Second-order NLO switches from molecules to polymer films based on photochromic cyclometalated platinum(II) complexes

Article abstract of DOI:10.1021/ja4131615

Novel photochromic dithienylethene-based platinum(II) complexes (C ^{^}N^{^}N)Pt(C≡C-DTE-C₆H₄-D) ((C^{^}N^{^}N) = 4,4′-di(n-hexyl)-6-phenyl-2,2′- bipyridine; D = H, NMe₂) were prepared and characterized. Their excellent photochromic properties allow the photoinduced switching of their second-order nonlinear optical properties in solution, as measured by the EFISH technique, due to formation of an extended π-conjugated ligand upon suitable electromagnetic radiation. Insights into the electronic structures of the complexes and the nature of their excited states have been obtained by DFT and TD-DFT calculations. These novel Pt(II) complexes were nanoorganized in polymer films which were poled, affording new materials characterized by a good second-order NLO response that can be easily switched, with an excellent NLO contrast. To the best of our knowledge, our compounds allowed designing the very first examples of switchable NLO polymer films based on metal complexes.

Full text of DOI:10.1021/ja4131615

Article
pubs.acs.org/JACS
Second-Order NLO Switches from Molecules to Polymer Films Based
on Photochromic Cyclometalated Platinum(II) Complexes
Julien Boixel,† Veronique Guerchais,*,† Hubert Le Bozec,† Denis Jacquemin,*,‡,§ Anissa Amar,†,∥
́
Abdou Boucekkine,*,† Alessia Colombo,⊥ Claudia Dragonetti,⊥,# Daniele Marinotto,⊥,#
Dominique Roberto,*,⊥,# Stefania Righetto,⊥ and Roberta De Angelis∇
†UMR 6226 CNRS-Universite
‡UMR CNRS 6230, Universite
§Institut Universitaire de France, IUF, 103, Boulevard St Michel, F-75005 Paris cedex 5, France
́
de Rennes 1, Institut des Sciences Chimiques de Rennes, Campus de Beaulieu, 35042 Rennes, France
de Nantes, CEISAM, 44322 Nantes cedex 3, France
́
∥Laboratoire de Thermodynamique et Model
́ ́
isation Moleculaire USTHB BP 32, El Alia, 16111 Bab Ezzouar Alger, Algeria
⊥Dipartimento di Chimica and Centro di Eccellenza CIMAINA dell’Universita
̀
degli Studi di Milano, UdR dell’INSTM, 20133
Milano, Italy
#ISTM-CNR, 20133 Milano, Italy
∇Dipartimento di Ingegneria Industriale, Universita
̀
di Roma ‘Tor Vergata’, 00133 Roma, Italy
S
* Supporting Information
ABSTRACT: Novel photochromic dithienylethene-based platinum(II) complexes (C∧N∧N)Pt(CCDTEC6H4D)
((C∧N∧N) = 4,4′-di(n-hexyl)-6-phenyl-2,2′-bipyridine; D = H, NMe2) were prepared and characterized. Their excellent
photochromic properties allow the photoinduced switching of their second-order nonlinear optical properties in solution, as
measured by the EFISH technique, due to formation of an extended π-conjugated ligand upon suitable electromagnetic radiation.
Insights into the electronic structures of the complexes and the nature of their excited states have been obtained by DFT and
TD-DFT calculations. These novel Pt(II) complexes were nanoorganized in polymer films which were poled, affording new
materials characterized by a good second-order NLO response that can be easily switched, with an excellent NLO contrast. To
the best of our knowledge, our compounds allowed designing the very first examples of switchable NLO polymer films based on
metal complexes.
