Structure, absorption and fluorescence of (bi)thiophene substituted methylidene-pyrrolinones Dedicated to Professor Milo? Nepra? in occasion of his 80th birthday

Article abstract of DOI:10.1016/j.molstruc.2013.03.055

Four thiophene and bithiophene substituted methylidene-pyrrolinones were synthesized and their absorption and fluorescence properties were compared with previously reported behavior of their phenyl and biphenyl substituted analogues. Their structures were estimated theoretically by density functional theory (DFT) and proved by X-ray diffraction in two cases. Thiophene derivatives show a considerable bathochromic shifts in absorption with respect to phenyl analogues, in accordance with theoretical predictions based on time dependent DFT. Conjugation extension caused a bathochromic shift in absorption, too. All compounds show fluorescence in low temperature solvent glass, while only one bithiophene substituted derivative fluoresce weakly in room temperature solution. Three of four N-methylated derivatives show strong yellow, orange and red solid-state fluorescence. An ability of solid-state fluorescence was correlated with nearest neighbor out-of-plane interactions in crystal.

Full text of DOI:10.1016/j.molstruc.2013.03.055

Journal of Molecular Structure 1043 (2013) 43–51
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structure, absorption and fluorescence of (bi)thiophene substituted
methylidene-pyrrolinones
a,b,
a
b
c,d
a
⇑
ˇ
ˇ ˇ ˇ ˇ ˇ
, Stanislav Lunák Jr. , Jan Vynuchal , Antonín Lycka , Zdenka Padelková ,
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 heterocyclic rings were synthesized and compared with aromatic ones analogues.
Structures were estimated by DFT and proved by X-ray diffraction in two cases.
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:
Four thiophene and bithiophene substituted methylidene-pyrrolinones were synthesized and their
absorption and fluorescence properties were compared with previously reported behavior of their phenyl
and biphenyl substituted analogues. Their structures were estimated theoretically by density functional
theory (DFT) and proved by X-ray diffraction in two cases. Thiophene derivatives show a considerable
bathochromic shifts in absorption with respect to phenyl analogues, in accordance with theoretical pre-
dictions based on time dependent DFT. Conjugation extension caused a bathochromic shift in absorption,
too. All compounds show fluorescence in low temperature solvent glass, while only one bithiophene
substituted derivative fluoresce weakly in room temperature solution. Three of four N-methylated deriv-
atives show strong yellow, orange and red solid-state fluorescence. An ability of solid-state fluorescence
was correlated with nearest neighbor out-of-plane interactions in crystal.
Received 3 February 2013
Received in revised form 26 March 2013
Accepted 26 March 2013
Available online 2 April 2013
Dedicated to Professor Miloš Nepraš in
occasion of his 80th birthday
Keywords:
Absorption
Fluorescence
X-ray diffraction
Density functional theory
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
rigidity may play crucial role [8]. The relation between the ability
of a molecule to fluoresce in solution and in crystal is not straight-
Solid-state (ss) fluorescence of organic conjugated compounds
is of general interest because of its use in organic light-emitting
diodes and field-effect transistors, solid-state lasers and lumines-
cent sensors [1]. Except the intramolecular competitive radiation-
less processes given by Jablonski diagram, the intermolecular ones
(like electron and energy transfer [1] or triplet fission [2]) must be
taken into account. Consequently, the relation between crystal
structure and fluorescence ability has to be considered in ss fluoro-
phores research. The transfer processes are usually correlated with
local nearest-neighbor interactions [3–7], nevertheless sometimes
this model is insufficient and the global properties like crystal
forward. The molecules strongly fluorescent in solution usually
save this property in solid, if the strong intermolecular interactions
like p–p stacking are absent [9,10]. On the other hand, a lot of com-
pounds non- or weakly fluorescent in solution show aggregation-
or crystallization-induced emission enhancement [11]. One of the
possible mechanisms of such enhancement (restriction of excited
state intramolecular rotation) is probably responsible for ss fluo-
rescence of aryl-methylidene-pyrrolinones, reported by our group
[12]. Multicolor emission obtained by substituent tuning forms
one of the challenges in the area of ss emitters [1]. We have
recently shown, that the variations of peripheral substituents on
diketo-pyrrolo-pyrrole (DPP) chromophore can tune the ss fluores-
cence maximum in a broad range over 200 nm in a visible and
near-infrared region (568–784 nm) [13]. On the other hand, vinyl-
ene and phenylene-vinylene conjugation extension of phenyl-
methylidene-pyrrolinones, coming from the same precursor as
⇑
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 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.03.055

44
Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
OC2H5
O
R N
O
Ar
Ar
R
H
Compound
1
1m
2
CH3
H
S
CH3
H
2m
3
CH3
H
3m
4
S
S
4m
CH3
Scheme 1. Compounds under study.
unsymmetrical DPPs (i.e. pyrrolinone ester), lead only to relatively
narrow region of ss fluorescence maxima (561–630 nm) [12]. Thus
the main aim of the presented study was to modify the structure of
aryl-methylidene-pyrrolinones to shift the ss fluorescence maxima
to longer wavelengths.
by 3J(13C, 1H) coupling constants on methylidene proton. All com-
pounds show s-trans conformation of carboxy ester group accord-
ing to methyl and methylene 1H NMR chemical shifts [12].
