Synthesis, crystal structures, and optical properties of a novel imidazole derivative and its Zn(II) complex

Article abstract of DOI:10.1016/j.molstruc.2006.06.025

A novel imidazole derivative, 1-[trans-4-(4-diethylaminostyryl)phenyl]imidazole (abbreviated as L), and its complex Zn(SCN)₂L₂ have been synthesized and fully characterized. Their crystal structures have been determined by X-ray diffraction. The molecular structure of L is approximately coplanar, the bond lengths of C-C and C-N are located between the normal double and single bonds, which show that L has a delocalized π-electron system in the molecule. In the molecular structure of Zn(SCN)₂L₂, zinc atom is four coordinated by two nitrogen atoms of the ligands (L) and two nitrogen atoms of thiocyanate to form a distorted tetrahedral geometry. The one-dimensional structure of Zn(SCN)₂L₂ is formed by C-H?S interactions and π-π interactions. They both crystallize in the noncentrosymmetric space group P2₁, and exhibit powder second-harmonic generation (SHG) efficiency approximately twenty times high as that of urea, indicating promising potential applications as useful nonlinear optical (NLO) materials.

Full text of DOI:10.1016/j.molstruc.2006.06.025

Journal of Molecular Structure 829 (2007) 202–207
www.elsevier.com/locate/molstruc
Synthesis, crystal structures, and optical properties of a novel imidazole
derivative and its Zn(II) complex
Feng Jin a,b, Jian-feng Li a, Hong-ping Zhou a, Jie-ying Wu a, Jia-xiang Yang a,d,
Yu-peng Tian a,c,d,¤, Min-hua Jiang d
a Department of Chemistry, Anhui University, Hefei 230039, PR China
b Department of Chemistry, Fuyang Normal College, Fuyang 236041, PR China
c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China
d State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
Received 16 May 2006; received in revised form 24 June 2006; accepted 26 June 2006
Available online 22 August 2006
Abstract
A novel imidazole derivative, 1-[trans-4-(4-diethylaminostyryl)phenyl]imidazole (abbreviated as L), and its complex Zn(SCN)2L2 have
been synthesized and fully characterized. Their crystal structures have been determined by X-ray diVraction. The molecular structure of L
is approximately coplanar, the bond lengths of C–C and C–N are located between the normal double and single bonds, which show that
L has a delocalized ꢀ-electron system in the molecule. In the molecular structure of Zn(SCN)2L2, zinc atom is four coordinated by two
nitrogen atoms of the ligands (L) and two nitrogen atoms of thiocyanate to form a distorted tetrahedral geometry. The one-dimensional
structure of Zn(SCN)2L2 is formed by C–H,S interactions and ꢀ–ꢀ interactions. They both crystallize in the noncentrosymmetric space
group P21, and exhibit powder second-harmonic generation (SHG) eYciency approximately twenty times high as that of urea, indicating
promising potential applications as useful nonlinear optical (NLO) materials.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Imidazole; ꢀ-Conjugated system; SHG; Noncentrosymmetric
1. Introduction
occurs in a noncentrosymmetric space group as design. It is
not possible to predict crystal geometries from molecular
geometries, which has greatly hampered the progress of
crystal engineering of functional solids. Therefore, design
and synthesis of acentric crystals have gained scientiWc and
technological interest [7–11]. The second factor is the
requirement of delocalized ꢀ-electron system of molecules.
Accordingly, the rational design and synthesis of such acen-
tric crystal with high SHG eYciency is still a challenge to
synthetic chemists and material scientists.
Moreover, the organic–inorganic hybridized coordina-
tion complexes containing imidazole derivatives, which
were designed based on topologies, have been extensively
reported [12], but the ꢀ-conjugated imidazole derivatives
with functional group and their metal complexes, which
possess excellent optical properties, are relatively scarce.
