and column temperature of 30 ꢂC were used in all analysis. The
separation was performed in following chromatographic system:
An octadecyl silica cartridge column, Zorbax Eclipse XDB C18
(150 ꢀ 2.1 mm i.d., 5
mm particle size) purchased from Agilent
O
O
O
O
Ar
N2+ Cl-
O
H
Ar
O
Ar
pH 1-7
+
+
N
N
N
N
N
H
N
N
H
H
O
H
O
R
O
R
R
1
Ar= Ph
Ar=2-Naphthyl
Ar= Ph , R=H 1a(Z)
Ar= Ph , R=H 1a(E)
2
Ar=2-Naphthyl, R=H 2a (Z)
Ar=2-Naphthyl, R=H 2a (E)
Scheme 1. Syntheses of the studied compounds.

172
T. Aysha et al. / Dyes and Pigments 91 (2011) 170e176
Table 2
(HPST Prague, Czech Republic) was used for the separation of
samples. Gradient was employed, with 20 mM ammonium acetate
in water as solvent A (pH 4) and acetonitrile as solvent B: from 5% B
in 0e3 min, then 5e100% in 3e20 min to 100% B in 20e30 min.
The 1H-NMR spectra were recorded on a Bruker Avance II 400
spectrometer (400.13 MHz). The samples were dissolved in hex-
adeuteriodimethyl sulfoxide. The 1H chemical shifts were refer-
1H chemical shifts in Z and E isomers of the dyes 1a-e and 2a-e measured in hex-
adeuteriodimethyl sulfoxide.
Compound
1H Chemicalshift (ppm)
eNHN]
eCONHe
OCH2CH3
Aromatic
protons
%
1a (Z)
(E)
1b (Z)
(E)
13.10
13.01
13.15
13.01
13.10
13.31
13.18
12.97
13.14
13.00
13.15
13.06
13.05
13.20
13.16
13.32
13.26
13.03
13.22
13.04
11.40
11.31
11.49
11.43
11.52
11.52
11.32
11.20
11.35
11.24
11.53
11.42
11.63
11.56
11.63
11.63
11.45
11.32
11.47
11.35
4.20, 1.25
4.09, 0.92
4.19, 1.23
4.08, 0.91
4.20, 1.24
4.09, 0.92
4.19, 1.23
4.06, 0.89
4.20, 1.24
4.08, 0.91
4.22, 1.21
4.09, 0.82
4.21, 1.20
4.09, 0.82
4.22, 1.21
4.10, 0.84
4.21, 1.21
4.07, 0.82
4.21, 1.22
4.08, 0.82
7.08e7.66
7.37e7.85
7.39e8.30
6.80e7.63
7.01e7.65
7.09e8.22
7.39e8.26
7.42e8.32
6.84e8.22
7.00e8.22
62
38
55
45
62
38
69
31
80
20
70
30
71
29
60
40
85
15
75
25
enced to the central signal of the solvent (
d
¼ 2.55).
2.2. Synthesis
1c (Z)
(E)
1d (Z)a
(E)a
Starting pyrrolinone esters, ethyl 4,5-dihydro-5-oxo-2-phe-
nyl(1H)pyrrole-3-carboxylate (1) and ethyl-4,5-dihydro-5-oxo-2-
naphthyl(1H)pyrrole-3-carboxylate (2), were synthesized accord-
ing to published procedures [13,17].
4-Aminophenol, 4-nitroaniline and 4-cyanoaniline were all
purchased from Aldrich Chemical Company.
All ten dyes were prepared by diazotization of the primary
aromatic amines followed by a coupling reaction. The aromatic
amine (10 mmol), concentrated HCl (35%, 4 mL) and water (30 mL)
were stirred until amine was completely dissolved. Then the
mixture was cooled (external cooling) at the temperature 0e5 ꢂC
and NaNO2 (10 mL of 1M solution) were dropwise added during
30 min. When the reaction was completed, the solution of the
diazonium compound was used for the next coupling reaction.
