Photoswitching of the second-order nonlinearity of a tetrahedral octupolar multi DTE-based copper(i) complex

Article abstract of DOI:10.1039/c2cc34999j

The modulation of the quadratic NLO response of an octupolar metal-based chromophore featuring four photochromic dithienylethene units is reported. Quantum mechanical simulations are consistent with a full switching of the DTE units and reproduce the strong enhancement of the NLO response.

Full text of DOI:10.1039/c2cc34999j

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COMMUNICATION
Photoswitching of the second-order nonlinearity of a tetrahedral
octupolar multi DTE-based copper(I) complexw
Hiroyuki Nitadori,ab Lucie Ordronneau,a Julien Boixel,ac Denis Jacquemin,*d
a
c
b
c
a
´
Abdou Boucekkine, Anu Singh, Munetaka Akita, Isabelle Ledoux, Veronique Guerchais
and Hubert Le Bozec*a
Received 12th July 2012, Accepted 31st August 2012
DOI: 10.1039/c2cc34999j
The modulation of the quadratic NLO response of an octupolar
metal-based chromophore featuring four photochromic dithienyl-
ethene units is reported. Quantum mechanical simulations are
consistent with a full switching of the DTE units and reproduce
the strong enhancement of the NLO response.
reported so far. Therefore, we have designed a new bis
bipyridine copper(I) complex featuring four DTE units, and
such systems are expected to display at strong contrast to the
NLO response. It should be noted that only a very few number
of single molecules containing four or more DTE have been
synthesised up to now,8 and in the majority of them, two
vicinal DTE units cannot be entirely closed8b–d whereas the
two reported cases with full switching8a,e use saturated or
poorly conjugated bridging units to connect the different
photochromic DTE fragments. In this communication, we
demonstrate that the quadruple DTE copper complex is
converted into its fully closed isomer upon UV irradiation,
and we present for the first time the modulation of the
quadratic NLO activity of an octupolar chromophore.
The synthesis of Lo,o was achieved by a controlled functio-
nalization of the 4,40 positions of 4,40,6,60-tetramethyl-2,20-
bipyridine 1. Thus, dilithiation with LDA in THF at ꢀ78 1C,
followed by reaction with chlorotrimethylsilane and subsequent
chlorination with hexachloroethane afforded selectively the resulting
bis-chloromethyl derivative 2 in good yields (Scheme 1). Finally,
treatment of the diethylphosphonatomethyl-2,20-bipyridine 3,
readily obtained from 2 through an Arbuzov reaction, with
the DTE-aldehyde a4 under normal Wadsworth–Emmons
conditions6b afforded the target bipyridine ligand Lo,o in 70%
yield after purification by recrystallization.
In the past years, much attention has been paid on the reversible
switching or modulation of NLO properties.1,2 As most of the
NLO chromophores are based on push–pull (dipolar) systems,
photochromic molecules incorporating dithienylethene (DTE)
architectures are particularly suitable for the on/off modulation
of the NLO response, due to reversible interconversion between
a non-conjugated open form and a p-conjugated closed form
by alternate irradiation with UV and visible light.3 In this context,
we have recently reported the first efficient phototriggered NLO
enhancement of dipolar metal complexes featuring a bis-DTE
bipyridine ligand.4 Beyond the classical dipolar approach,
non-dipolar (octupolar) molecules have emerged as an important
and promising class of NLO chromophores, giving rise to two- and
three-dimensional architectures.5 Metal ions are versatile templates
to build up tetrahedral (Td, D2d) and octahedral (D3) octupolar
arrangements.6 For example, we have already shown that the
association of d10 Cu(I), Ag(I) and Zn(II) ions with tetrasubstituted
4,40,6,60-bipyridyl ligands gives rise to pseudo-tetrahedral metal
complexes with fairly large quadratic hyperpolarizabilities.7
The corresponding copper complex [Cu(Lo,o)2]PF6 was
formed almost quantitatively after mixing 2 equiv. of Lo,o with
[Cu(MeCN)4]PF6 at room temperature in dichloromethane–
acetonitrile solution (Fig. 1).
To the best of our knowledge, the photoregulation of the
NLO properties of octupolar chromophores has not been
a
´
UMR 6226 CNRS-Universite de Rennes 1, Institut des Sciences
The ligand and complex were fully characterized by NMR
and microanalysis (see ESIw). The copper(I) complex exhibits
Chimiques de Rennes, Campus de Beaulieu, 35042 Rennes, France.
E-mail: lebozec@univ-rennes1.fr; Fax: +33 223236939;
Tel: +33 223236729
b Chemical Resources Laboratory, Tokyo Institute of Technology,
R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
c
´
Laboratoire de Photonique Quantique et Moleculaire, UMR CNRS
8531, Institut d’Alembert, ENS Cachan, 61 avenue du President
´
Wilson, 94235 Cachan, France
´
d
UMR CNRS 6230, Universite de Nantes, CEISAM,
2 rue de la Houssinie`re, 44322 Nantes cedex 3, France.
E-mail: Denis.Jacquemin@univ-nantes.fr; Fax: +33 251125712;
Tel: +33 251125564
w Electronic supplementary information (ESI) available: Synthesis
and characterisations of ligands and complexes; details of theoretical
calculations, extra TD-DFT analysis and NLO measurements. See
DOI: 10.1039/c2cc34999j
Scheme 1 Synthesis of the photochromic bipyridine Lo,o
.
c
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Chem. Commun., 2012, 48, 10395–10397 10395

