Design and synthesis of 1,10-phenanthroline based Zn(II) complexes bearing 1D push-pull NLO-phores for tunable quadratic nonlinear optical properties

Article abstract of DOI:10.1016/j.jorganchem.2006.01.043

Three Zn(II) complexes bearing 1,10-phenanthroline and one-dimensional (1D) push-pull NLO-phores with various acceptor strength as well as π-conjugation length have been synthesized in high yields for two-dimensional (2D) nonlinear optical response. The q

Full text of DOI:10.1016/j.jorganchem.2006.01.043

Journal of Organometallic Chemistry 691 (2006) 2512–2516
www.elsevier.com/locate/jorganchem
Design and synthesis of 1,10-phenanthroline based Zn(II)
complexes bearing 1D push–pull NLO-phores for tunable
quadratic nonlinear optical properties
a
a
a
a
b
Sanjib Das , Atanu Jana , V. Ramanathan , Tapas Chakraborty , Sampa Ghosh ,
b
a,
*
Pushpendu K. Das , Parimal K. Bharadwaj
a
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, Uttar Pradesh, India
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India
b
Received 14 December 2005; received in revised form 17 January 2006; accepted 19 January 2006
Available online 3 March 2006
Abstract
Three Zn(II) complexes bearing 1,10-phenanthroline and one-dimensional (1D) push–pull NLO-phores with various acceptor
strength as well as p-conjugation length have been synthesized in high yields for two-dimensional (2D) nonlinear optical response.
The quadratic optical nonlinearity of the ligands and the complexes are measured by the HRS technique. The ligands show small sec-
ond-order optical nonlinearity (b) comparable to the standard, para-nitroaniline (pNA). However, upon complexation with Zn(II), each
complex exhibits large b values showing the importance of metal ion in enhancing the optical nonlinear effect.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Zn(II) complex; Push–pull NLO-phore; Metal ion template; NLO modulation
1. Introduction
ity is accompanied by a bathochromic shift of the elec-
tronic transition, leading to undesirable dispersion,
fluorescence and re-absorption effects [3]. Beside, the
second-order molecular optical nonlinearity (bijk) of such
classical dipolar push–pull NLO-phores is mostly one-
dimensional in character and dominated by one hyperpo-
larizability b tensor component. Several design principles
have been adopted during the past decade to circumvent
these drawbacks by extending the charge transfer from
one to higher dimensions (2D,3D) [4]. Thus, recognition
of molecules with multi-dimensional (nD) charge-transfer
characteristics in nonlinear optics have triggered various
synthetic research activities [5–7] with multi-polar mole-
cules for second-order NLO applications. Compared to
1D dipolar NLO-phores, they offer several advantages such
as increased b responses due to multi-dimensional b tensor
components, easier non-centrosymmetric arrangements
and an improved nonlinearity trade-off. Among the miscel-
laneous goals and strategies, one attractive approach is the
Synthesis of molecules displaying large second-order
nonlinear optical (NLO) properties is of interest for their
potential applications [1] in telecommunication, optical
computing, optical data storage and so on. Efficiency of
this property is determined by measuring the molecular
first hyperpolarizability, b. At the molecular level, com-
pounds likely to exhibit large b values must have polariz-
able electrons (i.e., p-electrons) spread over a large
distance. Thus, organic dipolar compounds with extended
p systems having terminal donor and acceptor groups are
likely to exhibit large b values [2]. However, a major prob-
lem associated with these one-dimensional (1D) dipolar
chromophores is the nonlinearity-transparency trade-off,
such that the increase in the second-order hyperpolarizabil-
*
Corresponding author.
E-mail address: pkb@iitk.ac.in (P.K. Bharadwaj).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.01.043

S. Das et al. / Journal of Organometallic Chemistry 691 (2006) 2512–2516
2513
design of compounds in which the assembly of several
(identical or complementary) 1D NLO-phore leads to an
improved optical response to the resulting multi-chromo-
phoric compound with respect to its monomeric counter-
part. There are few examples based on calix [4] arenes [8],
b-cyclodextrin [9], polyamides [10], dendrimers [11] where
1D dipolar NLO-phores are incorporated via covalent
linkages to show significantly large b values in comparison
to the monomeric dipolar counterparts.
