Synthesis, structure and luminescence of Co-crystals with hexagonal channels: Arranging disposition and π-π Interactions

Article abstract of DOI:10.1039/c2ce25759a

Co-crystallization of asymmetric building blocks 5-(4-pyridyl)pyrimidine (A) and 5-(4-(1-imidazolyl)phenyl)pyrimidine (B) with resorcinol and hydroquinone, respectively, was investigated. Four new organic co-crystals (1-4) were generated. The X-ray single crystal analysis indicates that the crystal packing in 1, 2 and 4 generates open hexagonal channels with resorcinol or hydroquinone molecules encapsulated through hydrogen bonding interactions. In 3, {B₂(resorcinol)₂} macrocycle is generated from clip-like resorcinol and asymmetric B molecules. In addition, the luminescent properties of A, B and 1-4 were investigated primarily in the solid state. The luminescent property is dependent on the molecular vertical arranging disposition and the intermolecular π-π contacting characteristic. The results display more interesting tunability of the emission intensity of the building modules by the incorporation of coformers into the co-crystals.

Full text of DOI:10.1039/c2ce25759a

CrystEngComm
View Article Online
View Journal
PAPER
Synthesis, structure and luminescence of Co-crystals
with hexagonal channels: arranging disposition and p–p
interactions3
Cite this: DOI: 10.1039/c2ce25759a
Gui-Ge Hou,* Hui-Juan Zhao, Ju-Feng Sun, Dong Lin, Xian-Ping Dai, Jing-Tian Han
and Hui Zhao
Co-crystallization of asymmetric building blocks 5-(4-pyridyl)pyrimidine (A) and 5-(4-(1-imidazolyl)phe-
nyl)pyrimidine (B) with resorcinol and hydroquinone, respectively, was investigated. Four new organic co-
crystals (1–4) were generated. The X-ray single crystal analysis indicates that the crystal packing in 1, 2 and
4 generates open hexagonal channels with resorcinol or hydroquinone molecules encapsulated through
hydrogen bonding interactions. In 3, {B2(resorcinol)2} macrocycle is generated from clip-like resorcinol and
asymmetric B molecules. In addition, the luminescent properties of A, B and 1–4 were investigated
primarily in the solid state. The luminescent property is dependent on the molecular vertical arranging
disposition and the intermolecular p–p contacting characteristic. The results display more interesting
tunability of the emission intensity of the building modules by the incorporation of coformers into the co-
crystals.
Received 15th May 2012,
Accepted 23rd August 2012
DOI: 10.1039/c2ce25759a
www.rsc.org/crystengcomm
obtaining potential organic co-crystals with outstanding
luminescent properties is a challenge for scientists. Recent
Introduction
Hydrogen bonding has been widely used in the construction of
supramolecular aggregation because of their strength and
directionality.1,2 During past decades, Etter’s set of rules for
hydrogen bonding interaction in organic crystals,1a Desiraju’s
concept of ‘supramolecular synthons1b and the definition of
‘‘co-crystal’’2c have been brought forward. A co-crystal is one of
the crystalline materials composed of different molecular
species held together by non-covalent forces in which all
components are solid under ambient conditions when in their
pure form.2c Co-crystals offer aggregation to modify the
physical properties of the subset involved without changing
the compound structure. Some of them exhibit encouraging
potential for application in pharmaceutical science and solid-
state organic synthesis.3 Investigation of co-crystals as
luminescent materials, however, is extremely rare.4 In terms
of fundamental studies and practical applications, the ability
to tune and control the luminescent color or intensity of an
organic material is important to achieve multi-color materials
or intensity-tunable fluorescent materials, and further to meet
the need of next generation light-emitting materials.5 So,
advances in organic co-crystals have indicated that the
intermolecular interactions or 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. Incorporation of
different coformers with the different functional groups and
orientations may profoundly influence the structural assem-
blies and luminescent properties because of their different
hydrogen bonding capabilities and steric/electronic effects. So,
the exploration of the synthesis of new and patentable co-
crystals, and furthermore their luminescent properties are very
significant.
Previously, some symmetric pyrimidine derivatives as
‘‘linear’’ building blocks, such as 5,59-dipyrimidine and 1,2-
bis(59-pyrimidyl)ethyne, have occasionally been used in supra-
Binzhou Medical University, Yantai, 264003, P. R. China.
E-mail: guigehou@163.com
Electronic supplementary information (ESI) available. CCDC reference
3
numbers 855279, 855280, 874492 and 874493. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2ce25759a
Scheme 1 Building modules A, B and co-crystals 1–4.
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm

View Article Online
Paper
CrystEngComm
molecular chemistry and the construction of co-crystals.6 In
contrast, hydrogen bonding-driven organic co-crystals based
on asymmetric pyrimidine derivatives have not received much
attention.7 Thus, the inclusion of different functional groups,
such as pyrimidine, pyridine and imidazole, can lead to
different and patentable co-crystals with versatile structures
and potential properties.
Additionally, some organic hydroxybenzene and aromatic
carboxylic acids have been selected as hydrogen-bond donors
to investigate hydrogen-bond topology and dimensionali-
ty.4c,7,8 In this contribution, an effective strategy for getting
novel organic co-crystals by designing and controlling the
orientation of coformers and dimension of building blocks is
carried out and we report four new co-crystals 1–4 (Scheme 1),
7.69 (d, 2H, –C6H4), 7.57–7.54 (d, 2H, –C6H4), 7.34 (s, 1H, –
C3H3N2), 7.24 (s, 1H, –C3H3N2). Elemental analysis (%) cald for
C13H10N4: C 70.26, H 4.53, N 25.21; found: C 69.89, H 4.62, N
25.35.