2.1.1. Synthesis of ethyl (4Z)-5-oxo-2-phenyl-4-(thiophen-2-
Inspired by that fact, that a substitution of 3,6-phenyl rings of
DPP chromophore by oligothiophenes brings a higher bathochro-
mic shift in absorption and fluorescence [14] compared to conjuga-
tion extension based solely on hydrocarbons [15], we have
synthesized compounds 2 and 4 (Scheme 1) as heteroanalogues
of previously reported compounds 1 and 3 [16]. Furthermore, their
better soluble N-methyl derivatives 2m and 4m were synthesized.
Comparable compound 1m was reported in our previous study
[12] and the compound 3m was newly synthesized to complete
the series and to cover the effect of phenylene conjugation exten-
sion in N-methylated series. Five new compounds were fully
characterized and X-ray diffraction was applied in two cases (2m
and 4m), in which the monocrystals of sufficient quality were pre-
pared, in order to confirm the molecular structure and to evaluate
the intermolecular interactions in solid state. An effect of hetero-
substitution and conjugation extension on experimental absorp-
tion spectra was supported by theoretical calculations of
excitation energies based on time dependent density functional
theory (TD DFT) including the solvent effect by polarized contin-
uum model (PCM). The fluorescence ability was studied in solution,
in low temperature solvent glass and in polycrystalline solid-state.
ylmethylidene)-4,5-dihydro-1H-pyrrole-3-carboxylate (2)
Toluene (25 ml), ethyl 5-oxo-2-phenyl-4,5-dihydro-1H-pyrrole-
3-carboxylate (pyrrolinone ester; 1.25 g; 5.42 mmol) and
thiophene-2-carbaldehyde (0.61 g; 5.42 mmol) were added into
round bottom flask. The reaction mixture was heated under reflux
for 4 h in the catalytic presence of piperidine (c. 1.5 ml). Solid portion
was separated in the reaction mixture after 20 min of heating and
thustoluene(10 ml)wasaddedinorderto improvestirring. Theprod-
uct was filtrated and dried. It was obtained with 82% yield. The melt-
ing point was 214–216 °C. Elemental analysis: Calculated: C (66.44),
H (4.65), N (4.30), S (9.85) Found: C (66.36), H (4.54), N (4.27), S
(9.93). MS analysis M = 325. Positive-ion MS: m/z 326 [M+H]+, 100%.
2.1.2. Synthesis of ethyl (4Z)-5-oxo-2-phenyl-4-(2,20-bithiophen-5-
ylmethylidene)-4,5-dihydro-1H-pyrrole-3-carboxylate (4)
Toluene (15 ml), pyrrolinone ester (0.63 g; 2.7 mmol) and 2,20-
bithiophene-5-carbaldehyde (0.6 g; 2.7 mmol) were added into
round bottom flask. The reaction mixture was heated under reflux
for 6 h in the catalytic presence of piperidine (c. 0.75 ml). The reac-
tion was monitored by TLC (n-hexane:acetone 3:2). Toluene
(10 ml), pyrrolinone ester (0.53 g) and piperidine (0.5 ml) were
added and the mixture was heated under reflux next 6 h due to
presence of starting aldehyde. Then more pyrrolinone ester
(0.25 g) and toluene (10 ml) were added and reaction mixture
was heated for the following 6 h under reflux. The product was fil-
trated and dried. It was obtained with 95% yield. The melting point
was 224–226 °C. Elemental analysis: Calculated: C (64.84), H (4.20),
N (3.44), S (15.74), Found: C (64.83), H (4.17), N (3.51), S (15.51). MS
analysis M = 407. Positive-ion MS: m/z 408 [M+H]+, 100%.
