The influence of pyrrole linked to the π-conjugated polyene on crystal characteristics and polymorphism

Article abstract of DOI:10.1016/j.dyepig.2009.12.011

Novel pyrrole-based polyene derivatives bearing various different substituents were synthesized to investigate the influence of molecular conformational change (rotational isomerization) of a π-conjugated bridge on crystal characteristics and polymorphism. The pyrrole-based polyene crystals exhibited strong second harmonic generation efficiency that was ～two orders of magnitude larger than that of urea. Chromophores bearing asymmetric pyrrole readily formed polymorphs whereas crystals having a symmetric dimethylaminophenyl group displayed only one crystal structure. The observed polymorphism in pyrrole-based chromophores was attributed to the existence of rotational isomerization in which the minor rotamer exhibits a self-additive effect during the formation of the crystals.

Full text of DOI:10.1016/j.dyepig.2009.12.011

Dyes and Pigments 86 (2010) 149e154
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The influence of pyrrole linked to the
p-conjugated polyene on crystal
characteristics and polymorphism
,
b
c
b
Pil-Joo Kim a, O-Pil Kwon a , Mojca Jazbinsek , Hoseop Yun , Peter Günter
*
a Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea
b Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland
c Division of Energy Systems Research and Department of Chemistry, Ajou University, Suwon 443-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel pyrrole-based polyene derivatives bearing various different substituents were synthesized to
investigate the influence of molecular conformational change (rotational isomerization) of a -conju-
Received 17 October 2009
Received in revised form
17 December 2009
Accepted 21 December 2009
Available online 6 January 2010
p
gated bridge on crystal characteristics and polymorphism. The pyrrole-based polyene crystals exhibited
strong second harmonic generation efficiency that was wtwo orders of magnitude larger than that of
urea. Chromophores bearing asymmetric pyrrole readily formed polymorphs whereas crystals having
a symmetric dimethylaminophenyl group displayed only one crystal structure. The observed poly-
morphism in pyrrole-based chromophores was attributed to the existence of rotational isomerization in
which the minor rotamer exhibits a “self-additive effect” during the formation of the crystals.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Crystal engineering
Nonlinear optics
Isomerization
Polymorphism
Self-additive effect
Polyene
1. Introduction
configurationally locked polyene (CLP) chromophores having
a plane-rigidified cyclohexene ring were developed [13e15]. Such
Organic crystal engineering is devoted to the design of a solid
to achieve desired molecular orientation and physical proper-
ties, including crystal characteristics such as polymorphism,
morphology and habit [1e5], for which, knowledge of the
influence of molecular conformational change on crystal char-
acteristics is important. Although the influences of molecular
CLP molecules can assume an acentric crystal structure, which is
required for second-order nonlinear optics and exhibit large
macroscopic second-order nonlinearity [16e18]. For example, the
CLP chromophore having the dimethylaminophenyl electron donor,
MM1 ((2-(3-(2-(4-dimethylaminophenyl)vinyl)-5,5-dimethylcy-
clohex-2-enylidene)malononitrile), see Fig. 1a) forms an acentric
crystal structure with P21 space group and exhibits strong macro-
scopic nonlinearity [16].
conformational changes in non-p-conjugated bridges on crystal
characteristics have been extensively investigated [6,7], only
a few studies discuss the influence conformational changes of
To obtain acentric crystal structures for second-order nonlinear
optical applications, understanding the conformational changes
(i.e., isomerism) of the core molecular structure, namely the
p
-conjugated bridges on crystal character, addressing mainly
cisetrans isomerization [8,9], but rarely rotational isomerization.
Organic pushepull, -conjugated, polyene molecules that
p
p-conjugated bridge of a typical nonlinear optical molecule, is
posses both electron donor and electron acceptor groups have been
extensively studied for photo- and electro-luminescent applica-
tions [10] and second-order nonlinear optical applications [11,12].
