Second order molecular nonlinearities in new orthopalladated push-pull chromophores

Article abstract of DOI:10.1016/j.ica.2003.06.020

Three new orthopalladated chromophores based on push-pull π conjugated ligands have been prepared and characterized. The second order molecular NLO activity has been determined by EFISH measurements giving μ _gβ values up to 610 × 10^-48

Full text of DOI:10.1016/j.ica.2003.06.020

Inorganica Chimica Acta 357 (2004) 913–918
www.elsevier.com/locate/ica
Second order molecular nonlinearities in new orthopalladated
push–pull chromophores
a,
b
c
a
a
Roberto Centore *, Alain Fort , Barbara Panunzi , Antonio Roviello , Angela Tuzi
a
Dipartimento di Chimica, Universitꢀa degli Studi di Napoli ‘‘Federico II’’, Via Cinthia, 80126 Napoli, Italy
b
IPCMS-CNRS, Groupe dÕOptique Nonlinꢁeaire et dÕOptoꢁelectronique, 23 Rue du Loess, 67037 Strasbourg Cedex, France
c
Dipartimento di Scienza degli Alimenti, Universitꢀa degli Studi di Napoli ‘‘Federico II’’, Via Universitꢀa 100, 80055 Napoli, Italy
Received 17 March 2003; accepted 18 June 2003
Abstract
Three new orthopalladated chromophores based on push–pull p conjugated ligands have been prepared and characterized. The
second order molecular NLO activity has been determined by EFISH measurements giving lgb values up to 610 ꢀ 10ꢁ48 esu at an
incident laser wavelength of 1.907 lm. The X-ray structures of two different polymorphs of one chromophore are also reported and
polymorphism is discussed in terms of conformational changes.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Nonlinear optics; EFISH; Metallorganic; Palladium; Complexes
1. Introduction
properly functionalized for use as monomers in the
synthesis of NLO active polymers or networks.
There has been an increased interest in recent years in
metal containing compounds for applications in second
order nonlinear optics (NLO) [1]. As compared with all
organic p conjugated push–pull compounds, metal co-
ordination introduces additional chemical variables
(oxidation number, coordination geometry, nature of
the ligands, ionic charge etc.) that may be potentially
used for improving molecular nonlinearities. Most of
the studies reported to date deal with the use of orga-
nometallic fragments attached at the end of p conju-
gated organic systems as electron donor [2] or acceptor
[3] groups. Less investigated is the use of metals as
conjugation bridges along the p conjugated organic
system [4]. In the latter case the choice of the metal and
of the coordination geometry must be compatible with
an efficacious p-conjugation path passing across the
metal atom (e.g. through dp–pp interactions), square
planar coordinating metals being suitable candidates.
Furthermore, metal containing chromophores must be
We have recently reported the synthesis of cyclopal-
ladated compounds containing Schiff bases functional-
ized as acrylates. Starting from these compounds, side-
chain homo and copolymers were obtained having
suitable properties (solubility, transparency in thin
films) for the measurement of bulk second order NLO
activity [5]. The crystal structure and molecular non-
linearity (EFISH technique) of some model chromo-
phores have also been reported [6]. In that study the
electron donor and acceptors groups were placed in
opposite directions with respect to the metal so that
conjugation across the metal atom was a necessary
condition for NLO activity. The modesty of the mea-
sured NLO response (probably due also to nonoptim-
ized positions of donor and acceptor groups along the
conjugation path) prompted us to study compounds in
which donor/acceptor groups are placed on the same
side of the metal. The results of this investigation are
reported here, regarding three complexes of formula:
(see Scheme 1).
The (N, O) chelating moieties on Pd(II) are push–pull
conjugated systems closely related to well investigated
NLO active compounds [7]. An improved behaviour as
* Corresponding author. Tel.: +39-081-674090; fax: +39-081674450.