2. Experimental and theoretical procedures
2.1. Syntheses and analyses
Compounds 2 and 4 were synthesized by piperidine catalyzed
condesation of 2-thiophene and 2-bithiophene carboxaldehydes
with pyrrolinone ester (ethyl 5-oxo-2-phenyl-4,5-dihydro-1H-
pyrrole-3-carboxylate) as reported previously [12,16]. Methyl
iodide was used an agent in N-methylation of 2, 3 and 4 to form
2m, 3m and 4m according to Ref. [12]. All compounds arose solely
as Z isomers on newly formed exocyclic double bond as confirmed
2.1.3. General procedure for the synthesis of methylated derivatives
DMF (60 ml), starting non-methylated compound (2.15 mmol)
and potassium carbonate (3 mmol) were given to the three-necked

Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
45
MS analysis M = 421. Positive-ion MS: m/z 422 [M+H]+, 100%;
m/z 314 [M+HAC2H5OH]+.
Table 1
DFT dihedral angles and relative energies (DE) of isomers/conformers of compound 2.
a
, b and c describe the CAC@CAC(@O), C@CAC(OEt)@O and C@CAC@C(Th) dihedral
angles, respectively, i.e. Z/E isomers on C@C and s-trans/s-cis conformers on CAC
leading to ethyl ester and thiophene groups.
2.2. Instrumental equipment
Isomer Conformer
(ester)
Conformer
(thiophene)
a
(°)
b (°)
c
(°)
DE
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).
(kcal molꢁ1
)
E
Z
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
ꢁ171 ꢁ142
ꢁ177 ꢁ137
176 6.510
ꢁ10 6.233
ꢁ163
ꢁ163
38 ꢁ169 6.908
43
ꢁ174 ꢁ179 0.000
ꢁ173 0.276
ꢁ13 ꢁ179 0.859
ꢁ15 1.130
25 6.397
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
0
0
1
The X-ray data for crystals of 2m (yellow) and 4m (orange)
were obtained at 150 K using Oxford Cryostream low-temperature
ꢁ1
ꢁ1
0
device on a Nonius KappaCCD diffractometer with Mo K
a
radiation
scan
(k = 0.71073 Å), a graphite monochromator, and the and
u
v
mode. Data reductions were performed with DENZO-SMN [17].
The absorption was corrected by integration methods [18]. Struc-
tures were solved by direct methods (Sir92) [19] and refined by full
matrix least-square based on F2 (SHELXL97) [20]. Hydrogen atoms
were mostly localized on a difference Fourier map, however to
ensure uniformity of treatment of crystal, all hydrogens were
recalculated into idealized positions (riding model) and assigned
temperature factors Hiso(H) = 1.2 Ueq (pivot atom) or of 1.5 Ueq
for the methyl moiety with CAH = 0.96, 0.97, 0.93 and 0.93 Å for
methyl, methylene, vinylidene hydrogen atoms and atoms in aro-
matic rings, respectively.
Crystallographic data for structural analysis have been deposited
with the Cambridge Crystallographic Data Centre, CCDC Nos.
921116 and 921117 for 2m and 4m, respectively. Copies of this
information 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).
The absorption spectra were recorded at room temperature in
DMSO using a Perkin-Elmer Lambda 35 spectrophotometer in
1 cm pathlength quartz cuvettes. A Perkin-Elmer (PE) LS55 was
used for measuring fluorescence spectra. 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 PE. This accessory 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 instru-
ment equipped with a PE accessory for solid state measurements.
flask with thermometer and refluxing condenser. After 20 min of
stirring was heated to 80 °C and iodomethane (12.9 mmol) was
added. The reaction mixture was heated at 80 °C for 2 h. Product
was isolated by precipitation to 50 ml distilled water and was
filtered off.
2.1.4. Ethyl (4Z)-1-methyl-5-oxo-2-phenyl-4-(thiophen-2-
ylmethylidene)-4,5-dihydro-1H-pyrrole-3-carboxylate (2m)
Product was obtained with 90% yield. The melting point was
125–128 °C. Elemental analysis: Calculated: C (67.27), H (5.05), N
(4.13), S (9.45) Found: C (67.09), H (4.89), N (4.08), S (9.36). MS
analysis M = 339. Positive-ion MS: m/z 340 [M+H]+, 100%.
2.1.5. Ethyl (4Z)-4-(biphenyl-4-ylmethylidene)-1-methyl-5-oxo-2-
phenyl-4,5-dihydro-1H-pyrrole-3-carboxylate (3m)
Product was obtained with 90% yield. The melting point was
125–128 °C. Elemental analysis: Calculated: C (79.20), H (5.66), N
(3.42), Found: C (78.53), H (5.49), N (3.37). MS analysis M = 409.
Positive-ion MS: m/z 410 [M+H]+, 100%.
