Nature of Color Diversity in Phenylenevinylene-Based Polymorphs

Article abstract of DOI:10.1021/acs.cgd.9b00328

In this work we present a joint experimental and theoretical study of the light emitting properties of (E,E)-2,5-dimethoxy-1,4-bis[2-(4-carboxylatestyryl)]benzene, a phenylenevinylene (PV, from now on) derivative in the solid state. Careful crystal growth under different conditions of this PV luminophore yields three different crystals with dissimilar color emissions with maxima ranging between 504 and 565 nm. Single crystal structure elucidations indicate that they correspond to a DMF solvate, denoted 2·DMF (yellow) and two conformational polymorphs named 2α (yellow) and 2β (orange). Analysis of the close contacts and the framework topologies help us to rationalize the different colors observed, not only on the basis of different intramolecular conformations but also of the different intermolecular interactions observed. Particularly, the presence of different C-H···πand ?···πinteractions in 2α and in 2β polymorphs, respectively, give rise to different supramolecular frameworks with hex and bcu topologies. Theoretical studies reinforce the hypothesis that cooperative ?···πinteractions in the crystal lattice are responsible for the red-shifted color observed in the polymorph 2β. In fact, in contrast to what is observed with the polymorph 2α, as we incorporate more monomers in the calculations of the optical properties of polymorph 2β, the estimated energy for the vertical excitation from the ground state to the first excited state moves to lower values.

Full text of DOI:10.1021/acs.cgd.9b00328

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Article
On the Nature of Color Diversity in Phenylenevinylene-based Polymorphs
Juan C. Cárdenas, Lina M. Aguirre-Díaz, Johan F. Galindo, Jorge Ali-Torres, Cristian Ochoa-Puentes,
Marcelo Echeverri, Berta Gómez-Lor, M. Ángeles Monge, Enrique Gutiérrez-Puebla, and Cesar A. Sierra
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00328 • Publication Date (Web): 04 Jun 2019
Downloaded from http://pubs.acs.org on June 5, 2019
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On the Nature of Color Diversity in
Phenylenevinylene-based Polymorphs
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Juan C. Cárdenas, † Lina M. Aguirre-Díaz, ‡* Johan F. Galindo, † Jorge Alí-Torres, † Cristian
Ochoa-Puentes, † Marcelo Echeverri, ‡ Berta Gómez-Lor, ‡ M. Ángeles Monge, ‡ Enrique
Gutiérrez-Puebla‡ and Cesar A. Sierra. †*
†
‡
Departamento de Química, Universidad Nacional de Colombia, Bogotá D.C., Colombia.
Departamento de Nuevas Arquitecturas en Química de Materiales, Materials Science Factory,
Instituto de Ciencia de Materiales de Madrid-Consejo Superior de Investigaciones Científicas
(ICMM-CSIC), Sor Juana Inés de la Cruz 3, Cantoblanco 28049, Madrid, Spain.
KEYWORDS: Luminescence, fluorescence, polymorphism, supramolecular, topology.
ABSTRACT: In this work we present a joint experimental and theoretical study of the light
emitting properties of the E,E-2,5-dimethoxy-1,4-bis[2-(4-carboxylatestyril)]benzene, a
phenylenevinylene (PV, from now on) derivate in solid state. Careful crystal growth under
different conditions of this PV luminophore yields three different crystals with dissimilar color
emissions with maxima ranging between 504 and 565 nm. Single crystal structure elucidation
indicates that they correspond to a DMF solvate denoted 2∙DMF (yellow) and two conformational
polymorphs named 2 (yellow) and 2 (orange). Analysis of the close contacts and the framework
topologies help us to rationalize the different colors observed, not only on the base of different
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intramolecular conformations, but also of the different intermolecular interactions observed.
