Polymorphism in crystals of bis(4-bromophenyl)fumaronitrile through vapour phase growth

Article abstract of DOI:10.1039/c7ce01543g

Polymorphic selectivity within crystals grown via physical vapour transport (PVT) is dependent on the thermodynamic stabilities of differing molecular conformations and the kinetic regime within the growth apparatus. Crystals of bis(4-bromophenyl)fumaroni

Full text of DOI:10.1039/c7ce01543g

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Polymorphism in crystals of bisIJ4-bromo-
phenyl)fumaronitrile through vapour phase
Cite this: DOI: 10.1039/c7ce01543g
growth†
Torsten T. Jensen, ab Jason Potticary, ac Lui R. Terry,a
ac
Hannah E. Bruce Macdonald, a Jan Gerit Brandenburg de and Simon R. Hall
*
Polymorphic selectivity within crystals grown via physical vapour transport (PVT) is dependent on the ther-
modynamic stabilities of differing molecular conformations and the kinetic regime within the growth appa-
ratus. Crystals of bisIJ4-bromophenyl)fumaronitrile have been grown for the first time via this method, with
the formation of both the conventional polymorph and a new, unforseen polymorph. Analysis suggests that
the conventional form is less thermodynamically stable, with this form crystallising at higher temperature
than the newly discovered form due to the release of binding energy of intermolecular interactions during
the growth process. Fluorometry reveals the new form to exhibit weaker, red-shifted fluorescence emis-
sion owing to greater intermolecular π–π overlap.
Received 24th August 2017,
Accepted 10th October 2017
DOI: 10.1039/c7ce01543g
rsc.li/crystengcomm
Bis(4-Bromophenyl)fumaronitrile (subsequently Br-FN, Fig. 1)
is a convenient precursor for bisIJ4-(N-(1-naphthyl)phenylamino)-
Introduction
Physical vapour transport (PVT), in which a powdered sample
is sublimed and recrystallised from the vapour phase, is a
commonly used method of growing single crystals of organic
semiconducting molecules, due to the size and purity of crys-
tals produced when compared with other methods of crystal
growth.1 Multiple polymorphs of the same material can be
produced, dependent on the growth conditions, resulting in
crystals with starkly different physical properties including
electronic transport and optical emission.2 Thus, an under-
standing of polymorphism in PVT growth is critical to pro-
duce crystals with optimal properties for optoelectronic appli-
cations such as transistors, LED's and lasing.
phenyl)-fumaronitrile(NPAFN) and N-methyl-3,4-bisIJ4-(N-(1-
naphthyl)phenylamino)phenyl)maleimide (NPAMLMe), effi-
cient emitters for non-doped red organic light-emitting di-
odes.3 It has also garnered attention for forming single crys-
tals which fluoresce intensely under exposure to ultra-violet
light, with comparatively weak emission in solution. This is
due to aggregation induced emission (AIE), in which the intra-
molecular rotations that quench excited states in solution are
effectively prevented in the solid state. In Br-FN, the cyano
groups force the aromatic rings out of plane in the crystalline
state, preventing intermolecular π–π stacking and further re-
ducing quenching and increasing luminescence efficiency.4
Only one polymorph of Br-FN has been documented in the
a Complex Functional Materials Group, School of Chemistry, University of Bristol,
Bristol, BS8 1TS, UK. E-mail: simon.hall@bristol.ac.uk
b Centre for Doctoral Training in Condensed Matter Physics, HH Wills Physics
Laboratory, Tyndall Avenue, Bristol, BS8 1TL, UK
c Bristol Centre for Functional Nanomaterials, HH Wills Physics Laboratory,
Tyndall Avenue, Bristol, BS8 1TL, UK
d Department of Chemistry, University College London, 20 Gordon Street, London
WC1H 0AH, UK
e Thomas Young Centre, University College London, Gower Street, London WC1E
6BT, UK
† Electronic supplementary information (ESI) available: Crystal structure (cif)
data for form 1 and form 2 at 100 K, SEM images of grown crystals, pXRD pat-
terns of form 1 crystals extracted from the PVT growth tube, DSC data for form
1 and form 2, Low temperature pXRD data for form 1 and form 2, details and re-
sults of vacuum sublimation growth, crystal data and structure refinement for
form 1 and 2, and additional details on DFT calculations. CCDC 1568482 and
1563251. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c7ce01543g
Fig.