Metal ions being excellent templates, can gather organic
1D dipolar chromophores around to form predetermined
2D and 3D NLO-phores with various symmetries and
charge-transfer dimension tunable by virtue of the coordi-
nated metal center as well as the presence of polarizable d
electrons can also contribute [12] to greater nonlinear activ-
ity (Scheme 1).
to the metal ion resulting in large enhancement of b values
with respect to the corresponding 1D dipolar ligand.
2. Experimental section
Reagent grade 4-dimethylaminobenzoic acid (L1), 4-
(dimethylamino)cinnamic acid (L2), 4-dimethylaminobenz-
aldehyde, tert-butyl-cyanoacetate and 1,10-phenanthroline
monohydrate were acquired from Lancaster. Reagent
grade zinc(II) acetate dihydrate was from SD Fine Chemi-
cals, India. All these chemicals were used as received. All
solvents (SD Fine Chemicals) were freshly distilled prior
to use.
3. Analysis and measurements
Other advantages of using metal complexes as NLO
materials include higher damage threshold, fast response
time and easy synthesis. Recent reports are available in
the literature on Zn(II), Cu(I) and Ru(III) complexes of
4,40-bis(dialkylaminostyryl)-[2,20]-bipyridine ligands [13]
with large b values of 140–340 · 10ꢀ30 esu. As the Zn(II)
ion usually prefers tetrahedral coordination geometry, it
can serve as an excellent 3D template. We describe here
the synthesis of three 1,10-phenanthroline based Zn(II)
complexes (Fig. 1) for two-dimensional nonlinear optical
response where 1D dipolar NLO-phores with different
length of p-conjugation and acceptor strength are bonded
Spectroscopic data were collected as follows: IR (KBr
disk, 400–4000 cmꢀ1) Perkin–Elmer Model 1320; UV–Vis
spectra were recorded on a JASCO V-570 spectrophotom-
1
eter in CH3CN at 298 K. H NMR spectra were recorded
on a JEOL JNM-LA400 FT (400 MHz) instrument in
CDCl3 with Me4Si as the internal standard. The electro-
spray mass spectra (ES-MS) were recorded on a MICRO-
MASS QUATTRO Quadruple Mass Spectrometer. The
samples, dissolved in acetonitrile were introduced into the
ESI source through a syringe pump at the rate of 5 ll/
min. The ESI capillary was set at 3.5 kV and the cone volt-
age was 40 V. The spectra were collected in 6 s scans and
the print outs were average spectra of 6–8 scans. Melting
points were determined with an electrical melting point
apparatus by PERFIT, India and were uncorrected. Micro-
analyses for the complexes were obtained from CDRI,
Lucknow, India.
Mn+
Mn+
4. Synthesis
R
Mn+= Metal ion
Among the three ligands used in this study, the first two
(i.e., L1 and L2) are commercially available (Lancaster).
The ligand 4-dimethylamino-a-cyanocinnamic acid (L3)
was synthesized via Knoevenagel condensation as shown
in Scheme 2.
D- -A
π
1D dipolar structure
2D dipolar structure
low β value
high β value
Scheme 1. A schematic representation of designing 2D dipolar structure
through metal–ligand coordination for tunable NLO responses. R,
receptor for metal ion; Mn+, Zn(II) ion.
Typically, to a 50 mL absolute ethanolic solution of 4-
dimethylaminobenzaldehyde (0.6 g; 4 mmol) was added
N
N
N
O
O
O
NC
O
O
O
N
Zn
N
O
N
N
N
N
Zn
Zn
O
O
NC
O
O
O
N
N
N
1
2
3
Fig. 1. Schematic representation of the Zn(II) complexes used in the study.

2514
S. Das et al. / Journal of Organometallic Chemistry 691 (2006) 2512–2516
CHO
CN
CN
(ii)
(i)
COOH
COO-tert-butyl
N
N
N
L3
Scheme 2. Synthetic scheme for L3. Reagents and conditions: (i) tert-butyl cyanoacetate, 2 drops of piperidine and absolute ethanol (ii) trifluoroacetic
acid.
