Synthesis, structure and luminescence of novel 2D co-crystals based on asymmetric 5-substituted pyrimidines and isophthalic acid: Alternate arrangement and π-π Interactions

Article abstract of DOI:10.1016/j.molstruc.2012.11.039

Two novel organic co-crystals (A) (isophthalic acid) (1) and (B) (isophthalic acid) (2), were generated based on asymmetric building blocks 5-(4-pyridyl)pyrimidine (A) and 5-(4-(1-imidazolyl)phenyl)pyrimidine (B) with isophthalic acid, respectively. The building blocks interlinked with isophthalic acid through intermolecular H-bonding interactions to generate infinite chains, which further extended 2D planar networks. In addition, the luminescent properties of A, B and 1, 2 were investigated primarily in the solid state. Compared with the free building blocks, the emission maxima of 1 and 2 have not been changed, but emission intensities of 1 and 2 have decreased. The structure-property relationship indicates the luminescent property is dependent on the intermolecular-ABAB-alternate arrangement and the intermolecular π-π contacting characteristic. By incorporation of conformers into the co-crystals, the results display more interesting tunability of the emission intensities of the building modules in this study.

Full text of DOI:10.1016/j.molstruc.2012.11.039

Journal of Molecular Structure 1035 (2013) 203–208
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, structure and luminescence of novel 2D co-crystals based on
asymmetric 5-substituted pyrimidines and isophthalic acid: Alternate
arrangement and
p–p
interactions
⇑
⇑
Xian-Ping Dai, Gui-Ge Hou , Ju-Feng Sun, Dong Lin, Jing-Tian Han
School of Pharmacy, Binzhou Medical University, Yantai, Shandong 264003, PR China
h i g h l i g h t s
"
"
"
"
Two novel co-crystals (A) (isophthalic acid) (1) and (B) (isophthalic acid) (2) were generated and characterized.
The crystal packing in 1, 2 generated infinite zigzag chains, further 2D planar networks.
Emission intensities of 1, 2 distinctly decrease compared with 5-substituted pyrimidines.
The luminescent property is dependent on the intermolecular – ABAB – alternate arrangement and p–p contacting characteristic.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2012
Received in revised form 13 November 2012
Accepted 19 November 2012
Available online 28 November 2012
Two novel organic co-crystals (A) (isophthalic acid) (1) and (B) (isophthalic acid) (2), were generated
based on asymmetric building blocks 5-(4-pyridyl)pyrimidine (A) and 5-(4-(1-imidazolyl)phenyl)pyrim-
idine (B) with isophthalic acid, respectively. The building blocks interlinked with isophthalic acid through
intermolecular H-bonding interactions to generate infinite chains, which further extended 2D planar net-
works. In addition, the luminescent properties of A, B and 1, 2 were investigated primarily in the solid
state. Compared with the free building blocks, the emission maxima of 1 and 2 have not been changed,
but emission intensities of 1 and 2 have decreased. The structure–property relationship indicates the
luminescent property is dependent on the intermolecular – ABAB – alternate arrangement and the inter-
Keywords:
Co-crystals
Asymmetric building blocks
Crystal structures
Luminescence properties
molecular p–p contacting characteristic. By incorporation of conformers into the co-crystals, the results
display more interesting tunability of the emission intensities of the building modules in this study.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
intensity of an organic material is important to achieve multi-color
materials or intensity-tunable fluorescent materials, and ulti-
Due to their strength and directionality, hydrogen bonds have
been widely used in the construction of supramolecular aggrega-
tion [1–5]. During the past few decades, Etter’s set of rules for
hydrogen bonding interaction in organic crystals [6], Desiraju’s
concept of ‘supramolecular synthons’ [7] and Zaworotko’s defini-
tion of ‘‘co-crystal’’ [8] have been brought forward. Co-crystals
are able to offer the aggregation to modify the physical properties
of subset involved without changing the compound structure.
