Molecular switches containing transition metals

Article abstract of DOI:10.2533/000942903777679569

The synthesis and the characterization of a novel dithienylethene-based molecular switch bearing metal complexes are described.

Full text of DOI:10.2533/000942903777679569

LAUREATES: AWARDS AND HONORS (SCS)
169
CHIMIA 2003, 57, No. 4
Chimia 57 (2003) 169–172
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Molecular Switches Containing
Transition Metals
Vincent Adamo* and Peter Belser
Abstract: The synthesis and the characterization of a novel dithienylethene-based molecular switch bearing
metal complexes are described.
Keywords: Bridging ligand · Molecular device · Molecular switch · Photochromism · Ruthenium complex
1. Introduction
the different types of systems employed like W or Re were used. On the other hand,
so far, dithienylethenes containing a perflu- the inclusion of ruthenium led to a different
The aim of miniaturization in nanotechnol- orocyclopentene moiety integrated in the situation in which the photochromism of
ogy has led over the last few years to a huge structure, have been proved to display ex- the system was blocked. The blockage
improvement in the knowledge of organic cellent photochromic properties and low acted as a chemical lock of the information
photochromic systems [1]. In general, pho- photofatigue. Nevertheless, a major prob- and therefore data storage devices could
tochromic molecules have been widely stud- lem can arise from read-out instability since be envisioned.
ied in order to develop new molecular de- the irradiation needed for the memory
The present communication shows the
vices able to use optical recording/reading process can interact with the switch itself synthesis of a thiophene-based diarylethene
procedures. These laser-based techniques and therefore alter the initial situation. Pos- unit, covalently linked to bipyridine moi-
applied to photochromic switches could re- sible solutions have been attempted but fur- eties through phenylene spacers (Schemes
duce the memory spot size even to some- ther improvement is still required. One pos- 1 and 2). This basic framework has been
thing as small as half of the wavelength of sibility could be the introduction of metal- used as a ditopic ligand for the preparation
the incoming light. The applicability of or- lic centers like ruthenium or osmium. The of the homodinuclear metal complex with
ganic photochromic molecules is restricted presence of these centers could allow, in the ruthenium as the metal center. The prelimi-
to a certain kind of structure. They must ful- first instance, a sensitized performance of nary properties showed the possibility of
fill the following basic properties: Thermal the switching unit by energy transfer after full opening/closing cycles of the switching
stability of both bistable isomers, fast re- metal-based MLCT irradiation. This new part in any structures reported herein, either
sponse, and low fatigue resistance. Among approach could shift the irradiation wave- by direct irradiation of the switching unit
length from the UV to the visible and this itself or by irradiation into the MLCT band
would improve photofatigue. Moreover, the of the covalently bonded ruthenium com-
presence of a strong emitting metallic cen- plex. Moreover, the presence of an emitting
ter like Ru(bpy)32+ could provide a way to co-ordination center could lead to light-
achieve non-destructive read-out. The emis- triggered fluorescence behavior depending
sion could be tuned by the different states of on the state of the switching unit.
the switching unit.
Different research groups have attempt-
ed the introduction of metallic centers at- 2. Preparation of the Free
tached to the dithienylethene unit. For Ligand and the Corresponding
instance, Launay and co-workers [2] have Metal Complex
reported a bimetallic system where the
switching unit has been symmetrically func- 2.1. Ligand LSL
tionalized by two cyclometallated rutheni-
The synthesis of the ligand 1 (Scheme 1)
um complexes using acetylene groups as was performed first by bromination of
spacers. In this case a photostationary state 2-methylthiophene followed by treatment
of 75% has been achieved after irradiation with n-butyllithium/triisopropylborate lead-
at 254 nm. Lehn and co-workers [3] report- ing to 3-bromo-2-methyl-5-thiopheneboron-
ed in 1999 a system where different transi- ic acid. The 4-(p-bromophenyl)-2,2’-bipyri-
tion metals like tungsten, rhenium, and dine was obtained by a condensation reac-
ruthenium were attached. Light-triggered tion from the corresponding ‘Kröhnke salt’
fluorescence was achieved when metals [4] and 4-bromo-(3-carboxyl-3-oxoprop-
*Correspondence: V. Adamo
Department of Chemistry
University of Fribourg
Pérolles
CH–1700 Fribourg
Tel.: +41 26 300 87 33
Fax: +41 26 300 97 38
E-Mail: vincent.adamo@unifr.ch