Preparation of co-crystals 1–4
A CH2Cl2 and CH3CN solution (10 mL, 1 : 1, v/v) of A (15.7 mg,
0.1 mmol) or B (22.2 mg, 0.1 mmol) with resorcinol (11.0 mg,
0.1 mmol) and hydroquinone (11.0 mg, 0.1 mmol), respec-
tively, was kept at room temperature. Upon slow evaporation
of the solvent about 5 days, colorless crystals 1–4 were
obtained.
Co-crystal 1
Yield: 80%. IR (KBr pellet cm21): 3049(s), 2972(s), 1603(s),
1568(s), 1485(s), 1410(s), 1327(s), 1292(s), 1240(m), 1180(s),
1142(m), 964(m), 870(m), 822(m), 771(m), 723(m). 1H NMR
(400 MHz, DMSO, 25 uC, TMS, ppm): 9.29 (d, 3H, –C4H3N2),
9.16 (s, 1H, C6H6O2), 8.74 (d, 2H, –C5H4N), 7.85 (d, 2H,
–C5H4N), 6.91 (t, 0.5H, C6H6O2), 6.17 (m, 1.5H, C6H6O2).
Elemental analysis (%) calcd for C12H10N3O: C 67.91, H 4.74, N
19.80; found: C 67.54, H 4.87, N 19.89.
namely [(A)?(resorcinol)0.5
] (1), [(A)?(hydroquinone)0.5] (2),
[(B)?(resorcinol)] (3) and [(B)2?(hydroquinone)] (4), based on
A, B with hydroquinone and resorcinol, respectively.
Experimental
Materials and methods
Co-crystal 2
A was prepared according to the literature.9 Infrared (IR)
samples were prepared as KBr pellets and spectra were
obtained in the 400–4000 cm21 range using a Bruker Tensor-
27 FTIR spectrometer. Elemental analyses were performed on
a Perkin-Elmer Model 240c analyzer. 1H NMR data were
collected using Bruker Avance-300 and 400 spectrometers.
Chemical shifts are reported in d relative to TMS. All
fluorescence measurements were carried out on a Cary
Eclipse Spectrofluorimeter (Varian, Australia) equipped with
a xenon lamp and quartz carrier at room temperature.
Yield: 82%. IR (KBr pellet cm21): 3041(s), 2833(s), 2702(s),
1603(s), 1568(s), 1473(s), 1417(s), 1331(m), 1221(s), 1070(m),
825(s), 753(m). H NMR (400 MHz, DMSO, 25 uC, TMS, ppm):
9.28 (d, 3H, –C4H3N2), 8.73 (d, 2H, –C5H4N), 8.60 (s, 1H,
C6H6O2), 7.87 (d, 2H, –C5H4N), 6.54 (s, 2H, C6H6O2). Elemental
analysis (%) calcd for C12H10N3O: C 67.91, H 4.74, N 19.80;
found: C 67.68, H 4.85, N 19.92.
1
Co-crystal 3
Yield: 87%. IR (KBr pellet cm21): 3125(br), 1614(s), 1559(s),
1525(s), 1485(s), 1400(s), 1311(m), 1175(s), 1149(s), 1059(m),
1006(m), 826(s), 773(m), 749(m). 1H NMR (400 MHz, DMSO, 25
uC, TMS, ppm): 9.22 (d, 3H, –C4H3N2), 9.14 (s, 2H, C6H6O2),
8.39 (s, 1H, –C3H3N2), 7.98 (d, 2H, –C6H4), 7.87 (s, 1H,
–C3H3N2), 7.85 (d, 2H, –C6H4), 7.15 (s, 1H, –C3H3N2), 6.91 (t,
1H, C6H6O2), 6.18 (m, 3H, C6H6O2). Elemental analysis (%)
calcd for C19H16N4O2: C 68.66, H 4.85, N 16.85; found: C 68.32,
H 4.67, N 16.98.
Preparation of B
5-Bromopyrimidine (1.59 g, 10.0 mmol), 4-bromophenylboro-
nic acid (2.00 g, 10.0 mmol), [Pd(PPh3)4] (0.23 g, 0.2 mmol) and
K2CO3 (4.14 g, 30.0 mmol) in EtOH/H2O (v/v, 3 : 1, 50 mL) were
placed into a flame-dried Schlenk flask, then heated to reflux
for 48 h. After removal of the solvent under vacuum, the
residue was purified on silica gel using column chromato-
graphy with CH2Cl2/THF (v/v, 10 : 1) as the eluent to give 5-(4-
bromophenyl)pyrimidine as a colorless crystalline solid (yield
Co-crystal 4
1
85%). H NMR (300 MHz, CDCl3, 25 uC, TMS, ppm): 9.23 (s,
Yield: 83%. IR (KBr pellet cm21): 3128(s), 1610(m), 1556(m),
1526(s), 1468(s), 1407(s), 1358(m), 1333(m), 1314 (m), 1251(s),
1H, –C4H3N2), 8.95 (s, 2H, –C4H3N2), 7.68–7.65 (d, 2H, –C6H4),
7.51–7.47 (d, 2H, –C6H4). Elemental analysis (%) cald for
C10H7BrN2: C 51.09, H 3.00, N 11.92; found: C 51.43, H 3.08, N
11.78.
