Synthesis, structure, absorption and fluorescence of Pechmann dye heteroanalogues

Article abstract of DOI:10.1016/j.dyepig.2013.04.010

Three N-analogues of Pechmann dye with identical aryl groups in the 5, 5′-positions (aryl = phenyl, 2-naphthyl, 2-thienyl) and carboxy ester groups in the 3,3′-positions were synthesized by oxidative dimerization of the corresponding 5-aryl-pyrrolinone es

Full text of DOI:10.1016/j.dyepig.2013.04.010

Dyes and Pigments 98 (2013) 530e539
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, structure, absorption and fluorescence of Pechmann dye
heteroanalogues
,
b
,
c
d
,
e
f
,f
ꢀa
Tarek Aysha a , Stanislav Lunák Jr. , Antonín Lycka , Jan Vynuchal , Zdenek Eliás
*
ꢀ
ꢀ
a
ꢀ
ꢀ
,
c
c
ꢀ ꢁꢀ ꢀ
ꢀ
ꢀ
Ales Ruzicka , Zdenka Padelková , Radim Hrdina
a Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-53210 Pardubice,
Czech Republic
b National Research Centre, Dokki, PO Box 12311, Cairo, Egypt
cFaculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-53210 Pardubice, Czech Republic
d Research Institute for Organic Syntheses (VUOS), Rybitví 296, CZ-533 54 Pardubice, Czech Republic
e University of Hradec Králové, Faculty of Science, Rokitanského 62, CZ 500 03 Hradec Králové 3, Czech Republic
fSynthesia a.s., Pardubice, Semtín 103, CZ-532 17 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Three N-analogues of Pechmann dye with identical aryl groups in the 5, 50-positions (aryl ¼ phenyl,
2-naphthyl, 2-thienyl) and carboxy ester groups in the 3,30-positions were synthesized by oxidative
dimerization of the corresponding 5-aryl-pyrrolinone esters. Derivatives with improved solubility in
most organic solvents were prepared by subsequent N-methylation. The dyes were confirmed as E
isomers, i.e. holding an all-trans 1,6-diaryl-1,3,5-hexatriene backbone, with slight torsion of central C]C
bond by X-ray diffraction and density functional theory (DFT) calculations. No E/Z photoisomerization
was observed. Full assignment of 1H and 13C NMR signals was performed. The absorption spectra are
quite similar with the maxima in the range 572 nm (aryl ¼ phenyl)e643 nm (aryl ¼ 2-thienyl) in
accordance with time dependent DFT calculations of excitation energies. No fluorescence was observed
in solution, while all compounds fluoresce in low temperature solvent glass (77 K) and a weak solid-state
fluorescence of phenyl and 2-naphthyl derivatives in red/IR region was detected.
Article history:
Received 5 December 2012
Received in revised form
5 April 2013
Accepted 9 April 2013
Available online 18 April 2013
Keywords:
X-ray diffraction techniques
NMR spectroscopy
Absorption
Fluorescence
Density functional theory
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
recently performed in e.g. quinacridones [5], isoindigos [6,7],
benzodifuranones [8] and benzodipyrrolidones [9].
Reinvestigation of established organic chromophores has
resulted in such chromophores being employed as key components
in organic electronic devices. Diketopyrrolopyrroles (DPPs), in
which the (substituted) phenyl rings in the 3,6-positions are
replaced by (substituted) thiophenes, form probably the most
prominent class of dyes from this point of view. Considering only
“small”, i.e. non-polymeric DPPs, their efficiencies as donors in bulk
heterojunction (BHJ) organic photovoltaic cells (OPV) with C70 [1]
or C60 [2] fullerene or alternative [3] acceptors are remarkable.
Likewise, surprisingly high hole mobilities were found in a field-
effect transistor (FET) device [4]. Such success stimulated a
considerable effort to modify other classes of organic dyes and
pigments to meet the specific requirements of a given technological
area. Replacing or substituting phenyl rings with thiophenes was
A thiophene-containing analogue of legendary of Pechmann dye
(exo-dilactone) [10] was also recently presented [11]. These dyes
are generally not very stable and undergo a rearrangement to endo-
6,6-dilactones [11,12]. The only known way to prepare their iso-
indigoid N-analogues (exo-5,5-dilactams), i.e. by direct (alkyl)
amination is somewhat controversial. Whilst no problems were
mentioned in the patent literature [13], low yield and poor selec-
tivity were reported in a scientific paper [14]. If the derivatives with
carboxy ester substituents in the 3,30-positions are considered, the
situation is much more clear. Such Pechmann dye derivative was
synthesized from a furanone ester using Lawesson’s reagent [15].
N-Analogue 1 of Pechmann dye (Scheme 2, Table 1) was obtained
by oxidative dimerization of 5-phenylepyrrolinone ester under
considerably milder reaction conditions [13,16]. The route to the
corresponding thiophene-containing analogue of 1, i.e. compound
2, was recently reported in patents [17,18], describing the syntheses
of 5-(thiophen-2-yl)-pyrrolinone ester.
* Corresponding author. Institute of Organic Chemistry and Technology, Faculty
of Chemical Technology, University of Pardubice, Studentská 95, CZ-53210 Pardu-
bice, Czech Republic.
Our interest in the dyes coming from pyrrolinone esters lead
recently to the studies of their reactions with aromatic nitriles,
E-mail address: tarekaysha@hotmail.com (T. Aysha).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.04.010

T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
531
Table 1
Structure of prepared dyes.
O
O
O
S
S
NaH
OR
+
CH3OCOOR
O
EtOOC
70 OC
R
Ar
R
I
N
BrCH2COOEt
N
Reflux 20 h
Na2CO3 / Aceton
Ar
COOEt
O
O
S
O
S
Dye
Ar
R
CH3COONH4
CH3COOH
OR
COOR
PES I
H N
O
OEt
1
H
O
R= CH3
1m
2a
CH3
H
R= CH2CH3 PES II
S
S
Scheme 1. Synthesis of thienyl derivatives of pyrrolinone esters.
2ma
3
CH3
H
leading to asymmetrical DPPs [19,20], condensations with aromatic
aldehydes, leading to methylidene-pyrrolinones [21,22], and
coupling with diazotized aromatic amines, forming hydrazones
[23,24]. Consequently, we decided to prepare known parent com-
pound 1 and report for the first time the thiophene containing
analogues 2 and 3, differing by ester group in the 3,30-positions, and
to study the relation between their structure and spectral proper-
ties by a combined experimental/theoretical approach. As these
compounds are essentially insoluble, their more soluble N-methyl
derivatives 1m, 2m and 3m were also described. As a 2-naphthyl
substituted pyrrolinone ester was available from our previous
studies [23,24], we also synthesized the new compound 4 and its
soluble derivative 4m in order to study the effect of conjugation
extension.
3m
4
CH3
H
4m
CH3
a
COOMe instead of COOEt in 3,30 positions.
symmetrical structures with an E-arrangement on the central
double bond were investigated and C2 symmetry constraint was
applied in all cases. No imaginary frequencies were found by
vibrational analysis, confirming that the computed geometries
were real minima on the ground state hypersurfaces.