Particularly, the presence of different, C-H∙∙∙ and ∙∙∙ interactions in 2 and in 2 polymorphs
respectively, give rise to different supramolecular frameworks with hex and bcu topologies
correspondingly. Theoretical studies reinforce the hypothesis that cooperative ∙∙∙ interactions in
the crystal lattice are responsible for the red shifted color observed in the polymorph 2. In fact,
contrary to what it is observed with the polymorph 2, as we incorporate more monomers in the
calculations of the optical properties of polymorph 2, the estimated energy for the vertical
excitation from the ground state to the first excited state moves to lower values.
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1. INTRODUCTION
Controlling the optical properties of organic molecular aggregates in -conjugated systems has
acquired great relevance during the last few years. Assemblies of -conjugated units have been
found to offer an efficient pathway for excitons, allowing the development of new materials such
as molecular crystals, gels, and liquid crystals with stimuli responsible light emitting properties.
Besides, efficient light emitting of -conjugated organic materials arouse much interest in
close relation to the development of novel organic electronic devices such as organic light emitting
diodes (OLEDs), or chemical sensors, which take advantage of the interesting optical properties
of -conjugated molecules as active layers.1 However, although an enormous number of efficient
fluorophores with different chemical structures are known and there are well established structure-
property relationships to tune their optical properties at the molecular level (mainly by varying the
extension of the -system and the electronic nature of attached functional groups), the knowledge
on the factors that control bulk light emitting properties is still very limited.2 In fact, the design of
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molecules that are highly emissive in solid and crystalline states (states in which these compounds
are intended to be used in thin-film devices) has proved challenging since strong intermolecular
interaction and close packing usually lead to quenching of emission.
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Single crystal analysis provides invaluable information on how each molecule is arranged
and how it interacts with the surrounding molecules in the solid-state, and offers an excellent
opportunity to investigate the macroscopic properties that depend on the molecular arrangement.
In the last few years many efforts have been dedicated to tune the emission color of organic
fluorophores through crystal engineering and to establish clear design rules for optimize light
emission by analyzing intermolecular interactions in single crystals.3-4 In this context, light-
emitting polymorphs, are of particular interest as they allow correlating the changes in the solid-
state luminescent properties, exclusively on the basis of the molecular conformation and packing,
without the interference of potential effects exerted by different functional groups. 5-7
In the search of highly emitting organic materials, PVs occupy a distinctive place. These
molecules not only present very efficient light emitting properties in solid state, but also exhibit
very high optical and thermal stability and film forming proclivity.8-12 Due to their favorable
properties, these molecules have been incorporated in different optoelectronic devices.13
Furthermore, their tendency to self-assemble in supramolecular aggregates has been successfully
exploited to develop soft materials with tunable light emitting properties through a supramolecular
approach. Thereby, understanding the key factors that govern the light emitting properties of
phenylenevinylenes in crystalline solids, may led to novel strategies to control these properties and
therefore broaden their potential applications.14-17
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Herein we present the solid state light emitting properties of the PV derivative E,E-2,5-dimethoxy-
1,4-bis[2-(4-carboxylatestyril)] benzene (2) which crystallize in two conformational polymorphs
(denoted 2 and 2) and a DMF solvate (named 2∙DMF). The three crystals showed different
color emissions when excited at 360 nm. Single crystal analysis and computational calculations
allow us to correlate the emission colors observed with intramolecular conformational and/or
supramolecular differences found in the different crystals.
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2. METHODOLOGY
2.1. Experimental Section. Commercially available precursors were used as received
without further purification and solvents were dried according to standard procedures. Melting
points were determined using a Stuart SMP10 melting point apparatus and the reported values are
uncorrected. Infrared spectra were recorded with KBr discs on a Shimadzu IR Prestige-21 FT-IR
spectrophotometer. 1H and 13C NMR spectra were taken on a Bruker Advance 400 spectrometer
using CDCl3 or DMSO-d6 as solvents, chemical shifts are reported as  values in units of ppm.
UV-Vis spectra were measured in a double channel Thermo Scientific Evolution 300 UV-Vis
spectrophotometer with xenon lamp based platform. Fluorescence spectra were registered in
DMSO solutions on a PTI QuantaMaster 300 spectrofluorometer with xenon lamp.