A mixture of 5-(4-bromophenyl)pyrimidine (1.88 g, 8.0
mmol), imidazole (5.40 g, 80.0 mmol), CuSO4 (0.06 g, 0.24
mmol) and K2CO3 (5.52 g, 40.0 mmol) was heated neat with
stirring at 190 uC. After 6 h, the reaction was stopped and the
crude product was dissolved in CH2Cl2/MeOH (v/v, 10 : 1) and
filtrated. The filtrate was removed from the solvent under
vacuum and the residue was purified on silica gel by using
column chromatography with CH2Cl2/THF (v/v, 5 : 1) as the
1
1217(s), 1060(s), 962(m), 835(s), 759(m), 728(m). H NMR (400
MHz, DMSO, 25 uC, TMS, ppm): 9.22 (d, 3H, –C4H3N2), 8.66 (s,
1H, C6H6O2), 8.41 (s, 1H, –C3H3N2), 7.96 (d, 2H, –C6H4), 7.88
(s, 1H, –C3H3N2), 7.85 (d, 2H, –C6H4), 7.16 (s, 1H, –C3H3N2),
6.57 (s, 2H, C6H6O2). Elemental analysis (%) calcd for
C32H26N8O2: C 69.30, H 4.72, N 20.20; found: C 69.03, H
4.61, N 20.36.
Single-crystal structure determination
Suitable single crystals of 1–4 were selected and mounted in
air onto thin glass fibers. X-ray intensity data of 1–4 were
measured at 293 K on a Bruker SMART APEX CCD-based
diffractometer (Mo-Ka radiation, l = 0.71073 Å). The raw frame
1
eluent to give B as a colorless crystalline solid (yield 65%). H
NMR (300 MHz, CDCl3, 25 uC, TMS, ppm): 9.23 (s, 1H,
nC4H3N2), 8.97 (s, 2H, –C4H3N2), 7.92 (s, 1H, –C3H3N2), 7.72–
CrystEngComm
This journal is ß The Royal Society of Chemistry 2012

View Article Online
CrystEngComm
Paper
Table 1 Crystallographic data for 1–4
C
1
12H10N3O
C12H10N3O
2
C19H16N4O2
3
C32H26N8O2
4
Formula
CCDC
fw
Crystal size/mm
Cryst. syst.
a/Å
b (Å)
c/Å
874492
212.23
874493
212.23
0.45 6 0.45 6 0.05
Triclinic
7.352(2)
9.030(3)
9.219(3)
116.322(4)
93.538(4)
108.236(4)
506.3(3)
¯
P1
2
855280
332.36
855279
554.61
0.38 6 0.35 6 0.35
Triclinic
9.8525(19)
10.375(2)
13.982(3)
72.383(3)
89.789(2)
80.764(3)
1343.0(4)
0.26 6 0.24 6 0.18
Monoclinic
9.865(2)
15.655(4)
14.016(3)
90
101.089(3)
90
2124.3(8)
C2/c
0.40 6 0.10 6 0.10
Monoclinic
7.599(2)
9.335(3)
23.306(7)
90
97.817(5)
90
1637.9(8)
P21/n
a (u)
b (u)
c (u)
V/Å3
¯
P1
2
1.371
0.090
Space group
Z value
8
4
r calc. (g cm23
)
1.327
0.089
1.392
0.093
1.348
0.091
m (Mo-Ka)/mm21
T/K
298(2)
1982
0.0594, 0.1398
298(2)
1842
0.0660, 0.1693
298(2)
3058
0.0790, 0.1463
298(2)
4770
0.0546, 0.1343
No. of observations (I . 3s)
Final R indicesa [I . 2s(I)]:R; Rw
a
2
R1 = S ||Fo| 2 |Fc||/S |Fo|. wR2 = {S [w(Fo 2 Fc2)2]/S [w(Fo2)2]}1/2
.
Table 2 Relevant hydrogen-bonding geometries (Å, u) found in 1–4
…
…
…
D–H
A
d(D–H)
d(H A)
d(D A)
,(DHA)
Symmetry code
1
…
…
…
O(1)–H(1A) N(1)
C(1)–H(1) N(3)
C(5)–H(5) N(2)
0.82
0.93
0.93
1.95
2.75
2.63
2.773(3)
3.678(4)
3.492(3)
176.1
172.2
155.2
i
i: 0.5 + x, 20.5 + y, z;
ii: 20.5 + x, 20.5 + y, z
ii
2
i
…
…
…
O(1)–H(1A) N(1)
C(5)–H(5) N(3)
C(9)–H(9) N(2)
0.82
0.93
0.93
1.96
2.59
2.65
2.780(3)
3.516(5)
3.408(4)
173.0
169.0
138.8
i: 2x + 1,2y + 1,2z + 2
ii: x,21 + y, z;
iii: 2x, 2 2 y, 1 2 z
ii
iii
3
i
…
…
…
O(1)–H(1A) N(1)
O(2)–H(2A) N(4)
C(12)–H(12) O(2)
0.82
0.82
0.93
1.94
1.96
2.55
2.736(4)
2.761(4)
3.436(5)
165.1
165.5
160.5
i: 2x + 1,2y + 2,2z + 2
ii: 2x + 1/2, y 2 1/2, 2z + 3/2
ii
4
i
…
O(1)–H(1A) N(5)
C(1)–H(1) N(3)
C(12) –H(12) O(1)
O(2)–H(2A) N(1)
0.82
0.93
0.93
0.82
0.93
0.93
0.93
0.93
1.97
2.49
2.69
1.99
2.63
2.66
2.55
2.50
2.785(3)
3.391(3)
3.480(2)
2.809(3)
3.542(3)
3.470(2)
3.443(2)
3.329(3)
173.6
163.4
143.5
174.6
168.7
146.1
160.4
148.2
i: x + 1, y, z
i: x + 1, y, z
i: x + 1, y, z
ii: x 2 1, y + 1, z
ii: x 2 1, y + 1, z
ii: x 2 1, y + 1, z
iii: 1 + x, 21 + y, z
iv: 21 + x, y, z
i
…
i
i
…
ii
…
…
ii
C(15)–H(15) N(8)
C(25)ii–H(25)ii O(2)
…
iii
…
C(2)–H(2) N(4)
C(14)–H(14) N(7)
iv
…
data for 1–4 were integrated into SHELX-format reflection files
and corrected for Lorentz and polarization effects using
SAINT.10 None of the crystals showed evidence of crystal decay
combination of direct methods and difference Fourier synth-
eses and refined against F2 by the full-matrix least squares
technique. Crystal data, data collection parameters and refine-
ment statistics for 1–4 are listed in Table 1. Relevant hydrogen-
bonding geometries for 1–4 are given in Table 2. CCDC 874492
(1), 874493 (2), 855280 (3) and 855279 (4) contain the
during data collection. All structures were solved by
a
supplementary crystallographic data for this paper .
