SHG active crystals of a remote functionalized achiral NLO-phore assembled through zinc(II) complexation

Article abstract of DOI:10.1021/ic061145v

An achiral nonlinear optical chromophore with a remote functionality that can act as a ligand is developed on the basis of 4-nitroaniline derivatized with pyridine. The molecules are assembled through complexation with simple achiral zinc(II) salts and the H-bond network mediated by the counterions, to generate noncentrosymmetric materials exhibiting optical second harmonic generation (SHG). The crystal structures of the new complexes are determined; the counterion strongly influences the ligand orientations and lattice structure. SHG of the microcrystalline materials is investigated. Correlation between the structure and SHG is rationalized using semiempirical quantum chemical estimation of the hyperpolarizabilities of molecules and molecular clusters. The metal complexation plays a significant role in molecular assembly but affects the SHG very little, enabling simplified analysis of the bulk property in terms of molecular responses. Organization of remote functionalized molecules by metal ion complexation thus offers a convenient approach to the rational design of quadratic NLO materials.

Full text of DOI:10.1021/ic061145v

Inorg. Chem. 2006, 45, 9758−9764
SHG Active Crystals of a Remote Functionalized Achiral NLO-phore
Assembled through Zinc(II) Complexation
M. Jaya Prakash and T. P. Radhakrishnan*
School of Chemistry, UniVersity of Hyderabad, Hyderabad-500 046, India
Received June 23, 2006
An achiral nonlinear optical chromophore with a “remote functionality” that can act as a ligand is developed on the
basis of 4-nitroaniline derivatized with pyridine. The molecules are assembled through complexation with simple
achiral zinc(II) salts and the H-bond network mediated by the counterions, to generate noncentrosymmetric materials
exhibiting optical second harmonic generation (SHG). The crystal structures of the new complexes are determined;
the counterion strongly influences the ligand orientations and lattice structure. SHG of the microcrystalline materials
is investigated. Correlation between the structure and SHG is rationalized using semiempirical quantum chemical
estimation of the hyperpolarizabilities of molecules and molecular clusters. The metal complexation plays a significant
role in molecular assembly but affects the SHG very little, enabling simplified analysis of the bulk property in terms
of molecular responses. Organization of remote functionalized molecules by metal ion complexation thus offers a
convenient approach to the rational design of quadratic NLO materials.
Introduction
of a specific NLO-phore unit into different lattice structures
and examination of their SHG responses would provide
useful insight into the efficacy of the various assembly
Development of novel molecular materials for quadratic
nonlinear optical (NLO) applications such as second har-
monic generation (SHG) is realized through two major steps.
The first is the synthesis of the molecular unit, preferably
with a large hyperpolarizability (â), and the second involves
the assembly of these molecular building blocks with optimal
orientation of the â tensor components that maximizes the
NLO response of the bulk material. From the applications
perspective, it is highly desirable that the materials have a
wide transparency window in the wavelength range of
interest and good thermal and chemical stability. While the
first level of molecule (NLO-phore) synthesis has been
widely explored both theoretically and experimentally,1 the
more complex problem of assembly involves fundamental
questions of intermolecular interactions and molecular
organization in crystals and thin films. The even order nature
of the NLO effect of SHG necessitates a noncentrosymmetric
structure for the material; optimal orientation of the mol-
ecules can lead to enhanced responses.2 Several chemical3
and physical4 approaches have been implemented to fabricate
molecular materials exhibiting SHG response. Organization
(2) (a) Chemla, D. S., Zyss, J., Eds. Nonlinear Optical Properties of
Organic Molecules and Crystals; Academic Press: New York, 1987;
Vol. 1. (b) Nie, W. AdV. Mater. 1993, 5, 520. (c) Zyss, J.; Nicoud, J.
F. Curr. Opin. Solid State Mater. Sci. 1996, 1, 533.
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B. F.; Bethea, C. G.; Thurmond, C. D.; Lynch, R. T.; Bernstein, J. L.
J. Appl. Phys. 1979, 50, 2523. (c) Lipscomb, G. F.; Garito, A. F.;
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1991, 253 , 281. (e) Zyss, J. J. Phys. D 1993, 26, B198. (f) Dalton, L.