2.2. Single crystal X-ray structure determination. Single-crystal X-ray data were
obtained in a Bruker four circle kappa-diffractometer equipped with a Cu INCOATED
microsource, operated at 30 W power (45kV, 0.60mA) to generate Cu Kα radiation (λ = 1.54178
Å), and a Bruker VANTEC 500 area detector (microgap technology). Single crystal X-Ray
diffraction data were collected exploring over a hemisphere of the reciprocal space in a
combination of  and  scans to reach a resolution of 0.86 Å, using a Bruker APEX218 software
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suite (each exposure of 40 s covered 1º in . Unit cell dimensions were determined for least-
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squares fit of reflections with I > 20 . A semi-empirical absorption and scale correction based on
equivalent reflection was carried out using SADABS.19 The space group determination was carried
out using XPREP.20 The structures were solved by direct methods. The final cycles of refinement
were carried out by full-matrix least-squares analyses with anisotropic thermal parameters of all
non-hydrogen atoms. The hydrogen atoms were fixed at their calculated positions using distances
and angle constraints. All calculations were performed using SMART21 and APEX218 software for
data collection, SAINT21 for data reduction, SHELXTL20 and OLEX222 to resolve and refine the
structure respectively. CCDC 1895790, CCDC 1895648, CCDC 1895649 and CCDC 1895650
contain the supplementary crystallographic data for 1, 2, 2 and 2∙DMF, respectively.
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2.3. Linear Spectroscopy. UV-Visible studies were carried out on a PerkinElmer Lambda
XLS+ spectrometer. Fluorescence spectra were recorded on an Aminco SLM 8000
spectrophotometer.
2.4. Theoretical Calculations. In order to understand the interaction that drives the crystal
growth and optical responses of crystals and monomers in solution and solid state, computational
calculations using methods based on density functional theory (DFT) and time-dependent DFT
were performed. The calculations were carried out on the oligomers (up to 10 monomers) for the
different crystal structures.
DFT and TDDFT calculations: Electronic structure calculations were carried out at the
B3LYP-D3BJ/6-31+G(d) level of theory. This combination of functional and basis set has been
proved to provide a proper geometrical description.23 B3LYP-D3BJ considers the hybrid nonlocal
B3LYP density functional24-25 complemented with empirical dispersion corrections computed as
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a sum of two and three-body energies, as defined by Grimme et al.26-27 This correction allows
including long-range dispersion contributions to the computed ab initio B3LYP energy and
gradients. The inclusion of these dispersion terms has been shown to be relevant in determining
interaction occurring in crystal structure, which are in some cases the differential interaction.28 All
the electronic structure calculations were performed by using the Gaussian09 software package.29
TDDFT offers a good solution for the vertical excitation of the system from the ground state to the
different excited states which can be related with the absorption peaks in an experimental
spectrum.23, 30-31 For the TDDFT calculations we have computed the first fifteen single excited
state for every system.
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3. RESULTS AND DISCUSSION
Considering that the luminescence characteristics of the PV derivatives greatly depend on their
configuration/conformation relationships, we have focused in synthetic methodologies that yield
pure trans configuration, as it promotes the ideal arrangement of π orbitals for an uninterrupted
electron flow through the structure. Therefore, the synthesis of 1 was performed using a previously
optimized solvothermal Heck reaction, which produces the compound with the desired
stereoselectivity in good yield (Scheme 1).32
COOR
OMe
OMe
Pd(dba)2, P(OPh)3, NEt3
I
+
DMF, 110C,24h
I
OMe
ROOC
COOMe
OMe
1.
2.