properties using an external trigger has been extended to the
field of nonlinear optics, and in recent years, there have been
numerous works devoted to the switching of the second-order
NLO response at the molecular level. The quadratic hyper-
polarizability of chromophores may be manipulated by
reversibly modifying the properties of specific parts of active
molecules. The on/off switching may involve reducing the
donor capacity of the electron-rich fragment of a typical
donor(D)−acceptor(A) species, D-linker-A, by oxidation or
INTRODUCTION
■
During the last two decades, compounds with large second-
order nonlinear optical (NLO) properties have been
extensively investigated in view of their large panel of
applications, for example, as molecular building blocks for
optical communications, optical data processing and storage, or
electrooptical devices.1 Among them molecular species with
commutable NLO properties are of growing interest because
incorporation of switchability into the NLO behavior of such
molecular materials increases their potential for novel
applications in emerging optoelectronic and photonic tech-
nologies.2 Thus, modulating the electronic and optical
Received: December 27, 2013
Published: March 17, 2014
© 2014 American Chemical Society
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Scheme 1. Photocyclization of Complexes 1a,b
protonation.2 Conversely, the acceptor behavior of A can be
altered by reduction or deprotonation. Alteration of the
quadratic hyperpolarizability may also involve structural or
chemical modification of the bridging group, thereby interfering
with the communication between D and A. Thus, second-order
NLO switches can be achieved by a pH variation, by a redox
process, or by interaction with an electromagnetic radiation.2
Remarkably, coordination complexes may offer additional
flexibility when compared to organic NLO chromophores by
introducing NLO active charge-transfer transitions between the
metal and the ligands, tunable by virtue of the nature, oxidation
state, and coordination sphere of the metal center and even by
the number of f electrons.3 Considerable effort has been made
in designing and preparing coordination complexes with
effective redox- or photoswitchable second-order NLO
response in solution.2,4 Because metal complexes showing
both luminescent and second-order NLO properties are
particularly appealing as molecular multifunctional molecular
materials,5 we have been exploring in the past decade the
design and synthesis as well as the linear and nonlinear
properties of several metal complexes as NLO and
luminophores. In particular, we developed reversible metallo-
NLO switches using coordination chemistry as a strategy
allowing elaboration of multiphotochromic metal complexes,
presenting an efficient phototriggered quadratic NLO enhance-
ment in the liquid phase. For this purpose, we used the
ubiquitous photochromic dithienylethene (DTE) unit6 and
demonstrated the photomodulation of the NLO responses in
solution of several DTE-based bipyridine metal complexes7
(Zn, Cu, and Fe). In addition, luminescence properties can be
photomodulated using the same photochromic bipyridine
ligand coordinated to a bis-cyclometalated Ir(III) moiety.8
Strikingly, we found that in the case of (bipyridyl)Re and Ru
complexes both kinds of optical properties can be photo-
regulated within the same complex.9
It is known that cyclometalated platinum complexes display
rich photophysical properties,10 and preparation of related
photochromic complexes for photoregulation of their lumines-
cence properties has been intensively developed.11 However,
the NLO activity of cyclometalated Pt(II) complexes remains
less explored. Investigations on cyclometalated phenylpyridine
complexes of the type (C∧N)Pt(II), featuring a β-(diketonate)
coligand, and cyclometalated dipyridylbenzene complexes of
the type (N∧C∧N)Pt(II), fascinating for their excellent
brightness and luminescent properties,12 produce large NLO
responses.13,14 Interestingly, the charge transfer in these
systems that controls both the linear and the nonlinear optical
properties can be readily modulated by chemical modification
of the ligands. The ability to switch on and off the second-order
NLO response of cyclometalated Pt(II) complexes is
particularly appealing, and a density functional theory
investigation showed that it can be achieved using well-
designed photochromic DTE ligands.15
On the other hand, since the electronic mechanisms which
yield the NLO response in molecular systems are well defined,
it is now crucial to face their molecular engineering in order to
obtain organized molecular materials showing a temporal stable
and high bulk second-order NLO response.16 While at the
molecular level there is an increasing number of scientific
contributions about the NLO switching of coordination
compounds,2 at the macroscopic level this aspect has been
scarcely demonstrated.17 Actually, very few examples of
organized molecular materials with a high and switchable
NLO response are reported to date, such as the redox switching
of the NLO response of Langmuir−Blodgett films based on
Ru(II) complexes.17c,d
Surprisingly, although a large panel of redox-active2 and
photochromic metal complexes18 has appeared, there are, to the
very best of our knowledge, no examples of reversible metallo-
NLO switch in the solid state so far. Nevertheless, DTE
derivatives exhibit good photochromic properties in the solid
state, in thin films, or in the crystalline state,19 and their linear
optical properties have been successfully switched in polymer
films. For instance, high contrast fluorescence switching and
erasable optical recording was demonstrated in high-loaded
solid media.20 Photoswitching of the second-harmonic
generation (SHG) from poled phenyl-substituted DTE thin
films in a polystyrene matrix and their second-order NLO
properties in solution, measured by the electric-field-induced
second-harmonic generation (EFISH) technique,21 have been
recently reported by some of us.22
With our expertise in the luminescence studies of cyclo-
metalated platinum(II) alkynyls,23 we reasoned that this class of
compounds may be appealing candidates for metallo-NLO
switches in both solution and polymer films, as they provide
access to multifunctional materials. Moreover, mono-DTE-
based systems appear more appropriate for this first study than
the multiphotochromic bipyridyl metal complexes previously
reported by our groups. We describe herein the synthesis of the
new photochromic DTE-based platinum(II) complexes
(C∧N∧N)Pt(CCDTEC6H4D) 1a,b ((C∧N∧N) =
4,4′-di(n-hexyl)-6-phenyl-2,2′-bipyridine; a, D = H; b, D =
NMe2; Scheme 1). Their excellent photochromic properties
allow photoinduced switching of their second-order NLO
properties. A significant enhancement of the NLO response μβ
measured by the EFISH technique is observed as a result of the
formation of an extended π-conjugated ligand. Addition of the
electron-donating amino substituent on the alkynyl ligand in 1b
leads to an increase of the quadratic hyperpolarizability of the
closed DTE form. Insights into the electronic ground- and
excited-states structures of the complexes have been obtained
by DFT and TD-DFT calculations.24
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the fundamental and SH beams were p polarized. The poled film was
alternately irradiated by a He−Cd laser (Kimmon IK5352R-D) at 325
nm (continuous wave (cw) mode power, 15 mW) and visible light
with a cutoff filter at 550 nm (cw mode power, 157 mW) in order to
change the state of the photochromic molecules. The two beams
crossed the poled film in noncollinear configuration with respect to the
fundamental beam, and beam widths were about 1 cm.