Some of them have shown encouraging potential in pharmaceuti-
cal application and solid-state organic synthesis [9–12]. However,
investigation of co-crystals as luminescent materials is extremely
rare [13–15]. In terms of fundamental studies and practical appli-
cations, the ability to tune and control the luminescent color or
mately meet the requirement for next generation light-emitting
materials [16]. To obtain potential organic co-crystals with out-
standing luminescent properties is a challenge for scientists. Re-
cent advances in organic co-crystals have indicated that the
intermolecular interactions and molecular stacking patterns in
the solid state play a key role in the observed bulk luminescent
characteristics. Co-crystals are molecular solids composed of at
least two types of neutral chemical species. In organic co-crystals,
incorporation of different coformers with the different functional
groups and orientations may profoundly influence the structural
assemblies and luminescent properties because of their different
hydrogen bonding capability and steric/electronic effect. Therefore,
the exploration of novel and patentable co-crystals with lumines-
cent properties is very significant.
Previously, some symmetric pyrimidine derivatives, such as
5,50-dipyrimidine,1,2-bis(50-pyrimidyl)ethyne, were occasionally
used as ‘‘linear’’ building blocks in supramolecular chemistry and
⇑
Corresponding authors.
E-mail addresses: guigehou@163.com (G.-G. Hou), hanjingtian002@163.com
(J.-T. Han).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.11.039

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X.-P. Dai et al. / Journal of Molecular Structure 1035 (2013) 203–208
Table 1
Crystallographic data for 1–2.
the construction of co-crystals [17–19]. However, the hydrogen
bonding driven organic co-crystals based on asymmetric pyrimi-
dine derivatives have not been received much attention [20]. Thus
the inclusion of different functional groups, such as pyrimidine,
pyridine and imidazole, may lead to the different and patentable
co-crystals with versatile structures and potential properties.
In addition, some aromatic carboxylic acids as hydrogen-bond
donors have been selected to investigate hydrogen-bond topology
and dimensionality [21,22]. In this study, we report two novel co-
crystals, namely [(A) (isophthalic acid)] (1) and [(B) (isophthalic
acid)] (2) based on A, B with isophthalic acid.
Compound
1
2
Empirical formula
CCDC Deposit no.
Color/shape
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
(C9H7N3)ꢁ(C8H6O4)
875334
Yellow, block
323.30
(C13H10N4)ꢁ(C8H6O4)
855281
Colorless, plan
388.38
298(2)
Monoclinic
P21/c
10.5685(19)
13.080(2)
13.537(2)
90
104.649(3)
90
1810.5(6)
4
1.425
298(2)
Triclinic
P1ꢀ
6.6918(16)
7.4746(18)
16.150(4)
80.283(4)
88.014(3)
68.827(3)
742.2(3)
2
b (Å)
c (Å)
a
(°)
b (°)
2. Experimental
c
(°)
Volume (Å3)
2.1. Materials and methods
Z
q
l
calc. (g/cm3)
(Mo K
) (mmꢀ1
1.447
0.106
336
A and B was prepared according to a literature [23]. Infrared (IR)
samples were prepared as KBr pellets, and spectra were obtained
in the 400–4000 cmꢀ1 range using a Bruker tensor-27 FTIR spec-
trometer. 1H NMR data were collected using a Bruker Avance-
400 spectrometer. Chemical shifts were reported in d relative to
TMS. Elemental analyses were performed on a Perkin–Elmer Model
240c analyzer. All fluorescence measurements were carried out on
a Cary Eclipse Spectrofluorimeter (Varian, Australia) equipped with
a xenon lamp and quartz carrier at room temperature. XRD pattern
were obtained on a Rigaku D/Max-rB X-ray powder diffraction
a
)
0.102
808
F(000)
Limiting indices
ꢀ7 6 h 6 8, ꢀ9 6 k 6 8, ꢀ12 6 h 6 10,
ꢀ19 6 l 6 19
ꢀ15 6 k 6 15,
ꢀ15 6 l 6 16
0.30 ꢂ 0.22 ꢂ 0.08
1.99–25.50
Crystal size (mm)
Theta range for data
collection (°)
Completeness to h
Reflections collected
Independent
0.30 ꢂ 0.14 ꢂ 0.06
2.56–25.49
98.2%
3916
2705 [R(int) = 0.0274]
99.7%
9348
3357 [R(int) = 0.0472]
reflections
Data/restraints/
parameters
2705/0/219
3357/0/263
(XRD) with Cu Ka radiation (k = 1.5405 & Aring). The yields were
calculated from the crystalline samples after removal of the solvent
under vacuum and the molar amounts initially introduced.
