Structure, Z/E photoisomerization and an effect of (phenylene-)vinylene conjugation extension on absorption and fluorescence of methylidene-pyrrolinones

Article abstract of DOI:10.1016/j.molstruc.2012.04.061

Six methylidene-pyrrolinones with a conjugation chain extended by vinylene and phenylene-vinylene groups were studied; five of them were synthesized for the first time. Their structures were estimated theoretically by density functional theory (DFT) and proved by X-ray diffraction techniques in two cases. Structural changes upon Z/E photoisomerization in solution were studied by a combination of ¹H NMR spectroscopy and DFT GIAO method. Both types of conjugation extension caused a bathochromic shift in absorption. Vinylene extension produced the compounds fluorescent in solution. N-methylated derivatives have shown solid-state fluorescence.

Full text of DOI:10.1016/j.molstruc.2012.04.061

Journal of Molecular Structure 1022 (2012) 159–166
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structure, Z/E photoisomerization and an effect of (phenylene-)vinylene
conjugation extension on absorption and fluorescence of methylidene-pyrrolinones
a
a,b,
b
c,d
a
⇑
ˇ
ˇ
ˇ ˇ ˇ ˇ
, Jan Vynuchal , Antonín Lycka , Zdenka Padelková ,
Stanislav Lunák Jr. , Zdenek Eliáš
Radim Hrdina a
a Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-532 10 Pardubice, Czech Republic
b Synthesia, a.s., Pardubice, Semtín 103, CZ-532 17 Pardubice, Czech Republic
c Research Institute for Organic Syntheses (VUOS), Rybitví 296, CZ-533 54 Pardubice 20, Czech Republic
d University of Hradec Králové, Faculty of Education, Rokitanského 62, CZ 500 03 Hradec Králové 3, Czech Republic
h i g h l i g h t s
"
"
"
"
Methylidene-pyrrolinones with extended conjugation chains were synthesized.
Structures were estimated by X-ray diffraction.
Structural changes upon Z/E photoisomerization were studied by NMR and DFT.
N-methylated derivatives show solid-state fluorescence.
a r t i c l e i n f o
a b s t r a c t
Article history:
Six methylidene-pyrrolinones with a conjugation chain extended by vinylene and phenylene-vinylene
groups were studied; five of them were synthesized for the first time. Their structures were estimated
theoretically by density functional theory (DFT) and proved by X-ray diffraction techniques in two cases.
Structural changes upon Z/E photoisomerization in solution were studied by a combination of 1H NMR
spectroscopy and DFT GIAO method. Both types of conjugation extension caused a bathochromic shift
in absorption. Vinylene extension produced the compounds fluorescent in solution. N-methylated
derivatives have shown solid-state fluorescence.
Received 15 March 2012
Received in revised form 19 April 2012
Accepted 20 April 2012
Available online 3 May 2012
Keywords:
Photoisomerization
Conjugation
Ó 2012 Elsevier B.V. All rights reserved.
X-ray diffraction techniques
Density functional theory
NMR spectroscopy
1. Introduction
on double C@C bond is preferred giving cyclopenta-pyrrolinones
[2]. The products of piperidine catalyzed condensation of (substi-
Ethyl 5-oxo-2-phenyl-4,5-dihydro-1H-pyrrole-3-carboxylate
(pyrrolinone ester, Scheme 1) is an interesting intermediate in
colorant syntheses. Due to strongly activated methylene group in
position 4, it can undergo base catalyzed reaction with aromatic
nitriles [1,2], a condensation with aromatic aldehydes [3–5], car-
boxy esters and chlorides [6], azo-coupling with diazonium salts
[7–10] and oxidative dimerization [1,4]. The reaction with various
aromatic nitriles catalyzed by t-amyl alcoholate gives usually
asymmetrical diketo-pyrrolo-pyrroles (DPP) [11–13]. The excep-
tion is the reaction with trans cinnamonitrile, for which an addition
tuted) benzaldehydes with pyrrolinone ester are (substituted)
phenyl-methylidene-pyrrolones (compound 1) [5].
The aim of the presented work was to study whether the reac-
tion of trans cinnamaldehyde with pyrrolinone ester under piperi-
dine catalysis involves also an addition on C@C bond or the
condensation is preferred. As expected, the reaction conditions al-
lowed only the condensation forming compound 2 (Scheme 1).
The role of side carboxyester group in position 3 of methyli-
dene-pyrrolinone ring is crucial for final structure. Compound 1
arises from a reaction solely as the Z isomer, i.e. with EE arrange-
ment of its 1,4-diphenyl-butadiene (DPB) hydrocarbon backbone
[5], on the contrary to 3-unsubstituted methylidene-pyrrolinones
with EZ DPB backbone [14]. Compound 1 easily undergoes Z/E
photoisomerization [5]. The X-ray structures of 3-methylester ana-
logues of 1 (E isomer) and 1m (Z isomer) are known [15].
