Green-, Red-, and Infrared-Emitting Polymorphs of Sterically Hindered Push–Pull Substituted Stilbenes

Article abstract of DOI:10.1002/chem.202004419

The synthesis, XRD single-crystal structure, powder XRD, and solid-state fluorescence of two new DPA-DPS-EWG derivatives (DPA=diphenylamino, DPS=2,5-diphenyl-stilbene, EWG=electron-withdrawing group, that is, carbaldehyde or dicyanovinylene, DCV) are described. Absorption and fluorescence maxima in solvents of various polarity show bathochromic shifts with respect to the parent DPA-stilbene-EWGs. The electronic coupling in dimers and potential twist elasticity of monomers were studied by density functional theory. Both polymorphs of the CHO derivative emit green fluorescence (527 and 550 nm) of moderate intensity (10 % and 5 %) in polycrystalline powder form. Moderate (5 %) red (672 nm) monomer-like emission was also observed for the first polymorph of the DCV derivative, whereas more intense (32 %) infrared (733 nm) emission of the second polymorph was ascribed to the excimer fluorescence.

Full text of DOI:10.1002/chem.202004419

Accepted Article
Accepted Article
Title: Green, red and infrared emitting polymorphs of sterically hindered
push-pull substituted stilbenes
Authors: Karel Pauk, Stanislav Luňák Jr., Aleš Růžička, Aneta
Marková, Anna Mausová, Matouš Kratochvíl, Klára Melánová,
Martin Weiter, Aleš Imramovský, and Martin Vala
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To be cited as: Chem. Eur. J. 10.1002/chem.202004419
Link to VoR: https://doi.org/10.1002/chem.202004419
01/2020

10.1002/chem.202004419
Chemistry - A European Journal
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Green, red and infrared emitting polymorphs of sterically
hindered push-pull substituted stilbenes
Karel Pauk,[b] Stanislav Luňák Jr.,[a] Aleš Růžička,[c] Aneta Marková,[a] Anna Mausová,[b] Matouš
Kratochvíl,[a] Klára Melánová,[d] Martin Weiter,[a] Aleš Imramovský,[b] Martin Vala*[a]
[a]
Dr. S. Luňák Jr., MSc. A. Marková, MSc. M. Kratochvíl, Prof. M. Weiter, Assoc. Prof. M. Vala
Materials Research Centre, Faculty of Chemistry
Brno University of Technology
Purkyňova 464/118, CZ-612 00 Brno, Czech Republic
E-mail: vala@fch.vut.cz
[b]
[c]
[d]
Dr. K. Pauk, Ms A. Mausová, Assoc. Prof. A. Imramovský
Institute of Organic Chemistry and Technology , Faculty of Chemical Technology
University of Pardubice
Studentská 95, CZ-530 09 Pardubice, Czech Republic
Prof. A. Růžička
Department of General and Inorganic Chemistry, Faculty of Chemical Technology
University of Pardubice
Studentská 573, CZ-532 10 Pardubice, Czech Republic
Dr. K. Melánová
Joint Laboratory of Solid State Chemistry, Faculty of Chemical Technology
University of Pardubice
Studentská 84, CZ-532 10 Pardubice, Czech Republic
Supporting information (full experimental and computational details for synthesis and analytics, single crystal XRD, powder XRD, spectral and
photophysical measurements and quantum chemical calculations) for this article is given via a link at the end of the document.
Abstract: The synthesis, XRD single crystal structure, powder XRD
2010 Shimizu and Hiyama expressed an essential requirement to
obtain red fluorophores with PLQY over 30 % from neat solids and
fulfilled it in 2012 by for first time observed emission with
maximum over 700 nm (702 nm) and PLQY over 30 % (33 %) at
the same time [11]. Although, since that time the impressive
values of NIR emission were reported for polycrystalline powders,
e.g. 735 nm / 11 % [9b], water dispersions of nanoparticles, e.g.
