Competition between methoxy-based and pyrazine-based synthons in methoxy-substituted distyrylpyrazines

Article abstract of DOI:10.1021/cg101523f

Four new methoxy-substituted derivatives of E,E-2,5-bis(2-phenylethenyl) pyrazine have been synthesized. The supramolecular structures of the resulting set of five polymorphs have been studied using single-crystal X-ray diffraction to gauge the influence of the position of the methoxy groups on the organization of the molecules in the solid state, as part of an attempt to dispense with the particular polymorphism of the parent compound. The crystal packing patterns were analyzed in terms of the two pyrazine-based synthons found in the parent compoundss crystal structure, the π_pyrazine...π _phenyl stacking synthon, and the pyrazine hydrogen-bonded synthon, as well as in terms of weak intermolecular interactions such as CH...O, CH...N, and CH...π. The analysis shows that the introduction of methoxy groups in positions other than only the para position of the peripheral benzene rings successfully switches off the two synthons seen in the parent compound and that the new compounds adopt other packing strategies, based on methoxy...methoxy and -OCH₃...π contacts. Polymorphism, however, remains.(Figure Presented)

Full text of DOI:10.1021/cg101523f

ARTICLE
pubs.acs.org/crystal
Competition between Methoxy-Based and Pyrazine-Based Synthons
in Methoxy-Substituted Distyrylpyrazines
Alain Collas,† Izabela Bagrowska,‡ Krzysztof Aleksandrzak,‡ Matthias Zeller,§ and Frank Blockhuys*,†
†Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium
‡Faculty of Chemistry, Nicolaus Copernicus University, Gagarina 7, PL-87100 Toruꢀn, Poland
§Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555-3663, United States
S Supporting Information
b
ABSTRACT: Four new methoxy-substituted derivatives of E,E-
2,5-bis(2-phenylethenyl)pyrazine have been synthesized. The
supramolecular structures of the resulting set of five polymorphs
have been studied using single-crystal X-ray diffraction to gauge
the influence of the position of the methoxy groups on the
organization of the molecules in the solid state, as part of an
attempt to dispense with the particular polymorphism of the
parent compound. The crystal packing patterns were analyzed in
terms of the two pyrazine-based synthons found in the parent
compound’s crystal structure, the πpyrazine πphenyl stacking synthon, and the pyrazine hydrogen-bonded synthon, as well as in
3 3 3
terms of weak intermolecular interactions such as CH O, CH N, and CH π. The analysis shows that the introduction of
3 3 3
3 3 3
3 3 3
methoxy groups in positions other than only the para position of the peripheral benzene rings successfully switches off the two
synthons seen in the parent compound and that the new compounds adopt other packing strategies, based on methoxy methoxy
3 3 3
and -OCH3 π contacts. Polymorphism, however, remains.
3 3 3
solid-state structure of the compounds is indispensable. How-
ever, until today no systematic study has been conducted: the
only two distyrylpyrazines (DSPs) of which structures have been
reported are the parent compound (1)10,11 and 2,12 and in each
case two polymorphs were reported.
1. INTRODUCTION
Organic semiconductors of the distyrylbenzene type (DSB),
oligomers of poly(p-phenylene vinylene) (PPV), have great
potential as active materials in opto-electronic devices in which
they can be used, among others, for the construction of organic
light-emitting devices (OLEDs)1-5 and as memory and non-
linear optical (NLO) materials.6,7 To improve their relevant prop-
erties, such asthe efficiency of light emissionfor OLED materials, a
number of molecular engineering strategies have been developed.
One of these is to lower the barrier to electron injection by
increasing theelectron affinityofthematerial. This can be achieved
by substituting the DSB backbone with electron-withdrawing
groups, but since such operations are very often accompanied by
a decrease in solubility and volatility, which severely hampers
device construction, a more convenient strategy is to integrate
electron-withdrawing heteroatoms into the DSB backbone. This
incorporation strategy has been successfully applied by Nohara
et al.8 and Grimsdale et al.9 resulting in a dimethoxy derivative of
E,E-2,5-bis(2-phenylethenyl)pyrazine (1, Figure 1), that is,
E,E-2,5-bis[2-(4-methoxyphenyl)ethenyl]pyrazine (2), which in-
deed displays a higher electron affinity than PPV.
Even though from a structural chemistry point of view the
presence of polymorphism may be seen as advantageous, since it
provides an opportunity to study and understand the nature of
the interactions influencing a molecule during crystal packing, it
is clear that from a materials chemistry point of view it is not: the
fact that 1 and 2 suffer from polymorphism holds a number of
serious repercussions in particular for the construction and
evaluation of the above-mentioned devices. Conclusions with
respect to, for example, the efficiency of light emission or
mechanistic aspects, drawn from the results of electrical and
optical measurements, may be erroneous as 1 can exist in a form
(its R-form) that is capable of undergoing a solid-state photo-
polymerization. Indeed, the noncovalent interactions between
the aromatic moieties, which will be discussed below in greater
detail, help to establish the typical face-to-face stacking
(Figure 2a) and, in this way, to meet the conditions to undergo
a cycloaddition reaction: when the ethenyl spacers are separated
Even though the favorable molecular properties of these
nitrogen-containing DSBs have been demonstrated, it is clear
that using these materials in the above-mentioned applications
means that not only the molecular but also the supramolecular
aspect is of importance and that a detailed knowledge of the
Received: November 16, 2010
Revised:
January 17, 2011
Published: February 10, 2011
r
2011 American Chemical Society
1299
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Crystal Growth & Design
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Figure 1. The parent DSP (1) and its five methoxy-substituted derivatives (2-6). The structural formula of 1 presents the numbering scheme of the five
compounds; in polymorph 1a the molecules have no inversion symmetry and the numbering in the second half of the molecule (containing ring B)
mirrors the numbering in the first (containing ring A) with a suffix D added to the atom label.
since it is clear from the fact that 1 and 2 display similar
polymorphism that one methoxy group positioned para on each
of the peripheral rings is incapable of having a large enough effect.