Goodness-of-fit on F2 1.061
1.106
Final R indices
[I > 2sigma(I)]
R indices (all data)
R1 = 0.0524,
R1 = 0.0725, wR2 = 0.1456
wR2 = 0.1326
R1 = 0.0634,
wR2 = 0.1419
R1 = 0.1148, wR2 = 0.1628
0.182 and ꢀ0.230
2.2. Preparation of co-crystals 1–2
Largest diff. peak and 0.189 and ꢀ0.247
hole e.Åꢀ3
A CH2Cl2 and CH3CN solution (10 mL, 1:1, v/v) of A (15.7 mg,
0.1 m mol) with isophthalic acid (16.6 mg, 0.1 m mol) or
B
P
P
P
P
2
2
1=2
a R1
¼
jjFoj ꢀ jFcjj= jFoj. wR2 ¼ f ½wðF2oÞ ꢀ Fc2Þ ꢃ= ½wðFo2Þ ꢃg
.
(22.2 mg, 0.1 m mol) with isophthalic acid (16.6 mg, 0.1 m mol),
was kept at room temperature. Upon slow evaporation of the sol-
vent about 5 days, colorless crystals 1–2 were obtained,
respectively.
radiation, k = 0.71073 Å). The raw frame data for 1–2 were inte-
grated into SHELX-format reflection files and corrected for Lorentz
and polarization effects using SAINT [24]. None of the crystals
showed evidence of crystal decay during data collection. All struc-
tures were solved by a combination of direct methods and differ-
ence Fourier syntheses and refined against F2 by the full-matrix
least squares technique. Crystal data, data collection parameters,
and refinement statistics for 1, 2 are listed in Table 1. Relevant
hydrogen-bonding geometries for 1–2 are shown in Table 2.
2.2.1. Co-crystal 1
Yield: 80%. IR (KBr Pellet cmꢀ1): 3076(s), 2814(s), 2457(br),
1880(br), 1705(s), 1603(s), 1575(s), 1415(s), 1281(s), 1254(s),
1188(s), 1070(s), 999(m), 827(s), 725(s). 1H NMR (400 MHz, DMSO,
25 °C, TMS, ppm): 13.26 (s, 2H, –COOH), 9.29 (d, 3H, –C4H3N2), 8.74
(d, 2H, –C5H4N), 8.48 (s, 1H, –C6H4), 8.17 (d, 2H, –C6H4), 7.89 (d, 2H,
–C5H4N), 7.64 (m, 1H, –C6H4). Elemental analysis (%) calcd. for
C17H13N3O4 (323.30): C 63.15, H 4.05, N 12.99; Found: C 63.43, H
4.01, N 12.78.
3. Results and discussion
2.2.2. Co-crystal 2
3.1. Structural analysis
Yield: 85%. IR (KBr Pellet cmꢀ1): 3133(s), 2493(br), 1891(br),
1704(s), 1613(m), 1529(m), 1402(s), 1322(s), 1280(s), 1118(s),
1056(s), 828(s), 760(s), 733(m), 683(m). 1H NMR (400 MHz, DMSO,
25 °C, TMS, ppm): 13.32 (s, 2H, –COOH), 9.22 (d, 3H, –C4H3N2), 8.49
(s, 1H, –C6H4), 8.40 (s, 1H, –C3H3N2), 8.18 (d, 2H, –C6H4), 7.98 (d,
2H, –C6H4), 7.87 (m, 1H, –C3H3N2, 2H, –C6H4), 7.66 (m, 1H, –
C6H4), 7.16 (s, 1H, –C3H3N2). Elemental analysis (%) calcd. for
3.1.1. Co-crystal [(A) (isophthalic acid)] (1)
Single-crystal structure reveals that co-crystal 1 exhibits an 1:1
stoichiometric ratio of components, corresponding to the formula
(A) (isophthalic acid) (Fig. 1). In 1, the desired O–Hꢁ ꢁ ꢁN (O(2)–
H(2A)ꢁ ꢁ ꢁN(1) and O(3)–H(3)ꢁ ꢁ ꢁN(2)) hydrogen bonding systems
are formed between A and isophthalic acid into a zigzag chain with
a C22(16) motif extended along the crystallographic [01ꢀ1] axes
(Fig. 2a). In A molecule, pyrimidyl and pyridyl groups are not fully
coplanar, and the dihedral angle between them is approximately
23.8°.