685 nm / 20.7 % [12], and neat films, e.g. 756 nm / 17 % [13] or
750 nm / 21 % [14], the ˃700 nm / ˃30 % limit still remains a
challenge [15]. There are generally two ways to tune the position
of emission maxima. The first one, intramolecular, modifies the
strength of (usually) electron acceptor [14], e.g. by adding an
additional CN group [10] or by a conjugation extension [16], while
the second, intermolecular, modifies the structure in order to form
emitting aggregates, e.g. of J-type [4c] or X-type [16]. Structural
change, transforming the continuous H-type dimeric stacking to
discrete dimers, showing intense (54.8 %) excimer fluorescence
with a maximum at 690 nm, seems also promising [17].
and solid-state fluorescence of two new DPA-DPS-EWG derivatives
(DPA
= diphenylamino,
DPS = 2,5-diphenyl-stilbene,
EWG = carbaldehyde or dicyanovinylene) is described. Absorption
and fluorescence maxima in solvents of various polarity show
bathochromic shift with respect to parent DPA-stilbene-EWGs.
Electronic coupling in dimers and potential twist elasticity of
monomers were studied by density functional theory. Both
polymorphs of CHO derivative emit green fluorescence (527 and
550 nm) of moderate intensity (10 % and 5 %) in polycrystalline
powder form. Moderate (5 %) red (672 nm) monomer-like emission
was also observed for the first polymorph of DCV derivative, while
more intense (32 %) infrared (733 nm) emission of the second
polymorph was ascribed to the excimer fluorescence.
Introduction
Stilbene derivatives, substituted on central vinylene group by
phenyls (tetraphenylethene, TPE), CN group (α-cyanostilbene,
CS) or both (2,3,3-triphenylacrylonitrile, TPAN) form main
substructures of the compounds showing aggregation-induced
emission (AIE) or aggregation-induced enhanced emission
(AIEE) [18]. If suitably substituted, their derivatives can show FR
/ NIR solid-state fluorescence (SSF) [19]. Earlier attempt to
construct red emitting system of D-π-A type, using DPA donor and
dicyanovinylene (DCV) complex acceptor, connected by 3D
sterically hindered 9,9´spirobifluorene, preventing the stacking,
lead to intense SSF near 650 nm [20]. Thus, inspired by a stilbene
versatility and DPA-π-DCV concept, we designed and
synthesized DPA-DPS-DCV, with stilbene π-system substituted
at acceptor part by a pair of phenyls (Scheme 1). 2,5-Diphenyl-
stilbene (DPS) is thus a regioisomer of TPE. From another point
Solid-state organic luminophors emitting in the far red / near
infrared spectral region (FR / NIR, 650–900 nm [1] or 1000 nm [2])
are especially important in the fields of OLED and bioimaging [3].
Such low band-gap organic materials usually show lower
photoluminescence quantum yields (PLQY) than emitters in
shorter wavelength regions, because of intramolecular non
radiative processes, preferred according to so-called „energy-
gap“ law, and intermolecular formation of detrimental cofacial
aggregates [2,4]. Common design of FR / NIR emitters consists
of linking an electron donor and acceptor parts into one molecule
in D-π-A [5], D-π-A-π-D [6] or A-π-D-π-A [7] fashion. D in these
structures usually means diphenylamino or triphenylamino (DPA,
TPA) type donor, while A is either heterocyclic acceptor [8], two
or more cyano (CN) groups [9] or a combination of both [10]. In
1
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


N
O



N
N
N
Figure 1. Normalized experimental PXRD of a) polymorph R, b) polymorph IR
and c) recomputed from single crystal XRD of polymorph IR.
Scheme 1. Compounds under study, with highlighted D-π-A character by colour.
Dihedral angles describe the rotation of a donor (δ) and acceptor (α) substituted
parts of a stilbene core and the rotation of a substituent (ε).