EXPERIMENTAL SECTION
■
Synthesis. Compounds L′a11a and [(C∧N∧N)PtCl]10a ((C∧N∧N):
6-phenyl-4,4′-bis(n-hexyl)-2,2′-bipyridine) were prepared following
reported procedures. The new compound L′b was prepared similarly
to L′a, whereas the novel DTE-based platinum(II) complexes
(C∧N∧N)Pt(CCDTEC6H4D) 1a,b were prepared by
coupling the appropriate chloro precursor (C∧N∧N)PtCl with the
DTE-based alkyne L′ (see Supporting Information for details of the
syntheses and characterization of the various compounds, Scheme S1).
Polymer Films: Preparation and Characterization. Composite
films were produced by casting assisted by a control coater on glass
substrates from a dichloromethane solution of a poly(methyl
RESULTS AND DISCUSSION
■
Synthesis of the Complexes. The DTE-based platinum-
(II) complexes (C∧N∧N)Pt(CCDTEC6H4D) 1a,b
were prepared by coupling the appropriate chloro precursor
(C∧N∧N)PtCl10a with the DTE-based alkyne L′ (see
Supporting Information for details of the syntheses and
characterization of the various compounds, Scheme S1).
Compound L′b was synthesized following the procedure
reported for L′a.11a Complexes 1 were isolated as orange
powders in good yields and characterized by 1H and 13C NMR
spectroscopy, mass spectrometry, and elemental analysis. The
1H NMR spectrum (CDCl3) of 1a (see Figure S1, Supporting
Information) in its open form (o) shows two characteristic
singlets for the nonequivalent methyl protons (δ = 2.01, 1.87
ppm).
methacrylate) (PMMA, M ≈ 15000; Tg = 86.5 °C as determined
̅
w
by differential scanning calorimetry) matrix (75 mg/mL) and complex
1a or 1b in closed form (6% w/w relative to the polymer). The
spinning parameters were set at the following values: RPM 1 = 700;
ramp 1 = 1 s, time 1 = 5 s; RPM 2 = 2000; ramp 2 = 5 s, time 2 = 80 s.
Complexes 1a(c) and 1b(c) (c = closed form) film thickness was
measured by ellipsometry to be 1.49 and 1.7 μm, respectively. UV−vis
absorption spectra of photochromic films were carried out using a
JASCO V570 spectrometer. Concentrations of the photochromic
complexes in the films were calculated from their optical spectra to be
0.15 × 1020 (complex 1a(c) film) and 0.22 × 1020 cm−3 (complex
1b(c) film).
Corona Poling Setup. The fundamental incident light was
generated by a 1064 nm Q-switched Nd:YAG (Quanta System
Giant G790−20) laser with a pulse of 7 ns and 20 Hz repetition rate.
The output pulse was attenuated to 0.57 mJ and focused with a lens (f
= 600 mm) on the sample, placed over the hot stage. Corona poling
process was performed inside a drybox in a N2 atmosphere. The
fundamental beam was polarized in the incidence plane (so-called p
polarized) with an angle of about 55° with respect to the sample in
order to optimize the SHG signal. The hot-stage temperature was
controlled by a GEFRAN 800 controller, while the corona-wire voltage
(up to 8.5 kV across a 10 mm gap) was applied by a TREK 610E high-
voltage supply. After rejection of the fundamental beam by an
interference filter and a glass cutoff filter, the p-polarized SHG signal at
532 nm was detected with a UV−vis photomultiplier (PT, Hamamatsu
C3830). The output signal from the PT was set to a digital store
oscilloscope and then processed by a computer with dedicated
software.
Photophysical Properties and Photocyclization Re-
actions. Measured absorption spectra of complexes 1a,b in
their open forms are presented in Figure 2A, whereas the
characteristic data are collected in Table 1. The spectrum of
1a(o) displays an intense band (365 nm in CH2Cl2) in the UV
region corresponding to a mixed character of IL(π → π*)
transitions of the C∧N∧N and the DTE-acetylide ligands,
consistent with the calculations (see below). In the visible
region, a broad band is observed at 450 nm, tailing up to 540
nm, corresponding to the metal-to-ligand charge transfer
[MLCT dπ(Pt) → π*(C∧N∧N)] with some mixing of ligand
to ligand charge transfer transition [L′LCT (π(CCDTE
Ar) → π*(C∧N∧N)]. For 1b(o), an additional intense band is
observed at 325 nm which is ascribed to the IL(π → π*)
transitions of the (dimethylamino)aryl-substituted thiophene
fragment as a result of the presence of a pushing end group.
