A new approach to suppress nonlinearity-transparency trade-off through coordination chemistry: Syntheses and spectroscopic study on second-order nonlinear optical properties of a series of square-pyramidal zinc(II) complexes

Article abstract of DOI:10.1016/S1386-1425(02)00289-5

Five new square-pyramidal coordination compounds L · Zn(acac)₂ (1-5) (acac = acetylacetonate; L is a variety of thiosemicarbazones: p-dimethylaminobenzaldehyde thiosemicarbazone (1), p-hydroxy-o-methoxybenzaldehyde thiosemicarbazone (2), p-meth

Full text of DOI:10.1016/S1386-1425(02)00289-5

Spectrochimica Acta Part A 59 (2003) 1095Á1101
/
www.elsevier.com/locate/saa
A new approach to suppress nonlinearity-transparency
trade-off through coordination chemistry: syntheses and
spectroscopic study on second-order nonlinear optical
properties of a series of square-pyramidal zinc(II) complexes
Peng Ren a, Tao Liu a, Jingui Qin a,*, Chuangtian Chen b
a Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
b Beijing R&D Center for Synthetic Crystals, Chinese Academy of Science, Beijing 100080, People’s Republic of China
Received 9 November 2001; received in revised form 7 August 2002; accepted 7 August 2002
Abstract
Five new square-pyramidal coordination compounds L ×
/
Zn(acac)2 (1Á
/
5) (acacꢀacetylacetonate; L is a variety of
/
thiosemicarbazones: p-dimethylaminobenzaldehyde thiosemicarbazone (1), p-hydroxy-o-methoxybenzaldehyde thio-
semicarbazone (2), p-methoxybenzaldehyde thiosemicarbazone (3), p-hydroxybenzaldehyde thiosemicarbazone (4), o-
hydroxybenzaldehyde thiosemicarbazone (5)) have been synthesized and characterized by 1H NMR, IR, and elemental
analysis. All of these compounds exhibit pretty wide transparent ranges in the visible region. Their electronic absorption
spectra have been studied experimentally, and theoretically by ZINDO/S calculation. The latter has also been utilized to
estimate the extent of intramolecular charge transfer. The MOPAC software package has been used to evaluate their
first-order molecular hyperpolarizabilities (b). All b values of the five coordination compounds are larger than those of
the corresponding thiosemicarbazones. And complex 1 shows the largest b0 (39.1ꢁ
/
10ꢂ30 esu) in the series.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Calculation; Electronic spectrum; Second-order nonlinear optics; Structure; Syntheses
1. Introduction
parameter oscillator (OPO) [3]. It has long been
recognized that molecular chromophores could
exhibit a second-order NLO response several
orders of magnitude larger than that of inorganic
compounds, such as potassium dihydrogen phos-
phate (KDP) and lithium niobate (LN) [4,5]. To
search for new molecular chromophores people
have paid attention to coordination systems in
recent years, as they can offer a large variety of
molecular structures by changing metals, ligands
Second-order nonlinear optical (NLO) materials
have been attracting considerable attention due to
their broad applications in optoelectronics, such as
optical frequency conversion [1,2] and optical
* Corresponding author. Tel.: ꢃ
27-87647617.
E-mail address: jgqin@whu.edu.cn (J. Qin).
/86-27-87219044; fax: ꢃ86-
/
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 2 8 9 - 5

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P. Ren et al. / Spectrochimica Acta Part A 59 (2003) 1095Á
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Scheme 1. Synthetic route of the complexes.
or coordination numbers, to enhance thermal
stability, and to diverse electronic behaviors by
of ten times as large as that of KDP, is a successful
application of the empirical rule.
As a continuation of the research on finding new
coordination compounds exhibiting both large
second-order NLO effects and good transparency,
herein we report on a study of utilizing thiosemi-
carbazones, which have been proposed as coordi-
nation ligands for nonlinear optical materials
because of their pale colors, thermally stability
and excellent coordination capabilities [11], as
ligands, to form a series of novel pyramidal
virtue of the coordinated metal center [6Á8]. Up to
/
now, many organometallic and coordination com-
pounds exhibiting large first-order molecular hy-
perpolarizabilities have been synthesized [6]. Most
of them have absorption bands in visible region
due to the so-called optical nonlinearityÁ
/transpar-
ency trade-off.
