Synthesis and characterization of a new metal-organic NLO material: Dibromo bis(triphenylphosphine oxide) mercury(II)

Article abstract of DOI:10.1016/j.saa.2008.11.018

A new metal-organic nonlinear optical dibromo bis(triphenylphosphine oxide) mercury(II) (HgBr₂(TPPO)₂, TPPO = triphenylphosphine oxide) crystal has been synthesized. Single-crystal X-ray diffraction reveals that HgBr₂(TPPO

Full text of DOI:10.1016/j.saa.2008.11.018

Spectrochimica Acta Part A 72 (2009) 816–818
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and characterization of a new metal-organic NLO material:
Dibromo bis(triphenylphosphine oxide) mercury(II)
Li Lia,b, Zhengping Wangc, Xinyu Songa, Sixiu Suna,∗
a Department of Chemistry, Shandong University, Jinan, Shandong 250100, PR China
b College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, Ningxia 750021, PR China
c State Key Lab of Crystal Materials, Shandong University, Jinan, Shandong 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 September 2008
Accepted 14 November 2008
A
new metal-organic nonlinear optical dibromo bis(triphenylphosphine oxide) mercury(II)
(HgBr2(TPPO)2, TPPO = triphenylphosphine oxide) crystal has been synthesized. Single-crystal X-
ray diffraction reveals that HgBr2(TPPO)2 crystallizes in the orthorhombic system, space group Pna21,
a = 21.174 Å, b = 9.1979 Å, c = 17.468 Å, and Z = 4. The crystal was also characterized by FTIR spectroscopy,
differential scanning calorimetry (DSC), thermal gravity analysis (TGA), and UV–vis–IR spectroscopy.
Thermal analyses confirmed that the crystal is stable up to 151 ◦C. The transmission spectrum of
the crystal shows that the lower cut off wavelength lies at 340 nm. The nonlinear optical (NLO)
property of HgBr2(TPPO)2 has been estimated by Kurtz-powder second harmonic generation (SHG)
Keywords:
NLO materials
Optical transmittance
SHG efficiency
test.
Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
complex, the group IIB divalent d10 ions, Zn2+, Cd2+, and Hg2+ com-
plexes have attracted our interest for their unique characteristics
1. Introduction
In the last several decades, there has been considerable inter-
est in the nonlinear optical (NLO) materials. In now days, NLO
materials not only play an important part in solid state laser tech-
nique as frequency-conversion materials, but also have promising
applications in the technology of information transmission, storage,
extraction, processing and display [1–4]. Among various second
order NLO materials, metal-organic coordination compounds have
attracted much more attention due to their capability of combin-
ing the advantage of both organic and inorganic materials, such
as high NLO coefficients, stable physico-chemical properties and
better mechanical intension [5–7]. NLO material capable of fre-
quency conversion is generally composed of an electron donor (D),
an acceptor (A) and a conjugated ␲-system as a bridge providing
the electronic communication between the donor and acceptor [8].
And for the second order NLO materials, the bridged system units
must be packed in a non-centrosymmetric way. In metal-organic
compounds, metal centers can act as both donors and the bridging
moiety in D–␲–A system, and the metal–ligand bond is expected
to display large molecular hyperpolarizability because of the trans-
fer of electron density between the metal atom and the conjugated
ligand system [9]. The ferrocene derivative [10] is probably the best
example. Furthermore, in the case of metal-organic coordination
of pale color and high thermal stability. In this paper, we intro-
duce the synthesis, characterization and NLO properties of a new
metal-organic NLO crystal of HgBr2(TPPO)2.
2. Experimental
2.1. Material synthesis and crystal growth
All the starting materials were analytical reagent grade and used
as purchased. HgBr2(TPPO)2 was synthesized by stoichiometric
incorporation of HgBr2 and TPPO. The calculated amount of salt was
dissolved in absolute ethanol at room temperature. The solution
was refluxed for 2 h, and precipitates were filtered off. The filtrate
was then allowed to evaporate at room temperature. HgBr2(TPPO)2
salt was synthesized according to the reaction:
HgBr2 + 2TPPO → HgBr2(TPPO)2
The purity of the synthesized salt was improved by success-
fully recrystallization. Colorless and transparent single crystal
of dimension 4 mm × 3 mm × 3 mm were obtained by slow
evaporation technique from the saturated solution at room
temperature. The crystal had good compositional stability and
showed no degradation when stored in the open air for several
months.
∗
Corresponding author. Tel.: +86 531 8836 4879; fax: +86 531 8856 446.
E-mail address: ssx@sdu.edu.cn (S. Sun).
1386-1425/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.11.018