3
Results and discussion
Structural analysis
CO-CRYSTAL [(A)?(RESORCINOL)0.5] (1). The single-crystal structure
reveals that co-crystal 1 exhibits a 2 : 1 stoichiometric ratio of
Fig. 1 The ORTEP figure of 1 (displacement ellipsoids with 30% probability).
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm

View Article Online
Paper
CrystEngComm
i
…
Fig. 2 (a) The two-dimensional net with hexagonal grids extended in the crystallographic ab plane in 1, interlinked intermolecular C(1)–H(1) N(3) and C(5)–
ii
…
H(5) N(2) hydrogen bonds (symmetry codes: i: 0.5 + x, 20.5 + y, z; ii: 20.5 + x, 20.5 + y, z). (b) The double-network with resorcinol molecules encapsulated through
…
O(1)–H(1A) N(1) hydrogen bonds in 1.
components, corresponding to the formula (A)?(resorcinol)0.5
distances between adjacent planes are 3.85–3.90 Å, which
confirms the intermolecular p–p interactions.
In the double-network, open hexagonal capsules exist and
…
…
(Fig. 1). In 1, the desired weak C–H N (C(1)–H(1) N(3) and
11
…
C(5)–H(5) N(2)) hydrogen bonding systems
are formed
between A molecules into a two-dimensional net consisting
of hexagonal grids with a diagonal distance of approximately
8.7 Å (Fig. 2a). In this hexagonal grid, four unique molecules
have a separation of approximately 8.7 Å, which hopefully form
…
a perfect host pocket to trap resorcinol molecule with an O
O
separation of approximately 4.7 Å. As shown in Fig. 2b,
symmetrical resorcinol molecules are actually encapsulated in
…
are connected by four groups of C–H N bonds to generate an
the host capsule13 through stronger O(1)–H(1A) N(1) hydro-
…
R44 (22) motif. The nets are extended in the crystallographic ab
plane. Interestingly, under the pull from the intermolecular
hydrogen bonding between resorcinol and pyridine, adjacent
two nets are closed through an -AA- manner into a double-
network (Fig. 2b). Just as for the hydrogen bonding pull from
resorcinol, two A molecules are found in a parallel orientation
in head-to-head fashion.12 The corresponding face-to-face
gen bonds. Relevant hydrogen-bonding geometries are given
in Table 2.
CO-CRYSTAL [(A)?(HYDROQUINONE)0.5] (2). To explore the role of the
hydrogen bonding donor orientation of coformers in the
hydrogen bonding supramolecular recognition and further
crystal packing, hydroquinone was investigated primarily. The
asymmetric unit of co-crystal 2 is shown in Fig. 3. It contains
one A and half of hydroquinone in 2. It is different from 1 in
that the A molecules in 2 are linked through the desired weak
…
… …
CnH
N (C(5)–H(5) N(3) and C(9)–H(9) N(2)) hydrogen
bonds into a one-dimensional double-chain extended in the
crystallographic b axis (Fig. 4a). Different hexagonal grids to 1
can be found in 2. Two adjacent pyrimidine rings are
connected by an R44 (6) motif, so the hexagonal grid displays
an R44 (20) motif. The diagonal distance is approximately 9.9 Å.
The linear hydroquinone coformers as linkages link these
double-chains into a two-dimensional sheet (Fig. 4b). The
…
corresponding O(1)–H(1A) N(1) geometries are given in
Table 2. An R66 (36) motif is now formed by two hydroquinone
coformers and two chains. Compared to the bent-shaped
resorcinol in 1, the linear hydroquinone might be the reason
for the formation of the two-dimensional sheet in 2 different
from 1 during the co-crystallization.
Besides H–bonding interactions, the inter-sheet p–p inter-
actions (the centroid–centroid distances, 3.74–3.75 Å) also play
an important role in the crystal packing of 2. As shown in
Fig. 4c, these inter-sheet p–p interactions give rise to a three-
dimensional arrangement with hexagonal channels. The
symmetrical hydroquinone molecules are actually encapsu-
lated in the channels. In another word, the channel frame is
significantly reinforced by the hydrogen bonding interactions
and the inter-sheet p–p interactions.
Fig. 3 The ORTEP figure of 2 (displacement ellipsoids with 30% probability).