0.6 mL (4.4 mmol) of tert-butylcyanoacetate through a syr-
inge all at a time followed by catalytic amount (2 drops) of
piperidine. The orange yellow solid that appeared on stir-
ring overnight at room temperature, was collected by filtra-
tion, washed thoroughly with ethanol and dried under
vacuum. This solid was stirred for 2 h, with 3 mL of triflu-
oroacetic acid and then poured into 50 mL of water. The
orange yellow solid that separated, was collected by filtra-
tion, washed with ethanol and finally dried under vacuum.
Yield: 90%. Anal. Calc. for C12H12N2O2: C, 66.65; H, 5.59;
N, 12.96. Found: C, 62.88; H, 5.72; N, 13.08%.
1181vs, 1064w, 985vs, 946m, 864vs, 813vs, 728vs,
704s cmꢀ1 (vw, very weak; w, weak; m, medium; s, strong;
vs, very strong).
4.3. [Zn(phen)(L3)2] (3)
Synthesis of this complex is also accomplished following
a similar procedure taking 4-dimethylamino-a-cyanocin-
namic acid (L3), (0.43 g; 2 mmol) in place of L2. The bright
yellow solid that separated upon stirring overnight was col-
lected by filtration, washed with methanol and dried under
1
vacuum. Yield: 87%. H NMR (400 MHz, CDCl3, TMS,
4.1. [Zn(phen)(L1)2] (1)
25 ꢁC): d 2.86 (s, 6H), 2.98 (s, 6H), 6.47 (d, J = 9.03,
2H), 6.55 (d, J = 9.03, 2H), 7.65 (d, J = 8.79, 4H), 7.85–
7.78 (m, 4H), 7.96 (s, 2H), 8.41 (d, J = 6.79, 2H), 9.21
(bs, 2H); ES-MS (m/z): 675(18%) [MꢀH]+. Anal. Calc.
for C36H30N6O4Zn: C, 63.96; H, 4.47; N, 12.43. Found:
C, 64.11; H, 4.55; N, 12.57%. IR (KBr phase): 3108vw,
2905w, 2207vs, 1769m, 1601vs, 1526vs, 1351vs, 1235s,
1182vs, 945m, 814s, 719vs, 544m cmꢀ1 (vw, very weak; w,
weak; m, medium; s, strong; vs, very strong).
A methanolic solution (15 mL) containing 4-dimethyla-
minobenzoic acid (L1), (0.33 g; 2 mmol), Zn(OAc)2 Æ 2H2O
(0.22 g; 1 mmol) and 1,10-phenanthroline monohydrate
(0.2 g; 1 mmol) were allowed to stir for 24 h at room tem-
perature. An off-white solid separated which was collected
by filtration, washed with methanol and finally dried under
vacuum. The same product is obtained on using different
molar ratios of the reactants signifying stability of the
product under the reaction conditions. Yield: 80%. 1H
NMR (400 MHz, CDCl3, TMS, 25 ꢁC): d 2.72 (s, 12H),
6.24 (d, J = 8.79, 4H), 7.55–7.52 (m, 1H), 7.60 (d,
J = 8.79, 4H), 7.64–7.62 (m, 1H), 7.69 (bs, 2H), 8.26 (d,
J = 8.03, 2H), 9.06 (bs, 2H); ES-MS (m/z): 572(35%)
[MꢀH]+. Anal. Calc. for C30H28N4O4Zn1: C, 62.78; H,
4.92; N, 9.76. Found: C, 62.88; H, 5.01; N, 9.85%. IR
(KBr phase): 3082vw, 2911vw, 2814vw, 1605vs, 1523s,
1475m, 1441s, 1396vs, 1362vs, 1318m, 1194vs, 782vs,
611s cmꢀ1 (vw, very weak; w, weak; m, medium; s, strong;
vs, very strong).