However, the low thermal and photochemical stability resulting
from easy cisetrans isomerization of the polyene bridge presents
serious limiting factors for many applications. To overcome this,
a starting point for crystal engineering. A series of asymmetric,
N-substituted pyrrole, nonlinear optical CLP chromophores having
a pyrrole electron donor instead of a dimethylaminophenyl donor
group were described [19], in which the existence of rotational
isomerization [20] in the
cally in the single bond between the pyrrole and the hexatriene
bridge, which links different types of -conjugation (hetero-
p-conjugated bridge was observed, typi-
p
aromatic ring and polyene) [19]. In quantum chemical calculations,
the pyrrole-based CLP molecules show a strong asymmetry and
a large angle q(m,b) between the main charge-transfer direction
* Corresponding author. Tel.: þ82 31 219 2462; fax: þ82 31 219 1610.
E-mail address: opilkwon@ajou.ac.kr (O.-P. Kwon).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.12.011

150
P.-J. Kim et al. / Dyes and Pigments 86 (2010) 149e154
Fig. 1. Chemical structure of the investigated pyrrole-based chromophore and their abbreviations.
b
and the dipole moment
m
. This is expected to reduce the proba-
2-(3-(2-(1H-pyrrol-2-yl)vinyl)-5-methylcyclohex-2-enylidene)
malononitrile (HP2) 1H NMR (CDCl3,
): 1.16 (m, 3H, eCH3), 2.0e2.15
bility for antiparallel arrangement of first hyperpolarizabilities in
bulk materials [19] and therefore increase the probability for
acentric crystal structures. To investigate acentric crystal-forming
possibilities, new pyrrole-based CLP derivatives with different N-
substituted groups (X: eH, eCH3) on pyrrole and with different
number of methyl substituents in cyclohexene ring have been
designed and synthesized (see Fig. 1b). This paper concerns the
d
(m, 2H, eCH2), 2.28 (m, 1H, eCHeCH3), 2.77 (d, 1H, eCH2), 2.94 (d,
1H, eCH2), 6.31 (m, 1H, eC]CHe), 6.51 (d, 1H, J ¼ 16.0 Hz, eCH]
CHe), 6.55 (m, 1H, PyeH), 6.68 (s, 1H, PyeH), 6.96 (d, 1H,
J ¼ 15.6 Hz, eCH]CHe), 6.99 (s, 1H, PyeH), 8.68 (s, 1H, NH).
Elemental analysis for C16H15N3: (%) Calcd. C 77.08, H 6.06, N 16.85;
Found C 77.02, H 6.02, N 16.79. Yield: 12%.
influence of pyrrole linked to the
characteristics and polymorphism.
p-conjugated polyene on crystal
2-(3-(2-(1H-pyrrol-2-yl)vinyl)cyclohex-2-enylidene)malononitrile
(HP3) 1H NMR (CDCl3,
d): 2.0e2.15 (m, 2H, eCH2), 2.28 (m, 1H,
eCHeCH3), 2.77 (d,1H, eCH2), 2.94 (d,1H, eCH2), 6.31 (m,1H, eC]
CHe), 6.51 (d, 1H, J ¼ 15.6 Hz, eCH]CHe), 6.55 (m, 1H, PyeH), 6.68
(s, 1H, PyeH), 6.96 (d, 1H, J ¼ 16.0 Hz, eCH]CHe), 6.99 (s, 1H,
PyeH), 8.68 (s, 1H, NH). Elemental analysis for C15H13N3: (%) Calcd.
C 76.57, H 5.57, N 17.86; Found C 76.61, H 5.56, N 17.83. Yield: 21%.
2. Experimental
2.1. Synthesis of chromophores
The pyrrole-based chromophores were synthesized using
a condensation reaction with pyrrole-2-carbaldehyde or N-
methyl-pyrrole-2-carbaldehyde and a corresponding interme-
diate, 2-(3,5,5-trimethylcyclohex-2-enylidene)malononitrile,
2-(3,5-dimethylcyclohex-2-enylidene)malononitrile, 2-(3-methyl-
cyclohex-2-enylidene)malononitrile [16,19]. The synthesis of HP1
and MP1 were reported [19]; newly synthesized HP2, HP3, MP2,
and MP3 were synthesized according to the literature [16,19].