CrystEngComm
This journal is ß The Royal Society of Chemistry 2012

View Article Online
CrystEngComm
Paper
ii
Fig. 4 (a) The 1D double chain of 2 extended in the crystallographic b axis, interlinked intermolecular C(5)–H(5) N(3) and C(9)–H(9) N(2)iii hydrogen bonds. (b) The
…
…
i
…
2D sheet linked by linear hydroquinone coformers through O(1)–H(1A) N(1) hydrogen bonds in 2. (c) The inter-sheet p–p interaction-driven 3D network with
hydroquinone molecules encapsulated in 2. (Symmetry codes: i: 1 2 x, 1 2 y, 2 2 z; ii: x,21 + y, z; iii: 2x, 2 2 y, 1 2 z).
In the solid state, these {B2(resorcinol)2} macrocycles
connect each other through O(2)–H(2A) N(4) bonds formed
CO-CRYSTAL [(B)?(RESORCINOL)] (3). One of the important issues in
determining the dimensions of porous frameworks is the scale
of the building blocks. In principle, the longer the spacers
used, the larger pore dimensions obtained would be. To
achieve frameworks with larger-sized channels, we designed
and synthesized building block B, which has two asymmetric
pyrimidine and imidazole groups.
…
by pyrimidine and resorcinol into a two-dimensional sheet
(Fig. 6b). Viewed along the crystallographic b direction, these
{B2(resorcinol)2} macrocycles arrange in an -AA- stacking
fashion along the crystallographic b axis to generate rectan-
gular channels with an approximate effective cross-section of
ca. 16 6 4 Å2. The detailed hydrogen bonding chain can be
found in Fig. 6c. The pyrimidine groups and resorcinol
The asymmetric unit of co-crystal 3 is shown in Fig. 5, which
contains one B and one resorcinol. The bent resorcinol
coformer as the hydrogen bonding donor and acceptor looks
like an ‘‘organic clip’’14 with two groups of intermolecular
…
molecules arrange alternatively through C(12)–H(12) O(2)
2
…
and O(2)–H(2A) N(4) bonds to generate a C2 (5) motif.
…
…
hydrogen bonds (O(1)–H(1A) N(1) and C(12)–H(12) O(2))
from imidazole and pyrimidine, respectively. As shown in
Fig. 6a, two clip-like resorcinol coformers link two B molecules
into a {B2(resorcinol)2} macrocycle with an R44 (20) motif. It is
noteworthy that two B molecules are almost vertical with the
resorcinol coformer. Actually, the dihedral angles between the
imidazole, pyrimidine and resorcinol plane are 72.5u and
64.7u, respectively. In addition, two B molecules are parallel
and closer to each other through the weak p–p interaction
between adjacent aromatic rings (centroid–centroid distances,
3.70–3.83 Å), whereas two pyrimidine groups are located on
the opposite direction in head-to-tail fashion.12 This demon-
strates that such a clip-like resorcinol plays an important role
in constructing the unusual macrocyclic compound.
CO-CRYSTAL [(B)2?(HYDROQUINONE)] (4). The single-crystal struc-
ture reveals that co-crystal 4 contains two B and one
hydroquinone (Fig. 7). Changing the scale and substituent
groups of the building block compared with 1, similar
hydrogen-bonding two-dimensional net and larger-sized hex-
agonal grids with an R44 (29) motif are found in 4 (Fig. 8a). But
the diagonal distance of hexagonal grids are approximately
10.3 Å, which is distinctly longer than that of 1. In addition,
adjacent two nets are closed in an -AB- manner through inter-
net p–p interactions (the centroid–centroid distances, 3.75–
3.80 Å) into a double-network, which also have open hexagonal
…
capsules with a Nimidazole Nimidazole separation of approxi-
mately 9.4 Å. So, symmetrical hydroquinone molecules in 4 are
encapsulated through different hydrogen bonds from 1, 2.
…
…
Actually, O(1)–H(1A) N(5), C(12)–H(12) O(1) and O(2)–
…
…
H(2A) N(1), C(25)–H(25) O(2) hydrogen bonds are shown in
Fig. 2b. Their relevant hydrogen-bonding geometries are given
in Table 2.
As described in the experimental section, co-crystals 1–4
were prepared by direct assembly in CH2Cl2 and CH3CN
solution under the ambient conditions and crystallization via
solvent evaporation. 1H NMR spectra were collected on
1
crystalline samples of A, B and 1–4. H NMR spectra indicate
that A forms co-crystals in a 2 : 1 stoichiometric ratio with
both resorcinol and hydroquinone, whereas B forms co-
Fig. 5 The ORTEP figure of 3 (displacement ellipsoids with 30% probability).
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm

View Article Online
Paper
CrystEngComm
Fig. 6 (a) The H–bonding-driven {A2(resorcinol)2} macrocycle in 3. (b) A top view of the H–bonded two-dimensional network with the rectangular channels in 3. (c)
The C22 (5) hydrogen bonding chain in 3. (Symmetry codes: i: 2x + 1, 2y + 2, 2z + 2; ii: 2x + 1/2, y 2 1/2, 2z + 3/2).
crystals in a 1 : 1 ratio with resorcinol, but in 2 : 1 with
hydroquinone, corresponding to their molecular formulas
obtained from crystal analysis. In 1 and 4, structurally similar
2D nets with hexagonal grids [R44 (22) and R44 (29)] have been
constructed primarily based on the asymmetric building
blocks. Due to the introduction of the additional coformers,
one resorcinol or hydroquinone can link two adjacent nets to
give rise to a double-network and this is encapsulated in the
hexagonal space. So, 2 : 1 co-crystals are generated in spite of
the experimental 1 : 1 stoichiometric ratio (building block and
coformer). In 2, substantial hexagonal grids R44 (20) are found
in a double chain. Differently, hydroquinone only connects
two double chains into a single-network in 2, but not the
double-network in 1 and 4. However, a similar arrangement
with hexagonal channels with 1 and 4 also determines 2 : 1 co-
crystal in 2. But in 3, the substantial R44 (20) macrocycle is
generated from two resorcinol molecules and two B molecules.