⇑
Corresponding author at: Faculty of Chemical Technology, University of
Pardubice, Studentská 95, CZ-532 10 Pardubice, Czech Republic. Fax: +420
466823900.
E-mail address: elias.zd@seznam.cz (Z. Eliáš).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.04.061

ˇ
160
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
OC2H5
O
OC2H5
COOEt
CH3I
piperidine
O
Ar-CHO
H3C N
H N
H N
O
O
O
Ar
Ar
1, 1m:
Ar =
2, 2m:
3, 3m:
Ar =
Ar =
Scheme 1. Syntheses of the compounds under study.
Consequently, the second aim of the presented work was to
mained unchanged during syntheses and N-methylation had no ef-
fect on the arrangements on both C@C bonds.
study the structural details of 2, including its eventual photoisom-
er(s). In order to make this study more general, we synthesized
also another derivative with two exocyclic C@C double bonds of
a different type (compound 3). The effect of vinylene (1 ? 2) and
phenylene-vinylene (1 ? 3) conjugation extension on absorption
and fluorescence was evaluated both on compounds 1–3 and their
N-methylated (1m–3m) derivatives (Scheme 1).
2.1.1. Synthesis of ethyl (4Z)-5-oxo-2-phenyl-4-aryl-4,5-dihydro-1H-
pyrrole-3-carboxylates (2 and 3)
Toluene (75 ml), ethyl 5-oxo-2-phenyl-4,5-dihydro-1H-pyrrole-
3-carboxylate (pyrrolinone ester; 15 mmol) and aromatic aldehyde
(15 mmol) were added into round bottom flask. The reaction mix-
ture was heated under reflux for 4 h in the catalytic presence of
piperidine (cca. 0.5 ml). The product was filtrated and dried.
2. Experimental and theoretical procedures
2.1. Syntheses and analyses
2.1.2. Ethyl (4Z)-5-oxo-2-phenyl-4-[(2E)-3-phenylprop-2-en-1-
ylidene]-4,5-dihydro-1H-pyrrole-3-carboxylate (2)
The synthesis of 1 was reported previously [5], other five com-
pounds synthesized according to Scheme 1 are new, so they were
fully characterized. Compounds 2 and 3 arose solely as Z isomers
on newly formed exocyclic double bond as confirmed by 3J(13C,
1H) coupling constants on methylidene proton (Table 2) in the
same way as for 1 [5]. The trans isomer on the C@C double bonds
coming from cinnamaldehyde and stilbene-4-carbaldehyde re-
After recrystallization in mixture n-hexane: acetone (3:2) was
obtained product with 56% yield with m.p. 220–223 °C. Elemental
analysis: Calculated: C (76.50), H (5.54), N (4.06), Found: C (76.43),
H (5.35), N (3.96). MS analysis M = 345. Positive-ion MS: m/z 346
[M+H]+, 100%; m/z 300 [M+H-C2H5OH]+.
2.1.3. Ethyl (4Z)-2-phenyl-5-oxo-4-{4-[(E)-2-
phenylethenyl]benzylidene}-4,5-dihydro-1H-pyrrole-3-carboxylate
(3)
Table 1
Crystallographic data for 1m and 2.
Recrystallization was performed in methanol and product was
obtained with 36% yield. The melting point was 224–227 °C. Ele-
mental analysis: Calculated: C (79.79), H (5.50), N (3.32), Found:
C (79.76), H (5.62), N (3.24). MS analysis M = 421. Positive-ion
MS: m/z 422 [M+H]+, 100%.
Compound
1m
2
Empirical formula
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
21H19NO3
C22H19NO3
Triclinic
P ꢀ 1
Monoclinic
P21/c
10.2600(5)
11.4941(7)
17.7968(8)
90
123.886(5)
90
10.7030(8)
12.2839(8)
15.4090(10)
103.264(5)
109.141(6)
102.715(5)
4
a
(°)
2.1.4. Synthesis of N-methylated derivatives (1m–3m)
b (°)
DMF (60 ml), starting non-methylated compound (3 mmol) and
potassium carbonate (4.2 mmol) were given to the three-necked
flask with thermometer and refluxing condenser. After 20 min of
stirring was added iodomethane (18 mmol). The reaction mixture
was heated to 80 °C for 2 h. Product was isolated by precipitation
to 50 ml distilled water and was filtered off.