Therefore, we have systematically carried out research on
There has been intensive research on organic, metal-
organic materials exhibiting second-order nonlinear optical
(NLO) properties [1]. Such materials are of potential appli-
cations in electrooptic devices and second harmonic gener-
ation (SHG), which are of importance in areas such as
optical communications and data storage [2–4]. Two main
factors determine the NLO response of molecules. The Wrst
important factor is the requirement of noncentrosymmetric
arrangement of molecules in the crystal, because the sec-
ond-order susceptibility (ꢀ2) will vanish in a centrosymmet-
ric environment [5,6]. However, crystallization often rarely
*
Corresponding author. Tel.: +86 551 5108151; fax: +86 551 5107342.
E-mail address: yptian@ahu.edu.cn (Y.-p. Tian).
0022-2860/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.06.025

F. Jin et al. / Journal of Molecular Structure 829 (2007) 202–207
203
the rational design and synthesis of noncentric imidazole
derivatives and their complexes containing good electron
donor (D) and acceptor (A) through a conjugated bridge,
which is original and diVerent from other related reports. In
this article, we report the synthesis, crystal structures, ther-
mal stabilities, photoluminescence and SHG eYciency of a
novel D-ꢀ-A structural imidazole derivative (L) and its
complex Zn(SCN)2L2 with noncentrosymmetric space
group.
(HM+). IR (KBr, cm¡1): 3416 (m), 1601 (s), 1521 (s), 1480
(w). Single crystals of L were obtained by slow evaporation
of a benzene solution at room temperature.
2.4. Synthesis of Zn(SCN)2L2
The tetrahedral zinc complex was readily prepared by
layering method at room temperature. A clear methanol
solution (15 mL) of Zn(NO3)2 · 6H2O (29.80 mg,
0.1 mmol) and NaSCN (16.40 mg, 0.2 mmol) was carefully
layered onto a solution of L (73.40 mg, 0.2 mmol) in chlo-
roform (15 mL). Yellow needle-shaped crystals suitable
for X-ray diVraction were obtained by slow interlayer
diVusion. Yield: 70%. Anal. calcd. (%) for ZnC44H46N8S2:
C, 64.73; H, 5.68; N, 13.77. Found (%): C, 64.78; H, 5.64;
N, 13.81. IR (KBr, cm¡1): 3421 (m), 2073 (s), 1595 (s),
1524 (s), 1425 (w).
2. Experimental
2.1. General
All commercially available chemicals are of analytical
grade and used without further puriWcation. The solvents
were puriWed by conventional methods. 4-(Diethyla-
mino)benzaldehyde, 4-(diethylamino)benzalcohol, and
[4-(diethylamino)benzyl]triphenylphosphinium iodide
were synthesized according to the methods reported [13–
15]. IR spectra were recorded on a Nicolet FT-IR instrument
with KBr discs in the 400–4000 cm¡1 range. Elemental
analysis was carried out on Perkin-Elmer 240 analyzer.
UV–vis spectra were measured in DMF solutions
(10¡5 mol L¡1) with a UV-265 spectrophotometer. The
solid-state luminescence spectra were recorded on a Per-
kin-Elmer LS55B Xuorescence spectrophotometer at
room temperature.
2.5. X-ray crystal structure determination
The X-ray diVraction measurements were performed on
a Bruker SMART CCD area detector using graphite
monochromated MoKꢁ radiation (ꢁ D0.71073 Å) at
298(2)K. Intensity data were collected in the variable ꢂ-
scan mode. The structures were solved by direct methods
and diVerence Fourier syntheses. The non-hydrogen atoms
were reWned anisotropically and hydrogen atoms were
introduced geometrically. Calculations were performed
with SHELXTL-97 program package [16]. Details of the
crystal parameters, data collections and reWnements are
listed in Table 1, and selected bond distances and angles are
given in Table 2.
2.2. Synthesis of 4-imidazolyl-benzaldehyde
DMF (60 mL), imidazole (6.80 g, 100 mmol), anhydrous
potassium carbonate (13.80 g, 100 mmol), 4-Xuorobenzalde-
hyde (12.40g, 100 mmol) and three drops of aliquate-336
were added to a round-bottomed Xask. The reaction mix-
ture turned yellow and was kept stirring for 24h at 85 °C.