E-mail address: centore@chemistry.unina.it (R. Centore).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.06.020

914
R. Centore et al. / Inorganica Chimica Acta 357 (2004) 913–918
Scheme 1.
compared with the pure organic ligands may be ex-
pected in all the three complexes. In fact, for 1 a reduced
influence of photochemically or thermally induced
trans–cis isomerization of the azobenzene moiety should
be observed due to coordination. For 2, although the
sole electronic effect of binding to metal is expected to
increase proneness to nucleophile attack, in the whole a
higher general stability of the imine can be prompted by
coordination (see for instance [8]). Finally, for 3 the
coordinated 2-phenyl-benzoxazole ligand is constrained
in the planar conformation while deviations from pla-
narity can be observed in noncoordinated compounds
[9].
2. Experimental part
2.1. Synthesis
The compounds were prepared according to the
general scheme reported below by reacting dinuclear Pd
complex A with the appropriate chelating agent (C1, C2,
C3) in stoichiometric amounts (Scheme 2).
The Schiff base CH3O–p–C6H4O–p–COO–p–C6H4–
p–CH@N–p–C6H4–p–OCH3 (B) was obtained following
the general procedure already described [10]. The dinu-
clear complex A was obtained by reaction of palla-
dium(II) acetate with the stoichiometric amount of B
It should also be noted that the reported complexes
may be considered as models of monomers for the
synthesis of NLO active polymers. Monomers could be
easily obtained, as an example, by replacing the p-
methoxybenzoate group of the orthopalladated ligand
by p-acryloyloxy-alcoxybenzoate according to proce-
dures we have already reported in similar systems [5].
following
a
general procedure already reported
[11].Yield: 40% (mp 260 °C dec.). Anal. calc. for
C48H42N2O12Pd2: Pd 20.23%. Found: Pd 21.62%. 1H
NMR (CDCl3) d: 1.83 (s, 3H); 3.58 (s, 3H); 3.99 (s, 3H);
6.51 (d, 1H); 6.72 (d, 2H); 7.01 (d, 2H); 7.10 (dd, 2H); 7.30
(d, 2H); 7.69 (s, 1H); 8.28 (d, 2H). The dinuclear species
was used as crude product in the further reactions.
Scheme 2.

R. Centore et al. / Inorganica Chimica Acta 357 (2004) 913–918
915
Synthesis of C1: 2-(4-nitrophenylazo)-3-diethylamin-
ophenol. C1 was synthesized according to literature
methods [12] by addition of sodium nitrite to a cold
solution of 4-nitroaniline in hydrochloric acid (6 M) and
coupling of the diazonium salt with 3-(diethylamino)-
phenol in the presence of sodium acetate. After crys-
tallization in chloroform/hexane pure C1 was obtained
(yield 80%, mp 225 °C).
Synthesis of C2: 2-(4-dimethylaminophenyliminom-
ethyl)-5-nitrophenol. To a solution of 1.50 g (10 mmol)
of N; N-dimethyl-4-nitrosoaniline in 20 ml of ethanol/
water (3:1) was added a large excess (about 7 g) of so-
dium hydrosulfite, the colour turning from brown to
yellow. To this suspension, 1.67 g (10 mmol) of 4-ni-
trosalicyl-aldehyde [13,14] were added and, after about 1
h stirring, a dark red solid precipitated. This solid was
recovered by filtration and recrystallized from chloro-
form as red crystals. Yield 60%.
dark to brick red. After stirring about 1 h at room
temperature the mixture was filtered and concentrated in
vacuum to ꢂ40 ml. A brick red precipitate was recovered
by filtration and crystallized twice in dichloromethane/
hexane. Yield 45%. 2 and 3 both crystallized in dichlo-
romethane/n-hexane, were obtained as light brown (yield
55%) and light orange crystals (yield 40%), respectively.