1 Chemical structure of bisIJ4-bromophenyl)fumaronitrile,
including atom numbers.
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Cambridge Structural Database. The previous report on the
synthesis and characterisation of Br-FN in the solid state
was obtained by crystallisation from solution and reports
only a single polymorph being crystallised.3
tube after 5 hours at distances between 120 and 300 mm
from the powder.
Fluorometry
In this work, we synthesise Br-FN via solution and
recrystallise from both solution and physical vapour trans-
port (PVT). When recrystallising from solution only the previ-
ously known polymorph (form 1) could be obtained, however
both the conventional and a new, undocumented polymorph
(form 2) could be grown via PVT at different regions over a
temperature gradient within the growth apparatus. Density
functional theory (DFT) lattice calculations performed on
both polymorphs reveal the two forms to be close in lattice
energy, with a slight tendency of form 2 to be more stable.
Form 1 forms at higher temperature to form 2, which is at-
tributed to kinetic factors during growth. Differential scan-
ning calorimetry (DSC) measurements and low temperature
powder X-ray diffraction (pXRD) reveal the two forms to be
stable within the temperature range 12 to 493 K. Fluorometry
shows form 2 to exhibit red-shifted fluorescence when com-
Measurements were obtained from an Agilent Cary Eclipse
fluorescence spectrometer after mounting crystalline samples
on the end of a quartz plate using paraffin oil. The quartz
plate, cut at an 8° angle, allowed for emitted light to be
detected but not directly reflect the incident beam.
Crystallography
Powder X-ray diffraction. pXRD data were gathered using
a Bruker D8 Advance diffractometer (Cu-Kα radiation – wave-
length of 1.5418 Å) with a PSD LynxEye Detector. Step size
was 0.0114 2θ and hold time was 1 s. Samples were mounted
on a low-background sample holder with silicon wafer. Low
temperature pXRD: a scan from 7 to 40 2θ with a step size of
0.01968 over 41.25 minutes was taken at 300 K. The sample
was cooled to 12 K with an Oxford Cryosystems PheniX cryo-
stat as rapidly as possible and was held at 12 K for 15 mi-
nutes, after which a scan with the same parameters was
taken. The sample was then heated back to 300 K and an-
other scan was taken.
Single crystal X-ray diffraction. Single crystal X-ray diffrac-
tion data (SC-XRD) for form 1 and 2 were collected on a
Bruker Apex II CCD diffractometer using Mo Kα radiation (λ
= 0.71073 Å) at a temperature of 100(2) K. Intensities were
integrated in SAINT5 and absorption corrections based on
equivalent reflections were carried out using SADABS.6 The
structure was solved using Superflip7,8 and refined against F2
in ShelXL9 using Olex2.10 All of the non-hydrogen atoms were
refined anisotropically, while all of the hydrogen atoms were
located geometrically and refined using a riding model. Crys-
tallographic and refinement details are available in the ESI†
in Table S1. Crystallographic data for form 1 and 2 have been
deposited with the Cambridge Crystallographic Data Centre
as supplementary publication CCDC 1568482 and 1563251
respectively.
pared to form
stacking.
1 due to increased intermolecular π–π
Experimental
Synthesis
Synthesised as previously reported3 (4-bromophenyl)-
acetonitrile (0.1 mol) and iodide (0.1 mol) were dissolved in
diethyl ether (400 mL) under an inert nitrogen atmosphere.