c
(°)
Z
4
V (Å3)
1742.29(19)
1.271
1764.1(2)
1.300
Dc (g cmꢀ3
)
Crystal size (mm)
Crystal shape
0.37 ꢁ 0.23 ꢁ 0.20
0.38 ꢁ 0.37 ꢁ 0.24
Block
Block
l
(mmꢀ1
)
0.085
0.087
F[000]
704
728
h; k; l range
ꢀ12, 13; ꢀ14, 14; ꢀ22,
ꢀ13, 13; ꢀ14, 15; ꢀ19,
2.1.5. Ethyl (4Z)-4-benzylidene-1-methyl-5-oxo-2-phenyl-4,5-
dihydro-1H-pyrrole-3-carboxylate (1m)
Product was obtained with 95% yield. The melting point was
125–128 °C. Elemental analysis: Calculated: C (75.66), H (5.74), N
(4.20), Found: C (75.41), H (5.59), N (4.12). MS analysis M = 333.
Positive-ion MS: m/z 334 [M+H]+, 100%.
22
18
h Range (°)
Reflections
measured
Independent [Rint
Observed [I > 2
Parameters refined
2.24–26.7
20,674
1.48–27.30
30,256
a
]
3656
2348
226
7823
5062
469
r
(I)]
Max/min
GOFb
D
q
(eÅꢀ3
)
0.719/ꢀ0.385
0.264/ꢀ0.327
1.031
1.176
Rc/wRc
0.1031/0.2484
0.0698/0.1300
2.1.6. Ethyl (4Z)-1-methyl-5-oxo-2-phenyl-4-[(2E)-3-phenylprop-2-
en-1-ylidene]-4,5-dihydro-1H-pyrrole-3-carboxylate (2m)
Product was obtained with 94% yield. The melting point was
177–179 °C. Elemental analysis: Calculated: C (76.86), H (5.89), N
(3.90), Found: C (75.87), H (5.67), N (3.81). MS analysis M = 359.
Positive-ion MS: m/z 360 [M+H]+, 100%; m/z 314 [M+H-C2H5OH]+.
P
P
jF2o ꢀ F2o;meanj= F2o.
a
b
c
Rint
¼
P
2
1=2
S ¼ ½ ðwðF2o ꢀ Fc2Þ Þ=ðNdiffers ꢀ NparamsÞꢂ
.
h
iꢀ1
h
2
Weighting scheme: w ¼ r2ðFo2Þ þ ðw1PÞ þ w2P
,
where P ¼ maxðF2oÞþ
ꢀ
ꢁ
2
P
P
P
P
2
1=2
2F2c ꢂ; RðFÞ ¼ jjFoj ꢀ jFcjj= jFoj; wRðF2Þ ¼ ½ ðwðFo2 ꢀ F2c Þ Þ=ð
w
F2o Þꢂ
.

ˇ
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
161
Fig. 1. The molecular structure (ORTEP 40% probability level) of 1m. Selected interatomic distances (Å) and angles (°): O1AC1 1.223(5), O2AC12 1.210(5), O3AC12 1.318(5),
O3AC13 1.448(6), C1AN1 1.399(5), N1AC2 1.367(6), C2AC3 1.403(6), C3AC4 1.458(6), C4AC1 1.485(6), C4AC15 1.349(6), C15AC16 1.465(6), C4AC15AC16 134.5(4),
C1AC4AC3 106.1(3), C1AN1AC2 111.1(3).
2.1.7. Ethyl (4Z)-1-methyl-5-oxo-4-{4-[(E)-2-
phenylethenyl]benzylidene}-2-phenyl-4,5-dihydro-1H-pyrrole-3-
carboxylate (3m)
tal (GIAO) NMR computations. Solvent effect on chemical shifts
was not taken into account. Absolute shielding for reference TMS
computed at the same level of theory was 32.00.
Product was recrystallized from methanol. 1.37 g (yield 66%) of
a product was obtained. The melting point was 132–135 °C. Ele-
mental analysis: Calculated: C (79.98), H (5.79), N (3.22), Found:
C (79.71), H (5.85), N (3.13). MS analysis M = 435. Positive-ion
MS: m/z 436 [M+H]+, 100%.
Time dependent (TD) DFT calculations of excitation energies
were carried out with the same XC functional and the broader basis
set 6-311+G(2d,p). Solvent effect was taken into account through
non-equilibrium polarized continuum model (PCM).
All calculation codes came from Gaussian03W program suite
[20].
2.2. Instrumental equipment
3. Results and discussion
The 1H and 13C NMR spectra were recorded on a Bruker Avance
II 400 spectrometer (400.13 MHz for 1H, and 100.62 MHz for 13C) in
DMSO-d6. The 1H and 13C chemical shifts were referenced to inter-
nal TMS (d = 0.00). All 2D experiments (gradient-selected gs-COSY,
gs-HMQC, gs-HMBC) were performed using manufacturer’s soft-
ware (Topspin 2.1).