These compounds are E,E-2,5-bis[2-(3,4-dimethoxyphenyl)-
ethenyl]pyrazine (3), E,E-2,5-bis[2-(2,4-dimethoxyphenyl)ethenyl]-
pyrazine (4), E,E-2,5-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]pyrazine
(5),andE,E-2,5-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]pyrazine (6);
their structural formulas are shown in Figure 1. The introduction of
these substituents results in the exploitation of two effects. First,
the extra methoxy groups on the peripheral rings necessarily
increase the intermolecular distance between the molecules in
the solid, leading to the switching off of the typical pyrazine
hydrogen-bonded synthon (Figure 2b) seen in the photostable
γ-forms of 1 and 2. Second, methoxy groups tend to favor
reciprocal contacts among each other: this has been demon-
strated in our previous work on methoxy-substituted distyrylben-
zenes,15,16 but it can also be seen clearly in the solid-state structure
of both polymorphs of 2 (see below). Because of these favored
methoxy methoxy contacts π π stacking becomes increas-
Figure 2. (a) The πpyrazine πphenyl stacking synthon and (b) the
pyrazine hydrogen-bonded synthon.
3 3 3
by less than 4.2 Å, these crystals yield, after irradiation with
UV-light with an appropriate wavelength, photoproducts with a
well-defined yet altered stereochemical and supramolecular
structure;13 the photo-oligomerization and -polymerization of 1
have been studied in detail.14 In contrast, the second modification
of 1 (its γ-form) is photostable. Compound 2 displays similar
polymorphism as it too has a modification that is capable of
undergoing a solid-state photopolymerization (its R-form), as well
as a photostable modification (its γ-form). The former also
3 3 3
3 3 3
ingly unlikely, thereby also switching off the πpyrazine πphenyl
3 3 3
synthon (Figure 2a). These two effects are expected to lead to the
disappearance of the polymorphs observed for 1 and 2 and force
the molecules of the new compounds into a different supramole-
cular organization.
displays the πpyrazine πphenyl synthon (Figure 2a). Both the
3 3 3
photostable modifications of 1 and 2, on the other hand, display
the typical pyrazine hydrogen-bonded synthon (Figure 2b). The
fact that both compounds in this series are found in the same types
of polymorphs suggests that both synthons presented in Figure 2
have a similar stabilizing effect on the supramolecular structure,
despite the fact that they are based on different weak interactions.
In an attempt to deal with this unfortunate polymorphism in
order to take full advantage of the compounds’ molecular
properties, a series of new DSPs were prepared and characterized
containing two or three methoxy groups per peripheral ring,
2. EXPERIMENTAL SECTION
2.1. Syntheses. All reagents and solvents were obtained from
ACROS and used as received. All NMR spectra were recorded in CDCl3
on a Bruker Avance II spectrometer at frequencies of 400 MHz for 1H
and 100 MHz for 13C with tetramethylsilane (TMS) as internal standard.
Chemical shifts are given in ppm and coupling constants are in Hz. UV/vis
absorption spectra were recorded on a Cary 5 spectrometer for solutions
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Crystal Growth & Design
ARTICLE
(about 20 ꢀ 10-6 M) of the oligomers in CH2Cl2. Melting points were
obtained with an open capillary electrothermal melting point apparatus and
are uncorrected. Preparative details and spectroscopic characterization of
compounds 2 and 4 can be found in the Supporting Information.
mixture of acetonitrile and chloroform. The yield was 3.5%. Mp > 523 K.
UV/vis λmax = 419 nm (log ε = 4.65). δ1H 3.85 (s, 6H, 4-OCH3), 3.91
(s, 12H, 2-OCH3 and 6-OCH3), 6.17 (s, 4H, H3 and H5), 7.53 (d, J =
16.4, 2H, H8), 8.03 (d, J = 16.4, 2H, H7), 8.54 (s, 2H, H10). δ13C 55.34
(4-OCH3), 55.77 (2-OCH3 and 6-OCH3), 90.70 (C3 and C5), 107.74
(C1), 124.56 (C8), 125.28 (C7), 142.87 (C10), 150.22 (C9), 160.32
(C4), 161.22 (C2 and C6). Crystals of 6 suitable for a diffraction
experiment were obtained from the slow evaporation of a CH2Cl2
solution.
2.2. X-ray Crystallography. Data collection for compounds 5
(two polymorphs, 5a and 5b) and 6 was performed on an Enraf-Nonius
Mach 3 diffractometer using Mo-KR radiation (λ = 0.71073 Å). The
CAD-4 EXPRESS software17 was used for data collection and cell
refinement and XCAD-4 for data reduction.18 For compound 3 (two
polymorphs, 3a and 3b), data collection was performed on a Bruker AXS
SMART APEX CCD diffractometer using Mo-KR radiation (λ =
0.71073 Å) by ω scans; the data reduction and multiscan absorption
correction was performed with the Apex2 v2008 2.4 software.19 The
structures were solved by direct methods using SHELXS-9720 and
refined using SHELXL-97:20 hydrogen atoms were restrained to occupy
2.1.1. E,E-2,5-Bis[2-(3,4-dimethoxyphenyl)ethenyl]pyrazine (3). A
round-bottomed flask was charged with 4.2 g (25 mmol) of 3,4-
dimethoxybenzaldehyde and 1.4 g (12.5 mmol) of 2,5-dimethylpyrazine
in 20 mL of DMF. After 30 min, 2.8 g (50 mmol) of KOH was added.
The mixture was heated to 100 °C and stirred at this temperature for 4
days. After cooling to room temperature, the mixture was cooled in an ice
bath and 60 mL of cold methanol was added. The mixture was stored
overnight in the refrigerator and the resulting orange precipitate was
washed with water (2 ꢀ 20 mL) and ethanol (2 ꢀ 20 mL). The crude
product was isomerized for 4 h in refluxing p-xylene containing a catalytic
amount of I2. The precipitate was collected by filtration and washed with
water (2 ꢀ 20 mL) and ethanol (2 ꢀ 20 mL) and recrystallized from a 5:
1 mixture of ethanol and propanol. The yield was 5.8%. Mp 502
(polymorph 3a), 487 K (polymorph 3b). UV/vis λmax = 410 nm (log
ε = 4.74). δ1H 3.93 (s, 6H, 4-OCH3), 3.96 (s, 6H, 3-OCH3), 6.89 (d, J =
8.8 Hz, 2H, H5), 7.04 (d, J = 16.0 Hz, 2H, H8), 7.16 (m, 4H, H2 and H6),
7.67 (d, J = 16.0, 2H, H7), 8.57 (s, 2H, H10). δ13C 55.91 (4-OCH3),
55.97 (3-OCH3), 109.24 (C2), 111.28 (C5), 121.21 (C6), 122.26 (C8),
129.47 (C1), 133.91 (C7), 142.94 (C10), 149.01 (C4), 149.28 (C3),
150.01 (C9). Crystals of 3a and 3b suitable for a diffraction experiment
were crystallized concomitantly via the slow evaporation of an acetone
solution and separated manually under a microscope.