For example, MP2: N-Methyl-pyrrole-2-carbaldehyde (3.11 mL,
29 mmol) was mixed with an equimolar amount of 2-(3,5-
dimethylcyclohex-2-enylidene)malononitrile (5.0 g, 29 mmol) in
ethanol (80 mL). With the catalyst piperidine, the solution was
stirred for 16 h at room temperature. A crystalline solid obtained
by filtration was purified by recrystallization in methylene chlo-
ride/methanol. In 1H NMR spectra the chemical shifts (Varian
2.2. Absorption and photoluminescence measurement
UV/vis. absorption and photoluminescent spectra in solution
were recorded by a Jasco V-570 and FP-6500 spectrometer,
respectively. Photoluminescence spectra in solid state were
measured using a custom built luminescence spectrometer based
on an OceanOptics USB4000 CCD spectrometer (range:
350e1000 nm) for detection and a green cw laser with 532 nm for
excitation.
2.3. X-ray crystallographic data
2.3.1. HP2-III
C17H19N3O, Mr ¼ 281.35, triclinic, space group P1, a ¼ 8.4670
400 MHz NMR spectrometer) are reported in ppm (
(CH3)4Si.
d) relative to
(5) Å, b ¼ 8.7508(6) Å, c ¼ 12.3055(7) Å,
a
¼ 106.224(2)ꢀ,
b
¼ 91.827(2)ꢀ,
g
¼ 112.269(2)ꢀ, V ¼ 800.36(8) Å3, Z ¼ 2, T ¼ 290
2-(5-Methyl-3-(2-(1-methyl-1H-pyrrol-2-yl)vinyl)cyclohex-2-
(2) K,
m
(MoK
a
) ¼ 0.075 mmꢁ1. Of 7871 reflections collected in the
enylidene)malononitrile (MP2) 1H NMR (CDCl3,
d
): 1.20 (m, 3H,
q u scans on a Rigaku R-axis Rapid S
range 3.14ꢀe27.45ꢀ using an
eCH3), 2.04e2.10 (m, 1H, eCHeCH3), 2.11e2.18 (m, 1H, eCH2),
2.27e2.34 (m, 1H, eCH2), 2.77e2.82 (m, 1H, eCH2), 2.96e3.00 (m,
1H, eCH2), 3.74 (s, 3H, eNCH3), 6.23 (m, 1H, eC]CH), 6.69e6.78
(m, 3H, PyeH), 6.72e6.74 (d, 1H, J ¼ 16.0 Hz, eCH]CH), 6.94e6.98
(d,1H, J ¼ 16.0 Hz, eCH]CHe). Elemental analysis for C17H17N3: (%)
Calcd. C 77.54, H 6.51, N 15.96; Found C 77.56, H 6.46, N 15.98. Yield:
44%.
diffractometer, 3635 were unique reflections (Rint
¼
0.018,
completeness ¼ 99.1%). The structure was solved and refined
against F2 using SHELX97 [21], 192 variables, wR2 ¼ 0.1432,
R1 ¼ 0.0468 (Fo2 > 2
s
(Fo2)), GOF ¼ 1.123, and max/min residual
electron density 0.716/ꢁ0.265 eÅꢁ3, CCDC-733790.
2.3.2. MP3-II
2-(3-(2-(1-Methyl-1H-pyrrol-2-yl)vinyl)cyclohex-2-enylidene)
C16H15N3, Mr ¼ 249.32, triclinic, space group P1, a ¼ 6.9858
malononitrile (MP3) 1H NMR (CDCl3,
d): 1.97 (m, 2H, eCH2), 2.62 (t,
(5) Å, b ¼ 7.4647(5) Å, c ¼ 13.4359(11) Å,
a
¼ 87.512(3)ꢀ,
2H, J ¼ 12, eCH2), 2.79 (t, 2H, J ¼ 12.8, CH2), 3.73 (s, 3H, eNCH3),
6.23 (m, 1H, eC]CH), 6.69e6.78 (m, 3H, PyeH), 6.71e6.75 (d, 1H,
J ¼ 16.4 Hz, eCH]CH), 6.94e6.98 (d, 1H, J ¼ 15.6 Hz, eCH]CHe).