Notably, the 1 : 1 form is preferred under the conditions
tested. It corresponds to the experimental 1 : 1 stoichiometric
ratio of components.
As a matter of fact, the phase purity of the bulk crystalline
solid is well confirmed by powder X-ray diffraction (PXRD)
technique. As shown in Fig. S1–S2 (Supporting Information ),
3
the PXRD patterns of 1–4 obtained from the bulk crystalline
Fig. 8 (a) The 2D larger-sized hexagonal net extended in the crystallographic ab
i
ii
…
…
plane in 4, interlinked intermolecular C(1)–H(1) N(3) and C(2)–H(2) N(4)
hydrogen bonds. (b) The double-network with hydroquinone molecules
i
i
i
…
…
encapsulated through O(1)–H(1A) N(5) , C(12) –H(12) O(1) and O(2)–
iii
iii
iii
…
…
O(2) hydrogen bonds in 4 (symmetry code: i: 1 +
H(2A) N(1) , C(25) –H(25)
x, y, z; ii: 1 + x, 21 + y, z; iii: 21 + x, 1 + y, z).
Fig. 7 The ORTEP figure of 4 (displacement ellipsoids with 20% probability).
CrystEngComm
This journal is ß The Royal Society of Chemistry 2012

View Article Online
CrystEngComm
Paper
investigated primarily at room temperature. Fig. 9 and 10
present the comparison of the emission spectra of co-crystals
1–2 with A and 3–4 with B. Upon excitation at l = 389 nm,
compound A exhibits an emission maximum at 484 nm. At the
same time, B provides a red-shifted emission band at 506 nm
upon excitation at 394 nm (Fig. 10). The lengthening
conjugated spacer by insertion of an additional phenyl space
in B may result in the increasing electron-transporting effect of
extended conjugated systems along with the red-shifted color
(approximately 22 nm) compared with A.
As shown in Fig. 9, the emission spectra show that the co-
crystals of 1–2 exhibit emission maxima at ca. 511 and 498
nm upon excitation at 389 nm, respectively. Compared to A,
the emission maxima of 1–2 are lightly red-shifted about 27
and 14 nm, and the emission intensity of 1–2 distinctly
decrease, respectively. Not alone, almost identical emission
bands are observed in B and 3–4 (Fig. 10). In detail, the
emission maxima are not changed at 508 nm for 3, 509 nm
for 4 compared with 506 nm in B, but the emission intensities
of 3–4 distinctly decrease to varying degrees. The different
emission intensity of A and 1–2 might be caused by the
following two reasons.
Fig. 9 Solid-state photoinduced emission spectra of A and 1–2 at room
temperature.
On the one hand, almost vertical arranging disposition
between building modules and their coformers in their co-
crystals badly influences the intermolecular electron-transport
in spite of the hydrogen-bonding interactions. In co-crystals,
coformers are hardly planar with the hydrogen-bonding
pyrimidine, pyridine and imidazole groups. The correspond-
ing dihedral angles are ca. 65.3u in 1, ca. 86.3u in 2, ca. 72.5u
and 64.7u in 3, ca. 78.2u and 85.5u, 54.2u and 55.5u in 4,
respectively. By comparing the data, we can conclude that the
interplanar dihedral angles between building modules and
their coformers are close to intermolecular vertical forms,
resulting in poor conjugated effect, further decreasing their
emission intensities. Therefore, by introduction of the
coformers, the arranging disposition of the components
within the original co-crystals has been changed at different
levels and thus the solid-state luminescent properties can be
further tuned. Actually, 1–4 display distinctly weaker emission
intensity than A, B in the solid state because of the
introduction of resorcinol or hydroquinone.
Fig. 10 Solid-state photoinduced emission spectra of B and 3–4 at room
temperature.
solid at room temperature are basically identical to those of
simulated ones based on single-crystal X-ray diffraction data.
On the other hand, their different intermolecular p–p
contacting characteristics in the co-crystals can influence p
and p* electron-transport. It is well known that the intermole-
cular p–p interaction in the organic molecular packing structure
is the critical factor that determines the solid state emission
colours and emission intensity.15 The parallel stack and the
centroid–centroid distances of two building modules through
the intermolecular p–p interactions are listed in Fig. 11.
For 1, these building module A molecules are parallel
packing in -AABB- stacking fashion with the centroid–
centroid distances ca. 3.85 Å, 3.90 Å and 4.17 Å (Fig. 11).
But the -ABAB- alternate disposition can be found in 2, and
the centroid–centroid distances between adjacent pyrimidine
and pyridine planes only are ca. 3.73–3.74 Å (Fig. 11), which is
obviously shorter than the corresponding distances in 1. In
other words, 2 displays slightly stronger intermolecular p–p
stacking interactions than 1. The stronger p–p stacking
interactions enhance the rigidification of p-conjugated
Luminescent properties of A, B and 1–4
Synthesis of organic co-crystals by the judicious choice of
conjugated organic spacers and coformers has been proven to
be an efficient method for obtaining new types of luminescent
materials. Some of them display good electron-transporting/
hole-blocking properties in organic light-emitting materials,
lasers, sensors and two-photon fluorescent materials.4 An
effective strategy for getting novel organic co-crystals by
designing and controlling the orientation of coformers and
dimension of building blocks has been carried out and four
novel organic co-crystals discussed above are generated.
