Zinc(II) as a versatile template for the design of dipolar and octupolar NLO-phores

Article abstract of DOI:10.1021/ja025705a

Thermally stable dipolar and octupolar (D_2d, D₃) NLO-phores are readily accessible by combining one, two, or three 4,4′-bis(dialkylaminostyryl)-[2,2′]-bipyridyl ligands with zinc(II) salts. The off-resonant β₀ values point out the superiority of octupoles versus dipoles in terms of nonlinearity/transparency tradeoff. The octahedral tris(bipyridyl)zinc(II) complex exhibits a very large β₀ value (241 × 10^-30 esu), which is the largest ever reported for octupolar molecules. Copyright

Full text of DOI:10.1021/ja025705a

Published on Web 04/04/2002
Zinc(II) as a Versatile Template for the Design of Dipolar and Octupolar
NLO-phores
Katell Se´ne´chal,† Olivier Maury,† Hubert Le Bozec,*,† Isabelle Ledoux,‡ and Joseph Zyss‡
Laboratoire de Chimie de Coordination et Catalyse, UMR 6509 CNRS-UniVersite´ Rennes 1, Institut de Chimie,
Campus de Beaulieu, 35042 Rennes Cedex, France, and Laboratoire de Photonique Quantique Mole´culaire,
UMR 8537, ENS Cachan, 61 aVenue du pre´sident Wilson, 94235 Cachan
Received January 25, 2002
Scheme 1
Interest in nonlinear optics has led to the design of numerous
chromophores with large molecular hyperpolarizability â, mainly
monodimensional (1D) dipolar chromophores.1 Recognition in the
early 1990s of the enlarged potential for nonlinear optics of
octupolar molecules has triggered the emergence of a new field of
research.2 Basically, purely octupolar symmetries can be derived
from a cubic Td structure either by projection along a C3 axis giving
rise to the D3h or D3 symmetries (“TATB route”) or by fusion of
one type of charge in the barycenter leading to the D3h, D3, Td, or
D2d symmetries (“guanidinium route”) (Scheme 1). Thus, an
increasing number of 2D and 3D nondipolar molecular structures
have recently appeared in the literature with the revival and
development of the harmonic light scattering technique (HLS).3
Metal ions are powerful 3D templates, which can gather ligands
to build a variety of predetermined stereochemical (octupolar)
arrangements. In this context, 4,4′-bis(dialkylaminostyryl)-[2,2′]-
bipyridine ligands have been recently used for the molecular
engineering of octupolar D3 ruthenium(II)4 and D2d copper(I)5
complexes, due to their well-defined coordination with these metal
ions. By contrast, since there is no ligand field stabilization effect,
zinc(II) has the ability to expand its coordination sphere and
therefore is known to give tetrahedral complexes or octahedral
complexes with diimine ligands. Herein we would like to show
that this characteristic provides a unique opportunity to design either
dipolar or octupolar D2d and D3 molecules by simple controlled
combination of one, two, or three bipyridyl ligands with zinc(II).
In addition, these new octupolar chromophores display very large
off-resonant first-order hyperpolarizabilities and present an im-
proved transparency/nonlinearity tradeoff as compared to the
corresponding dipolar molecule.
The dipolar zinc(II) complex 1 has already been described
elsewhere6 and was obtained upon room-temperature treatment of
ZnCl2‚2H2O with one equivalent of 4,4′-bis(dibutylaminostyryl)-
[2,2′]-bipyridine a in dichloromethane. The tris(bipyridyl) zinc(II)
complex 3 was readily obtained in 83% yield by refluxing 3 equiv
of a with ZnCl2‚2H2O in ethanol, followed by an exchange of the
chloride with hexafluorophosphate counterions. The synthesis of
the bis(bipyridyl) zinc(II) complex 2 was quantitatively achieved
by reacting at room temperature Zn(OTf)2 with 2 equiv of the R,R′-
methyl substituted bipyridyl ligand b, which was used in place of
a in order to ensure the pseudotetrahedral D2d symmetry (Scheme
1).7 The structure of 2 and 3 was unambiguously confirmed by 1H,
and 13C NMR spectroscopy and elemental analysis.
