K. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 54–62
55
development of photonic devices [1–6]. Efforts have been particu-
Experimental and computational section
larly focused on the third-order NLO properties of metalorganic
molecules, which are promising materials for all-optical photonic
switching and have optical limiting capabilities [7–14].
General
Metalorganic materials have attracted a lot of attention, due to
the possibility of tuning their NLO response by appropriate design
at the molecular level. The most important consideration is the
careful selection of a suitable organic ligand and functional metal
ion to prepare a viable inorganic–organic composite material.
Among the organic ligands, large conjugated ligands, such as
imidazole derivatives, have been frequently used, due to their
interesting electronic and photonic properties.
Benzoin and salicylaldehyde were purchased from Aldrich
Chemical Company and were used as received; ammonium acetate
and glacial acetic acid were purchased from Nanjing Chemistry
Reagent Limited Company. Cu(CH COO) ꢃH O, Co(CH COO) ꢃ4H O,
3
2
2
3
2
2
Zn(CH COO) ꢃ3H O, AR, were purchased from Shanghai Chemistry
3
2
2
Reagent Limited Company. Solvents were purified and dried
according to standard procedure. Benzil was synthesized in our
laboratory.
In the past two decades, much effort has been made in the
application of transition metal coordination compounds toward
the development of advanced functional materials [15]. Greater
design flexibility can be achieved by varying the metal, its oxida-
tion state, the ligand environment and the geometry. In addition,
many complexes are known to possess low-lying charge transfer
transitions such as intraligand charge transfer (LLCT), metal-to-
ligand charge transfer (MLCT) and ligand-to-metal charge transfer
Synthesis
Synthesis of 2-(4,5-diphenyl-1-H-imidazole-2-yl)-phenol
The ligand was synthesized according to Scheme 1. Benzil
(
5 mmol, 1.05 g), salicylaldehyde (5 mmol, 0.610 g) and ammo-
nium acetate (80 mmol, 6.16 g) were refluxed in glacial acetic acid
20 mL) for 4 h. The mixture was poured into 100 mL deionized ice
(
(
LMCT) [12], which can be associated with large second-order non-
water after cooling, and then the pH was adjusted to about 7 with
aqueous ammonia. Gray-red solid was generated and recrystallized
with ethanol. Yield 90%, m. p.: 213.0–213.8 °C, Elemental Analysis
linearities (b). Recently, considerable investigations have been car-
ried out on the third-order NLO properties of a variety of materials,
especially metalorganic-based materials.
(
(
%): C, 80.75; H, 5.16; N, 8.97. Found: C, 80.04; H, 4.86; N, 8.66. IR
KBr, cm ): 3417.09 (s), 3208.27 (vs), 3058.89 (vs), 1602.23 (vs),
As early as 1987, Ho and co-workers started to study the third-
order NLO properties of phthalocyaninato complexes involving third
main group metals such as gallium and aluminum for the first time
ꢂ1
1
1
474.96 (vs), 1383.88 (m), 1294.47 (m). H NMR (DMSO, d,
ppm): 13.05 (s, 1H), 12.95 (s, 1H), 8.03 (d, J = 9.1, 2.4 Hz 1H),
[
16]. Hou and co-workers [17] reported that d10 metal-organic clus-
7
2
.53 (t, J = 7.2 Hz 2H), 7.49 (t, J = 7.2 Hz 4H), 7.42 (d, 1H), 7.35 (t,
H), 7.27 (t, 2H), 6.98 (d, J = 9.1 Hz 1H), 6.94 (d, J = 9.1 Hz 1H).