After being cooled to room temperature, it was poured into
ice-water (200 mL). The pale-yellow lamellar crystals
formed immediately. The product was Wltered and dried in
vacuo. Yield: 90%. Anal. calcd. (%) for C10H8N2O: C, 63.58;
H, 5.96; N, 9.27. Found (%): C, 63.55; H, 5.94; N, 9.26. MS:
m/z D172.
Table 1
Crystallographic for L and Zn(SCN)2L2
Compound
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
L
Zn(SCN)2L2
C44 H46 Zn N8 S2
816.38
C21 H23 N3
317.42
298(2)
0.71073
Monoclinic
P21
8.777(9)
7.701(8)
13.64(1)
106.31(2)
885.0(2)
2
298(2)
0.71073
Monoclinic
P21
19.413(3)
10.471(1)
21.760(3)
108.411(3)
4197(1)
4
b (Å)
c (Å)
ꢃ (°)
2.3. Synthesis of 1-[trans-4-(4-diethylaminostyryl)phenyl]
imidazole (L)
Volume (Å3)
Z
[4-(Diethylamino)benzyl]triphenylphosphinium iodide
(5.51 g, 10mmol), 4-imidazolyl-benzaldehyde (1.72g,
10mmol) and NaOH (2.00 g, 50mmol) were placed in a
mortar. The mixture was ground for 10 min, then poured
into distilled water (500 mL). The product was extracted
twice with CH2Cl2, and the organic layer was dried over-
night over anhydrous MgSO4. The solvent was removed
with a rotary evaporator to give the crude product. It was
puriWed by recrystallization from methanol. Yield: 80%.
Anal. calcd. (%) for C21H23N3: C, 79.21; H, 7.60; N, 13.20.
Found (%): C, 79.19; H, 7.63; N, 13.17. ESI-MS: m/z D318.3
ꢄCalcd (g/cm3)
F(000)
1.191
340
1.292
1712
Crystal size (mm)
ReXections collected
Unique reXections
Parameters
S on F2
0.35 £ 0.29 £ 0.14
0.22 £ 0.13 £ 0.06
4342
21643
2659
230
1.001
7408
496
1.015
R1 [I > 2ꢅ(I)]
wR2 [I > 2ꢅ(I)]
R1
0.0710
0.1387
0.2165
0.1846
0.0163/¡0.0187
0.0508
0.1112
0.1158
0.1423
wR2
ꢂꢄmin/max [e. nm¡3
]
0.0517/¡0.0298

204
F. Jin et al. / Journal of Molecular Structure 829 (2007) 202–207
Table 2
DMF
K2CO3
N
NH
F
CHO
CHO
CHO
+
Selected bond lengths (Å) and angles (°) for L and Zn(SCN)2L2
N
N
L
A
N(3)–C(20)
N(3)–C(18)
C(18)–C(19)
N(1)–C(3)
C(2)–C(3)
N(2)–C(4)
C(2)–C(3)
C(10)–C(11)
N(2)–C(2)
1.559(12)
1.573(11)
1.370(12)
1.355(10)
1.323(9)
1.397(7)
1.323(9)
1.168(9)
1.341(9)
N(3)–C(15)
C(20)–C(21)
N(2)–C(1)
N(1)–C(1)
C(11)–C(12)
C(7)–C(10)
1.388(8)
1.440(13)
1.359(8)
1.355(9)
1.516(10)
1.497(10)
1.497(10)
1.516(10)
DMSO
F
CHO
(C2H5)2N
(C2H5)2NH
NaBH4 CH3OH
C(7)–C(10)
C(11)–C(12)
C(15)–N(3)–C(20)
C(20)–N(3)–C(18)
CHCl3
KI, PPh3
(C2H5)2N
CH2P+Ph3I-
(C2H5)2N
CH2OH
122.2(7)
113.6(7)
C(15)–N(3)–C(18)
122.4(7)
S
Zn(SCN)2L2
Zn(1)–N(1)
Zn(1)–N(7)
N(1)–C(1)
N(2)–C(1)
N(2)–C(4)
N(3)–C(18)
N(4)–C(22)
N(7)–C(43)
C(2)–C(3)
C(10)–C(11)
C(12)–C(17)
C(13)–C(14)
C(15)–C(16)
N(4)–C(22)
N(5)–C(22)