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Fig. 2 (left) Experimental UV-vis absorption spectral change of
[Cu(Lo,o)2]PF6 in dichloromethane upon excitation at 350 nm. (right)
Fig. 1 Chemical structure of [Cu(Lo,o)2]PF6.
Simulated UV-vis spectra of [Cu(Lo,o)2]+
,
[Cu(Lo,o)(Lo,c)]+
,
Table 1 Electronic absorption data in the open and closed (PSS)
forms, HRS data before and after UV irradiationa
[Cu(Lo,c)2]+
, ,
[Cu(Lo,o)(Lc,c)]+ [Cu(Lc,c)(Lo,c)]+ and [Cu(Lc,c)2]+
isomers.
b
d
d
d,e
labs (ec)
closed form before UV
(PSS) irradiation
b1.91 (bxyzd,e) b1.91 (bxyz
)
labs (ec)
after UV
irradiationf
b
Compound open form
L
340 (96 000) 340 (61 000)
395 (34 000)
431 (27 000
668 (45 000)
[Cu(L)2]PF6 344 (166 000) 343 (119 000) 266 ꢁ 52
511 (24 000) 399 (sh)
(352 ꢁ 70)
450 (47 000)
1672 ꢁ 160
(2212 ꢁ 220)
701 (95 000)
a
e
b
c
d
At 298 K in CH2Cl2. In nm. In Mꢀ1 cmꢀ1
.
In 10ꢀ30 esu.
.
65% ring-closing determined by 1H NMR
2
2 f
hbHLS i = 4/7 bxyz
spectroscopy.
Fig. 3 Optimized geometry of [Cu(Lo,o)2]+ (left) and [Cu(Lc,c)2]+
only one set of signals as expected for D2d complexes. The
UV-visible spectrum of [Cu(Lo,o)2]PF6 (in CH2Cl2) shows like
(right) complexes.
L
o,o an intense band at around 350 nm assigned to (IL) p - p*
is independent of the number of closed units), nor energetic
problem (each closure of a DTE unit implies the same increase
in the total energies) that would prevent the electrocyclisation of
the four DTE units.
transition of the bipyridyl moieties with some mixing of the
DTE units (Table 1), and another band in the visible at 511 nm
corresponding to the dp(Cu) - p*(bipy) MLCT transition.9
The photocyclization process for both ligand and complex was
monitored by UV-vis and 1H NMR spectroscopy in dichloro-
methane. Upon irradiation at 350 nm, the yellow solution of
Lo,o and red solution of [Cu(Lo,o)2]PF6 turn green, due to the
formation of the fully closed forms with a photostationary
state (PSS) >90% in both cases. A new broad band emerges in
the visible at 668 and 701 nm, which is attributed to the
intraligand (IL) S0 - S1 transition of the closed DTE unit of the
free and coordinated ligand, respectively (Fig. 2 and Fig. S1
(ESIw)). Upon irradiation at 650 nm, the quantitative regeneration
of the fully open isomer was observed, indicating the fully
reversible closed-to-open photoisomerization.
The simulated spectra of all different isomers can be found
in Fig. 2, the numerical values for the dominant vertical
transition wavelengths being listed in Table S1 (ESIw). It is
striking that TD-DFT foresees that the successive cyclizations
of the different DTE’s imply: (i) the emergence of a strongly
intense visible band at large wavelength; (ii) a decrease in the
intensity of the stronger UV band when two DTE units are
closed on the same ligand. These trends are in good qualitative
agreement with experimental measurements. Quantitatively,
theory is on the spot for the visible band (ca. 690 nm instead of
701 nm), and the position of the 450 nm band, but is less
accurate for the position of the UV band, as well as for the
511 nm peak of the [Cu(Lo,o)2]+ structure, which theory foresees at
470 nm. An analysis of the orbital component of this hallmark
band confirms that it corresponds to a MLCT transition from
the copper(I) center to the complexed bipyridines. Nevertheless,
TD-DFT provides a series of insights. First, there is no
bathochromic shift accompanying the successive switching
on, an outcome indicating nearly independent DTE units,
which corroborates the ground-state energetic analysis above.
Second, for the visible part, there is absolutely no difference
between the [Cu(Lo,c)2]+ and [Cu(Lo,o)(Lc,c)]+ isomers, which
is consistent with a peak related to closed DTE rather than the
central core of the molecule. Third, the virtual orbitals typical
of the DTE core present the same energy for each open DTE
