Remarkable effect of iridium cyclometalation on the nonlinear absorption properties of a quadrupolar imine ligand

Article abstract of DOI:10.1021/ic4012313

A new type of dinuclear iridium complex, based on a quadrupolar Schiff base ligand, is synthesized and its structure fully characterized. Its linear and nonlinear spectroscopic properties are investigated, evidencing a strong contribution of the metal-to-ligand transitions not only to the linear absorption but also to the two- and three-photon absorption properties.

Full text of DOI:10.1021/ic4012313

Communication
pubs.acs.org/IC
Remarkable Effect of Iridium Cyclometalation on the Nonlinear
Absorption Properties of a Quadrupolar Imine Ligand
Julien Massue,†,‡ Joanna Olesiak-Banska,§ Erwann Jeanneau,⊥ Christophe Aronica,†
Katarzyna Matczyszyn,§ Marek Samoc,§ Cyrille Monnereau,*,† and Chantal Andraud*,†
†CNRS UMR-5182, Universite
́
Claude Bernard, Ecole Normale Super
́
ieure de Lyon, 46 allee
́
d’Italie, 69364 Lyon, France
́
§Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw,
Poland
⊥Centre de Diffractomet
́
rie Henri Longchambon, Universite
́
Claude Bernard Lyon 1, 5 rue de la Doua, 69100 Villeurbanne, France
S
* Supporting Information
particularly intense MLCT transitions, which can be tuned over a
wide range of wavelengths depending on the nature of the metal
and ligand.6
ABSTRACT: A new type of dinuclear iridium complex,
based on a quadrupolar Schiff base ligand, is synthesized
and its structure fully characterized. Its linear and
nonlinear spectroscopic properties are investigated,
evidencing a strong contribution of the metal-to-ligand
transitions not only to the linear absorption but also to the
two- and three-photon absorption properties.
In this work, we describe the synthesis and spectroscopic study
of a new type of cyclometalated iridium complexes with strongly
enhanced multiphoton absorption properties. In order to do so, a
classical quadrupolar molecule with an imine bridge (Schiff
base), functionalized at both extremities with aniline donors,7 is
used as a scaffold, which complexes two iridium(III) cations by
coordination to the nitrogen atoms of the imine bridge, and bis-
cyclometalation to the central phenyl ring.
onlinear-optical processes are among the most intriguing
Nphenomena in physics, being of much interest as both a
fundamental and applied research topic. The properties that arise
from the nonlinear-optical processes can be advantageously used
in a plethora of practical applications, in the fields of
telecommunications, data storage, optical switches, optical
power-limiting devices, and bioimaging.1 Owing to the very
active work of molecular engineering by various research groups
worldwide, patterns have emerged in the last 3 decades that have
allowed the design of molecules with constantly improving
second- or third-order nonlinear-optical properties.2
The quadrupolar ligand 1 (Figure 1, left) could be readily
obtained in three steps from commercially available starting
A common feature of all molecules with high nonlinearities is
the presence of low-lying electronic transitions of the charge-
transfer type, which lead to an important redistribution of the
electronic density of the molecule between its ground and excited
states. Thus, a general molecular engineering strategy consists of
using a large π-conjugated backbone, with strong electron-donor
and/or -acceptor groups at their extremities, resulting in
molecules with so-called intramolecular charge-transfer (ICT)
excited states. Alternatively, it has also been proposed that the
specific spectroscopic properties of transition-metal complexes
and organometallics, with multiple low-lying electronic
transitions of the metal-to-ligand (MLCT), ligand-to-metal
(LMCT), or interligand (LLCT) charge-transfer type, could be
advantageously used in the context of nonlinear optics.3
Figure 1. Structure of the ligand (1) and complex (1·Ir) involved in the
study.
materials (see the experimental section in the Supporting
Information). Briefly, nucleophilic aromatic substitution on 4-
fluoronitrobenzene, using N,N-dioctylamine as a nucleophile,
afforded the corresponding 4-nitro-N,N-dioctylaniline with good
yield. The nitro function was then reduced to amino, and the
resulting compound was involved in condensation with
terephthaldehyde, leading to a mixture of mono- and bis-
condensation products, from which the target compound could
easily be recovered. The treatment of 1 with a dichloro-
(pentamethylcyclopentadienyl)iridium(III) dimer, in the pres-
ence of a base (potassium acetate), following reported
procedures8 afforded the target complex 1·Ir (Figure 1, right).
Crystals suitable for X-ray diffraction were obtained as dark-
reddish needles by slow cooling of a boiling acetonitrile solution
of 1·Ir.
Surprisingly, apart from a few noticeable exceptions,3f,4
cyclometalated complexes of heavy metals (iridium, ruthenium,
and platinum) have been only very sparingly considered within
this framework, although their exceptional potential has been
increasingly acknowledged in recent years for related applica-
tions, such as electroluminescent materials.5 Indeed, it has been
shown in multiple examples that cyclometalation results in a
strong mixing between the ligand and metal orbitals, resulting in
Received: May 27, 2013
Published: September 10, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
The centrosymmetric crystal structure indicates that the
consecutive cyclometalation reactions selectively take place in
anti positions on the central phenyl ring, with the corresponding
ancillary ligands (Cp* and Cl) locating opposite to one another
with respect to this same ring (Figure 2). It also appears that the
In order to investigate the nonlinear absorption properties of 1
and 1·Ir, Z-scan measurements of dichloromethane solutions of
these compounds were performed in a range of wavelengths