C21H16N4O4 (388.37): C 64.94, H 4.15, N 14.42; Found: C 64.56, H
4.34, N 14.67.
2.3. Single-crystal structure determination
In the solid state, adjacent chains are connected to each other
through weak interchain C(9)–H(9)ꢁ ꢁ ꢁO(4) [25,26] hydrogen bonds
into a H-bonding-driven parquet-like network extended in the
crystallographic ac plane (Fig. 2b). The parquet-like grid displays
Suitable single crystals of 1–2 were selected and mounted in air
onto thin glass fibers. X-ray intensity data of 1–2 were measured at
293 K on a Bruker SMART APEX CCD-based diffractometer (Mo K
a

X.-P. Dai et al. / Journal of Molecular Structure 1035 (2013) 203–208
205
Table 2
Relevant hydrogen-bonding geometries (Å, °) found in 1–2.
D–Hꢁ ꢁ ꢁA
d (D–H)
d
d
<(DHA) Symmetry code
(Hꢁ ꢁ ꢁA) (Dꢁ ꢁ ꢁA)
1
O(2)–H(2A)ꢁ ꢁ ꢁN(1)i
O(3)–H(3)ꢁ ꢁ ꢁN(2)ii
C(9)iii–H(9)iiiꢁ ꢁ ꢁO(4)
0.82
0.82
0.93
1.83
1.88
2.36
2.650(2) 176.9 i: ꢀx, ꢀy, ꢀz + 1
2.702(2) 178.8 ii: ꢀx, ꢀy + 1, ꢀz
3.209(2) 150.8 iii: ꢀx + 1, ꢀy + 1,
ꢀz
2
O(1)–H(1A)ꢁ ꢁ ꢁN(1)i
0.82
0.82
1.78
2.602(3) 175.4 i: x + 1, ꢀy + 3/2,
z + 1/2
O(4)–H(4)ꢁ ꢁ ꢁN(3)ii
1.86
2.49
2.678(3) 173.5 ii: x ꢀ 1, y, z
3.279(3) 142.4 iii: ꢀx + 2, ꢀy + 1,
ꢀz + 1
C(20)–H(20)ꢁ ꢁ ꢁO(2)iii 0.93
C(2)iv–H(2)ivꢁ ꢁ ꢁO(1)
C(3)iv–H(3)ivꢁ ꢁ ꢁO(3)
0.93
0.93
2.38
2.41
2.72
3.298(3) 168.2 iv: ꢀx + 1, y + 1/
2, ꢀz + 1/2
3.328(3) 166.5 iv: ꢀx + 1, y + 1/
2, ꢀz + 1/2
C(8)iv–H(8)ivꢁ ꢁ ꢁN(4)ii 0.93
3.521(4) 144.6 ii: x ꢀ 1, y, z iv:
ꢀx + 1, y + 1/2,
ꢀz + 1/2
Fig. 2. (a) The C22(16) zigzag chain extended along the crystallographic [01ꢀ1] axes.