The reaction was cooled to −78 °C and sodium methoxide
(0.2 mol) was dissolved in dry and cold methanol, before be-
ing added drop-wise to the solution over 30 minutes. The so-
lution was left in a dry ice bath for 2 hours and then an ice
bath for the following 3 hours. After this, the solution was
stirred for 3 hours at 10 °C before quenching with 3–6%
hydrochloric acid. The solution was filtered and the filtrate
washed with a cold methanol–water solution. The product
was purified using silica gel column chromatography and
were separated using methanol, and recrystallized.
Differential scanning calorimetry
Crystal growth
DSC data were collected with a TA-Instruments Q100 DSC.
5.3 mg of each sample was hermetically sealed in an alumin-
ium pan and subject to a temperature ramp from 190 °C to
240 °C at a rate of 10.00 °C min−1, then ramped back down
to 190 °C at the same rate.
From solution.
A
saturated solution of Br-FN in
dichloromethane (3 ml) was left to evaporate in an atmo-
sphere of methanol. Crystals spontaneously nucleated from
the solution and were collected after eight days when the sol-
vent had completely evaporated.
Via PVT. A glass tube (inner diameter 39.5 mm) contain-
ing an inner growth tube (inner diameter 17 mm) with bisIJ4-
bromophenyl)fumaronitrile powder placed at one end was
inserted into a quartz tube. This was then inserted into a
Carbolite CTF horizontal tube furnace such that the powder
was positioned at the centre of the furnace. Nitrogen gas was
passed through the glass tube at a flow rate of 0.1 L min−1.
The temperature of the central point of the furnace was set
to 240 °C. Crystals formed on the inner walls of the growth
Density functional theory
Lattice energy calculations were performed in a developer ver-
sion of the CRYSTAL14 program package,11 with a screened
exchange hybrid functional (HSE-3c).12 The Brillouin zone
was sampled with γ-centred grid with 3 × 3 × 3 and 6 × 3 × 2
k-points for form 1 and form 2, respectively. Standard thresh-
olds were applied for self-consistent field convergence, geom-
etry optimization and integral screening. The experimental
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unit cell was used and atomic positions were relaxed within
space group constraints. Additional information on DFT cal-
culations available in the ESI.†
Results and discussion
Crystals of Br-FN grown via physical vapour transport have
needle-like morphology, with sizes ranging from approxi-
mately 1 mm to 2 cm in length (Fig. 2). Three distinct
“growth zones” appear in the growth tube: a 5 mm zone with
a temperature of approximately 210 °C (zone 1), with white
needles closest to the Br-FN growth powder which itself was
placed at the hottest area of the growth tube; green needles
with smaller white crystals over a range of 50 mm with tem-
peratures between 200 °C and 180 °C (zone 2); and a white
powder covering the outside of the inner and inside of the
outer tube over a 100 mm range (zone 3). Under exposure to
365 nm UV light, the distinction between zones becomes
more apparent: intense blue fluorescence is observed from
the white crystals and powder, and weaker green emission
from the green needles. This is in contrast to solution-grown
crystals of Br-FN (Fig. 3), which have a three-dimensional
block-like morphology and only fluoresce blue under excita-
tion. SEM images (Fig. S1, ESI†) of green PVT-grown needles
display smooth, parallel faces, in contrast to the solution-
grown crystals which have rough, uneven faces.
Fig. 3 Comparison between a solution-grown crystal of form 1 (left)
and a PVT-grown crystal of form 2 (right) under a) ambient light, and
b) 365 nm UV light. Squares are 0.5 cm2.
needle-like crystals revealed a novel polymorph which also
¯
adopted a triclinic unit cell with space group P1, but had con-
trasting lattice parameters (Table 1), subsequently denoted as
form 2. This polymorph also contains two molecules in the
unit cell (Z = 2) but this time the asymmetric unit consists of
two half molecules (Z′ = 2 × 0.5).