On the basis of more than 10 years’ research on
organometallic and coordination NLO materials,
we have summarized an empirical rule [9] indicat-
ing that the molecular configuration is definitely
an important factor to affect the linear and non-
linear optical properties of coordination com-
pounds. It seems that involvement of the metal
ions with d10 electron structure (such as Zn(II),
Cd(II) and Hg(II)) can cause excellent transpar-
ency in the visible region, as the metal ions do not
form an efficient electron conjugation system with
the ligands, and thus is hardly involved in the
intramolecular charge transfer. In addition, they
zinc(II) coordination compounds 1Á
1). We hope to provide valuable information to
the optimization of the optical nonlinearityÁ
/
5 (Scheme
/
transparency trade-off.
2. Experimental
The general synthetic route contains mainly
three steps, including the formation of acetone
zinc diacetylacetonate (acetone-ZDA); the forma-
tion of thiosemicarbazones; and the preparation of
the final coordination compounds (Scheme 1). All
of the chemicals used were of analytical reagent
(A.R.) grade. Acetone-ZDA was synthesized ac-
cording to the reference [12].
do not perform dÁd excitation themselves [9].
/
Phenylthiourea zinc diacetylacetonate (PZDA)
[10], a square-pyramidal coordination compound,
which shows excellent transparency in the visible
region and a second harmonic generation (SHG)

P. Ren et al. / Spectrochimica Acta Part A 59 (2003) 1095Á
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1101
1097
2.1. Physical measurements
2.2.4. p-Hydroxybenzaldehyde thiosemicarbazone
IR data (KBr, cmꢂ1): 3465.8 (s, OH); 3365.6,
Infrared (IR) spectra were recorded on a NICO-
LET 170SX FT-IR spectrometer. Nuclear mag-
netic resonance (NMR) was performed on a
MERCURY-VX300 spectrometer. The elemental
3122.5 (s, NÃ
1606.6 (s, CÄN); 1587.3, 1548.7, 1508.2 (s, CÃ
phenyl); 958.6 (m, CÄS). Elemental Anal. (%)
/
H, NH2); 3210.8 (m, NÃ
/
H, NH);
/
/C,
/
Calc.: C, 49.2; H, 4.6; N, 21.5. Found: C, 49.5;
analysis was obtained on a CaloÁErba elemental
/
H, 4.1; N, 21.8.
analyzer (Model 106). The electronic spectra of the
complexes in the ultraviolet-visible (UV-vis) region
were recorded in a SCHIMADZU UV-160A
spectrophotometer.
2.2.5. o-Hydroxybenzaldehyde thiosemicarbazone
IR data (KBr, cmꢂ1): 3445.5 (s, OH); 3324.0,
3142.7 (s, NÃ
1615.2 (s, CÄN); 1543.8, 1493.7 (s, CÃ
1063.6 (s, CÄS). Elemental Anal. (%) Calc.: C,
/
H, NH2); 3177.4 (s, NÃ
/
H, NH);
/
/
C, phenyl);
/
2.2. Preparation of thiosemicarbazones
49.2; H, 4.6; N, 21.5. Found: C, 49.3; H, 5.0; N,
21.2.
Thiosemicarbazones were prepared by conden-
sation of the appropriate substituted benzaldehyde
with thiosemicarbazide (1:1 molar ratio) in etha-
nol, and purified by recrystallization from metha-
2.3. Preparation of the complexes
The complexes were synthesized by reaction of
corresponding thiosemicarbazones with acetone
solution of acetone-ZDA in 40 8C. The products
nol (yields 80Á90%).
/
2.2.1. p-Dimethylaminobenzaldehyde
thiosemicarbazone
were purified by recrystallization from acetoneÁ
/
acetylacetone (2:1 volume ratio) (yields 50Á60%).