(b) The H-bonded network with R66(40) grids in 1. (Symmetry code: i: ꢀx, ꢀy, ꢀz + 1;
ii: ꢀx, ꢀy + 1, ꢀz; iii: ꢀx + 1, ꢀy + 1, ꢀz.)
hydrogen bonds. This chain exhibits a C22(19) motif. However, obvi-
ous structural difference of the 2D hydrogen-bonding sheet along
the crystallographic ab plane from robust 1 can be found in 2
(Fig. 4b). Complicated hydrogen bonding motifs are shown in
Fig. 4b. Two isophthalic acids connect each other through two
groups of C(20)–H(20)ꢁ ꢁ ꢁO(2) bonds to generate an R22(10) motif.
Two hydroxy groups of two isophthalic acids participate in an
intermolecular C(2)–H(2)ꢁ ꢁ ꢁO(1) and O(1)–H(1A)ꢁ ꢁ ꢁN(1) hydrogen
bonds with the imidazole groups to form an R44(10) ring. And
R44(28) grids are formed in the 2D sheet, lightly smaller than the
R66(40) grids of 1. In addition, the same isophthalic acid links an
imidazole group to an R22(11) motif. Therefore, isophthalic acid
and imidazole plane are almost coplanar, proved by the dihedral
angle 6.6° between them. In the 2D sheet, an R33(14) motif is formed
through intermolecular hydrogen bonds. So isophthalic acid and
pyrimidine group are almost coplanar (the dihedral angle ca.
4.4°). The auxiliary O–Hꢁ ꢁ ꢁN and C–Hꢁ ꢁ ꢁO interactions may hold
out the rigidity of such hydrogen-bonding motifs and thus result
in the coplanarity of B and isophthalic acid. Detailed hydrogen
bonds are shown in Fig. 4 and relevant hydrogen-bonding geome-
tries are given in Table 2. For B, the asymmetric hydrogen-bonding
interactions of both terminal groups influence its coplanarity,
which can be confirmed by the dihedral angles from imidazole
and pyrimidine compared with central phenyl ring, 20.8° and
21.2°, respectively. Due to the longer dimension of B, relatively
Fig. 1. The ORTEP figure of 1 (displacement ellipsoids with 30% probability).
an R66(40) motif. Relevant hydrogen-bonding geometries are given
in Table 2. In this network, the isophthalic acid plane is basically
parallel from H-bond linked pyrimidyl groups (the dihedral angle
approximately 2.3°), while it is angulate with H-bond linked pyri-
dyl group (the dihedral angle approximately 22.0°).
3.1.2. Co-crystal [(B) (isophthalic acid)] (2)
One of the important issues in determining the dimensions of
porous frameworks is the scale of the building blocks. In principle,
the lengthening conjugated spacers used, the larger pore dimen-
sions would be obtained. To achieve novel frameworks with versa-
tile structures and potential properties, we selected building block
B with longer dimension, which has asymmetric pyrimidine and
imidazole groups.
As shown in Fig. 3, co-crystal 2 consists of one crystallographi-
cally independent B and one isophthalic acid molecule, corre-
sponding to the formula (B) (isophthalic acid). A similar zigzag
chain with 1 along the crystallographic a axes can be found in
Fig. 4a. Isophthalic acid links two B molecules to generate a zigzag
chain through O–Hꢁ ꢁ ꢁN (O(1)–H(1A)ꢁ ꢁ ꢁN(1) and O(4)–H(4)ꢁ ꢁ ꢁN(3))
Fig. 3. The ORTEP figure of 2 (displacement ellipsoids with 30% probability).

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X.-P. Dai et al. / Journal of Molecular Structure 1035 (2013) 203–208
Fig. 4. (a) The C22(19) zigzag chain along the crystallographic a axes in 2. (b) The H-
bonded network of 2. (Symmetry code: i: x + 1, ꢀy + 3/2, z + 1/2; ii: x ꢀ 1, y, z; iii:
ꢀx + 2, ꢀy + 1, ꢀz + 1; iv: ꢀx + 1, y + 1/2, ꢀz + 1/2.)
Fig. 5. The PXRD patterns (black lines) obtained from the as-synthesized solids of 1
and 2 and the simulated PXRD patterns (blue lines) from single crystals of 1 and 2.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
complicated hydrogen bonding systems are build in 2 but not the
simple parquet-like network in 1. Whereas, these hydrogen-bond-
ing self-assembly systems can provide an excellent advantage to
study the interesting luminescent properties.