Elemental analysis for C16H15N3: (%) Calcd. C 77.08, H 6.06, N 16.85;
Found C 77.04, H 6.09, N 16.75. Yield: 15%.
b
¼ 80.833(3)ꢀ,
g
¼ 81.2109(17)ꢀ, V ¼ 683.44(9) Å3, Z ¼ 2,
T ¼ 293(2) K,
m(MoK
a
) ¼ 0.072 mmꢁ1. Of 6390 reflections
collected in the
q u scans on a Rigaku
range 2.0ꢀe27.5ꢀ using an
R-axis Rapid
S diffractometer, 3026 were unique reflections
(Rint ¼ 0.0171, completeness ¼ 96.9%). The structure was solved

P.-J. Kim et al. / Dyes and Pigments 86 (2010) 149e154
151
and refined against F2 using SHELX97 [21], 173 variables,
wR2 ¼ 0.1721, R1 ¼ 0.0477 (1778 reflections having Fo2 > 2
s
(Fo2)),
GOF ¼ 1.080, and max/min residual electron density 0.329/
ꢁ0.238 eÅꢁ3. CCDC-733791.
3. Results and discussion
3.1. Rational design of pyrrole-based chromophores
Fig. 1 shows the chemical structures of the investigated
pyrrole-based chromophores. The chromophores consist of the
p-conjugated configurationally locked polyene bridge linked to
pyrrole electron donor and dicyanomethylidene electron acceptor.
Since dicyanomethylidene electron acceptor group can also act as
hydrogen bond acceptor, we choose un-substituted pyrrole and
N-methyl-pyrrole on the electron donor site to introduce different
intermolecular hydrogen bonds in solid state. We also varied the
number of methyl substituents on the cyclohexene ring because
crystal structures can strongly depend on the number of the
methyl substituents in CLP crystals [17].
The wavelength of maximum absorption lmax of the synthesized
chromophores with un-substituted pyrrole and N-methyl-pyrrole
groups are similar; about 470 nm in the methanol solution. As
expected, this suggests that the substituents introduced do not
considerably affect the electronic molecular properties and
molecular nonlinearity, and may only be important for crystal
formation characteristics. All pyrrole-based chromophores exhibit
high thermal stability; the initial weight-loss temperature Ti of
about 270 ꢀC at the heating rate of 10 ꢀC/min was determined by
thermogravimetric analysis (TGA) measurements.
To screen the macroscopic nonlinearity, the Kurtz and Perry
powder second harmonic generation (SHG) test was performed at
the fundamental wavelength of 1.9 mm with crystalline powders
obtained from methylene chloride (MC)/methanol solution by slow
evaporation method. Among the six pyrrole-based compounds,
HP2 and MP3 crystals exhibited a strong SHG efficiency: 2.2 times
and 1.3 times larger than that of MM1 powder, which is about two
orders of magnitude larger than urea [16]. Therefore, HP2 and MP3
chromophores may pack in acentric structures promising for
second-order nonlinear optical applications.
Fig. 2. Powder X-ray diffraction patterns for polymorph crystals of HP2 (a) and MP3 (b).