/
IR data (KBr, cmꢂ1): 3373.3, 3151.5 (s, NÃ
/
H,
H, NH); 2900.7, 2858.3,
H, CH3, CH); 1605.5 (s, CÄN);
1595.0, 1504.4 (s, CÃC, phenyl); 1182.3 (s, CÄS).
NH2), 3249.8 (s, NÃ
/
2.3.1. p-Dimethylaminobenzaldehyde
thiosemicarbazone-ZDA (1)
2815.9 (m, CÃ
/
/
IR data (KBr, cmꢂ1): 3431.1, 3147.6 (s, NÃ
/
H,
H, NH); 2989.5, 2960.5,
H, CH3, CH); 1610.5 (s,
N); 1591.2, 1562.2, 1519.8 (CÃC, phenyl);
1180.4 (s, CÃO); 10l6.4 (m, CÄ
S).1H NMR
(DMSO-d6, ppm): 1.8 (s, 12H); 2.9 (s, 6H); 5.2
(s, 2H); 6.6Á6.7 (d, 2H); 7.5Á7.6 (d, 2H); 7.8 (s,
/
/
Elemental Anal. (%) Calc.: C, 54.0; H, 6.3; N, 25.2.
Found: C, 54.3; H, 6.5; N, 25.7.
NH2); 3263.3 (m, NÃ
2918.1, 2850.6 (m, CÃ
CÄ
/
/
/
/
/
/
2.2.2. p-Hydroxy-o-methoxybenzaldehyde
thiosemicarbazone
IR data (KBr, cmꢂ1): 3525.6 (s, OH); 3435.0,
/
/
2H); 7.9 (s, 1H); 11.1 (s, 1H). Elemental Anal. (%)
Calc.: C, 49.2; H, 6.2; N, 11.5. Found: C, 50.4; H,
6.5; N, 11.9.
3153.4 (s, NÃ
2968.2, 2933.5, 2839.0 (m, CÃ
1591.2, 1544.9, 1514.0 (s, CÃC, phenyl); 941.2 (s,
S). Elemental Anal. (%) Calc.: C, 48.0; H, 4.9;
/
H, NH2), 3276.8 (s, NÃ
/H, NH);
/H, CH3, CH);
/
CÄ
/
2.3.2. p-Hydroxy-o-methoxybenzaldehyde
thiosemicarbazone-ZDA (2)
IR data (KBr, cmꢂ1): 3514.1 (s, OH); 3425.3,
N, 18.7. Found: C, 47.7; H, 5.4; N, 18.6.
2.2.3. p-Methoxybenzaldehyde thiosemicarbazone
3184.3 (s, NÃ
2968.2, 2927.7 (w, CÃ
1523.7, 1460.0 (CÃC, phenyl); 1271.0, 1199.6 (s,
CÃO); 927.7 (m, CÄ
S).1H NMR (DMSO-d6,
ppm): 1.8 (s, 12H); 3.8 (s, 3H); 5.2 (s, 2H); 6.7Á
/
H, NH2); 3290.3 (m, NÃ
/
H, NH);
IR data (KBr, cmꢂ1): 3402.2, 3153.4 (s, NÃ
/
H,
H, NH); 2962.5, 2925.8 (m,
H, CH3, CH); 1602.7, 1535.2, 1508.2 (s, CÃC,
phenyl); 952.8 (m, CÄS). Elemental Anal. (%)
/
H, CH3, CH): 1587.3,
NH2); 3288.4 (s, NÃ
/
/
CÃ
/
/
/
/
/
/
Calc.: C, 51.7;H, 5.3; N, 20.1. Found: C, 51.2; H,
5.4; N, 19.8.
6.8 (d, 1H); 7.0 (d, 1H); 7.4 (s, 1H); 7.9 (s, 1H); 8.0
(s, 2H); 9.3 (s, 1H); 11.2 (s, 1H). Elemental Anal.

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P. Ren et al. / Spectrochimica Acta Part A 59 (2003) 1095Á1101
/
(%) Calc.: C, 46.5; H, 5.5; N, 8.5. Found: C, 46.0;
H, 4.8; N, 8.2.
