Effect of metal cation complexation on the nonlinear optical response of an electroactive bisiminopyridine ligand

Article abstract of DOI:10.1016/j.dyepig.2013.10.006

We report on the third order nonlinearities of a bis-iminopyridine- tetrathiafulvalene appended ligand and of its corresponding zinc metal complex. The measurements have been carried out using the Z-scan technique employing 30 ps, 532 nm laser excitation. The nonlinear optical response of the zinc complex has been found to be significantly enhanced with respect to the ligand. We illustrate the possibility to control the nonlinear optical response of this electroactive bisiminopyridine ligand. Moreover the obtained results are discussed and compared with previous findings reported in the literature.

Full text of DOI:10.1016/j.dyepig.2013.10.006

Accepted Manuscript
Effect of metal cation complexation on the Nonlinear Optical response of an
Electroactive Bisiminopyridine ligand
K. Iliopoulos, I. Guezguez, A.P. Kerasidou, A. El-Ghayoury, D. Branzea, G. Nita, N.
Avarvari, H. Belmabrouk, S. Couris, B. Sahraoui
PII:
S0143-7208(13)00378-1
10.1016/j.dyepig.2013.10.006
DYPI 4127
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 July 2013
Revised Date: 30 September 2013
Accepted Date: 1 October 2013
Please cite this article as: Iliopoulos K, Guezguez I, Kerasidou AP, El-Ghayoury A, Branzea D, Nita G,
Avarvari N, Belmabrouk H, Couris S, Sahraoui B, Effect of metal cation complexation on the Nonlinear
Optical response of an Electroactive Bisiminopyridine ligand, Dyes and Pigments (2013), doi: 10.1016/
j.dyepig.2013.10.006.
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ACCEPTED MANUSCRIPT
Effect of metal cation complexation on the Nonlinear Optical response of an
Electroactive Bisiminopyridine ligand
K. Iliopoulos,a,b,* I. Guezguez,a,c A. P. Kerasidou,a A. El-Ghayoury,a D. Branzea,a G. Nita,a N.
Avarvari,a H. Belmabrouk,a S. Couris,b,d and B. Sahraouia,*
aLUNAM Université, Université d’Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou,
2 Bd Lavoisier, 49045 Angers Cedex, France.
bInstitute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas
(FORTH/ICE-HT), 26504 Patras, Greece.
cLaboratory Electronic and Microelectronic, University of Monastir, Tunisia
dDepartment of Physics, University of Patras, Rio 26504, Patras, Greece.
*corresponding author.
E-mail address: bouchta.sahraoui@univ-angers.fr, Tel: +00332 41735489
*corresponding author.
E-mail address: kostiliopoulos@gmail.com, Tel: +0033241735423
Abstract
We report on the third order nonlinearities of a bis-iminopyridine-tetrathiafulvalene appended
ligand and of its corresponding zinc metal complex. The measurements have been carried out
using the Z-scan technique employing 30 ps, 532 nm laser excitation. The nonlinear optical
response of the zinc complex has been found to be significantly enhanced with respect to the
ligand. We illustrate the possibility to control of the nonlinear optical response of this
electroactive bisiminopyridine ligand. Moreover the obtained results are discussed and
compared with previous findings reported in the literature.
Keywords: metal cation complexation, Z-scan, nonlinear optical response, TTF-based
molecular systems

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1. Introduction
Recently, there has been intense interest concerning the investigation of organic
materials exhibiting sizeable optical nonlinearities in order to be employed in various opto-
electronic and photonic applications [1–6]. Tetrathiafulvalene (TTF)-based materials are of
great interest because of their low-first oxidation potential and their unusual stability and solid
state properties of the radical cations resulting from the one-electron oxidation of their parent
molecules [7]. They have been widely used as the π-donor components in the preparation of
molecular conductors and superconductors [8,9]. In addition, they have also found use as
redox switches in molecular and supramolecular architectures [10-12]. An active ongoing
research theme is the covalent linking of the electroactive TTF unit with a coordinating group
in order to produce multifunctional materials which can exhibit interplay between two or
more physical properties such as magnetism and conductivity [13-15]. Pyridine based Schiff
bases ligands such as the chelating 2,6-bis(imino)pyridyl unit are well known to form stable
complexes, with various transition metals, that have been extensively used as catalysts for
olefin polymerization [16,17].
Recently we have demonstrated that the nature of the metal cations play an important
role in the nonlinear absorption of orthogonal TTF-based units where an increase of the
nonlinearity was observed in the case of platinum as compared with palladium [18].
Moreover, in the case of TTF-based iron and ruthenium metal complexes we have shown that
the incorporation of metal cations results in switching of the character of the nonlinear
absorption of the systems from Saturable Absorptive (SA) to Reverse Saturable Absorptive
(RSA) [19].
As a continuation of our work, we report herein on the nonlinearity of a TTF-based
bisiminopyridine ligand 1 and its corresponding zinc complex 2 (scheme 1) [20]. The
objective of this work is to find out the impact of the complexation of the ligand by
introducing a metal cation on the nonlinear optical response.
2. Experimental
2.1 Z-scan technique