These complexes exhibit intense intraligand charge transfer
(ILCT) in the visible region as depicted in Figure 1. The
complexation is accompanied by a bathochromic shift which is
sensitive to the structure of the Zn(II) complexes. Compared to
the free ligands a (λmax ) 401 nm) or b (λmax ) 397 nm), the neutral
Zn(II) complex 1 has a red-shift (∆λ ) 58 nm) smaller than the
dicationic complexes 2 (∆λ ) 131 nm) and 3 (∆λ ) 65 nm).
Concerning 2 and 3, the bathochromic shift depends on the number
of coordinated ligands and decreases substantially as the ligand-
to-metal ratio increases. Moreover, complexes 1-3 are intense dyes
exhibiting high molecular extinction coefficients. The oscillator
strengths8 (Table 1) follow a 1:1:1.8:2.9 ratio for a, 1, 2, and 3,
respectively, as expected for noninteracting subchromophores (1:
1:2:3).3c-d This is in agreement with the absence of an extended
delocalization through Zn2+ between the bipyridyl subchromophoric
units in 2 and 3. These new octupolar complexes possess good
thermal stability with 10% weight losses (Td determined by TGA)
10
occurring above 300 °C.
The molecular hyperpolarizability coefficient â and the corre-
sponding dispersion free hyperpolarizability â0 for compounds 1-3
are reported in Table 1. For the neutral dipolar compound 1, µâ
was determined by the electric-field-induced second harmonic
generation (EFISH) method at λ ) 1.34 µm as fundamental
wavelength,6 and â was then deduced by measuring the dipole
moment µ by means of the Guggenheim model. â measurements
of octupolar complexes 2 and 3 were performed by the harmonic
light scattering (HLS) method at 1.91 µm. As the harmonic
wavelength at 955 nm is far enough from the λcut-off of these
complexes, any contribution of two-photon-induced fluorescence
* To whom correspondence should be addressed. E-mail: lebozec@
univ-rennes1.fr.
† Laboratoire de Chimie de Coordination et Catalyse, UMR 6509 CNRS-
Universite´ Rennes 1, Institut de Chimie, Campus de Beaulieu.
‡ Laboratoire de Physique Quantique Mole´culaire.
9
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J. AM. CHEM. SOC. 2002, 124, 4560-4561
10.1021/ja025705a CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
In conclusion, this study shows that the possibility of expanding
the coordination number renders Zn2+ very effective for the simple
and easy design of (1D) dipolar structures or (3D) octupolar
structures. Moreover, the results illustrate the superiority of
octupoles versus dipoles in terms of nonlinearity without significant
cost of transparency. Their good thermal stability and large first
hyperpolarizability make them attractive candidates for nonlinear
optical materials.
Acknowledgment. We thank the Re´gion Bretagne (PRIR
99CC10) and the CNRS for financial support. K.S. is grateful to
the French Ministry of Education for a fellowship.
References
Figure 1. UV-visible spectra of complexes 1-3.
Table 1. Linear and Nonlinear Optical Data
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λmax
ꢀ
ν1/2
(cm-1
â
â0
T
d
10
cmpd
(nm) (mol‚L-1‚cm-1
)
)
f
(10-30esu) (10-30esu) (°C)
(2) Zyss, J.; Ledoux, I. Chem. ReV. 1994, 94, 77.
a
b
401
397
65000
61000
62000
125000
175000
4280 1.22
4348 1.15
4490 1.22
3825 2.07
4544 3.48
380
368
399
308
330
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F.; Matschiner, R.; Wortmann, R. Angew. Chem., Int. Ed. 2000, 39, 1436-
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aZnCl2 1 459
b2Zn2+ 2 529
a3Zn2+ 3 466
172a
245b
340b
62
157
241
a Measured by EFISH (precision (10%) at 1.34 µm in a 10-3 mol‚L-1
chloroform solution; µ‚â ) 1830 × 10-30 esu5 with µ ) 10.65 D.
