New [(D-terpyridine)-Ru-(D or A-terpyridine)][4-EtPhCO2] 2 complexes (D = electron donor group; A = electron acceptor group) as active second-order non linear optical chromophores

Article abstract of DOI:10.1039/c2dt30183k

The dipolar and octupolar contributions of the second order nonlinear optical properties of [(4′-(C₆H₄-p-D)-2,2′: 6′,2′′-terpyridine)-Ru-(4′-(C₆H ₄-p-A)-2,2′:6′,2′′-terpyridine)]Y ₂ heteroleptic complexes (D and A are donor and acceptor groups, respectively), and related free terpyridines and homoleptic complexes, have been obtained by means of a comprehensive combination of Electric Field Induced Second Harmonic generation, Third Harmonic Generation, and Harmonic Light Scattering measurements. These results evidence how a metal can act as a bridge between two π-delocalized terpyridine moieties bearing a D and an A group, respectively, leading to a large quadratic hyperpolarizability hugely dominated by the octupolar contribution.

Full text of DOI:10.1039/c2dt30183k

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PAPER
New [(D-terpyridine)-Ru-(D or A-terpyridine)][4-EtPhCO2]2 complexes
(D = electron donor group; A = electron acceptor group) as active
second-order non linear optical chromophores
Alessia Colombo,a Danika Locatelli,a Dominique Roberto,*a,b Francesca Tessore,a Renato Ugo,a
Marco Cavazzini,b Silvio Quici,b Filippo De Angelis,*c Simona Fantacci,c Isabelle Ledoux-Rak,*d
Nicolas Tancrezd and Joseph Zyssd
Received 24th January 2012, Accepted 3rd April 2012
DOI: 10.1039/c2dt30183k
The dipolar and octupolar contributions of the second order nonlinear optical properties of [(4′-(C6H4-
p-D)-2,2′:6′,2′′-terpyridine)-Ru-(4′-(C6H4-p-A)-2,2′:6′,2′′-terpyridine)]Y2 heteroleptic complexes (D and
A are donor and acceptor groups, respectively), and related free terpyridines and homoleptic complexes,
have been obtained by means of a comprehensive combination of Electric Field Induced Second
Harmonic generation, Third Harmonic Generation, and Harmonic Light Scattering measurements. These
results evidence how a metal can act as a bridge between two π-delocalized terpyridine moieties bearing a
D and an A group, respectively, leading to a large quadratic hyperpolarizability hugely dominated by the
octupolar contribution.
group releasing charge, upon charge transfer. Such a situation is
sometimes referred to as one whereby there is more charge trans-
Introduction
For π-delocalized nitrogen donor ligands, such as push–pull pyr-
idines, bipyridines, and terpyridines, it is now well known that
the dipolar contribution to the quadratic hyperpolarizability
βEFISH, which is measured by the solution-phase dc Electric
Field Induced Second Harmonic generation (EFISH) method,1
increases significantly upon coordination to a metal center,
which, according to its oxidation state, may display an ambiva-
lent acceptor or donor key role.2 This enhancement of βEFISH is
due either to a red shift of the intraligand charge transfer (ILCT)
transition upon coordination (when the π-delocalized nitrogen
donor ligand bears a strong electron-donor substituent) or to a
significant contribution to βEFISH of metal-to-ligand (MLCT) or
ligand-to-metal (LMCT) charge transfer transitions.2 In these
latter cases, βEFISH may assume a negative sign, as a conse-
quence of a negative value of Δμeg, which is the difference
between the dipole moment in the excited and ground state.2,3 In
other words, a negative Δμeg value reflects a decrease of the
dipole moment upon excitation, in contrast with the more fre-
quent case of the acceptor group acquiring charge, or the donor
fer in the ground than in the excited state. Therefore, in the
design of organometallic compounds with relevant nonlinear
optical (NLO) properties, a metal center, due to its ambivalent
acceptor or donor role, appears as an interesting bridge between
two π-delocalized nitrogen ligands bearing an electron donor and
an electron acceptor substituent, respectively. Ambivalence
between the donor and acceptor natures had been pointed-out
already at an early stage of molecular nonlinear optics, in the
case of para-substituted pyridine-1-oxide derivatives. In this
case, following a route by Katrizky et al.,4 the N-oxide group
was shown to act either as a donor or as an acceptor, as triggered
by the reciprocal acceptor or donor nature of the substituent
group in the para position of the pyridine ring. This approach
led to the investigation of the 3-methyl-4-nitropyridine-1-oxide
(POM) as second order NLO molecular crystals.5
We have now tried to follow a new approach, by studying the
second order NLO features of cationic complexes of the type
[(D-terpyridine)-Ru-(A-terpyridine)]2+ (D and A are electron
donor and acceptor groups, respectively) that would combine the
peculiar characteristics of traditional push–pull organic chromo-
phores (presence of a donor and an acceptor group linked by a
bridge) with those of organometallic complexes (introduction of
possible low energy metal to ligand or ligand to metal charge
transfer transitions). Here, we report findings on the second
order NLO properties of push–pull heteroleptic ([TD1RuTA]-
[EtPhCO2]2, [TD2RuTA][EtPhCO2]2, [TD1RuTD3][EtPhCO2]2)
and homoleptic ([TD1RuTD1][EtPhCO2]2 and [TD2RuTD2]-
[EtPhCO2]2) cationic Ru(II) complexes (where TD1, TD2 and TD3
are 4′-(C6H4-p-D)-2,2′:6′,2′′-terpyridine with DvN(C16H33)2,
N(C16H33)2PhCHvCH, and CH3, respectively, whereas TA
aDipartimento di Chimica Inorganica, Metallorganica e Analitica
“Lamberto Malatesta” dell’Università degli Studi di Milano, UdR di
Milano dell’INSTM, I-20133 Milano, Italy.