R.; Harper, A. W.; Robinson, B. H. Proc. Natl. Acad. Sci. U.S.A. 1997,
94, 4842. (g) Gangopadhyay, P.; Sharma, S.; Rao, A. J.; Rao, D. N.;
Cohen, S.; Agranat, I.; Radhakrishnan, T. P. Chem. Mater. 1999, 11,
466. (h) Gangopadhyay, P.; Rao, S. V.; Rao, D. N.; Radhakrishnan,
T. P. J. Mater. Chem. 1999, 9, 1699. (i) Gangopadhyay, P.;
Radhakrishnan, T. P. Chem. Mater. 2000, 12, 3362. (j) Gangopadhyay,
P.; Srinivas, N. K. M. N.; Rao, D. N.; Radhakrishnan, T. P. Opt. Mater.
2002, 21, 55.
(4) (a) Weissbuch, I.; Lahav, M.; Leiserowitz, L.; Meredith, G. R.;
Vanherzeele, H. Chem. Mater. 1989, 1, 114. (b) Serbutovicz, C.;
Nicoud, J. F.; Fischer, J.; Ledoux, I.; Zyss, J. Chem. Mater. 1994, 6,
1358. (c) Ramamurthy, V.; Eaton, D. F. Chem. Mater. 1994, 6, 1128.
(d) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994, 94,
31. (e) Ashwell, G. J.; Jackson, D. P.; Crossland, W. A. Nature
(London) 1994, 368, 438. (f) Hoss, R.; Konig, O.; Kramer-Hoss, V.;
Berger, U.; Rogin, P.; Hulliger, J. Angew. Chem., Int. Ed. Engl. 1996,
35, 1664. (g) Ulman, A. Chem. ReV. 1996, 96, 1533. (h) Miyata, S.,
Sasabe, H., Eds. Poled Polymers and their Applications to SHG and
EO DeVices; Gorden and Breach Science Publishers: Amsterdam,
1997. (i) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (j) Lvov, Y.; Ariga,
K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146,
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* To whom correspondence should be addressed. E-mail: tprsc@
uohyd.ernet.in. Phone: 91-40-2301-1068. Fax: 91-40-2301-2460.
(1) (a) Zyss, J.; Ledoux, I. Chem. ReV. 1994, 94, 77. (b) Kanis, D. R.;
Ratner, M. A.; Marks, T. J. Chem. ReV. 1994, 94, 195. (c) Marks, T.
J.; Ratner, M. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 155.
9758 Inorganic Chemistry, Vol. 45, No. 24, 2006
10.1021/ic061145v CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/02/2006

SHG ActiWe Crystals
Table 1. Crystallographic Data for PINA-C, PINA-N, ZPA, and ZPP
param
PINA-C
PINA-N
ZPA
ZPP
empirical formula
C12H11N3O2
orthorhombic
Pbca
11.3657(8)
7.7206(6)
24.7787(18)
90.0
2174.3(3)
8
1.401
C12H11N3O2
monoclinic
Pn
4.5747(18)
10.194(4)
12.037(5)
99.730(6)
553.3(4)
2
C28H28N6O8Zn
monoclinic
Pn
10.406(4)
10.387(4)
14.203(6)
108.928(6)
1452.2(10)
2
C30H32N6O8Zn
orthorhombic
C2221
13.6852(9)
16.1279(11)
13.7491(9)
90.0
3034.6(3)
4
1.466
cryst system
space group
a/Å
b/Å
c/Å
â/deg
V/Å3
Z
F
calc/g cm-3
1.376
0.097
298(2)
1.468
0.906
298(2)
µ/mm-1
0.099
298(2)
0.870
298(2)
temp/K
λ/Å
0.710 73
0.8917, 0.9853
0.710 73
0.6829, 0.9923
a
2546
154
0.710 73
0.5677, 0.8991
0.053(8)
6574
390
0.933
0.710 73
0.7208, 0.9028
0.011(13)
3641
205
1.085
min, max transm
Flack param
no. of reflcns
no. of params
GOF
2631
154
1.082
1.172
R (for I g 2σI)
0.0586
0.1394
0.206, -0.326
0.0692
0.1598
0.219, -0.220
0.0486
0.0649
0.555, -0.293
0.0407
0.0915
0.585, -0.200
wR2
largest diff peak, hole/e Å-3
a Not refined.
motifs. In this context, we have considered a general
approach on the basis of what can be termed as “remote-
functionalized NLO-phores”. Since the assembly of these
units is primarily mediated by the remote-functional group
that is not in conjugation with the π-electron system of the
NLO-phore, the variations in the SHG response of the bulk
materials will be dominated by the assembly patterns with
the molecular response remaining largely invariant.
ation; the resulting molecule is N-(4-picolyl)-4-nitroaniline,
PINA. Zinc(II) was chosen as the metal ion since it affords
materials with good transparency window as well as thermal
stability.9,10 This paper presents, first, the polymorphic
structures of centrosymmetric as well as noncentrosymmetric
crystals formed by the ligand, PINA, designated respectively
as PINA-C and PINA-N. Investigation of complexes of PINA
with a variety of achiral Zn(II) salts showed that several
(acetate, propionate, succinate, maleate) form SHG active
materials. The crystals of the complexes derived from the
acetate and propionate salts of Zn(II), Zn(PINA)2X2 (X )
acetate, ZPA, and X ) propionate, ZPP) were amenable to
structure analysis and are presented here. SHG measurements
on the microcrystalline powders of PINA as well as the Zn-
(II) complexes are reported. The trends are examined using
quantum chemical estimations of the hyperpolarizability of
molecules and molecular clusters and qualitative consider-
ations of the NLO-phore orientations that are influenced by
the extended H-bonded network formations in the two
crystals. The study illustrates the utility of metal complex-
ation of remote functionalized molecules as a convenient and
rational approach to the assembly of novel molecular SHG
materials.
We have investigated several organic molecular materials
based on remote functionalized chromophores and NLO-
phores. The interesting phenomena demonstrated include
charge transfer occurring exclusively in the solid state,5
enhanced fluorescence in the solid state and doped polymer
films,6 and the realization of noncentrosymmetric crystal
lattices and second harmonic generation from achiral7 as well
as chiral molecular units.8 In this study, we have explored a
new avenue on the basis of metal complexation for the
assembly of remote functionalized NLO-phores to fabricate
SHG active materials. Metal complexation is a convenient
approach to assemble molecular NLO materials; however,
the metal ions bound to the chromophore units affect their
electronic structure and hence NLO response, making any
analysis of the SHG from the bulk material and further
design/development complicated. In many instances, the
metal ion complexation may have the detrimental effect of
reducing the hyperpolarizability of the molecular NLO-
phores and thus the SHG response of the material. The
remote functional approach outlined above is expected to
circumvent these difficulties. We have chosen the classical
NLO-phore on the basis of the push-pull system, 4-nitroa-
niline (pNA), and introduced 4-picolyl group as the remote
functionality to aid in the assembly through metal complex-
Experimental Section
Synthesis and Characterization. N-(4-Picolyl)-4-nitroaniline
(PINA). A 2.0 g amount of K2CO3 was added to 30 mL of DMSO,
and the solution was stirred for 15 min. A 0.713 g (0.67 mL, 6.6
mmol) amount of 4-picolylamine and 0.913 g (0.68 mL, 6.6 mmol)
of 4-fluoronitrobenzene were added, and the reaction mixture was
(9) (a) Qin, J.; Liu, D.; Dai, C.; Chen, C.; Wu, B.; Yang, C.; Zhan, C.
Coord. Chem. ReV. 1999, 188, 23. (b) Xiong, R.; Zuo, J.; You, X.;
Fun, H.; Raj, S. S. S. New J. Chem. 1999, 23, 1051. (c) Xiong, R.;
Zuo, J.; You, X.; Abrahams, B. F.; Bai, Z.; Che, C.; Fun, H. Chem.
Commun. 2000, 2061. (d) Tian, Y.; Yu, W.; Zhao, C.; Jiang, M.; Cai,
Z.; Fun, H. Polyhedron 2002, 21, 1217. (e) Chen, Y.; Zhang, J.; Cheng,
J.; Kang, Y.; Li, Z.; Yao, Y. Inorg. Chem. Commun. 2004, 7, 1139.
(f) Kim, Y.; Martin, S. W.; Ok, K. M.; Halasyamani. P. S. Chem.
Mater. 2005, 17, 2046.
(5) Jayanty, S.; Radhakrishnan, T. P. Chem. Mater. 2001, 13, 2072.
(6) Jayanty, S.; Radhakrishnan, T. P. Chem.sEur. J. 2004, 10, 791.
(7) (a) Jayanty, S.; Gangopadhyay, P.; Radhakrishnan, T. P. J. Mater.
Chem. 2002, 12, 2792. (b) Jayanty, S.; Radhakrishnan, T. P. Chem.s
Eur. J. 2004, 10, 2661. (c) Prakash, M. J.; Radhakrishnan, T. P. Chem.
Mater. 2006, 18, 2943.
(8) Prakash, M. J.; Radhakrishnan, T. P. Cryst. Growth Des. 2005, 5, 1831.
(10) Evans, C.; Luneau, D. J. Chem. Soc., Dalton Trans. 2002, 83.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9759

Prakash and Radhakrishnan
stirred for 12 h at 30 °C. Ice mixed with water was added and
stirred for 15 min. The yellow precipitate formed was filtered out,
washed with cold water, and dried under vacuum; yield ) 49.6%.
The crude product was recrystallized by diffusing hexane vapors
into a solution in ethyl acetate yielding crystals of PINA-C: mp/
°C ) 145-147; FT-IR (KBr) νj/cm-1 ) 3231.0, 3013.1, 1603.0,
1
1570.0, 1510.0, 1475.7, 1113.0; H NMR (DMSO-d6) δ/ppm )
4.47 (d, 2H), 6.64 (d, 2H), 7.30 (d, 2H), 7.86 (t, 1H), 7.96 (d, 2H),
8.50 (d, 2H); 13C NMR (DMSO-d6) δ/ppm ) 45.21, 111.70, 122.61,
126.61, 136.83, 148.33, 150.17, 154.63. Needle-shaped crystals of
PINA-N were grown from a mixture of acetonitrile and water (6:4
v/v).
Zn(PINA)2(CH3COO)2 (ZPA). A solution of 0.025 g (0.11
mmol) of PINA in 2 mL of methanol was mixed with a solution of
0.024 g (0.11 mmol) of zinc acetate in 2 mL of methanol and kept
at 30 °C. Crystals of the complex formed in 3 days were filtered
out, washed, and dried: yield ) 84.5%; mp/°C ) 162-164; FT-
IR (KBr): νj/cm-1 ) 3312.1, 3134.6, 3069.0, 1603.0, 1560.3, 1504.6,
1477.6, 1111.1; 1H NMR (DMSO-d6) δ/ppm ) 1.80 (s, 6H), 4.50
(d, 4H), 6.63 (d, 4H), 7.39 (d, 4H), 7.87(t, 2H), 7.96 (d, 4H), 8.52-
(d, 4H); 13C NMR (DMSO-d6) δ/ppm ) 22.89, 45.15, 111.75,
123.01, 126.62, 136.90, 149.70, 149.90, 154.56, 171.43.
Figure 1. Molecular structure of PINA in (a) PINA-C and (b) PINA-N
from single-crystal X-ray analysis. H atoms are omitted for clarity, and
90% probability thermal ellipsoids are indicated. C (gray), N (blue), and O
(red) atoms are indicated.
Zn(PINA)2(C2H5COO)2 (ZPP). A solution of 0.025 g (0.11
mmol) of PINA in 2 mL of methanol was mixed with a solution of
0.025 g (0.11 mmol) of zinc propionate in 3 mL of methanol and
kept at 30 °C. Crystals of the complex formed in 4 days were
filtered out, washed, and dried: yield ) 80.6%; mp/°C ) 136-
138; FT-IR (KBr): νj/cm-1 ) 3315.9, 3120.0, 3069.0, 2949.4,
1610.0, 1595.3, 1506.5, 1483.4, 1109.2; 1H NMR (DMSO-d6)
δ/ppm ) 0.95 (t, 6H), 2.10 (m, 4H), 4.52 (d, 4H), 6.64 (d, 4H),
7.42 (d, 4H), 7.89 (m, 2H), 7.96 (d, 4H), 8.53 (d, 4H); 13C NMR
(DMSO-d6) δ/ppm ) 10.98, 28.91, 45.15, 111.76, 123.16, 126.70,
136.94, 149.79, 150.49, 154.52, 180.46.
Figure 2. H-bonded chains of PINA formed along (a) the a axis in PINA-C
and (b) the c axis in PINA-N. H atoms except those involved in H-bonding
are omitted for clarity. C (gray), H (white), N (blue), and O (red) atoms
and an H-bond (broken cyan line) are indicated.
Crystallography. X-ray diffraction data were collected on a
Bruker Nonius Smart Apex diffractometer (with CCD detector).
Mo KR radiation with a graphite crystal monochromator in the
incident beam was used. Data were reduced using SAINT; all non-
hydrogen atoms were found using the direct method analysis in
SHELXTL, and after several cycles of refinement, positions of the
hydrogen atoms were calculated and added to the refinement
process.11 The Flack parameter was refined for the zinc complexes.
Details of data collection, solution, and refinement, as well as the
crystallographic information files, are submitted as Supporting
Information.
the reference (1 U ) SHG of urea). Calibration measurements were
carried out using N-4-nitrophenyl-(S)-prolinol (NPP).
Semiempirical Computations. Geometries of the molecules and
molecular clusters for the computations were taken from the crystal
structure, and hydrogen atoms alone were optimized using the
semiempirical AM1 method13 in the program suite MOPAC93.14
Hyperpolarizabilities were computed using the TDHF method15
available in MOPAC93; the values reported are the magnitude of
â
vec, conventionally defined using the x, y, and z projections of the
various tensorial components. â0, the static value, as well as â1.17
for excitation wavelength of 1064 nm are presented.
SHG Measurement. Second harmonic generation from micro-
crystalline powders was examined using the Kurtz-Perry12 method.
Identity of the powder samples was ensured and presence of
polymorphic structures ruled out by verifying the agreement
between experimental powder X-ray patterns and those simulated
from the single-crystal structure (see Supporting Information).
Particle sizes were graded using standard sieves; sizes ranging from
40 to 300 µm were studied. Samples were loaded in glass capillaries
having an inner diameter of 600 µm. The fundamental beam (1064
nm) of a Q-switched ns-pulsed (6 ns, 10 Hz) Nd:YAG laser (Spectra
Physics model INDI-40) was used. The second harmonic signal
was collected using appropriate optics and detected using a
monochromator, PMT, and oscilloscope (Tektronix model TDS 210,
60 MHz). Filters were used to bring the signals from all the samples
in the same range. Urea with particle size ∼150 µm was used as
Results and Discussion
Two polymorphs of PINA could be crystallized depending
on the solvent employed. Ethyl acetate-hexane yielded
centrosymmetric crystals (PINA-C) belonging to the orthor-
hombic space group Pbca, whereas noncentrosymmetric
crystals (PINA-N) belonging to the monoclinic space group
Pn were obtained from acetonitrile-water solution. The
crystallographic data for the two are collected in Table 1.
The molecular structures in the two crystals are very similar
(Figure 1) except for the link between the 4-picolyl group
and pNA moiety; the dihedral angle C12-N2-C11-C8 is
respectively 121.9 and 80.7° in PINA-C and PINA-N. In
(13) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am.
Chem. Soc. 1985, 107, 3902.
(14) MOPAC93; Fujitsu, Inc.: Tokyo, 1993.
(11) (a) SAINT-Plus, version 6.45; Bruker AXS: Madison, WI, 2003. (b)
SHELXTL, version 6.14; Bruker AXS: Madison, WI, 2003.
(12) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798.
(15) Dupuis, M.; Karna, S. J. Comput. Chem. 1991, 12, 487.
9760 Inorganic Chemistry, Vol. 45, No. 24, 2006

SHG ActiWe Crystals
Figure 4. Schematic diagram of the (a) the extended net and (b)
interpenetrating network (the two nets colored differently for clarity) formed
through the H-bond assembly of ZPA molecules. The vertexes represent
the Zn(II) ions and the edges the connectivity between corresponding
molecules mediated by H-bonds: (I) rZn‚‚‚Zn ) 10.094 Å (N8-H8‚‚‚O12);
(II) rZn‚‚‚Zn ) 12.188 Å (N4-H4‚‚‚O13).
Figure 3. (a) Molecular structure of ZPA from single-crystal X-ray
analysis. H atoms are omitted for clarity, and 75% probability thermal
ellipsoids are indicated. (b) H-bonding around each molecule of ZPA. H
atoms except those involved in H-bonding are omitted for clarity. C (gray),
H (white), N (blue), O (red), and Zn (green) atoms and an H-bond (broken
cyan line) are indicated.
Table 2. Significant Bond Lengths in ZPA and ZPP
cryst
bond
bond length (Å)
ZPA
Zn1-N1
Zn1-N2
Zn1-O11
Zn1-O12
Zn1-N1
Zn1-O6
2.042(2)
2.038(2)
1.962(2)
2.040(2)
2.050(2)
1.9670(17)
ZPP
both the crystals, the PINA molecules are assembled into
extended chain structures through intermolecular H-bonds
formed between the amino nitrogen and pyridine nitrogen
atoms: rN2‚‚‚N1 ) 2.967 Å, θN2-H2‚‚‚N1 ) 147.1° in PINA-C;
rN2‚‚‚N1 ) 2.966 Å, θN2-H2‚‚‚N1 ) 165.2° in PINA-N. Figure
2a shows the adjacent chains formed along the a axis, related
by a center of inversion in PINA-C; the chains formed along
the c axis in PINA-N are shown in Figure 2b.
Crystals of Zn(II) acetate complex, ZPA grown from
methanol are found to belong to the monoclinic space group
Pn with one molecule in the asymmetric unit (Figure 3a,
Table 1). Zn(II) is tetrahedrally coordinated by the pyridine
nitrogen of two PINA molecules and the oxygen of two
acetate ions; the significant bond lengths are collected in
Table 2. The pNA moieties in the PINA ligands are oriented
away from the direction of the acetate ligands; the torsion
angle between the bisector of the O11-Zn1-O12 angle and
the dipole vector of the pNA moieties N3-N5 and N4-N6
is respectively 110.3 and 137.3°. Each ZPA molecule is
engaged in four H-bonds around it (Figure 3b) involving the
Figure 5. (a) Molecular structure of ZPP from single-crystal X-ray
analysis. H-atoms are omitted for clarity, and 75% probability thermal
ellipsoids are indicated. (b) H-bonding around each molecule of ZPP.
H-atoms except those involved in H-bonding are omitted for clarity. C
(gray), H (white), N (blue), O (red), and Zn (green) atoms and an H-bond
(broken cyan line) are indicated.
amino and acetate groups; the independent ones correspond
to rN3‚‚‚O12 ) 3.017 Å, θN3-H3‚‚‚O12 ) 149.0° and rN4‚‚‚O13
)
Inorganic Chemistry, Vol. 45, No. 24, 2006 9761

Prakash and Radhakrishnan
Table 3. Powder SHG Data for PINA-N, ZPA, and ZPP (1 U ) SHG
of Urea with Particle Size > 150 µM)
SHG (U)
av particle size (µm)
PINA-N
ZPA
ZPP
40
75
125
175
225
275
2.09
2.85
2.97
3.14
3.35
3.38
5.36
6.53
11.63
12.43
14.81
15.05
0.21
0.22
0.23
0.24
0.25
0.28
‚‚O7 ) 2.897 Å, θN2-H2‚‚‚O7 ) 167.3°. These H-bonds connect
the molecules into a noninterpenetrating network structure
(Figure 6) that extends parallel to the bc plane. Thus, the
H-bonded assembly of ZPA and ZPP share some common
features but exhibit significant differences as well.
Figure 6. Schematic diagram of the (a) the extended net and (b)
noninterpenetrating adjacent nets (the two nets colored differently for clarity)
formed through the H-bond assembly of ZPP molecules. The vertexes
represent the Zn(II) ions and the edges the connectivity between the
corresponding molecules mediated by H-bonds: rZn‚‚‚Zn ) 11.911 Å (N2-
H2‚‚‚O7).
Electronic absorption spectra of PINA as well as the zinc
complexes are shown in Figure 7. The three compounds show
nearly identical solution spectra with a peak at ∼370 nm.
The absorption is much broader in the solid state, as expected
from the impact of intermolecular interactions on the
electronic structure; it falls off near 500 nm in all cases so
that the materials are nearly transparent at the wavelength
at which the SHG is measured in our study (532 nm). Results
of the Kurtz-Perry powder SHG studies12 on the different
materials are collected in Table 3. The particle size depen-
dence of the SHG indicates that all the compounds show
phase-matchable behavior. The SHG observed in ZPA (∼15
U) is considerably enhanced with respect to that of PINA-
N; however, on changing the counterion from acetate to
propionate in ZPP, the SHG is reduced below that of PINA-
N. Even though a judicious choice of metal ion can enhance
the molecular hyperpolarizability,16 metal complexation in
many instances has the opposite impact and/or enforces
nonoptimal orientation of the NLO-phores resulting in low
SHG in the bulk material. Among the SHG active zinc
complexes reported earlier, those showing strong response
are rare;10,17 the majority of them exhibit SHG well below
10.0 U9,18 (see Supporting Information for a more extensive
list of references). The relatively high value of ZPA is notable
and indicates that the remote functionality approach has
enabled an efficient exploitation of the NLO response of the
molecular units. As described below, this approach also
facilitates convenient analysis of the basis of the observed
SHG response of the two zinc complexes. Even though the
powder SHG is an average measure and does not provide
Figure 7. Electronic absorption spectra of PINA and its zinc(II) complexes
in (a) solution state and (b) solid state.
2.901 Å, θN4-H4‚‚‚O13 ) 149.7°. These H-bonds assemble ZPA
molecules into a 3-dimensional interpenetrating network
structure (Figure 4).
(16) (a) Roberto, D.; Ugo, R.; Bruni, S.; Cariati, E.; Cariati, F.; Fantucci,
P.; Invernizzi, I.; Quici, S.; Ledoux, I.; Zyss, J. Organometallics 2000,
19, 1775. (b) Lacroix, P. G. Eur. J. Inorg. Chem.. 2001, 339. (c)
Roberto, D.; Ugo, R.; Tessore, F.; Lucenti, E.; Quici, S.; Vezza, S.;
Fantucci, P.; Invernizzi, I.; Bruni, S.; Ledoux-Rak, I.; Zyss, J.
Organometallics 2002, 21, 161. (d) Lacroix, P G.; Averseng, F.;
Malfant, I.; Nakatani, K. Inorg. Chim. Acta 2004, 357, 3825. (e)
Maury, O.; Viau, L.; Senechal, K.; Corre, B.; Guegan, J.; Renouard,
T.; Ledoux, I.; Zyss, J.; Le Bozec, H. Chem.sEur. J. 2004, 10, 4454.
(f) Tancrez, N.; Feuvrie, C.; Ledoux, I.; Zyss, J.; Toupet, L.; Le Bozec,
H.; Maury, O. J. Am. Chem. Soc. 2005, 127, 13474. (g) Tessore, F.;
Roberto, D.; Ugo, R.; Pizzotti, M.; Quici, S.; Cavazzini, M.; Bruni,
S.; De Angelis, F. Inorg. Chem. 2005, 44, 8967.
Crystals of ZPP grown from methanol belong to the
orthorhombic space group C2221 with half the molecule in
the asymmetric unit (structure of the full molecule is shown
in Figure 5a); crystallographic data are provided in Table 1.
Coordination around Zn(II) is similar to that in the case of
ZPA (the significant bond lengths are listed in Table 2);
however, the pNA moieties in the ligands are now oriented
in the direction of the propionate ligands so that the torsion
angle between the bisector of the O6-Zn1-O6′ angle and
the dipole vector of the pNA moieties, N2-N3, is 28.0°.
Each ZPP molecule is engaged in four similar H-bonds
(Figure 5b) involving the amino and propionate groups: rN2‚
(17) (a) Lin, W.; Evans, O. R.; Xiong, R.; Wang, Z. J. Am. Chem. Soc.
1998, 120, 13272. (b) Evans, O. R.; Lin, W. Chem. Mater. 2001, 13,
3009. (c) Ye, Q.; Li, Y.; Song, Y.; Huang, X.; Xiong, R.; Xue, Z.
Inorg. Chem. 2005, 44, 3618.
9762 Inorganic Chemistry, Vol. 45, No. 24, 2006

SHG ActiWe Crystals
Table 4. AM1/TDHF Computed Hyperpolarizabilities (â0 and â1.17) of ZPA and ZPP Molecules, the Molecular Clusters in Their Unit Cells (Figure
8a,c), PINA Ligand Pair in ZPA and ZPP Molecules, and PINA Ligand Clusters in the Unit Cells (Figure 8b,d)a
â0 (10-30 esu)
â
1.17 (10-30 esu)
system
ZPA
8.222
ZPP
9.035
ZPA
16.337
ZPP
molecule
17.711
molecular cluster in the unit cell
ligand pair in the molecule
ligand cluster in the unit cell
10.623 (5.312)
8.425 (4.213)
11.081 (2.770)
9.805 (2.451)
11.648 (5.824)
11.245 (1.406)
21.315 (10.658)
17.364 (8.682)
22.725 (5.681)
19.497 (4.874)
22.273 (11.137)
23.605 (2.951)
a The value per molecule in the case of molecular clusters or per PINA unit in the case of ligand clusters is indicated in parentheses.
the zinc salt; the clusters of ligands in the unit cells (Figure
8b,d) were also considered in the study. Table 4 provides
the computed values for these systems as well. The relative
orientation of the pNA units leads to a slightly larger â in
ZPP compared to ZPA; the torsion angle between the dipole
vectors is 60.2 and 117.3° in ZPP and ZPA, respectively. It
is seen that the NLO response of the PINA ligands is very
close to that of the respective complexes, supporting the
assumption of the remote nature of metal complexation. The
slight decrease in the â of the ligands on complexation arises
due to the oppositely directed nonlinear response of the
pyridine-Zn(II) unit with respect to that of the pNA moiety.
The most significant finding from the study is the following.
