Efficient photoswitching of the nonlinear optical properties of dipolar photochrome zinc(II) complexes

Article abstract of DOI:10.1002/anie.200704138

(Chemical Equation Presented) Flipping the switch: A new type of bipyridine-based ligand functionalized by phenyl- and dimethylaminophenyldithienylethene groups allows the preparation of photochromic dipolar zinc(II) complexes. For the first time, efficient on/off photoswitching of the NLO response of metallochromophores is observed.

Full text of DOI:10.1002/anie.200704138

Angewandte
Chemie
DOI: 10.1002/anie.200704138
Nonlinear Optics
Efficient Photoswitching of the Nonlinear Optical Properties of
Dipolar Photochromic Zinc(II) Complexes**
Vincent Aubert, VØronique Guerchais, ElØna Ishow, Khuyen Hoang-Thi, Isabelle Ledoux,
Keitaro Nakatani, and Hubert Le Bozec*
A current challenge is the development of efficient strategies
for the design of switchable nonlinear optical (NLO) materi-
als.[1] As most molecules with large first hyperpolarizability
values b comprise p systems that are unsymmetrically end-
capped with donor and acceptor moieties, various strategies
have been explored to alter the electron-donor (or acceptor)
capacityof the end groups using external stimuli such as redox
methods[2] and protonation/deprotonation reactions.[3]
Another elegant approach to the reversible switching of
NLO properties is the use of photochromic compounds.[1b,4]
Among them, dithienylethene (DTE) derivatives are the
most promising because of their good fatigue resistance, the
remarkable thermal stabilityof both isomers, and the rapid
Scheme 1. Target photochromic complexes.
response time, which are prerequisite conditions for practical
applications.[5] Typically, DTE derivatives undergo reversible
interconversion between an unconjugated open form and a p-
conjugated closed form when irradiated in the UVand visible
spectral ranges, respectively.
switching the NLO properties, we designed a new type of 4,4’-
bis(ethenyl)-2,2’-bipyridine ligand functionalized by phenyl-
and dimethylaminophenyl DTE groups. These ligands
allowed us to prepare the corresponding photochromic
dipolar (bipyridyl)zinc(II) complexes (Scheme 1, type II).
Herein, we report the synthesis and photochromic properties
of these new compounds, as well as the efficientlyphoto-
triggered enhancement of the NLO activityof dipolar
complexes.
The target bipyridine derivatives 1a(o) (D = H, (o)
indicates open form) and 1b(o) (D = NMe2) were readily
prepared in 55–60% yield by a double Horner–Wadsworth–
Emmons condensation between the bisphosphonate bipyr-
idine and the aryl-substituted DTE aldehydes 6 (Scheme 2).
The latter compounds were obtained bya multistep proce-
dure. The two thienyl fragments were prepared independently
and were successivelyconnected to C 5F8 (see the Supporting
Information for details).
In recent years, considerable effort has been devoted to
the development of transition-metal complexes as NLO
chromophores, and their large second-order nonlinearities
have been demonstrated.[6] The photoswitching of their NLO
properties has not been reported to date, although recent
studies have highlighted the potential of metal-containing
DTE ligands for luminescence or electron-transfer photo-
switching.[7]
We previouslydescribed the use of 4,4 ’-bis(dialkylami-
nostyryl)-2,2’-bipyridine compounds for the molecular engi-
neering of noncentrosymmetric dipolar[8] (Scheme 1, type I)
or octupolar metal complexes.[9] With the aim of photo-
[*]V. Aubert, Dr. V. Guerchais, Dr. H. Le Bozec
OrganomØtalliques et MatØriaux MolØculaires
UMR 6226 CNRS-UniversitØ de Rennes 1
Campus de Beaulieu, 35042 Rennes (France)
Fax: (+33)2-2323-6939
For both ligands, the photocyclization process was con-
1
firmed by H NMR spectroscopyexperiments. The colorless
solutions of 1a(o) and 1b(o) in CD2Cl2 turn blue and green,
1