E-mail: dominique.roberto@unimi.it
bISTM del CNR, Via Venezian 21, I-20133 Milano, Italy
cIstituto di Scienze e Tecnologie Molecolari del CNR, Via Elce di Sotto
8, I-06100 Perugia, Italy. E-mail: filippo@thch.unipg.it
dLaboratoire de Photonique Quantique et Moléculaire UMR CNRS 8531
– Institut d’Alembert – ENS Cachan, 61 avenue du Président Wilson,
94235 Cachan, France. E-mail: ledoux@ens-cachan.fr
This journal is © The Royal Society of Chemistry 2012
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Chart 1
Table 1 Electronic spectra, μβ1.907 EFISH, μ, γ1.907 EFISH and γ1.907 THG of the free terpyridine ligands and their related Ru(II) complexes, working in
CHCl3 with an incident radiation wavelength of 1.907 μm
b
λmax
(nm)
μβ1.907 EFISH
β1.907 EFISH
β0,EFISH (β0,EFISH, theo)
γ1.907 EFISH
γ1.907 THG
Compounda
(×10−48 esu)
μexp (μtheo) (D) (×10−30 esu)
(×10−30 esu)
(×10−33 esu)
(×10−33 esu)
TD1
TD2
TD3
TA
362c
397c
280f
280f
525g
247
350
80
−17
525
—
3.6 (2.1d, 2.4e) 69
57 (29)
54
39
−9
33 (118)
—
1.2
1.7
0.4
−0.08
2.6
0.9
0.27
0.22
0.04
0.07
0.44
0.50
0.14
5.1
69
1.8
44
1.7
10
−10
52
[TD1RuTA]Y2
[TD1RuTD1]Y2 547g
[TD1RuTD3]Y2 529g
Ca 0
7.8
—
68
520
43 (96)
2.6
(7.5d,h, 9.2e,h
10.3
Ca 0
10.2
)
[TD2RuTA]Y2
507g
793
—
77
—
53
51
—
36
3.7
1.5
2.8
0.38
1.62
0.32
[TD2RuTD2]Y2 528g
[TD2RuTD3]Y2 500g
540
a Y = 4-EtPhCO2. b The experimental value of μ was used to calculate β1.907 EFISH c ILCT transition. d Calculated in vacuo with 3-21g* basis set.
.
e Calculated in CHCl3 solution with 3-21g* basis set. f π → π* transition. g There is superposition of the MLCT and ILCT transitions. h Calculated
with N(n-butyl)2 instead of N(n-hexadecyl)2.
is 4′-(C6H4-p-NO2)-2,2′:6′,2′′-terpyridine; see Chart 1). Their
second order NLO response, along with that of the free terpyri-
dines, was measured in solution by means of a comprehensive
combination of EFISH, THG (Third Harmonic Generation), and
HLS (Harmonic Light Scattering) methods in order to evaluate
both the dipolar and octupolar contributions to the quadratic
hyperpolarizability.6,7 An incident wavelength of 1907 nm was
used, whose second harmonic 2ω and third harmonic 3ω lie at
953 and 636 nm, respectively, which means they are in a trans-
parent region with respect to the absorption electronic spectra of
all the ruthenium cationic complexes and terpyridines investi-
gated. To gain insight into the electronic structure and optical
properties of the investigated complexes we also performed
Density Functional Theory (DFT) and Time-Dependent DFT
(TDDFT) calculations. Since in CHCl3 solutions all the investi-
gated cationic complexes generate strong ion-pairing, we were
able to measure the dipole moments of the ion pairs by the
Guggenheim method.8
complexes were prepared following the method reported in the
literature for related compounds.11a 4′-(C6H4-p-D)-2,2′:6′,2′′-ter-
pyridine (TD1 or TD2) was reacted with a stoichiometric amount
of RuCl3·nH2O, in ethanol under reflux, to give [RuCl3(4′-
(C6H4-p-D)-2,2′:6′,2′′-terpyridine)]. This was followed by treat-
ment, in methanol under reflux, with the other terpyridine (TD1
,
TD2, TD3 or TA), in the presence of a catalytic amount of N-ethyl-
morpholine as a reducing agent. The cationic complexes were
finally purified and crystallized by addition of 4-EtPhCO2−Ag at
room temperature. It is worth pointing out that 4-EtPhCO2 was
chosen as the counterion because it gives a good solubility to all
the cationic species investigated in CHCl3, a relatively low polar
solvent that allows, by strong ion-pairing, EFISH measurements
to be carried out even for ionic species.12
1
All ligands and complexes were characterized by H NMR,
electronic absorption spectroscopy, mass spectrometry (Fast
Atom Bombardment, FAB+), and elemental analysis (see Exper-
imental section).
Electronic absorption spectra (Table 1) of the free terpyridine
ligands TD1 and TD2, in addition to π–π* transitions at
ca. 280 nm, show one strong absorption band at 362 and
397 nm, respectively, which can be attributed to the intraligand
charge transfer (ILCT) transition that emanates from the dialky-
lamino group. Upon coordination to a Ru(II) center, this band is
significantly red-shifted (λmax is shifted to ca. 500–547 nm) and
Results and discussion
Synthesis and spectroscopic characterization
The terpyridines ligands were prepared as previously
described,9,10 whereas the heteroleptic and homoleptic Ru(II)
6708 | Dalton Trans., 2012, 41, 6707–6714
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becomes broader due to superposition of the ILCT and
MLCT11,13,14 transitions, see below.