Pyrrolinone ester intermediate (10 mmol) was dissolved in
methanol (100 mL) by heating, the mixture was externally cooled to
1e (Z)b
(E)c
2a (Z)
(E)
2b (Z)
(E)
2c (Z)
(E)
2d (Z)d
(E)e
2e (Z)f
(E)g
a
10.80 (OH) broadened signal.
3.80 (OCH3).
3.79 (OCH3).
9.46 (OH).
9.38 (OH).
b
c
d
e
f
3.81 (OCH3).
3.80 (OCH3).
0
ꢂC and the solution of diazonium salt was added dropwise. The
g
reaction mixture was stirred at 0e5 ꢂC for 4 h while warming
gradually to room temperature and then stirred overnight. The
precipitated dye was filtered off and washed with methanol
and water. Final products were recrystallized, characterized by
elemental analysis, mass spectrometry and melting points, which
together with reaction yields are presented in the following text.
Data from 1H NMR spectroscopy are presented in section 3.1.
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.91(3H, t, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.08 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.37e7.85 (9H, m, aromatic protons); 11.43 (1H, br. s,eCONHe);
13.01 (1H, br. s, eNHN]).
MS analysis M ¼ 360. Positive-ion MS: m/z 361 [M þ H]þ, 100%;
m/z 315 [M þ HeC2H5OH]þ, Negative-ion MS: m/z 359 [M ꢁ H]ꢁ,
100%.
2.3. Ethyl 5-oxo-2-phenyl-4-(2-phenylhydrazono)-4,5-dihydro-1H-
pyrrole-3-carboxylate (1a)
Elemental analysis:calculated(C20H16N4O3):C (66.66%), H(4.84%),
N(15.55%). Found: C (66.39%), H(4.70%), N(15.16%).
Preparative yield 63%; purified by recrystallization from ethanol,
mp ¼ 193e195 ꢂC.
2.5. Ethyl 5-oxo-2-phenyl-4-[2-(4-nitrophenyl)hydrazono]-4,5-
dihydro-1H-pyrrole-3-carboxylate (1c)
1H NMR (400 MHz, DMSO-d6,
d, ppm) Z isomer: 1.25 (3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.20 (2H, q, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.08e7.66
(10H, m, aromatic protons); 11.40 (1H, br. s,eCONHe); 13.10 (1H, br.
s, eNHN]).
Preparative yield 90%; purified by recrystallization from ethanol,
mp ¼ 231e233 ꢂC.
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.92 (3H, t, 3H,
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.09 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.08e7.66 (10H, m, aromatic protons); 11.31 (1H, br. s,eCONHe);
13.01 (1H, br. s, eNHN]).
MS analysis, M ¼ 335. Positive-ion MS: m/z 336 [M þ H]þ, 100%;
m/z 290 [M þ HeC2H5OH]þ, Negative-ion MS: m/z 334 [M ꢁ H]ꢁ,
100%; m/z 288 [M ꢁ HeC2H5OH]ꢁ.
Elemental analysis: calculated (C19H17N3O3): C(68.05%),
H(5.11%), N(12.53%). Found: C (68.10%), H(5.28%), N(12.63%).
2.4. Ethyl 5-oxo-2-phenyl-4-[2-(4-Cyanophenyl)hydrazono]-4,5-
dihydro-1H-pyrrole-3-carboxylate (1b)
Preparative yield 56%; purified by recrystallization from ethanol,
mp ¼ 234e236 ꢂC.
1H NMR (400 MHz, DMSO-d6,
d, ppm) Z isomer: 1.23 (3H, t, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz CH3); 4.19 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.37e7.85 (9H, m, aromatic protons); 11.49 (1H, br. s,eCONHe);
13.15 (1H, br. s, eNHN]).
Fig. 1. Absorption spectra of 1ae1e in NMP.

T. Aysha et al. / Dyes and Pigments 91 (2011) 170e176
173
Table 4
Solvent effect on absorption maxima of compounds 1a, 1c and 1e.