To further understand the photochromic behaviour of this multi-
DTE compound, theoretical calculations have been performed
using a methodology that has been successful for studying
multi-DTE molecules.10 We have focused our analysis on the
different isomers using all possible closed–open combinations
for the four photochromes, the fully open [Cu(Lo,o)2]+, the
mono-closed [Cu(Lo,o)(Lo,c)]+, the two doubly-open-doubly-
closed [Cu(Lo,c)2]+ and [Cu(Lo,o)(Lc,c)]+, the triply-closed
[Cu(Lc,c)(Lo,c)]+ and finally the fully closed [Cu(Lc,c)2]+ isomers.
As shown in Fig. 3, [Cu(Lo,o)2]+ and [Cu(Lc,c)2]+ belong to the
D
2d symmetry and the angles between the two ligands are nearly
901, and our calculations also demonstrate that there is no steric
hindrance (the distance between the two reactive carbon atoms
c
10396 Chem. Commun., 2012, 48, 10395–10397
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financial support. This research used resources of the GENCI-
CINES/IDRIS and of the CCIPL. AS acknowledges the
C’Nano Ile-de-France Program for her PhD grant. HN is
grateful to ITP and JSPS programs for a travel grant.
Notes and references
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hints for a complete cyclisation of the compound. Fourth, the
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remained unchanged afterwards, which also clearly corroborates
a relatively weak coupling between the different DTE units. The
HOMOs of Fig. 4 and Fig. S3 (ESIw) are clearly related to the
DTE core, once they are closed, whereas the LUMOs are located
on the central bipyridine and partly delocalized on the DTE.
The Harmonic Light Scattering (HLS) technique was used
for the molecular first hyperpolarizability b measurements.
They were performed in dichloromethane at a fundamental
wavelength of 1.91 mm. The Td (or D2d) symmetry reduces the
nonlinear microscopic coefficient to only one non-vanishing
term, bxyz, which can be easily deduced from the following
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2
2 5b
relation: hbHLS i = 4/7 bxyz
.
The experimentally determined
b1.91 and the corresponding off-diagonal bxyz coefficients for the
Cu(I) complex before and after UV irradiation are given in
Table 1. Before photocyclisation, [Cu(Lo,o)2]+ shows a fairly large
b value which can be reasonably attributed to the low energy
MLCT dp(Cu) - p*(bipy) transition. A substantial enhancement
of the second-order NLO activity is then observed after UV
irradiation and subsequent accumulation of the ring-closed
isomer, a result which is consistent with an NLO response
mainly controlled by the red-shifted ILCT transition. The
molecular first hyperpolarisability is increased almost six fold
after only 65% of ring-closing, to reach an impressive bxyz
value of 2212 ꢂ 10ꢀ30 esu, which is to our knowledge the largest
off-diagonal hyperpolarizability coefficient ever reported for octu-
polar chromophores having Td or D2d symmetry.11 Theoretical
calculations of the static bxyz component using a range-separated
hybrid DFT scheme (see ESIw for details) also predict a large
enhancement of the nonlinear response by a factor of eight when
switching all four DTE units.
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In summary, we have described here a novel approach to the
first molecular hyperpolarisability (b) modulation in an octupolar
chromophore featuring four photochromic DTE units. Joint
theory–experiment works are under way to design and con-
struct other octupolar metal-based switches having an octa-
hedral geometry and containing up to six DTE units.
The authors thank the ANR program (BLANC COMET)
for financial support. D. J. acknowledges the European
´
Research Council and the Region des Pays de la Loire for
11 T. Ishizuka, L. E. Sinks, K. Song, S.-T. Hung, A. Nayak, K. Clays
and M. Therien, J. Am. Chem. Soc., 2011, 133, 288.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10395–10397 10397