The low-energy MLCT/L′LCT band remains unaffected by the
presence of the dialkylamino group, consistent with a poorly
conjugated open form.
Maker Fringe and Second-Harmonic Photoswitch. In the
Maker fringe experiment, the second-harmonic (SH) intensity was
detected as a function of the incidence angle of the fundamental beam
and normalized with respect to that of a calibrated quartz crystal wafer
(X-cut) 1 mm thick whose d11 is 0.46 pm/V. The incidence angle was
changed by rotating the poled film along the Y axis (Figure 1), while
The photocyclization reactions have been monitored by
1
UV−vis absorption and H NMR spectroscopies. Figure 2B
depicts the electronic spectra of the closed isomers at the
photostationary state (PSS) upon irradiation at 350 nm of a
CH2Cl2 solution of 1a and 1b, and the corresponding
numerical data are listed in Table 1. The emergence of a new
low-energy band in the visible domain is typical of formation of
closed DTE.11 The transition band attributed to the closed-
DTE unit of 1a(c) appears at 616 nm, while introducing an
amino group in 1b induces both a small red shift (633 nm) and
a broadening of this hallmark band. Moreover, in both cases,
1
the H NMR experiments show quantitative formation (95%
Figure 1. Set-up photoswitch experiment and maker fringe (XYZ is the
macroscopic coordinate system).
conversion) of the closed form 1a,b(c) (Figure S1, Supporting
Information). Excitation into the visible (MLCT band) leads to
the same photochromic behavior: a similar rate of conversion to
1a,b(c) is observed. This feature, already reported in related
systems, suggests a possible cyclization pathway via the 3MLCT
excited state.11
DFT and TD-DFT study. Starting from the optimized
geometries obtained at the PBE0/LanL2DZP level (computa-
tional details in the Supporting Information) vertical TD-DFT
the polarization of the fundamental and SH beam could be changed by
a half-wave plate and a cube beam splitter, respectively, as previously
reported.25 In order to determine the nonzero independent
components of the susceptibility tensor for poled films (C∞v
symmetry) Maker fringe measurements were conducted with different
polarizations: p → p, s → p, and 45 → s. In the SHG photoswitch
experiment the poled film was rotated at an incidence angle of 57° and
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Figure 2. (Top) UV−vis absorption spectra of 1a and 1b in their (A) open (o) and (B) closed (c) (PSS) forms. (Bottom) TD-DFT-computed
UV−vis spectra of 1a and 1b in their (A′) open (o) and (B′) closed (c) forms.
electronic spectra are displayed in Figure 2 (bottom: Figure 2A′
and 2B′), and the molecular orbitals (MO) involved in the
relevant electronic transitions are shown in Figure S2 (see
Supporting Information).
Table 1. Absorption Data for 1a,b in Their Open and Closed
(PSS) Forms Measured in CH2Cl2
a
a
λabs [nm] (ε × 103[dm3 mol−1cm−1]), λabs [nm], closed form
b
complex
open form
(PSS)
TD-DFT results confirm the assignments given above for the
different absorption bands. The lowest energy absorption band
of 1a(o) and 1b(o) in their open forms is computed at 476 and
481 nm, corresponding mainly to a HOMO → LUMO and
HOMO-1 → LUMO transition, respectively. This band can be
assigned as L′LCT/MLCT transitions, the HOMO being
delocalized on the platinum and the acetylide−thienyl fragment
with a weight of 12% (13%) for the metal atom, whereas the
LUMO is located on the bipyridine of the cyclometalated
ligand (see Figure S2, Supporting Information). In agreement
with the experimental data and chemical intuition, this
absorption band is similar for both derivatives (D = H,
NMe2). A moderate to intense band in the UV region in the
spectrum of 1a(o) (1b(o)) is computed at 385 nm (340 nm).
Experimentally, photocyclization can be triggered by irradiation
at 350 nm for both complexes. For 1a, this process involves a
HOMO → LUMO+2 electronic transition from a transition
assigned as IL′CT/ML′CT transitions involving the alkynylth-
ienyl fragment (left part of L′ vs C5F6). By contrast, the high-
energy band of 1b results from a HOMO → LUMO+4
transition, attributed to an IL′(π → π*) transition that involves
the (aminophenyl)thiophene group (right part of the open
DTE-based ligand L′) as a result of the presence of a terminal
electron-donating group. The LUMO+2 of 1a and LUMO+4 of
1b exhibit a significant density on the reactive carbons atoms
1a
1b
365 (11.5), 450 (4.9)
325 (38.8), 450 (4.9)
365, 385, 388sh, 616
329, 365, 395, 422sh,
633
a
b
At 298 K in 2.5 × 10−5 M. PSS = photostationary state; 95% ring
closing determined by H NMR spectroscopy.