Herein, the luminescent properties of co-crystals 1–4 based
on asymmetric building modules A and B are explored.
The solid-state emission spectra of A, B and co-crystals 1–4
with approximate 25 mg crystalline samples, respectively, were
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm

View Article Online
Paper
CrystEngComm
Fig. 11 A view of the characteristics of intermolecular p–p stacking interactions in co-crystals 1–4, respectively.
systems for the realisation of p and p* electron-transport. So,
the emission intensity of 2 distinctly increases compared with
Conclusions
In this study, two asymmetric building modules A and B were
applied to construct new organic co-crystals with resorcinol
and hydroquinone, respectively, and four new 2D organic co-
crystals were generated. The asymmetric building modules A
and B with different functional groups, such as pyrimidine,
pyridine and imidazole, provide great advantages for the
hydrogen-bonding self-assembly system. However, the main
discrepancy of the coformers is in their substituent groups,
which can profoundly affect the hydrogen bonding supramo-
lecular recognition and further crystal packing. As described
above, the asymmetric building modules A and B interlink
through intermolecular H-bonding interactions to generate a
1D infinite double-chain in 2 or 2D network in 1, 4 all with
hexagonal grids, further extending to 3D networks with open
hexagonal channels. Resorcinol or hydroquinone molecules
are encapsulated in the hexagonal channels through hydrogen
bonding interactions. The results show that the asymmetric
building modules are suited to prepare the solid materials
containing tube-like channels, which are not easily achievable
by the symmetric building blocks. In addition,
{B2(resorcinol)2} macrocycles in 3 are gained from two clip-
like resorcinol coformers and two B molecules. This demon-
strates that such a clip-like resorcinol plays an important role
in constructing unusual macrocyclic compound.
All co-crystals display interesting photoluminescence in the
solid state. The emission intensity of the asymmetric building
modules is significantly influenced by their incorporation of
conformers into the co-crystals. The structure–property rela-
tionship of these co-crystal systems is summarized, which
indicates that the solid state emission intensity is dependent
on the molecular arranging disposition and the intermolecular
p–p contacting characteristics. What is important is that the
results display more interesting tunability of the emission
intensity of the building modules by the incorporation of
that of 1.
For 3, 4, the -ABAB- parallel stack of B building modules in
the weak intermolecular p–p interactions are found in Fig. 11.
The centroid–centroid distances between adjacent pyrimidine
and imidazole planes are ca. 3.82 Å in 3, ca. 3.75 Å, 3.80 Å in
4. But for 3, the centroid–centroid distances between two
adjacent phenyl planes only is ca. 3.69 Å, which is obviously
shorter than the corresponding distance (ca. 4.10 Å) in 4. By
comparing the centroid–centroid contact distances in 3 and
4, we can conclude that
3 displays slightly stronger
intermolecular p–p stacking interactions than 4. So, the
emission intensity of 3 distinctly increases compared with
that of 4.
Except for the different molecular arranging disposition
and the different intermolecular p–p contacting character-
istics, other factors (such as the different molecular con-
formations, hydrogen bonding interactions and particle size
of crystalline samples) can also affect the emission properties
of the co-crystals 1–4 in the solid state.16 Although we cannot
give further explanations for the relationship between the
solid-state emission properties and the molecular packing,
conformations and coformers, it is clear that the lumines-
cence properties of co-crystals are basically related to the
introduction of the coformers. The introduction of the
coformers can change the geometric arrangement of the
components in the cocrystal, which present new insight into
the structure-property relationship of these cocrystal systems.
Notably, they display more interesting tunability of the
emission intensity of the building modules by the incorpora-
tion of conformers into the H-bonding-driven co-crystals
compared to the pure building modules. The strategy
provides a facile way to design and develop new types of
solid fluorescent materials.
CrystEngComm
This journal is ß The Royal Society of Chemistry 2012

View Article Online
CrystEngComm
Paper
M. Anderson, M. R. Probert, C. N. Whiteley, A. M. Rowland,
A. E. Goeta and J. W. Steed, Cryst. Growth Des., 2009, 9,
1082; (g) L. R. MacGillivray, J. Org. Chem., 2008, 73, 3311.
4 (a) T. Lee and P. Y. Wang, Cryst. Growth Des., 2010, 10,
1419; (b) A. D. G. D. Mauro, M. Carotenuto, V. Venditto,
V. Petraccone, M. Scoponi and G. Guerra, Chem. Mater.,
2007, 19, 6041; (c) G.-G. Hou, J.-P. Ma, L. Wang, P. Wang,
Y.-B. Dong and R.-Q. Huang, CrystEngComm, 2010, 12,
4287.
conformers into the H-bonding-driven co-crystals. It might
have potential applications for organic crystal engineering to
construct patentable crystals with interesting luminescent
properties.
Acknowledgements
We are grateful for financial support from the National
Natural Science Foundation of China (grant No. 30970298),
the Foundation of Shandong province (No. J11LF27,
ZR2010HL065), the Foundation of Yantai City (No. 2011076)
and the Scientific Foundation of Binzhou Medical University
(No. BY2011KYQD08, BY2011KJ085).
ˇˇ ´
5 D.-P. Yan, A. Delori, G. O. Lloyd, T. Friscic, G. M. Day,
W. Jones, J. Lu, M. Wei, D. G. Evans and X. Duan, Angew.
Chem., Int. Ed., 2011, 50, 12483.