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For the determination of the third order nonlinear optical properties of the systems 1, 2
the well-known Z-scan technique has been utilized [21], employing a 30 ps diode pumped
Nd:YVO4 laser with a repetition rate of 10 Hz. The Z-scan setup will not be presented in
detail here as the information can be found in the literature [21,22]. This setup is widely used
for the determination of the nonlinearities as it allows the investigation of the magnitude of
the nonlinear absorption and refraction of a material, as well as their sign. It is based on
measuring the transmittance of a focused laser beam through the sample as it moves along the
transmission axis. The transmittance is measured in two different ways, which take place
simultaneously. The “open aperture” Z-scan includes detection of the totality of the laser
beam after the sample, while concerning the “closed aperture” Z-scan, only the part of the
beam which passes through a narrow aperture is detected. By the former the imaginary part of
the third order nonlinear susceptibility (Imχ(3)) related to the nonlinear absorption can be
determined. The latter includes the contribution of both the nonlinear refraction and
absorption to the nonlinearity. Under certain limitations the division of the “closed aperture”
by the “open aperture” Z-scan traces (called “divided” Z-scan) can provide the real part of the
third order nonlinear susceptibility (Reχ(3)), which is related to the nonlinear refraction of the
investigated system. In the case of the nonlinear absorption a minimum or maximum of the
transmittance around the focal point corresponds to positive (Reverse Saturable Absorption
(RSA)) or negative (Saturable Absorption (SA)) response. In the case of the nonlinear
refraction a “valley-peak” or “peak-valley” configuration correspond to positive (self-
focusing) or negative (self-defocusing) response.
More specifically from the “open aperture” Z-scan the nonlinear absorption coefficient
(β) can be obtained by fitting the experimental data according to the following equation:

m

∞
−
β
I L / 1+ z2 z2
(
)
0
eff
0

T =
(1)
∑
3/2
m +1
m=0
(
)
where T is the normalized transmittance, L = 1−exp −α 0
L
a0 is the effective thickness
(
)
)
(
eff
of the sample, α0 is the linear absorption coefficient at the laser excitation wavelength and I0
is the on-axis irradiance at the focus. Additionally from the fitting of the divided Z-scan data
by means of the following equation:
4∆Φ0 x
T =1−
(2)
x2 + 9 x2 +1
(
)(
)

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the on-axis nonlinear phase shift at the focus ꢀΦ0 can be obtained. The latter is related with
′
. By using the
∆Φ0 = kI0γ Leff
the nonlinear refractive index parameter through the equation
following equations the Reχ(3) and Imχ(3) can be determined from the nonlinear refractive
index parameter and the nonlinear absorption coefficient respectively:
10−6 cn02
480π 2
3
)
Re χ ( esu =
γ
cm2 W −1
′
(
)
(
)
(3)
10−7 c2n02
96π 2ω
3
)
Im χ ( esu =
β
cmW −1
(
)
(
)
where c is the speed of light in cm s-1 and ω is the fundamental frequency in cycles s-1. The
second order hyperpolarizability (γ) can be determined from the third order nonlinear
susceptibility (χ(3)) using the equation:
3
)
χ (
(4)
γ
=
NL4
where N is the number density and L is the local field correction factor.
3. Results and discussion
3.1 UV-VIS absorption studies
The electronic absorption spectrum of the ligand (1) was recorded in dichloromethane
solution (~2×10−5M at room temperature; black line in Figure 1). This ligand exhibits two
strong electronic absorption bands at 269 and 336 nm, which are assigned to the π→π*
absorption bands of the TTF and the phenyl ring respectively. The broad band observed in the
visible region at about 433 nm is characteristic of the intramolecular charge transfer transition
(ICT) from the highest occupied molecular orbital in the two TTFs to the lowest unoccupied
molecular orbital in the electron-accepting pyridyl unit [23,24] and is responsible for the dark
orange-red color of this compound.
In our previous report, we prepared and characterized single crystals of the resulting
zinc metal complex 2 [20]. The UV-Vis electronic absorption spectrum of the dissolved
crystals is shown in figure 1, red line). The complex 2 exhibits the same features as the free
ligand in the UV region (<400 nm). In the visible region the ICT transition is red shifted