2.1.2. E,E-2,5-Bis[2-(3,4,5-trimethoxyphenyl)ethenyl]pyrazine (5). A
round-bottomed flask was charged with 4.9 g (25 mmol) of 3,4,5-
trimethoxybenzaldehyde and 1.4 g (12.5 mmol) of 2,5-dimethylpyrazine
in 30 mL of DMF. After 30 min, 2.8 g (50 mmol) of KOH was added.
The mixture was heated to 100 °C and stirred at this temperature for 6
days. After cooling to room temperature, the mixture was cooled in an
ice bath and 70 mL of cold methanol was added. The mixture was stored
overnight in the refrigerator and the resulting orange precipitate was
washed with water (2 ꢀ 20 mL) and ethanol (2 ꢀ 20 mL). The crude
product was isomerized for 4 h in refluxing p-xylene containing a
catalytic amount of I2. The precipitate was collected by filtration and
washed with water (2 ꢀ 20 mL) and ethanol (2 ꢀ 20 mL) and
recrystallized from acetonitrile. The yield was 3.7%. Mp 484
(polymorph 5a), 490 K (polymorph 5b). UV/vis λmax = 402 nm (log
ε = 4.63). δ1H 3.90 (s, 6H, 3-OCH3), 3.92 (s, 12H, 3-OCH3 and
5-OCH3), 6.84 (s, 4H, H2, and H6), 7.09 (d, J = 16.00, 2H, H8), 7.66
(d, J = 16.00, 2H, H7), 8.60 (s, 2H, H10). δ13C 56.16 (3-OCH3 and
5-OCH3), 61.00 (4-OCH3), 104.40 (C2 and C6), 123.51 (C8), 131.89
(C1), 134.27 (C7), 139.05 (C4), 143.13 (C10), 148.96 (C3 and C5),
153.49 (C9). Crystals suitable for a diffraction experiment were grown by
introducing small needle-shaped crystals obtained from a sublimation
experiment into a saturated CHCl3 solution. Slow evaporation of this
solution yielded larger, suitable needle-shaped crystals of polymorph 5a. A
small amount of block-shaped crystals (polymorph 5b) was found in the
same batch and was used in the measurement. Since that time, however,
we have been unable to obtain more crystals of polymorph 5b.
the calculated positions and defined as riding [CH = 0.93 Å and Uiso
-
(H) = 1.2 Ueq(C) for aromatic CH]. For the analysis of the supramo-
lecular structures, CH bond lengths were normalized to the value
derived from neutron diffraction (1.083 Å). After this normalization
procedure, PLATON21 was used to calculate all intra- and intermole-
cular contacts. Figures were prepared using MERCURY (version 2.3).22
MCE 200523 was used to visualize the residual electron density between
the two ethenylic spacers (Figure 11). The experimental details includ-
ing the results of the refinements are given in Table 1. The numbering
scheme can be found in Figure 1. CCDC-797361 (3a), 797362 (3b),
797363 (5a), 797364 (5b), and 797365 (6) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre.
3. RESULTS AND DISCUSSION
Attempts were made to grow single crystals suitable for XRD
of all oligomers using various crystallization techniques: taking
into account the solubilities of the oligomers, these comprise the
slow evaporation of CH2Cl2, CHCl3, and THF solutions, and
vapor diffusion of diethyl ether into saturated CH2Cl2, CHCl3,
and THF solutions. However, only for compounds 3, 5, and 6
crystals of sufficient size and quality were obtained. Details of the
data collection and structural refinement of compounds 3 (two
polymorphs, 3a and 3b), 5 (two polymorphs, 5a and 5b), and 6
are collected in Table 1. The necessary data for the structures of
compounds 1 and 2 were obtained from the CSD,24 refcodes
STPYAZ01 (polymorph 1a, γ-form),10 STPYAZ (polymorph 1b,
R-form),11 FUGYAH (polymorph 2a, γ-form),12 and FU-
GYAH01 (polymorph 2b, R-form).12 No crystals suitable for a
diffraction experiment could be obtained for compound 4. For all
structures, the carbon-hydrogen distances were normalized to
1.083 Å after the refinement, and the resulting geometrical
parameters have been used in the following discussion of the
different intermolecular short contacts. The details of these
contacts have been summarized in Table 2. The numbering
scheme of the molecules is given in Figure 1.
2.1.3. E,E-2,5-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]pyrazine (6). A
round-bottomed flask was charged with 4.9 g (25 mmol) of 2,4,6-
trimethoxybenzaldehyde and 1.4 g (12.5 mmol) of 2,5-dimethylpyrazine
in 30 mL of DMF. After 30 min, 2.8 g (50 mmol) of KOH was added. The
mixture was heated to 100 °C and stirred at this temperature for 7 days.