diffraction, powder SHG test and photoluminescence. The crystal
growing method and the physical properties of several HP2 and
MP3 polymorphs obtained are listed in Table 1. We obtained four
polymorphs of HP2 and two of MP3, whose presence was
confirmed by powder X-ray diffraction (see Fig. 2). HP2-III and HP2-
IV crystals include a solvent, methanol and methyl ethyl ketone
(MEK), respectively, as determined by 1H NMR measurement. In the
powder SHG test, HP2-I and MP3-I phases are SHG active, while the
other HP2 and MP3 polymorphs are SHG inactive. Therefore, HP2-I
3.2. Crystal characteristics and polymorphism
Crystal-forming characteristics were investigated in more detail
for HP2 and MP3 due to the observed large macroscopic nonline-
arity. We grew crystals from different solvents and characterized
their properties by differential scanning calorimetry (DSC), X-ray
Table 1
Growth method, physical, and chemical data of CLP crystals: labs is the wavelength of the maximum absorption in methanol solution, lem,solution and lem,solid are the
wavelength of the maximum emission in methanol solution and solid, respectively, Ttr the phase transition temperature, and Tm the melting temperature (peak position in DSC
scan). Powder SHG efficiency was measured at a fundamental wavelength of 1.9 mm relative to that of MM1 powder (about two orders of magnitude larger than urea) from
Ref. [16].
Solvent (growing method)b
labs (nm)
lem,solution (nm)
lem,solid (nm)
Ttr (ꢀC)
Tm (ꢀC)
Powder SHG
HP2-I
MC/MeOH, (SE)
MeOH, (SE)
MeOH, (RC)
MEK, (SE)
MC/MeOH, (SE)
AcCN, (SE)
468
468
468
468
470
470
500
575
575
575
575
575
575
626
625
635
e
170
143
187
187
184
184
201
201
236
2.2
0
0
0
1.3
0
This work
This work
This work
This work
This work
This work
Ref. [16]
HP2-II
HP2-IIIa
HP2-IVa
MP3-I
MP3-II
MM1
58, 170
60, 158, 170
134, 190
134, 184
Not
e
643
710
711
MeOH (SE, RC), MG/VG
1.0
observed
a
HP2-III and HP2-IV crystals include the solvent, methanol and MEK, respectively, while all other crystals listed here do not include the solvent in the crystal structure.
MC (methylene chloride), MeOH (methanol), MEK (methyl ethyl ketone), AcCN (acetonitrile), RC (rapid cooling), SE (slow evaporation), MG (melt growth),
b
VG (vapor growth).

152
P.-J. Kim et al. / Dyes and Pigments 86 (2010) 149e154
Fig. 4. DSC thermodiagrams of HP2 and MP3 crystals (10 K/min scan rate): Ttr is the
phase transition temperature.
Fig. 3. Absorption and photoluminescence (PL) spectra (normalized scale) of pyrrole-
based CLP derivatives in solution and in different crystalline solids: (a) HP2 and (b)
MP3.
chromophores. Not only different rotational isomers can exist in
these crystals, rotamers can lead to an additive-like effect as
discussed below. The pyrrole-based chromophores having an
extended
and MP3-I phases have a noncentrosymmetric arrangement of
molecules, while the other HP2 and MP3 polymorphs have
a centrosymmetric arrangement of molecules. Fig. 3 shows pho-
toluminescence spectra of HP2 and MP3 polymorphs in solution
and solid state. In solid state, the polymorphs exhibit different
photoluminescence spectra: the difference of the wavelength of the
maximum emission lem,solid is 10 nm (106 cmꢁ1) between HP2-I
and -II polymorphs and 67 nm (1.5 ꢂ 105 cmꢁ1) between MP3-I
and -II polymorphs (see also Table 1). In DSC measurement we
observe even more polymorphs of HP2 and MP3 crystals. Before the
meting temperature, both crystals reveal several thermally induced
solidesolid phase transitions (see Fig. 4).
p-conjugation bridge have a rotatable single bond between the
pyrrole and the hexatriene bridge as illustrated in Fig. 5a [19]. The
existence of rotamers was confirmed by 1H NMR measurement of
HP2 chromophore as shown in Fig. 5 and single-crystal X-ray
analysis of HP2 and MP3 chromophores as shown in Fig. 6. In
Fig. 5bc, 1H NMR spectrum in CDCl3 solution with different
concentration of high purity materials reveals two structural
features (i.e., rotamers) [19]. Fig. 6 shows the molecular structures
of HP2-III and MP3-II crystals, which reveal a different confor-
mation of the
p-conjugation bridge linking pyrrole and polyene
These measurements show that the pyrrole-based chromo-
phores having asymmetric pyrrole easily form many polymorphs.