3.1. Molecular and crystal structure
Single crystals of 1m and 2 were of sufficient quality for X-ray
diffraction techniques (Table 1). The only molecular structure of
1m is shown on Fig. 1. On the contrary to 1m, there were found
two structures in a crystal cell of 2 (Fig. 2). The distance of the sug-
gested double bond C2@C3 in 1m and 2 (1.403(6) for 1m and
1.368(3) and 1.372(3) Å for 2) is significantly elongated when com-
pared to the standard C@C distance (1.331 Å) [21], which is a proof
of a conjugation within the central ring. Furthermore, compounds
1m and 2 each display a shortening of the N1AC1 and N1AC2 bond
distances (1.399(5), 1.367(6) Å for 1m and 1.373(3), 1.392(3) and
1.368(3), 1.394(3) Å for both independent molecules of 2), with re-
spect to the standard N(sp3)AC(sp2) distance of 1.44 Å [21], attrib-
utable to a peptide-like bonding character. Z (1m), resp. EZ (2)
arrangement on exocyclic C@C double bonds was confirmed. Side
carboxyester group is in s-trans conformation in both cases. Both
structures of 2 show similar rotation of 2-phenyl out of pyrrolinone
ring (37–38°), which is considerably increased (79°) by a steric ef-
fect of N-methyl group in 1m. Opposite pendant phenyl is almost
coplanar with methylidene-pyrrolinone ring in a structure A of
compound 2, while considerably deviated from planarity in a
structure B (16°) and especially in compound 1m (23°), proving a
remarkable influence of crystal packing forces on a planarity of
chain conjugated systems. Torsion of carbonyl group out of pyrroli-
none plane lies in an interval 13–19°.
The X-ray data for crystals of 1m (yellow) and 2 (orange) were
obtained at 150 K using Oxford Cryostream low-temperature de-
vice on a Nonius KappaCCD diffractometer with Mo K
(k = 0.71073 Å), a graphite monochromator, and the / and
a
radiation
scan
v
mode. Data reductions were performed with DENZO-SMN [16].
The absorption was corrected by integration methods [17]. Struc-
tures were solved by direct methods (Sir92) [18] and refined by full
matrix least-square based on F2 (SHELXL97) [19]. Hydrogen atoms
were mostly localized on a difference Fourier map, however to en-
sure uniformity of treatment of crystal, all hydrogen were recalcu-
lated into idealized positions (riding model) and assigned
temperature factors Hiso(H) = 1.2 Ueq (pivot atom) or of 1.5Ueq for
the methyl moiety with CAH = 0.96 Å, 0.97, 0.93 and 0.93 Å for
methyl, methylene, vinylidene hydrogen atoms and atoms in aro-
matic rings, respectively, and 0.86 Å for NAH bonds.
Crystallographic data for structural analysis have been depos-
ited with the Cambridge Crystallographic Data Centre, CCDC nos.
827776 and 827777 for 1m and 2, respectively. Copies of this infor-
mation may be obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EY, UK (fax: +44 1223 336033; e-
mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
There were detected two dimer-forming specific intermolecular
interactions in a crystal of compound 2 (Fig. 3). First, A and B mol-
ecules are connected by two CO-HN hydrogen bonds with a length
about 1.96–1.99 Å, i.e. a bit longer than a similar type of H-bond in
the simplest DPP pigment (C.I.P.R.255) with the H-bond (OAH)
length about 1.82 Å [22]. Second, two A molecules (but not B) are
2.3. Quantum chemical calculations
The theoretical calculation based on density functional theory
(DFT) were carried out. The geometry was optimized using B3LYP
functional in combination with 6-311G(d,p) basis set. The same
functional and basis set was used for gauge-including atomic orbi-
p–p stacked with the interplane distance about 3.18–3.19 Å.

ˇ
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
162
Fig. 2. The molecular structure (ORTEP 50% probability level) of 2. Selected interatomic distances (Å) and angles (°): for molecule A: O1AC1 1.232(3), O2AC20 1.211(3),
O3AC20 1.338(3), O3AC21 1.453(3), C1AN1 1.373(3), N1AC2 1.392(3), C2AC3 1.368(3), C3AC4 1.460(3),C4AC1 1.484(4), C4AC5 1.358(4), C5AC6 1.430(4), C6AC7 1.342(4),
C7AC8 1.463(3); C4AC5AC6 125.1(2), C5AC6AC7 123.1(2), C6AC7AC8 125.9(2), C1AC4AC3 105.5(2), C1AN1AC2 111.7(2); for molecule B: O4AC101 1.231(3), O5AC120
1.209(3), O6AC120 1.340(3), O6AC121 1.452(3), C101AN2 1.368(3), N2AC102 1.394(3), C102AC103 1.372(3), C103AC104 1.459(3), C104AC101 1.488(4), C104AC105
1.359(4), C105AC106 1.435(4), C106AC107 1.338(4), C107AC108 1.465(4); C104AC105AC106 124.9(3), C105AC106AC107 122.9(3), C106AC107AC108 125.4(3),
C101AC104AC103 105.5(2), C101AN2AC102 111.8(2).