R=Me
R=H
KOH, MeOH, THF, H2O
Reflux,20h
Scheme 1. Heck synthesis of compound 1 and conditions for synthesis of compound 2
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The catalytic system offered by Pd(dba)2 and P(OPh)3 has been found to be ideal for obtaining PV
systems with 2½ electronic conjugation length and trans-trans configuration, selectively. This
high selectivity can be attributed to the steric hindrance and electronic properties of the phosphite
ligand, which results in an enhanced orientation control of the substrates during the C – C coupling
step and also in an improvement of the catalytic cycle kinetics. Moreover, the phosphite ligand
confers to the Pd in the catalyst high stability, preventing the aggregation and precipitation of Pd(0)
black.33 Spectroscopic characterization confirms the trans-trans configuration of this compound
(see S. I.). Hydrolysis of 1, under basic conditions result in the obtaining of 2 with excellent yield,
without affecting the trans-trans configuration according to the spectroscopic characterization (see
S. I.).
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The UV-Vis absorption and fluorescence spectra of 2 were initially determined in a 10-5 M
DMSO solution. As can be observed in figure 2, compound 2 has two absorption bands at 337 and
408 nm which can attributed to   * electronic transition and to a n  * transition respectively.
When irradiated at the absorption maximum the solution emits in the blue region (475nm) showing
a quantum yield (FL) of 0.50 (Figure 1).
Figure 1. Experimental UV-Vis absorption and fluorescence spectra of 2 in DMSO.
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3.1. Crystal Growth and Structure Determination by single crystal X-ray Diffraction
With the aim of understanding the origin of the color variations in the solid state, we have
attempted to obtain single crystals of sufficient quality for structure determination. Interestingly,
by systematically varying the growing conditions we could obtain three different crystals of 2,
showing different colors (Figure 2).
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Figure 2. Above: crystal morphology for different compounds of 2 (left to right: 2, 2
polymorphs and 2DMF solvate). Down: view of the same set of crystals under UV light.
Slow evaporation at room temperature of a DMF solution of 2, give rise to the DMF solvate
(2DMF) which grows as elongated plate-like yellow crystals. Conversely, crystallization of 2
from a hot CH2Cl2 solution give rise to a mixture of the two polymorphs 2 and 2 as large yellow
prismatic crystals and small plate-liquid orange crystal respectively (Figure 2). The three
crystalline materials of 2 emit strongly when irradiated with UV light (λexc: 400nm) showing
maxima at 504, 522 and 565 nm for 2 polymorph, 2∙DMF solvate and the 2 polymorph
respectively (Figure 3).
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Figure 3. Experimental fluorescence spectra for the three derivates of 2 in solid state irradiated
at 400 nm.
After a careful crystal growth process, it was possible to obtain selectively the different crystals.
Thus, crystals of 2 were obtained exclusively in a mixture of CH2Cl2/EtOH/DMF 1:1:1.5, heated
up to 60ºC for 3 days and then cooled down to room temperature. A variation in the solvent mixture
(H2O/EtOH/DMF 1:1:1) allowed us to obtain exclusively orange crystals of polymorph 2.
Interestingly the solubility of the two polymorphs is also very different. Once crystallized, the
orange crystals of polymorph 2 are practically insoluble while yellow crystals of polymorph 2
are highly soluble in most organic solvents. Furthermore, we were able to recrystallize compound
1, its structure determination and the supramolecular analysis are placed in the SI section 3.
Single crystal structure determination shows that the solvate 2DMF crystallizes in the
monoclinic P21/n space group. Meanwhile, the two conformational polymorphs 2 and 2
crystallize in the triclinic P-1 and in the Monoclinic P21/n space group respectively (Table 1).