2.1.6. Ethyl (4Z)-4-(2,20-bithiophen-5-ylmethylidene)-1-methyl-5-
oxo-2-phenyl-4,5-dihydro-1H-pyrrole-3-carboxylate (4m)
Product was obtained with 92% yield. The melting point was
139–141 °C. Elemental analysis: Calculated: C (65.53), H (4.54), N
(3.32), S (15.21) Found: C (65.32), H (4.42), N (3.27), S (14.88).
Fig. 1. The molecular structure (ORTEP 50% probability level) of 2m. Selected interatomic distances (Å); angles and interplanar angle (°): O1 C1 1.230(3), O2 C10 1.202(4), C1
N1 1.375(4), N1 C2 1.407(4), C2 C3 1.364(4), C3 C4 1.463(4), C4 C1 1.477(4), C4 C5 1.355(4), C5 C6 1.431(4), C6 C9 1.478(4), S1 C6 1.726(3), S1 C7 1.672(4); C1 N1 C2 110.6(2),
N1 C1 C4 106.8(2), C1 C4 C5 127.9(3), C4 C5 C6 133.6(3), C5 C6 S1 129.0(3) and interplanar angle of phenyl ring vs. heterocyclic ring containing amido group is 50.76.

46
Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
Fig. 2. The molecular structure (ORTEP 50% probability level) of 4m. Selected interatomic distances (Å); angles and interplanar angles (°): for molecule a: O1 C1 1.235(4), O2
C15 1.206(5), C1 N1 1.387(5), N1 C2 1.400(4), C2 C3 1.355(5), C3 C4 1.470(5), C4 C1 1.454(5), C4 C5 1.360(5), C5 C6 1.441(5), C6 S1 1.741(4), S1 C7 1.730(4), C6 C9 1.379(5);
C1 N1 C2 109.8(3), N1 C1 C4 106.7(3), C1 C4 C5 128.0(3), C4 C5 C6 132.7(4), C5 C6 S1 128.8(3) and phenyl ring vs. heterocyclic ring amido containing 87.82; for molecule b:
O4 C101 1.218(4), O5 C115 1.204(4), C101 N2 1.387(5), N2 C102 1.391(5), C102 C103 1.355(5), C103 C104 1.466(5), C104 C101 1.481(5), C104 C105 1.351(5), C105 C106
1.438(5), C106 S3 1.742(4), S3 C107 1.733(4), C107 C110 1.453(5), C110 S4 1.727(4), S4 C111 1.705(4); C101 N2 C102 110.4(3), N2 C101 C104 105.9(3), C101 C104 C105
127.2(3), C104 C105 C106 133.9(4), C105 C106 S3 127.9(3) and interplanar angle of phenyl ring vs. heterocyclic ring containing amido group is 72.71.
Polycrystalline samples were placed under quartz plate and the
emission spectra were recorded using front face geometry.
the most and the least stable conformers of Z isomer is
1.13 kcal molꢁ1, while only 0.67 kcal molꢁ1 for E isomers. Finally,
Z isomer can be expected as the reaction product in the syntheses
of compound 2, preferably in s-trans conformations, while the ex-
pected conformation of eventual E photoisomer is not so clear, as
2.3. Quantum chemical calculations
three of four conformers differ mutually less than 0.28 kcal molꢁ1
.
The theoretical calculation based on DFT were carried out. The
ground state geometry was optimized using B3LYP functional in
combination with 6-311G(d,p) basis set. TD DFT calculations of exci-
tation energies were carried out with the same xc functional and the
broader basis set 6-311+G(2d,p). Solvent effect of dimethylsulfoxide
was taken into account through non-equilibrium PCM. The same xc
functional, but only 6-311G(d,p) basis set was used for excited state
TD DFT optimization of model compounds 1, 2 and 4 with only
hydrogens (not ethyl esters) in positions 3 of pyrrolinone ring. All
calculation codes came from Gaussian09W program suite [21].
The detailed screening the ground state energy minima with re-
spect to various conformations of compound 4 was not done, but
the computed energy difference between Z and E isomers (both
in s-trans/s-trans arrangements) is 6.685 kcal molꢁ1, i.e. a destabi-
lization of E with respect to Z isomer is similar as for compound
2 (6.510 kcal molꢁ1).