Table 1
Optical absorption maxima of the five complexes in solvents of
different polarities
2.3.3. p-Methoxybenzaldehyde thiosemicarbazone-
ZDA (3)
lmax (nm)
IR data (KBr, cmꢂ1): 3377.1, 3172.7 (s, NÃ
/
H,
H, NH); 2964.4, 2839.0 (m,
H, CH3, CH); 1608.5 (s, CÄN); 1587.3, 1523.7,
1487.0 (CÃC, phenyl); 1257.5, 1174.6 (s, CÃO);
927.7 (s, CÄ
S). 1H NMR (DMSQ-d6, ppm): 1.8 (s,
12H); 3.8 (s, 3H); 5.2 (s, 2H); 6.9, 7.0 (d, 2H); 7.6Á
Toluene
CHCl3
Ethyl acetate
CH3OH
DMF
NH2); 3151.2 (m, NÃ
/
1
2
3
4
5
365
281
335
306
331
287
337
283
354
288
364
282
332
288
327
310
331
290
350
286
361
282
330
292
325
305
327
294
354
271
362
305
328
303
319
306
322
300
367
270
361
308
334
306
326
306
330
304
382
276
CÃ
/
/
/
/
/
/
7.7 (d, 2H); 7.9 (s, 2H), 8.0 (s, 1H); 11.2 (s, 1H).
Elemental Anal. (%) Calc.: C, 48 1; H, 5.7; N, 8.8.
Found: C, 48.8; H, 5.4; N, 8.3.
2.3.4. p-Hydroxybenzaldehyde thiosemicarbazone-
ZDA (4)
IR data (KBr, cmꢂ1): 3416.2 (m, OH): 3392.0,
3115.5 (m, NÃ
2851.5, 2823.0 (w, CÃ
N); 1611.0, 1584.8, 1517.8, 1443.4 (CÃ
1234.8, 1170.2 (s, CÃO); 927.7 (s, CÄ
S).1H NMR
(DMSO-d6, ppm): 1.8 (s, 12H); 5.2 (s, 2H); 6.7Á6.8
(s, 2H); 7.5Á7.6 (d, 2H); 7.7 (s, 1H); 7.9 (s, 2H); 9.8
/
H NH2); 3219.5 (s, NÃ
H, CH3, CH); 1642.6 (m, CÄ
C, phenyl);
/
H, NH);
method for calculating electronic spectra and
second-order molecular nonlinear optical proper-
ties can be found elsewhere [14]. The molecular
hyperpolarizabilities were calculated using PM3
parameterization in MOPAC software package
/
/
/
/
/
/
/
(7.0) [15]. The closed-shell restricted HartreeÁ
/
(s, 1H); 11.2 (s, 1H). Elemental Anal. (%) Calc.: C,
46.9; H, 5.5; N, 9.1. Found: C, 46.4; H, 5.3; N, 8.5.
Fock (RHF) formalism was employed. The mono-
excited configuration interaction (MECI) approx-
imation was used to describe the excited states.
The ten highest occupied molecular orbitals and
the ten lowest unoccupied molecular orbitals were
involved in CI mixing. The molecular structure
was optimized utilizing AM1 parameterization
[16].
2.3.5. o-Hydroxybenzaldehyde thiosemicarbazone-
ZDA (5)
IR data (KBr, cmꢂ1): 3452.3 (m, OH); 3359.8,
3166.9 (m, NÃ
2980.5, 2920.7 (w, CÃ
N); 1550.7, 1515.9, 1446.5 (CÃ
1205.4, 1157.2 (m, CÃO); 1043.4 (m, CÄ
NMR (DMSO-d6, ppm): l.8 (s, 12H); 5.2 (s, 2H);
6.7Á6.9 (m, 2H): 7.0Á7.2 (m, 3H); 8.2 (s, 2H); 8.4
/
H, NH2): 3278.8 (m, NÃ
H, CH3 CH); 1606.6 (s, CÄ
C, phenyl); 1278.7,
S).1H
/
H, NH);
/
/
/
/
/
/
/
(s, 1H); 9.8 (s, 1H); 11.3 (s, 1H). Elemental Anal.