N(5)–C(25)
N(6)–C(41)
C(23)–C(24)
C(32)–C(33)
C(41)–C(42)
N(8)–Zn(1)–N(7)
N(7)–Zn(1)–N(4)
N(7)–Zn(1)–N(1)
2.007(3)
1.940(4)
1.306(4)
1.327(5)
1.438(4)
1.442(6)
1.319(5)
1.150(5)
1.345(6)
1.261(6)
1.374(6)
1.398(5)
1.392(6)
1.319(5)
1.340(5)
1.430(5)
1.450(6)
1.342(6)
1.465(5)
1.503(7)
118.33(18)
103.81(15)
108.97(16)
Zn(1)–N(4)
Zn(1)–N(8)
N(1)–C(3)
2.006(3)
1.916(4)
1.356(6)
1.369(5)
1.371(5)
1.643(11)
1.450(6)
1.148(6)
1.482(6)
1.485(6)
1.386(6)
1.388(6)
1.370(5)
1.361(5)
1.367(5)
1.377(5)
1.457(6)
1.462(5)
1.317(5)
1.446(8)
111.99(17)
104.44(15)
109.14(13)
N
N
NaOH
A + S
N
N(2)–C(2)
N(3)–C(15)
N(3)–C(20)
N(6)–C(41)
N(8)–C(44)
C(7)–C(10)
L
Scheme 1. Synthesis route for L.
C(11)–C(12)
C(12)–C(13)
C(14)–C(15)
C(16)–C(17)
N(4)–C(24)
N(5)–C(23)
N(6)–C(36)
N(6)–C(39)
C(28)–C(31)
C(31)–C(32)
C(39)–C(40)
N(8)–Zn(1)–N(4)
N(8)–Zn(1)–N(1)
N(4)–Zn(1)–N(1)
complex Zn(SCN)2L2, the strong absorption peak at
2073 cm¡1 is for ꢆas (NCS), which is consistent with the
occurrence of thiocyanate–N coordination and agrees well
with the molecular structure [18]. UV–vis spectrum of L
displays absorption peak at 369 nm, which is assigned as K
band absorption arising from ꢀ–ꢀ* transitions within the
extended ꢀ-conjugated system [19]. The large molar absorp-
tion coeYcient (log D 4.29) is an indication of high elec-
tronic delocalization system. Zn(SCN)2L2 displays
absorption peaks at 368 nm (log D 4.60), arising from ꢀ–ꢀ*
transitions of the ligands.
3.2. Crystal structures
3. Results and discussion
3.1. Synthesis and characterization
The molecular structures of L and Zn(SCN)2L2 are
shown in Figs. 1a and 2a, respectively. The selected bond
distances and angles are listed in Table 2. It is important to
note that, L and its complex Zn(SCN)2L2 both crystallize in
the acentric monoclinic space group P21 as expected.
The synthetic route of 1-[trans-4-(4-diethylaminosty-
ryl)phenyl]imidazole (L) is shown in Scheme 1. It was syn-
thesized by solvent free Wittig reaction with high yield.
Wittig reaction is a widely used method to synthesize trans-
conWguration alkene. Conventional Wittig reactions are
carried out in solution and in nitrogen atmosphere using
strong base, such as MeONa, t-BuOK, NaH, NaNH2, and
so on. In our experiments, we ground the reaction mixture
in a mortar, using the commom base – NaOH. The reaction
could be accomplished within 10 min and the product could
be handled easily and the yield was above 50%. Therefore,
solvent free Wittig reaction is an inexpensive and conve-
nient method to synthesize trans-conWguration alkene.