Solvent
Dielectric
constant
lmax [nm]
1a
1c
1e
Dioxan
Acetone
NMP
Acetonitrile
DMSO
2.21
20.7
32.0
37.5
47.0
433
434
441
434
442
461
460
471
461
472
455
449
457
450
457
MS analysis M ¼ 335. Positive-ion MS: m/z 351 [M þ H]þ,100%; m/z
306 [M þ HeC2H5OH]þ, Negative-ion MS: m/z 350 [M ꢁ H]ꢁ,100%; m/
z 288[M ꢁ HeC2H5OH]ꢁ.
Elemental analysis: calculated (C19H17N3O4):
C (64.95%),
H(4.88%), N(11.96%). Found: C (64.81%), H(4.92%), N(11.75%).
2.7. Ethyl 5-oxo-2-phenyl-4-[2-(4-methoxyphenyl)hydrazono]-4,5-
dihydro-1H-pyrrole-3-carboxylate (1e)
Fig. 2. Absorption spectra of 2ae2e in NMP.
Preparative yield 82%; purified by recrystallization from ethanol,
mp ¼ 177e179 ꢂC.
1H NMR (400 MHz, DMSO-d6,
d
, ppm) Z isomer: 1.24 (3H, t, 3H, t,
1H NMR (400 MHz, DMSO-d6,
d, ppm) Z isomer: 1.24 (3H, t, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.20 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.39e8.30 (9H, m, aromatic protons); 11.52 (1H, br. s,eCONHe);
13.10 (1H, br. s, eNHN]).
3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 3.80 (3H, s, OCH3); 4.20 (2H, q, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.01e7.65 (9H, m, aromatic protons); 11.35
(1H, br. s, eCONHe); 13.14 (1H, br. s, eNHN]).
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.92 (3H, t, 3H,
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.91 (3H, t, 3H,
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.09 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.39e8.30 (9H, m, aromatic protons); 11.52 (1H, br.s,eCONHe);
13.31 (1H, br. s, eNHN]).
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 3.79 (3H, s, OCH3); 4.08(2H, q, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.01e7.65 (9H, m, aromatic protons); 11.24
(1H, br. s, eCONHe); 13.00 (1H, br. s, eNHN]).
MS analysis M ¼ 396. Positive-ion MS: m/z 397 [M þ H]þ, 100%;
m/z 355 [M þ HeC2H5OH]þ, Negative-ion MS: m/z 395 [M ꢁ H]ꢁ,
100%.
MS analysis M ¼ 335. Positive-ion MS: m/z 351 [M þ H]þ, 100%;
m/z 306 [M
þ
HeC2H5OH]þ, Negative-ion MS: m/z 350
[M ꢁ H]ꢁ,100%; m/z 288[M ꢁ HeC2H5OH]ꢁ.
Elemental analysis: calculated (C19H16N4O5): C(60%), H(4.24%),
N(14.73%). Found: C (60.19%), H(4.33%), N(14.78%).
Elemental analysis: calculated (C20H19N3O4):
C (65.74%),
H(5.24%), N(11.5%). Found: C (65.81%), H(5.62%), N(11.65%).
2.8. Ethyl 5-oxo-2-(naphthalen-2-yl)-4-(2-phenylhydrazonyl)-4,5-
dihydro-1H-pyrrole-3-carboxylate(2a)
2.6. Ethyl 5-oxo-2-phenyl-4-[2-(4-hydroxyphenyl)hydrazono]-4,5-
dihydro-1H-pyrrole-3-carboxylate (1d)
Preparative yield 87%; purified by recrystallization from ethyl-
acetate, mp ¼ 338e340 ꢂC.
Preparative yield 57%; purified by recrystalization from ethanol,
mp ¼ 237e239 ꢂC.
1H NMR (400 MHz, DMSO-d6,
d, ppm) Z isomer: 1.21 (3H, t, 3H, t,
1H NMR (400 MHz, DMSO-d6,
d, ppm) Z isomer: 1.23 (3H, t, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.22 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.09e8.22 (12H, m, aromatic protons); 11.53 (1H, br. s, eCONHe);
13.15 (1H, br. s, eNHN]).
3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.19 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
6.80e7.63 (9H, m, aromatic protons); 10.80 (1H, s, OH); 11.32 (1H,
br. s, eCONHe); 13.18 (1H, br. s, eNHN]).
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.82 (3H, t, 3H,
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.89 (3H, t, 3H,
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.09 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.09e8.22 (12H, m, aromatic protons); 11.42 (1H, br. s,eCONHe);
13.06 (1H, br. s, eNHN]).
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.06 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
6.80e7.63 (9H, m, aromatic protons); 10.80 (1H, s, OH); 11.20 (1H,
br. s, eCONHe); 12.97 (1H, br. s, eNHN]).
MS analysis M ¼ 385. Positive-ion MS: m/z 386 [M þ H]þ, m/z
340 [M þ HeC2H5OH]þ,100%, Negative-ion MS: m/z 384 [M ꢁ H]ꢁ,
100%.
Table 3
Elemental analysis: calculated (C23H19N3O3):
C (71.67%),
Absorption maxima
temperature.
(lmax) and molar absorptivities (3max) in NMP at room
H(4.97%), N(10.90%). Found: C (71.50%), H(5.01%), N(10.88%).
Dye
lmax [nm] E,
Z mixture
3max [L molꢁ1 cmꢁ1
E, Z mixture
]
2.9. Ethyl 5-oxo-2-(naphthalen-2-yl)-4-[2-(4-cyanophenyl)
hydrazono]-4,5-dihydro-1H-pyrrole-3-carboxylate (2b)
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
441
443
460
463
457
448
449
465
470
463
22700
36500
30800
22800
30100
34200
40000
34000
31300
35400
Preparative yield 69%; purified by reprecipitation from dioxan,
cyclohexane, mp ¼ 273e275 ꢂC. 1H NMR (400 MHz, DMSO-d6,
d,
ppm) Z isomer: 1.20 (3H, t, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.21 (2H, q,
3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.39e8.26 (11H, m, aromatic protons);
11.63 (1H, br. s,eCONHe); 13.05 (1H, br. s, eNHN]).
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.82 (3H, t, 3H,
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.09 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);

174
T. Aysha et al. / Dyes and Pigments 91 (2011) 170e176
ppm) Z isomer: 1.21 (3H, t, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.21 (2H, q,
1
0.8
0.6
0.4
0.2
0
3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2), 6.84e8.22 (11H, m, aromatic protons);
9.46 (1H, s, OH); 11.45 (1H, br. s, eCONHe); 13.26 (1H, br. s,
eNHN]).
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.82 (3H, t, 3H,
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.07 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
6.84e8.22 (11H, m, aromatic protons); 9.48 (1H, s, OH); 11.32 (1H,
br. s, eCONHe); 13.03 (1H, br. s, eNHN]).
MS analysis M ¼ 401. Positive-ion MS: m/z 402 [M þ H]þ, m/z
356 [M þ HeC2H5OH]þ,100%, Negative-ion MS: m/z 400 [M ꢁ H]ꢁ,
100%.
300
400
500
600
700
800
nm
Absorbance in 2-MeTHF at 300 K
Fluorescence at 300K
Fluorescence at 77K
Excitation at 300k
Excitation at 77K
Fluorescence in solid state
Elemental analysis: calculated (C23H19N3O4):
C (68.82%),
H(4.77%), N(10.47%). Found: C (68.99%), H(4.62%), N(10.50%).
Fig. 3. Absorption at room temperature (MTHF) and excitation and emission spectra at
low temperature (MTHF) of compound 1c together with its solid-state emission at
room temperature.
2.12. Ethyl 5-oxo-2-(naphthalen-2-yl)-4-[2-(4-methoxyphenyl)
hydrazono]-4,5-dihydro-1H-pyrrole-3-carboxylate (2e)
Preparative yield 71%; purified by reprecipitation from dioxin,
cyclohexane, mp ¼ 230e232 ꢂC. 1H NMR (400 MHz, DMSO-d6,
7.39e8.26 (11H, m, aromatic protons); 11.56 (1H, br. s, eCONHe);
13.20 (1H, br. s, eNHN]).
d
, ppm) Z isomer: 1.22 (3H, t, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 3.81
(3H, s, OCH3); 4.21 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.00e8.22
(11H, m, aromatic protons); 11.47 (1H, br. s, eCONHe); 13.22 (1H,
br. s, eNHN]).