Single crystals of 2m and 4m were of sufficient quality for X-ray
diffraction techniques. Crystallographic data for 2m: C19H17NO3S,
M = 339.40, monoclinic, P21/c, a = 12.0090(8), b = 7.7050(6), c =
17.9859(9) Å, b = 90.373(5)°, Z = 4, V = 1664.19(19) Å3, Dc = 1.355 -
g cmꢁ3
,
l
= 0.211 mmꢁ1
,
Tmin/Tmax = 0.952/0.969; ꢁ12 6 h 6 15,
ꢁ10 6 k 6 9, ꢁ23 6 l 6 20; 11,032 reflections measured (hmax
27.15°), 10,955 independent (Rint = 0.0307), 2890 with I > 2 (I),
3. Results and discussion
=
r
3.1. Molecular structure
217 parameters, S = 1.096, R1(obs. data) = 0.0691, wR2(all data) =
0
0.1743; max., min. residual electron density = 1.381, ꢁ0.571 eAÅꢁ3
Crystallographic data for 4m: C23H19NO3S2, M = 421.51, tri-
.
There are two possible isomers of compound 1 and each of
them can theoretically exist in two conformational arrangements
with respect to an orientation of side carboxy ester group with re-
spect to pyrrolinone ring. Among them s-trans conformer is pre-
ferred for Z isomer and s-cis for E isomer from sterical reasons
[12]. In the case of (bi)thiophene derivatives the third structural
(conformational) aspect has to be taken into account – an orienta-
tion of thiophene ring, which can be also described by s-trans and
s-cis conformations. Thus the dependence of relative molecular en-
ergy on DFT optimized geometry was carried out for all eight pos-
sible isomers/conformers of compound 2 (Table 1).
clinic, P-1, a = 8.6640(6), b = 11.3269(6), c = 21.3571(15) Å,
a =
75.581(4), b = 86.090(6),
c
= 80.784(4)°, Z = 4, V = 2002.8(2) Å3,
Dc = 1.398 g cmꢁ3
,
l
= 0.291 mmꢁ1, Tmin/Tmax = 0.924/0.981; ꢁ11
6 h 6 11, ꢁ14 6 k 6 14, ꢁ27 6 l 6 27; 33,050 reflections measured
(hmax = 27.4°), 32,963 independent (Rint = 0.1211), 5403 with
I > 2r(I), 523 parameters, S = 1.119, R1(obs. data) = 0.0756, wR2(all
data) = 0.1176; max., min. residual electron density = 0.430,
0
ꢁ0.661 eAÅꢁ3
.
Compound 2m (Fig. 1) crystallize in the monoclinic space group
P21/c with four molecules within the unit cell, on the other hand
the compound 4m crystallizes in the triclinic space group P-1 with
four molecules within the unit cell and two independent molecules
(Fig. 2). In the molecules of both compounds, the interatomic
angles inside of central heterocyclic rings confirm that these rings
are planar with a high degree of conjugation while the C4AC5
bonds are attributed as a multiple ones. The direct comparison of
the structure of 2m and 4m with recently obtained and in detail
described structure of 1m [12] with the same central ring can be
done.
Z isomers of 2 are planar in the area of exocyclic C@C and CAC
bonds; the most stable conformer is s-trans both on ester and thi-
ophene. The energy destabilization induced by conformational
change is almost additive, i.e. 0.27 kcal molꢁ1 per ester group rota-
tion and 0.86 kcal molꢁ1 per thiophene rotation. Distorted
E
isomers are at least 6.23 kcal molꢁ1 less stable, i.e. considerably
more than for compound 1 (2.38 kcal molꢁ1) [12], and their confor-
mational preferences are a bit complicated, as they depend on
gentle sterically driven equilibria. The energy difference between

Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
47
Table 2
1H and 13C NMR chemical shifts for compounds 2, 2m, 3m, 4 and 4m in DMSO-D6
3´
4´
1´
2´
3´
4´
2
´
2´
1´
O
3
S
´
2´
O
H
5
´
2
3
S
2
´
´
3´
´
R =
3
´
´
4
´
1´´
2´´
3´´
4´´
1
5´
N
S
R
4
4´
´´
R
5
´
O
H/C No.