Fig. 3. The dimers in crystal of compound 2. (a) The H-bond dimer of A and B molecules, (b) the p–p stacked dimer of two A molecules.
Although the lateral displacement (Fig. 3b) is considerable, this
interaction is responsible for planarization of A molecule of 2, on
the contrary to B molecule of 2 and all molecules of 1m, in which
crystal no such specific intermolecular interactions were found.
signals in each case, i.e. only one new isomer arised in the com-
pounds with two exocyclic C@C double bonds (2, 2m, 3, 3m). Eval-
uation of 13C and 1H chemical shifts established, that all changes
correspond to Z/E isomerization on methylidene-pyrrolinone C@C
bond as for 1 and 1m. Fig. 4 shows convincingly that the splitting
pattern of the key proton H-4-20 (@CHACH@CHA) in the 1H NMR
spectrum does not change after irradiation and, thus, the mutual
all-trans proton arrangement must be same both in EZ-2m and
EE-2m. Full assignment can be found in Table 2. Trans arrangement
3.2. Z/E photoisomerization in solution
Standing of DMSO-d6 solutions of all six compounds under
study on daylight for 1 week brought only one new set of proton

ˇ
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
163
OC2H5
O
OC2H5
O
light
H
N
H
N
O
O
EZ-2
EE-2
OC2H5
O
O
OC2H2
light
H
N
O
H
N
O
EZ-3
EE-3
1
Fig. 4. A part of H NMR spectrum showing the signal of H-4-20 (@CHACH@CHA)
Scheme 2. Photochemical Z/E isomerization.
proton in compound EZ-2m (upper trace) and signals of H-4-20 protons in a mixture
of compounds EZ-2m and EE-2m formed after irradiation (bottom trace). 1H
chemical shifts are given in ppm.
Table 3
Theoretical relative ground state energies (Erel [kcal molꢀ1]) and 1H chemical shifts of
methyl (d(CH3)) and methylene (d(CH2)) protons together with their experimental
values.
Table 2
1H and 13C chemical shifts and nJ(13C, 1H) and nJ(1H, 1H) coupling constants (Hz,
0.3 Hz) of compounds EZ-2m and EE-2m measured in hexadeuteriodimethyl
sulfoxide.
Compound Isomera Conformerb Theoretical
Experimental
c
Erel
d(CH3)d d(CH2)e d(CH3) d(CH2)
H/C no.
EZ-2m
d(1H)
EE-2m
d(1H)
1
Z
E
Z
E
Z
E
Z
E
Z
E
Z
E
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
0.000 0.905
1.060 1.525
4.670 0.948
2.384 0.494
0.000 0.660
1.662 1.473
4.730 0.637
3.219 0.401
0.000 0.697
0.487 1.440
2.334 0.622
3.974 1.252
0.000 0.948
1.097 1.487
2.139 0.828
4.551 1.262
0.000 0.911
1.054 1.519
5.031 0.966
2.706 0.550
0.000 0.673
1.686 1.492
5.079 0.631
3.570 0.503
4.103
4.203
4.013
3.668
3.917
4.083
3.739
3.586
3.901
4.057
3.915
4.118
4.095
4.166
4.084
4.207
4.115
4.206
4.046
3.743
3.916
4.085
3.754
3.644
1.06
0.66
0.86
0.57
0.92
0.81
1.11
1.03
1.07
0.71
0.88
0.62
4.11
3.66
3.95
3.58
3.97
3.97
4.12
4.12
4.12
3.77
3.96
3.68
d(13C)
d(13C)
2
3
4
5
–
–
–
–
153.5
103.0
125.7
166.4
–
–
–
–
155.2
104.3
125.3
167.9
4.5 (CH)a
9.4 (CH)a
1m
2
12.0 (CH)a,
2.5 (CH3)
6.8 (CH)a,
2.8 (CH3)a
a
NCH3
2-10
2.903
–
7.51
7.57
7.51
–
3.966
0.92
7.90
8.59
7.32
–
7.66
7.51
7.43
27.5
2.906
–
c
c
c
–
3.968
0.80
7.40
8.15
7.37
–
c
c
c
28,0
130.4
129.2
128.2
129.7
162.7
59.2
130.5
129.2
128.3
129.7
163.6
59.8
2-20
2-30
2-40
3-COO
OCH2
CH3
4-10
2.8 (CH2)a
2.8 (CH2)a
2m
3
13.6
13.4
139.8
124.7
143.9
136.4
127.7
129.2
129.7
11.9b, 0.9b
15.6b, 11.9b
15.6b
135.6
125.9
144.2
136.3
127.5
129.2
129.7
12.0b, 0.9b
15.1b, 12.0b
15.1b
4-20
4-30
4-40
4-50
4-60
3m
4-70
a
b
c
nJ(13C, 1H).
nJ(1H, 1H).
a
b
c
On methylidene-pyrrolinone C@C bond.
