A redox- and photo-responsive quadri-state switch based on dimethyldihydropyrene-appended cobalt complexes

Article abstract of DOI:10.1039/c5tc04277a

A multi-addressable quadri-state molecular switch based on the redox and photochromic properties of a dimethyldihydropyrene-substituted terpyridine cobalt complex has been designed and fully characterized.

Full text of DOI:10.1039/c5tc04277a

Journal of
Materials Chemistry C
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COMMUNICATION
A redox- and photo-responsive quadri-state
switch based on dimethyldihydropyrene-
Cite this:DOI: 10.1039/c5tc04277a
appended cobalt complexes†
Received 16th December 2015,
Accepted 11th January 2016
ab
ab
abc
ab
Assil Bakkar, Saioa Cobo,* Frederic Lafolet,
´ ´
Diego Roldan,
ab
ab
Eric Saint-Aman and Guy Royal*
DOI: 10.1039/c5tc04277a
www.rsc.org/MaterialsC
A multi-addressable quadri-state molecular switch based on the of combining photo-responsive derivatives with electron-responsive
redox and photochromic properties of a dimethyldihydropyrene- transition metal complexes displaying at least two well-defined
8
substituted terpyridine cobalt complex has been designed and fully and accessible redox states. From previously reported studies
characterized.
on molecular architectures binding a polypyridyl metal complex
and a photochromic subunit, the following requirements for
9
The use of photochromic couples as key components in light- obtaining four-state switches can be drawn: (i) the photochromic
powered molecular switches has been shown to be one of the most system must be fully reversible with a short response time, and
attractive and straightforward strategies towards the development (ii) the complexed metallic centre must be switched reversibly
1
,2
of useful molecules or materials in molecular electronics. In between its different redox states without overlap with the
order to improve the photo-conversion rate, the isomerization electrochemical response of the photochromic unit. Taking into
quantum yield or the stability of both states, extensive research account these pre-requisites, a DHP-appended bis-terpyridine
has been undertaken on different photochromic compounds cobalt complex has been designed using a pyridinium linker
3
4
such as the well-known spiropyrans, fulgides or dithienyl- between the two moieties (Chart 1). Indeed, we have previously
5
ethenes. In this context, the less explored dimethyldihydropyrene reported that the substitution of the DHP core with electron-
(
DHP)/cyclophanediene (CPD) couple, mainly studied by Mitchell withdrawing pyridinium groups enhances the quantum yield of
1
et al. and Nishihara et al., is an appealing synthetic platform the opening reaction and allows fast and quantitative conversions
10
that can be easily functionalized to endow additional switching between the photo-states.
properties with the photochromic compound. The aromatic planar The synthetic route leading to the ligand Lc is depicted in
4 p-electron DHP isomer, also designated as the ‘‘closed-ring’’ Scheme 1. The procedure starts with the bromination of the
+
1
form, absorbs light in the visible range. Upon visible irradiation, known 2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene
this species is readily converted into a thermodynamically less in the presence of 1.1 equivalents of N-bromosuccinimide in a
stable and colourless CPD isomer, the ‘‘open-ring’’ form, through a mixture of CH Cl /DMF. The mono-brominated intermediate
2 2
complex mechanism leading to the opening of the central carbon– was then subjected to a Suzuki–Miyaura coupling reaction in
carbon bond and giving two non-coplanar benzyl rings connected the presence of 4-pyridylboronic acid and tetrakis(triphenyl-
6
through olefin bridges. The reverse transformation, the CPD - phosphine)palladium to yield the mono-pyridine-substituted
DHP conversion, best described as a six p-electron electrocyclization,
may be accomplished optically by irradiation in the UV range,
or thermally.
The development of multi-addressable molecular switches
able to display multiple functionalities in response to different
7
stimuli is still a challenging issue. An appealing strategy consists
a
Univ. Grenoble Alpes, DCM UMR 5250, F-38000 Grenoble, France
b
CNRS, DCM UMR 5250, F-38000 Grenoble, France. E-mail: saioa.cobo@ujf-grenoble.fr,
guy.royal@ujf-grenoble.fr
c
Univ. Paris Diderot, Sorbonne Paris Cit ´e , ITODYS, UMR 7086 CNRS,
II
4+
2
1
5 rue Jean-Antoine de Ba ¨ı f, 75205 Paris Cedex 13, France
Chart 1 Targeted Co (L
c
)
complex. (A racemic mixture of trans isomers
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c5tc04277a
is expected.)
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3
wavelength at around 740 nm in CH CN due to the partial
radiative desactivation of the excited state DHP*.