MS analysis M ¼ 410. Positive-ion MS: m/z 411 [M þ H]þ, m/z 365
[M þ HeC2H5OH]þ,100%, Negative-ion MS: m/z 409 [M ꢁ H]ꢁ, 100%.
Elemental analysis: calculated (C24H18N4O3):
C (70.23%),
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.82 (3H, t, 3H,
H(4.42%), N(13.65%). Found: C (69.99%), H(4.32%), N(13.60%).
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 3.80 (3 ꢃ H, s, OCH3); 4.08 (2H, q, 3H, t,
3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.00e8.22 (11H, m, aromatic protons); 11.35
(1H, br. s,eCONHe); 13.04 (1H, br. s, eNHN]).
2.10. Ethyl 5-oxo-2-(naphthalen-2-yl)-4-[2-(4-nitrophenyl)
hydrazono]-4,5-dihydro-1H-pyrrole-3-carboxylate (2c)
MS analysis M ¼ 415. Positive-ion MS: m/z 416 [M þ H]þ, m/z 370
[M þ HeC2H5OH]þ,100%, Negative-ion MS: m/z 414 [M ꢁ H]ꢁ, 100%.
Elemental analysis: calculated (C24H21N3O4): C (69.39 %), H(5.10 %),
N(10.11 %). Found: C (69.17 %), H(4.95 %), N(10.90 %).
Preparative yield 86%; purified by reprecipitation from dioxan,
cyclohexane, mp ¼ 272e275 ꢂC. 1H NMR (400 MHz, DMSO-d6,
d,
ppm) Z isomer: 1.21 (3H, t, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.22 (2H, q,
3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2); 7.42e8.32 (11H, m, aromatic protons);
11.63 (1H, br. s, eCONHe); 13.16 (1H, br. s, eNHN]).
3. Results and discussions
1H NMR (400 MHz, DMSO-d6,
d, ppm) E isomer: 0.84 (3H, t, 3H,
3.1. Synthesis and structure of the dyes
t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH3); 4.10 (2H, q, 3H, t, 3J ¼ 7.1 ꢃ 0.2 Hz, CH2);
7.42e8.32 (11H, m, aromatic protons); 11.63 (1H, br. s, eCONHe);
13.32 (1H, br. s, eNHN]).
Five dyes were synthesized from each pyrrolinone ester
(Scheme 1, Table 1). Reaction yields were relatively high, always
over 50%. The colorants synthesized from 2-naphthyl pyrrolinone
ester 2 were less soluble than those derived from the phenyl analog
in common solvents.
MS analysisM ¼ 446. Positive-ionMS: m/z447 [M þ H]þ,100%;m/z
405 [M þ HeC2H5OH]þ, Negative-ion MS: m/z 445 [M ꢁ H]ꢁ, 100%.
Elemental analysis: calculated (C23H18N4O5): C(64.18%),
H(4.22%), N(13.02%). Found: C (64.39%), H(4.33%), N(13.08%).
All compounds exist in DMSO-D6 solution as a mixture of two
components the content of which can change slightly during time.