Thus, the synthesis was completely repeated and further purifying
step (column chromatography) was included both, for DPA-DPS-
DCV and for -CHO, before final crystallizations (batch 2). We were
pleasently surprised, that 1) the impurity emission disapiered, 2)
DPA-DPS-DCV crystallized in a new modification with more
intense SSF shifted to NIR and 3) the single crystal with sufficient
parameters for X-ray diffractometry (XRD) was obtained. The
crystal structure was also determined for DPA-DPS-CHO
polymorph from batch 1. The existence of DPA-DPS-DCV
polymorphs, coming from batch 1 (polymorph R) and batch 2
(polymorph IR) was confirmed by room temperature powder XRD
(PXRD). The PXRD pattern, computed from low temperature
single crystal XRD, agrees well with the experimental one for
polymorph IR at lower θ (Fig. 1), while at the higher ones the
differencies caused by different temperature are more remarkable.
The existence of two polymorhs of DPA-DPS-CHO, coming from
batch 1 (polymorh DG) and batch 2 (polymorph G), was also
confirmed by PXRD (Fig. S1).
of view, accenting the sterical hindrance between ortho phenyl
and vinyl hydrogens, the compound can be considered as once
by TPA and twice by CN substituted 2,5-diphenyl-1,4-divinyl-
benzene (PVB), i.e. TPA-PVB-(CN)2. The polymorphism of
propeller-like TPA containing fluorescent compounds is known
[21] and is often used in stimuli-responsive materials [22]. Side
phenyl substitution of trans,trans-distyrylbenzene lead to the
formation of two polymorphs, showing either slipped-stacked or
uncommon cross-stacked (X-type) packing [23]. As both these
structural features are contained in the molecules under study,
polymorphism is quite expectable.
Results and Discussion
The synthesis of the target compound was carried out according
to Scheme 2. Although both final DPA-DPS-DCV and the key
intermediate DPA-DPS-CHO have shown acceptable analytics
(batch 1), the residual short wavelength fluorescence of an
impurity in acetonitrile, where the infrared fluorescence of DPA-
DPS-DCV is almost completely quenched, was observed.
Scheme 2. Synthesis of DPA-DPS-CHO and DPA-DPS-DCV. Compound 3, used in the second reaction step, is dimethyl [4-(diphenylamino)benzyl]phosphonate
(see SI).
2
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Figure 2. Crystal structure of polymorph IR. Overall view (a) on the alternation of the dimers in a slipped stacked column. Dimer 1 consists from yellow and green
monomers, dimer 2 from yellow and red. Detail of dimer 1 (b, c) with highlighted stack (b) in a main conjugated chain (φ = 32.9 °, CC = 3.539 Å) and (c) between a
pair of side phenyls (φ = 13.1 °, CC = 4.237 Å). Detail of dimer 2 (d, e) with highlighted stack (d) in a main conjugated chain (φ = 0 °, CC = 4.255 Å) and (e) between
a side phenyl and phenyl from DPA group (φ = 14.2 °, CC = 4.054 Å).
Figure 3. Splitted orbitals in dimer 1: LUMO+1 (ag) (a, b), HOMO (au) (c, d), front view (a, c), side view (b, d), Isovalue = 0.02 was used for drawing.
3
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DPA-DPS-CHO polymorph DG crystalizes in non-
centrosymmetrical P212121 space group, while DPA-DPS-DCV
polymorph IR in centrosymmetrical P1 group. Consequently, in
polymorph DG the charge separation, given by its push-pull
character, is not compensated and the polymorph should show
an activity in the second order nonlinear optics. On the other hand,
the key structural feature of polymorph IR is the formation of
highly slipped columns with two alternating types of
centrosymmetrical dimers (Fig. 2a). The dominant type of packing
is a formation of parallel offset face-to-face stacks [24], either
between main conjugated chains or between side phenyls. The
stacks were characterized by the angles φ between the planes of
benzene rings (Fig. 2c-2e) or benzene ring and DCV group
(Fig. 2b) and center-to-center (CC) distances (Fig. 2b-2e) and
compared to density functional theory computed (DFT: ωB97X-D
/ 6-31G(d,p) [25]) optimized structure of benzene dimer (φ = 0 °,
CC = 4.233 Å, Fig. S2a). Simply said, dimer 1 is more closely π-
stacked along the main conjugated chain with plane-to-plane (PP,
defined by the central rings of PVB substructure) distance 3.378 Å
(Fig. S3a), while the dimer 2 is more loosely stacked with PP,
defined by TPA phenyls, equal to 3.952 Å (Fig. S3b), but a bit
better stacked between side phenyl and DPA phenyl (Fig. 2e),
than dimer 1 between side phenyls of PVB (Fig. 2c).