As described in the experimental section, co-crystals 1 and 2
were prepared by direct assembly in CH2Cl2 and CH3CN solution
under the ambient condition and crystallization via solvent evapo-
ration. 1H NMR spectra of the solution were collected on crystalline
samples of 1 and 2. 1H NMR spectra indicated that the chemical
shift values were completely identical to that of the original mate-
rials [23], indicating the formation of the neutral and discrete mol-
the hydrogen bonding interactions. In 1, pyrimidyl and pyridyl
groups of A molecule are not fully coplanar, proved by the dihedral
angle approximately 23.8°. Isophthalic acid plane is basically angu-
late with H-bond linked pyridyl group (the dihedral angle approx-
imately 22.0°). Therefore, after introduction of the coformer,
electron transfer is obviously affected herein due to intramolecular
and intermolecular distortion. So, 1 displays distinctly weak
emission intensity than A in the solid state. The similar – ABAB –
alternate arranging disposition can be found in 2. However, the
molecular conformations and the planar characteristic of 2D
hydrogen-bonding sheet could even more affect the emission
property of 2 in the solid state. In other words, the complex hydro-
gen bonding characteristic affects the intramolecular and intermo-
lecular coplanar level as well as the mutual rigidification of
components. As described above, several hydrogen-bonding mo-
tifs, such as C22(19), R22(10) and R44(10) motifs hold out the rigidity
of such hydrogen-bonding network and thus result in the coplan-
arity of B and isophthalic acid. The isophthalic acid planes are al-
most coplanar from imidazole and pyrimidine, respectively,
proved by the dihedral angles ca. 6.6° and ca. 4.4°. However, com-
pared with middle phenyl group in B molecule, larger dihedral an-
gles are formed from imidazole and pyrimidine, respectively, ca.
20.8° and ca. 21.2°. Actually, 2 displays distinctly weak emission
intensity than B in the solid state because of the introduction of
isophthalic acid. Therefore, by introduction of the coformers, the
arranging disposition and coplanarity of components within the
original co-crystals have been changed at different levels, and thus
the solid-state luminescent properties can be further tuned.
ecules in solution. Both
A and B form co-crystals in 1:1
stoichiometric ratio with isophthalic acid, corresponding to their
molecular formulas obtained from crystal analysis. As a matter of
fact, the phase purity of the bulk crystalline solid was well con-
firmed by powder X-ray diffraction (PXRD) technique. As shown
in Fig. 5, the PXRD patterns of 1 and 2 obtained from the bulk crys-
talline solid at room temperature are basically identical to those of
simulated ones based on single-crystal X-ray diffraction data.
3.2. Luminescent properties of A, B, 1 and 2
The solid-state emission spectra of co-crystals 1 and 2 with
approximate 25 mg crystalline samples, respectively, were investi-
gated primarily at room temperature. Fig. 6 presents the compari-
son of the emission spectra of co-crystal 1 with A and 2 with B,
respectively. In the solid state, co-crystal 1 exhibits emission max-
imum at ca. 487 nm upon excitation at k = 389 nm. Compared to A
[23] and 1, the emission maxima of B [23] and 2 are clearly
red-shifted and 2 exhibits emission maximum at ca. 510 nm upon
excitation at k = 394 nm. The lengthening conjugated spacer by
insertion of an additional phenyl space in B may result in the
increasing electron-transporting effect of extended conjugated sys-
tems along with the red-shifted color in B and 2. Compared to
emission maxima of A (484 nm) or B (506 nm) [23], the emission
maxima are not any change, but emission intensities of 1 and 2 dis-
tinctly decrease.