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CHIMIA 2003, 57, No. 4
1-enyl)benzene. A palladium(O) catalyzed at 120 oC leads after an ion exchange and dif-
Suzuki cross-coupling reaction between the ferent purification steps to the desired com- or the corresponding H-NMR spectra in-
Measuring either the UV-vis spectra
1
two precursors gives the desired 3-bro- pound 3 ([(bpy)2Ru-LSL-Ru(bpy)2](PF6)4). dicates clearly the evolution of the closing
mothiophene derivative 1.
Treatment of 1 with n-butyllithium fol-
lowed by addition of perfluorocyclopen- 3. Photophysical Properties
tene leads to the expected ligand 2 (LSL)
(Scheme 2).
or opening process. The Table shows the
chemical shifts of the methyl and the thio-
phene protons, respectively, in the open and
closed form of the switching unit in 2 and 3.
Furthermore, the λmax and ε-values of the
absorption bands are also given.
3.1. General Remarks Concerning
the Photochromism
2.2. Preparation of the Dinuclear
Ruthenium Complex
Upon irradiation with UV-light the mo-
lecular switch in the ‘open form’undergoes induced ring closing process is very high.
The efficiency of the photochemical-
A
bis-metallation reaction using a light driven cyclization process to the For the compounds 2 (LSL) and 3 we found
Ru(bpy)2Cl2 as precursor and the ligand 2 ‘closed form’. Irradiation into the absorp- photostationary states of 98% and 96%,
gives the homodinuclear metal complex [5]. tion band lying at about 600 nm gives rise respectively. Such high values for photosta-
Heating the reactants in 2-methoxyethanol to a ring opening process (Scheme 3).
tionary states on metal complex containing
Scheme 1. Synthesis of the precursor compound 1
Scheme 2. Synthesis of the ligand 2 (LSL)

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CHIMIA 2003, 57, No. 4
Scheme 3. Photoconversion of the prepared
metal complex.
Table. 1H NMR chemical shifts of selected protons and extinction coefficients for the bridging
ligand 2 (LSL) and the metal complex 3 (Ru-LSL-Ru).
Compound
o (open); c (closed)
1H NMR
Me
UV/Vis
max [nm] (ε x 10-3/cm–1 M–1
)
thiophene
λ
2 o (LSL)a
2.02
2.22
2.09
2.26
7.35
6.77
7.52
6.99
322 (40)
331 (33), 603 (14)
289 (139)
2 c (LSL)a
3 o (Ru-LSL-Ru)b
3 c (Ru-LSL-Ru)b
289 (132), 617 (32)
aIn CDCl3, bIn CD3OD.
switching molecules can be explained by a closing process can be given by the large quenches drastically the excited state of the
nearby complete separation of the absorp- peripheral groups attached to the switching metal complex. In contrast the isomer in the
tion bands, which are well suited for photo- unit. In fact, the dithienylethene derivative open form possesses an energy level which
chemical reactions [6].
can exist in a parallel or antiparallel con- does not perturb the emission from the
formation in which only the latter is able ruthenium center.
3.2. Photochromism of the Dinuclear to undergo a photocyclization process [7].
Metal Complex An increase of steric hindrance favors the
The absorption spectrum (Fig.) shows antiparallel conformation. Indeed, the de- 4. Conclusion
a broad band centered at 610 nm corre- scribed switching unit contains bulky side
sponding to the closed isomer of the ruthe- groups and holds them in the preferred an-
nium complex. The ring opening process tiparallel conformation.
Dithienylethene derivatives containing
transition metal complexes were synthe-
is achieved by irradiation into the 600 nm
Emission spectra were measured on the sized.Acomplete and reversible conversion
band. The closing/opening process could be metal complex [(bpy)2Ru-LSL-Ru(bpy)2]4+ from the open to the closed form (and vice
repeated at least 10 times without chemical in the open and closed forms using an exci- versa) of the bistable switching molecule
decomposition. In the case of the dinuclear tation wavelength of 332 nm. Depending on was observed. An incorporation of a co-
ruthenium complex, a very efficient ring the state of the switch a large discrimination ordination center like Ru(bpy)32+ as sensi-
closing process (φ = 0.93) takes place, either in the fluorescence intensity can be observed. tizer enables the use of visible light to in-
by direct irradiation of the switching unit This different behavior in the emission duce a ring closure process of the switching
with UV-light or by triplet sensitization of properties indicates the strong influence on system. The quantum yield for the photocy-
the ruthenium complex by irradiation into the energy level of the switching unit in the clization process of the compounds 2 and 3
the MLCT band at 450 nm and subsequent open and closed form, respectively. Appar- is close to 100% probably due to the pre-
energy transfer process from the excited ently, the switching unit in the closed form ferred antiparallel conformation adopted
metal center to the switching unit. A strong represents a more conjugated molecule and by the system. A pronounced discrimina-
argument for the high quantum yield of the has a low lying triplet energy state, which tion of the emission intensity was observed

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CHIMIA 2003, 57, No. 4
Fig. Absorption and emission spectra of the complex 3 (Ru-LSL-Ru) in a 10-5 M acetonitrile solution.
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depending on the open and closed form of
the dithienylethene subunit in the metal
complex [(bpy)2Ru-LSL-Ru(bpy)2](PF6)4.
Such a behavior can open interesting appli-
cations in the field of information storage.
Acknowledgements
We thank the National Swiss Science Foun-
dation for funding through the NFP 47 program.
Received: February 7, 2003
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