Geometry of both alternating dimers 1 and 2 was recomputed
from crystal structures by density functional theory (DFT) and
compared with the experimental data (Fig. S3), in order to
understand 1) how stand-alone are both dimers in an isolated
dimer approximation, and 2) how their arrangement affects the
electronic structure (Fig. 3). Computed structure of dimer 1
matches the experiment in an excellent way. The crucial stack
between benzene ring and vinylene of DCV gives CC = 3.485 Å
(exp. CC = 3.539 Å), an overall length of a conjugated system of
a dimer, characterized by a distance between the nitrogens of
TPA groups (NN), is 22.508 Å (exp. NN = 22.536 Å). The plane-
to-plane (PP) distance between planes, defined by central
benzene rings of PVB, is 3.238 Å (exp. PP = 3.378 Å), indicating
a bit overestimated attractive forces in DFT calculations.
Consequently, the computed PP = 3.395 Å and CC = 3.795 Å in
a benzene parallel offset stack (Fig. S2a) may be also lower than
the real ones and thus the experimental values of CCs between
side phenyls (Fig. 2c, e) may be relatively close to the ideal
phenyl-phenyl stack. On the other hand, there is a considerable
difference between the computed and experimental parameters
of dimer 2: PP = 3.660 Å (exp. PP = 3.952 Å) and NN = 8.210 Å
(exp. NN = 7.824 Å), i.e. the computed geometry is compressed
in a perpedicular direction and prolonged along the direction of a
conjugated system (Fig. S3b). The dimer 1 thus can be
considered as a discrete dimer [26], while the dimer 2 is not stand-
alone, as its structure significantly depends on interactions with
further neighbours, omitted in a dimer approximation.
Figure 4. Crystal structure of polymorph DG. Overall view (a) on the stacked
column. Detail of a dimer (b) with highlighted stack in a main conjugated chain
(φ = 12.3 °, CC = 4.888 Å).
dimer, so as any of splitted LUMO orbitals for dimer 2, but the
LUMOs in dimer 1 share a common space at the vinylene-
benzene part, as seen from the side view on Fig. 3. After the
excitation to LUMO electron can easily move between both
monomers of dimer 1, but not out of them, while hole must stay
located on one of TPA groups. Such situation is ideal for
a formation of an excimer, localized on dimer 1.
The packing of DPA-DPS-CHO in polymorph DG is
considerably different. Molecules form infinite columns, consisting
of the same type of non-centrosymmetrical dimers (Fig. 4). The
key intermolecular interaction is an edge-to-face y-shaped [24]
arrangement of the side phenyls (Fig. S5), in which all these
phenyls serve both as donors and acceptors in C-H…π
interactions. The effective stack between main conjugated chains
of adjacent molecules in a column is disabled, because of big CC
distance (Fig. 4). Consequently, there is no efficient electronic
coupling between conjugated chains, and the molecules can be
considered as electronically isolated monomers. If the parameters
of side phenyl edge-to-face arrangement (Fig. S5) are compared
with DFT optimized structure of corresponding benzene dimer
(φ = 75.9 °, CC = 4.841 Å, Fig. S2b), we see that the values from
crystal are quite close to the ideal phenyl-phenyl y-shaped
arrangement.
Both frontier Kohn-Sham orbitals of a monomer are splitted in
a dimer. These splitted orbitals in a centrosymmetrical dimer are
of ag and au symmetry. Their shape thus differs mainly by the
opposite parity and their energy difference relates to electronic
coupling, i.e. hole and electron transfer integrals [27]. These
splittings are relatively small in dimer 2 (ΔHOMO = 22 meV,
ΔLUMO = 51 meV), for dimer 1 ΔHOMO = 45 meV remains also
small, while ΔLUMO = 234 meV is considerably higher. The size
of orbital energy splittings is reflected in a shape of these MOs
(Figs. 3, S4). HOMO orbitals on monomers do not overlap in any
Molecular geometries of both DPA-DPS-DCV and DPA-
DPS-CHO in crystals are quite similar (Fig. 5). Both crystals
contain only one independent molecule, side phenyls are nearly
coplanar, rotated about 50 ° out-of-plane for both molecules and
show anti arrangement on acceptor substituted part of stilbene,
characterized (Scheme 1) by dihedral angles α = 40.1 ° and
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Figure 5. Molecular structure of DPA-DPS-CHO (a) and DPA-DPS-DCV (b),
ORTEP view, 50% probability level.