1
calculations have been performed to simulate the UV−vis
spectra in solution (CH2Cl2) of complexes 1a,b in their open
and closed forms. The calculated absorption wavelengths for
the studied complexes are reported in Table 2. The simulated
Table 2. Computed Absorption Wavelengths at the PCM-
PBE0/LANL2DZP Level in CH2Cl2
λmax
(nm)
oscillator strength
compound
(f)
main transition (weight)
1a(o)
385
476
317
680
340
481
378
699
0.12
0.27
0.28
0.79
1.06
0.25
0.42
1.04
HOMO → LUMO+2 (+91%)
HOMO → LUMO (+97%)
HOMO-7 → LUMO (+80%)
HOMO → LUMO (+99%)
HOMO → LUMO+4 (+88%)
HOMO-1 → LUMO (+69%)
HOMO-3 → LUMO (+67%)
HOMO → LUMO (+98%)
1a(c)
1b(o)
1b(c)
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Table 3. μβEFISH and βEFISH of the Investigated Pt(II) Complexes 1a,b in Their Open and Closed (PSS) Forms
ab
,
ab
c
c
de
,
de
,
,
complex
μβEFISH
,
open
μβEFISH
,
closed (PSS)
−1150
−2635
μ, open
μ, closed
βEFISH
,
open
βEFISH
,
closed (PSS)
1a
1b
−180
−220
15.70
11.30
18.71
12.31
−11
−19
−61
−214
a
b
c
Measured in CH2Cl2 at a concentration of 10−3 M; estimated uncertainty in EFISH measurements is 10%. 10−48 esu. DFT-calculated dipole
d
e
moments. Values obtained using DFT-calculated dipole moments. 10−30 esu.
and the required bonding character of the interacting orbitals
for the ring-closure reaction (Figure S2, Supporting Informa-
tion), the so-called photochromic shape.26 The two reactive
carbons of these LUMOs present a weight of 8.05%, 13.70% for
1a (LUMO) and 2.90%, 16.60% for 1b (LUMO+2),
respectively. Therefore, electrocyclization could process directly
from the excited states corresponding to these UV bands,
though relaxation to a lower triplet MLCT state followed by
cyclization could also be an effective process.
reported in Table 3. Like the cyclometalated Pt(II) complexes
(C∧N)Pt(acac)13a,b and (N∧C∧N)Pt(X),13c both complexes 1a
and 1b are characterized by a negative value of μβEFISH
,
irrespective of the form of the DTE unit (open or closed), in
agreement with a negative value of Δμeg (difference between
the dipole moments of the excited and ground states). This
behavior can be attributed to a quadratic NLO response
dominated by a CT from the Pt−acetylide DTE moiety to the
cyclometalated ligand. Such CT character appears clearly in the
S1−S0 density differences plots computed with TD-DFT (see
Figure S3, Supporting Information).
Let us now turn to the closed forms: the absorption bands in
the visible region of 1a(c) and 1b(c) are computed at 680 and
699 nm, respectively. The red shift experimentally observed (17
nm) is well reproduced, illustrating the influence of the
terminal end donor group. The corresponding excitation
induces the reopening process; experimentally, cycloreversion
is triggered upon irradiation at 650 nm. From Figure S2,
Supporting Information, it can be seen that this excitation
corresponds to promotion of an electron from the HOMO
delocalized along the π-conjugated acetylide ligand (L′)
containing the closed DTE fragment to the LUMO delocalized
over the entire molecule, assigned to a blend of IL′(π → π*)
and L′LCT transitions. Moreover, the LUMOs of both
molecules are suitable for the ring-opening process; they are
characterized by an antibonding interaction between the two
carbons to be separated, the Mulliken overlap population
between them presenting a negative value equal to −0.015
(−0.009) e for 1a (1b). A net decrease of the platinum
contribution to the HOMO is found for the two closed forms
compared to that of the open isomers, with a weight of 3%
(2%) for 1a (1b). The UV absorption band is calculated at 317
nm (378 nm), which originates from a HOMO-7 → LUMO
(HOMO-3 → LUMO) transition assigned to L′LCT/IL′
excitations.
As evidenced in Table 3, the absolute values of μβEFISH and
βEFISH of the two Pt(II) complexes in their open forms are very
weak, in the same range of those already reported for DTE-
based bipyridine metal complexes in the same form.7−9
A
dramatic increase of the second-order NLO activity is observed
after UV irradiation and subsequent accumulation of the ring-
closed isomers: in the case of 1a the βEFISH value is increased
almost 6-fold, while a 12-fold enhancement is observed for 1b.
The large enhancement of the quadratic NLO response upon
ring closing clearly reflects the delocalization of the π-electron
system in the closed forms, accompanied by an important
decrease of the HOMO−LUMO gap (Table S2, Supporting
Information), the largest NLO enhancement being observed for
the complex featuring the electron-donor dimethylamino end
group. Although the ground-state dipole moment of the closed
form of 1b is lower than that of 1a, its μβEFISH value is more
than 2-fold enhanced. The 3.5 times larger βEFISH of 1b (PSS)
vs 1a (PSS) can be due to a greater charge transfer distance
(dCT multiplied by a factor of 2.6; see Table S2, Supporting
Information) and therefore to a larger Δμeg.