After cooling to room temperature, the mixture was cooled in an ice bath
and 90 mL of cold methanol was added. The mixture was stored overnight
in the refrigerator and the resulting orange precipitate was washed
with water (2 ꢀ 20 mL) and ethanol (2 ꢀ 20 mL). The crude product
was isomerized for 4 h in refluxing p-xylene containing a catalytic amount
of I2. The precipitate was collected by filtration and washed with water
(2 ꢀ 20 mL) and ethanol (2 ꢀ 20 mL) and recrystallized from a 5:2
All oligomers adopt the anti conformation in which the two
ethenyl spacers are oriented in opposite directions with respect
to the central pyrazine ring. Also, they adopt the conformation in
which the N N axis in the central ring is aligned with the
3 3 3
orientations of the ethenylic links, which was earlier determined
to be the more stable.25 Disorder associated with the typical pedal
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Crystal Growth & Design
ARTICLE
Table 1. Details of the Data Collection and Structural Refinement of 3a, 3b, 5a, 5b, and 6
3a
3b
5a
5b
6
A. Crystal Data
chemical formula
chemical formula weight (g mol-1
C24H24N2O4
404.45
C24H24N2O4
404.45
C26H28N2O6
464.50
C26H28N2O6
464.50
C26H28N2O6
464.50
)
cell setting
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
space group
crystal form
plate
prism
needle
block
block
crystal size (mm)
0.05 ꢀ 0.38 ꢀ 0.55
yellow-green
2867
0.70 ꢀ 0.52 ꢀ 0.46
orange
0.24 ꢀ 0.18 ꢀ 0.18
orange
0.34 ꢀ 0.32 ꢀ 0.32
yellow
0.21 ꢀ 0.21 ꢀ 0.42
orange
crystal color
no. of reflections for cell param
845
25
25
25
θ-range (°)
F(0 0 0)
T (K)
a (Å)
b (Å)
c (Å)
2.89-31.08
428
2.60-22.09
428
5.20-13.01
492
5.84-16.52
492
5.53-11.50
492
295(2)
295(2)
14.118(3)
9.424(2)
7.8733(18)
90
293(2)
4.371(2)
13.193(4)
20.695(5)
90
293(2)
8.040(2)
10.806(4)
15.268(5)
90
293(2)
13.0095(10)
7.0463(6)
12.2219(9)
90
4.389(4)
15.4880(10)
17.215(4)
90
R (°)
β (°)
γ (°)
V (Å3)
115.8140(10)
90
90.938(5)
90
98.52(3)
90
117.533(19)
90
103.90(3)
90
1008.57(14)
2
1047.4(4)
2
1180.2(7)
2
1176.2(7)
2
1136.0(11)
2
Z
Dx (Mg m-3
μ (mm-1
)
1.332
1.282
1.307
1.312
1.358
)
0.091
0.088
0.093
0.094
0.097
B. Data collection
diffractometer
Bruker AXS SMART
APEX CCD
ω scans
Bruker AXS SMART
APEX CCD
ω scans
Enraf-Nonius
Enraf-Nonius
Mach3
Enraf-Nonius
Mach3
Mach3
data collection method
no. of measured reflections
no. of independent reflections
no. of observed reflections
criterion for observed reflections
θmax (°)
ω/2θ scans
ω/2θ scans
4503
ω/2θ scans
4639
9843
5406
4801
2496
2566
2162
2164
2084
1930
1352
945
776
1330
I > 2σ(I)
28.28
I > 2σ(I)
28.28
I > 2σ(I)
I > 2σ(I)
25.61
I > 2σ(I)
25.34
25.33
range of h
-17 e h e 17
-9 e k e 9
-16 e l e 16
0
-11 e h e 18
-12 e k e 12
-10 e l e 10
0
-5 e h e 0
0 e h e 9
-13 e k e 13
-18 e l e 16
3
0 e h e 5
-18 e k e 18
-20 e l e 20
3
range of k
-15 e k e 15
range of l
-24 e l e 24
no. of standard reflections
3
1
5
frequency of standard reflections (h)
1
1
Iintensity decay (%)
59
1
C. Refinement
refinement on
F2
F2
F2
F2
F2
GOF
1.046
0.1260
0.0421
0.0569
2496
138
1.047
0.1740
0.0612
0.1170
2566
0.994
0.1557
0.0483
0.1571
2162
157
0.933
0.2494
0.0736
0.2265
2164
157
1.019
0.1008
0.0343
0.0748
2084
157
wR2
R1
Rall
no. of reflections used in refinement
no. of parameters used
138
(Fo2) þ (AP)2 þ BP] where P = (Fo þ 2Fc2)/3
0.0625
0.1472
0.00
2
weighting scheme
A
0.0660
0.0637
0.00
0.0594
0.1671
0.00
0.1245
0.00
0.0424
0.1907
0.00
B
(Δ/σ)max
ΔFmax
ΔFmin
0.00
0.252
0.186
0.161
0.477
-0.235
0.132
-0.180
-0.163
-0.205
-0.124
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Table 2. Details (distances d in Å, angles θ in degrees and symmetry codes) of the Short Intermolecular Contacts D-H
A
3 3 3
Given in the Texta
D
H
A
dH 3 3 3
dD 3 3 3
θ
symmetry code
A
A
1a
C10
C10D
C6D
C5D
C6
H10
H10D
H6D
H5D
H6
N1D
N1
2.57
2.55
2.80
2.70
2.66
2.85
2.50
2.49
2.47
2.58
2.64
2.70
2.76
2.48
2.68
2.75
2.58
2.57
2.46
2.55
2.85
2.37
2.62
2.70
2.67
2.53
2.75
2.68
2.45
2.58
2.61
2.28
2.12
2.70
2.52
2.52
2.96
2.99
3.452
3.460
3.536
3.682
3.719
3.876
139
140
125
150
167
159
136
145
155
163
179
147
165
152
126
128
158
155
135
154
139
136
123
172
116
135
130
155
154
140
139
100.9
118.5
151
147
147
142
137
x, y, -1 þ z
x, y, 1 þ z
Cg(Ar)
N1D
N1
-x, -y, -z
1/2 - x, 1/2 þ y, -z
-x, 1/2 þ y, -1/2 - z
1/2 - x, -y, -1/2 þ z
x, 1 þ y, z
1/2 - x, 1/2 - y, 1 - z
x, -y, -1/2 þ z
1/2 - x, y, -1/2 þ z
x, 3/2 - y, -1/2 þ z
-1/2 - x, y, 1/2 þ z
x, y, -1 þ z
1 - x, 1 - y, 1 - z
x, 1/2 - y, -1/2 þ z
-x, -1/2 þ y, 3/2 - z
2 - x, 2 - y, -z
1b
2a
C4
H4
Cg(Ar)
N1
C10
C41
C41
C6
H10
H41b
H41a
H6
3.366(2)
3.429(1)
3.484(2)
3.633(2)
3.725(2)
3.653(2)
3.809(2)
3.475(2)
3.433(2)
3.518(2)
3.614(3)
3.585(5)
3.316(5)
3.553(3)
3.742(3)
3.230(5)
3.346(5)
3.774(4)
3.292(4)
3.389(4)
3.540(6)
3.688(8)
3.460(7)
3.478(7)
3.503(8)
2.700(2)
2.805(2)
3.684(3)
3.479(3)
3.482(3)
3.873(4)
3.860(4)
O4
Cg(Ar)
N1
2b
C41
C10
C41
C41
C6
H41b
H10
H41c
H41b
H6
O4
C7dC8
C7dC8
O3
3a
3b
Cg(ArN)
Cg(Ar)
O3
C10
C41
C31
C31
C6
H10
H41b
H31b
H31c
H6
O4
2 - x, -1/2 þ y, 1/2 - z
x, 3/2 - y, 1/2 þ z
x, 5/2 - y, -1/2 þ z
x, 5/2 - y, -1/2 þ z
-1 þ x, y, z
O4
N1
C41
C41
C41
C2
H41c
H41a
H41c
H2
Cg(Ar)
O4
5a
O3
-1 þ x, y, z
O4
-x, 1/2 þ y, 1/2 - z
-x, 1/2 þ y, 1/2 - z
x, 1/2 - y, 1/2 þ z
-1 - x, -1/2 þ y, 1/2 - z
x, 1/2 - y, -1/2 þ z
x, 1/2 - y, -1/2 þ z
-x, 1/2 þ y, 1/2 - z
-x, 1/2 þ y, 1/2 - z
intramolecular
C31
C10
C41
C2
H31a
H10
H41b
H2
O4
O3
C7dC8
O5
5b
6
C8
H8
O5
C6
H6
O4
C7
H7
O4
C7
H7
O6
C8
H8
O2
intramolecular
C41
C41
C21
C21
C61
H41b
H41c
H21a
H21c
H61c
O6
-1 þ x, -1/2 - y, -1/2 þ z
-1 þ x, -1/2 - y, -1/2 þ z
1 - x, -y, -z
N1
O4
Cg(Ar)
Cg(Ar)
-1 þ x, y, z
1 þ x, y, z
a The lack of an esd on a number of the parameters is due to the normalization of the CH distances and/or the absence of esds in the CIFs of 1a and 1b.