the μβ1.907 EFISH value of the more π-delocalized terpyridine TD2
is higher than that of TD1 due to a higher value of the dipole
moment. The terpyridine TA, bearing a withdrawing NO2 group,
is characterized by a small and negative μβ1.907 EFISH value, as
previously reported.9
EFISH and THG investigation
The significant red shift of the ILCT transitions that is pro-
duced upon coordination of the terpyridines TD1 and TD2 (see
Table 1) is consistent with the perturbation due to the acidic
character of the Ru(II) center, which enhances the strength of the
pyridine acceptor part of the ligands and so induces an increase
of their second order NLO response.2,9 Heteroleptic complexes
In order to investigate their nonlinear optical properties, the
various free terpyridines and complexes were first investigated
by EFISH and THG (see Table 1). It is known that the EFISH
technique1 can provide direct information on the intrinsic mol-
ecular second order NLO properties through eqn (1):
[TD1RuTA][EtPhCO2]2 and [TD2RuTA][EtPhCO2]2 show
a
γEFISH ¼ ðμβEFISH=5kTÞ þ γðꢀ2ω; ω; ω; 0Þ
ð1Þ
μβ1.907 EFISH value that is higher (enhancement factor = 2.1–2.3)
than that of free TD1 and TD2, respectively, but this enhancement
is mainly due to an increase of the dipole moment. The
where μβEFISH/5kT is the dipolar orientational contribution,
βEFISH is the projection along the dipole moment axis of the vec-
torial component of the tensor of the quadratic hyperpolarizabil-
ity, and γ(−2ω; ω, ω, 0), which is a third order term at frequency
ω of the incident light, is a purely electronic cubic contribution
to γEFISH. The cubic contribution is usually negligible when
studying (with the EFISH technique) the second order NLO
properties of push–pull dipolar compounds such as terpyri-
dines.2,9 Therefore, for the terpyridine ligands and related hetero-
leptic complexes investigated here, the cubic contribution can be
reasonably neglected, also because the cubic γTHG values are
less than 5–20% of the γEFISH values, allowing therefore the
μβ1.907
value of [TD2RuTA][EtPhCO2]2 is higher (by a
EFISH
factor of 1.5) than that of [TD2RuTD3][EtPhCO2]2, which has a
similar dipole moment. This shows that the presence of a strong
electron-withdrawing group on the second terpyridine may exalt
the second order NLO response, like in traditional push–pull
organic chromophores. However, this enhancement is relatively
small and not observed for the couple [TD1RuTA][EtPhCO2]2/
[TD1RuTD3][EtPhCO2]2.
determination of μβ1.907
(Table 1). On the contrary, the
EFISH
HLS investigation
γTHG values of the homoleptic complexes [TD1RuTD1]-
[EtPhCO2]2 and [TD2RuTD2][EtPhCO2]2 are quite similar to
those of the related γEFISH values, in agreement with a negligible
dipolar contribution and a relevant role of γ(−2ω; ω, ω, 0),
which is the cubic contribution.
In order to have a complete understanding of the second order
NLO properties of the terpyridines and the related cationic
ruthenium(II) complexes investigated, and in particular to evalu-
ate also the octupolar contribution to the quadratic hyperpolariz-
ability, we completed our EFISH and THG measurements using
an HLS study, working at 1907 nm. The results are presented in
Table 2. The dipolar (J = 1) and octupolar (J = 3) contributions
In order to obtain βEFISH from EFISH measurements, it is
necessary to know the value of the dipole moments. These were
measured for the terpyridines and their complexes using the
Guggenheim method,8 working in chloroform solution. It should
be pointed out that the dipole moments measured in the case of
the cationic complexes could be overestimated, since we
measured the dipole moment of strong ion-pairs.15 Although this
can lead to an underestimation of the value of the β1.907 EFISH
values of the ionic complexes, it is not problematic in the
present study, as the dipolar contribution is expected to be much
lower than the octupolar one in these π-delocalized systems, as
described below.
ˉ
and the modulus of the quadratic hyperpolarizability (kβ1.907k)
have been calculated using the following equations,6,7,16 assum-
ing that all the compounds, in good approximation, have a C2v
symmetry.
J¼1
J¼3
2
2
2
kβk ¼ kβ k þ kβ
k
ð3Þ
ð4Þ
ꢀ
ꢀ
ꢀ
ꢀ
μð0Þ
J¼1
ꢀ
pffiffiffiffiffi
γ0;C
¼
β
ꢀ
2ðor1Þv
15kBT
As evidenced in Table 1, we applied the two-level model1 to
calculate the zero-frequency static quadratic hyperpolarizability
β0 from βEFISH, measured at 1.907 μm, using the following
expression:
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
2þ
β
ð5Þ
2
9
2
J¼1
J¼3
ꢀ
ꢀ
ꢀ
2
2
kβ2HLSl ¼ kjβXXXj l þ kjβZXXj l ¼
β
ꢀ
21
As evidenced in Table 2, all of the terpyridines and related cat-
ionic complexes investigated are characterized by a remarkably
2
2
β0 ¼ βEFISHð1 ꢀ ð2λmax=λÞ Þð1 ꢀ ðλmax=λÞ Þ
ð2Þ
ˉ0
high value of kβ k. This is due to a very high contribution of the
where λ is the fundamental wavelength of the incident photon
(1907 nm) and λmax is the maximum absorption value of the
charge transfer transition that is mainly responsible for the
second order NLO response (Table 1).
For the terpyridine ligands bearing a donor substituent, where
the second order NLO response can be ascribed mainly to the
ILCT transition,9 the μβ1.907 EFISH value of TD3 is lower than
that of the related TD1, due to both a lower value of the dipole
moment and a lower β1.907 EFISH, since a methyl group is not a
strong donor when compared to a dialkylamino group. Besides,
octupolar component, as expected for very rich electron systems.
For the free terpyridines bearing an electron donor group, the
J = 3
ˉ
octupolar contribution (kβ1.907
k) is 10 to 15 times higher
J = 1
ˉ
than the dipolar contribution (kβ1.907
k), whereas for the het-
eroleptic complexes the enhancement is even higher (by a factor
of 20 to 66). This result is of particular relevance because it
confirms that the quadratic hyperpolarizability of a dipolar com-
pound can be largely dominated by the octupolar component, as
recently observed in the case of cyclometallated neutral Ln(III)17
or cationic Ir(III)18 complexes.