We found it interesting to carry out computations of the first
hyperpolarizabilities of the considered species using the
ωB97X/LANL2DZP level of theory. The ωB97X functional
presents a correct asymptotic behavior (exact exchange going to
100% when the electron separation goes to infinity), and this is
known to be a requirement for accurate NLO properties. On
the other hand, we remind that for electronic spectra PBE0 is
known to be one of the best compromises, at least within the
vertical approximation (see the Supporting Information for
more details). The obtained total static βStat and dynamic βDyn
first hyperpolarizabilities, computed at the ωB97X/LANL2DZP
level of theory, are listed in Table S3, Supporting Information.
The theoretical results, in agreement with the experimental
ones, reproduce very well the important increase of the NLO
response when going from open to closed forms of the
complexes and when going from 1a (c) to 1b (c).
Quadratic NLO Studies. In order to investigate the
second-order NLO properties of the Pt(II) complexes 1a,b,
the EFISH method21 has been used (see Supporting
Information) as it can provide direct information on the
intrinsic dipolar molecular second-order NLO properties
through
γEFISH = (μβEFISH/5kT) + γ(−2ω; ω, ω, 0)
(1)
where μβEFISH/5kT is the dipolar orientational contribution and
γ(−2ω; ω, ω, 0), a third-order term at frequency ω of the
incident light, is a purely electronic cubic contribution which
can usually be neglected when studying the second-order NLO
properties of strongly dipolar molecules.1 The μβEFISH values
were measured in CH2Cl2 solution (concentration = 10−3 M)
working with a nonresonant incident wavelength of 1.907 μm.
To obtain βEFISH, the projection along the dipole moment axis
of the vectorial component of the tensor of the quadratic
hyperpolarizability, it is necessary to know the dipole moment,
μ. In the present study we used the DFT-computed dipole
moments (Table 3; Table S5 and Figure S4, Supporting
Information). The values of μβEFISH as well as the
corresponding βEFISH, before and after UV irradiation, are
To study the second-order NLO properties of the Pt(II)
complexes 1a and 1b in the solid state, we prepared thin films
of the chromophores dispersed in a polymethylmethacrylate
(PMMA) matrix as reported in the Experimental Section. The
complexes were deposited in the closed form, due to a higher
dipole moment in the closed than in the open form (Table 3),
in a way to optimize the orientation of the chromophores
during the poling. UV−vis absorption spectra of the complex
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Figure 3. Thin film of the complex 1b in closed form (PSS): (A) Poling experiment; (B) UV−vis absorption spectra before and after poling.
Figure 4. Photoswitch of the poled thin film: (A) complex 1a and (B) complex 1b.
1a(c) and 1b(c) films before poling are illustrated in Figure
S5B, Supporting Information, and Figure 3B, respectively. Since
they show absorption bands very similar to that reported for
the same complex in solution (Figure 2B), one can consider the
molecules of complexes 1a(c) and 1b(c) inside the films as
isolated and only weakly interacting with the polymer matrix.
The poling measurement of the complex 1b(c) film is shown
in Figure 3A, in which the optimized poling parameter
temperature (50 °C) and electric field (8.5 kV) have permitted
us to obtain a sufficiently high and stable second-harmonic
generation (SHG) signal. The UV−vis absorption spectrum of
the complex 1b(c) film after poling is illustrated in Figure 3B;
the decrease of the absorption band (at peak 637 nm)
compared to that observed before poling (so-called dichroic
effect) is due to the partial orientation of molecules along the
direction of the electric poling field.27 No appreciable Stark
shift27 of the absorption peaks was noted after poling. A very
similar behavior is observed for the poled complex 1a(c) film
(Figures S5A and S5B, Supporting Information), although we
point out a lower SHG signal after poling.
Figure S6, Supporting Information, presents Maker fringes
measurements for the films based on complexes 1a and 1b in
closed and open forms. These measurements have been
performed after poling for the closed form while after poling
and irradiation at λ > 550 nm (at least 30 min) for the open
form. We note that 30 min is a sufficient irradiation time to
reach the photostationary state of the open form for complex
1a film (see, for example, the first photoswitch in Figure 4A),
whereas this irradiation time is not sufficient to convert
completely the chromophores from closed to open form for the
complex 1b film (Figure 4B). In particular, in the latter case we
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observed that the photostationary state is not reached even
after a much longer time (3 h).