On the other hand, for MM1 crystals having a symmetric dime-
thylaminophenyl electron donor we only observed one crystal
structure, which was obtained by various methods, e.g., by solution
growth method [16,22] with slow evaporation, rapid cooling, and
slow cooling in various solvents such as methanol, ethanol, acetone,
methylene chloride and acetonitrile, by melt [16,23] and vapor
growth [24], as well as in DSC thermodiagram measurement.
In many cases polymorphism can be induced by additives
[25e28]. The reason for the occurrence of many polymorphs in
pyrrole-based chromophores compared to MM1 crystal is attrib-
uted to the existence of rotational isomerization in these
by rotational isomerization.
In solution the ratio between the NMR intensity of the minor
rotamer (Iminor) and the major rotamer (Imajor) decreases with
increasing the solution concentration [19]. However, even in high
concentration the ratio of the minor rotamer is not going to zero;
for example, 6% of the minor rotamer exists in 0.1 M solution (see
Fig. 5). The presence of the minor rotamer can act as a “tailor-made”
additive for the major rotamer in solution. Tailor-made additives
consist of the binder part with similar chemical structures and the
perturber part with slightly different chemical structure disturbing
hydrogen bonds of host molecules [26e28]. The fixed ratio of the
rotamers at a certain solution concentration suggests that the two
rotamers are in equilibrium in solution. Therefore, the minor

P.-J. Kim et al. / Dyes and Pigments 86 (2010) 149e154
153
Fig. 6. Molecular structures of chromophores in HP2-III and MP3-II crystals as deter-
mined by single-crystal X-ray analysis.
4. Conclusions
To investigate acentric crystal-forming possibilities and the
influence of molecular conformational change of
p-conjugated
bridge on crystal characteristics and polymorphism, new pyrrole-
based configurationally locked polyene (CLP) derivatives with
different N-substituted groups (X: eH, eCH3) on pyrrole and with
different number of methyl substituents in cyclohexene ring are
designed and synthesized. The pyrrole-based HP2 crystals exhibit
a large macroscopic nonlinearity with strong second harmonic
generation efficiency of about two orders of magnitude larger than
urea, which is promising for second-order nonlinear optical
applications. We investigate the influence of pyrrole linked to the
p-conjugated polyene on crystal characteristics and polymorphism.
The pyrrole-based CLP chromophores having asymmetric pyrrole
easily form many polymorphs, while for the CLP crystals having
a symmetric dimethylaminophenyl group only one crystal struc-
ture is observed. The reason for the occurrence of complex poly-
morphs in pyrrole-based chromophores is attributed to the
existence of rotational isomerization in these chromophores. The
minor rotamer exists even at high solution concentrations and
exhibits
hydrogen bonds with major rotamers. Therefore, in crystal engi-
neering of molecules having -conjugated bridge, the influence of
a “self-additive effect”, which induce complicated
p
rotational isomerization is an important parameter.
Acknowledgement
This work has been supported by the National Research Foun-
dation of Korea Grant (NRF) funded by the Korea government
(MEST) (No. 2009-0071457), Priority Research Centers Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2009-
0093826), Ajou University Research fellowship of 2009
(20095332), and the Swiss National Science Foundation.
Fig. 5. The presence of rotational isomers of HP2 chromophore: (a) rotatable bond, 1
H
NMR spectrum in CDCl3 solution with different concentrations of (b) 0.06 M and (c)
0.001 M. (d) The relative NMR ratio is the ratio between the NMR intensity of the
minor rotamer (Iminor) and the major rotamer (Imajor). The asterisk (*) notices the minor
rotamer.
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complex polymorphism. Therefore, to control the molecular
ordering of asymmetric pyrrolic chromophores in the solid state,
the influence of rotational isomerization plays an important role in
crystal characteristics and polymorphism.
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