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Table 1. Main crystallographic and refinement data for the different isolated crystals of
compounds 1 and 2
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Compound
Formula
Molecular
Weight /gmol-1
Temperature/K
Wavelength/Ǻ
Crystal System
Space Group
a/Ǻ
1
2α
2β
2·DMF
C28H26O6
C52H44O12
C26H22O6
C32H36N2O8
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458.49
860.87
430.44
576.63
296(2)
1.54178
Triclinic
P-1
296(2)
1.54178
Triclinic
P-1
296(2)
296(2)
1.54178
Monoclinic
P21/n
1.54178
Monoclinic
P21/n
6.2418(6)
7.6927(7)
12.4052(11)
75.9000(5)
79.585(5)
76.023(6)
7.7009(11)
11.0569(19)
14.529(2)
13.1512(11)
9.6461(10)
17.3222(14)
90
11.2332(5)
12.0858(5)
11.3519(5)
90
b/Ǻ
c/Ǻ
/
83.033(12)
81.212(12)
89.103(11)
101.203(6)
90
106.384(3)
90
β/
γ/
V/Ǻ3
555.87(7)
1
1213.6(3)
1
2155.6(3)
4
1478.58(11)
2
Z
Dx/ g.cm-3
μ/mm-1
1.370
0.784
242
1.178
0.688
452
1.326
0.719
724
1.259
0.768
596
F(000)
GOF F2
Final
1.198
0.997
0.923
1.075
R1: 0.0776
R1: 0.1185
R1: 0.0700
R1: 0.0525
R indexes
[I˃2σ(I)]
R indices
(all data)
wR2: 0.2227
wR2: 0.2988
wR2: 0.1823
wR2: 0.1413
R1: 0.0880
R1: 0.2145
R1: 0.1529
R1: 0.0813
wR2: 0.2649
wR2: 0.3481
wR2: 0.2520
wR2: 0.1796
At a molecular level, the main structure differences between the three crystalline materials of 2
rely in their torsion angles () and the dihedral angles between the planes of the aromatic rings
(Table 2, Figure 4). Among the two polymorphs (2 and 2) significant changes between the plane
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of the central ring and those of the peripheral rings are appreciated (35.75º, 31.38º for polymorph
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2 and 11.97º, 17.02º for polymorph 2).
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Table 2. Experimental angle values for the different arrangements of 2
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Polymorph
Solvate
2DMF
Polymorph
Angle
2
2
107.61º
120.40º
35.75º
31.38º
150.54º
146.81º
146.35º
11.97º
17.02º
 (C11, C10, C5, C3)
(C31, C30, C25, C23)
Plane 1
-
12.40º
-
Plane 2
The planarity of the molecules will determine the effective -conjugation length which affects the
emission color, and could explain the red-shifting observed in case of polymorph 2, which is
planarized when compared to polymorph 2. However, in case of 2DMF solvate, the dihedral
angle between the ring planes (12.40º) is closer to the values shown by polymorph 2 but emits in
yellow, and therefore the color of this species cannot be explained solely on the basis of
intramolecular conformation. This points to significant contribution of supramolecular interactions
in the origin of the red-shifting observed in polymorph 2. Note that the origin of red shift
commonly observed in the spectra of aggregated π-conjugated molecules has been attributed to
the delocalization of electronic excitation over several molecular units because of the favorable π-
overlap between neighboring molecules. 34
3.2. Supramolecular Analysis
In order to understand the influence of the supramolecular arrangement in the colors observed for
the polymorphs, we have studied the interaction network of the crystals and analyzed the nature of
the interactions among the neighboring molecular units (see SI section 3 the summary and strength
classification of the intermolecular hydrogen bonds and weak interactions).
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Figure 4. View of the angle between the planes of the aromatic rings for 2, 2∙DMF and 2.
(Drawings and angle measurements performed with Mercury Crystallographic program35).
In case of the supramolecular network of 2DMF, the following intermolecular hydrogen bonds
and other weak interactions are involved (the values presented are d, D and  respectively): i)
synthon by i) O1-H1∙∙∙O20DMF (1.627Å, 2.608Å and 176.78º) and ii) C20-H20∙∙∙O2 (2.538Å,
3.219Å and 123.06º) growing the framework along b axis, iii) C13ii-H13Cii∙∙∙ (3.074Å, 3.961Å
and 152.77º), iv) C13iii-H13Biii∙∙∙O20DMF (2.790Å, 3.371Å and 121.45º) and v) C8=C9∙∙∙ring
(3.912Å, off-center parallel stacking (offset 29.35º)) growing the structure along a axis, than
together with vi) C4i-H4i∙∙∙O2 (2.518Å, 3.476Å and 156.77º), vii) C22-H22ADMF∙∙∙O1(2.812Å,
3.453Å and 123.58º) connects the framework along c axis giving rise to a 3D supramolecular
arrangement. This framework can be described as an 8-connected uninodal net (considering as
node the single molecule) with bcu (body centered cubic; 8/4/c1; sqc3) topology (Figure 5).