between 750 and 1600 nm, as described in ref 11 (see the
experimental section in the Supporting Information).
Figure 4a shows the results of Z-scan measurements of
absorptive nonlinearities of 1 in dichloromethane together with
Figure 2. Perspective view of the iridium complex 1·Ir with
displacement ellipsoids plotted at the 30% probability level. Hydrogen
atoms were omitted for clarity.
π-conjugated backbone of the quadrupolar ligand is significantly
distorted from planarity with a C5−C6−N2−C7 dihedral angle of
51°. Upon examination of the crystal structure, it is clear that this
distortion arises from steric hindrance between the Cp* ancillary
ligand on each iridium cation and the terminal aniline rings,
which lie parallel to one another.
The effect of this distortion and of the consequent decreased
conjugation along the molecule axis is clearly observed when the
respective absorption spectra of 1 and 1·Ir in dichloromethane
(Figure 3) are compared. In the case of 1·Ir, the ICT transition is
Figure 4. Plot of the two- and three-photon nonlinear-optical
absorption spectra of (a) 1 and (b) 1·Ir. The corresponding
wavelength-doubled and -tripled linear absorption spectra (normalized,
au) are displayed as red and gray dashed lines, respectively, for
comparison.
the one-photon absorption spectrum plotted against two and
three times the wavelength. We identified the nonlinear
absorption in the range 700−1100 nm as being dominated by
a two-photon absorption process (2PA), whereas three-photon
absorption (3PA) was seen to dominate for longer wavelengths
(for details about the methodology and calculations, see refs 11b
and 11c). The highest 2PA cross section, with a value ∼250 GM,
is present at 825 nm, which is markedly shorter than twice the
wavelength of the one-photon absorption peak. Simultaneously,
3PA is found to peak at 1300 nm, with a value of σ3 of 1.0 × 10−79
cm6·s2. The apparent blue shift of the two-photon absorption
band can be expected, since in centrosymmetric molecules the
selection rules for the one- and two-photon absorption are
mutually exclusive. On the other hand, the three-photon
absorption selection rule is the same as that for one photon.
Here some shift between the positions of the peaks is observed,
which may be due to different contributions of vibrational
substates in the case of one- and three-photon transitions.
The iridium complex 1·Ir exhibits an intense 2PA band with a
maximum value of the two-photon absorption cross section of
Figure 3. Absorption spectra of 1 and 1·Ir in dichloromethane at 298 K.
significantly blue-shifted (λmax = 430 nm), in comparison to that
of 1 (λmax = 452 nm), and its molar extinction coefficient
(εICT,1·Ir= 2.2 × 104 M−1·cm−1) is significantly reduced (εICT,1
=
4.8 × 104 M−1·cm−1). Moreover, while the ICT band is broad and
almost structureless in the case of the ligand, a longer-wavelength
shoulder (λ = 477 nm) is clearly observed in the case of the
complex. By analogy with literature data on related phenyl-
pyridine cyclometalated complexes,9 this characteristic shape can
be reasonably ascribed to a mixed ICT− 1MLCT character of this
electronic transition, which is a classical feature for coordination
compounds and is particularly intense in cyclometalated
complexes.3f Additionally, a less intense band is also observed
at lower energies (λmax = 598 nm), which we ascribe, by analogy
with related compounds in the literature,10 to a mixed
1MLCT−3MLCT transition with a unusually large molar
extinction coefficient for that range of wavelengths (ε = 6900
M−1·cm−1).
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Inorganic Chemistry
Communication
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1180 GM at 825 nm (Figure 4b). The 2PA band is blue-shifted
with respect to the absorption bands of the linear spectrum
plotted at twice the wavelength, similar to the spectrum of a pure
ligand. No feature is seen because of two-photon absorption in
the range of the MLCT bands, but that may again be expected
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band at ∼1300 nm, with the 3PA cross section equal to 5.8 ×
10−79 cm6·s2 in the case of the complex. The highest values of the
intrinsic 3PA cross sections are on the order of 150 × 10−79 cm6·
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or organometallic11c molecules of similar size.
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dinuclear cyclometalated Schiff bases constitutes a very
promising approach toward material with enhanced multiphoton
absorption properties in the near-IR, up to telecommunication
wavelengths.
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ASSOCIATED CONTENT
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X-ray crystallographic data in CIF format, material and methods,
experimental details, synthetic procedures and characterization,
1H and 13C NMR spectra of all new compounds, IR spectra of 1
and 1·Ir, and HR-mass spectrum of 1·Ir. This material is available
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: cyrille.monnereau@ens-lyon.fr.
*E-mail: chantal.andraud@ens-lyon.fr.
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■
Present Address
‡J.M.: Laboratoire de Chimie Molec
́
ulaire et Spectroscopies
s (LCOSA), ICPEES−LCOSA, UMR CNRS 7515,
Ecole Europeenne de Chimie, Polymeres et Materiaux (ECPM).
Notes
Avancee
́
́
́
̀
́
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Arago,
Samoc, M.; Zafra, J. L.; Lop
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́
J.; Ortí, E.; Pappenfus, T. M.; Mann, K. R.; Matczyszyn, K.;
This work was supported by the Foundation for Polish Science,
“Welcome” Program, the Iuventus Plus grant, and a statutory
activity subsidy from the Polish Ministry of Science and Higher
Education for the Faculty of Chemistry of WUT.
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