Z-2
E-2
Z-4
d(1H)
d(1H)
d(13C)
d(13C)
d(1H)
d(13C)
NH/NCH3
C@O
1
2
3
11.07
–
–
–
–
–
10.96
–
–
–
–
–
4.01
0.93
–
11.12
–
–
–
–
–
166.6
123.9
102.6
149.6
163.2
59.5
13.7
131.0
130.4
129.2
127.8
130.0
–
137.9
138.0
127.7
134.4
–
–
–
–
–
168.6
125.9
104.6
149.2
164.2
60.4
13.4
166.7
123.4
102.8
149.5
163.2
59.5
COO
CH2
CH3
@CHR
3-10
3-20
3-30
3-40
R-10
R-20
R-30
R-40
R-50
R-100
R-20
R-300
R-40
R-500
–
–
4.12
1.08
8.37
–
7.61
7.52
7.52
–
4.13
1.09
8.30
-
7.62
7.52
7.52
–
13.7
130.8
130.4
129.3
127.9
130.1
–
136.4
139.9
124.3
139.9
–
–
–
–
–
7.94
7.29
7.94
–
–
–
7.82
7.52
-
–
–
7.56
7.21
7.68
–
–
–
–
–
–
–
–
–
–
136.7
125.5
128.8
127.2
–
–
Z-2 m
d(1H)
Z-3 m
d(1H)
Z-4 m
d(1H)
d(13C)
d(13C)
d(13C)
NH/NCH3
C@O
1
2
3
2.95
–
–
–
–
27.5
165.2
121.9
102.9
152.1
162.8
59.1
13.4
132.0
130.6
129.1
128.0
129.4
–
137.8
138.6
127.8
134.9
–
–
–
–
–
2.91
–
–
–
–
–
3.96
0.86
8.28
–
a
a
a
–
8.40
7.82
–
27.7
165.2
127.1
103.6
153.4
163.3
59.2
13.5
140.6
130.7
b
3.37
–
–
–
–
27.6
165.3
121.4
103.1
151.9
162.8
59.2
COO
CH2
CH3
@CHR
3-10
3-20
3-30
3-40
R-10
R-20
R-30
R-40
R-50
R-100
R-20
R-300
R-40
R-500
–
–
3.96
0.88
8.51
–
7.49
7.55
7.55
–
3.97
0.90
8.44
–
7.50
7.56
7.56
–
13.5
131.8
130.6
129.2
128.1
129.5
–
136.3
140.4
124.3
144.2
–
b
b
133.4
132.6
b
142.0
–
139.2
b
b
–
–
7.97
7.31
8.00
–
–
–
7.84
7.52
–
–
–
7.58
7.22
7.69
–
–
a
a
a
–
136.6
125.6
128.8
127.3
–
–
b
a 7.80–7.84 and 7.44–7.60 (10H, m).
b 132.6, 129.6, 129.1, 129.0, 128.2, 126.8, 126.4 (all @CHA).
Generally, the experimental geometry of Z isomers of 2m and
4m is planar on exocyclic carbon–carbon bonds. Both compounds
adopt s-trans arrangement on carboxy ester and thiophene as pre-
dicted from DFT conformational screening on compound 2. Rota-
tion of pendant phenyl rings with respect to pyrrolinone in
compounds 2m and 4m strongly depends on crystal packing.
While theoretical DFT dihedral angles are 74° for both 2m and
4m, the experimental ones cover a wide range from 51° for 2m,
over 72° for 4m(b) to 88° for 4m(a). The calculated exocyclic
C@C and CAC bond lenghts in S0 state are 1.364 Å and 1.456 Å,
1.364 Å and 1.432 Å, 1.367 Å and 1.427 Å for 1m, 2m, 4m, respec-
tively, while the corresponding experimental values are 1.349 Å

48
Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
Table 3
DFT (S0) and TD DFT (S1) computed exocyclic bond lengths of model compounds 1, 2
and 4 without the side carboxy ester group. Corresponding dihedral CAC@CAC and
C@CAC@C angles are in parentheses.
1
0.8
0.6
0.4
0.2
0
Ar
State
S0
LC@C (Å)
LCAC (Å)
1.364(179°)
1.453 (179°)
S1
S0
1.409 (180°)
1.364 (180°)
1.423 (179°)
1.432 (179°)
S1
S0
1.410 (166°)
1.367 (180°)
1.400 (177°)
1.427 (179°)
350 400 450 500 550 600 650 700 750 800 850
λ (nm)
Fig. 3. From the left to right: absorption in DMSO, fluorescence in DMSO,
fluorescence in MTHF at 77 K and ss fluorescence of compound 4.
S1
1.410 (175°)
1.391 (180°)
and 1.465 Å, 1.355 Å and 1.431 Å, 1.351 Å and 1.438 Å (the last
ones for b molecule of 4m). DFT calculations thus a bit overesti-
mate the delocalization in the ground electronic state.
As all methylidene-pyrrolinone derivatives in our previous stud-
ies (including 1, 1m and 3) [12,16] all five new derivatives 2, 2m,
3m, 4 and 4m also arose solely as Z-isomers from reaction by
NMR analysis of their fresh DMSO-d6 solutions (Table 2). In some
cases the rise of the second set of signals corresponding to E-isomer
was observed (e.g. compound 2 in Table 2). The values of methyl
and methylene 1H chemical shifts of all compounds indicate s-trans
conformation of side carboxy ester group in Z-isomers [12], con-
firmed by X-ray structures for 1m [12], 2m and 4m.