motion of the ethenylic spacers26 could not be identified in any of
the five new solid-state structures. In the following, the crystal
structures of 1 and 2 will be reviewed and compared in detail,
after which the structures of the new compounds (3-6) will be
analyzed.
3.1. E,E-2,5-Bis(2-phenylethenyl)pyrazine (1). Molecules
in the γ-form (1a) of 1 are noncentrosymmetric and twisted
along the molecular axis (Figure 3). This can be seen most
clearly from the different dihedral angles between the least-
squares planes through the rings. The angle between the plane
through peripheral ring A and the plane through the central
ring is about 27°, while the angle between the plane through
the central ring and the plane through peripheral ring B is
about 25°. In contrast, molecules in the R-form (1b) of 1 are
virtually planar and centrosymmetric: the central pyrazine ring
is twisted out of the plane of the peripheral benzene ring by
about 12°.
In the γ-form (1a) typical pyrazine chains based on the
synthon in Figure 2b are formed in the [0 0 1] direction, and
the peripheral rings are twisted out of the plane of the pyrazine
ring to accommodate the C10-H10 N1D and C10D-
3 3 3
H10D N1 hydrogen bonds (Figure 3). These chains are then
3 3 3
held together two by two by a CH π interaction [C6D-
3 3 3
H6D Cg(Ar)] and form stacks perpendicular to the chains.
3 3 3
Finally, these stacks of coupled chains are linked by weak
hydrogen bonds [C5D-H5D N1D] (not given in Figure 3).
3 3 3
In the R-form (1b) the molecules organize themselves into
sawtooth ribbons in the [0 1 0] direction through weak CH
N
3 3 3
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Figure 3. The typical pyrazine chain in the [0 0 1] direction and the
CH πinteraction between the layers in the γ-form of 1(polymorph 1a).
3 3 3
hydrogen bonds also involving the nitrogen atoms of the pyrazine
rings [C6-H6 N1], but these are different from the typical
3 3 3
synthons given in Figure 2b (Figure 4a). Perpendicular to these
ribbons the π π stacks based on the synthon in Figure 2a are
3 3 3
formed which are responsible for the photoactivity of the structure:
the Cg(ArN) Cg(Ar) distance is 3.940 Å (Figure 4b,c). Finally,
3 3 3
these stacks assemble in a herringbone pattern held together by a
weak hydrogen bond in which H4 contacts the centroid of a phenyl
ring [C4-H4 Cg(Ar)] (Figure 4a,c).
3 3 3
3.2. E,E-2,5-Bis[2-(4-methoxyphenyl)ethenyl]pyrazine (2).
Molecules in both the γ- (2a) R-forms (2b) of 2 are rela-
tively planar: the least-squares plane through the central pyrazine
ring makes an angle of about 30° with the plane through the
peripheral benzene ring in 2a, while this angle is limited to about
15° in 2b.
In the γ-form (2a), typical pyrazine chains based on the synthon
in Figure 2b are formed in the [0 1 0] direction and the peripheral
rings are twisted out of the plane of the pyrazine ring to accom-
modate the C10-H10 N1 hydrogen bonds (Figure 5a). These
3 3 3
chains are then interconnected through methoxy methoxy
3 3 3
dimer formation [C41-H41b O4] in the direction perpendi-
cular to the chains, constructing sheets of molecules. Finally, these
3 3 3
sheets are organized into shifted stacks through -OCH3
π
3 3 3
interactions [C41-H41a Cg(Ar)] (Figure 5b).
3 3 3
In the R-form (2b) the molecules organize themselves into
Figure 4. (a) Sawtooth organization of the molecules, (b) weak
sawtooth ribbons through weak hydrogen bonds also involving the
interactions between the molecules, and (c) πpyrazine πphenyl stacking
3 3 3
in the R-form of 1 (polymorph 1b).
nitrogen atoms of the pyrazine ring [C6-H6 N1], but these are
3 3 3
different from the typical synthons given in Figure 2b (Figure 6a).
Perpendicular to these ribbons the π π stacks based on the
corresponding synthon (Figure 2a) are formed which are responsible
3.3. The Analogy between the Packings of 1 and 2. The
2,5-distyrylpyrazine unit has two particular properties which
determine its supramolecular behavior. First, there is the C-H
bond on the central ring which is activated by the presence of the
electron-withdrawing nitrogen atom. This directly leads to the
3 3 3
for the photoactivity of the structure: the Cg(ArN) Cg(Ar) dis-
3 3 3
tance is 3.988 Å (Figure 6b,c). Finally, these stacks assemble in a
herringbone pattern held together by weak CH O hydrogen
3 3 3
bonds between the methoxy groups [C41-H41b O4]
possibility of generating relatively strong reciprocal CH
N
3 3 3
3 3 3
(Figure 6a,c). As a consequence of this three-dimensional organiza-
tion, two additional but very weak interactions can be identified: H41b
and H10 contact the π-system of one of the vinyl spacers (Figure 6a).
hydrogen bonds between two molecules in the synthon depicted
in Figure 2b. Second, it consists of an electron-poor central
pyrazine ring (at least with respect to the peripheral benzene
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Figure 5. (a) The combination of the hydrogen-bonded synthon with
the methoxy methoxy interactions leading to sheets, and (b) the
3 3 3
CH π interactions stacking these sheets in the γ-form of 2
3 3 3
(polymorph 2a).
rings) and two electron-rich peripheral benzene rings (at least
with respect to the central pyrazine ring). This directly leads to
the possibility of generating favorable π π interactions in the
3 3 3
synthon depicted in Figure 2a, by stacking the molecules
perpendicularly and shifting them the length of a styryl fragment.