On the other hand, their different intermolecular
ing characteristic in the co-crystals can influence
tron-transport. It is well known that the intermolecular
p
–
p
contact-
p
and pꢄ elec-
p–p
interaction in the organic molecular packing structure is the criti-
cal factor that determines the solid state emission colours and
emission intensities [27]. The parallel stack and the centroid–
centroid distances of the two building modules through the
On the one hand, the – ABAB – alternate arranging disposition
between building modules and isophthalic acids in co-crystals
badly influenced the intermolecular electron-transport in spite of
intermolecular p–p interactions are listed in Fig. 7. These building
module molecules are parallel packing in – ABAB – stacking
fashion from isophthalic acids. The centroid–centroid distances

X.-P. Dai et al. / Journal of Molecular Structure 1035 (2013) 203–208
207
Fig. 6. Solid-state photoinduced emission spectra of A with 1 (excitation 389 nm) and B with 2 (excitation 394 nm) at room temperature.
H-bonding-driven co-crystals compared to the pure building mod-
ules. The strategy provides a facile way to design and develop new
types of solid fluorescent materials.
4. Conclusions
In this study, two novel 2D organic co-crystals were generated
based on two asymmetric 5-substituted pyrimidines A and B with
isophthalic acid, respectively. A and B with different functional
groups, such as pyrimidine, pyridine and imidazole, provide great
advantage on the hydrogen-bonding self-assembly system. In
other words, the asymmetric structures, the different functional
groups and the different dimensions of the building modules can
profoundly affect the hydrogen bonding supramolecular recogni-
tion and further crystal packing. At the same time, isophthalic acid
as hydrogen-bond donor also shows its particular advantage in
investigating hydrogen-bond topology and dimensionality. As de-
scribed above, the asymmetric building modules A and B interlink
with isophthalic acid through intermolecular H-bonding interac-
tions to generate infinite chains, and further extend 2D planar net-
works in 1, 2.
Both co-crystals display interesting luminescence in the solid
state. The emission intensities of the asymmetric building modules
are significantly influenced by their incorporation of conformers
into the co-crystals. The structure–property relationship of these
co-crystal systems is summarized, which indicates that the solid
state emission intensity is dependent on the molecular arranging
Fig. 7. View of the characteristic of intermolecular
and 2, respectively.
p–p stacking interactions in 1
are ca. 3.76 Å, 3.79 Å in 1 and 3.81 Å, 3.94 Å in 2. The weak
stacking interactions enhance the rigidification of -conjugated
systems for the realization of
and pꢄ electron-transport.
Whereas, the separated effect from isophthalic acids in – ABAB –
stacking chain maybe seriously affect
and pꢄ electron-transport
p–p
p
disposition and the intermolecular
p–p contacting characteristic.
p
The results also display more interesting tunability of the emission
intensity of the building modules by the incorporation of conform-
ers into the H-bonding-driven co-crystals. It might have potential
applications for the organic crystal engineering to construct pat-
entable crystals with interesting luminescent properties.
p
between building modules. Actually, the emission intensities of
the co-crystals distinctly decrease.
Except for the different molecular arranging disposition and the
different intermolecular
p–p contacting characteristic, other fac-
Acknowledgments
tors (such as the different molecular conformations, hydrogen
bonding interactions and particle size of crystalline samples) could
also affect the emission properties of co-crystals in the solid state
[28]. Although we cannot give further explanations for the
relationship between the solid-state emission properties and
the molecular packing, conformations, coformers, it is clear that
the luminescent properties of co-crystals are basically related
to the introduction of the coformers. The introduction of the co-
formers can change the geometric arrangement of components in
the co-crystal, which present new insight into the structure–prop-
erty relationship of these co-crystal systems. Notably, they display
more interesting tunability of the emission intensity of the
building modules by the incorporation of conformers into the
We are grateful for financial support from the National Natural
Science Foundation of China(Grant Nos. 30970298, 31170320), the
Foundation of Shandong province (No. J11LF27), the Foundation of
Yantai City (No. 2011076) and the Scientific Foundation of Binzhou
Medical University (No. BY2011KYQD08).
Appendix A. Supplementary material
CCDC 875334 (1), 855281 (2) contain the supplementary crys-
tallographic data for this paper. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge

208
X.-P. Dai et al. / Journal of Molecular Structure 1035 (2013) 203–208
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