Terpyridine Zn(II), Ru(III) and Ir(III) complexes as new asymmetric chromophores for nonlinear optics: First evidence for a shift from positive to negative value of the quadratic hyperpolarizability of a ligand carrying an electron donor substituent upon

Article abstract of DOI:10.1039/b201221a

The synthesis of 4′-(C₆H₄-p-NBu₂)-2,2′:6′, 2″-terpyridine and the strongly enhanced second-order NLO response of its Zn(II), Ru(III) and Ir(III) complexes are reported, evidencing for the first time a shift from positive t

Full text of DOI:10.1039/b201221a

Terpyridine Zn(II), Ru(III) and Ir(III) complexes as new asymmetric
chromophores for nonlinear optics: first evidence for a shift from
positive to negative value of the quadratic hyperpolarizability of a ligand
carrying an electron donor substituent upon coordination to different
metal centres
Dominique Roberto,*a Francesca Tessore,a Renato Ugo,a Silvia Bruni,a Amedea Manfredib and Silvio
Quicib
a Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR “CSSSCMTBSO”,
Università di Milano, Via G. Venezian, 21, I-20133 Milano, Italy. E-mail: dominique.roberto@unimi.it
b Dipartimento di Chimica Organica e Industriale, and, Centro CNR sulla Sintesi e Stereochimica di
Speciali Sistemi Organici, Via C.Golgi, 19, I-20133 Milano, Italy
Received (in Cambridge, UK) 1st February 2002, Accepted 21st February 2002
First published as an Advance Article on the web 19th March 2002
The synthesis of 4A-(C6H4-p-NBu2)-2,2A:6A,2B-terpyridine
and the strongly enhanced second-order NLO response of its
Zn(II), Ru(III) and Ir(III) complexes are reported, evidencing
for the first time a shift from positive to negative value of the
ligand quadratic hyperpolarizability by varying the nature of
the metal centre.
This ligand was prepared (60% yield) by condensation of
4-(dibutylamino)benzaldehyde with 2-acetylpyridine (molar
t
ratio 1+2) in the presence of BuOK and NH4OAc.6 Further
reaction with a stoichiometric amount of ZnY2 (Y = Cl,
CF3CO2) or MCl3 (M = Ir, Ru) afforded [ZnY2L] and
[MCl3L].7 Because the ruthenium complex was not soluble
enough in CHCl3 for an accurate determination of either the
dipole moment8 or bvec, the more soluble trifluoroacetyl
derivative was prepared by exchange of the chloride ligand with
silver trifluoroacetate.7
Metal complexes of various pyridines,1a–e bipyridines2 and
phenanthrolines1b,f have been studied as molecules with
enhanced second-order nonlinear optical (NLO) responses.
These ligands, when bearing an electron donor substituent, upon
coordination, show increased quadratic hyperpolarizability bvec
as measured by the Electric Field Induced Second Harmonic
generation (EFISH) method.3 This is due to a red shift of the
intraligand charge transfer (ILCT) transition1d,e,4a which can be
enhanced by increasing the acceptor properties of the metal
centre.1e Besides chelation, imposing a rather planar arrange-
ment of the p-delocalized system of flexible bipyridines, can
lead to a stronger effect.1f,2b Therefore we investigated
complexes with metal ions in relatively high oxidation state of
other polychelated nitrogen donor p-delocalized ligands such as
terpyridines carrying an electron donor substituent. Metal
complexes of terpyridines have been studied for their photo-
chemical and photophysical properties,5 but to our knowledge,
their NLO properties have never been investigated. Here, we
report preliminary findings on an unexpected effect on bvec of
4A-(C6H4-p-NBu2)-2,2A:6A,2B-terpyridine (L) by coordination to
Ru(III) and Ir(III).