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Table 2 〈β1.907 HLS〉, 〈β0HLS〉, dipolar and octupolar contributions and the modulus of the quadratic hyperpolarizability of some free terpyridine
ligands and their related Ru(II) complexes, working in CHCl3 with an incident radiation wavelength of 1.907 μm
〈β1.907 HLS
〉
〈β0
〉
kβ1.907
k
kβ1.907
k
kβ1.907
k
kβ k
ˉ0
J = 1
J = 3
ˉ
ˉ
ˉ
HLS
Compound
(×10−30 esu)
(×10−30 esu)
(×10−30 esu)
(×10−30 esu)
(×10−30 esu)
(×10−30 esu)
TD1
TD2
TD3
TA
178
215
162
260
511
147
170
145
232
330
263
205
801
542
53
53
34
−8
41
ca. 0
53
59
571
692
522
842
1655
ca. 1384
1034
3890
2741
573
694
523
842
1655
ca. 1384
1035
3890
2741
473
549
468
751
1068
854
662
2594
1756
[TD1RuTA]Y2
[TD1RuTD1]Y2 427
[TD1RuTD3]Y2 320
[TD2RuTA]Y2
[TD2RuTD2]Y2 846
1201
Ca 0
J = 3
ˉ
It is worth pointing out that the kβ1.907
k value of
Hartree–Fock (HF) exchange on the electronic and optical prop-
erties of the investigated systems. Indeed, the B3LYP func-
tional21 contains 20% HF exchange, the PBE022 25% HF
exchange and the MPW1K23 41.7% of HF exchange. As will be
clear in the following, for the more extended and π-delocalized
TD2 system only the MPW1K functional provides reasonable
[TD1RuTD3][EtPhCO2]2 is similar to the sum of those of the two
J = 3
ˉ
free terpyridines (TD1 + TD3), and that the kβ1.907
k values of
[TD1RuTD1][EtPhCO2]2 and [TD1RuTA][EtPhCO2]2 are slightly
higher (by a factor of 1.2 times) than the sum of the two terpyri-
dines (TD1 + TD1 and TD1 + TA, respectively). This suggests the
absence of an effect on the second order NLO response due to a
charge transfer through the ruthenium center, so that a simple
additive model could be used to give approximately the octupo-
lar contribution of the quadratic hyperpolarizability of these Ru
results. Static hyperpolarizabilities were calculated for the related
2+
[TD1RuTA]2+ and [TD1RuTD3
]
by a finite field differentiation
of analytic polarizabilities, employing the B3LYP functional.24
We only report the zzz component of the static hyperpolarizabil-
ity tensor (β0zzz), i.e. the hyperpolarizability component oriented
along the dipole axis, since all the other components are at least
one order of magnitude smaller than the zzz one, so that β0zzz can
be directly compared with the static hyperpolarizability derived
from EFISH experiments.
J = 3
ˉ
(II) complexes. However, the kβ1.907
k value of [TD2RuTD2]-
[EtPhCO2]2 is 1.6 times higher than the sum of the value of the
J = 3
ˉ
two terpyridines, whereas the kβ1.907
k value of [TD2RuTA]-
[EtPhCO2]2 is 2.5 times higher than the sum of the value of the
two terpyridines (TD2 and TA). This suggests that, with the ter-
pyridine TD2, the metal plays a role in the second order NLO
response. Similarly, an increase (although much lower, a factor
The optimized geometrical structures for [TD1RuTA]2+ and
[TD2RuTA]2+ do not show significant differences in the coordi-
nation sphere of the metal center and will not be discussed in
detail. A schematic representation of the molecular orbitals
obtained by the MPW1K functional in CHCl3 solution for
[TD1RuTA]2+ and [TD2RuTA]2+ is reported in Fig. 1.
In the investigated complexes, the Highest Occupied Molecu-
lar Orbital (HOMO) is a π-bonding orbital localized on the ter-
pyridine system of the TDx (x = 1, 2) ligand bearing the NBu2
substituent, with substantial contribution from the latter, whereas
the Lowest Unoccupied Molecular Orbital (LUMO) is a π*
orbital localized on the TA ligand, see Fig. 1. A 3.44 eV
HOMO–LUMO gap is calculated for [TD1RuTA]2+, which
reduces to 2.69 eV in [TD2RuTA]2+, see Fig. 1. This is mainly
the result of the strong destabilization of the occupied molecular
orbitals found in [TD2RuTA]2+, due to the increased
J = 1
ˉ
of 1.3 times) of the dipolar contribution value, kβ1.907
k, of
[TD2RuTA][EtPhCO2]2 is observed with respect to the sum of the
two terpyridines (TD2 and TA), but this is not the case for
[TD1RuTA][EtPhCO2]2. Clearly, the role of the metal as a trans-
mitter seems to be relevant only with complexes bearing a
ligand with a more π-delocalized push–pull system, such as ter-
pyridine TD2
.
Computational analyses
The representative
[TD1RuTA]2+,
[TD1RuTD3
]
and
2+
[TD2RuTA]2+ complexes have been investigated by means of
Density Functional Theory (DFT) and Time Dependent DFT
(TDDFT) to gain insight into their electronic and optical
properties.
electron richness and π-delocalization of TD2 compared to TD1
;
The geometry, electronic structure and TDDFT calculations
have been performed with the Gaussian03 program package.19
Geometry optimizations, performed at the B3LYP/3-21G* level
without any symmetry constraint, are in good agreement with
the crystallographic data reported in the literature.11a For the cal-
culation of the optical absorption spectra by TDDFT we investi-
gated the effect of the exchange-correlations functional and
included solvation effects by means of the non-equilibrium
C-PCM implementation20 on the geometries optimized in vacuo.