Clearly, each of these two properties lies at the basis of one of the
three-dimensional structures observed in the two polymorphs, in
this case 1a and 1b, respectively. The introduction of methoxy
groups in both 4-positions of the benzene rings will increase the
polarization of the molecules, as they enhance the electron-
richness of the peripheral rings. How they influence the expres-
sion of the two properties mentioned above can be seen from the
comparison of the supramolecular structures of 1 and 2.
In 1a and 2a the pyrazine hydrogen-bonded synthon
(Figure 2b) generates the main building blocks of the crystal in
the form of the chains (Figures 3 and 5a). Then, the CH
interactions holding these chains together in 1a are replaced by -
π
3 3 3
OCH3 π interactions in 2a (Figures 3 and 5b). Finally, the
3 3 3
CH N interactions interconnecting the stacks in 1a are re-
3 3 3
placed by methoxy methoxy contacts (Figure 5a) in 2a; this
3 3 3
tendency of methoxy groups to favor reciprocal contacts is known,
as it has been identified in the crystal structures of methoxy-
substituted distryrylbenzenes.15,16 As such, the methoxy groups in
2a take over the role of a number of hydrogen atoms in 1a, but do
not fundamentally change the supramolecular organization.
Similarly, from a juxtaposition of Figures 4 and 6 the resem-
blance of the packing motifs of 1b and 2b becomes clear. In these
Figure 6. (a) Sawtooth organization of the molecules, (b) weak
interactions between the molecules, and (c) πpyrazine πphenyl stacking
3 3 3
in the R-form of 2 (polymorph 2b).
and 2b, despite the difference in length of the molecules, can
also be recognized in the similarity of the unit cell parameters,
since the main sawtooth motif runs along the longest cell
axis: 20.638(7) vs 26.061(3) Å, 7.655(6) vs 7.3280(10) Å, and
9.599(6) vs 9.1240(10) Å for 1b and 2b, respectively, keeping
in mind that the former was studied at room temperature and
the latter at 150 K. As in 2a, the methoxy groups in 2b take
over the role of a number of hydrogen atoms in 1b, but do not
fundamentally change the supramolecular organization. In the
following the supramolecular structures of the new compounds
containing more than one methoxy groups per peripheral ring
will be discussed.
polymorphs the πpyrazine πphenyl synthon (Figure 2a) gener-
ates the main building blocks of the crystal in the form of the
stacks (Figures 4b and 6b). Then, in both 1b and 2b, these stacks
3 3 3
are interconnected into stacked ribbons via CH N contacts,
3 3 3
which are, as mentioned earlier, different from the synthons given
in Figure 2b (Figures 4a and 6a). Finally, the stacked ribbons
are organized into a herringbone pattern based on different
T-shaped interactions. These are the CH π interactions in 1b
3 3 3
involving the para-CH moiety from the peripheral benzene rings
(Figure 4a,c), which are replaced by methoxy methoxy con-
3 3 3
tacts involving the para-methoxy groups (Figure 6a,c) in 2b;
again, the methoxy groups favor the formation of reciprocal
contacts.15,16 The similarities between the crystal packings of 1b
3.4. E,E-2,5-Bis[2-(3,4-dimethoxyphenyl)ethenyl]pyrazine
(3). Compound 3 crystallized as a mixture of two polymorphs
(3a and 3b). In 3a the angle between the pyrazine and benzene
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Figure 7. The infinite chains held together by two CH π contacts in 3a.
3 3 3
Figure 8. The infinite chains held together by CH π, CH O, and CH N contacts in 3b.
3 3 3
3 3 3
3 3 3
planes is about 6°. The methoxy group in the 3-position is twisted
out of the plane of the benzene ring (about 19°) to a greater
extent than the one in the 4-position (about 8°); this may be
linked to the former’s involvement as hydrogen bond acceptor
(see below). In 3b the pyrazine ring is twisted about 23° out of
the plane of the benzene ring. The methoxy groups are quasi
coplanar with the benzene ring, as torsion angles of about 1° and
about 5° are measured for the groups in the 3- and 4-positions,
respectively.
the molecules along the molecular axis, as in the previously
discussed compounds and polymorphs, but rather a consequence
of the peculiar Z shape of the molecules which was also seen
in both E,E-1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene
(CSD refcode JACBIY27) and E,E-2,5-dimethoxy-1,4-bis-[2-(3,
4,5-trimethoxyphenyl)ethenyl]benzene (CSD refcode GAKZAU15);
these two compounds can be considered the DSB analogues of
5. Because of the 3,4,5-trisubstitution on the peripheral rings,
the methoxy groups in the 4-position are almost perpendicular
(about 85°) to the benzene ring, while those in the 3- and 5-
positions remain in the plane, making angles of about 5° and 6°,
in 5a; in 5b these values are about 86°, 3°, and 4°, respectively.
In the structure of 5a (Figure 9) shifted stacks of molecules are
In the structure of 3a (Figure 7), chains of molecules are
formed in the [1 0 1] direction through two mutual methoxy
methoxy contacts with the methoxy group in the 4-position as
hydrogen bond donor and the one the 3-position as acceptor:
H41b contacts the oxygen atom of the methoxy group in the
3 3 3
generated through two methoxy methoxy weak hydrogen
3 3 3
bonds [C41-H41a O4 and C41-H41c O3] (Figure 9a).
3-position [C41-H41b O3]. These chains are then organized
3 3 3
3 3 3
3 3 3
These stacks are then connected to each other by three weak
into the three-dimensional structure via two different CH
π
3 3 3
hydrogen bonds [C2-H2 O4, C31-H31a O4, and C10-
interactions: H6 contacts the pyrazine ring[C6-H6 Cg(ArN)],
whereas H10 contacts the methoxy-substituted ring [C10-H10
3 3 3
3 3 3
3 3 3
H10 O3] (Figure 9b). In addition, H41b of the methoxy group
3 3 3
in the 4-position contacts the π-system of the ethenyl spacer of a
molecule above (not given in Figure 9). All three hydrogen atoms
of this para-methoxy group are used in developing the crystal
structure, while the methoxy group in the 5-position remains
unused.
Cg(Ar)].