The dipole moment enhancement (m EF) upon coordination
of L to Zn(II) with different ancillary ligands Y (Table 1; m EF
= 3.8 and 4.7 for Y = Cl and CF3CO2, respectively, in accord
with the increased electron withdrawing strength of Y) is higher
than that occurring for other chelating nitrogen donor p-
delocalized ligands (m EF = 1.5–2.7).1f,2b This could be
attributed to the trigonal bipyramidal geometry of terpyridine
Zn(II) complexes9 instead of the typical tetrahedral geometry of
other chelated Zn(II) complexes. UV–visible spectra in CHCl3
of all complexes (Table 1) show one strong absorption band
around 416–465 nm attributed to the intraligand charge transfer
(ILCT) transition, which emanates from the NBu2 donor,1d,e,4a
red-shifted (Dlmax = 56–105 nm) with respect to the free
ligand ILCT band (lmax = 360 nm) related to the increased
acceptor properties of its p* orbitals upon coordination.1d The
charge transfer character of these absorptions is confirmed by
their relevant solvatochromism. Coordination to ZnCl2 causes a
red-shift of the ILCT band (Dlmax = 65 nm) similar to that for
Table 1 Electronic spectra, dipole moments, EFISH b1.34 and bCT, 1.34 of 4A-(C6H4-p-NBu2)-2,2A+6A,2B-terpyridine (L) and its complexes with Zn(II), Ru(III
)
or Ir(III
)
af
n
lmax em
lmaxa/nm /nm
mb1.34ai/10230
mag/D m EFh cm5 esu21
D
b1.34 (b0)j/10230
cm5 esu21
b1.34
(EF)k
bCT, 1 .34 /
Molecule
fal
Dmegm/D
10230 cm5 esu21
L
360b
425b
427b
465bc
533d
449
503
512
624
2.1
8.0
10
—
46
538
877
22 (15)
67 (36)
88 (47)
2109
—
0.40
0.33
0.34
0.81
0.31
8.0
10.8
8.2
20.1
22.9
43
98
79
ZnCl2L
Zn(CF3CO2)2L
IrCl3L
3.8
4.7
3.8
3.0
4.0
5.0
7.9
2861
24
279
283
48
Ru(CF3CO2)3L
416b
508e
911e
590
9.2
4.3
2642
270
3.2
0.15
0.20
0.04
12.7
5.4
15.8
75
2208
285
a In CHCl3. b Attributed to the ILCT of the ligand. c Due to the anomalous high intensity and the slightly negative Dmeg, an expected MLCT transition13 is
under the ILCT transition. d Assigned to MLCT according to ref. 13. e Assigned to LMCT according to refs. 14 and 15. f lmax of the emission. g Measured
by the Guggenheim method according to ref. 8. h m EF is the m enhancement factor (m complex/m free terpyridine. i Measured by the EFISH technique. j In
parentheses b0 calculated using the equation b0 = bl(1 2 (2lmax/l)2)(1 2 (lmax/l)2) of ref. 1e. k b1.34 EF is b1.34 enhancement factor (b1.34 complex/b1.34
free terpyridine. l f is the oscillator strength obtained from the integrated absorption coefficient. m Dmeg is the difference between excited- and ground-state
molecular dipole moments, obtained from solvatochromic data; see ref. 4. n Quadratic hyperpolarizability tensor along the charge transfer direction.
846
CHEM. COMMUN., 2002, 846–847
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other nitrogen donor chelating ligands with p-delocalized
bridges connecting the chelating aromatic structure and the
donor substituent such as 4-(trans-CHNCHC6H4-p-NBu2)-4A-
We thank Dr Rak-Ledoux and the École Normale Supérieure
de Chachan for their kindness to allow F. T. to carry out EFISH
measurements. This work was supported by CNR (PFMSTAII,
year 1999, Research Title: Sintesi e sviluppo di composti
molecolari organometallici e di coordinazione con proprietà di
ottica non lineare (NLO) e con proprietà elettriche aniso-
tropiche e isotropiche).
CH3-2,2A-bipy2b (Dlmax
=
72 nm), 4,4A-bis(trans-
CHNCHC6H4-4A-NBu2)-2,2A-bipy2c (Dlmax
58 nm) or
5-(trans-CHNCHC6H4-4A-NMe2)-1,10-phen1f (Dlmax
55
=
=
nm). However it is much higher than that for a phenanthroline
with a donor substituent directly bound to the aromatic system
such as 5-NMe2-1,10-phenanthroline (Dlmax = 13 nm).1f
Therefore this large red-shift suggests a strong stabilizing effect
on the terpyridine p* orbitals due to chelation.
Notes and references
1 (a) L. T. Cheng, W. Tam, G. R. Meredith and S. R. Marder, Mol. Cryst.
Liq. Cryst., 1990, 189, 137; (b) L. T. Cheng, W. Tam and D. F. Eaton,
Organometallics, 1990, 9, 2856; (c) D. W. Bruce and A. Thornton, Mol.
Cryst. Liq. Cryst., 1993, 231, 253; (d) D. R. Kanis, P. G. Lacroix, M. A.
Ratner and T. J. Marks, J. Am. Chem. Soc., 1994, 116, 10089; (e) D.
Roberto, R. Ugo, S. Bruni, E. Cariati, F. Cariati, P. C. Fantucci, I.