TDDFT calculations have been performed with the larger DZVP
basis set for all atoms; the same B3LYP functional used for geo-
metry optimizations as well as the PBE0 and MPW1K func-
tionals have been tested, to check the effect of the amount of
the LUMOs lie at essentially the same energy in the two com-
plexes (3.63 vs. 3.64 eV in [TD1RuTA]2+ and [TD2RuTA]2+,
respectively).
Substantial differences between the [TD1RuTA]2+ and
[TD2RuTA]2+ complexes are found also in the lower lying
occupied orbitals. Indeed, while in [TD1RuTA]2+ the HOMO −
1/HOMO − 3 have mainly Ru-t2g character, with a small contri-
bution coming from the C and N p orbitals of both terpyridines,
in [TD2RuTA]2+ substantial mixing of the Ru-t2g orbitals with
TD2 π-bonding character takes place, see the HOMO − 1 in
Fig. 1. The lower-lying HOMO − 2/HOMO − 4 maintain a
similar mixing of metal and ligand character, while no t2g metal
character is found in the lower-lying frontier molecular orbitals.
6710 | Dalton Trans., 2012, 41, 6707–6714
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Fig. 1 A schematic representation of the molecular orbitals for the [TD1RuTA]2+ and [TD2RuTA]2+ complexes, calculated in CHCl3 solution by the
MPW1K functional and the DZVP basis set. Also shown are the isodensity plots of selected molecular orbitals for the [TD1RuTA]2+ and [TD2RuTA]2+
complexes.
The LUMO + 1 of [TD1RuTA]2+ is essentially delocalized on
the terpyridine of the TA ligand, with some involvement of the
t2g Ru orbitals, reflecting π-back donation from the metal, while
in [TD2RuTA]2+ the LUMO + 1 is a π* orbital localized on the
excitation energy. Based on the above results, we now discuss
only the results obtained in solution with the MPW1K func-
tional. The lowest excitation energy in [TD2RuTA]2+ is calcu-
lated, as mentioned above, at 2.25 eV (550 nm). This is a strong
(f = 1.62) Intra Ligand Charge Transfer (ILCT) transition invol-
ving the TD2 ligand: all the involved orbitals are mainly localized
on the TD2 ligand, with the main contribution from the HOMO
→ LUMO + 1 excitation and a small but sizable contribution
arising from the HOMO − 1 → LUMO + 1 excitation, which is
of mixed Ru-TD2 character, see above. The HOMO → LUMO
excitation is calculated to be the second excited state at 2.31 eV
(536 nm), this state involving a TD2 to TA charge transfer exci-
tation. Due to the orthogonality of the TD2 and TA terpyridine
ligands, this transition has, however, zero oscillator strength and
does not contribute to the absorption spectrum and (dipolar
second-order) non-linear optical properties. The lowest transition
is essentially the only relevant transition up to 400 nm, where
some weak Metal to Ligand Charge Transfer (MLCT) excitations
are computed. The calculated transition energy for the first
excited state (550 nm, 2.25 eV) nicely compares with the exper-
imental absorption maximum measured for [TD2RuTA]2+ in
CHCl3 solution (507 nm, 2.45 eV), allowing us to assign the
experimental feature to an ILCT transition of the TD2 ligand,
red-shifted compared to that of the free ligand by coordination to
the metal. The shift between calculated and experimental data is
well within the accuracy of the employed methodology;
TD2 ligand, see Fig. 1.
It is interesting to notice the remarkable effect of solvation
and exchange-correlation functional on the calculated electronic
structure of [TD2RuTA]2+. As an example of the effect of sol-
vation, we notice that in vacuo a substantial reduction of the
HOMO–LUMO gap is calculated compared to the system in sol-
ution (1.75 vs. 2.69 eV, respectively). Comparing the results in
solution using the PBE0 and MPW1K exchange-correlation
functionals, we notice again a remarkable effect: the HOMO–
LUMO gap is calculated to be 1.63 eV with PBE0 vs. 2.69 with
MPW1K, due to a concomitant HOMO stabilization (0.60 eV)
and LUMO destabilization (0.46 eV) taking place with the latter
functional.
We then performed TDDFT excited state calculations to gain
insight into the optical properties of the investigated systems and
to assign their absorption spectra. As found above for the
HOMO–LUMO gaps, the excited states are extremely sensitive
to solvation and functional effects: as an example, the lowest
excitation energy in solution is computed to be 2.25 (1.43) eV
with the MPW1K (PBE0) functional. The substantial functional
effect is essentially due to the increased amount of HF exchange
in MPW1K, which increases the HOMO–LUMO gap and
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HLS measurements
differences between calculations and experimental measurements
can, however, be also originated because calculations have been
performed on the free cationic Ru complexes, while experimen-
tal measurements are performed on the corresponding strong ion
pair.
The HLS technique6,7,26 involves the detection of the incoher-
ently scattered second harmonic generated by a solution of the
molecule under irradiation with a laser of wavelength λ, leading
to the measurement of the mean value of the β × β tensor
product, 〈βHLS〉. All HLS measurements were carried out at the
École Normale Supérieure de Cachan in CHCl3 at a concen-
tration of 1 × 10−3 M, working with a low energy non resonant
incident radiation of 1.907 μm.
We notice that the peculiar electronic structure of the
[TD2RuTA]2+ complex, which translates into a substantial mixing
of the ligand and metal orbitals also in the excited state, might
be responsible of the communication between the donor and
acceptor ligands that is evidenced by the non additive non-linear
optical response of this system.25 For the “conventional”
[TD1RuTA]2+ system, such mixing was not evidenced and a
purely additive model of the non-linear optical response was
found to hold for this system. Finally, we found that, in agree-
ment with the experimental data, similar β0,EFISH are computed
for the couple [TD1RuTA][EtPhCO2]2/[TD1RuTD3][EtPhCO2]2.