complete NMR analysis was performed for 15N-labeled
2.11. Ethyl 5-oxo-2-(naphthalen-2-yl)-4-[2-(4-hydroxyphenyl)
hydrazono]-4,5-dihydro-1H-pyrrole-3-carboxylate (2d)
A
compound 1a in our previous study [19]. It was shown there, that it
is possible to differentiate between E and Z isomers of the hydra-
zone tautomer, since especially 1H chemical shifts (Table 2) of the
ethoxy group protons can be used as a decisive indicator. Further-
more, the 1H chemical shifts of the (CO) NH protons in Z- forms are
Preparative yield 80%; purified by reprecipitation from dioxan,
cyclohexane, mp ¼ 256e258 ꢂC. 1H NMR (400 MHz, DMSO-d6,
d,
1
0.8
0.6
0.4
0.2
0
1.2
1
0.8
0.6
0.4
0.2
0
300
500
700
350
400
450
500
550
600
650
700
nm
nm
Excitaion in 2-MTHF at 300K
Fluorescence in solid state
Fluorescence at 77K
A in 2-MeTHF
Absorbance at 300 K
Fluorescence at 77 K
Excitation at 77 K
Fluorescence at 300K
Excitation at 77 K
Fluorescence in solid state
Fig. 4. Absorption at room temperature (MTHF) and excitation and emission spectra at
low temperature (MTHF) of compound 2c together with its solid-state emission at
room temperature.
Fig. 5. Absorption at room temperature (MTHF) and excitation and emission spectra at
low temperature (MTHF) of compound 2a together with its solid-state emission at
room temperature.

T. Aysha et al. / Dyes and Pigments 91 (2011) 170e176
175
shifted to marginally higher field with respect to those in the E
isomer in all cases. The % of E and Z was calculated by comparing the
intensity of the peak related to aliphatic protons (ethoxy group).
The fraction of Z isomer in a mixture is always higher. We mention
the opposite conformation of 3-carbethoxy group in both isomers
(s-trans for Z- and s-cis for E isomer), which was proved by NMR in
our previous study [19].
All compounds fluoresce in low temperature MTHF glass (e.g.
compound 2a and 1e on Figs. 5 and 6). Low temperature fluores-
cence data for the 2-phenyl set are summarized in Table 5. A
bathofluoric shift of low temperature fluorescence emission
maxima of their 2-(2-naphth-2-yl) analogs is only marginal (e.g.
3 nm when going from 1a to 2a). All compounds show resolved
vibronic structure in their low temperature emission spectra with
similar intensity of 0e0 and 0e1 vibronic sub-bands. The shape of
fluorescence excitation spectra depends more on a character of
substituent. In all cases the maximum corresponds to 0e1 vibronic
transition. 0e0 vibronic sub-band is a local maximum for electron-
donor substituted derivatives and is detectable only as a shoulder
for neutral or electron-acceptor substituted derivatives.
Compounds 1b, 2b, 1c and 2c fluoresce in solution at room
temperature with moderate intensity (Table 5). The spectra are
rather blurred, and it suggests that the emission maxima corre-
spond to 0e1 vibronic sub-band. The excitation maxima are
shifted few nanometers to higher wavelength with respect to the
absorption maxima in most cases, a feature which may indicate
a dominant role of the fluorescence of the isomer absorbing at
longer wavelengths (probably the more planar Z isomer according
to DFT calculations for 1a [19]). Such difference in fluorescence
efficiency between E and Z isomers is not rare; it has recently been
observed for arylmethyleneoxindoles [22]. Finally, the ability of
the compounds with exocyclic double bond to fluoresce only in
a rigid medium preventing the competitive E/Z isomerization
process is quite general; noted for both arylmethylidene
eoxindoles [22] and epyrrolinones [13]. Furthermore, the positive
effect of an electron-accepting substituent on the fluorescence
efficiency in solution was observed not only for the above
mentioned arylmethylidene epyrrolinones [13,23], but also for
arylmethylidene eoxazolones [20,23], eimidazolones and
ebutenolides [23]. The explanation probably comes from the
study on hemithioindigo E/Z photoisomerization, in which the
authors have shown, that electron-accepting substituent on stil-
bene half of the molecule decreases the reaction rate by several
orders of magnitude with respect to a suitably placed electron-
donor group [24]. Consequently, the higher 4F (Table 5) of 1c (2c)
compared to 1b (2b) may attributed to the stronger electron-
accepting character of a nitro compared to a cyano group. Unfor-
tunately, the exact evidence of E/Z photoisomerization of the
compounds under study was not possible because of relatively fast
back (dark) thermal isomerization.