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Figure 5. Above: Atomic representation of 2∙DMF showing its asymmetric unit (drawings
performed with Mercury Crystallographic program).35 enter: view along c axis of the
supramolecular (left) and an augmentation view of the principal weak interactions between
molecules (right). Down: supramolecular packing showing the 8-connected node (central
molecule) and the resulting topological bcu representation of this connectivity (topology
simplification made using TOPOS program).36
For both polymorphs of 2, we found as a common characteristic the presence of a synthon among
the terminal carboxylic acid of both independent molecules. For 2: O21-H21∙∙∙O2 (1.676Å, 2.630
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and 158.67º), and O1-H1∙∙∙O22 (1.656Å, 2.629Å and 159.76º Å) grows the framework along a
axis. Meanwhile the synthon in 2: O21-H21∙∙∙O2 (1.563Å, 2.623Å and 166.83º), and O1-
H1∙∙∙O22 (1.647Å, 2.633Å and 166.64º) grows the framework parallel and perpendicular to the
(011) plane (Figure 6).
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In case of 2 supramolecular arrangement, the following additional intermolecular weak
interactions are observed (the values presented are d, D and  respectively): i) C26iii-H26iii∙∙∙O22
(2.666Å, 3.482Å and 146.84º), ii) C27iii-H27iii∙∙∙O1 (2.604Å, 3.405Å and 157.52º), iii) C6-
H6∙∙∙O2iii (2.663Å, 3.498Å and 138.12º), iv) C7-H7∙∙∙O21iii (2.679Å, 3.659Å and 132.05º), v) C24-
H24iii∙∙∙ (2.966Å, 3.554Å and 122.59º) and vi) C4-H4iv∙∙∙ (2.997Å, 3.676Å and 131.22º) which
grow the framework along c and a axis, building a layered supramolecular structure in the plane
(0 2 1). Finally, the presence of the weak interaction vii) C13ii-H13ii∙∙∙O23 (2.682Å, 3.421 and
134.08º) grows the crystal along b axis, giving rise to its 3D supramolecular framework, that can
be described as an 8-connected uninodal net (considering each single molecule as a node) with
hex (hexagonal primitive 8/3/h4, sqc4) topology (Figure 6).
The 2 supramolecular framework is formed besides with the synthon growing along a by
the following intermolecular weak interactions (the values presented are d, D and  respectively):
i) C33i-H33Bi∙∙∙O22 (2.634Å, 3.312 Å and 128.00º), ii) C31i-H31i∙∙∙O22 (2.507Å, 3.405 Å and
162.26º), iii) C13-H13C∙∙∙O1 (2.895Å, 3.408Å and 114.58º), iv) C33-H33A∙∙∙O3 (2.811Å, 3.450Å,
124.72º), v) C13-H13B∙∙∙C1 (2.791Å, 3.357 Å and 118.41º), vi) C3-H3∙∙∙C8 (2.883Å, D:3.791 Å
and 165.71º), vii) C29-H29∙∙∙ring (3.224Å, 3.614Å, 107.51º), viii)C8-H8∙∙∙ring (3.338Å, 3.385Å
and 84.91º) and ix) C28=C29∙∙∙ring (3.506Å, off-center parallel stacking (offset 10.83º)),which
grow the framework along b and c axis giving rise to a 3D supramolecular framework, that can be
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Figure 6. Above: an augmented view of the common synthon founded for both polymorphs, followed by the ORTEP 37 representation
of the asymmetric unit for each polymorph (Left: 2 and right: 2). Centre: supramolecular arrangement for each case, showing an
augmentation of the weak interactions involved in the process. Below: Supramolecular topology representation of the 8-connected net
for each case.