The IR spectrum of free L shows absorption at
1480cm¡1, attributing to the vibration of the CBN of imid-
azole. Compared with the double peaks at 1489 and
1425cm¡1 assigned to the CBN of imidazole in
Zn(SCN)2L2, the shift and split of the band suggests coordi-
nation of the imidazole group in the complex [17]. In the
In the molecular structure of L, least-square plane calcu-
lations show that the dihedral angle between the two ben-
zene rings is 9.5°, indicating they are nearly coplanar. The
sum of the three C–N–C angles taking nitrogen atom as
center (C(15)–N(3)–C(20), 122.2(7)°; C(18)–N(3)–C(20),
113.6(7)°; C(15)–N(3)–C(18), 122.4(7)°) is 358.2°, therefore
the trigonal NC3 is approximately coplanar. The bond
lengths of N(3)–C(18) (1.57(1) Å) and N(3)–C(20)
(1.56(1)Å) are longer than that of N(3)–C(15) (1.388(8) Å),
conWrming two electrons on N(3) are partial to the adjacent
phenyl ring. It can be seen from Table 2 that all the bond
lengths of C–C are located between the normal CBC dou-
ble bond (1.34 Å) and C–C single bond (1.54 Å), which
show that L has a highly delocalized ꢀ-electron system in
the molecule. Because crystallographic disorder appears in
C(10) and C(11), the bond length of C(10)–C(11)
(1.168(9) Å) is shorter than the normal CBC bond length.
In conclusion, the departure of C–N and the average bond

F. Jin et al. / Journal of Molecular Structure 829 (2007) 202–207
205
Fig. 1. (a) Molecular structure of L (hydrogen atoms are omitted for clarity). (b) Packing diagram of L.
lengths show that there is a ꢀ-conjugated system in the mol-
ecule, which is very important for the optical nonlinearities
of the compound.
As shown in Fig. 2a, in the molecular structure of
Zn(SCN)2L2, zinc atom is four coordinated by two nitro-
gen atoms of the ligands (L) and two nitrogen atoms of
thiocyanate to form a distorted tetrahedral geometry.
The Zn–N bond lengths are in the range between
1.914(4) Å and 2.007(3) Å. The bond angles around the
zinc atom are in the range of 103.81(15)–118.33(18)°. The
strong distortion in the tetrahedral environment of the
four donor atoms is explained on the basis of strong ste-
reo interactions between the ligands. The planar, corre-
sponding bond lengths and bond angles of the two
ligands (L) in the complex are diVerent with those of the
free ligand. Least-square plane calculations show that the
Fig. 2. (a) The ZnII coordination environment in the complex of Zn(SCN)2L2 (hydrogen atoms are omitted for clarity). (b) C–H,S intermolecular
interactions in crystal of Zn(SCN)2L2. (c) Perspective view of molecular packing of Zn(SCN)2L2.

206
F. Jin et al. / Journal of Molecular Structure 829 (2007) 202–207
dihedral angles between the two benzene rings are 3.7°
(for planes containing C25, C26, C27, C28, C29, C30) and
C33, C34, C35, C36, C37, C38), and 16.7° (for planes con-
taining C25, C26, C27, C28, C29, C30 and C33, C34, C35,
C36, C37, C38), respectively. While, the dihedral angle
between the two benzene rings is 9.5° in the free ligand.
The bond lengths and bond angles being comparable
with those of free L are listed in Table 2. From Table 2,
we can see that the C–C and C–N bond lengths in
Zn(SCN)2L2 are more even than those of the correspond-
ing free L. In the crystal structure, the molecules are
stacked through C(9)–H(9),S(2) (the distance of H,S is
2.871 Å and the C–H,S angle is 170.4°) and C(24)–
H(24),S(1) (the distance of H,S is 2.944 Å and the C–
H,S angle is 137.5°) interactions (see Fig. 2b) along the
b axis, to form one-dimensional structures (the van der
Waals radii of H and S are 1.00 Å and 2.05 Å, respec-
tively). The C–H,S hydrogen bond is very important in
supramolecular self-assembly [20–22]. In addition to the
hydrogen bonds as mentioned above, there are ꢀ–ꢀ inter-
actions between the neighboring imidazole rings in the
one-dimensional structures. The two imidazole rings
from two diVerent ligands are nearly parallel with a dihe-
dral angle of 7.3° and a centroid-to-centroid distance of
3.802 Å. As shown in Fig. 2c, the one-dimensional chains
are stacked through C(18)–H,S(1) (the distance of H,S
is 2.939 Å and the C–H,S angle is 164.1°) interactions,
to form a noncentrosymmetric crystal.