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indicating an increase of the electron acceptor behavior of ligand 1 upon complexation. In
addition, the presence of a new shoulder around 385 nm which corresponds to a ligand
centered (LC, πꢀπ* and nꢀπ*) absorption is probably due to conformational change of the
ligand after complexation with zinc chloride. In order to confirm the presence in solution of a
single zinc complex specie, UV-Visible electronic absorption titration was performed by
adding increasing amounts of a zinc chloride solution into a CH2Cl2 solution of the ligand 1
(Figure 1). These additions result in a decrease of the bands located at 281 and 335 nm and
the appearance of the new band around 385 nm and a red shift of the ICT band. The presence
of three isobestic points at 365, 437 and 490 nm clearly indicates the formation of single zinc
complex specie. Note that no additional absorption changes were observed upon introduction
of more than one equiv. of ZnCl2, as expected for the formation of the 1:1 complex.
3.2 Nonlinear optical investigation
For the characterization of the nonlinear refraction and absorption, the samples have
been put in 1 mm thick quartz cells and studied by means of the Z-scan technique (see
experimental section). Several different concentrations ranging from 0.5 to 2 mM, in DMF
have been prepared for each molecule. For each investigated concentration, energy-dependent
measurements have been carried out. The solvent has been separately investigated showing
that its contribution to the nonlinear absorption has been negligible, while the contribution to
the nonlinear refraction has been significant under the same experimental conditions.
Concerning the nonlinear absorption of the samples the “open aperture” Z-scans obtained
revealed a decrease of the transmittance around the focal point of the lens, which is attributed
to Reverse Saturable Absorption (RSA)-type behavior of the sample. In figure 2 two
characteristic “open aperture” Z-scans are shown corresponding to solutions of the ligand 1
and complex 2. To facilitate direct comparison both curves, which are shown in Figure 2
correspond to 1 mM concentration in DMF and were obtained using the same incident laser
energy (i.e. 1ꢁJ). It becomes obvious that the nonlinear absorption of the Zn complex is
higher than that of the ligand and this has been verified by a plethora of scans in several
different energies and concentrations.
Then, the nonlinear absorption parameter β and the corresponding imaginary part of the
third order nonlinear susceptibility (Imχ(3)) have been determined from several “open

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aperture” Z-scans and are presented in Tables 1 and 2. In Figure 3, the variation of the Imχ(3)
of both compounds 1 and 2, as a function of concentration is shown. The large difference
between the slopes of the two curves in this graph demonstrates unambiguously the fact that
the nonlinear absorption of the Zinc complex 2 is much larger than that of the ligand 1. From
the slopes shown in Figure 3 the imaginary part of the second order hyperpolarizability has
been determined and is shown in Tables 1 and 2.
However the nonlinear refraction of ligand 1 has been found to be negligible as the
“closed aperture” and “divided” Z-scans have shown that the only contribution has been
emanating from the solvent. For this reason, the second order hyperpolarizability presented in
Table 1 in the case of ligand 1 is equal to Imγ. On the contrary, in the case of the zinc
complex 2, a nonlinear refractive character of the sample has been found as can be seen in
figure 4, where two “divided” Z-scans of the solvent and of a of 1.0 mM solution of the zinc
complex 2 in DMF, under the same experimental conditions (i.e. laser energy of 1.6 ꢁJ), are
presented. It is clear that the solvent exhibits a positive nonlinear refraction (self-focusing) as
the divided Z-scan reveals a “valley-peak” configuration, while in the solution of the complex
2 a “peak-valley” configuration has been found, corresponding to negative nonlinear
refraction (self-defocusing). As a result we can deduce that compound 2 has a negative
nonlinearity, which (at least for the specific laser energy and concentration) is not only
cancelling the positive contribution of the solvent, but it is also adding up to the self-
defocusing character of the sample, resulting finally to a peak-valley configuration. The
magnitude of the ꢀΤp-v (the difference between the valley and the peak of the divided Z-scan)
was dependent upon the concentration and the incident laser energy, and in several cases low
or even zero ꢀΤp-v values have been obtained, which corresponded to cancellation of the two
contributions to the nonlinearity. By adjusting the concentration/incident laser energy we have
achieved to obtain sufficient signal to noise ratios, as shown as an example in Figure 4. This
behavior clearly indicates a high nonlinear refraction for the zinc complex. The second order
hyperpolarizability has been determined for the zinc complex 2 and can be seen in Table 2.