2. Experimental and computational procedures
2.1. Quantum chemical calculations
Time dependent (TD) DFT computations of the vertical excita-
tion energies were carried out on the computed ground state ge-
ometries. The same exchange-correlation functional (B3LYP) was
used with rather broader basis set (6-311þG(2d,p)). The solvent
effect of DMSO was involved by non-equilibrium polarized con-
tinuum model (PCM). The same xc functional, but only 6-311G(d,p)
basis set was used for the excited state TD DFT optimisation of
model compound 1 with only hydrogens in the 3,30-positions.
All methods were taken from Gaussian09W program suite [25],
and the default values of computational parameters were used. The
results were analysed using GaussViewW from Gaussian Inc.
The ground state geometry of six compounds 1e3m was opti-
mized using quantum chemical calculations based on density
functional theory (DFT). Hybrid three-parameter B3LYP functional
in combination with 6-311G(d,p) basis set was used. No constraints
were preliminarily employed, but, if the nonconstrained compu-
tations converged to symmetrical structures, the final computations
were carried out with symmetry constraint. Practically, only
Ar
Ar
C2H5
1
Ar = phenyl, R =
H
COOR
O
N
COOR
H
N
oxidation / air
nitrobenzene
2
Ar = 2-thienyl, R = CH3
Ar = 2-thienyl, R = C2H5
O
2.2. Materials and equipments
O
3
4
N
PES
ROOC
H
Ar = 2-naphthyl, R =
C2H5
2-Acetylthiophene (98%), sodium hydride (60% dispersion in
mineral oil), anhydrous 1,2 dimethoxyethane (99.5%), dime-
thylcarbonate (98%), diethylcarbonate (98%) and ethyl bromoace-
tate (98%) were purchased from SigmaeAldrich. Solvents for
spectroscopic measurements (spectroscopic grade) were pur-
chased from Fluka.
The absorption spectra were recorded at room temperature in
DMSO using a PerkineElmer Lambda 35 spectrophotometer with
1 cm pathlength quartz cuvettes. A PerkineElmer (P.eE.) LS55 was
used for measuring fluorescence spectra at low temperatures.
2-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
Ar
70 °C K2CO3
DMF
MeI
Ar
Ar
C2H5
O
1m
Ar = phenyl, R =
H3C
COOR
O
N
2m
3m
Ar = 2-thienyl, R = CH3
Ar = 2-thienyl, R = C2H5
O
O
O
O
N
Ar
Pechmann dye
ROOC
CH3
4m Ar = 2-naphthyl, R =
C2H5
Ar
Scheme 2. Synthesis of N-analogues of Pechmann dyes and their N-methylated
derivatives.

532
T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
P.eE. This accessory contains an isolated box filled by liquid nitro-
gen 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.eE.
accessory for solid state measurements. Polycrystalline samples
were placed under quartz plate and the emission spectra were
recorded using front face geometry.
information may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EY, UK (fax: þ44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
2.3. Syntheses and analytics
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 to determine the purity of the pre-
pared intermediates and dyes.
Startingpyrrolinoneesters, ethyl4,5-dihydro-5-oxo-2-phenyl(1H)
pyrrole-3-carboxylate and ethyl-4,5-dihydro-5-oxo-2-naphthyl(1H)
pyrrole-3-carboxylate were synthesized according to published pro-
cedures and melting points were similar as the reported ones as
published [20,21].
Synthesis of thienyl pyrrolinone esters (PES I and PES II) based
Positive-ion and negative-ion atmospheric pressure chemical
ionization (APCI) mass spectra were measured on an ion trap
analyser Esquire 3000 (Bruker Daltonics, Bremen, Germany) in the
range m/z 50e1000. The samples were dissolved in acetonitrile and
on using a corresponding b-ketoester I, methyl 3-oxo-3-(thiophen-
2-yl) propanoate in the case of thienyl PES I and ethyl 3-oxo-3-(thi-
ophen-2-yl) propanoate in the case of thienyl PES II were prepared as
reported procedure [17,30].
analysed by direct infusion at the flow rate 100
m
L minꢀ1. The
selected precursor ions were further analysed by MS/MS analysis
under the following conditions: the isolation width m/z ¼ 4, the
collision amplitude in the range 0.7e1.0 V depending on the pre-
cursor ion stability, the temperature of drying gas was 330 ꢁC, the
APCI temperature was 400 ꢁC, the tuning parameter compound
stability was 100%, the flow rate and the pressure of nitrogen were
4 mL minꢀ1 and 45 psi, respectively.
2.3.1. Thienyl pyrrolinone esters
General procedure
Alkyl 3-oxo-3-(thiophen-2-yl) propanoate
(b-ketoester I)
(0.38 mol), ethyl bromoacetate (63.5 g, 0.38 mol), sodium carbonate
(56 g, 0.53 mol), acetone (400 mL) and 1,2 dimethoxyethane (95 mL)
were added to the three necked flask equipped with thermometer,
stirrer and reflux condenser. The reaction mixture was heated under
reflux for 20 h. Acetone was added to the reaction mixture to
maintain a constant volume due to slight evaporation. The cooled
mixture was filtered to remove inorganic salts which were washed
with acetone. The filtrate and acetone from the washing process
were combined and acetone was distilled off with other volatile
fractions up to 150 ꢁC under nitrogen. Acetic acid (285 mL, 5.0 mol)
and ammonium acetate (163 g, 2.11 mol) were added to the dark
residue. The mixture was kept under reflux at 120 ꢁC for 4 h. The
product was obtained by filtration from the mixture after cooling,
then washed with boiling methanol and boiling water.
The 1H and 13C NMR spectra were recorded on a Bruker Avance II
400 spectrometer at 400.13 MHz and 100.62 MHz, respectively. The
samples were dissolved in hexadeuteriodimethyl sulfoxide. The 1H
and 13C NMR chemical shifts were referenced to the central signal
of the solvent (
d
¼ 2.55 and 39.6, respectively).
The X-ray data for crystals of 1m and 2m were obtained at 150 K
using Oxford Cryostream low-temperature device on a Nonius Kap-
ꢁ
paCCD diffractometer with Mo Ka radiation (
l
¼ 0.71073 A), a graphite
monochromator, and the and
f
c
scan mode. Data reductions were
performed with DENZO-SMN [26]. The absorption was corrected by
integration methods [27]. Structures were solved by direct methods
(Sir92) [28] and refined by full matrix least-square based on F2
(SHELXL97) [29]. Hydrogen atoms were mostly localized on a differ-
ence Fourier map, however to ensure uniformityof treatment of crystal,
all hydrogen were recalculated into idealized positions (riding model)
and assigned temperature factors Hiso(H) ¼ 1.2Ueq (pivot atom) or of
2.3.1.1. Methyl 5-oxo-2-(thiophen-2-yl)-4,5-dihydro-1H-pyrrole-3-
carboxylate (thienyl PES I). Greenish needle precipitate, 50% yield
with mp ¼ 221e223 ꢁC.
1H NMR (400 MHz, DMSO-d6,
d, ppm): 3.56 (2H, s, CH2), 3.68
(3H, s, CH3), 7.22 (1H, dd, 3J(H, H) ¼ 5.0 Hz, 3J(H, H) ¼ 3.9 Hz, CH),
7.89 (1H, dd, 3J(H, H) ¼ 3.9 Hz, 4J(H, H) ¼ 1.2 Hz, CH), 7.93 (1H, dd,
3J(H, H) ¼ 5.0 Hz, 4J(H, H) ¼ 1.2 Hz, CH) three thiophene protons,
10.89 (1H, br.s, NH).
ꢁ
1.5Ueq for the methyl moiety with CeH ¼ 0.96, 0.97 and 0.93 A for
methyl, methylene hydrogen atoms and atoms in aromatic rings,
P
P
P
respectively. Rint
¼
rFo2 ꢀ Fo2,meanr/ Fo2, GOF ¼ [ (w(Fo2 ꢀ F2c)2)/
P
P
½
(Ndiffrs ꢀ Nparams)] for all data, R(F) ¼
rrFor ꢀ rFcrr/ rFor for
13C NMR (100 MHz, DMSO-d6,
d, ppm): 30.2, 51.0, 100.7, 126.9,
P
P
observed data, wR(F2) ¼ [ (w(Fo2 ꢀ Fc2)2)/( w(Fo2)2)] for all data.
129.6, 131.6, 132.5, 145.4, 163.4, 176.2.
½
Crystallographic data for 1m: C28H26N2O6, M ¼ 486.51 g/mol,
MS analysis, M ¼ 223 g/mol: positive-ion MS: m/z 224 [M þ H]þ;
m/z 192 [M þ H ꢀ CH3OH]þ 100%, negative-ion MS: m/z 222
[M ꢀ H]ꢀ 100%; m/z 190 [M ꢀ H ꢀ CH3OH]ꢀ 100%.
Elemental analysis; calculated (C10H9NO3S): C (53.80%) H (4.06%)
N (6.27%); found: C (54.40%) H (4.27%) N (6.17%).
ꢁ
monoclinic, P21/c, a ¼ 18.6501(12), b ¼ 7.5760(4), c ¼ 18.0710(10) A,
3
ꢁ
b
m
¼ 107.389(5) , Z ¼ 4, V ¼ 2436.6(3) A , Dc ¼ 1.326 g cmꢀ3
,
ꢁ
¼
0.094 mmꢀ1, Tmin/Tmax
¼ 0.979/0.987; ꢀ24 ꢂ h ꢂ 24,
ꢀ9 ꢂ k ꢂ 9, ꢀ23 ꢂ l ꢂ 21; 16,414 reflections measured (qmax ¼ 27ꢁ),
16,403 independent (Rint ¼ 0.0526), 3553 with I > 2
s(I), 325 parame-
ters, S ¼ 1.210, R1(obs. data) ¼ 0.0595, wR2(all data) ¼ 0.1082; max.,
2.3.1.2. Ethyl 5-oxo-2-(thiophen-2-yl)-4,5-dihydro-1H-pyrrole-3-
carboxylate (thienyl PES II). Yellowish precipitate, 54% yield with
mp ¼ 196e199 ꢁC.
ꢁꢀ3
min. residual electron density ¼ 0.263, ꢀ0.322 e A
.
Crystallographic data for 2m: C22H18N2O6S2, M ¼ 470.50 g/mol,
monoclinic, P21/c, a ¼ 13.0170(6), b ¼ 17.2491(10), c ¼ 9.8270(10) A,
1HNMR (400 MHz,DMSO-d6,
d
, ppm):1.26 (3H, t, 3J(H, H)¼ 7.1 Hz,
ꢁ
3
ꢁ
b
m
¼ 111.703(5) , Z ¼ 4, V ¼ 2050.0(3) A , Dc ¼ 1.524 g cmꢀ3
,
CH3), 3.45 (2H, s, CH2), 4.17 (2H, q, 3J(H, H) ¼ 7.1 Hz, OCH2), 7.23
(1H, dd, 3J(H, H) ¼ 5.0 Hz, 3J(H, H) ¼ 3.9 Hz, CH), 7.88 (1H, dd,
3J(H, H) ¼ 3.9 Hz, 4J(H, H) ¼ 1.2 Hz, CH), 7.93 (1H, dd, 3J(H, H) ¼ 5.0 Hz,
4J(H, H) ¼ 1.2 Hz, CH) three thiophene protons, 10.86 (1H, br.s, NH).
ꢁ
¼
0.305 mmꢀ1
,
Tmin/Tmax
¼
0.942/0.973; ꢀ16
ꢂ
h
ꢂ
16,
ꢀ22 ꢂ k ꢂ 22, ꢀ12 ꢂ l ꢂ 11; 15,774 reflections measured (qmax ¼ 27.4ꢁ),
15,558 independent (Rint ¼0.0674), 3170with I>2
s(I), 302parameters,
S ¼ 1.122, R1(obs. data) ¼ 0.0673, wR2(all data) ¼ 0.1150; max., min.
13C NMR (100 MHz, DMSO-d6,
d, ppm): 14.4, 38.3, 59.6, 101.2,
ꢁꢀ3
residual electron density ¼ 0.355, ꢀ0.538 e A
.
126.9, 129.7, 131.9, 132.4, 145.2, 163.1, 176.3.
Crystallographic data for structural analysis have been depos-
ited with the Cambridge Crystallographic Data Centre, CCDC nos.
912966 & 912967 for 1m and 2m, respectively. Copies of this
MS analysis, M ¼ 237 g/mol: Positive-ion MS: m/z 238 [M þ H]þ;
m/z 192 [M þ H ꢀ C2H5OH]þ 100%, Negative-ion MS: m/z 236
[M ꢀ H]ꢀ 100%; m/z 190 [M ꢀ H ꢀ C2H5OH]ꢀ 100%.

T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
533
Elementalanalysis;Calculated(C11H11NO3S):C(55.68%)H(4.67%)
Elemental analysis; Calculated (C22H18N2O6S2):
C
(56.16%)
N (5.90%); Found: C (55.91%) H (4.78%) N (5.92%).
H (3.86%) N (5.95%); Found: C (56.51%), H (3.91%), N (5.94%).
2.3.2.4. Ethyl-4-[1,2-dihydro-4-(ethoxycarbonyl)-5-(naphthalen-2-
yl)-2-oxo-3H-pyrrol-3-ylidene]-4,5-dihydro-2-(naphthalen-2-yl)-5-
oxo-1H-pyrrole-3-carboxylate (4). Preparative yield 52%, a sample
was purified by recrystallization from ethanol, mp < 300 ꢁC.
2.3.2. Synthesis of Pechmann dye analogues
General procedure
To 500 mL Keller flask equipped with stirrer, thermometer,
refluxing condenser and capillary for air supply, nitrobenzene
(275 mL), pyrrolinone ester (3.5 mmol) and sodium 9,10-
anthraquinone-1,5-disulfonate (0.75 g, 1.82 mmol) were added.
The reaction mixture was heated to 180 ꢁC under intensive air
bubbling and stirring. After 1 h of heating nitrobenzene was
distilled off under reduced pressure and to the blue residual, n-
hexane (230 mL) was added and the mixture was heated under
reflux for 30 min. The product was filtrated while remaining hot
and washed by 3 ꢃ 50 mL boiling methanol.
1HNMR (400 MHz, DMSO-d6,
d
, ppm):1.21 (6H, t, 3J(H, H)¼ 7.1 Hz,
2 ꢃ CH3), 4.21 (4H, q, 3J(H, H) ¼ 7.1 Hz, 2 ꢃ CH2), 7.67 (2H, m),
7.69 (2H, m), 7.72 (2H, m), 8.04 (2H, m), 8.06 (2H, m), 8.08 (2H,
m), 8.38 (2H, m) fourteen naphthalene protons, 11.34 (2H, br.s,
2 ꢃ NH).
13C NMR (100 MHz, DMSO-d6,
d
, ppm): 14.0 (2 ꢃ CH3), 60.3
(2 ꢃ CH2), 109.2, 126.1, 128.3, 132.1, 133.9, 151.7 (2 ꢃ all C), 125.2,
127.8, 127.9, 128.0, 128.3, 128.9, 129.3 (2 ꢃ all CH), 164.3 (2 ꢃ COO),
168.2 (2 ꢃ C]O).
MS analysis, M ¼ 558 g/mol: Positive-ion MS: m/z 559 [M þ H]þ;
m/z 513 [M þ H ꢀ C2H5OH]þ 100%, Negative-ion MS: m/z 557
[M ꢀ H]ꢀ 100%.
2.3.2.1. Ethyl-4-[1,2-dihydro-4-(ethoxycarbonyl)-5-phenyl-2-oxo-
3H-pyrrol-3-ylidene]-4,5-dihydro-2-phenyl-5-oxo-1H-pyrrole-3-
carboxylate (1). Preparative yield 69%, mp < 300 ꢁC.
Elemental analysis; Calculated (C34H26N2O6): C (73.11%), H (4.69%),
N (5.02%); Found: C (73.22%), H (4.85%), N (5.09%).
1H NMR (400 MHz, DMSO-d6,
d
, ppm): 1.19 (6H, t, 3J(H, H) ¼ 7.1 Hz,
2 ꢃ CH3), 4.16 (4H, q, 3J(H, H) ¼ 7.1 Hz, 2 ꢃ CH2), 7.56 (4H, m,
aromatic protons), 7.72 (6H, m, aromatic protons), 11.19 (2H, br.s,
2 ꢃ NH).
2.3.2.5. Ethyl-4-[1,2-dihydro-4-(ethoxycarbonyl)-1-methyl-5-
phenyl-2-oxo-3H-pyrrol-3-ylidene]-4,5-dihydro-1-methyl-2-phenyl-
5-oxo-1H-pyrrole-3-carboxylate (1m). DMF (60 mL), compound 1
(1.44 g, 3.14 mmol) and anhydrous potassium carbonate (0.6 g,
4.3 mmol) were added to the three-necked flask connected with
thermometer and refluxing condenser. After 20 min of stirring,
iodomethane (2.68 g, 8.84 mmol) was added. The reaction mixture
was heated to 100 ꢁC for 2 h. Coppery laminas in dark violet solu-
tion were obtained after precipitation by added distilled water
(50 mL). Product was extracted from the mixture by ether
(3 ꢃ 50 mL). Organic layer was isolated, additional ether (20 mL)
was added and the product was filtered off. Acquired crystals were
flushed by 15 mL of ether for removing mother liquor and dried.
0.3 g, 20% yield of the product was obtained, mp 198e202 ꢁC.
13C NMR (100 MHz, DMSO-d6,
d, ppm): 14.0 (2 ꢃ CH3), 60.3
(2 ꢃ CH2), 108.7, 128.3, 128.7, 151.8 (2 ꢃ all C), 128.6, 128.8, 131.3
(2 ꢃ all CH), 164.2 (2 ꢃ COO), 168.2 (2 ꢃ C]O).
MS analysis, M ¼ 458 g/mol: Positive-ion MS: m/z 459 [M þ H]þ;
m/z 413 [M þ H ꢀ C2H5OH]þ 100%, Negative-ion MS: m/z 457
[M ꢀ H]ꢀ 100%; m/z 411 [M ꢀ H ꢀ C2H5OH]ꢀ 100%.
Elemental analysis; Calculated (C26H22N2O6):
C (68.11%),
H (4.84%), N (6.11%); Found: C (67.88%), H (4.47%), N (6.11%).
2.3.2.2. Methyl-4-[4-(methoxycarbonyl)-2-oxo-5-(thiophen-2-yl)-
1,2-dihydro-3H-pyrrol-3-ylidene]-5-oxo-2-(thiophen-2-yl)-4,5-
dihydro-1H-pyrrole-3-carboxylate (2). Preparative yield 56%,
mp < 300 ꢁC.
1HNMR (400 MHz,DMSO-d6,
d
, ppm):1.03 (6H, t, 3J(H, H) ¼ 7.1 Hz,
1H NMR (400 MHz, DMSO-d6,
d
, ppm): 3.74 (6H, s, 2 ꢃ CH3), 7.30
2 ꢃ CH3), 2.96 (6H, s, 2 ꢃ N-CH3), 4.03 (4H, q, 3J(H, H) ¼ 7.1 Hz,
2 ꢃ CH2), 7.53 (4H, m, aromatic protons), 7.71 (6H, m, aromatic
protons).
(2H, dd, 3J(H, H) ¼ 5.0 Hz, 3J(H, H) ¼ 3.9 Hz, 2 ꢃ CH), 7.98 (2H,
dd, 3J(H, H) ¼ 3.9 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH), 8.07 (2H, dd,
3J(H, H) ¼ 5.0 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH) six thiophene protons,
11.39 (2H, br.s, 2 ꢃ NH).
13C NMR (100 MHz, DMSO-d6,
d
, ppm): 13.9 (2 ꢃ CH3), 28.1
(2 ꢃ N-CH3), 60.0 (2 ꢃ CH2), 109.0, 127.3, 129.4, 159.6 (2 ꢃ all C),
128.3, 128.4, 129.3 (2 ꢃ all CH), 163.3 (2 ꢃ COO), 166.6 (2 ꢃ C]O).
MS analysis M ¼ 486 g/mol, Positive-ion MS: m/z 487 [M þ H]þ,
100%; m/z 441 [M þ H ꢀ C2H5OH]þ.
13C NMR (100 MHz, DMSO-d6,
d
, ppm): 51.5 (2 ꢃ CH3), 107.3,
127.5, 129.9, 144.1 (2 ꢃ all C), 128.3, 132.8, 134.8 (2 ꢃ all CH), 164.7
(2 ꢃ COO), 167.7 (2 ꢃ C]O).
MS analysis, M ¼ 442 g/mol: Positive-ion MS: m/z 443 [M þ H]þ;
m/z 396 [M þ H ꢀ CH3OH]þ 100%, Negative-ion MS: m/z 441
[M ꢀ H]ꢀ 100%.
Elementalanalysis;Calculated(C28H20N2O6):C(69.12%),H(5.39%),
N (5.76%), Found: C (68.94%), H (5.21%), N (5.67%).
Elemental analysis; Calculated (C20H14N2O6S2): C (54.29%),
H (3.19%), N (6.33%); Found: C (55.01%), H (3.21%), N (6.50%).
2.3.3. N-Alkylated Pechmann dye analogues 2me4m
General procedure
DMF (80 mL), corresponding Pechmann dye analogue
(5.56 mmol) and anhydrous potassium carbonate (1.60 g,11.3 mmol)
were added to 250 mL three-necked flask connected with ther-
mometer and reflux condenser. After 30 min of stirring at room
temperature, iodomethane (6.32 g, 44.5 mmol) was added. The re-
action mixture was heated to 65e70 ꢁC for 1 h then the mixture was
cooled down to room temperature then added to iced water (50 mL),
ppt was filtrated off and washed with boiling water (150 mL).
2.3.2.3. Ethyl-4-[4-(ethoxycarbonyl)-2-oxo-5-(thiophen-2-yl)-1,2-
dihydro-3H-pyrrol-3-ylidene]-5-oxo-2-(thiophen-2-yl)-4,5-dihydro-
1H-pyrrole-3-carboxylate (3). Preparative yield 58%, mp < 300 ꢁC.
1H NMR (400 MHz, DMSO-d6,
d
, ppm):1.27 (6H, t, 3J(H, H)¼ 7.1 Hz,
2 ꢃ CH3), 4.23(4H, q, 3J(H, H) ¼ 7.1 Hz, 2 ꢃ CH2), 7.33 (2H, dd,
3J(H, H) ¼ 5.0 Hz, 3J(H, H) ¼ 3.9 Hz, 2 ꢃ CH), 7.96 (2H, dd,
3J(H, H) ¼ 3.9 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH), 8.06 (2H, dd,
3J(H, H) ¼ 5.0 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH) six thiophene protons,
11.35 (2H, br.s, 2 ꢃ NH).
2.3.3.1. Methyl (4E)-4-[4-(methoxycarbonyl)-1-methyl-2-oxo-5-(thi-
ophen-2-yl)-1,2-dihydro-3H-pyrrol-3-ylidene]-1-methyl-5-oxo-
13C NMR (100 MHz, DMSO-d6,
d
, ppm): 14.1 (2 ꢃ CH3), 60.4
(2 ꢃ CH2), 107.7, 127.4, 130, 143.9 (2 ꢃ all C), 128.2, 132.6, 134.6
2-(thiophen-2-yl)-4,5-dihydro-1H-pyrrole-3-carboxylate
(2m).
Dark blue precipitate with 91% yield was obtained, mp < 300 ꢁC.
(2 ꢃ all CH), 164.3 (2 ꢃ COO), 167.7 (2 ꢃ C]O).
MS analysis, M ¼ 470 g/mol: Positive-ion MS: m/z 471 [M ꢀ H]þ
100%; m/z 425 [M þ H ꢀ C2H5OH]þ 100%.
1H NMR (400 MHz, DMSO-d6,
d, ppm): 3.17 (6H, s, 2 ꢃ N-CH3),
3.66 (6H, s, 2 ꢃ CH3), 7.35 (2H, dd, 3J(H, H) ¼ 5.0 Hz, 3J(H, H) ¼ 3.8 Hz,

534
T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
3
2 ꢃ CH), 7.74 (2H, dd, J(H, H) ¼ 3.8 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH),
8.07 (2H, dd, 3J(H, H) ¼ 5.0 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH) six thio-
phene protons.
the presence of air and nitrobenzene as a solvent with a good yield
over 50% for all compounds (Scheme 2). Compounds 1e4 are highly
insoluble in a variety of solvents and generally improved solubility
was noted for their N-methyl derivatives 1me4m were prepared by
N-methylation with methyl iodide using potassium carbonate as a
base in DMF. The reaction yield was over 90% for all compounds
except compound 1m which was 20%.
13C NMR (100 MHz, DMSO-d6,
d, ppm): 28.9 (2 ꢃ N-CH3), 51.6
(2 ꢃ CH3), 109.5, 126.6, 127.7, 148.2 (2 ꢃ all C), 128.1, 1132.6, 33.4
(2 ꢃ all CH), 164.0 (2 ꢃ COO), 166.6 (2 ꢃ C]O).
MS analysis, M ¼ 470 g/mol: Positive-ion MS: m/z 471 [M þ H]þ;
m/z 425 [M þ H ꢀ CH3OH]þ 100%, Negative-ion MS: m/z 469
[M ꢀ H]ꢀ 100%.
3.2. X-ray diffraction and DFT calculated structures
Calculated (C22H18N2O6S2): C (56.16%) H (3.86%) N (5.95%)
Found: C (56.45%), H (3.91%), N (5.99%).
Pechmann dye was established as the strictly planar E isomer
(180ꢁ) both by X-ray diffraction [31] and our DFT B3LYP/
6-311G(d,p) calculations. These calculations also give planar
arrangement on the central exo-5,5-dilactam in its O / NH and
NMe analogues, but the phenylepyrrolinone dihedral angle is 21ꢁ
and 38ꢁ respectively. Substitution of the 3,30 positions by ethyl
ester groups (compound 1) leads to planarity distortion on the
central C]C bond (167ꢁ, resp. 168ꢁ) in a slightly more stable
2.3.3.2. Ethyl (4E)-4-[4-(ethoxycarbonyl)-1-methyl-2-oxo-5-(thio-
phen-2-yl)-1,2-dihydro-3H-pyrrol-3-ylidene]-1-methyl-5-oxo-2-(thi-
ophen-2-yl)-4,5-dihydro-1H-pyrrole-3-carboxylate (3m). Dark blue
precipitate with 92% yield of the product was obtained,
mp < 300 ꢁC.
1HNMR(400MHz, DMSO-d6,
d
, ppm):1.16(6H, t, 3J(H, H)¼ 7.1Hz,
2 ꢃ CH3), 3.15 (6H, s, 2 ꢃ N-CH3), 4.13 (4H, q, 3J(H, H) ¼ 7.1 Hz,
2 ꢃ CH2), 7.35 (2H, dd, 3J(H, H) ¼ 5.0 Hz, 3J(H, H) ¼ 3.8Hz, 2 ꢃ CH), 7.74
(2H, dd, 3J(H, H) ¼ 3.8 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH), 8.07 (2H, dd,
3J(H, H) ¼ 5.0 Hz, 4J(H, H) ¼ 1.2 Hz, 2 ꢃ CH) six thiophene protons.
(D
E ¼ 0.39 kcal.molꢀ1) s-trans rotamer (carboxy ester rotated 115ꢁ
out of pyrrolinone ring), resp. s-cis rotamer (carboxy ester rotated
39ꢁ). Sterical effect of carboxy ester groups also leads to an increase
of phenylepyrrolinone rotation (35ꢁ and 38ꢁ for s-trans and s-cis,
respectively). N, N0-Dimethylation (compound 1m) essentially
leaves both the central (167ꢁ, resp. 169ꢁ) and carboxy ester (117ꢁ,
resp. 39ꢁ) torsions unaffected, but the rotations of the pendant
phenyls are considerably bigger (50ꢁ, resp. 60ꢁ for the s-trans, resp.
s-cis rotamer). The s-trans rotamer of 1m with respect to carboxy
ester orientation is the slightly more stable rotamer by theory
13C NMR (100 MHz, DMSO-d6,
d
, ppm): 14.1 (2 ꢃ CH3), 28.8
(2 ꢃ N-CH3), 60.5 (2 ꢃ CH2), 110.1, 126.6, 127.9, 148.2 (2 ꢃ all C),
128.1, 132.5, 133.2 (2 ꢃ all CH), 163.6 (2 ꢃ COO), 166.6 (2 ꢃ C]O).
MS analysis, M ¼ 498 g/mol: Positive-ion MS: m/z 499 [M þ H]þ;
m/z 453 [M þ H ꢀ C2H5OH]þ 100%, Negative-ion MS: m/z 497
[M ꢀ H]ꢀ 100%.
Calculated (C24H22N2O6S2): C (57.82%) H (4.45%) N (5.62%);
Found: C (57.91%), H (4.51%), N (5.60%).
(D
E ¼ 0.22 kcal molꢀ1).
Generally, the experimental molecular structure of 1m obtained
by single crystal X-ray diffractometry fully supports the DFT pre-
dictions. An agreement in central double bond torsion is absolute
(167ꢁ), the molecule is found as an s-trans rotamer with respect to
the asymmetrically rotated ester groups (120ꢁ and 131ꢁ) and with
pendant phenyl groups rotated by 56ꢁ and 59ꢁ (Fig. 1).
2.3.3.3. Ethyl-4-[1,2-dihydro-4-(ethoxycarbonyl)-1-methyl-5-(naph-
thalen-2-yl)-2-oxo-3H-pyrrol-3-ylidene]-4,5-dihydro-1-methyl-2-
(naphthalen-2-yl)-5-oxo-1H-pyrrole-3-carboxylate (4m). 1H NMR
(400 MHz, DMSO-d6,
d
, ppm): 1.01 (6H, t, 3J(H, H) ¼ 7.1 Hz, 2 ꢃ CH3),
3.05 (6H, s, 2 ꢃ N-CH3), 4.21 (4H, q, 3J(H, H) ¼ 7.1 Hz, 2 ꢃ CH2), 7.68
(2H, m), 7.70 (2H, m), 7.72 (2H, m), 8.08 (2H, m), 8.10 (2H, m), 8.13 (2H,
m), 8.27 (2H, m) fourteen naphthalene protons.
Looking for the absolute energy minima with respect to possible
conformations of four thienyl substituted compounds 2, 2m, 3 and
3m was more exhaustive, as not only the minima with respect to s-
trans/s-cis rotamerism of methyl (ethyl) carboxy ester, but also of
thiophene s-trans/s-cis orientation relative to pyrrolinone had to be
calculated, i.e. four optimisations for each of these four compounds
were carried out. The trend is quite clear with respect to ester group
orientation: s-cis arrangement (i.e. opposite to that of the phenyl
substituted compounds 1 and 1m) is strongly preferred (5.23e
7.38 kcal molꢀ1) for all four compounds 2, 2m, 3, 3m (i.e. irre-
spective of whether methyl or ethyl ester is in position 3,
irrespective to N-methylation and irrespective to the orientation
of thiophene ring). The s-cis rotamers with respect to thiophene
arrangement are also preferred in all cases, but this type of
rotamerism affects the total energy only moderately (1.78e
2.27 kcal molꢀ1). The most stable conformations of thienyl de-
rivatives show more planar structures, as compared to phenyl
substituted analogues (1, 1m). Typically, the central bond torsion is
169e170ꢁ for all and thiophene torsions are about 29ꢁ for non-
methylated (2, 3) and 42ꢁ for N-methylated examples (2m, 3m).
The comparison of theoretical geometry of compound 2m with
the data coming from X-ray diffractometry is not as straightforward
13C NMR (100 MHz, DMSO-d6,
d
, ppm): 14.0 (2 ꢃ CH3), 28.3
(2 ꢃ N-CH3), 60.0 (2 ꢃ CH2), 109.3, 126.0, 130.0, 132.1, 133.5, 155.9
(2 ꢃ all C), 126, 127.1, 127.8, 128.0, 128.3, 128.7, 128.8 (2 ꢃ all CH),
163.4 (2 ꢃ COO), 166.7 (2 ꢃ C]O).
MS analysis, M ¼ 586 g/mol: Positive-ion MS: m/z 587 [M þ H]þ;
m/z 541[M þ H ꢀ C2H5OH]þ 100%.
Elementalanalysis;Calculated(C36H30N2O6):C(73.71%),H(5.15%),
N (4.78%), Found: C (73.42%), H (5.30%), N (5.03%).
3. Results and discussions
3.1. Synthesis
The synthesis of various pyrrolinone esters (phenyl, 2-naphthyl)
[20,21] commenced from
condensation of the corresponding esters. This method was not
successful for thienyl derivatives, thus the -ketoester of thienyl
b-ketoesters obtained through Claisen
b
derivatives I were prepared by the reaction of 2-acetylthiophene
with dimethylcarbonate in case of thienyl PES I or diethylcar-
bonate in the case of thienyl PES II in the presence of sodium hy-
as in the case of 1m. First, 2m crystallizes as a centrosymmetrical
ꢁ
dride. The process was very efficient giving
b-ketoester I with yields
pep
stacked dimer with the interplane distance about 3.79 A
over 90% (Scheme 1) and the overall yield of thienyl pyrrolinone
ester was higher than 50%. During the last amidation/cyclization
step in syntheses of thienyl PES I (Scheme 1) ethanol was the only
leaving product and no pyrrolinone ethyl ester was formed.
Four Pechmann dye analogues were successfully prepared by an
oxidative dimerization of the corresponding pyrrolinone esters in
(Fig. 2) and a stacking interaction may partially deform a molecular
structure, and, second, there is a conformational disorder on the
pendant (unstacked) part of a molecule, where thiophenes in both
s-trans and s-cis arrangement are present. Experiment confirms the
preferred s-cis conformation of methyl ester substituent and, as in
the case of 1m, gives an absolute agreement with theory on central

T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
535
Fig. 1. The molecular structure (ORTEP 50% probability level) of compounds 1m (a) and 2m (b).
bond torsion angle (169ꢁ) for 2m. On the other hand the theoreti-
cally preferred s-cis thiophene rotamer is not present in a stacked
part of a molecular dimer and only partially present in a pendant
part. Definitely, both rotamers should be considered in an inter-
pretation of spectral data in solution.
(exo-5,5-dilactone), the solution of which decolorizes on exposure
to daylight during few hours by our experience.
Full assignment of NMR chemical shifts is summarized in
Table 2 as the numbering of atoms of prepared dyes as shown in
Fig. 3. 1H chemical shifts of methyl and methylene protons of
carboxy ester group of compounds 1 (1m) and 3 (3m) do not show
considerable differences observed for s-trans and s-cis arrange-
ment of aryl-methylidene-pyrrolinones [21,22], that may be
caused by a remarkable out-of-plane torsion of this side group in
all Pechmann dye heteroanalogues, suggestive of relatively free
rotation on NMR time scale. Similar lability of pendant thiophenes
orientation may explain an absence of distinct signals of s-trans
and s-cis rotamers of compounds 2e3m., i.e., the signals of thio-
phene localized protons and carbons correspond to an average
over both conformers.
3.3. NMR spectroscopy
Combination of 1H and 13C NMR spectroscopy is a useful tool to
detect E/Z photoisomerization on exocyclic C]C bond and s-cis/
s-trans conformational changes on carboxy ester group of aryl-
methylidene-pyrrolinones based on pyrrolinone esters [21,22]. So
the same procedure was applied to the Pechmann dyes under
study. Nevertheless, in this case no changes of NMR spectra were
found even after several weeks of standing on daylight. So we
conclude, that E isomers of Pechmann dye heteroanalogues do not
undergo photoisomerization. Furthermore, they are also highly
stable with respect to light induced decomposition, which is in
sharp contrast with the behaviour of original Pechmann dye
3.4. Absorption and fluorescence
Absorption spectra of all prepared compounds were measured
in DMSO and 2-MeTHF at room temperature (Table 3). TD DFT
computed excitation energies for various conformations of com-
pounds 1e3m including the solvent effect of DMSO by polarized
continuum model (PCM) are summarized in Table 4. None of the
compounds fluoresce in solution at room temperature, while all of
them show fluorescence at low temperature in MTHF solvent glass
(Table 3). Solid-state fluorescence of the powders was detected in
some cases (1m, 4, 4m), but never for thiophene substituted de-
rivatives 2e3m.
All compounds show only one strong HOMOeLUMO absorption
band in the visible region. The room temperature absorption
spectra in both solvents show similar shape and a bathochromic
Fig. 2. X-ray structure of
conformational disorder on pendant thiophenes (A e perpendicular, B e along a
pyrrolinone plane).
pep stacked dimer of compound 2m with highlighted

536
T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
Table 2
shift of 9e23 nm in more polar DMSO compared with MTHF
(Table 3). The vibronic structure is detectable for more planar non-
alkylated compounds (Figs. 4 and 5) with the absolute maximum
corresponding to 0e1 vibronic transition for 1 and 4 (Fig. 4) and to
0e0 band for relatively more planar thienyl substituted 2 (Figure 5)
and 3. Further deviation from planarity caused by N-alkylation is
manifested by more blurred spectra with 0e1 absolute maxima for
all compounds and 0e0 vibronic transition observed only as a
barely discernible long-wavelength shoulder (Figs. 6 and 7). On the
other hand, fluorescence excitation spectra in a low temperature
MTHF glass show considerably better resolved vibronic structure
with 0e0 transitions detectable as a (usually) absolute maximum
in all eight cases, enabling the comparison with theoretical exci-
tation energies. Although the PCM parameters describing the sol-
vent effect of highly polar [32] MTHF glass are not available in
Gaussian software package [25], there was recently shown a good
correlation between experimental 0e0 vibronic maxima in this
glass and theoretical PCM TD DFT excitations energies obtained for
DMSO parameterization as a representative of highly polar envi-
ronment [33]. The theoretical results generally reflect all trends
observed experimentally, i.e. bathochromic (and hyperchromic)
effect accompanying heterosubstitution (1e3), marginal effect of
the type (methyl- vs. ethyl) of alkyl in ester group in position 3
(2 vs. 3) and a bathochromic (but hypochromic) effect of N-alkyl-
ation. Quantitatively, the theoretical wavelengths are under-
estimated by 13 and 19 nm for phenyl derivatives 1 and 1m,
respectively, while an agreement between theory and experiment
for thienyl substituted derivatives 2e3m is outstanding: absolute
deviation is less than 5 nm for (experimentally detected) 2m
rotamer with s-cis ester and s-trans thiophene arrangement. A
bathochromic and hyperchromic effect of conjugation extension
(1e4) was not treated theoretically, because of considerably higher
computational costs for compounds 4 and 4m in their possible
rotamers of 2-naphthyl [20]. Compound 1m absorbs in MTHF at
room temperature at 560 nm, i.e. shows only 2 nm hypsochromic
shift with respect to the reported derivative without carboxyesters
in the 3.30-positions, although the planarity of 1m is more distorted
(50ꢁ vs. 38ꢁ by DFT). We ascribe this effect to a compensation of the
non-planarity on single bond (phenylepyrrolinone) by a non-
planarity on central double bond, causing bathochromic shift
through the BruningseCorvin effect [34], observed e.g. for sterically
hindered indigo derivatives [35].
1H and 13C chemical shifts NMR for compounds 1e4 (A) and for N-methylated
compounds 1me4m (B) in DMSO-D6.
(A) For compounds 1e4
H/C no.
1
2
3
4
d
(1H)
d
(
13C)
d
(1H)
d
(
13C)
d
(1H)
d
(
13C)
d
(1H)
d(
13C)
(NH)
C]O
1
2
3
11.19
e
e
11.39
e
e
11.35
e
e
11.34
e
e
168.2
128.3
108.7
151.8
164.2
60.3
14.0
128.7
128.8
128.6
131.3
e
167.7
129.9
107.3
144.1
164.7
e
167.7
130.0
107.7
143.9
164.3
60.4
168.2
128.3
109.2
151.7
164.3
60.3
e
e
e
e
e
e
e
e
e
e
e
e
COO
CH2
CH3
1/
e
e
e
e
4.16
1.19
e
e
4.23
1.27
e
4.21
1.21
8.38 129.3
3.74
e
51.5
e
14.1
e
14.0
2/
7.72
7.56
7.59
e
e
127.5
e
127.4
e
126.1
7.67 125.2
8.08 128.0
3/
7.98 132.8
7.33 128.3
7.96 132.6
7.33 128.2
4/
4a/
5/
e
e
e
e
e
133.9
8.04 127.8
7.72 128.3
7.69 127.9
8.06 128.9
7.56
7.72
e
128.6
128.8
e
8.07 134.8
e
8.06 134.6
e
6/
e
e
e
e
e
e
e
e
7/
e
e
e
e
e
e
8/
e
e
8a/
e
e
e
132.1
(B) For N-methylated compounds 1me4m
1m
2m
3m
4m
3.05
e
e
e
e
e
(NeCH3) 2.96
C]O
1
2
3
28.1
166.6
127.3
109.0
155.9
163.3
60.0
13.9
128.3
129.4
128.4
130.7
e
3.17
28.9
166.6
127.7
109.5
148.2
164.0
e
3.15
28.8
28.3
166.7
127.4
109.3
155.9
163.4
60.0
e
e
e
e
e
e
e
e
e
e
e
e
166.6
127.9
110.1
148.2
163.6
60.5
e
e
e
COO
CH2
CH3
1/
e
4.03
1.03
e
a
4.13
1.16
4.21
1.01
8.27 130.0
3.66
e
51.6
e
14.1
e
14.0
e
2/
e
126.6
e
126.6
e
126.0
7.72 126.0
8.13 127.8
3/
7.74 133.4
7.35 128.1
7.71 133.2
7.34 128.1
a
a
4/
4a/
5/
e
a
e
e
e
e
e
133.5
8.08 127.8
7.70 128.0
7.68 127.1
8.10 128.7
128.4
129.4
e
8.07 132.6
e
8.06 132.5
e
6/
e
e
e
e
e
e
e
e
a
7/
e
e
e
e
e
e
e
e
e
8/
e
8a/
e
e
132.1
a
7.53e7.71 (10H, m, aromatic protons).