6000000
5000000
4000000
3000000
2000000
1000000
0
485 nm (complex)
476 nm
(L)
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 3. The emission spectra of L and the complex Zn(SCN)2L2.
cules of Zn(SCN)2L2 [22,23]. The photoluminescence
properties indicate that the complex has more extensive
of ꢀ conjugation in the system.
3.5. Second-order nonlinear optical properties
The imidazole derivative (L) and Zn(SCN)2L2, with
noncentrosymmetric space group P21, were expected to
give rise to eYcient SHG eVects for NLO application.
Preliminary experimental results conWrmed this and
showed they both displayed strong powder SHG eYcien-
cies. The SHG intensity was determined by the powder
technique of Kurtz and Perry [24], in which the second
harmonic output was generated by irradiating powder
samples of random a Q-switched and mode-locked
Nd:YAG laser. The 532 nm second harmonic intensities
of L and Zn(SCN)2L2 powders were estimated as about
20 times that of NLO urea powders by direct frequency
doubling of a laser diode at 1064 nm at room tempera-
ture. This result is easily understood, since the ligand in
the free state and in Zn(SCN)2L2 both present extensive
electron delocalization in whole system with long conju-
gation length between the donor and acceptor [8c]. More-
over, the solid-state electronic spectra study show that
3.3. Thermal stability
Thermal stabilities for the imidazole derivative and its
zinc complex were determined by TG technique. The TG
curves exhibited that L and Zn(SCN)2L2 were stable up to
their decomposition temperature of 289.3 °C and 350.6 °C,
respectively, without showing any melting process, indi-
cating the high stability of them. The decomposition tem-
perature of Zn(SCN)2L2 is much higher than that of the
free ligand. So coordination of the ligand to the metal ion
results in greater thermal stability, which indicates the
potential applications of Zn(SCN)2L2 due to its higher
thermal stability.
there is no absorption above 400 nm for
L and
Zn(SCN)2L2, which make them to be promising candi-
date materials for generation of blue–violet light.
3.4. Photoluminescence
4. Conclusion
The photoluminescence properties of
L
and
Zn(SCN)2L2 were investigated in the solid state at room
temperature (Fig. 3). Zn(SCN)2L2 exhibits emission band
at 485 nm, which is red-shifted by 9 nm compared with
that of the free ligand (L), accompanied by increased
luminescent intensity. The maximum emission observed
in the free ligand is assigned to the ꢀ–ꢀ* transition of the
ꢀ-conjugated system, and the emission maximum of the
complex is also assigned to the ꢀ–ꢀ* tentatively intrali-
gand Xuorescence due to its close resemblance to that of
the free ligand. The small red shift may be a result of the
above-mentioned weaker ꢀ–ꢀ interactions in the mole-
A new imidazole derivative with D-ꢀ-A structure has
been synthesized by solvent free Wittig reaction, and its
zinc complex was obtained by self-assembly. They both
possess better thermal stability. X-ray diVraction investiga-
tions reveal that they crystallize in the same noncentrosym-
metric space group P21. The SHG intensities of them were
observed to be much stronger than that of urea powders. In
summary, the new ꢀ-conjugated imidazole derivative
and its metal complex Zn(SCN)2L2 possess mechanical,
thermal, chemical stabilities and excellent optical proper-

F. Jin et al. / Journal of Molecular Structure 829 (2007) 202–207
207
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We thank the National Natural Science Foundation of
China (50532030, 50335050, and 50325311), Person with
Ability Foundation of Anhui Province (2002Z021), Doc-
toral Program Foundation of the Ministry of Education of
China, Education Committee of Anhui Province
(2006KJ032A, 2006KJ135B), The Team for ScientiWc Inno-
vation Foundation of Anhui Province (2006KJ007TD),
Key Laboratory of Opto-Electronic Information Acquisi-
tion and Manipulation, (Anhui university), Ministry of
Education.
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Crystallographic data for the structural analysis have
been deposited at the Cam-bridge Crystallographic Data
Center, CCDC No. 255807 for L and CCDC No. 255805
for complex Zn(SCN)2L2. Copies of this information may
be obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB221EZ, UK (fax: +44 12 23/
336 033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk). Structural factors are available on
request from the authors. Supplementary data associated
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