3 3 3
A similar situation is found in the second polymorph 3b
(Figure 8). Again, chains of molecules are formed through in-
volvement of the methoxy groups in the 3- and 4-positions as
acceptors and donors, respectively [C41-H41b O3]. These
3 3 3
In 5b, however, no methoxy methoxy contacts are present
chains are now organized into the three-dimensional structure
through additional methoxy...methoxy contacts with the meth-
oxy group in the 3-position acting as donor: H31b and H31c
contact the oxygen atoms of two 4-methoxy groups from two
different molecules (Table 2). In addition to these, further later-
al CH N and CH π contacts can be seen between H6 and
3 3 3
(Figure 10). Aromatic and ethenylic hydrogen atoms H2 and H8
contact O5, while on the other side of the molecule H6 and H7
contact O4 (Table 2). Because of the presence of the face-to-face
synthon (Figure 2a), the relatively acidic H10 does not take part
in the intermolecular network. The pronounced πpyrazine πphenyl
3 3 3
3 3 3
3 3 3
interactions with a Cg(ArN) Cg(Ar) distance of 3.859 Å
the nitrogen atom of the pyrazine ring [C6-H6 N1] and
3 3 3
3 3 3
are easily recognized and are the reason for this structure to meet
the criteria for a solid-state polymerization: the ethenyl spacers are
only 3.367 Å apart which is well below the 4.2 Å criterion. Indeed,
during the measurement a large intensity decay of the control
reflections could be observed (Table 1). During the refinement
residual electron density could be located in the neighborhood of
the ethenyl spacers and this is graphically represented in Figure 11.
Furthermore, upon inspection of the crystal after the measurement
it was found to have changed from a yellow block to a white mass.
between H41c and the π-system of the methoxy-substituted
ring [C41-H41c Cg(Ar)], respectively. Note that the rela-
3 3 3
tively acidic H10 does not participate in this packing, while it
did in 3a.
3.5. E,E-2,5-Bis[2-(3,4,5-trimethoxyphenyl)ethenyl]pyra-
zine (5). Compound 5 crystallized as a mixture of two poly-
morphs (5a and 5b). In 5a the plane of the pyrazine ring makes
an angle of about 10° with the plane of the benzene ring; in 5b
this angle is about 15°. Neither angle is actually due to a twist of
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All these observations clearly indicate this polymorph’s photosen-
sitivity. There was not enough material for any kind of further
analysis of the photoproduct.
3.6. E,E-2,5-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]pyra-
zine (6). In 6 the plane of the pyrazine ring and the plane of
the benzene ring make an angle of about 9°, and this seems to be
the result of the combination of a twist along the molecular axis,
as in 1, 2, and 3, and a slight Z shape, as in 5. The three methoxy
groups deviate by no more than about 5° from the plane of the
benzene ring, and for those in the 2- and 6-positions this may be
linked to their involvement in intramolecular CH O interac-
3 3 3
tions (Table 2).28
The molecules in 6 are interconnected by the methoxy group
in the 4-position which simultaneously contacts N1 and O6
(Figure 12a). The methoxy group in the 2-position forms a
typical methoxy methoxy interaction with the oxygen atom of
3 3 3
the methoxy group in the 4-position [C21-H21a O4]
3 3 3
(Figure 12b). Stairlike stacks are then formed by CH
contacts involving the methoxy groups in the 2-and 6-positions
π
3 3 3
and the C21-H21a O4 contact mentioned above (Figure 12b).
3 3 3
Again, H10 is not used.
3.7. The Supramolecular Structures of Methoxy-Substi-
tued Distyrylpyrazines. The first expected effect of the intro-
duction of more than one methoxy group on each of the two
peripheral rings, that is, the switching off of the typical pyrazine
hydrogen-bonded synthon (Figure 2b) seen in 1a and 2a, has
clearly been achieved: in none of the five structures of com-
pounds 3, 5, and 6 this synthon can be observed. In fact, the
nitrogen atom has been relegated to a very subordinate role. Only
in 3b and 6 is it involved as acceptor in CH N contacts: in the
3 3 3
former the donor is the same methine group of one of the
peripheral benzene rings as in 1b and 2b, and in the latter the
donor is a methoxy group. Note that to form the CH
N
3 3 3
hydrogen bonds of the pyrazine chain (Figure 2b), 1, 2 and E,E-
2,5-bis[2-(2,3,4,5,6-pentafluorophenyl)ethenyl]pyrazine25 (the
decafluoro derivative of 1) twist their peripheral rings out of
the plane of the pyrazine ring to reduce the intermolecular
Figure 11. Residual electron density appearing between the ethenyl
spacers of two neighboring molecules in polymorph 5b; contour
surfaces are plotted at 0.8 e Å-3 (blue) and -0.8 e Å-3 (red). Note
that only one-half of each of the molecules is represented.
Figure 9. (a) The shifted stacks based on methoxy methoxy inter-
3 3 3
actions and (b) the herringbone pattern generated by CH O contacts
3 3 3
in 5a.
Figure 10. The polymerizable chains based on the πpyrazine πphenyl synthon, and the CH O contacts which connect the chains in 5b.
3 3 3 3 3 3
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One such interaction is present in 2a, in 3b one works together
with the CH N and the two methoxy methoxy interac-
3 3 3
3 3 3
tions mentioned above, and in 6 two of them join the methox-
y
methoxy contact in forming the stairlike stacks. These -
3 3 3
OCH π interactions clearly also play a part in preventing the
formation of the πpyrazine πphenyl synthon, thereby assisting
the methoxy methoxy interactions.
3 3 3
3 3 3
3 3 3
Polymorph 5b clearly sets itself apart from the other four.
There are neither methoxy methoxy nor -OCH3 π con-
3 3 3 3 3 3
tacts, despite the large number of such groups; two of the
methoxy groups are involved as acceptor in four CH
O
3 3 3
interactions with C-H bonds of the peripheral rings and the
ethenyl spacers. The absence of methoxy methoxy or -
3 3 3
OCH3 π contacts allows the structure to switch on the
3 3 3
3 3 3
πpyrazine πphenyl synthons, leading to its observed photoinst-
ability. As 5 and 6 contain the same number of methoxy groups,
their relative positions must be the reason for their different
behavior. Indeed, both 5 and 6 enjoy the enhanced polarization
of the DSP backbone due to the increased number of electron-
donating groups, favoring the π π synthon, but in 6 the
3 3 3
methoxy group in the 6-position collaborates with the nitrogen
atom of the pyrazine ring to create a “pocket” of increased
electron density in which the 4-methoxy group gladly positions
itself, thereby efficiently eliminating the possibilities for π
stacking. The structure then reverts to using -OCH3
π
π
3 3 3
3 3 3
contacts as the main organizational tool, just like E,E-2,5-
dimethoxy-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]benzene
(CSD refcode BEZYAH28), its most closely related DSB analo-
gue. The absence of methoxy groups in the 2-and 6-positions and
the rather awkward orientation of the methoxy group in the
4-position prevent 5 from adopting a similar structure, which
leaves the door wide open for π π stacking in 5b.