tersplayavarietyof rolesinthe third-orderNLOproperties. Themetal
ions and ligandsaffect the NLO properties bydelocalizing the – elec-
p
tron cloud. If metalionsmakea greater contribution tothedelocaliza-
tion than ligands, then the effect of the metal ions on the NLO
properties will be greater; and vice versa if ligands make a greater
contribution. Paul and coworkers [18] synthesized and characterized
a series of complexes with arylalkynyl iron/ruthenium and found
large changes in third-order nonlinearities due to oxidation. Wang
et al. [19] studied third-order NLO properties of a single crystal con-
taining cadmium ions. The magnitudes of the third-order nonlinear
refraction index, n2, and the secondorder molecular hyperpolarizabil-
Synthesis of complex Cu(DPIP)
All complexes weresynthesizedaccording toScheme 2. The ligand
1 mmol, 0.3121 g) and Cu(CH COO) O (0.5 mmol, 0.0998 g) were
dissolved in ethanol (70 mL) in the molar ratio of 2:1 and heated
under reflux for 2 h. The solution stands for 15 days at room temper-
ature and after filtering, black-brown rod-shaped crystals are formed.
The crystals were isolated by filtration, washed with methanol and
2
1
(
3
ꢃH
2 2
2
dried in a vacuum desiccator using anhydrous CaCl . Elemental Anal-
ysis (%): C, 70.04; H, 4.45; N, 7.78. Found: C, 69.77; H, 4.32; N, 7.56. IR
ꢂ18
2
ꢂ30
ity,
c, were 9.409 ꢁ 10
m W and 2.774 ꢁ 10
esu, respectively,
ꢂ1
(
cm ): [
m
(Cu–O)] 915.89 (vs), [
m
(Cu–N)] 473.25 (vs). UV–Vis (etha-
which suggests that the crystal is a potential candidate for all-optical
switching devices in the blue-green region of light.
4
ꢂ1
ꢂ1
nol) kmax (nm): 340 (
e
= 3.09 ꢁ 10 L mol cm ).
It is therefore likely that researchers will continue to find
Synthesis of complex Co(DPIP)
The aubergine rod-shaped crystals of 2 were prepared by a pro-
cedure similar to that used for 1 with Co(CH COO) O instead
2
2
metallorganic compounds containing
p-systems which exhibit
NLO properties. For instance, four, tetra-nuclear heterobimetallic
’
’
3
2
ꢃ4H
2
3 3 3 3
clusters [MOS M Y(PPh ) ] (M = Mo, W; M = Ag, Cu; Y = Br, I), have
of Cu(CH
3
COO)
2
ꢃH
2
O. Elemental Analysis (%): C, 73.51; H, 4.79; N,
been recently synthesized, and were found to exhibit effective non-
linear absorption, self-focusing effects and large optical limiting
capabilities; additionally, the influence of the complex composi-
tion on the NLO properties of some neutral cubane-like heterothio-
metallic complexes has been found to be an important factor [20].
The structure of the title ligand has been researched for many
years [21–23], but the NLO properties of the complexes have not
been reported. To further develop this promising field, and also
to search for better NLO materials, we selected the ligand 2-(4,5-
diphenyl-1-H-imidazole-2-yl)-phenol (DPIP) as a candidate with
potential NLO properties because DPIP has a large conjugated sys-
tem and strong coordination with a nitrogen atom and an oxygen
atom, and can complex with a variety of metals. Herein, we have
synthesized a series of complexes 1–3 with benzyl and imidazole
skeletons by the Davidson method, and have characterized their
structures by X-ray crystallography. Moreover, their IR, UV–Vis,
fluorescence, thermogravimetry, and NLO properties have been
investigated experimentally and theoretically.
ꢂ1
7
9
3
.46. Found: C, 72.95; H, 4.53; N, 7.29. IR (cm ): [
31.50 (vs), [
m(Co–O)]
m(Co–N)] 462.60 (vs). UV–Vis (ethanol) kmax (nm):
4
ꢂ1
ꢂ1
34 (
e
= 4.83 ꢁ 10 L mol cm ).
2
Synthesis of complex Zn(DPIP) 3
The structure of complex 3 had been reported [23]. In order to
study the NLO and other properties, the complex was synthesized
O
O
CHO
OH
NH
OH R:CH3COONH4, S:CH3COOH
120 , 4 h
N
Scheme 1. Synthetic route to ligand.