b Measured by HRS (precision (15%) in a (1-5)‚10-2 mol‚L-1 dichlo-
romethane solution at 1.91 µm.
to the HLS signal can be considered as negligible. HLS measure-
ments were carried out in concentrated dichloromethane solution
(1-5 × 10-2 mol‚L-1) with a concentrated solution of ethyl-violet
as reference (â1.91 ) 170 × 10-30 esu). Interestingly, the off-
resonant â0 values (Table 1) monotonically increase from 1 to 3
with respect to the number of subchromophores organized around
the Zn(II) center. The â0 value of 2 is about 2-fold larger than that
+
of the recently reported tetrahedral CuI(b)2 complex (â0 ) 78 ×
(4) (a) Dhenaut, C.; Ledoux, I.; Samuel, I. D. W.; Zyss, J.; Bourgault, M.;
Le Bozec, H. Nature 1995, 374, 339. (b) Vance, F. W.; Hupp, J. T. J.
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Chem. Commun. 2001, 2430.
10-30 esu).5 This result is consistent with the better acceptor strength
of Zn2+ versus Cu+ and the absence of an antagonist MLCT
transition in 2. An improved transparency/nonlinearity tradeoff is
reached for the octahedral complex 3 as compared to that for the
tetrahedral complex 2. Moreover, these results point out the
efficiency of the octupolar strategy since 3 exhibits a â0 value that
is roughly 4 times larger than that of the corresponding dipolar
derivative 1, without the undesirable bathochromic shift of the ILCT
transition [∆λ(3 vs 1) ) 7 nm]. It is also worth noting that 3 exhibits
a very large â0 value of 241 × 10-30 esu (comparable to that of
the ruthenium analogous complex9), which is to the best of our
knowledge the largest ever reported for octupolar molecules.10 The
enhanced molecular NLO activity of D3 metallo-octupoles as
compared to those of the most efficient D3h organic octupoles can
be related to the ability of Zn2+ to organize three subchromophores
or six intraligand charge transfers, whereas the central benzene ring
in D3h organic octupoles allows the spatial arrangement of only
three charge transfers.
(5) Renouard, T.; Le Bozec, H.; Ledoux, I.; Zyss, J. Chem. Commun. 1999,
871.
(6) Hilton, A.; Renouard, T.; Maury, O.; Le Bozec, H.; Ledoux, I.; Zyss, J.
Chem. Commun. 1999, 2521.
(7) Pallenberg, A. J.; Marschner, T. M.; Barnhart, D. M. Polyhedron 1997,
16, 2711.
(8) The oscillator strength f is estimated by the following relationship: f )
4.319 × 10-9‚ꢀ‚ν1/2 where ν1/2 represents the half-height bandwidth (cm-1).
(9) More recent measurements concerning [a3Ru][PF6]2 give fluorescence-
free â0 of ca. 240 × 10-30esu see ref 4c.
(10) The most efficient D3h organic octupoles featuring extended conjugated
pathway are: 1,3,5-triscyano-2,4,6-tris(dibutylaminobistyryl)benzene
(λmax ) 470 nm, â1.56 ) 219 × 10-30esu, â0 ) 116 × 10-30esu), see ref
3
g-h; 1,3,5-tris(methylsulfonylbistyryl)benzene (λmax ) 377 nm,
â1.34 ) 250 × 10-30esu, â0 ) 157 × 10-30esu), see ref 3j; and 4,9,14-
tris(4′-(di-4-methoxyphenyl)aminophenylethynyl)truxenone (λmax ) 509
nm, â0 ) 169 × 10-30esu, â0 ) 104 × 10-30esu), see ref 3b.
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