The efficiency and the kinetics of the photo-isomerization of
+
II
4+
Lc and Co (L ) have been investigated upon monitoring the
c 2
changes occurring in their absorption spectra at low temperature
(B281 K) during a visible irradiation (l 4 630 nm) under an
inert atmosphere. The ring opening process was revealed by
the disappearance of the absorption bands characteristic of
the closed-ring isomer at the expense of new signals growing
at 252 and 274 nm (Fig. 1a), along with the gradual decrease
of fluorescence emission (see ESI†). Under our experimental
+
Scheme 1 Synthesis of DHP-substituted terpyridine ligand L
as a racemic mixture of trans isomers.
c
, obtained
conditions, the quantitative conversion of L + into the open-ring
c
form L + was achieved after less than 1 minute of irradiation
o
ꢀ
5
(
10 M in CH
3
CN). The well-defined isosbestic point at 310 nm
1
0
DHP as a green solid. The latter was then reacted with one molar in the array of spectra also supports the conclusion that the
0
0
0
00
11
equivalent of 4 -(4-bromo-methyl-phenyl)-2,2 :6 ,2 -terpyridine.
The crude reaction product was subjected to an anion exchange
procedure to afford the targeted pyridinium salt L
c
isomerization of L + is the only reaction triggered by irradiation.
The isomerization quantum yield has been determined by
+
ꢀ
isolated actinometry using 2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-
c
, BF
4
ꢀ
2
as a red powder. The synthesis of the bis-terpyridine metal dihydropyrene as standard. A value of Fc/o = 3 ꢁ 10 has been
II
4+
complex Co (L
c
)
2
has been carried out by mixing two molar found close to the value reported for other pyridinium-substituted
+
10
equivalents of the ligand Lc with one molar equivalent of Co(BF4)2 DHP derivatives. The same behaviour was observed with
in deoxygenated acetonitrile medium. The metal complex is Co (L
II
4+
c
)
2
3
into CH CN solution. Upon irradiation, the main
isolated by precipitation upon addition of diethyl ether. All the absorption bands in the visible region gradually vanished, the
compounds were characterized (see ESI†), by FTIR, HRMS, only band which remained unaffected being the d–d transition
fluorescence and NMR spectroscopies (except for the cobalt(II) at 530 nm. A complete photo-conversion has been reached for
complex because of its paramagnetic behaviour). The inter- the metal complex but with a lower isomerization quantum
ꢀ3
conversion between photo-states has been followed by UV-vis yield (Fc/o = 4.6 ꢁ 10 ) attributed to the d–d contribution at
II
4+
spectroscopy and electrochemistry under an inert atmosphere the excitation wavelength. The fatigue resistance of Co (L )
c 2
1
2
to avoid endoperoxide formation.
during successive ON/OFF switch cycles was monitored by
+
The absorption spectrum of the free ligand Lc exhibits five absorption spectroscopy (see ESI†). Up to at least 10 cycles,
absorption bands at 672, 527, 424, 337 and 269 nm (Fig. 1a). no degradation has been observed.
+
c
The bands in the 300–800 nm range are related to p–p*
The electrochemical behaviour of an electrolytic L
solution
3
transitions involving the four singlet electronic states of the (CH CN + 0.1 M TBAP) has been studied by cyclic voltammetry
+
DHP core whereas the band observed at 269 nm is attributed to (Fig. 2). The response of the closed-ring form L
c
exhibits two
p–p* transitions centred on the terpyridine motif. Metalation successive anodic waves accounting for one-electron transfers
+
2+
10
of L
characteristic d–d band at 530 nm (Fig. 1b), seen at 513 nm reversible whereas the second wave at +1.17 V is irreversible,
(
c
with Co was revealed through the emergence of the centred on the DHP core. The first anodic wave at +0.45 V is
ꢀ
1
ꢀ1
2+
e = 2700 L mol cm ) in the simple Co(tolyl-terpy)2 reference its cathodic counter-part being observed on the reverse scan at
complex. This d–d transition band partially overlaps the S –S
transition band of the ligand (e = 5400 L mol cm ). The final
0
2
ꢀ
1
ꢀ1
ꢀ
1
ꢀ1
absorbance at 530 nm (e = 14900 L mol cm ) is roughly the
sum of both components taking into account one Co(tolyl-terpy)
2
+
2
subunit and two DHP subunits. Both investigated closed-ring
compounds display fluorescence spectra with a maximum
+
II
4+
Fig. 1 UV-vis spectrum evolution during irradiation (l 4 630 nm, 500 W,
Fig. 2 Cyclic voltammograms of L
c
(a), and Co (L
)
c 2
(b) before (black)
+
2
81 K, from t = 0 up to 30 min) of a CH
3
CN solution of free L
c
(a) and
and after (grey) irradiation (l 4 630 nm, 500 W, 281 K, t = 30 min); 1 mM in
II
4+
ꢀ1
Co (L
c
)
2
(b), from the closed-ring (black) to the open-ring form (grey).