7
6
4
5
8a
CH3
4a
4
4
R
3
4
CH3
1
O
3
O
5
O
2
5
2
3
O
2
S
3
O
2
O
1
O
1
O
3
6
O
3
2
1
2
1
N
R
1
R
N
N
R
N
R
R
N
R
N
O
O
O
O
O
S
O
O
O
O
H3C
R
H3C
1 - 4 R= H
1m - 4m R= CH3
R= CH3 (2)
R= C2H5 (3)
Fig. 3. The structures of prepared dyes and numbering of atoms as NMR data in Table 2.

T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
537
Table 3
1
Absorption, fluorescence emission (Fl.), excitation (Ex.) spectra and molar absorp-
tion coefficients of prepared compounds.
0.8
0.6
0.4
0.2
0
Compound 2-MeeTHF
Abs. max Fl. max
DMSO
Fl. max.
solid (nm)
Ex. max. Abs. max.
3
(L cmꢀ1
molꢀ1
(nm)
77 K (nm) 77 K (nm) (nm)
)
1
1m
2
2m
3
3m
4
556, 585 642, 694 568, 613 572, 605 15,700
560 653, 698 578, 626 573 13,400
589, 627 680, 731 608, 654 605, 643 34,300
593 693, 746 615, 665 602 27,600
589, 627 683, 731 617, 663 607, 644 39,100
591 689, 741 617, 666 609 31,200
571, 604 656, 707 579, 630 589, 619 28,800
e
755
e
e
350
450
550
650
750
850
(nm)
e
A in DMSO
Excitation at 77 K
A in 2-MeTHF
Fluorescence in solid state
Fluorescence at 77 K
e
705
(sh)
569
(sh)
Fig. 4. Absorption and fluorescence spectra of compound 4.
4m
663, 708 579, 634 581
23,800
763
The underlined values are the absolute absorption maxima.
undergo rotations leading to a proximity of S1 and S0 hypersurfaces
near the excited state equilibrium geometry and consequent
internal conversion. An absence of eventual cis isomer in solution,
confirmed by NMR, means either that rotation in excited state is
limited (hindered), or the raised cis isomer is thermally highly
unstable in the ground state.
An absence of fluorescence in solution for all eight compounds is
probably based upon considerable extension of the central carbone
carbon bond in the lowest S1 excited state. The DFT computed
ꢁ
length of the central C]C bond of 1m (1.374 A) in S0 agrees quite
ꢁ
well with experiment (1.369 A) giving thus a good chance for the
Last, but not the least, an absence of solid-state fluorescence in
thiophene containing derivatives should be mentioned. The crystal
environment can be considered as rigid similarly to the solvent
glass that should limit the excited state geometrical relaxation, but
also enables various bimolecular specific interactions like energy
and electron transfer [36], absent in diluted (frozen) solutions.
We will discuss only the difference between compounds 1m
and 2m, for which the structural interactions in crystal are known
from X-ray diffractometry. In the case of compound 1m, there are
relevance of theoretical modelling based on (TD) DFT. As the
excited state geometry optimisations by TD DFT are extremely time
consuming, they were carried out only for the model NH analogue
of Pechmann dye without the side chain carboxy ester groups.
When going from 1m to 1, the lengths of the central C]C bond in S0
ꢁ
state is only slightly lengthened (1.376 A) by DFT, while the absence
of 3,30substituents makes the model compound planar in central
ꢁ
part and, consequently, the C]C bond is shorter (1.372 A). TD DFT
optimisation gives fully planar molecule in the S1 state, i.e. the
central part remains planar, although the central bond is remark-
no pep stacking and other strong interactions, thus its solid-state
fluorescence can be considered as a result of conserved proper-
ties of the individual molecules in the rigid environment [37]. On
ꢁ
ably lengthened (1.410 A) and phenylepyrrolinone dihedral angle is
near 180ꢁ, as the length of corresponding exocyclic single bond is
the other hand, compound 2m crystallizes as
pep stacked dimer,
ꢁ
ꢁ
considerably shortened (from 1.458 A in S0 to 1.442 A in S1). By our
opinion, the relatively weak central bond in excited state can
which in fact may not automatically mean the absence of fluores-
cence [38]. We carried out single point energy calculation of the
dimer of 2m at DFT B3LYP/6-311G(d,p) level on experimental
geometry (Fig. 2) and evaluated HOMO and LUMO splitting-in-
dimer as these energy splittings in the case of a centrosymmetrical
dimers relate to electron transfer integrals and, consequently, to
the probability of electron-transfer inside the dimer [39]. We found
HOMO splitting of 107 meV and LUMO splitting 80 meV, i.e., the
values are sufficiently high to enable the transfer in both splitted
orbitals after excitation. The formation of radical cation/radical
anion pair is thus possible in a dimer of 2m and its subsequent
recombination may form the main fluorescence quenching channel
in crystal.
Table 4
PCM (DMSO) TD DFT excitation energies recomputed to wavelengths (l00) and
oscillator strengths (fosc) of the longest wavelength HOMOeLUMO absorption band.
The data for more probable ester conformation are bolded.
Compound Ester conformation Thiophene conformation l00 [nm] fosc
1
trans
cis
e
e
600
603
0.658
0.613
1m
2
trans
cis
e
e
607
599
0.453
0.384
trans
cis
trans
cis
trans
cis
647
640
658
657
0.774
0.816
0.748
0.735
1.2
1
2m
3
trans
cis
trans
cis
trans
cis
653
648
669
659
0.612
0.607
0.616
0.601
0.8
0.6
0.4
0.2
0
trans
cis
trans
cis
trans
cis
648
642
659
658
0.778
0.804
0.733
0.719
3m
trans
cis
trans
cis
trans
cis
654
649
670
660
0.609
0.597
0.603
0.587
350
450
A in DMSO
550
650
750
850
(nm)
A in 2-MeTHF
Fluorescence at 77 K
Excitation at 77 K
Fig. 5. Absorption and fluorescence spectra of compound 2.

538
T. Aysha et al. / Dyes and Pigments 98 (2013) 530e539
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550
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Fluorescence at 77 K
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