Strong overlap of proton signals in the range of 7.62–7.42 ppm.
With respect to carboxy ester group.
Value 0.000 kcal molꢀ1 was ascribed to the most stable isomer/conformer.
Arithmetic mean of values for three protons.
d
e
of XAC6H4ACH@CHAPh protons was retained after irradiation as
clear from the practically same 3J(1H, 1H) coupling constants in
compounds 3 and 3m being 15.3 0.1 Hz. Definitely, photochemi-
cal isomerization undergoes according to Scheme 2, no side reac-
tion to ZZ isomer or consecutive (photo)isomerization to ZE
isomer was detected.
There was shown on pyrrolinone ester based hydrazone dyes
[9], that the change in Z/E arrangement can force the side ester
group to conformational change (s-trans/s-cis). So the recently val-
idated combined experimental/theoretical procedure [9] was ap-
plied to a model set of six methylidene-pyrrolinones. In the first
Arithmetic mean of values for two protons.
step the structure of both experimentally found isomers (Z and E
for 1 and 1m, resp. EZ and EE for 2, 2m, 3 and 3m) each with both
possible conformations of side carboxy ester group was optimized
using by DFT and the relative ground state energies with respect to
the most stable isomer/conformer were summarized in Table 3.
The calculated structures of 1 and 2 are shown on Figs. 5 and 6
for illustration. Z isomers for all six compounds are always more
stable than E isomers irrespective to carboxy ester conformation.
More stable conformations of Z isomer are s-trans for all six

ˇ
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
164
Table 4
Theoretical excitation energies of the first two ppꢃ transitions (converted to k00) of Z isomers in s-trans conformation together with experimental absolute absorption and
fluorescence maxima in DMSO at room temperature.
PCM (DMSO) TD-DFT
S0–S1
Experimental
Absorption
kabs
S0–S2
Fluorescence
k00
fosc
Coeff.
k00
fosc
Coeff.
emax
kemis
Stokes
1
1m
2
2m
3
3m
421
421
472
462
488
476
0.179
0.343
0.987
0.752
1.527
1.308
0.63 (1-10)
0.62 (1-10)
0.62 (1-10)
0.62 (1-10)
0.70 (1-10)
0.70 (1-10)
352
326
356
359
396
399
0.663
0.026
0.335
0.610
0.117
0.311
0.18 (1-10)
0.53 (2-10)
0.63 (2-10)
0.65 (2-10)
0.68 (2-10)
0.68 (2-10)
412
409
434
435
435
429
11,900
8700
22,800
20,200
30,000
25,300
–
–
575
587
–
–
–
5650
5950
–
–
–
kabs (nm) – measured absolute absorption maxima.
emax (l molꢀ1 cmꢀ1) – measured molar absorptivity.
kemis (nm) – fluorescence emission maximum.
Stokes (cmꢀ1) – Stokes shift.
k00 (nm) – computed excitation energy converted to wavelength.
fosc – computed oscillator strength of 0–0 vibronic band.
Coeff. – expansion coefficient of the monoexcited configurations in TD DFT.
compounds (confirmed by X-ray for 1m and 2), while s-cis confor-
mation is more probable for E isomer of 1, 1m, 3 and 3m, but not
for 2 and 2m. Finally, the thermodynamical viewpoint prefers the
conformational change after isomerization for the derivatives with
phenyl ring bonded directly to methyliden-pyrolinone. These com-
pounds could be thus interesting in photoswitching [23], if e.g. ree-
sterified with a suitable biopolymer.
tween the experimental values for both isomers must mean, that
the conformation of carboxy ester is different for them, and vice-
versa. The sets of theoretical values of methyl and methylene 1H
chemical shifts match the experimental ones significantly better
for the most stable arrangement (Table 4). Theoretical values of
methyl protons are systematically underestimated by 0.11–
0.22 ppm, while the deviation of theoretical chemical shifts of
methylene protons is only ꢀ0.01 to 0.07 ppm. Relatively small dis-
agreement goes probably on account to imperfect theoretical
methodology; any other assignment gives generally less consistent
set of results. Thus, we consider the photochemical isomerizations
in Scheme 2 including the conformation of carboxy ester group
definitely proved. The s-trans arrangement of carboxyester group
is more stable than the s-cis one in all cases, when this group is
more or less coplanar with pyrrolinone ring. This is the case of iso-
mers with Z arrangement on methylidene-pyrrolinone double
bond for all six compounds. The sterical interaction of the vinylene
Only one value of 1H chemical shift was found experimentally
for two (three) methylene (methyl) protons of ethyl group, which
means that a free rotation of ethyl on the NMR time scale must ex-
ist. On the other hand, the different average values of d(CH3), resp.