L. Li et al. / Spectrochimica Acta Part A 72 (2009) 816–818
817
Table 1
Crystallographic parameters for HgBr2(TPPO)2.
Empirical formula
C36 H30 Br2 O2 P2 Hg
Formula weight
Temperature
Crystal system
Space group
a (Å)
916.95
298
orthorhombic
Pna21
21.174(2)
9.1979(10)
17.468(2)
90.00
b (Å)
c (Å)
beta (◦)
Volume (Å3)
3402.1(7)
4
1.790
6.998
Z
Calculated density (mg m−3
)
Absorption coefficient (mm−1
F(0 0 0)
)
Fig. 1. The asymmetric unit of HgBr2(TPPO)2.
1768
Range of h, k, l
−27/23, −11/11, −22/13
Total reflections
Independent reflections
Papameters
19138
2081
388
R indices (I >2(I))
R (all data)
0.0261, 0.0470
0.0431, 0.0513
0.960
Goodness of fit on F2
3. Characterization
Data collection were performed on
a
Bruker-Nonius
SMART APEX II CCD diffractometer equipped with graphite-
monochromated Mo K␣ radiation (ꢀ = 0.71073 Å) at 298 K. The
structure of the crystal was solved by direct method and refined by
the full matrix-least-squares technique using the program SHELXL
[11]. The FTIR spectrum was recorded in the 400–4000 cm−1
region, using KBr pellets on a Bruker vector-22 spectrometer.
Differential scanning calorimetry and thermal gravity analysis
(DSC/TGA) were carried out from room temperature to 600 ◦C
by using a TA Instruments SDT Q600 under an N2 atmosphere
at a heating rate of 10 K min−1. The transmission spectrum was
recorded on a Hitachi U-4100 UV–vis–IR spectrophotometer, which
can operate over the range 200–1200 nm at room temperature.
The NLO property of HgBr2(TPPO)2 was tested by Kurtz powder
SHG test using an Nd:YAG laser.
Fig. 2. FTIR spectrum of HgBr2(TPPO)2.
analysis have been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 700822. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk.
4. Results and discussion
4.2. FTIR studies
4.1. Single crystal XRD
The Fourier transform infrared (FTIR) spectrum of HgBr2(TPPO)2
is shown in Fig. 2 and compared with the standard spectrum of
the TPPO ligand. The assignments along with TPPO are shown
in Table 3. The stretching vibration of P=O (1190 cm−1) shifts to
lower frequency and splits into three sharp peaks (1187, 1164, and
1151 cm−1), which clearly indicates the presence of oxygen to metal
bonds in the coordination compound [12]. This is also confirmed by
single crystal X-ray diffraction analysis. On coordination through
oxygen, we can see from the FTIR spectrum, the nature of other
vibration bands changes very slightly. The peak at 3433 cm−1 may
be assigned for the stretching vibrations of the O–H bonds of water
molecules absorbed by KBr.
X-ray single crystal diffraction reveals that the grown crystal
crystallizes in orthorhombic space group, Pna21 symmetry. The lat-
tice parameters are a = 21.174 Å, b = 9.1979 Å, c = 17.468 Å, and Z = 4.
A summary of the crystallographic data is given in Table 1. Selected
bond distance and bond angle data are summarized in Table 2. The
asymmetric unit is presented in Fig. 1. As depicted in Fig. 1, the
central mercury atom was coordinated by two O atoms from TPPO
ligand and two Br atoms. Crystallographic data for the structural
Table 2
Selected bond distances and angles for HgBr2(TPPO)2.
Moiety
Distance (Å)
Moiety
Angle (◦)
Table 3
Comparison of the main FTIR spectral data of HgBr2(TPPO)2 with TPPO.
Hg1–O1
Hg1–O2
Hg1–Br2
Hg1–Br1
O1–P1
2.433 (3)
2.442 (3)
2.4406 (6)
2.4491 (5)
1.487 (3)
1.493 (4)
O1–Hg1–Br2
O1–Hg1–O2
Br2–Hg1–O2
O1–Hg1–Br1
Br2–Hg1–Br1
O2–Hg1–Br1
P1–O1–Hg1
P2–O2–Hg1
107.14 (8)
90.49 (10)
102.11 (8)
93.47 (7)
151.305 (19)
97.34 (8)
IR band (cm−1
TPPO
)
Assignment
HgBr2(TPPO)2
1190
1187, 1164, 1151
1436, 997
1588, 1484
3054
VP=O
VP–C
VC=C
VC–H
P2–O2
1439, 995
1591, 1485
3058
133.38 (19)
124.03 (18)

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L. Li et al. / Spectrochimica Acta Part A 72 (2009) 816–818
and 1200 nm. The unpolished crystal with thickness of 3 mm was
used. The transmittance spectrum was shown in Fig. 4. The cut-off
wavelength of HgBr2(TPPO)2 occurs at 340 nm. The absence of the
absorption at 1064 and 532 nm shows it could be used for opti-
cal window applications. The transparency of the crystal is low
(30–40%), which can be improved by growing of higher optical
quality single crystal.
4.5. Second harmonic generation
Kurtz powder technique [13] was used to estimate the rela-
tive second harmonic generation (SHG) activity of HgBr2(TPPO)2.
A pulse energy of 4 mJ/pulse, pulse width of 10 ns, and repetition
rate of 10 Hz are used. Powdered samples of standard KDP and com-
pound HgBr2(TPPO)2 with the same particle size were considered
for powder SHG measurements. It was found that the SHG effi-
ciency of HgBr2(TPPO)2 is comparable with that of KDP. During the
measurement, we also found that the crystal of HgBr2(TPPO)2 can
reach phase matching. The SHG measurements on the crystal of
HgBr2(TPPO)2 indicate the potential application of the material for
frequency conversion process.
Fig. 3. DSC/TGA curves of HgBr2(TPPO)2.
5. Conclusion
Single crystal of HgBr2(TPPO)2 was synthesized and grown from
ethanol solution. The grown crystal was characterized by single-
crystal XRD analysis. The study reveals that the crystal belongs to
the orthorhombic system. The thermal behavior of the grown crys-
tal was studied by using DSC/TGA. The Kurtz powder tests prove
that the SHG efficiency of HgBr2(TPPO)2 is comparable with that
of KDP. The SHG measurements on the crystal indicate that it is a
potential candidate for NLO applications.
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Fig. 4. Optical transmission spectrum of HgBr2(TPPO)2.
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TGA and DSC have been carried out for the grown crystals. TGA
and DSC curves are shown in Fig. 3. HgBr2(TPPO)2 is thermally sta-
ble up to 151 ◦C. There is no phase transition till the material melts
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the decomposition temperature; therefore, the endothermic peak
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4.4. UV–vis studies
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To find the transmission range of HgBr2(TPPO)2, we recorded
the optical transmission spectrum of HgBr2(TPPO)2 between 200