(%) Calc.: C, 46.9; H, 5.5; N, 9.1. Found: C, 47.3;
H, 6.2; N, 9.3.
3. Results and discussion
The synthesis of the thiosemicarbazones can be
monitored by IR spectrum to verify the disap-
pearance of the carbonyl signals. Formation of the
complexes also can be monitored by IR spectrum,
2.4. Calculation of the second-order NLO response
All calculations were performed on a PC
Pentium III 750MHz. The all-valence ZINDO/S
(intermediate neglect of differential overlap)
method in HYPERCHEM 5.11 software package
(Hypercube, Florida) [13] was employed to calcu-
late electronic spectra. Details of the Z1NDO
in which the CÄ
/
S signals had red shifts of about 20
cmꢂ1, and by appearance of the resonance peaks
1
in 1.8 ppm (s, 12H) and 5.2 (s, 2H) in H NMR
spectra, which indicate the inclusion of acetylace-
tone.

P. Ren et al. / Spectrochimica Acta Part A 59 (2003) 1095Á
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1099
Table 2
ZINDO/S derived maximal absorption (lmax in nm), oscillator strengths (f), dipole moment changes between ground state and excited
state (Dm in D), and composition of the first excited states
Complex
lmax
F
Dm
5.21
Composition of Cl expansionf
1a
311
271
322
273
319
305
273
323
305
274
321
307
273
1.107
0.620
0.87
ꢂ
ꢂ/
/
0.665x78 Á 81
0.508x80 Á 85
0.599x78 Á 81
0.480x80 Á 85
0.554x75 Á 78
0.374x77 Á 78
0.448x77 Á 82
0.516x72 Á 75
0.376x74 Á 75
0.437x74 Á 79
0.528x72 Á 75
0.360x74 Á 75
0.446x74 Á 79
ꢂ
/
1.69
2.22
2.25
3.37
3.14
2.96
3.55
3.38
3.27
4.51
2.42
4.48
2b
3c
ꢂ
/
0.71
ꢂ
/
ꢂ/
ꢃ
/
0.326x79 Á 84
0.774
0.335
0.715
0.632
0.424
0.704
0.67
ꢂ
/
ꢂ
/
ꢂ
/
ꢂ
/
ꢂ/
ꢂ
/
0.323x78 Á 81
4d
5e
ꢂ
/
ꢂ
/
ꢂ
/
0.349x72 Á 75
ꢂ
/
ꢂ/
ꢂ
/
0.23
0.74
ꢂ
/
ꢂ/
a
Orbital 80 is the HOMO, and 81 is the LUMO.
HOMO, 80; LUMO, 81.
HOMO, 77; LUMO, 78.
b
c
d
e
f
HOMO, 74; LUMO, 75.
HOMO, 74; LUMO, 75.
Configuration interaction.
3.1. Solvatochromism and electronic spectra
NLO properties of organometallic and coordina-
tion NLO materials, in which several optical
transitions may be involved in the NLO response
[17]. Anyway, it is reasonable to say that the
The UV-vis absorption wavelengths of the
complexes in some solvents of different polarities
are listed in Table 1. It is obvious that all
absorption maxima of the five complexes are
located in UV region, which is in agreement with
the colors of the five complexes listed in Table 3.
All of the compounds exhibit solvatochromism,
that is, their maximal absorption peaks show
hypsochromic or bathochromic shifts with increas-
ing solvent polarity. This means negative or
positive dipole moment change between the
ground and the excited state, respectively, and
hence indicate obvious charge-transfer (CT) char-
acter for the associated optical transition. It is
notable that all of the five complexes have two
bands showing solvatochromism. It is well known
that solvatochromism is based on Two-Level
Model in which only HOMO and LUMO, i.e.
ground and first excited-state, are involved in CT
character, and is relatively valid in a large number
of organic NLO chromophores. However, it is
quite complicated in predicting the extent of the
solvatochromism in 1Á5 indicates the change of
/
the dipole moments in the ground and the excited
states (Dm) which verifies the existence of intra-
molecular charge transfer.