Although these values are overestimated, compared to their
experimental counterpart, they agree as order of magnitude,
which considering the rather simplified level of theory, can be
considered more than satisfactory.
Preparation of ruthenium complexes
For the numbering used in the attribution of the 1H-NMR
signals see below.
Experimental
General comments
RuCl3·xH2O was purchased from Engelhard. Terpyridine TD3
was purchased from Sigma Aldrich and used without further
10
10
9
purification, while ligands TD1
,
TD2
,
TA and the salt
RuCl3TD1 and RuCl3TD2
4-EtPhCOOAg9 were prepared as previously reported by some
of us. All solvents were used as purchased, without further
purification. Products were characterized by 1H-NMR (Bruker
DRX-300 spectrometer) and UV-vis (Jasco V-530 spectropho-
tometer) spectroscopy, mass spectrometry (Varian VG9090 spec-
trometer) and elemental analysis. Dipole moments, μ, were
A solution of TD1 or TD2 (1 mmol) in MeOH or absolute EtOH
was added to a solution of RuCl3·xH2O (1 mmol) in the same
solvent. The resulting dark red mixture was refluxed for 4 h and
then left at room temperature overnight. The brownish red pre-
cipitate thus collected was filtered, washed with a small amount
of cold MeOH or absolute EtOH and dried in vacuum, affording
the desired product in high yield (82% or 72%, respectively).
measured in CHCl3 (amylene stabilized) by using
a
WTW-DM01 dipole meter (dielectric constant) coupled with a
RX-5000 ATAGO Digital Refractometer (refractive index)
according to the Guggenheim method.8 Elemental analyses were
carried out at the Dipartimento di Chimica Inorganica, Metallor-
ganica e Analitica “Lamberto Malatesta” of the Università degli
Studi di Milano.
RuCl3TD1. MS-FAB+ m/z 943 (M
C53H80N4RuCl3 = 978). Anal. calcd (found): C, 64.90 (64.30);
H, 8.22 (7.76); N, 5.68 (5.71).
−
Cl)+ (calcd for
RuCl3TD2. MS-FAB+ m/z 1047 (M − Cl)+ (calcd for
C61H86N4RuCl3 = 1082). Anal. calcd (found): C, 67.66 (67.85);
H, 8.00 (8.14); N, 5.17 (5.66).
EFISH and THG measurements
All EFISH and THG measurements were carried out at the
Dipartimento di Chimica Inorganica Metallorganica e Analitica
“Lamberto Malatesta” of the Università degli Studi di Milano, in
CHCl3 solutions at a concentration of 1 × 10−3 M, working with
a non-resonant incident wavelength of 1.907 μm, obtained by
Raman-shifting the fundamental 1.064 μm wavelength produced
by a Q-switched, mode-locked Nd3+:YAG laser manufactured by
Atalaser. The apparatus for the EFISH and THG measurements
is a prototype made by SOPRA (France). The μβEFISH values
reported are the mean values of 16 successive measurements per-
formed on the same sample. The sign of μβ is determined by
comparison with the reference solvent (CHCl3).
Heteroleptic complexes [TD1RuTA][4-EtPhCO2]2 and [TD2RuTA]
[4-EtPhCO2]2
A solution of TA (1 mmol) in absolute EtOH (5 mL) and 5 drops
of N-ethylmorpholine were added to a solution of RuCl3TD1 or
RuCl3TD2 (1 mmol) in the same solvent (5 mL). The brown
mixture was refluxed for 1 h and subsequently hot filtered on
celite.
A solution of 4-EtPhCOOAg (2 mmol) in hot EtOH (10 mL)
was then added to the filtrate and the mixture was stirred at room
temperature overnight. After filtration of AgCl on celite and
evaporation of this second filtrate, the desired products were
6712 | Dalton Trans., 2012, 41, 6707–6714
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[TD2RuTD3][4-EtPhCO2]2. 1H-NMR (300 MHz, CD3OD,
25 °C, TMS): δ (ppm) 9.28 (s, 2H, H3′, H5′), 9.25 (s, 2H, Hc′,
He′), 8.88 (dd, 4H, J = 8.1 Hz, J = 4.9 Hz, Hc, Hc′′, H3, H3′′),
8.28 (d, 2H, J = 8.5 Hz, H12, H12′), 8.19 (d, 2H, J = 8.5 Hz, H7,
H7′), 8.02 (m, 4H, Hd, Hd′′, H4, H4′′), 7.88 (d, 4H, J = 8.5 Hz,
Hx, Hx′), 7.85 (d, 2H, J = 8.9 Hz, H7, H7′), 7.56 (m, 4H, H6, H6′′,
Hf, Hf′′), 7.48 (d, 2H, J = 8.9 Hz, H8, H8′), 7.28 (m, 5H, H9 or
H10, He, He′′, H5, H5′′), 7.23 (d, 4H, J = 8.5 Hz, Hy, Hy’), 7.10
(d, 1H, J = 15.9 Hz, H10 or H9), 6.70 (d, 2H, J = 8.5 Hz, H13,
H13′), 3.30 (t, 4H, J = 7.1 Hz, N–CH2), 2.68 (q, 4H, J = 7.7 Hz,
CH3–CH2 of 4-EtPhCO2−), 1.65 (m, –CH2–), 1.31 (m, –CH2–),
1.23 (t, 6H, J = 7.7 Hz, CH3 of 4-EtPhCO2−), 0.90 (t, 6H, J =
6.9 Hz, CH3). Anal. calcd (found): C, 73.73 (72.91); H, 7.57
(7.56); N, 7.15 (7.36).
obtained as bright dark red powders (70% and 23% yield,
respectively).