1. Anal. calc. for C38H35N5O7Pd: Pd, 13.66; C, 58.50; H,
4.52; N, 8.98%. Found: Pd: 13.43, C, 58.35; H, 4.49;
N: 8.87%.1H NMR (CDCl3) d: 1.30 (t, 6H); 3.48
(dd, 4H); 3.93 (d, 6H); 5.81 (dd, 2H); 6.36 (dd, 1H);
6.82 (dd, 1H); 7.01 (dd, 4H); 7.37 (m, 4H); 7.94 (d,
2H); 8.07 (d, 2H); 8.16 (m, 3H). UV–Vis (CHCl3):
kmax 518 nm, emax3:9 ꢀ 104 dm3 molꢁ1 cmꢁ1
.
2. Anal. calc. for C37H32N4O7Pd: Pd, 14.17%. Found:
Pd, 14.49%.1H NMR (CDCl3) d: 2.69 (s, 6H); 3.92
(s, 6H); 5.30 (d, 1H); 6.52 (d, 2H); 6.77 (dd, 1H);
6.99 (dd, 4H); 7.27 (m, 8H); 7.99 (d, 2H); 8.07 (d,
2H). UV–Vis (CHCl3): kmax 489 nm, emax1:4 ꢀ 104
1H NMR (CDCl3) d: 3.07 (s, 6H); 6.79 (d, 2H); 7.30
(d, 2H); 7.42 (d, 1H); 7.72 (dd, 1H); 7.80 (s, 1H); 8.71 (s,
1H); 14.25 (s, 1H).
Synthesis of C3. 2-[2-hydroxy-4-diethylaminiphenyl]-
6-nitrobenzoxazole. The synthesis was performed in two
steps.
dm3 molꢁ1 cmꢁ1
.
3. Anal. calc. for C39H34N4O8Pd: Pd, 13.42; C, 59.06;
H. 4.32; N 7.06%. Found: Pd, 13.84; C, 59.05; H,
4.32; N 6.94%.1H NMR (CDCl3) d: 1.32 (t, 6H);
3.45 (dd, 4H); 3.90 (s, 6H); 5.91 (d, 1H); 6.20 (dd,
1H); 6.42 (d, 1H); 6.70 (d, 1H); 6.83 (d, 1H); 7.17
(m, 4H); 7.31 (d, 1H); 7.54 (dd, 2H); 7.77 (dd, 2H);
8.22 (m, 4H). UV–Vis (CHCl3): kmax 460 nm,
(1) 2-[[[4-(diethylamino)-2-(hydroxy)phenyl]methylene]
amino]-5-nitrophenol. 5.00 g (32 mmol) of 2-amino-5-ni-
trophenol were dissolved in about 80 ml of hot acetic
acid, and 6.00 g (31 mmol) of 4-(diethylamino)salicylal-
dehyde were dissolved in about 60 ml of hot acetic acid.
The two solutions were mixed with stirring. After a few
minutes an orange solid of the Schiff base precipitated,
which was recovered by filtration, washed with ethanol,
and dried at 80 °C for 2 h. Yield 87 %, mp 279 °C.
(2) C3. To a suspension of 9.34 g (28 mmol) of the
iminic precursor in 140 ml of dry dioxane/acetic acid
(10:4) 13.20 g (29 mmol) of lead (IV) acetate were added
and the temperature was kept at 5–7 °C during the ad-
dition. After this time, the suspension was stirred for 4 h
at 8–10 °C, and finally for 1 h at room temperature. The
suspension was poured in about 400 ml of water. A
brown solid precipitate formed, which was recovered by
filtration, washed in water, then dried at 80 °C for 4–5 h.
It was purified by chromatography in dichloromethane
on a florisil 100–200 mesh column. From the concen-
trated solution bright orange crystals precipitated. Yield
e
max3:3 ꢀ 104 dm3 molꢁ1 cmꢁ1
.