5. NLO measurements
Second, harmonic measurements in solution were car-
ried out by the hyper-Rayleigh scattering (HRS) technique
[14]. The fundamental (1064 nm) of a Q-switched Nd:YAG
laser (Spectra Physics, DCR-3G, 8 ns) beam was focused
by a biconvex lens (f.1. 10 cm) to a spot 5 cm away after
passing through the glass cell containing the sample. The
scattered light in the perpendicular direction was collected
by a UV–Vis sensitive photomultiplier tube (PMT). For
wavelength discrimination, a monochromator (Czerny
Turner 0.25 m) was used and no other collection optics
was employed. The input power was monitored using a
power meter. All data were collected at laser power
624 mJ pulseꢀ1 that is below the threshold for stimulated
Raman, self-focusing/self-defocusing, Brillouin scattering,
and dielectric breakdown. The experimental set-up was
first standardized by measuring the b value for para-nitro-
aniline (pNA) in CH3CN by the external reference method
[15] and a value of 22.1 · 10ꢀ30 e.s.u. was obtained that was
close to the reported value for this compound. The mono-
chromator was scanned at intervals of 2 nm to find if the
signal at the second harmonic wavelength has any contri-
bution from two or multi-photon fluorescence in L1, L2,
L3 and 1–3. In fact, it was found that all the compounds
do not have any two-photon fluorescence around 532 nm.
All the complexes showed excellent stability under laser
4.2. [Zn(phen)(L2)2] (2)
This compound was isolated in a similar manner taking
4-dimethylamino cinnamic acid (L2) (0.38 g; 2 mmol) in
place of L1 keeping other reactants unchanged. The yellow
solid was collected by filtration, washed thoroughly with
1
methanol and dried under vacuum. Yield: 85%. H NMR
(400 MHz, CDCl3, TMS, 25 ꢁC): d 2.72 (s, 12H), 5.88 (d,
J = 15.87, 2H), 6.21 (d, J = 8.55, 4H), 6.9 (d, J = 8.79,
4H), 7.05 (d, J = 15.87, 2H), 7.59 (bs, 2H), 7.67 (bs, 2H),
8.26 (d, J = 7.79, 2H), 8.95 (bs, 2H); ES-MS (m/z):
625(20%) [MꢀH]+. Anal. Calc. for C34H32N4O4Zn1: C,
65.23; H, 5.15; N, 8.95. Found: C, 65.33; H, 5.23; N,
9.07%. IR (KBr phase): 3070vw, 2892vw, 2806vw,
1631vs, 1606vs, 1527vs, 1429vs, 1362vs, 1252vs, 1225s,

S. Das et al. / Journal of Organometallic Chemistry 691 (2006) 2512–2516
2515
1.8
irradiation and no sign of decomposition could be
detected. Blank experiments (without ligands) with metal
ions were performed, that showed no contribution to the
HRS intensity due to the metal salts. Fig. 2 displays the
plots of I2x=I2x vs. number density of 1, 2 and 3 as well
as reference standard pNA. From the ratio of the two
slopes (i.e., ratio of slope of compound and pNA) in
Fig. 2a and b, the b value of 1, 2 and 3 was determined
by external reference method.
1
1.5
1.2
0.9
0.6
0.3
0.0
2
3
L2
L1
L3
6. Results and discussion
All the complexes are stable in air and their solubility in
common organic solvents. The IR spectra of complexes 1–
3 show that the coordination geometry around the Zn(II)
ion are almost same for all the complexes. Although the car-
bonyl stretching frequency of complexes 1 and 3 are
1605 cmꢀ1 and 1601 cmꢀ1, respectively whereas in case of
complex 2 it is 1631 cmꢀ1 attributable to the more symmetric
binding nature [16a] of the carboxylate to the Zn(II) ion in
complex 2. The cyano group in complex 3 remains non-inter-
200
250
300
350
400
450
500
550
Wavelength (nm)
Fig. 3. Absorption spectra of the ligands and their Zn(II) complexes in
CH3CN (conc. 1 · 10ꢀ5 M).
sponding b0 static values are derived from the well-known
two-state model [18] and are listed as well. The first hyper-
polarizability (b) for L1–L3 are slightly lower comparable
to that of the standard, pNA (Table 1) although the trend
shows that the b value increases with higher conjugation
and acceptor strength causing greater polarization of the
entity which can stabilize the charge-separated form much
more as expected [2e,19].
acting with the metal and shows a strong peak at 2207 cmꢀ1
.