1
0.8
0.6
0.4
0.2
0
Fluorescence, as the key process in this study, comes (usually)
from relaxed lowest excited singlet state. Thus it is of interest to
know the geometrical relaxation in S1 compared S0 state, especially
with respect to the rigidity on exocyclic parts of the molecules. As
the TD DFT excited state geometry optimization require an ex-
treme computing costs, only the model compounds 1, 2 and 4 with-
out the side carboxy ester group were studied. A satisfactory
agreement between the experimental and theoretical values of
both exocyclic carbon–carbon bond lengths for compounds 1m,
2m and 4m in the ground state implies, that the values for model
compounds may be also realistic (Table 3). While these carbon–
carbon bond lengths show distinct single and double bond charac-
ter in S0 state, there is a considerable equalization of their lengths
in S1 excited state. The lost of bond lengths alternation, i.e. pro-
longing of originally double bond, thus opens the Z/E photoisomer-
ization route. From this point of view there is no difference among
the model compounds, as the excited state bond lengths, around
which the potential rotation occurs, are the same within 0.001 Å.
On the other hand, originally (in S0) single exocyclic carbon–carbon
bonds show a dramatic differences in S1. Its length decreases from
1.423 Å for model compound 1 to 1. 1.391 Å for 4. Furthermore,
although the bond lengths of originally double bond are the same
for compounds 2 and 4 in S1, there is a remarkably bigger torsion
around it for model 2, as a result of sterical hindrance between S
atom and O(@C) atom from pyrrolinone ring. Finally, the key meth-
ylidene carbon is fixed in compound 4 considerably more than in
other compounds. On assumption, that the excited state rotation
around the methylidene-pyrrolinone bond needs a conformational
support from its nearest intramolecular neighborhood (e.g. partial
rehybridization on methylidene carbon or a partial rotation around
the adjacent exocyclic bond), model compound 4 can be consid-
ered as the most resistant, because the bonding of exocyclic carbon
is the most rigid.
350
400
450
500
550
600
650
λ (nm)
Fig. 4. Absorption in DMSO and fluorescence in MTHF at 77 K of compound 2m.
1
0.8
0.6
0.4
0.2
0
350
400
450
500
550
600
650
700
750
800
λ (nm)
Fig. 5. From the left to right: absorption in DMSO, fluorescence in MTHF at 77 K and
ss fluorescence of compound 4m.

Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
49
Table 4
3.2. Absorption and fluorescence of isolated molecules
Absorption maxima and molar absorptivities in DMSO, fluorescence excitation and
emission maxima in MTHF at 77 K and fluorescence maxima in solid state of the
compounds under study. The bolded values of ss fluorescence correspond to intense
emission, easily detectable by naked eye.
The absorption spectra of all compounds under study in DMSO
show one structureless band in visible area (Figs. 3–5). The exper-
imental maxima are summarized in Table 4. Visible absorption re-
lates to allowed and strong HOMO–LUMO transition; the second
singlet–singlet transition is weak HOMO-1–LUMO and falls into
the UV region (Table 5). Theoretical excitation energies are a bit
lower with respect to the experimental ones, because the absorp-
tion maxima relate to higher vibronic transitions [12]. Both hetero-
substitution and conjugation extension brings a bathochromic and
hyperchromic shift, while the effect of N-methylation is opposite,
as predicted.
We have explained the usual absence of fluorescence of meth-
ylidene pyrrolinones in solution and its presence in low tempera-
ture solvent glass by an effect of environment rigidity hindering
the excited state rotation leading to Z/E isomerization [12,16]. Con-
sequently, all compounds studied here show fluorescence in MTHF
glass at 77 K (Figs. 3–5, Table 4), while only the most rigid (in S1)
compound 4, as supported by TD DFT calculations, but not more
distorted compound 4m, is the only one weakly fluorescent in
DMSO solution at room temperature (Figs. 3 and 5).
Abs.
DMSO
kmax (nm)
Molar Absorp.
Exc. MTHF Fluor.
Fluor.
solid kmax
(nm)
emax
kmax (nm)
MTHF kmax
(l mol cmꢁ1
)
(nm)
1
412
11,900
8700
408
406
441
432
420
430
491
462
522
535
526
537
538
547
582
587
–
1m 409
432
2m 422
425
3m 421
469
4m 462
561
544
–
606
590
710
651
2
19,500
20,100
16,500
14,300
52,700
33,900
3
4
Table 5
TD DFT excitation energies recomputed to wavelengths, oscillator strengths and
expansion coefficient of the dominating electronic configuration of the first two
singlet transitions. Computations were carried out for Z isomers/s-trans conformers
for all compounds.