Comparisons between the two forms reveal a starkly differ-
ent molecular arrangement of Br-FN (Fig. 4). In form 1, the
phenyl rings of each molecule are twisted with respect to
each other and near-perpendicular with an angle of 97.83°
between the mean planes calculated through the carbon
atoms of the phenyl rings and a (phenyl)C–CC–C(N) torsion
angle of ∼8°, whereas in the new polymorph (form 2) the
rings are parallel with (phenyl)C–CC–C(N) torsion angles of
∼2/−2°. Both structures display intermolecular π–π stacking
interactions which are more extensive in form 2, see Table 1.
In form 1 each of the two phenyl rings form one π–π interac-
tion to symmetry related phenyl rings on different molecules,
creating chains through the structure in approximately the
[101] direction. However, in form 2 the parallel orientation of
the phenyl rings with respect to each other enables both phe-
nyl rings on each unique molecule to interact with pairs of
phenyl rings on parallel molecules above and below with a
separation of 3.87 Å, creating π–π stacks in approximately the
[100] direction, which is aligned with the long axis of the
needles. The layered structure is similar to those seen in
other derivatives of diphenyl fumaronitriles, such as bisIJ4-
methoxyphenyl)fumaronitrile,3 whereas the conventionally
obtained structure, form 1, is distinctly nonplanar. DSC (Fig.
S3 and S4, ESI†) reveal that no phase change occurs between
the two polymorphs over the temperature range of 300 K to
their measured melting points of 493 K. The melting points
of the two polymorphs were measured to be equal within er-
ror margins, therefore any difference in lattice energy is be-
yond the detection limits of the DSC apparatus. Low tempera-
ture pXRD (Fig. S5 and S6, ESI†) performed on powder of
form 1 and form 2 also indicate that there is no phase
change between 300 K and 12 K. Thus, the polymorphs are
monotropic in the temperature range 12–493 K. This would
therefore suggest the presence of a large potential barrier
separating the energy minima of each polymorph. Static (0 K)
lattice energy values for the previously known and new forms
calculated via DFT (HSE-3c) were −148.3 kJ mol−1 and −151.3
kJ mol−1, respectively. This was confirmed by PBE-D3 (ref. 13,
14) single-point energies on the HSE-3c structures, evaluated
SC-XRD on solvent-grown crystals and pXRD performed
on the white crystals and powder from the PVT growth tube
matched the pattern of the conventional polymorph (Fig. S2,
ESI†), subsequently denoted as form 1. This polymorph has a
¯
triclinic system with space group P1, and contains two mole-
cules in the unit cell (Z′ = 1, Z = 2). SC-XRD on the green
Fig. 2 Schematic of PVT growth apparatus, including the temperature
profile of the furnace. a) Crystals grown in the growth tube under lab
light, and b) under 365 nm UV light.
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Table 1 π–π stacking interaction distances
Form
1
Ring 1
Ring 2
Centroid–centroid distance (Å)
Offset distance (Å)
C1–C6
C9–C14
C11–C61
3.70
4.07
1.50
1.25
C92–C142
2
C1–C6
C1–C6
C9–C14
C9–C14
C13–C63
C14–C64
C93–C143
C94–C144
3.87
3.87
3.87
3.87
1.39
1.39
1.64
1.64
1 = −x, 1 − y, −z, 2 = 1 − x, −y, 1 − z, 3 = −1 + x, +y, +z, 4 = 1 + x, +y, +z.
in a projector augmented plane wave basis set with energy
cut-off of 800 eV with the VASP 5.4 code15 yielding −150.2 kJ
mol−1 and −153.8 kJ mol−1. The accuracy of the HSE-3c func-
tional is to within approximately 5 kJ mol−1 for absolute lat-
tice energies of organic compounds, so both polymorphic
forms are equally stable within error values, with a slight ten-
dency of form 2 to be more stable. Form 2 has a 20 kJ mol−1
higher stabilization from London dispersion forces making it
competitive to form 1 and resulting in closer packing, evident
in the smaller unit cell volume (Table 2). The similarity in lat-
tice energies would explain how the two polymorphs grow
concomitantly in the growth tube.