Invernizzi, S. Quici, I. Ledoux and J. Zyss, Organometallics, 2000, 19,
1775; (f) D. Roberto, R. Ugo, F. Tessore, E. Lucenti, S. Quici, S. Vezza,
P. C. Fantucci, I. Invernizzi, S. Bruni, I. Ledoux-Rak and J. Zyss,
Organometallics, 2002, 21, 161.
2 (a) M. Bourgault, C. Mountassir, H. Le Bozec, I. Ledoux, G. Pucetti and
J. Zyss, J. Chem. Soc., Chem.Commun., 1993, 1623; (b) M. Bourgault,
K. Baum, H. Le Bozec, G. Pucetti, I. Ledoux and J. Zyss, New. J. Chem.,
1998, 517; (c) A. Hilton, T. Renouard, O. Maury, H. Le Bozec, I.
Ledoux and J. Zyss, Chem. Commun., 1999, 2521; (d) H. Le Bozec and
T. Renouard, Eur. J. Inorg. Chem., 2000, 229.
The enhanced bvec value of L, measured at a non-resonant
wavelength of 1.34 mm (b1.34),10,11 upon coordination to ‘ZnY2’
remains positive as in other Zn(II) complexes with related p
delocalized ligands.1f,2 The enhancement factor b1.34 EF is
higher for the more electron-withdrawing ancillary ligand
CF3CO2 (Table 1). However, upon coordination of L to IrCl3 or
Ru(CF3CO2)3, b1.34 increases significantly its absolute value, in
agreement with a relevant red shift of the ILCT transition, but
the sign becomes negative. This is a new observation: for the
first time different metal centres, which all behave as significant
Lewis acids according to the Dlmax red shift of the ligand
ILCT,1e can influence not only the absolute value but also the
sign of the quadratic hyperpolarizability of a nitrogen donor p-
delocalized ligand bearing a strong electron-donor group. A
solvatochromic investigation, carried out by use of absorption
and emission data4a and using a large series of solvents,4b
confirms that also bCT (the quadratic hyperpolarizability tensor
3 I. Ledoux and J. Zyss, Chem. Phys., 1982, 73, 203.
4 (a) S. Bruni, F. Cariati, E. Cariati, F. A. Porta, S. Quici and D. Roberto,
Spectrochim. Acta Part A, 2001, 57, 1417; (b) Absorption and emission
spectra of complexes and L were measured in toluene, chloroform,
anisole, methylene chloride, ethyl acetate, 1,2-dichloroethane, acetone;
the spectra of L were also measured in cyclohexane and in carbon
tetrachloride.
along the charge-transfer direction) is positive for Zn(II
)
complexes and negative for Ir(III) and Ru(III) complexes. An
analysis of the contributions of the various absorption bands to
bCT according to the two-level model,12 shows that the
enhancement of the quadratic hyperpolarizability of L upon
coordination to Zn(II) is due, as in other Zn(II) complexes,1f,2 to
both a red-shift of the ILCT transition and a positive Dmeg
(Table 1). However for the Ir(III) complex the second order
NLO response is not dominated by the ligand ILCT transition
only but there is a contribution of at least a weaker band at 533
nm and also of another band, probably a MLCT transition,13
located under the ILCT absorption at 465 nm. This hypothesis
is supported by the unexpected slightly negative Dmeg and the
high intensity of the absorption band at 465 nm. The band at 533
nm can be tentatively assigned to a MLCT (metal to ligand
charge transfer) transition according to literature13 and to its
5 (a) J.-P. Collin, S. Guillerez, J.-P. Sauvage, F. Barigelletti, L. De Cola,
L. Flamigni and V. Balzani, Inorg. Chem., 1991, 30, 4230; (b) J.-P.
Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez and C. Coudret,
Chem. Rev., 1994, 94, 993; (c) G. Albano, V. Balzani, E. C. Constable,
M. Maestri and D. R. Smith, Inorg. Chim. Acta, 1998, 277, 225; (d) P.
Lainé and E. Amouyal, Chem. Commun., 1999, 935; (e) M. Ziegler, V.
Monney, H. Stoeckli-Evans, A. Von Zelewsky, I. Sasaki, G. Dupic, J.-
C. Daran and G. G. A. Balavoine, J. Chem. Soc., Dalton Trans., 1999,
667; (f) J.-P. Collin, I. M. Dixon, J.-P. Sauvage, J. A. G. Williams, F.
Barigeletti and L. Flamigni, J. Am. Chem. Soc., 1999, 121, 5009; (g) F.