Metal-coordinated cyano groups show [16b] the stretching
frequency above 2250 cmꢀ1. The thermal analysis data
obtained on the complexes [17] show that each complex is
stable at least up to 250 ꢁC that make them potentially useful
as NLO materials. The complexes were characterized by 1H
NMR, IR and ES-Mass spectroscopy as well as by elemental
analysis [17]. The UV–Vis spectral data of the three ligands
and corresponding Zn(II) complexes recorded in dry
CH3CN are displayed in Fig. 3. The ligands exhibit intra-
ligand charge transfer absorption in the near UV-region.
The peak position is quite sensitive to the length of conjuga-
tion as well as acceptor strength and shifts monotonously to
higher wavelengths as the conjugation length and acceptor
strength increases. On complexation with Zn(II), a small
blue-shift is observed (Fig. 3) in each case offering better
transparency compared to the ligands.
Table 1
Linear and nonlinear optical data
Compound
k
max/nm (e/dm3 b (10ꢀ30 esu) b0 (10ꢀ30 esu) Td (ꢁC)a
molꢀ1 cmꢀ1
340
)
pNA
22.1
11.7
–
(standard)
L1
L2
L3
1
307 (45,781)
358 (44,959)
417 (44,479)
292 (158,733)
349 (98,256)
404 (71,577)
10.2
16.7
20.2
49.0
82.7
107.9
6.2
8.1
6.6
31.3
41.5
39.2
–
–
–
300
260
260
2
3
The molecular first hyperpolarizabilities of L1–L3 and
their Zn(II) complexes are collected in Table 1. The corre-
a
Decomposition temperature.
0.65
0.50
pNA
1
2
0.60
0.45
0.40
0.35
0.30
0.25
3
0.55
0.50
0.45
0.40
0.35
0.30
10
11
12
13
14
15
16
17
18
19
20
4.0
4.5
5.0
5.5
6.0
6.5
Number Density x 1015 molecules/cc
Number Density x 1015 molecules/cc
a
b
Fig. 2. (a) Plots of I2x=I2x vs. number density of 1, 2 and 3. (b) A plot of I2x=I2x vs. number density of reference standard pNA.

2516
S. Das et al. / Journal of Organometallic Chemistry 691 (2006) 2512–2516
(b) E.M. Breitung, C.F. Shu, R.J. McMahon, J. Am. Chem. Soc. 122
The absorption spectra of the Zn(II) complexes show
(2000) 1154;
blue shift with a broadening compare to its monomeric
counterparts L1–L3 that can attributable from several
interactions between the 1D NLO-phores such as electro-
static or p–p cofacial interactions [20], excitanic coupling
[21], hindered solvation or aggregation effects within the
Zn(II) complex. As the harmonic wavelength is far enough
from the kcut-off for the ligands as well as their Zn(II) com-
plexes, any contribution of two-photon-induced fluores-
cence to the HRS signal can be considered as negligible.
Table 1 shows that 2 possesses higher b value compared
to 1 whereas 3 exhibits the highest b value among the three.
The Zn(II) ion organizes two 1D NLO-phores ligands
around leads to strong dipolar interactions enhancing the
polarizability of the complex unit which can significantly
affect in the marked increase of the b values of the com-
plexes compared to the 1D chromophores underlying the
importance of metal ions in modulating the optical
nonlinearity.
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In conclusion, we have shown that a simple synthetic
strategy to obtained 2D dipolar Zn(II) complexes from
1D push–pull NLO-phoric ligands via metal-template effect
with very large second-order optical nonlinearity (b val-
ues). The monomeric 1D dipolar ligands themselves do
not exhibit any significant b values but readily form com-
plexes with Zn(II) ion resulting various 2D dipolar struc-
tures that shows significantly high b values in comparison
to the monomeric 1D dipolar counterparts underlying the
importance of the Zn(II) ion in the spontaneous self-orga-
nization of the 1D dipolar NLO-phores within the com-
plexes. The high thermal stability and large optical
nonlinearity of these complexes offer them potential candi-
date as a NLO materials. We are presently working with
metal mediated second as well as third-order nonlinear
optical properties where the metal ions are taken from both
the transition and the inner transition series.
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Acknowledgements
Financial support received from DRDO, India (to PKB)
is gratefully acknowledged.
[17] See supplementary material.
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