chrysene16 and α-quaterthiophene.17 In these cases, the poly-
morph that grew at the higher temperature was designated
as the “high temperature polymorph” and the polymorph
that grew at lower temperature as the “low temperature poly-
morph”. The temperature of the substrate on which a crystal
grows will influence the disturbance of the crystal lattice. A
higher temperature substrate will cause the release of bind-
ing energy of intermolecular interactions during the growth
process.18 This can therefore lead to the formation of multi-
ple polymorphs dependent on the temperature of the point
on the growth tube on which the crystal grows, with less ther-
modynamically stable polymorphs forming at higher temper-
ature. This matches with our observations that form 1, likely
being the less stable of the two polymorphs, forms at higher
temperature to form 2. Further evidence arises from vacuum
sublimation growth (Fig. S7 and S8, ESI†). pXRD on the
resulting powder confirmed the formation of form 2, with
crystallisation taking place on a substrate at lower tempera-
ture to the PVT tube.
The growth of multiple polymorphs of organic crystals in
physical vapour transport (PVT) has been reported previously
for crystals of 7,14-bisIJ(trimethylsilyl)ethynyl)dibenzoijb,def]-
Optical absorption measurements performed on crystals
of form 1 and form 2 (Fig. 5) display peaks at 328 and 425
nm respectively. Form 1 has a fluorescence emission peak at
441 nm, and form 2 at 476 nm. The crossover point between
the absorption and emission spectra, indicating the energy of
the 0–0 transition, is at 430 nm for form 1 and 445 nm for
form 2. Greater overlap between π bonds causes the forma-
tion of excimers, excitons delocalised over 2 molecules that
shift emission to lower energies. Excimers also quench fluo-
rescence, reducing emission efficiency.19 This would explain
why form 2 emits at lower energy and with less intensity com-
pared to form 1. The differences in fluorescence intensity
Table 2 Unit cell parameters of form 1 and form 2
Form 1
Form 2
Crystal system
Space group
Triclinic
P1
Triclinic
P1
¯
¯
a/Å
b/Å
c/Å
α/Å
β/°
γ/°
7.9926(9)
9.4626(12)
10.6914IJ15)
92.131(11)
110.063IJ10)
74.017(10)
728.699
2
3.87260IJ10)
10.7671(4)
16.5188(6)
90.217(3)
93.186(3)
98.738(2)
679.68(4)
2
Fig. 4 a) and b) Crystal structure of the previously known polymorph
(form 1) of Br-FN viewed slightly offset from the a and b axes, and
along the c axis respectively, with lines representing unit cell bound-
aries. Depth cueing used to show relative positions of molecules. c)
and d) Unit cell of the new polymorph (form 2) viewed slightly offset
from the a and b axes, and along the a-axis respectively.
Volume/Å3
Z
Temperature/K
100
100
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Hall for insightful discussions and assistance with collecting
experimental data, and Dr Hazel Sparkes and Dr Natalie
Pridmore for assistance with SC-XRD data refinement.
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Crystals of bisIJ4-bromophenyl)fumaronitrile have been grown
via solution and physical vapour transport, with two distinct
polymorphs being grown from the vapour phase, one of
which was previously unknown. Lattice energy calculations
suggest that this form has greater thermodynamic stability
compared to the previously known form, with the previously
known form crystallising at higher temperature within the
PVT growth tube due to the release of binding energy of
intermolecular interactions during the growth process. The
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
S. R. H., J. P. and T. T. J. acknowledge the Engineering and
Physical Sciences Research Council UK (grants EP/G036780/1
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