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Table 1: Nonlinear optical parameters of the Ligand 1 (532 nm, 30 ps)
Ligand 1
Conc.
(mM)
0.5
β
Imχ(3)
(×10-11 m/W) (×10-13 esu) (×10-32 esu)
γ (Imγ)
0.18
0.46
0.66
1.01
0.10
0.25
0.36
0.55
1.0
1.25
1.5
2.0
Table 2: Nonlinear optical parameters of the Zinc complex 2 (532 nm, 30 ps).
Zinc Complex 2
Conc.
(mM)
0.50
β
Imχ(3)
(×10-11 m/W) (×10-13 esu) (×10-32 esu) (×10-32 esu) (×10-32 esu)
Imγ
Reγ
γ
0.80
1.30
1.78
2.36
0.44
0.70
0.96
1.28
0.75
4.50
-9.30
10.24
1.00
1.50
The nonlinear optical parameters presented in Tables 1 and 2 indicate an increase of the
nonlinear absorption of the Zinc complex by about 4 times. This fact combined with the
appearance of nonlinear refraction in the case of the complex results to a total increase of the
second order hyperpolarizability by about 8 times. This significant enhancement might be
explained by the red shift of the intramolecular charge transfer occurring between the TTF
unit and bis-iminopyridine fragment upon complexation as clearly shown during the titration
of the ligand by adding increasing amount of zinc chloride (Figure 1).
In a previous publication we have demonstrated [18] that the nonlinear absorption of
orthogonal pyrrolo-TTF derivatives is significant if compared with that of the initial ligand,
which has been found to be negligible. This result is in agreement with our current findings.
Additionally in another recent publication [19] we have shown that the introduction of metal

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cations can be of great importance as far as the optical nonlinearity of TTF-based systems is
concerned. More specifically it has been found that there is a switching of the nonlinear
absorption of the molecular systems upon complexation of the ligand with ruthenium or with
iron. This behaviour has been attributed to the presence of the metal to ligand charge transfer
(MLCT) band in the complexes. It is worth noting that in the present case, although no MLCT
band is present in complex 2, the coordination of metal cation is beneficial for the nonlinear
optical response.
4. Conclusions
In conclusion, a comparative study of the nonlinear optical response between an
electroactive TTF based ligand and its corresponding zinc complex has been performed. The
obtained results demonstrate an enhancement of the nonlinearity upon complexation of the
ligand. In this study we have shown the possibility of controlling the nonlinear optical
response of these systems by a metal cation complexation. This fact opens new prospects for
photonic/optoelectronic applications.
Acknowledgements
K.I. acknowledges support from the European Commission and General Secretariat for
Research and Technology (Greece) for a National Strategic Reference Framework (NSRF)
project (PE3-(1612)).

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Figure Captions
Scheme 1. Chemical structures of ligand 1 and zinc complex 2
Figure 1. UV-Vis titration of ligand 1 (C= 2.5 10-5M) by ZnCl2 in a mixture
dichloromethane/acetonitrile, 1/1.
Figure 2: “Open aperture” Z-scans of the investigated compounds a) Ligand 1 b) Zn-complex
2.
Figure 3: Imχ(3) as a function of concentration for ligand 1 and complex 2.
Figure 4: “Divided” Z-scans obtained for the Zinc complex 2 (1.0 mM) and the solvent
(DMF).

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• Effect of metal cation complexation on the NLO response
• Control of the nonlinear Optical response of an Electroactive
Bisiminopyridine ligand
• High NLO properties of the studied compounds