S0–S1
S0–S2
k00
fosc
Coeff.
k00
fosc
Coeff.
3.3. Crystal packing and solid-state fluorescence
1
1m
2
2m
3
3m
4
431
421
436
428
447
438
496
484
0.563
0.343
0.629
0.417
0.914
0.630
1.048
0.887
0.63 (1–10)
0.62 (1–10)
0.70 (1–10)
0.69 (1–10)
0.70 (1–10)
0.69 (1–10)
0.71 (1–10)
0.70 (1–10)
325
326
331
332
352
353
385
389
0.018
0.026
0.147
0.394
0.303
0.586
0.070
0.195
0.54 (6–10)
0.53 (2–10)
0.57 (2–10)
0.67 (2–10)
0.67 (2–10)
0.68 (2–10)
0.65 (2–10)
0.68 (2–10)
Common presence of ss fluorescence of all N-methylated deriv-
atives was ascribed to the effect of crystal environment rigidity and
to an absence of strong intermolecular interactions leading to en-
ergy or electron transfers, quenching the fluorescence [12]. From
this point of view the intense yellow fluorescence of 1m, orange
fluorescence of 3m and the red one of 4m (Fig. 6) were generally
4m
1
2
3
4
1m
2m
3m
4m
1
2
3
4
1m
2m
3m
4m
Fig. 6. Polycrystalline samples of 1–4m under daylight (top) and under UV (365 nm) irradiation (bottom).

50
Z. Eliáš et al. / Journal of Molecular Structure 1043 (2013) 43–51
(a) Compound 1m
(b) Compound 2m
along plane
perpendicular to plane
Fig. 7. Crystal packing of 1m (top) and 2m (bottom). Molecular planes in dimers are defined by coplanar pyrrolinone rings. Planes are separated 3.644 Å (1m) and 3.606 Å
(2m). Short contacts (less than a sum of van der Waals radii) inside the dimer are pointed out in perpendicular view.
unexpected, the first time in a given set observed absence of ss
fluorescence of compound 2m. An interpretation is based on
(a) 4m (a)
assumption, that excited state out-of-plane rotation as the crucial
intramolecular deactivation channel in fluid environment can be
hindered in crystal, if the rotating (hetero)aryl-methylidene group
is fixed by interplane fixation. In-plane contacts in crystal are con-
sidered to be insufficient.
There is a considerable difference between crystal packing of
compounds 1m and 2m (Fig. 7). Both compounds show no
p–p
stacking between the adjacent coplanar molecules, so an energy
or electron transfer is not efficient. Both exocyclic methylidene car-
bon and hydrogen atoms in compound 1m are fixed by two short
contacts with respect to out-of-plane torsion in dimer. On the
other hand, there is no short contact in dimer of 2m with a partic-
ipation of either thiophene or methylidene atoms. Consequently
(partial), excited state torsion is not sufficiently hindered in 2m
and ss fluorescence is not observed, on account of non-radiative
monomolecular deactivation.
(b) 4m (b)
On the contrary to 4, compound 4m does not show fluorescence
in solution. So it’s extremely strong ss fluorescence must come
from hindering of excited state torsion in solid-state, but in the
same time the quenching by electron or energy transfers must be
limited. This situation exactly corresponds with the crystal packing
(Fig. 8). Both molecules a and b of 4m are strongly interacting in
interplanar dimers with respect to bithiophenyl torsion as each
of pendant thiophene rings is fixed by three (4m(a)) or two
(4m(b)) out-of-plane short contacts. On the other hand, central
coplanar pyrrolinone and thiophene rings of both molecules in
along plane
perpendicular to plane
Fig. 8. Crystal packing of 4m (molecule a on top, b bottom). Molecular planes in
dimers are defined by coplanar pyrrolinone rings. Planes are separated 3.574 Å
(4m(a)) and 3.669 Å (4m(b)). Short contacts (less than a sum of van der Waals radii)
inside the dimer are pointed out in perpendicular view.
expected as a consequence of crystal rigidity and a bathochromic
shift accompanying the conjugation extension. Relatively strong
solid-state fluorescence of compound 3 is exceptional within the
series, as its absence is more common for non-alkylated deriva-
tives. We expect the role of non-planar biphenyl part in hindering
the quenching intermolecular interactions (transfers), but as the
X-ray structure is not on disponse, we will let the nature of this
phenomenon open. The main discussion will thus be oriented on
both dimers show a minimal
shift and opposite orientation of both molecules.
p–p stacking, because of a lateral
4. Conclusion
Solid-state fluorophores of methylidene-pyrrolinone type were
studied. All compounds fluoresce in low temperature solvent glass

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