3.2. Absorption
The absorption spectra of all ten derivatives were measured in
NMP at room temperature. The spectra are shown on Figs. 1 and 2
and the spectral data are summarized in Table 3. The spectral
shapes are broad without any vibronic structure. The trends are
quite clear. There is a moderate bathochromic (5e8 nm) and
hyperchromic (3500e8500 L molꢁ1 cmꢁ1) shift when going from
the 2-phenyl to the 2-(2-naphthyl) derivatives. Both electron-
donating and electron-withdrawing substituents in para position of
phenyl-hydrazonyl moiety cause a significant bathochromic (19 nm
for 1c) and hyperchromic (13800 L molꢁ1 cmꢁ1 for 1b) shift with
respect to parent compounds 1a and 2a.
The dyes are essentially insoluble in non-polar solvents and
show slight positive solvatochromism in polar solvents (Table 4).
The longest wavelength maxima were found in DMSO and NMP,
although the latter shows a lower dielectric constant than aceto-
nitrile. Thus probably some specific solute-solvent interaction (e.g.
H-bonding) moderately affects the position of spectral maxima.
3.3. Fluorescence
Fluorescence spectroscopy was carried out in three different
environments: in solution, in a frozen solvent glass (MTHF) and in
the polycrystalline solid state. While the effect of the 2-aryl unit on
the fluorescence spectra was only marginal, the substituent effect
in the para position of phenyl-hydrazonyl unit was crucial and its
variation caused dramatic changes in fluorescence. Generally, all
ten compounds fluoresce in rigid environments (solid state and low
temperature solvent glass), but only the cyano and nitro derivatives
(1b, 2b and 1c, 2c) show fluorescence in solution at room
temperature. The ability to fluoresce in any environment is some-
what surprising for nitro substituted compounds absorbing at
wavelengths lower than 500 nm (Figs. 3 and 4), because the np*
triplet state localized on nitro group is in such cases usually lower
than the lowest pp* singlet state and intersystem crossing
according to El-Sayed rule dominates [21].
Least all of the compounds fluoresce in the polycrystalline
polycrystalline solid state (Figs. 3e6, Table 5). This phenomenon
supports our suggestion that competitive photoisomerization must
be hindered in a rigid environment. Unfortunately, as the X-ray
structure of molecular aggregation in crystal is not available, the
detailed discussion of the ability of solid-state fluorescence would
be only speculative.
1.2
1
0.8
0.6
0.4
0.2
0
Table 5
Fluorescence excitation and emission maxima [nm] of 0e0 and 0e1 vibronic bands
(MTHF at 77 K), solid-state fluorescence maxima at room temperature and fluo-
rescence maxima and quantum yields at room temperature.
Compound Excitation Emission
Emission (Solid) Emission (MTHF,
300 K)
(MTHF,
77 K)
(MTHF,
77 K)
300
500
700
900
nm
0e1 0e0 0e0 0e1
lmax [nm] 4F
Absorbance at 300 K
Fluorescence at 77 K
Excitation at 77 K
1a (2a)
1b (2b)
1c (2c)
1d (2d)
1e (2e)
449 sh
452 sh
501 534 554 (580)
492 524 565 (588)
e
e
Fluorescence at solid state
527 (540) 0.05 (0.06)
524 (545) 0.10 (0.18)
455 481 503 534 593 (599)
470 493 542 574 669 (672)
466 496 529 559 647 (609)
Fig. 6. Absorption at room temperature (MTHF) and excitation and emission spectra at
low temperature (MTHF) of compound 1e together with its solid-state emission at
room temperature.
e
e
e
e

176
T. Aysha et al. / Dyes and Pigments 91 (2011) 170e176
[8] Griffiths J. Prediction of colour change in dye equilibria I e azo and hydrazone
4. Conclusion
tautomers and anions of 4-phenylazo-1-naphthols. J Soc Dyers Colourists
1972;88:106e9.