Neve, A. Crispini, F. Loiseau and S. Campagna, J. Chem. Soc., Dalton
Trans., 2000, 1399; (h) N. W. Alcock, P. R. Barker, J. M. Haider, M. J.
Hannon, C. L. Painting, Z. Pikramenou, E. A. Plummer, K. Rissanen
and P. Saarenketo, J. Chem. Soc., Dalton Trans., 2000, 1447.
6 (a) E. C. Constable and D. R. Smith, Supramol. Chem., 1994, 4, 5; (b)
the synthesis and photophysical properties of phenyl-substituted
2,2A:6A,2B-terpyridine ligands have also been described by other groups:
see ref. 5 and: W. Spahni and G. Calzaferri, Helv. Chim. Acta, 1984, 67,
450; T. Mutai, J.-D. Cheon, S. Arita and K. Araki, J. Chem. Soc., Perkin
Trans. 2, 2001, 1045.
7 Products were characterized by 1H NMR (except the Ru(III) complex),
mass spectrometry and elemental analysis.
8 Dipole moments were determined in CHCl3 solution according to: (a) E.
A. Guggenheim, Trans. Faraday Soc., 1949, 45, 203; (b) B. Thompson,
J. Chem. Educ., 1996, 43, 66.
9 (a) F. W. B. Einstein and B. R. Penfold, Acta Crystallogr., 1966, 20,
924; (b) J. E. Douglas and C. J. Wilkins, Inorg. Chim. Acta, 1969,
635.
negative Dmeg. On the other hand, in the terpyridine Ru(III
)
complex, the negative sign of the quadratic hyperpolarizability
originates from the high contribution to the second order NLO
response of an absorption band at 911 nm, in opposition to the
positive contribution of another band at 508 nm and to the
expected positive contribution of the ILCT transition at 416 nm.
The new transitions, both characterized by a positive value of
Dmeg, can be tentatively assigned to LMCT (ligand to metal
charge transfer) transitions.14–16 However the absorption at 911
nm is located at lower energy then the second harmonic (l =
670 nm) and therefore produces a negative and dominating
contribution to the quadratic hyperpolarizability when this latter
is measured at an incident wavelength of 1.34 mm.
10 EFISH measurements were carried out in CHCl3 solutions of different
concentrations at 1.34 mm. EFISH b1.34 values (Table 1) are defined
Of course the assignment of the new absorption bands to
according to the ‘phenomenological’ convention.11
.
MLCT transitions in Ir(III) and to LMCT transitions in Ru(III
)
11 A. Willetts, J. E. Rice, D. M. Burland and D. P. Shelton, J. Chem. Phys.,
1992, 97, 7590.
12 J. L. Oudar, J. Chem. Phys., 1977, 67, 446.
complexes must be confirmed by a Raman-shift investigation
which is underway. In conclusion we have shown for the first
time that while the second order NLO response of p-delocalized
nitrogen donor ligands carrying a donor group in complexes
13 (a) J. Collin, I. M. Dixon, J. Sauvage, J. A. G. Williams, F. Barigelletti
and L. Flamigni, J. Am. Chem. Soc., 1999, 121, 5009; (b) F. Neve, A.
Crispini, F. Loiseau and S. Campagna, J. Chem. Soc., Dalton Trans.,
2000, 1399.
14 (a) D. D. Walker and H. Taube, Inorg. Chem., 1981, 20, 2828; (b) J. C.
Curtis, B. P. Sullivan and T. J. Meyer, Inorg. Chem., 1983, 22, 224.
15 K. Kalyanasundaram, S. M. Zakeeruddin and K. Nazeeruddin, Coord.
Chem. Rev., 1994, 132, 259.
with low oxidation state soft metal centres (e.g. Rh(
I
) 4d8, Ir(
I
)
5d8, Os(II) 5d6, W(0) 5d6)1c–e or even with relatively hard Zn(II
)
3d10 centres1f,2 is dictated only by their ILCT transition, in
borderline metal centres like Ir(III) 5d6 or Ru(III) 4d5 this
response is influenced also by LMCT or even MLCT transi-
tions, in such a way that it can change its sign (see the Ir(III
)
16 In the complex of L with RuCl3, not enough soluble for dipole moment
and EFISH measurements, the band at 911 nm shifts at 795 nm, as
complex). This change of sign was reported only in low
oxidation state metal complexes when the substituent of this
kind of ligands is an electron acceptor group.1b,d,e
2
expected for a LMCT transition, by substitution of CF3CO2 with
Cl2.
CHEM. COMMUN., 2002, 846–847
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