3. Characterization
The thermal behaviour was examined by differential
scanning calorimetry (Perkin–Elmer DSC-7, nitrogen
atmosphere, scanning rate 10 K/min). Temperature
controlled polarizing microscopy was used for optical
observations (Zeiss microscope combined with Mettler
FP5 microfurnace). Thermogravimetric analysis was
performed with Mettler TG50 apparatus. Proton NMR
spectra were recorded on Varian XL 200 spectrometer.
UV–Vis spectra were recorded with a Perkin–Elmer
Lambda 7 UV–Vis spectrophotometer.
4. X-ray analysis of 2
1
20%. (mp 218 °C). H NMR (CDCl3) d : 1.05 (t, 6H);
3.49 (dd, 4H); 6.22 (d, 1H); 6.36 (dd, 1H); 7.62 (d, 1H);
7.80 (d, 1H); 8.31 (dd, 1H); 8.40 (s, 1H); 11.07 (s, 1H).
Synthesis of complexes 1, 2, 3. 1, 2, 3 were prepared
from stoichiometric amounts of A and the correspond-
ing ligands C1, C2, C3 by the same general procedure.
As an example, in a typical preparation of 1, to 0.488 g
(1.7 mmol) of C1 dissolved in 40 ml of dichloromethane
and 20 ml of absolute ethanol were added 0.400 g of
sodium acetate, 0.600 g of potassium carbonate and
finally 0.900 g of A (0.85 mmol). The colour turned from
Single crystals of three polymorphs of 2, i.e. a; b; and c
forms, were obtained by evaporation from a chloroform/
n-hexane solution in form of well developed prismatic
needles, lozenges and square plates, respectively. Weiss-
enberg and oscillation photographs indicated monoclinic
system and the P21=c space group in all cases. Data were
collected on a Nonius MACH3 diffractometer (graphite
monochromated Mo Ka radiation). During data collec-
tions crystal specimens resulted stable as confirmed by
the intensity of control reflections periodically measured,

916
R. Centore et al. / Inorganica Chimica Acta 357 (2004) 913–918
Quartz
11
¼ 1:2 ꢀ 10ꢁ9 esu at 1.06 lm
Table 1
Crystal, collection and refinement data for a and b forms of 2
experimental value of d
was extrapolated to 1:1 ꢀ 10ꢁ9 esu at 1.907 lm. Ne-
glecting the purely electronic contribution compared to
the orientational part and allowing Kleinman symmetry
for rod-like molecules, the projection b of the vector part
of tensor bikj along the direction of the dipole moment is
finally determined. A detailed description of the experi-
mental apparatus has been given elsewhere [16]. Mea-
surements were performed in chloroform solution.
a
b
Chemical formula
Formula weight
T (K)
Crystal system
Space group
C
751.07
37H32N4O7Pd
293
monoclinic
P21=c
ꢂ
a (A)
15.457(2)
6.880(1)
34.120(6)
115.23(1)
3282.3(9)
4, 1.529
18.628(2)
9.292(1)
ꢂ
b (A)
ꢂ
c (A)
19.250(9)
93.31(2)
3327(2)
b (°)
3
ꢂ
V (A )
6. Results and discussion
Z, Dx (kg/m3)
4, 1.494
0.612
l (mmꢁ1
)
0.627
From the thermodynamic data reported in Table 2
for 1, 2 and 3 is observed solid state polymorphism for 1
and 2.
Theta range
Data/parameters
R1, wR2 on
1.32° to 27.97°
7819/442
0.0652, 0.1423
1.09° to 28.18°
8040/442
0.0489, 0.1143
F ðI > 2rðIÞ)
Samples of 2 crystallized from several solvents mix-
tures give well developed single crystals with habitus of
three different kinds (needles, lozenges and square tables).
Specimens of the different crystal forms have been in-
vestigated by thermal analysis at the polarizing micro-
scope which showed they correspond to different crystal
phases (named a; b and c respectively). The pattern of
phase transformations is sketched below (see Scheme 3).