d(CH2), for both isomers prove the stabilized position of the whole
carboxy ester group. The chemical shifts (Table 4) computed by
GIAO method are generally very similar for s-trans, resp. s-cis, con-
formers of both isomers, proving that 1H chemical shifts of ethyl
protons depend only on a conformation of carbethoxy group with
respect to pyrrolinone ring. Finally, a considerable difference be-
Fig. 5. DFT computed structure of 1 with both conformations of carbethoxy group of E and Z isomers, together with relative total energies and GIAO NMR and experimental
1H chemical shifts of the methylene and methyl groups.

ˇ
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
165
Fig. 6. DFT computed structure of 2 with both conformations of carbethoxy group of E and Z isomers, together with relative total energies and GIAO NMR and experimental
1H chemical shifts of the methylene and methyl groups.
does not correspond to 0–0 vibronic transition, but to the higher
one (Fig. 7 for illustration). Compounds 2 (Fig. 7) and 2m are the
only fluorescent ones in solution (Table 4). Their ability of fluores-
cence may relate to the sterically less demanding intramolecular
reorganization in excited state, not leading to conformational
change of 3-ethylester group. Stokes shift is relatively high in both
cases (Table 4), mainly because the absorption/fluorescence max-
ima do not correspond to 0–0 vibronic transitions, so the reabsorp-
tion should not affect the eventual application.
On the contrary, four of six derivatives (1m, 2, 2m and 3m) are
able to fluoresce strongly in polycrystalline solid-state, in which
the competitive photoisomerization is limited, with the emission
maxima at 561, 630, 577 and 608 nm, respectively. The presence
of solid-state (ss) fluorescence makes the compounds interesting
for some applications, but the interpretation of this phenomenon
is not straightforward [24]. A bit simplifying, the ss fluorescence
was expected for all three N-methylated derivatives 1m–3m by
analogy with N-alkylated DPPs [25], in which H-bonding is dis-
abled and
p–p stacking is considerably limited by molecular
Fig. 7. Absorption (left) and fluorescence (middle) of 2 in DMSO and its fluores-
cence in polycrystalline solid-state (right).
non-planarity. But the appearance of ss fluorescence of compound
2 is a bit surprising, as both types of (usually quenching) dimers
are present in its crystal (Fig. 3).
protons with these oxygens in E isomers of compounds 2 (Fig. 6)
and 2m also leaves the carboxy ester group almost coplanar. On
the contrary, the interation with ortho-phenyl hydrogens in E iso-
mers of 1 (Fig. 5), 1m, 3 and 3m forces carboxy ester group to
rotate out-of-plane considerably. This type of sterical hindrance
is the driving force of the conformational change upon
photoisomerization.
4. Conclusion
Ethyl 5-oxo-2-phenyl-4,5-dihydro-1H-pyrrole-3-carboxylate
was condensed with trans cinnamaldehyde and trans stilbene-4-
carbaldehyde to give the dyes with vinylene (2) and phenylene-
vinylene (3) conjugation extension with respect to parent ethyl
4-benzylidene-5-oxo-2-phenyl-4,5-dihydro-1H-pyrrole-3-carbox-
ylate (1) arose from a reaction with benzaldehyde. All three dyes
1–3 were isolated solely in Z arrangement on a newly created
exocyclic C@C double bond. Photochemical Z/E isomerization
underwent exclusively on this bond, giving thus only EE isomers
of 2 and 3. Steric effect accompanying photoisomerization forced
the molecule to change a conformation of 3-carboxy ester group
from s-trans to s-cis in compounds 1 and 3, but not in 2. Absorption
3.3. Absorption and fluorescence
Both vinylene and phenylene-vinylene conjugation extension
causes a comparable bathochromic and hyperchromic shift in
absorption (Table 4). PCM TD DFT theoretical computations predict
a considerably higher red shift; the disagreement may go on ac-
count to the fact, that for all compounds the absorption maximum

ˇ
S. Lunák Jr. et al. / Journal of Molecular Structure 1022 (2012) 159–166
166
ˇ
ˇ
ˇ
[12] S. Lunák Jr., L. Havel, J. Vynuchal, P. Horáková, J. Kucerík, M. Weiter, R. Hrdina,
Dyes Pigm. 85 (2010) 27.
spectra of both extended compounds 2 and 3 are red shifted with
respect to 1. Compound 2 is able to fluoresce both in solution and
in solid-state. N-methylated derivatives of 1–3 behave in a very
similar way as their precursors with respect to Z/E photoisomeriza-
tion and show solid-state fluorescence.