ε = −10.6 ° (CHO) and α = −16.5 ° and ε = 31.1 ° (DCV). On the
contrary electron donating TPA groups are rotated in an opposite
sense with respect to α, giving δ = 7.0 ° (CHO) and δ = 21.8 °
(DCV), but the sum of absolute values of α + δ does not differ
considerably for both compounds. On the other hand, crystal of
DPA-stilbene-CHO [28] contains two independent molecules, one
of them in anti (α = 8.9 ° and ε = −0.6 °, δ = 3.4 °) and the second
in syn (α = −1.9 ° and ε = 168.6 °, δ = 11.2 °) arrangement.
Molecular modelling (Tab. S2) shows marginal energy differences
between rotamers with syn and anti arrangements for DPA-
stilbene-EWG derivatives (˂0.012 kcal / mol) and more than the
order of magnitude higher for DPA-DPS-EWG derivatives
(˃3.055 kcal / mol). Thus the sterical hindrance of side phenyl
substituents causes two structural effects: more pronounced non-
planarity of a main conjugated chain and fixation a anti conformer.
The potential minimum, relating to the conformer with an opposite
sense of TPA rotation, was found only for both CHO derivatives
and the energy difference between both rotamers of DPA-DPS-
CHO was marginal (˂0.2 kcal / mol). Thus the conformation of
DPA-DPS-DCV molecule, observed in crystal, is the only one
preferred by theory, while the experimental conformation of DPA-
DPS-CHO is one of at least two favourable rotamers. The ability
to undergo substantial changes of a dihedral angle between donor
and acceptor parts of a molecule, depending on the external
stimuli, can be considered as „twist elasticity“ and may be
a source of polymorphism of DPA-DPS-CHO [29].
Figure 6. Fluorescence spectra of DPA-DPS-CHO (a) and DPA-DPS-DCV (b)
in various solvents, and all polymorphs in powder (c). Maximum of very weak
fluorescence of DPA-DPS-DCV in acetonitrile was near the end of a detection
range of a conventional spectrofluorometer, so it was extended with the laser
excited NIR emission detected with iCCD camera.
Absorption and fluorescence were studied in five solvents of
various polarity (Fig. 6, Tab. S1). DPA-DPS-CHO behaves in
solution in a similar way as other compounds of DPA-π-CHO
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structure [30]. The absorption maximum in toluene is 3 nm red
shifted with respect to DPA-stilbene-CHO [30a], molar
absorptivity is lower, as side phenyls do not contribute to the
oscillator strength and only increase a molecular weight,
fluorescence maximum is 13 nm red shifted and PLQY and
fluorescence lifetime (τF) are almost identical. The dependence of
the absorption and fluorescence on solvent polarity is quite similar
to DPA-biphenyl-CHO [30b], i.e. the absorption maxima are not
significantly affected by solvents, while the fluorescence shows a
remarkable red shift with increasing polarity, that is a common
feature of D-π-A molecules, arising from intramolecular charge-
transfer [9a]. The highest PLQY is reached for solvents with
medium polarity as for DPA-biphenyl-CHO [30b], but its decrease
in high polarity solvents is not so dramatic. On the other hand, the
differences between spectral and photophysical properties of
DPA-DPS-DCV and DPA-stilbene-DCV [31] are remarkable
(Tab. S1). First of all, the absorption maxima of DPA-DPS-DCV
in the same (dichlormethane) or similar (benzene and toluene)
solvents are dramatically red shifted (from 395 to 473 nm and
from 394 to 470 nm, respectively). Even bigger shifts are
observed for fluorescence maxima (from 524 to 752 nm in
dichlormethane and from 488 nm in benzene to 605 nm in
toluene). PLQYs of DPA-DPS-DCV are higher in less polar
solvents and lower in the more polar ones, as compared to DPA-
stilbene-DCV. Furthermore, theoretical calculation, based on time
dependent (TD) DFT / DFT (Tab. S2), predict exactly the opposite
effect, i.e. a hypsochromic shift in absorption, caused by the
decreased planarity of a conjugated chain between electron
donor and acceptor, due to side-phenyl hindrance. Strictly said,
we are unable to explain the slight (CHO) or dramatic (DCV)
bathochromic shifts, induced by side-phenyl substitutions. We
may only speculate, that the correct description of both absorbing
and emitting states of these push-pull stilbenes in solution
requires an involvement of charge-transfer structures, as in
merocyanines [31], contributing a bit (CHO) or considerably more
(DCV) in screwed side-phenyl substituted derivatives, as
compared to DPA-stilbene-EWGs.