The X-ray analysis performed on the three poly-
morphs has shown as a first noteworthy feature that
they all crystallize in the same crystal system and space
group (allowed that P21=c is very diffuse among mo-
lecular crystals); this may suggest that polymorphism is
mainly of conformational nature.
Molecular structures of a and b polymorphs are re-
ported in Figs. 1 and 2; selected bond and torsion angles
are reported in Table 3. The coordination around the Pd
atom is substantially square planar in all forms with
trans arrangement of the coordinated N atoms and
standard values for bond distances and angles involving
the metal [6].
h
i
8
<
9
=
1=2
ꢀ
ꢁ
P
2
P
w F02 ꢁ Fc2
w F02
jjF0j ꢁ jFcjj
h
i
P
R1 ¼
;
wR2 ¼
:
ꢀ
ꢁ
P
2
:
;
jF0j
which showed only random fluctuations. Semiempirical
absorption correction (w scans) was applied in all cases.
The structures were solved by direct methods (SHELXS
program of SHELX-97 package [15]) and refined by the
full matrix method (SHELXL program of the same
package) with anisotropic displacement parameters for
C, N, O, Pd atoms; H atoms were placed in calculated
positions and refined by the riding model (Uiso ¼ Ueq of
the carrier atom). Some crystal, collection and refine-
ment data are given in Table 1 for a and b forms. In the
case of the c form, the poor quality of the diffraction
pattern, tested on several crystal specimens, (reflections
on the Weissenberg photographs appeared very large
and diffuse in the x direction) prevented accurate in-
tensity measurements with diffractometer. Actually, al-
though the structure was still solved by direct methods,
refinement was rather unsatisfactory so the geometric
features of the structure will not be discussed here (they
may be furnished by the authors upon request).
The molecule of 2 has a limited number of confor-
mational degrees of freedom and the polymorphs are
characterized by differences in the corresponding torsion
angles, which are reported in Table 3. The observed
Crystal data for c form. C37H32N4O7Pd, M ¼ 751:07,
ꢂ
monoclinic, a ¼ 8:30ð1Þ, b ¼ 10:835ð7Þ, c ¼ 37:1ð1Þ A,
Table 2
Thermal data of complexes
3
ꢂ
b ¼ 91:60ð9Þ°, V ¼ 3335ð10Þ A , T ¼ 293 K, space
group P21=c, Z ¼ 4, q ¼ 1:496 kg/m3, l ¼ 0:614 mmꢁ1
.
Tm (°C)
Td (°C)a
1
2
3
211 (83, 160)b
262 (165, 174)b
271
260
292
285
5. NLO measurements
a Decomposition temperature calculated at 5% by weight loss.
b Solid–solid transitions.
The EFISH technique was used to determine the
nonlinearity lgb of the chromophores (lg stands for
the ground state permanent dipole moment and b for the
quadratic hyperpolarizability of the molecules). The light
source was a Q-switched Nd:Yag laser whose emission at
1.06 lm was shifted to 1.907 lm by stimulated Raman
scattering in a high pressure hydrogen cell. Measure-
ments were calibrated relative to a quartz wedge: the
Scheme 3.

R. Centore et al. / Inorganica Chimica Acta 357 (2004) 913–918
917
Fig. 1. The crystallographically independent molecule of 2 (solid character) in the a phase. Thermal ellipsoids are drawn at 30% probability level.
Fig. 2. The crystallographically independent molecule of 2 (solid character) in the b phase. Thermal ellipsoids are drawn at 30% probability level.
Table 3
Selected bond angles (°) and torsion angles (°) with e.s.d.Õs in paren-
theses
pulsions [6] between the aminophenyl ring and the
orthopalladated phenyl group (the distance C14ꢃ ꢃ ꢃC31 is
3.13(1), 3.308(7) in a and bÞ and to the fact that the ori-
a
b
entation of the latter is blocked by metal complexation. It
should be noted that similar steric effects are also present
in 1 for which, therefore, twist of the p-nitrophenyl group
may be inferred; in 3 the steric overcrowding is somewhat
smaller and, in any case, the orientation of the nitro-
benzoxazole group is fixed by coordination.