[TD1RuTA][4-EtPhCO2]2. 1H-NMR (300 MHz, CD3OD,
25 °C, TMS): δ (ppm) 9.35 (s, 2H, Hc′, He′), 9.19 (s, 2H, H3′,
H5′), 8.91 (d, 2H, J = 7.9 Hz, Hc, Hc′′), 8.86 (d, 2H, J = 7.7 Hz,
H3, H3′′), 8.61 (d, 2H, J = 8.7 Hz, Hh, Hh′), 8.53 (d, 2H, J =
8.7 Hz, Hg, Hg′), 8.20 (d, 2H, J = 8.4 Hz, H7, H7′), 8.03 (m, 6H,
Hd, Hd′′, H4, H4′′, H5, H5′′), 7.91 (d, 4H, J = 8.1 Hz, Hx, Hx′),
7.62 (d, 2H, J = 5.0 Hz, Hf, Hf′′), 7.52 (d, 2H, J = 5.2 Hz, H6,
H6′′), 7.33 (m, 2H, He, He′′), 7.26 (d, 4H, J = 8.0 Hz, Hy, Hy′),
6.99 (d, 2H, J = 8.7 Hz, H8, H8′), 3.51 (t, 4H, J = 7.2 Hz, N–
CH2), 2.70 (q, 4H, J = 7.6 Hz, CH3–CH2 of 4-EtPhCO2−), 1.74
(m, –CH2–), 1.26 (m, –CH2– and CH3 of 4-EtPhCO2−), 0.88 (t,
6H, J = 7.2 Hz, CH3). Anal. calcd (found): C, 72.36 (69.53); H,
7.39 (7.21); N, 7.33 (6.30).
Homoleptic complexes [TD1RuTD1][4-EtPhCO2]2 and
[TD2RuTD2][4-EtPhCO2]2
[TD2RuTA][4-EtPhCO2]2. 1H-NMR (300 MHz, CD3OD,
25 °C, TMS): δ (ppm) 9.36 (s, 2H, Hc′, He′), 9.30 (s, 2H, H3′,
H5′), 8.92 (dd, 4H, J = 7.9 Hz, J = 3.5 Hz, Hc, Hc′′, H3, H3′′),
8.59 (d, 2H, J = 8.9 Hz, Hh, Hh′), 8.53 (d, 2H, J = 8.7 Hz, Hg,
Hg′), 8.29 (d, 2H, J = 8.4 Hz, H12, H12′), 8.04 (m, 4H, Hd, Hd′′,
H4, H4′′), 7.92 (d, 4H, J = 8.1 Hz, Hx, Hx′), 7.86 (d, 2H, J = 8.4
Hz, H7, H7′), 7.61 (d, 2H, J = 5.0 Hz, Hf, Hf′′), 7.56 (d, 2H, J =
5.0 Hz, H6, H6 ′′), 7.49 (d, 2H, J = 8.9 Hz, H8, H8′), 7.30 (m,
5H, H9 or H10, He, He′′, H5, H5′′), 7.27 (d, 4H, J = 8.1 Hz, Hy,
Hy′), 7.08 (d, 1H, J = 16.3 Hz, H10 or H9), 6.72 (d, 2H, J = 8.9
Hz, H13, H13′), 3.33 (t, 4H, J = 7.1 Hz, N–CH2), 2.70 (q, 4H, J
= 7.6 Hz, CH3–CH2 of 4-EtPhCO2−), 1.65 (m, –CH2–), 1.31 (m,
–CH2– and CH3 of 4-EtPhCO2−), 0.91 (t, 6H, J = 6.4 Hz, CH3).
Anal. calcd (found): C, 72.36 (69.53); H, 7.39 (7.21); N, 7.33
(6.30).
A solution of TD1 or TD2 (1 mmol) in absolute EtOH (5 mL)
and 5 drops of N-ethylmorpholine were added to a solution of
RuCl3TD1 or RuCl3TD2 (1 mmol) in the same solvent (5 mL).
The brown mixture was refluxed for 1 h and subsequently hot
filtered on celite.
A solution of 4-EtPhCOOAg (2 mmol) in hot EtOH (10 mL)
was then added to the filtrate and the mixture stirred at room
temperature overnight. After filtration of AgCl on celite and
evaporation of this second filtrate, the desired products were
obtained as bright dark red powders (78% and 35% yield
respectively).
[TD1RuTD1][4-EtPhCO2]2. 1H-NMR (300 MHz, CD3OD,
25 °C, TMS): δ (ppm) 9.14 (s, 4H, H3′, H5′), 8.81 (d, 4H, J =
7.7 Hz, H3, H3′′), 8.17 (d, 4H, J = 8.3 Hz, H7, H7′), 7.97 (dd,
4H, J = 7.7 Hz, H4, H4′′), 7.90 (d, 4H, J = 8.4 Hz, Hx, Hx′), 7.51
(d, 4H, J = 5.2 Hz, H6, H6′′), 7.25 (d, 4H, J = 7.7 Hz, Hy, Hy′),
7.24 (m, 4H, H5, H5′′), 6.96 (d, 4H, J = 9.0 Hz, H8, H8′), 3.49 (t,
8H, J = 7.1 Hz, N–CH2), 2.69 (q, 4H, J = 7.7 Hz, CH3–CH2 of
4-EtPhCO2−), 1.72 (m, –CH2–), 1.42 (m, –CH2–), 1.27 (m,
–CH2–), 1.24 (t, 6H, J = 7.7 Hz, CH3 of 4-EtPhCO2−), 0.87 (t,
12H, J = 7.0 Hz, CH3). Anal. calcd (found): C, 76.54 (76.34);
H, 9.22 (9.20); N, 5.76 (5.71).