Even though at the molecular level the nonlinear response
of ZPP is stronger than that of ZPA, the response per
molecule or per PINA unit at the crystal level is considerably
weaker in ZPP. Examination of Figure 8 reveals that this
stems from the orientation of the pNA units that tends to
cancel out the response from each other to a large extent in
ZPP whereas, in ZPA, they are conducive to producing
appreciable SHG. The difference between ZPA and ZPP at
the molecular level is the counterion. This leads to the
different molecular conformations and extended H-bonded
network structures in the two crystal lattices that together
Figure 8. Molecular clusters considered in the computations: (a) unit cell
of ZPA; (b) PINA in the unit cell of ZPA; (c) unit cell of ZPP; (d) PINA
enforce the relative orientation of the NLO-phore units,
determining the bulk SHG response. The computational
exercise explains why the bulk SHG observed in ZPA is
stronger than that in ZPP. It demonstrates also that the
analysis of the bulk SHG in terms of the response of the
NLO-phore units is facilitated by the remote functional nature
of the ligand sites that ensured that the metal-mediated
assembly did not influence the NLO-phore unit considerably.
This provides a new direction for the systematic fabrication
of quadratic NLO materials through the metal complexation
route.
in the unit cell of ZPP. C (gray), N (blue), O (red), and Zn (green) atoms
are indicated.
details of the nonlinear susceptibility tensor components,
useful insight into the observed trends can be gained through
semiempirical quantum chemical computations.
Computations were carried out on molecular structures
determined from the single-crystal analysis. The hyperpo-
larizabilities computed for single molecules of ZPA and ZPP
as well as the cluster of symmetry-related ones in the unit
cell, two in ZPA and four in ZPP (Figure 8a,c), are collected
in Table 4. The NLO response is dominated by the ligands
since they bear the primary NLO-phore unit pNA; the
contribution of the metal link attached to the remote
functional units is likely to be small. This aspect was probed
through computations on the pair of PINA ligands in ZPA
and ZPP retaining their relative orientations, by excising out
Conclusions
A simple ligand involving the classical NLO-phore, pNA,
is designed with an appropriate remote functional group that
enables the assembly through metal complexation. The free
ligand is shown to form polymorphic structures, one of them
being noncentrosymmetric and SHG active. Assembly of the
ligand molecule through Zn(II) complexation and H-bonding
leads to noncentrosymmetric and SHG active materials with
the lattice structure sensitively dependent on the anion. The
complex with zinc(II) acetate shows an interpenetrating
3-dimensional network structure formed through intermo-
lecular H-bond interactions. The observed SHG of the metal
complexes is rationalized using semiempirical computations
(18) (a) Qin, J.; Su, N.; Dai, C.; Yang, C.; Liu, D.; Day, M. W.; Wu, B.;
Chen, C. Polyhedron 1999, 18, 3461. (b) Wang, X. Q.; Xu, D.; Yuan,
D. R.; Tian, Y. P.; Yu, W. T.; Sun, S. Y.; Yang, Z. H.; Fang, Q.; Lu,
M. K.; Yan, Y. X.; Meng, F. Q.; Guo, S. Y.; Zhang G. H.; Jiang M.
H. Mater. Res. Bull. 1999, 34, 2003. (c) Evans, C. C.; Masse, R.;
Nicoud, J.; Bagieu-Beucher, M. J. Mater. Chem. 2000, 10, 1419. (d)
Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511. (e) Han, L.;
Hong, M.; Wang, R.; Luo, J.; Lin, Z.; Yuan, D. Chem. Commun. 2003,
2580. (f) Ren, P.; Liu, T.; Qin, J.; Chen, C. J. Coord. Chem. 2003,
56, 125.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9763

Prakash and Radhakrishnan
of the hyperpolarizabilities of molecules and supramolecular
assemblies in the unit cell. This study suggests that noncen-
trosymmetric and SHG active crystal lattices of NLO-phores
may be constructed through appropriate ligand design and
metal complexation and that the structure and function are
affected by subtle changes in the linking units. The remote
functional approach allows convenient analysis of the bulk
SHG in terms of the molecular responses, facilitating the
design of novel quadratic NLO materials based on metal
complexes.
Acknowledgment. Financial support from the CSIR and
DST, New Delhi, and use of the National Single Crystal
X-ray Diffractometer Facility at the School of Chemistry and
the Universities with Potential for Excellence Program of
the UGC, New Delhi, are acknowledged. M.J.P. thanks the
CSIR, New Delhi, for a Senior Research Fellowship.
Supporting Information Available: Details of the crystal
structure determination, crystallographic tables, powder X-ray
patterns and simulations, a reference list, and computational details.
IC061145V
9764 Inorganic Chemistry, Vol. 45, No. 24, 2006