SSF of polycrystalline powders of both polymorphs of both
compounds is shown on Fig. 6c. DPA-DPS-CHO polymorphs
emit green fluorescence with the maxima at 527 nm (DG) and
550 nm (G) and with moderate PLQY 10 % (DG) and 5 % (G).
Polymorph DG with known XRD structure emits without doubts as
an isolated monomer, because of a marginal electronic coupling
between its molecules. As the packing in polymorph G is not
known, we may only speculate, that, due to the twist elasticity, the
bathofluoric shift and the decrease of PLQY may come from the
arrangement with less distorted monomer conformation and with
tighter packing as for bis-TPA substituted thiazolothiazol [21b].
PLQY of both polymorphs is lower than for any solvent, so neither
TPA group, as expected [18a], nor DPS bridge, on the contrary to
TPE regioisomer [18a], can be considered as AIEgens. DPA-
DPS-DCV polymorphs emit in target FR / NIR area with moderate
intensity for polymorph R (672 nm, 5 %) and more intensely for
polymorph IR (733 nm, 32 %), i.e. with parameters only rarely
reported, e.g. for four-coordinated boron compounds [33]. We
consider the red emission as coming from an isolated monomer
in analogy with both green emitters, while the NIR emission as
coming from excimer fluorescence located on dimer 1 (Fig. 2b).
As compared to fluorescence in solvents, the emissions of
electronically isolated monomers with 5 % PLQY (G and R) thus
lie at the wavelengths 20–30 nm shorter than in chloroform, due
to their limited ability of excited state relaxation, as compared to
fluid environment. Further support of this interpretation comes
from apparent similarity of the crystal structure and spectral and
photophysical behaviour of R and IR polymorphs with the recently
reported excimer type emitter BTA-TPA [19b]. The structural
similarity consists of the alternation of the stacked discrete dimer
(PP = 3.38 Å for IR polymorph and 3.50 Å for BTA-TPA) with the
unstacked one (PP = 3.95 Å for IR polymorph and 4.31 Å for BTA-
TPA). Both compounds show a considerable blue-shift of SSF, if
going from excimer type fluorescence to the monomer type (going
from polymorph IR to R in our case, from crystal to amorphous
film for BTA-TPA), accompanied by a decrease of PLQY (from
32 % for IR to 5 % for R, from 54.8 % for crystal to 21.9 % for
amorphous film) and fluorescence lifetime (from 4.52 ns for IR to
1.46 ns for R [34], from 6.8 ns for crystal to 3.7 ns for amorphous
film of BTA-TPA). Both, the polymorph IR, and BTA-TPA
fluorescence lifetime of excimer fluorescence is remarkably
longer than in any reported solvent, in which, in addition, the
decay is always monoexponential (Tab. S1).
We note only, that the IR polymorph is sufficiently stable.
There were found no significant changes in single crystal XRD
after several months standing in a deposit. SSF of polycrystalline
powder was also well reproduced. On the other hand, if the
powder is pressed between two glasses, its fluorescence
maximum shows immediate hypsochromic shift to about 670 nm,
i.e. near to the maximum of polymorph R, so IR polymorph can
be considered as stimuli-responsive.