6 (a) H.-Y. Peng, C.-K. Lam, T. C. W. Mak, Z.-W. Cai, W.-
T. Ma, Y.-X. Li and H. N. C. Wong, J. Am. Chem. Soc., 2005,
127, 9603; (b) I. Georgiev, E. Bosch, C. L. Barnes and
M. Draganjac, Cryst. Growth Des., 2004, 4, 235; (c) C.
L. Barnes and E. Bosch, Cryst. Growth Des., 2005, 4, 1049.
7 R. Horikoshi, C. Nambu and T. Mochida, New J. Chem.,
2004, 28, 26.
References
1 (a) M. C. Etter, Acc. Chem. Res., 1990, 23, 120; (b) G.
R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311; (c)
D. Braga, F. Grepioni and G. R. Desiraju, Chem. Rev., 1998,
98, 1375; (d) P. R. Schreiner, Chem. Soc. Rev., 2003, 32, 289;
(e) M. A. Mateos-Timoneda, M. Crego-Calama and D.
N. Reinhoudt, Chem. Soc. Rev., 2004, 33, 363; (f) A. G. Doyle
and E. N. Jacobsen, Chem. Rev., 2007, 107, 5713; (g) R.
M. McKinlay and J. L. Atwood, Angew. Chem., Int. Ed., 2007,
46, 2394.
2 (a) G. R. Desiraju, Acc. Chem. Res., 2002, 35, 565; (b)
A. Nangia, Acc. Chem. Res., 2008, 41, 595; (c) M.
J. Zaworotko, Cryst. Growth Des., 2007, 7, 4; (d) T. Kato,
N. Mizoshita and K. Kishimoto, Angew. Chem., Int. Ed.,
2006, 45, 38; (e) D. Braga and F. Grepioni, Acc. Chem. Res.,
2000, 33, 601; (f) Y. Furusho, Y. Tanaka, T. Maeda, M. Ikeda
and E. Yashima, Chem. Commun., 2007, 3174; (g)
B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101,
1629; (h) J.-J. Jang, L. Li, T. Yang, D.-B. Kuang, W. Wang
and C.-Y. Su, Chem. Commun., 2009, 2387; (i) J.-J. Jiang, S.-
R. Zheng, Y. Liu, M. Pan, W. Wang and C.-Y. Su, Inorg.
Chem., 2008, 47, 10692.
8 (a) L. Brammer, Chem. Soc. Rev., 2004, 33, 476; (b) M. Du,
X.-J. Jiang, X. Tan, Z.-H. Zhang and H. Cai, CrystEngComm,
2009, 11, 454; (c) D.-S. Zhou, F.-K. Wang, S.-Y. Yang, Z.-
X. Xie and R.-B. Huang, CrystEngComm, 2009, 11, 2548.
´
9 F. Thebault, S. A. Barnett, A. J. Blake, C. Wilson, N.
¨
R. Champness and M. Schrolder, Inorg. Chem., 2006, 45,
6179.
10 Bruker Analytical X-ray Systems, Inc. Madison, WI, 1998.
11 (a) G. R. Desiraju, Acc. Chem. Res., 1991, 24, 290; (b) G.
R. Desiraju, Acc. Chem. Res., 1996, 29, 441.
12 (a) G. K. Kole, G. K. Tan and J. J. Vittal, Org. Lett., 2010, 12,
ˇ
128; (b) D.-K. Bucar, A. Sen, S. V. S. Mariappan and L.
R. MacGillivray, Chem. Commun., 2012, 48, 1790.
13 (a) G.-G. Hou, Y. Liu, Q.-K. Liu, J.-P. Ma and Y.-B. Dong,
Chem. Commun., 2011, 47, 10731; (b) G.-G. Hou, Y. Wu, J.-
P. Ma and Y.-B. Dong, CrystEngComm, 2011, 13, 6850; (c) G.-
G. Hou, J.-P. Ma, T. Sun, Y.-B. Dong and R.-Q. Huang,
Chem.–Eur. J., 2009, 15, 2261.
14 (a) L. R. MacGillivray, J. L. Reid, J. A. Ripmeester and G.
S. Papaefstathiou, Ind. Eng. Chem. Res., 2002, 41, 4494; (b)
E. Elacqua, P. Kaushik, R. H. Groeneman, J. C. Sumrak, D.-
3 (a) M. L. Cheney, M. J. Zaworotko, S. Beaton and R.
D. Singer, J. Chem. Educ., 2008, 85, 1649; (b) M. Khan,
V. Enkelmann and G. Brunklaus, J. Am. Chem. Soc., 2010,
132, 5254; (c) D. Ajami and Jr. J. Rebek, J. Am. Chem. Soc.,
2006, 128, 5314; (d) M. K. Stanton and A. Bak, Cryst. Growth
Des., 2008, 8, 3856; (e) P. M. Bhatt, Y. Azim, T. S. Thakur
and G. R. Desiraju, Cryst. Growth Des., 2009, 9, 951; (f) K.
ˇ
K. Bucar and L. R. MacGillivray, Angew. Chem., Int. Ed.,
2012, 51, 1037; (c) Y.-B. Dong, T. Sun, J.-P. Ma, X.-X. Zhao
and R.-Q. Huang, Inorg. Chem., 2006, 45, 10613.
15 Y. Fan, Y.-F. Zhao, L. Ye, B. Li, G.-D. Yang and Y. Wang,
Cryst. Growth Des., 2009, 9, 1421.
16 B. Kesanli and W. Lin, Coord. Chem. Rev., 2003, 246, 305.
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm