Degenerate four-wave mixing determination of third-order optical nonlinearities of three mixed ligand nickel(II) complexes

Article abstract of DOI:10.1016/j.molstruc.2011.09.021

Three mixed ligand nickel(II) complexes were synthesized and characterized by UV-visible, IR, MS and elemental analysis. Their off-resonant third-order nonlinear optical properties were measured using femtosecond laser and degenerate four-wave mixing technique. The third-order nonlinear optical susceptibilities χ⁽³⁾ were 3.20-3.51 × 10^-13 esu. The nonlinear refractive indexes n₂ were 5.89-6.45 × 10^-12 esu. The second-order hyperpolarizabilities γ of the molecules were 3.20-3.50 × 10^-31 esu. The response times were 55-81 fs. The results show that these complexes have potential nonlinear optical applications.

Full text of DOI:10.1016/j.molstruc.2011.09.021

Journal of Molecular Structure 1006 (2011) 282–287
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Degenerate four-wave mixing determination of third-order optical nonlinearities
of three mixed ligand nickel(II) complexes
b
a
Zhibin Cai a, , Mao Zhou , Jinghua Xu
⇑
a Institute of Fine Chemical Industry, Zhejiang University of Technology, Hangzhou 310014, PR China
b Hangzhou Sharply Pharmaceutical Institute Co. Ltd., Hangzhou 310052, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2011
Received in revised form 8 September 2011
Accepted 8 September 2011
Available online 14 September 2011
Three mixed ligand nickel(II) complexes were synthesized and characterized by UV–visible, IR, MS and
elemental analysis. Their off-resonant third-order nonlinear optical properties were measured using
femtosecond laser and degenerate four-wave mixing technique. The third-order nonlinear optical
susceptibilities v(3) were 3.20–3.51 ꢀ 10ꢁ13 esu. The nonlinear refractive indexes n2 were 5.89–6.45 ꢀ
10ꢁ12 esu. The second-order hyperpolarizabilities
c
of the molecules were 3.20–3.50 ꢀ 10ꢁ31 esu. The
response times were 55–81 fs. The results show that these complexes have potential nonlinear optical
applications.
Keywords:
Nonlinear optics
DFWM technique
Mixed ligand nickel(II) complex
Synthesis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
photophysical and photochemical properties [11–18]. A Japanese
patent shows that the conjugated organic molecules, phenanthro
The design and synthesis of materials exhibiting large third-
order nonlinear optical (NLO) properties has been the focal point
of a large amount of recent research in great part due to their
potential applications in optical communication, optical data stor-
age, optical information processing, optical computing and so on.
Unlike second-order NLO materials, the structure–property rela-
tionships of third-order NLO materials are a little vague. Several
design principles have been adopted during the past few decades
to improve third-order optical nonlinearities of materials. One
attractive approach is the formation of organometallic and coordi-
nation materials by combining of conjugated organic molecules
with transition metal [1–5]. Many conjugated organic molecules
such as stilbene derivatives [6], azobenzene derivatives [7],
anthraquinone derivatives [8], squaraine derivatives [9] and tri-
phenylamine derivatives [10] have been proven to be effective
third-order NLO materials. Compared to purely organic materials,
organometallic and coordination materials can combine the advan-
tages of architectural flexibility, ease of fabrication and tailoring,
and high NLO properties of organics with good transmittancy,
temporal and thermal stability of inorganics. Such materials also
[9,10-d]imidazole derivatives, can exhibit high third-harmonic
generation and rapid response [19]. So they are highly promising
candidates for third-order NLO materials. Their analogs, imi-
dazo[4,5-f][1,10]phenanthroline (IP) and its 2-aryl-substituted
derivatives (PIP, TIP), also have
p-conjugated electron systems
and are able to act as bidentate ligands towards transition metals
such as nickel because of the replacement of phenanthrene by phe-
nanthroline (phen). We present here three mixed ligand nickel(II)
complexes a–c (Fig. 1), where different conjugated ligands (phen,
IP, PIP, TIP) are bonded to the central nickel ion in the form of
octahedral coordination geometry. Their structures were charac-
terized by UV–visible, IR, MS and elemental analysis. By using fem-
tosecond laser, the off-resonant third-order optical nonlinearities
of the complexes were measured with degenerate four-wave mix-
ing (DFWM) technique. The relationships between molecular
structure and optical property were analyzed. To our knowledge,
it is the first study of the NLO properties for these complexes and
the complex c is new.
2. Experimental
have enhanced second-order hyperpolarizabilities
cules by introducing a metal atom.
c of the mole-
2.1. Materials
In recent years, many nickel complexes have received consider-
able attention as third-order NLO materials due to their rich
The compound 1,10-phenanthroline-5,6-dione was prepared
according to the literature method [20]. Other materials were com-
mercially available and of reagent grade. All of them were used
without further purification.
⇑
Corresponding author. Tel.: +86 571 88320428.
E-mail address: caizb-h@tom.com (Z. Cai).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.09.021

Z. Cai et al. / Journal of Molecular Structure 1006 (2011) 282–287
283
N
N
phen
N
2+
N
N
N
N
N
N
N
Ni
N
H
N
N
H
N
IP
a
b
c
N
2+
N
N
N
N
N
N
N
Ni
N
H
N
H
N
N
PIP
N
N
N
N
N
N
N
2+
Ni
S
S
N
H
N
N
H
N
N
TIP
Fig. 1. Structures of the ligands and the nickel(II) complexes a–c.
2.2. Physical measurements
2.3. Synthesis
FT-IR spectra were recorded on a Vector 22 spectrometer using
KBr pellets. 1H NMR spectra were collected on a AVANCE III
500 MHz apparatus, with TMS as internal standard and DMSO-d6
as solvent. UV–visible spectra were recorded on a Shimadzu
UV-2550 UV–visible spectrometer. Elemental analyses were con-
ducted on a Thermo Finnigan Flash EA 1112 apparatus. Mass spec-
tra were taken on a Therm LCQ TM Deca XP plus ion trap mass
spectrometry instrument. Melting points were measured on an
X-4 micromelting point apparatus without correction.
2.3.1. 1H-Imidazo[4,5-f][1,10]phenanthroline (IP)
A mixture of 1,10-phenanthroline-5,6-dione (1.05 g, 5 mmol),
formaldehyde (0.18 g, 6 mmol), ammonium acetate (7.7 g,
100 mmol), and glacial acetic acid (16 mL) was heated under reflux
with stirring for 3 h. The cooled solution was diluted with H2O and
neutralized with concentrated aqueous ammonia. The precipitate
was collected and recrystallized from DMF to give a light yellow
crystalline powder. Yield: 0.8 g, 72.7%. m.p. >310 °C (Lit. m.p.
>310 °C [21]). 1H NMR (500 MHz, DMSO-d6), d (ppm): 13.75

284
Z. Cai et al. / Journal of Molecular Structure 1006 (2011) 282–287
(s, 1H), 9.03 (dd, 2H, J1 = 4.3 Hz, J2 = 1.5 Hz), 8.83 (dd, 2H, J1 = 8 Hz,
J2 = 1.5 Hz), 8.47 (d, 1H), 7.83 (dd, 2H, J1 = 8 Hz, J2 = 4.3 Hz).
measurement, the laser was very stable (rms < 0.1%). The input
beam was split into two beams k1 and k2 with nearly equal energy
by use of a beam splitter, then focused on the sample. The beam k2
passed through a delay line driven by a stepping motor in order
that the optical path length difference between the k2 and k1
beams could be adjusted during the measurement. The angle be-
tween the beams k1 and k2 was about 5°. When k1 and k2 were
overlapped spatially in the sample, the generated signal beam k3
or k4 passed through an aperture, recorded by a photodiode and
then analyzed by a Lock-in amplifier and computer.
2.3.2. 2-Phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (PIP)
This ligand was synthesized as above using benzaldehyde. The
product was a yellow granular crystal. Yield: 1.2 g, 81.1%. m.p.
>310 °C (Lit. m.p. >310 °C [21]). 1H NMR (500 MHz, DMSO-d6), d
(ppm): 13.79 (s, 1H), 9.05 (dd, 2H, J1 = 4.3 Hz, J2 = 1.8 Hz), 8.95
(dd, 2H, J1 = 7.6 Hz, J2 = 1.8 Hz), 8.30–8.32 (m, 2H), 7.86 (dd, 2H,
J1 = 7.6 Hz, J2 = 4.3 Hz), 7.64 (dd, 2H), 7.53–7.56 (m, 1H).
The experiments were performed at 22 °C. The samples dis-
solved in DMF at concentrations of 5 ꢀ 10ꢁ4mol/L were placed in
a 1 mm thick quartz cell. The solvent DMF has no nonlinear signal
under the light intensity adopted. So the third-order optical non-
linearities measured come from the ligands and the complexes
themselves.
2.3.3. 2-(2-Thienyl)-1H-imidazo[4,5-f][1,10]phenanthroline (TIP)
This ligand was synthesized as above using 2-thiophenecarbox-
aldehyde. The product was a brown-yellow crystalline powder.
Yield: 0.58 g, 76.8%. m.p. >320 °C (Lit. m.p. >320 °C [22]). 1H NMR
(500 MHz, DMSO-d6), d (ppm): 13.78 (s, 1H), 9.04 (dd, 2H,
J1 = 4.4 Hz, J2 = 1.5 Hz), 8.86 (dd, 2H, J1 = 7.9 Hz, J2 = 1.5 Hz), 7.93
(dd, 1H, J1 = 3.8 Hz, J2 = 0.8 Hz), 7.84 (dd, 2H, J1 = 7.9 Hz,
J2 = 4.4 Hz), 7.78 (dd, 1H, J1 = 4.8 Hz, J2 = 0.8 Hz), 7.30 (dd, 1H,
J1 = 4.8 Hz, J2 = 3.8 Hz).
3. Results and discussion
3.1. Synthesis and characterization
2.3.4. (1H-Imidazo[4,5-f][1,10]phenanthroline-
jN7,jN8)bis(1,10-
Among three nickel(II) complexes a–c, c was first synthesized
by our research group. a and b have been previously reported
[23]. Two steps are included in the published synthetic route: (1)
The synthesis of [Ni(phen)2Cl2] according to the literature method
[24]. (2) The complexation reaction of [Ni(phen)2Cl2] with the
appropriate ligands. In this paper, we adopted a simple one-pot
synthetic method. The desired complexes a–c were isolated as
the perchlorates and purified by recrystallization in relatively high
yield. They were characterized by FT-IR spectra, electrospray ion-
ization MS and elemental analysis. But they could not be further
characterized by 1H NMR spectra due to their paramagnetic
properties.
phenanthroline- N1, N10)-nickel(2+) (a)
j
j
A mixture of nickel dichloride hexahydrate (0.24 g, 1 mmol),
phen (0.4 g, 2.2 mmol) and methanol (20 mL) was heated under
reflux with stirring for 2 h. A solution of IP (0.26 g, 1.2 mmol) in
methanol (15 mL) was then added and further stirred under reflux
for 3 h. The reaction mixture resulted was cooled to room temper-
ature, and a saturated aqueous sodium perchlorate solution was
added. The precipitate was collected and recrystallized from DMF
twice to give a pink crystalline powder. Yield: 0.7 g, 79.2%. FT-IR
(KBr),
1422, 1122 (
m
(cmꢁ1): 3395 (mNH
ClO4), 855 (d@CH), 725 (d@CH), 625 (dClO4). Anal. Calc.
, mOH), 3050 (m@CH), 1614 (mC@C), 1514,
m
for C37H24Cl2N8NiO8ꢂ2.5H2O: C, 50.31; H, 3.31; N, 12.69. Found:
C, 50.45; H, 3.23; N, 12.42%. ESI-MS (CH3CN): m/z 637.1
([MA2ClO4AH]+), 319.3 ([MA2ClO4]2+).
3.2. The third-order NLO properties
The UV–visible absorption spectra of two ligands (phen, IP) and
three complexes a–c in DMF solutions are displayed in Figs. 3 and
4. Their maximum absorption peaks appear at 227 nm, 254 nm,
270 nm, 280 nm and 295 nm, respectively, which are attributed
2.3.5. (2-Phenyl-1H-imidazo[4,5-f][1,10]phenanthroline-
jN7,jN8)
bis(1,10-phenanthroline- N1, N10)-nickel(2+) (b)
j
j
This complex was synthesized using a procedure similar to that
described for a, with PIP instead of IP. The product was a light yel-
to intraligand p–
p⁄ transitions. Above 400 nm, their DMF solutions
low crystalline powder. Yield: 0.5 g, 53.1%. FT-IR (KBr),
3433 (mNH OH), 3045 ( @CH), 1639 ( C@C), 1518, 1455, 1118 (
809 (d@CH), 725 (d@CH), 634 (dClO4). Anal. Calc. for
m
(cmꢁ1):
ClO4),
are essentially transparent. The laser wavelength (800 nm) used in
the experiment of DFWM is out of the absorption region. Thus their
off-resonant third-order optical nonlinearities can be measured.
The third-order nonlinear optical susceptibility v(3) is measured
via a comparison with that of a reference sample CS2, calculated
from the DFWM signal (I), the linear refractive index (n), the sam-
ple thickness (L) and absorption correction factor using the follow-
ing equation [25]:
,
m
m
m
m
C
43H28Cl2N8NiO8ꢂ1.5H2O: C, 54.86; H, 3.32; N, 11.90. Found: C,
54.95; H, 3.57; N, 12.24%. ESI-MS (CH3CN): m/z 357.3
([MA2ClO4]2+).
2.3.6. [2-(2-Thienyl)-1H-imidazo[4,5-f][1,10]phenanthroline-
jN7,jN8]
bis(1,10-phenanthroline- N1, N10)-nickel(2+) (c)
j
j
This complex was synthesized using a procedure similar to that
ꢀ ꢁ
ꢀ
ꢁ
Is 1=2 Lr ns 2 aL expð
a
L=2Þ
vðs3Þ
¼
vðr3Þ
ð1Þ
described for a, with TIP instead of IP. The product was a light yel-
(cmꢁ1):
ClO4),
Ir
Ls nr 1 ꢁ expðꢁ
a
LÞ
low crystalline powder. Yield: 0.6 g, 63.9%. FT-IR (KBr),
3341 (mNH OH), 3070 ( @CH), 1606 ( C@C), 1522, 1422, 1126 (m
m
,
m
m
m
where the subscripts ‘‘s’’ and ‘‘r’’ represent the parameters for the
sample and CS2. And indicates the linear absorption coefficient.
850 (d@CH), 727 (d@CH), 629 (dClO4). Anal. Calc. for C41H26Cl2N8Ni
O8SꢂH2O: C, 52.48; H, 3.01; N, 11.94. Found: C, 52.70; H, 3.33; N,
12.33%. ESI-MS (CH3CN): m/z 719.0 ([MA2ClO4AH]+), 360.4
([MA2ClO4]2+).
a
The fraction a
L=2Þ comes from the sample absorption and equals
L expð
a
1ꢁexpðꢁ
a
to 1 approximatelyLÞwhile the sample has little absorption around
the employed laser wavelength. The values of vr(3) and nr for CS2
are 6.7 ꢀ 10ꢁ14 esu and 1.632, respectively [26].
2.4. Nonlinear optical measurements
The nonlinear refractive index n2 in isotropic media is estimated
through the equation [27]:
The third-order NLO properties were measured using femtosec-
ond DFWM technique, with a Ti: Sapphire laser. Fig. 2 shows the
experimental setup. The pulse width was determined to be 80 fs
on a SSA25 autocorrelator. The operating wavelength was centered
at 800 nm. The repetition rate of the pulses was 1 KHz. During the
n2ðesuÞ ¼ 12
p
vð3Þ=n2
ð2Þ
where n is the linear refractive index of solution, measured by 2WAJ
Abbe refractometer.

Z. Cai et al. / Journal of Molecular Structure 1006 (2011) 282–287
285
Fig. 2. Experimental setup of DFWM.
c
¼
vð3Þ=Nf4
ð3Þ
phen
where N is the number density of the solute per milliliter, related to
the molar concentration c by N = NAc with NA being Avogadro’s
number. And f4 is the local field correction factor which is
[(n2 + 2)/3]4 (n is the linear refractive index of solution).
The dependence of DFWM signal intensity on the delay time of
the input beam is shown in Fig. 5. The curves are obtained via fit-
ting the time convolution between autocorrelation function of
ꢂ
ꢃ
IP
t
pulse and single exponent decline function exp
. The response
ꢁT2
times of the samples can be obtained from Fig. 5.
The values of v(3), n2,
c and response times for the samples de-
duced and calculated from the experimental results are listed in
Table 1.
The
c values obtained for the mixed ligand nickel(II) complexes
a–c are large, and comparable with those of some known NLO orga-
Wavelength / nm
nometallic and coordination materials (for example, 6.5 ꢀ 10ꢁ32
–
1.9 ꢀ 10ꢁ31 esu for 1,3-dithiole-2-thione-4,5-dithiolate complexes
at 1064 nm [29,30], 1.21 ꢀ 10ꢁ31 esu for a heterometal cluster
containing iron at 532 nm [31], 2.6 ꢀ 10ꢁ34–1.7 ꢀ 10ꢁ32 esu for
Fig. 3. UV–visible absorption spectra of the ligands (phen, IP) in DMF.
alkynylruthenium complexes at 800 nm [32], 2.8 ꢀ 10ꢁ33
–
1.04 ꢀ 10ꢁ32 esu for fullerene-containing organometallics [33],
1.0 ꢀ 10ꢁ31 esu for azo-nickel chelate compound at 830 nm [34]).
Though a sufficient understanding of the effect of molecular struc-
ture on the third-order NLO response is still lacking, many studies
b
have indicated that delocalized p-conjugated electron systems play
an important role in determining the third-order NLO properties of
organic materials [35–40]. Thus the ligands phen and IP who pos-
sess two-dimensional delocalized
exhibit the
values in the order of 10ꢁ31 esu. Compared with
metal-free ligands phen and IP, the value of the nickel(II) complex
p-conjugated electron systems
c
c
c
a is distinctively greater. This may be due to the fact that the electron
a
configuration of Ni2+ is d8, the overlapping between the d orbitals of
central nickel ion and p-electron orbitals of ligands can allow the
three ligands to be in electronic communication with one another
through the nickel, leading to a larger polarizable system, and finally
better NLO properties.
Wavelength / nm
Among three nickel(II) complexes a–c, b possesses higher
value compared to a whereas c shows the highest value. This
result denotes that the extended delocalization of the ligands
c
Fig. 4. UV–visible absorption spectra of the complexes a–c in DMF.
c
p
The second-order hyperpolarizability
c
of a molecule in isotro-
exerts an effect on the third-order NLO properties of the
complexes. By introducing the benzene ring in 2-position of the
pic media is related to the solution v(3) by [28]:

286
Z. Cai et al. / Journal of Molecular Structure 1006 (2011) 282–287
2
IP
phen
2
1
1
0
0
-150
-100
-50
0
50
100
150
-150
-100
-50
0
50
100
150
Time delay (fs)
Time delay (fs)
5
4
3
2
1
0
4
3
2
1
b
a
0
-150
-100
-50
0
50
100
150
-150
-100
-50
0
50
100
150
Time delay (fs)
Time delay (fs)
6
5
4
3
2
1
c
0
-150
-100
-50
0
50
100
150
Time delay (fs)
Fig. 5. DFWM signal versus delay time for the ligands (phen, IP) and the complexes a–c in DMF solution.
Table 1
The values of v(3), n2,
complexes a–c.
c
and response times for the ligands (phen, IP) and the
contrast to the benzene ring (151 kJ/mol), the thiophene ring has
lower delocalization energy (121 kJ/mol), which results in stronger
intramolecular charge transfer and higher degree of electron delo-
calization of the ligand TIP. So the greater the electron delocaliza-
Sample
n
v(3)
n2
c
Response time
(10ꢁ13 esu)
(10ꢁ12 esu) (10ꢁ31 esu) (fs)
tion of the ligands is, the larger the
be. In addition, the bathochromic shift of the
in the complexes a–c also implies the increase in the degree of
electron delocalization, which in turn leads to the better third-or-
der optical nonlinearity.
c
values of the complexes will
phen
IP
a
b
c
1.4306 1.75
1.4319 1.96
1.4320 3.20
1.4308 3.35
1.4327 3.51
3.23
3.61
5.89
6.18
6.45
1.76
1.96
3.20
3.36
3.50
37
39
57
55
81
p–
p⁄ absorption band
p
-
Fig. 5 shows that the nickel(II) complexes a–c possess ultrafast
response times which are in the range of 55–81 fs. Such instanta-
neous responses also ensure that the measured NLO responses
are mainly contributed from the delocalized electrons [41].
imidazole unit, the electron delocalization of the ligand PIP is
enhanced owing to the increase of the conjugation length. In

Z. Cai et al. / Journal of Molecular Structure 1006 (2011) 282–287
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800 nm. The large
(
c
= 3.20–3.50 ꢀ 10ꢁ31 esu) and ultrafast
(s = 55–81 fs) nonlinearities showed by these complexes originate
from the extensive electronic delocalization and polarization.
Acknowledgement
This research was financially supported by the Natural Science
Foundation of Zhejiang Province, China (Grant No. Y4080370).
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