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described as an 8-connected uninodal net (considering each single molecule as a node) with bcu
topology (Figure 6). As we can determine through the topological analysis, all three derivates of 2
can be described as 8-connected nets, using the single molecule as node. After a careful analysis
of the supramolecular arrangements for all three crystalline materials of 2, the increment in the
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number of interactions involved (hydrogen bonds, ··· and CH ··· interactions) follows the
particular order: 2∙DMF < 2α < 2β, which can be related to the red-shifting observed in the light
emitting properties that together with the planarity behaviour (2α < 2∙DMF and 2β) could help
us to rationalize the colour variations of these PV derivates.
3.3. Computational Analysis
By using the experimental crystal structure coordinates of polymorphs 2 and 2 and of the DMF
solvate denoted 2∙DMF, the packing energies for the different crystal were calculated by
subtracting n times the energy of a single PV molecule to the energy of the packed PV with n
molecular units (Figure 7).
The relative packing energies for the packed PV, up to 10 monomers, extracted from the
2 polymorph structure are shown in Figure 8. In this figure we can see how the energy difference
resulting from adding a new molecular unity on the crystal remains essentially constant. It is worth
noting that for the trimer and pentamers (meaning three and five PV molecular units) of polymorph
2 we calculated two different packing energies (Figure 7).
As shown above, the crystal structure of polymorph 2 is mainly stabilized by CH∙∙∙
interactions and different PV packing units are linked among them by H-bond interactions. The
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Figure 7. Geometrical arrangement for the decamers and packing energies (kcal/mol) exhibiting
in 2 (above), solvate 2∙DMF (center) and 2 (below).
CH∙∙∙ interactions are 3,030 Å between CH3 group and benzene rings, while ranged among 2,967
and 3,129 Å between the H of the benzene ring and the centroid of the adjacent benzene ring of
the closest neighbor PV. Hydrogen bond interactions are among 3,077 and 3,991 Å; both distances
are in the range of the experimental reported data.38-39 Thus, the effect of adding a new PV
molecular unit in the crystal generates low cooperativity.
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The packing energy of the orange crystal resulting from polymorph 2β was calculated in an
analogous way (Figure 7). However, in this case the higher symmetry of the crystal structure
allows forming a unique PV packed structure. The packing energy, as in the previous case, remains
constant and low cooperativity was energetically observed. This crystal structure presents a more
regular arrangement, mainly stabilized by ∙∙∙ stacking interactions between the C-C bond and
the benzene rings with stacking distances of 3,613 Å, and 3,553 Å from the centroid of the terminal
benzene ring and the carboxyl group. These ∙∙∙ stacking interactions generate a more compact
structure, and the number of interactions by monomer explains the lower polymerization energy
compared with the yellow crystal structure (2α polymorph).
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Table 3. Theoretical UV-Vis highest excitation band for the oligomers formed in both polymorphs.
2α polymorph
Highest Excitation
2β polymorph
Highest Excitation
Number
Number
of Monomers
Band (nm)
469.8
of Monomers
Band (nm)
457.8
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2
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430.6
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441.0
5r
5t
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475.4
493.1
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475.3
488.0
506.3
10
492.6
The influence of the addition of a new molecular PV packing unit on the spectroscopic properties
of the two polymorphs has also been calculated, finding two effects as shown in Table 3 and Figure
8. The first effect is the red shifted displacement of the main absorption band when a new
molecular unit is added. This is consistent with the experimental evidences of the shift in the
maximum of absorption between the solution and crystals spectra (see Figures 1 and 3), assuming
that the electronic transitions of PV molecules in solution correspond to independent PV molecular
units. On the other hand, as can be appreciate in Table 4 and Figure 8, polymorph 2 has almost
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reached its maximum shift with the aggregation of 10 molecular units, while polymorph 2
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apparently can still be shifted more (Also see, S.I. Section 4).
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Figure 8. Vertical excitation spectra from the TDDFT calculations for each molecular PV unit in
2, solvate 2∙DMF and 2 where n indicates the number of monomers.