From an application point of view, an interesting aspect of
metal complexes is the possibility to switch their NLO
response.2,4,30,31 The good second-order NLO properties of
the polymeric films based on the Pt(II) complexes 1a and 1b
prompted us to investigate the switching of the NLO response
in the solid state. SHG photoswitching experiments have been
performed after poling for complex 1a,b films in closed form
(see Figure 3): the SHG signal increases after UV irradiation, as
the closed form was generated from the open one. Once a
stable plateau is reached, UV irradiation was stopped and visible
light was turned on. The SHG intensity decreased as the closed
form converted back to the open form. The first photoswitch
presents a contrast of about 80% and 50% for complexes 1a and
1b, respectively (Figure 4A and 4B). For complex 1b, the lower
contrast could be attributed, at least in part, to the not complete
conversion of the chromophores from the closed to the open
form. In any case, the observed NLO contrasts exceed those
recently reported for “all organic” phenyl-substituted DTE thin
films.22 A feature common to both measures is the loss of some
SHG signal during the switches, due to the irreversible loss of
orientation through the photoisomerization processes. This
behavior has already been observed in PMMA32 and could be
reduced in the near future, for example, using a cross-linking
technique for preparation of the polymer film.
As noted above, complexes 1a and 1b have a different
photoisomerization time to reach the PSS. For complex 1b,
conversion from the closed to the open form is slow and not
completed even after 3 h while the reverse conversion is very
fast (see Figure 4B); instead, an opposite behavior is observed
for complex 1a (see Figure 4A) with a fast conversion from the
closed to the open form (less than 30 min) and a relatively slow
reverse conversion. Such a behavior has been already observed
in solution for photochromic bipyridine ligands and Zn
complexes.7b
The standard expression28 used to fit the SHG intensity in a
Maker fringe measurement includes the absorption coefficient
of the film at the harmonic frequency. In this expression the
SHG intensity is proportional to the square of the effective
nonlinear optical coefficient (deff) that depends on polarizations
of the fundamental and SH beam. Considering the C∞v
symmetry expected for poled films and the polarizations of
the fundamental and SH beam, the coefficient deff assumes the
expression
deff = d31 sin θ2
for s → p configuration
(2a)
deff = d15 sin θ1
for 45 → s configuration
(2b)
deff = 2d15 sin θ1 cos θ1 cos θ2 +
sin θ2(d31 cos2 θ1 + d33 sin2 θ1)
for p → p configuration
(2c)
where θ1 and θ2 are, respectively, the angles of refraction inside
the poled film for the fundamental and SH beam with refractive
indices nω and n2ω (sin θm = sin θ/nmω, m = 1, 2). It is
worthwhile remembering that the components of the nonlinear
optics coefficient and the susceptibility tensor are related by the
(2)
formula 2dij = χij
.
By fitting the Maker fringe measurements using these
expressions (eqs 2a−2c) the three nonzero coefficients of the
(2)
second-order susceptibility tensor for a poled film χ33(2), χ31
,
and χ15(2) have been evaluated (Table 4); the error in these data
Table 4. Susceptibility Components of Poled Films
Complexes 1a,b in Closed Form and Switched Open Form
upon Irradiation at λ > 550 nm
CONCLUSIONS
■
The new cyclometalated (C∧N∧N)Pt(CCDTEC6H4
D) complexes investigated in the present work are a new family
of organometallic second-order NLO chromophores with a
response easily tunable by a rational approach, where the Pt
atom plays the role of a central bridge of the transfer process
from the donor to the acceptor moieties of the molecular
structure. Remarkably, use of a strong electron-donor group (D
= NMe2) leads to the best μβEFISH reported up to now for a
Pt(II) complex. Moreover, their excellent photochromic
properties allow photoinduced switching of their second-
order nonlinear optical properties.
χ33 (10−10 χ 31(2) (10−10 χ 15(2) (10−10
(2)
χ33(2)/ χ χ33(2)/ χ
31
(2)
(2)
15
complex
esu)
esu)
esu)
1a(c)
1a(o)
1b(c)
7.64
3.87
4.77
2.39
1.95
1.67
3.10
2.86
1.60
1.32
1.01
1.45
3.91
2.31
5.41
4.84
16.80
13.84
16.71
9.55
a
1b(o)
a
Note: 1b(o) has not reached the photostationary state of the open
form.
The presence of n-hexyl substituents on the cyclometalated
ligand is a tool for their straightforward nanoorganization in
polymer films which can be easily poled affording new materials
characterized by a good second-order NLO response that can
be easily switched, with an excellent NLO contrast, by
interaction with electromagnetic radiation.
To our knowledge, this study represents the first
demonstration of the photomodulation of the NLO response
of a photochromic metal complex in thin films, opening a new
avenue for preparation of convenient reversible-NLO switches.
can be estimated to be less than 20%. As shown in Table 4, for
both 1a and 1b the χ33(2), χ31(2), and χ15(2) values increase going
from the open to the closed form and the largest NLO
enhancement is observed for the complex featuring the strong
electron-donor dimethylamino end group, confirming what has
(2)
been seen in the EFISH measurements. Moreover, the χ33
/
χ31 and χ33(2)/χ15 ratios are not close to 3, the expected
value in a poled film containing chromophores having a one-
dimensional first hyperpolarizability tensor.29 This behavior of
the second-order susceptibility tensor, particularly emphasized
(2)
(2)
(2)
for the film based on complex 1b(c) in which χ31 is about
equal to χ33(2), suggests that the charge transfer inside the
chromophore is not only in the direction of the dipole moment
but that other contributions are present in other directions,22,30
in agreement with our theoretical data (see Figures S3 and S4,
Supporting Information).