Conclusion
Decorating of push-pull substituted stilbenes with a pair of side
phenyls fixes and screws the molecular structure and causes
bathochromic and bathofluoric spectral shift in solutions. Both
compounds form polymorphs, which show solid-state
fluorescence in a polycrystalline powder form. On the contrary to
its TPE regioisomer, 2,5-diphenyl-stilbene π-bridge is not an
AIEgen. Interactions of side phenyl pairs are decisive for a
packing in crystals and consequently for spectral position and
intensity of SSF. Their edge-to-face arrangement lead to minimal
electronic coupling of molecular main conjugated chains and an
emission of moderate intensity comes from relatively isolated
monomers in crystals. Parallel offset face-to-face packing of side
phenyls enabled to form crystal consisting of the electronically
coupled isolated dimers, showing rare excimer type intense
emission in near infrared region.
Experimental Section
Synthesis All reagents and solvents were purchased from commercial
sources (Sigma-Aldrich, Fluorochem, Acros Organics, TCI Europe, Merck,
and Lach-Ner). Commercial grade reagents were used without further
purification. Tetrahydrofuran (THF) was pre-dried under sodium and then
distilled under anitrogen atmosphere. Reactions were monitored using thin
layer chromatography (TLC) plates coated with 0.2 mm silica gel (60 F254,
Merck). TLC plates were visualized using UV irradiation (254 nm). All
melting points were determined using a Melting Point B-540 apparatus
(Büchi, Switzerland) and are given in their uncorrected form. The IR
spectra were recorded on a Nicolet 6700 FT-IR spectrometer (Thermo
Fisher Scientific, Wal-tham, MA, USA) over the range of 400–4000 cm-1
using the ATR technique. The NMR spectra were measured in DMSO-d6,
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CDCl3, CD2Cl2-d2, and Tetrahydrofuran-d8 (TDF) solutions at ambient
temperature on a Bruker AvanceTM III 400 spectrometer at frequencies
400 MHz (1H) and 100.26 MHz (13C), or on a Bruker AscendTM 500
spectrometer at frequencies 500.13 MHz (1H) and 125.76 MHz (13C{1H}).
The chemical shifts (δ) reported in SI are given in ppm and are related to
the following residual solvent peaks: -4.79 (D2O-d2), -5.32 (CD2Cl2-d2), -
7.27 (CDCl3), and -2.5 (DMSO-d6). Tetramethylsilane (TMS) was used as
an internal standard. The coupling constants (J) are reported in Hz.
Elemental analyses (C, H, and N) were performed on an automatic
microanalyser (Flash 2000 Organic elemental analyser). Mass
spectrometry with high resolution was determined by the “dried droplet”
method using a MALDI mass spectrometer LTQ Orbitrap XL (Thermo
Fisher Scientific) equipped with a nitrogen UV laser (337 nm, 60 Hz).
Spectra were measured in positive ion mode and in regular mass extent
with a resolution of 100,000 at a mass-to-charge ratio (m/z) of 400, with
2,5-dihydrobenzoic acid (DBH) used as the matrix. The FT-IR spectra were
recorded on FT-IR Nicolet iS50 using the ATR technique.
Acknowledgements
This work was supported by the Czech Science Foundation grant
No. 17-21105S. Access to computing and storage facilities owned
by parties and projects contributing to the National Grid
Infrastructure MetaCentrum provided under the programme
"Projects of Large Research, Development, and Innovations
Infrastructures" (CESNET LM2015042) is greatly appreciated by
S.L.
Keywords: Donor-acceptor systems • Fluorescence • Density
functional calculations • Crystal engineering • Polymorphism
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Entry for the Table of Contents
Intense near infrared shining: Push-pull substituted stilbenes, decorated with a pair of side phenyls, form solid-state emitting
polymorphs. Parallel offset face-to-face packing of side phenyls enabled to form crystal consisting of the electronically coupled
isolated dimers, showing rare excimer type intense emission in the near infrared region.
9
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