ˇ
ˇ
ˇ
ˇ
ˇ
[13] S. Lunák Jr., J. Vynuchal, P. Horácková, B. Frumarová, Z. Zák, J. Kucerík, O. Salyk,
J. Mol. Struct. 983 (2010) 39.
[14] M. Bourotte, M. Schmitt, A. Follenius-Wund, C. Pigault, J. Haiech, J.-J.
Bourguignon, Tetrahedron Lett. 45 (2004) 6343.
[15] D. Ramaiah, K. Ashok, R. Barik, D. Venugopal, N.P. Rath, K. Bhattacharyya, P.K.
Das, M.V. George, J. Org. Chem. 57 (1992) 6032 (Files KUSBOP and KUSBEF in
Cambridge structural, database (CSD)).
[16] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1997) 307.
[17] P. Coppens, in: F.R. Ahmed, S.R. Hall, C.P. Huber (Eds.), Crystallographic
Computing, Munksgaard, Copenhagen, 1970, pp. 255–270.
[18] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr. 26
(1993) 343.
[19] G.M. Sheldrick, SHELXL-97, University of Göttingen, Göttingen, 1997.
[20] W.M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M.
Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.
Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G.
Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D.
Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S.
Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,
C. Gonzalez, J.A. Pople, Gaussian 03, Gaussian, Inc., Wallingford, CT, 2004.
[21] F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpen, R. Taylor, J. Chem.
Soc., Perkin Trans. 2 (1987) S1–S19.
Acknowledgements
The work was supported by the Project VZ 0021627502, Minis-
try of Education, Youth and Sport, Czech Republic.
ˇ
ˇ
Zdenka Padelková was supported by the Ministry of Education,
Youth and Sport of the Czech Republic via Project VZ 0021627501.
References
[1] J. Pfenninger, A. Iqbal, A.C. Rochat, O. Wallquist, US4659775, Ciba-Geigy, 1987.
[2] C.J.H. Morton, R. Gilmour, D.M. Smith, P. Lightfoot, A.M.Z. Slawin, E.J. MacLean,
Tetrahedron 58 (2002) 5547.
[3] J.M. Adam, P. Dalvi, V.S. Ekkundi, J.P. Bacher, S. Tiwari, WO2004083170, Ciba
Specialty Chemicals, 2004.
[4] J.M. Adam, P. Dalvi, V.S. Ekkundi, J.P. Bacher, R. Sreenivasan, D.M. Rane,
WO2004089941, Ciba Specialty Chemicals, 2004.
ˇ
ˇ
ˇ
[5] J. Vynuchal, S. Lunák Jr., A. Hatlapatková, R. Hrdina, A. Lycka, L. Havel, K.
ˇ
Vynuchalová, R. Jirásko, Dyes Pigm. 77 (2008) 266.
[6] C.J.H. Morton, R.L. Riggs, D.M. Smith, N.J. Westwood, P. Lightfoot, A.M.Z. Slawin,
Tetrahedron 61 (2005) 727.
[7] T. Chamberlain, WO0174950, Sun, Chemical, 2001.
[8] T. Chamberlain, US7132518, Sun, Chemical, 2006.
[22] J. Mizuguchi, A. Grubenmann, G. Wooden, G. Rihs, Acta Crystallogr., Sect. B:
Struct. Sci. 48 (1992) 696 (File SAPDES01 in CSD).
[23] M.-M. Russew, S. Hecht, Adv. Mater. 22 (2010) 3348.
ˇ
ˇ
[9] A. Lycka, S. Lunák Jr., T. Aysha, R. Holuša, R. Hrdina, Tetrahedron Lett. 51 (2010)
3149.
[24] M. Shimizu, T. Hiyama, Chem. – Asian J. 5 (2010) 1516.
ˇ
ˇ
ˇ
[25] S. Lunák Jr., M. Vala, J. Vynuchal, I. Ouzzane, P. Horáková, P. Mozíšková, Z. Eliáš,
M. Weiter, Dyes Pigm. 91 (2011) 269.
ˇ
ˇ
[10] T. Aysha, S. Lunák Jr., A. Lycka, R. Hrdina, Dyes Pigm. 91 (2011) 170.
ˇ
ˇ
[11] S. Lunák Jr., J. Vynuchal, M. Vala, L. Havel, R. Hrdina, Dyes Pigm. 82 (2009) 102.