Finally, even though our crystal engineering approach suc-
ceeded in dispensing with the polymorphs based on the typical
3 3 3
Figure 12. (a) The intermolecular contacts between the methoxy
group in the 4-position and N1 and O6 (the top and bottom molecules
have been only partially represented), and (b) the stairlike stacks formed
pyrazine hydrogen-bonded synthon and the πpyrazine πphenyl
3 3 3
by CH π contacts in 6.
3 3 3
synthon (at least for two of the three compounds), the introduc-
tion of more methoxy substituents did not lead to the exlusion of
polymorphism, that is, a situation in which one polymorph is
considerably more energetically favorable than any other. The
two polymorphic modifications of 3 are remarkably similar: in
both structures infinite chains are formed via the methoxy groups
in the 4-positions contacting the methoxy groups in the 3-posi-
tions. Then, the main difference between 3a and 3b is the manner
in which these chains are held together: in 3a this is achieved by
two CH π contacts, while in 3b only one CH π contact
distance; this option is not used by any of the other DSPs under
investigation.
The second expected effect, the switching off of the πpyrazine
πphenyl synthon (Figure 2a) seen in 1b and 2b, as a result of
3 3 3
the increased number of favored reciprocal methoxy methoxy
3 3 3
contacts, has also been achieved, at least in part: in just one (5b)
of the five structures of compounds 3, 5, and 6 this synthon can
be observed. Indeed, our earlier suggestion15,16 that methoxy
groups tend to favor contacts among each other in methoxy-
substituted DSBs can now be extended to include three DSPs
(besides 2), as the supramolecular structures of 3a, 3b, 5a, and 6
3 3 3
3 3 3
exists next to two CH O and one CH N weak hydrogen
3 3 3
3 3 3
bonds. On the other hand, the two polymorphic modifications of
5 are considerably more different. While the structure of 5a is
mainly dictated by weak hydrogen bonds involving the methoxy
all depend heavily on intermolecular CH O contacts between
3 3 3
methoxy groups. In 3a and 3b these interactions lead to the
formation of the chains of molecules (Figures 7 and 8), and in 3b
groups, in 5b the πpyrazine πphenyl synthon is favored over
3 3 3
two extra methoxy methoxy contacts assist in the organiza-
methoxy methoxy interactions.
3 3 3
3 3 3
tion of these chains into the observed three-dimensional struc-
ture (Figure 8). In 5a one such contact assists in generating the
herringbone pattern which interlinks the shifted stacks of mole-
cules (Figure 9b), while these stacks themselves are based on two
4. CONCLUSIONS
A clear analogy between the structures and polymorphs
of E,E-2,5-bis(2-phenylethenyl)pyrazine (1) and E,E-2,5-bis-
[2-(4-methoxyphenyl)ethenyl]pyrazine (2) has emerged from
a detailed comparison of their supramolecular structures. The
addition of one methoxy group to each of the benzene rings upon
going from 1 to 2 proved to be a small enough change for 2 to
assimilate both possible crystal packings of 1, based on the
methoxy methoxy contacts (Figure 9a). In 6 one such inter-
3 3 3
action cooperates with the CH N contact mentioned above
3 3 3
(Figure 12a), and a second is involved in the formation of the
stairlike stacks (Figure 12b).
Besides these methoxy methoxy contacts these substitu-
3 3 3
ents are involved in a second synthon also seen in methoxy-
substituted DSBs, that is, -OCH3 π interactions.15,16,28,29
pyrazine chain and πpyrazine πphenyl synthons. The pyrazine
3 3 3
3 3 3
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Crystal Growth & Design
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hydrogen-bonded synthon is easily deactivated by the further
addition of one or two methoxy group(s) to each of the
peripheral rings in a position other than the para position.
(11) Sasada, Y.; Shimanou., H; Nakanish., H; Hasegawa, M. Bull.
Chem. Soc. Jpn. 1971, 44, 1262–1270.
(12) Scaccianoce, L.; Feeder, N.; Teat, S. J.; Marseglia, E. A.;
Grimsdale, A. C.; Holmes, A. B. Acta Crystallogr. C 2000, 56, 1277–1279.
(13) Schmidt, G. M. J.; Penzien, K. Angew. Chem., Int. Ed. 1969, 8,
608.
(14) Stezowski, J. J.; Peachey, N. M.; Goebel, P.; Eckhardt, C. J.
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However, the π π synthon, which brings about a photosensi-
3 3 3
tive structure, can only be switched off if the methoxy groups can
dominate the supramolecular structure through reciprocal inter-
actions, as in 3a, 3b and 5a, or via favorable interactions with the
nitrogen atom, as in the “pocket” of increased electron density in
the structure of 6. In general, though, the role of the nitrogen
atom has become subordinate, and it is no longer the driving
force behind the observed three-dimensional organization, which
it is in 1 and 2. Thus, the crystal engineering strategy in which
Des. 2006, 6, 241–246.
(17) CAD-4 EXPRESS; Enraf-Nonius: Delft, The Netherlands,1989.
(18) Harms, K.; Wocadlo, S. XCAD4; Universiteit Marburg:
Duitsland, 1996.
(19) APEX2, SADABS and SAINT-Plus; Bruker AXS Inc.: Madison,
Wisconsin, USA, 2008.
methoxy groups are used to generate intermolecular CH
O
3 3 3
and -OCH3 π contacts and switch off others has been
3 3 3
successful. On the other hand, polymorphism remains: based
on the fact that four sets of polymorphs were observed for five
compounds, it is clear that polymorphism and distyrylpyrazines
are closely associated.
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tEngComm 2011, 13, 702–710.
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis details for 2 and 4;
b
crystallographic information file. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax þ32.3.265.23.10, e-mail frank.blockhuys@ua.ac.be.
(28) Vande Velde, C. M. L.; Chen, L. J.; Baeke, J. K.; Moens, M.;
Dieltiens, P.; Geise, H. J.; Zeller, M.; Hunter, A. D.; Blockhuys, F. Cryst.
Growth Des. 2004, 4, 823–830.
(29) Vande Velde, C. M. L.; Baeke, J. K.; Geise, H. J.; Blockhuys, F.
Acta Crystallogr. C 2005, 61, o284–o287.
’ ACKNOWLEDGMENT
A.C. wishes to thank the Institute for the Promotion of
Innovation by Science and Technology in Flanders (IWT) for
his predoctoral grants. Financial support by FWO-Vlaanderen
under Grant G.0129.05 and by the University of Antwerp under
Grant GOA-2404 is gratefully acknowledged.
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