C8–O2–C9–C14
C15–N1–C16–C17
C29–N2–C30–C31
C23–C28–C29–N2
C28–C29–N2–Pd1
C28–C23–O5–Pd1
Pd1–N2–C30
)139.1(8)
)141.7(8)
135.8(8)
24(1)
61.8(6)
132.2(5)
)83.3(5)
5.3(8)
5(1)
8.8(7)
)38(1)
122.1(5)
116.4(7)
)12.8(7)
124.2(3)
113.1(4)
As shown in Table 3 and Fig. 3, the five membered Pd
coordinated ring is substantially planar in all forms
while deviations from planarity are observed for the six
membered one, mainly because the Pd atom is out of the
mean plane of the other five atoms, as we have already
found in a similar case [6]. Actually, this structural
feature is a very efficient way for reducing the steric
repulsions between amino-phenyl and orthopalladated-
phenyl rings and so the metal atom is directly involved
in the conformational changes that are responsible for
polymorphism. Coherently with this, the lower distance
of Pd from the mean plane of the other five atoms is
measured in the b form, in which the highest torsion
around N2–C30 is observed. On the basis of the above
discussion, the presence of solid state polymorphism
also in 1, as well its absence in 3, is not unexpected since
the involved steric features are similar in 1 and 2.
C30–N2–C29
torsion around the O2–C9 bond is consistent with the
corresponding torsional potential [17] which, in the range
of the observed torsions, is rather flat and low. Also for
the torsion around N1–C16 bond the data of the poly-
morphs are consistent with literature data for the non-
coordinated imines which always show phenyl twist as a
result of a presumably low torsional barrier [18]. The
twist is probably related to the need of relaxing the close
ꢂ
contact O5ꢃ ꢃ ꢃC17 (2.94(1), 2.857(6) A for a and bÞ. On
the other hand the torsion around N2–C30 which is ob-
served, to various extents, in the polymorphs (and mostly
in bÞ is not consistent with crystal data on free imines of
closely related chemical structure [18]. Actually the ob-
served effect should be due to intramolecular steric re-

918
R. Centore et al. / Inorganica Chimica Acta 357 (2004) 913–918
Acknowledgements
Financial support of PRIN 2002 (Italy) is gratefully
acknowledged. Thanks are also due to the Centro In-
terdipartimentale di Metodologie Chimico-Fisiche of the
ꢀ
Universita di Napoli ‘‘Federico II’’ for NMR, X-Ray
(MACH3 Nonius diffractometer) and CSD facilities.
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1997.
[16] T. Thami, P. Bassoul, M.A. Petit, J. Simon, A. Fort, M.
Barzoukas, A. Villaeys, J. Am. Chem. Soc. 114 (1992) 915.
[17] P. Coulter, A.H. Windle, Macromolecules 22 (1989) 1129.
[18] CSD version 5.23 April 2002 release, Cambridge Crystallographic
Data Centre, Cambridge, England, 2002.
Table 4
EFISH data of chromophores
[19] C.R. Moylan, R.J. Twieg, V.Y. Lee, S.A. Swanson,
K.M. Betterton, R.D. Miller, J. Am. Chem. Soc. 115 (1993)
12599.
Chromophore
Solvent
kmax (nm)
lgb (10ꢁ48 esu)
[20] L.R. Dalton, A. Harper, A. Ren, F. Wang, G. Todorova, J. Chen,
C. Zhang, M. Lee, Ind. Eng. Chem. Res. 38 (1999) 8.
[21] P. Ambrosanio, R. Centore, S. Concilio, B. Panunzi, A. Sirigu, N.
Tirelli, Polymer 40 (1999) 4923.
1
2
3
CHCl3
CHCl3
CHCl3
518
489
460
610
300
400