The results of ZINDO/S calculation are listed in
Table 2. The experimental optical spectra in
relatively less polar chloroform solution and
calculated ZINDO/S derived electronic spectrum
of complexes 1 and 2 are compared in Fig. 1. The
agreement between calculation and experiment is
rather good, although there exist 10Á40 nm error
/
in lmax between calculated and experimental data,
which is within the error range of ZINDO/S
calculation of some other types of complexes
[17,18]. The results show all complexes have
more than one maximal absorption peak. In the
case of complex 1, there are mainly two absorption
maxima at 311 nm (fꢀ
0.620). As for complex 2, they are at 322 nm (fꢀ
0.87) and 273 (fꢀ0.71).
/
1.107) and 271 nm (fꢀ
/
/
/

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P. Ren et al. / Spectrochimica Acta Part A 59 (2003) 1095Á1101
/
Fig. 2. AMI optimized molecular structure of complex 1.
3.2. Calculations of b values
MOPAC 97 software package has been used to
Fig. 1. ZINDO/S derived electronic transitions of the complex
1 (top) and 2 (bottom) and their comparison with experimental
data. The columns represent the oscillator strength (f) of the
various transitions, while the curves represent the experimental
optical spectrum.
calculate the b values of complexes 1Á5 and their
/
corresponding thiosemicarbazone ligands (Table
3). AM1 parameterization was utilized to optimize
the structures of the five compounds and found
that zinc, sulfur and four oxygens constitute
square-pyramidal configuration (Fig. 2), which is
consistent with the crystal structure of PZDA [10].
Table 3
The calculated b0 and lmax recorded in chloroform of the complexes and the corresponding thiosemicarbazone ligands
b0 (cal) (ꢁ
/
10ꢂ30 (esu))
lmax (nm)
Colors
Complex
Ligand
Complex
Ligand
Complex
Ligand
1
2
3
4
5
30.6
11.7
12.4
10.3
7.8
21.5
9.3
9.8
7.9
4.9
364
332
327
331
350
365
332
326
328
337
Light yellow
White
White
Light yellow
White
White
White
White
White
White

P. Ren et al. / Spectrochimica Acta Part A 59 (2003) 1095Á
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1101
1101
We can find that the calculated first molecular
hyperpolarizabilities (b) of all of the five coordi-
nation compounds are larger than that of p-
of square-pyramidal coordination compounds.
The involvements of Zn(II) ion with d10 electron
structure and colorless thiosemicarbazones in the
complexes result in obvious enhancements of the
first-order molecular hyperpolarizabilities, com-
paring with their ligands, while the colors un-
changed.
nitroaniline (6.8ꢁ
among which complex 1 shows the largest b
(39.1ꢁ
10ꢂ30 esu). Comparing with their corre-
/
l0ꢂ30 esu, calculated value),
/
sponding thiosemicarbazone ligands, the b values
of the five coordination compounds also increase
dramatically.
The experimental absorption maxima of longest
wavelength of the complexes and their correspond-
ing thiosemicarbazone ligands recorded in chloro-
form are listed in Table 3. It seems that the bands
of the complexes do not show obvious red shifts
comparing with their corresponding ligands. This
is due to the d10 electron structure of Zn(II) ion,
which makes it difficult to undergo intramolecular
charge transfer process through metal ion. So we
can infer that the zinc ion is hardly involved in the
intramolecular CT process [9]. On the other hand,
the metal complexation decreases the electron
density of S atom (changed from 6.2420 to
6.1603 for 1, MOPAC calculated result), thus
Acknowledgements
The authors are grateful to the National Key
Fundamental Research Program of China for
financial support.
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/
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/
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/
S group in thiosemicarbazone. This may be
the reason that the first-order molecular hyperpo-
larizabilities of the complexes have increased
comparing with the corresponding thiosemicarba-
zones, while the colors of the complexes are still
the same as those of their ligands.
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4. Conclusions
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A new method aiming at suppressing so-called
nonlinearityÁtransparency trade-off through co-
/
ordination chemistry has been delivered on the
basis of spectroscopic studies and first-order
molecular hyperpolarizabilities analyses of a series
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