ASSOCIATED CONTENT
* Supporting Information
■
S
Description of the synthesis, characterization, and photo-
isomerization of the new compounds in solution, EFISH and
Maker fringe measurements, and computational details. This
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Article
(4) As examples, see: (a) Coe, B. J.; Houbrechts, S.; Asselberghs, I.;
material is available free of charge via the Internet at http://
pubs.acs.org.
Persoons, A. Angew. Chem., Int. Ed. 1999, 38, 366−369. (b) Espa, D.;
̀
Pilia, L.; Marchio, L.; Mercuri, M. L.; Serpe, A.; Barsella, A.; Fort, A.;
Dalgleish, S. J.; Robertson, N.; Deplano, P. Inorg. Chem. 2011, 50,
2058−2060.
AUTHOR INFORMATION
■
Corresponding Authors
(5) As examples, see: (a) Margapoti, E.; Shukla, V.; Valore, A.;
Sharma, A.; Dragonetti, C.; Kitts, C. C.; Roberto, D.; Murgia, M.; Ugo,
R.; Muccini, M. J. Phys. Chem. C 2009, 113, 12517−12522. (b) Valore,
A.; Cariati, E.; Dragonetti, C.; Righetto, S.; Roberto, D.; Ugo, R.; De
Angelis, F.; Fantacci, S.; Sgamellotti, A.; Macchioni, A.; Zuccaccia, D.
Chem.Eur. J. 2010, 16, 4814−4825. (c) Todescato, F.; Fortunati, I.;
Carlotto, S.; Ferrante, C.; Grisanti, L.; Sissa, C.; Painelli, A.; Colombo,
A.; Dragonetti, C.; Roberto, D. Phys. Chem. Chem. Phys. 2011, 13,
11099−11109. (d) Zaarour, M.; Singh, A.; Latouche, C.; Williams, J.
A. G.; Ledoux-Rak, I.; Zyss, J.; Boucekkine, A.; Le Bozec, H.;
Guerchais, V.; Dragonetti, C.; Colombo, A.; Roberto, D.; Valore, A.
Inorg. Chem. 2013, 52, 7987−7994. (e) Oliveri, I. P.; Failla, S.;
Colombo, A.; Dragonetti, C.; Righetto, S.; Di Bella, S. Dalton Trans.
2014, 43, 2168−2175.
veronique.guerchais@univ-rennes1.fr
denis.jacquemin@univ-nantes.fr
abdou.boucekkine@univ-rennes1.fr
dominique.roberto@unimi.it
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.J. acknowledges both the European Research Council (ERC)
and the Reg
́
ion des Pays de la Loire for financial support in the
framework of a Starting Grant (Marches 278845) and
recrutement sur poste strategique, respectively. In France, this
(6) (a) Irie, M. Chem. Rev. 2000, 100, 1685−1716. (b) Morimoto,
M.; Irie, M. J. Am. Chem. Soc. 2010, 132, 14172−17178.
́
research used resources of (1) CCIPL (Centre de Calcul
Intensif des Pays de la Loire) and (2) the GENCI-IDRIS-
CINES National Computing Centers. In Italy, it was supported
by Fondazione Cariplo (Grant No. 2010-0525), by MIUR
(FIRB 2004, RBPR05JH2P), and by CNR. D.J. and V.G are
grateful to BRESMAT network for financial support.
(7) (a) Aubert, V.; Guerchais, V.; Ishow, E.; Hoang-Thi, K.; Ledoux,
I.; Nakatani, K.; Le Bozec, H. Angew. Chem., Int. Ed. 2008, 47, 577−
580. (b) Ordronneau, L.; Aubert, V.; Met
Nakatani, K.; Ibersiene, F.; Hammouten
́
ivier, R.; Ishow, E.; Boixel, J.;
e, D.; Boucekkine, A.; Le
̀
Bozec, H.; Guerchais, V. Phys. Chem. Chem. Phys. 2012, 14, 2599−
2605. (c) Nitadori, H.; Ordronneau, L.; Boixel, J.; Jacquemin, D.;
Boucekkine, A.; Singh, A.; Akita, M.; Ledoux, I.; Guerchais, V.; Le
Bozec, H. Chem. Commun. 2012, 48, 10395−10397. (d) Ordronneau,
L.; Aubert, V.; Guerchais, V.; Boucekkine, A.; Le Bozec, H.; Singh, A.;
Ledoux, I.; Jacquemin, D. Chem.Eur. J. 2013, 19, 5845−5846.
(8) Aubert, V.; Ordronneau, L.; Escadeillas, M.; Williams, J. A. G.;
Boucekkine, A.; Coulaud, E.; Dragonetti, C.; Righetto, S.; Roberto, D.;
Ugo, R.; Valore, A.; Singh, A.; Zyss, J.; Ledoux, I.; Le Bozec, H.;
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