E-mail: hubert.le-bozec@univ-rennes1.fr
Homepage: http://o2.univ-rennes1.fr/
respectively, upon UV irradiation. The H NMR spectra of
the closed forms 1a(c) and 1b(c) show the characteristic
upfield shift of the two thiophene protons, while the methyl
proton signals are shifted downfield. Integration of the
methyl-group resonances indicates approximately 95% con-
version to the ring-closed isomers. The photocyclization and
photocycloreversion processes were also monitored by
absorption spectroscopy(Figure 1, Table 1). The UV/Vis
spectra (CH2Cl2) of 1a(o) and 1b(o) show an intense band
at 343–348 nm, which is tentativelyassigned to intraligand
(IL) p!p* transitions of the bipyridyl moieties with some
mixing of p!p* transitions of the DTE units. Irradiation of
CH2Cl2 solutions at 365 nm resulted in a new broad band at
Dr. E. Ishow, Dr. K. Nakatani
Laboratoire de Photophysique et Photochimie SupramolØculaires et
MacromolØculaires
UMR CNRS 8531, ENS Cachan
61 avenue du PrØsident Wilson, 94235 Cachan (France)
K. Hoang-Thi, Prof. I. Ledoux
Laboratoire de Photonique Quantique et MolØculaire
UMR CNRS 8531, ENS Cachan
61 avenue du PrØsident Wilson, 94235 Cachan (France)
[**]We thank RØgion Bretagne for financial support (SIE 211-B3-11).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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nm red shift for the IL band,
which appears at 669 nm for
1b(c) (Figure 1).[10] Excitation at
588 nm, in the absorption band of
the closed forms 1a(c) and 1b(c),
led to the quantitative regenera-
tion of the open isomers. The
quantum yields for the ring-clo-
sure process (1a(o) f= 0.45,
1b(o) f= 0.24) were found to be
much higher than those for the
reverse process (1a(c) f= 8.4 ”
10ꢀ3, 1b(c) f= 1.9 ” 10ꢀ3), as has
been observed for related sys-
tems.[7f,g] No emission was
detected for 1a,b(o) and 1a,b(c)
in solution at room temperature.
The corresponding bipyridyl
zinc(II) bis(acetato) complexes
2a(o) and 2b(o) (Scheme 2)
were prepared quantitatively
upon
treatment
of
Zn-
(OAc)2·2H2O with one equiva-
lent of ligand 1a(o) or 1b(o) in
dichloromethane at room tem-
perature (Scheme 2). The zinc
complexes displayed identical
photochromic behavior, confirm-
ing that complexation of the
bipyridyl ligands to ZnII did not
alter the photoconversion pro-
cess. Upon illumination at
365 nm, new absorption bands at
629 nm and 687 nm appeared,
originating from the formation
of the closed isomers 2a(c) and
2b(c), respectively(Figure 2,
Table 1). Complexation to Zn-
(OAc)2 induces a typical bath-
Scheme 2. Synthesis and photochromic reactions of ligands and complexes. D=H (1a, 2a, 6a),
D=NMe2 (1b, 2b, 6b).
Table 1: UV/Vis absorption data.
Compound[a] labs [nm]
(e [mꢀ1 cmꢀ1])
E
max [ev] labs [nm]
Emax [ev]
(e [mꢀ1 cmꢀ1])
open form
closed form
1a
1b
343 (75600)
348 (89000)
3.61
3.56
347 (62000)
396 (22000)
623 (35000)
347 (54000)
395 (24000)
438 (17800)
669 (36000)
342 (60000)
400 (25000)
629 (32000)
343 (46000)
394 (26500)
450 (17200)
687 (34560)
3.57
3.13
1.99
3.57
3.14
2.83
1.85
3.62
3.1
1.97
3.61
3.14
2.75
1.80
Figure 1. Absorption spectra (visible region) of the PSS closed-ring
isomers 1a(c) (c&c) and 1b(c) (c”c) in CH2Cl2, from
which the influence of the terminal group D (H vs. NMe2) is apparent.
2a
2b
357 (55000)
360 (76000)
3.47
3.44
623 nm, which is attributed to the IL S0!S1 transition of the
closed form 1a(c), the red shift arising from the formation of a
p-conjugated system. Replacement of the H end group with
the stronglydonating NMe group induces an additional 46-
[a]Determined in CH 2Cl2.
2
578
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Chemie
2a(c) also displays quite a large NLO response even in the
absence of anyapparent stronglyelectron-donating group.
Thus, the substantial enhancement of mb0 clearlyreflects the
efficient delocalization of the p-electron system in the closed