The second observed effect corresponds to the number of bands observed in the absorption spectra;
for the molecular PV unit it is possible to observe three bands in both cases, while for the packed
system the number of bands decreases reaching a single band. This means that the stacking and
packing interactions decrease the excitation energy between the ground state surface and the
excited potential surfaces (the origin of the luminescent properties).
The inclusion of solvent molecules in the calculations does not modify the observed trends (Figure
7 and 8, crystal 2∙DMF). As in the previous cases, a clear displacement in the wavenumber in the
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emission spectra is observed upon the inclusion of monomeric unities. This result indicates that
the red shifting observed in the spectrum of this crystallographic form compared to that of
polymorph 2α, is due not only to the planarity of the molecule but also depends on the
intermolecular interactions. Additional computational data is presented in the S.I, section 4.
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4. CONCLUSIONS
The synthesis of the phenylenevinylene derivative 1 was carried out with high stereoselectivity
using a two-fold Heck reaction in the key step (trans – trans configuration) and its basic hydrolysis
yielded PV 2, which is highly fluorescent in solid state and in solution. Careful crystallization of
the phenylenevinylene derivate 2 resulted in three crystalline species (two polymorphs 2-yellow
and 2-orange) and a DMF solvate 2∙DMF with different colors and fluorescence properties in
solid state when irradiated at 400 nm.
The differences observed in the color emission have been rationalized on the basis of a
careful analysis of the supramolecular interactions as well as of the topology of the crystalline
frameworks. Although, the topological analysis is not frequently used in organic materials, by
using it in these compounds we show here its utility to explain supramolecular related properties.
In particular the high planarity of the polymorph 2 found by crystal structure
determination, implies that the  – electronic conjugation and the  –  stacking interactions are
much more favorable, resulting in a decreasing in the energy needed for the electrons to delocalize
along the structure and causing a red shift in the colour of the polymorph. Both experiments and
computational calculations show that the different spectroscopic properties of these polymorphs
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have an important intermolecular contribution as they are strongly influenced by the differences
in their packing interactions.
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/
 Synthesis and experimental characterization of A, B, 1 and 2.
 Single-crystal XRD crystal data and structure refinement for 1.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: casierraa@unal.edu.co
Lina.Aguirre@csic.es
ORCID
Juan C. Cárdenas: 0000-0001-8204-4004
Lina M. Aguirre-Díaz: 0000-0002-7510-1653
Johan F. Galindo: 0000-0001-9968-362X
Jorge Alí-Torres: 0000-0003-1354-8713
Cristian Ochoa-Puentes:
Marcelo Echeverri: 0000-0002-9290-4546
Berta Gómez-Lor: 0000-0002-2995-9624
M. Ángeles Monge: 0000-0003-2242-7593
Enrique Gutiérrez-Puebla: 0000-0002-2878-3540
Cesar A. Sierra A: 0000-0002-8727-7429
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Notes
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The authors declare no competing financial interest
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■ ACKNOWLEDGMENTS
J. A-T thanks DIEB-UNAL for financial support. JFG thanks to COLCIENCIAS for the “Es
Tiempo de Volver” scholarship. JCC thanks to the National Doctoral Program of COLCIENCIAS
(Grant No. 511-2010) for doctoral fellowship. L. M. A-D, M. E., B. G-L, M. A. M. and E. G-P,
thanks to the Spanish Government MINECO project MAT2016-78465-R and Fondo Social
Europeo from the European Union and CTQ2016-78557-R grants) and the Comunidad de Madrid
S2013/MIT-2740 (PHAMA_2.0).
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On the Nature of Color Diversity in Phenylenevinylene-based Polymorphs
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Juan C. Cárdenas, † Lina M. Aguirre-Díaz, ‡* Johan F. Galindo, † Jorge Alí-Torres, † Cristian
Ochoa-Puentes, † Marcelo Echeverri, ‡ Berta Gómez-Lor, ‡ M. Ángeles Monge, ‡ Enrique
Gutiérrez-Puebla‡ and Cesar A. Sierra. †*
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Synopsis: Experimental and theoretical study of the light emitting properties of a
phenylenevinylene derivate in solid state.
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