Continuing a systematic research of dyes derived from pyrroli-
none esters, ten azo dyes were prepared by a diazotization of para
substituted anilines and consequent azo coupling of these diazo-
nium salts with two pyrrolinone esters as passive components. All
dyes were confirmed as keto-hydrazone tautomers and were found
as a mixtures of E and Z isomers with respect to the exocyclic C]N
bond, in contrast to similar arylmethylidene epyrrolinones, that
arise from the condensation of aldehydes with pyrrolinones which
exist exclusively as Z isomers with respect to exocyclic C]C bond
[13]. The absorption spectra are quite similar irrespective of either
substituent or solvent. Fluorescence on the other hand is strongly
dependent on the character of the substituent. All compounds
fluoresce in a low temperature solvent glass and in the solid state
and only 4-cyano and 4-nitro derivatives show fluorescence in
room temperature solution. An explanation based on a comparative
E/Z isomerization and fluorescence after excitation, was proposed.
[9] Mustroph H, Bartel R. Studies on UV/Vis absorption spectra of azo dyes. vi. the
effect of substituents on the color derivatives of 1-phenylazo-2-naphtols.
J Prakt Chem 1982;324:743e52.
[10] Nikolov P, Fratev F, Stoyanov S, Polansky OE. Fluorescence and absorption
spectra of phenylazopyrazolone dyes. Z Naturforsch 1981;36a:191e6.
[11] Joshi H, Kamounah FS, Gooijer C, van der Zwan G, Antonov L. Excited state
intramolecular proton transfer in some tautomeric azo dyes and Schiff bases
containing an intramolecular hydrogen bond. J Photochem Photobiol A Chem
2002;152:183e91.
[12] Adam JM, Dalvi P, Ekkundi VS, Bacher JP, Tiwari S. WO patent 04/083170;
2004.
ꢀ
ꢀ
ꢀ
[13] Vynuchal J, Lunák Jr S, Hatlapatková A, Hrdina R, Lycka A, Havel L, et al. The
synthesis, absorption, fluorescence and photoisomerisation of 2-aryl-4-aryl-
methylidene-pyrroline-5-ones. Dyes Pigm 2008;77:266e76.
[14] Morton CJH, Riggs RL, Smith DM, Westwood NJ, Lightfoot P, Slawin AMZ.
Synthetic studies related to diketopyrrolopyrrole (DPP) pigments. Part 2: the
use of esters in place of nitriles in standard DPP syntheses: Claisen-type
acylations and furopyrrole intermediates. Tetrahedron 2005;61:727e38.
[15] Morton CJH, Gilmour R, Smith DM, Lightfoot P, Slawin AMZ, MacLean EJ.
Synthetic studies related to diketopyrrolopyrrole (DPP) pigments. Part 1: the
search for alkenyl DPPs. Unsaturated nitriles in standard DPP syntheses:
a
novel cyclopenta[c]pyrrolone chromophore. Tetrahedron 2002;58:
5547e65.
Acknowledgments
ꢀ
ꢀ
[16] Lunák Jr S, Vynuchal J, Vala M, Havel L, Hrdina R. The syntheses, absorption
and fluorescence of polar diketo-pyrrolo-pyrroles. Dyes Pigm 2009;82:102e8.
This research was supported by the grant project no VZ MSMT
No. 0021627501 sponsored by the Ministry of Education, Youth and
Sports of the Czech Republic.
ꢀ
ꢀ
ꢀ
[17] Lunák Jr S, Havel L, Vynuchal J, Horáková P, Kucerík J, Weiter M, et al. The
geometry and absorption of diketo-pyrrolo-pyrroles substituted with various
aryls. Dyes Pigm 2010;85:27e36.
[18] Chamberlain T. WO patent 01/74950; 2001.
[19] Lycka A, Lunák Jr S, Aysha T, Holusa R, Hrdina R. A 1H, 13C and 15N NMR
spectroscopic and GIAO DFT study of ethyl 5-oxo-2-phenyl-4-(2-phenyl-
hydrazono)-4,5-dihydro-1H-pyrrole-3-carboxylate. Tetrahedron Lett 2010;
51:3149e51.
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