forms and demonstrates efficient on/off switching of the NLO
responses.
In summary, we have prepared and studied the first
photochromic metal complexes that allow efficient switching
of the nonlinear optical properties. Owing to the ubiquityof
2,2’-bipyridine derivatives in coordination chemistry, this
work opens attractive perspectives for the construction of
photochromic multipolar NLO molecules and materials.
Figure 2. UV/Vis absorption change of 2b in CHCl3 (1.5”10ꢀ5 m) upon
*
*
irradiation. c c open form, c c photostationary state
(PSS) after irradiation at 365 nm, a”a PSS after irradiation at
588 nm (reverse reaction).
Experimental Section
Spectroscopic-grade chloroform and dichloromethane were used for
all optical measurements. Photoisomerization experiments and
kinetics were performed using a Hamamatsu UV Spot Light Source
(Xe–Hg lamp) as an excitation light source equipped with bandpass
filters (0.63 mWcmꢀ2 at 365 nm, 18.6 mWcmꢀ2 at 588 nm) and an
optical fiber, while simultaneous probing was performed with a
continuous Xe lamp (450 W) and a CCD camera coupled with a
spectrometer (Princeton Instruments). The values of mb for the
chromophores were measured using the EFISH technique. The
amplified nanosecond Nd3+:YAG laser at 1.06 mm and a repetition
rate of 10 Hz pumps a hydrogen Raman cell to obtain a larger
wavelength (1.907 mm) for which both the fundamental and harmonic
frequencies are far awayfrom the resonance of the investigated
molecule.
ochromic shift (Dl = 6–18 nm) of the intraligand charge-
transfer (ILCT) bands, as has been observed for related
bis(acetato) zinc(II) complexes.[11]
Upon photoexcitation at 588 nm, the (c) isomers com-
pletelyreverted to the corresponding (o) isomers without any
1
decomplexation (Figure 2). According to H NMR spectros-
copy, the degree of cyclization showed slightly lower con-
version efficiencies than those of the ligands (92% for 2a and
87% for 2b), whereas the quantum yields for the photo-
cyclization of 2a(o) (f= 0.24) and 2b(o) (f= 0.16) were
again much higher than those for the photocycloreversion
(2a(c) f= 2.4 ” 10ꢀ3, 2b(c) f= 0.6 ” 10ꢀ3).
The NLO figure of merit mb (where m is the dipole
moment and b the quadratic hyperpolarizability coefficient)
was evaluated byelectric-field induced second harmonic
generation (EFISH) measurement for the open and photo-
stationarystate (PSS) closed forms of 2a,b in concentrated
dichloromethane solution (10ꢀ3 m) with a nonresonant inci-
dent wavelength of 1.91 mm. The static mb0 values extrapo-
lated at zero frequencywere calculated using the two-level
model.[12]
Full details of the experimental procedures and compound
characterization are given in the Supporting Information.
Received: September 7, 2007
Published online: December 3, 2007
Keywords: molecular switches · nonlinear optics ·
.
photochromism · zinc
As indicated in Table 2, the mb0 values for the open forms
are verysmall, in agreement with the absence of p conjuga-
tion between the two thiophene rings of the DTE fragment.
Note that the larger NLO activityof 2b(o) relative to 2a(o)
can be tentativelyexplained bya dipolar contribution of the
dimethylaminophenylthienyl perfluoropentene fragment, the
perfluoropentene moietyacting as a weak electron-accepting
group. Upon conversion to the closed form in the photosta-
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Compound mb
[10ꢀ48 esu]
mb
[10ꢀ48 esu]
mb0
[10ꢀ48 esu]
mb0
[10ꢀ48 esu]
closed form
open form
closed form open form
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2020[c]
4220[c]
75[b]
1020[c]
1800[c]
200[b]
160[b]
[a]Determined at linc =1907 nm. [b]Error ꢁ20%. [c]Error ꢁ5%.
Angew. Chem. Int. Ed. 2008, 47, 577 –580
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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579

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