Heteroleptic complexes [TD1RuTD3][4-EtPhCO2]2 and
[TD2RuTD3][4-EtPhCO2]2
A solution of TD3 (1 mmol) in absolute EtOH (10 mL) and 5
drops of N-ethylmorpholine were added to a solution of
RuCl3TD1 or RuCl3TD2 (1 mmol) in the same solvent (5 mL).
The brown mixture was refluxed for 1 h and subsequently hot
filtered on celite. A solution of 4-EtPhCOOAg (2 mmol) in hot
EtOH (10 mL) was then added to the filtrate and the mixture
stirred at room temperature overnight. After filtration of AgCl on
celite and evaporation of this second filtrate, the desired products
were obtained as dark red powders (96% and 92% yield,
respectively).
[TD2RuTD2][4-EtPhCO2]2. 1H-NMR (300 MHz, CD3OD,
25 °C, TMS): δ (ppm) 9.29 (s, 4H, H3′, H5′), 8.90 (d, 4H, J =
8.3 Hz, H3, H3′′), 8.29 (d, 4H, J = 8.4 Hz, H12, H12′), 8.04 (dd,
4H, J = 7.8 Hz, H4, H4′′), 7.92 (d, 4H, J = 8.1 Hz, Hx, Hx′), 7.86
(d, 4H, J = 8.4 Hz, H7, H7′), 7.56 (d, 4H, J = 5.4 Hz, H6, H6′′),
7.49 (d, 4H, J = 8.9 Hz, H8, H8′), 7.30 (m, 6H, H9 or H10, H5,
H5′′), 7.27 (d, 4H, J = 8.0 Hz, Hy, Hy′), 7.08 (d, 2H, J = 16.2
Hz, H10 or H9), 6.73 (d, 4H, J = 8.7 Hz, H13, H13′), 3.32 (m, 8H,
J = 7.1 Hz, N–CH2), 2.70 (q, 4H, J = 7.6 Hz, CH3–CH2 of
4-EtPhCO2−), 1.41 (m, –CH2–), 1.26 (t, 6H, J = 7.6 Hz, CH3 of
4-EtPhCO2−), 0.92 (t, 12H, J = 7.0 Hz, CH3). Anal. calcd
(found): C, 78.20 (77.67); H, 8.91 (9.05); N, 5.21 (5.35).
[TD1RuTD3][4-EtPhCO2]2. 1H-NMR (300 MHz, CD3OD,
25 °C, TMS): δ (ppm) 9.24 (s, 2H, Hc′, He′), 9.17 (s, 2H, H3′,
H5′), 8.86 (dd, 4H, J = 8.4 Hz, Hc, Hc′′, H3, H3′′), 8.20 (d, 2H, J
= 8.4 Hz, H7, H7′, Hg, Hg′), 8.00 (m, 4H, Hd, Hd′′, H4, H4′′), 7.89
(d, 4H, J = 8.1 Hz, Hx, Hx′), 7.58 (m, 2H, Hf, Hf′′), 7.52 (d, 2H,
J = 5.2 Hz, H6, H6′′), 7.27 (m, 4H, H5, H5′′, He, He′′), 7.24 (d,
4H, J = 7.9 Hz, Hy, Hy′), 6.99 (d, 2H, J = 8.9 Hz, H8, H8′), 3.52
(t, 4H, J = 6.8 Hz, N–CH2), 2.69 (q, 4H, J = 7.5 Hz, CH3–CH2
of 4-EtPhCO2−), 2.56 (s, 3H, CH3 of TD3), 1.74 (m, –CH2–),
1.44 (m, –CH2– and CH3–CH2 of 4-EtPhCO2−), 0.91 (t, 6H, J =
6.2 Hz, CH3). Anal. calcd (found): C, 74.66 (74.49); H, 7.73
(7.75); N, 6.55 (6.69).
Conclusions
In conclusion, the [TD1RuTA][4-EtPhCO2]2 and [TD2RuTA]-
[4-EtPhCO2]2 heteroleptic complexes investigated in this work
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 6707–6714 | 6713

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Eur. J. Inorg. Chem., 1998, 1205; (c) R. A. Fallahpour, M. Neuburger
and M. Zehnder, Synthesis, 1999, 6, 1051; (d) R. A. Fallahpour,
M. Neuburger and M. Zehnder, New J. Chem., 1999, 53.
represent a nice example of push–pull systems where a metal,
instead of an organic fragment, acts as a bridge between the two
π-delocalized organic moieties bearing a donor and an acceptor
group, respectively. Their quadratic hyperpolarizability, along
with that of free terpyridines and related homoleptic [TD1RuTD1]-
[4-EtPhCO2]2 and [TD2RuTD2][4-EtPhCO2]2 complexes with
two terpyridines bearing a donor substituent, is remarkably high
and largely dominated by the octupolar contribution, even in the
case of the push–pull asymmetric cations. Interestingly, it
appears that, in the highly π-delocalized push–pull heteroleptic
[TD2RuTA][4-EtPhCO2]2 complex, the efficiency of the ruthe-
nium bridge on the octupolar contribution to the quadratic hyper-
polarizability is particularly high, while such a bridging effect is
not so relevant for the less π delocalized system [TD1RuTA]-
[4-EtPhCO2]2. In this latter case the octupolar contribution is
just the sum of the contribution of the two terpyridines.
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Acknowledgements
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P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M.
A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J.
A. Pople, GAUSSIAN 03 (Revision B.05), Gaussian, Inc., Pittsburgh, PA,
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This work was supported by MIUR (FIRB 2003:
RBNE033KMA, FIRB 2004: RBPR05JH2P and PRIN 2008:
2008FZK5AC_002) and by the Centro Nazionale delle Ricerche
(INSTM-PROMO). We deeply thank Dr Stefania Righetto for
EFISH measurements and Dr Sergio Rondena for some exper-
imental help.
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