CH
3
CN + 0.1 M TBAP; 0.1 V s ; WE = glassy carbon.
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pc B 0 V. In the region of negative potentials, the cyclic
Communication
E
voltammetry curve displays a single one-electron reversible
cathodic wave at E1 = ꢀ1.49 V attributed to the reduction of
/2
10
II
4+
c 2
the pyridinium unit. The electrochemical signature of Co (L )
can be roughly described as the superimposition of the response
attributed to the DHP core in the free ligand, and that of the
complexed cobalt metal centre, characterized by two distinct
reversible electron transfers at ꢀ1.03 V and ꢀ0.05 V attributed
II/I
III/II
to the Co and Co
redox couples, respectively, in agree-
II
4+
2
ment with the electrochemical response of the simple Co(tolyl- Scheme 2 Multi-addressable states for the Co (L
c
)
complex modulated
2
+
13
by photo- or thermal and electrochemical redox processes.
terpy)2 reference complex. The electrochemical signature
+
of the open-ring form Lo has been recorded in situ, after
submitting to visible light irradiation an electrolytic solution
+
II
4+
of L
c
under an inert atmosphere. The resulting cyclic voltammo- between four discrete states derived from Co (L
c
)
2
. The full
gram reveals the disappearance of the first anodic wave attributed process described as a square scheme combines the two
to the one-electron oxidation of the DHP core at the expense of distinguishable forms of the photochromic core which can be
a reversible anodic wave at E1/2 = + 0.56 V attributed to the addressed photo-chemically and the two distinct redox states of
oxidation of the CPD isomer. A second irreversible oxidation the complexed metal centre which can be addressed electro-
wave was also observed at the same potential as the second chemically (Scheme 2).
oxidation peak of the closed-ring isomer, suggesting a coupled
Each state can be unambiguously identified from a set of
electrochemically driven ring-closure reaction. Accordingly, on spectroscopic and electrochemical signatures and the inter-
the reverse scan, the cathodic response matches with that of the conversion between each of the four stable states can be triggered
closed-ring isomer. This hypothesis was further confirmed by the use of optical and/or electrochemical inputs. The key
when voltammetric scans were repeated between ꢀ0.3 and experimental features allowing us to identify each state
+
+
1.3 V (see ESI†): the CV curve of L
c
is progressively restored involved in the square scheme are shown in Fig. 3. Starting
II
4+
during cycling. Similar changes have been observed in the from a millimolar CH
3
CN (+0.1 M TBAP) solution of Co (L
c 2
)
II
4+
electrochemical response of a Co (L )
solution upon visible (state ‘‘0,0’’ in Fig. 3), its irradiation (l 4 630 nm) quantitatively
c 2
II
4+
light irradiation i.e. the signals attributed to the DHP core yields the open-ring form Co (L ) (state ‘‘1,0’’ in Fig. 3). This
o 2
underwent similar changes in the free ligand and in the Co(II) photo-conversion is revealed by the progressive disappearance
complex. The waves attributed to the electron transfers involving of the absorption bands at 337, 424, 527 and 672 nm along with
the complexed metal centre are not significantly affected by the the appearance of a reversible oxidation wave of the CPD form at
opening photo-process.
As previously stated for DHP derivatives, the open-ring form molecule) of this Co (L
can be either photo-chemically (UV irradiation) or thermally carried out to generate the Co (L
E1/2 = + 0.56 V. An exhaustive electrolysis (1 electron per
II
4+
)
o 2
solution at Eapp = 0.2 V was then
III
5+
)
o 2
complex (state ‘‘1,1’’ in
converted back to the stable DHP form. To avoid degradation of Fig. 3). During the electro-oxidation process, the emergence of
the photochromic core upon UV irradiation, thermal ring closure an intense absorption band at 338 nm, the characteristic of a
1
0
has been preferred.
The thermal ring-closure process yielding back the DHP cation in Co(tolyl-terpy)
form was checked upon studying the stability of compounds intensity of the d–d transition band at 530 nm are concomitantly
p–p* transition band of a tolyl-terpy coordinating to a trivalent
3+ 13
2
, and a slight decrease of the absorbance
+
II
4+
L
o
and Co (L
in the UV-vis spectra over time at three different temperatures and
assuming a first-order thermal relaxation process (see ESI†). The shows the disappearance of the oxidation wave attributed to
o 2
)
. It was assessed upon monitoring the changes observed.
The electrochemical behaviour of the resulting solution
ꢀ
1
II
III
calculated activation energies of about 20 (ꢂ5) kcal mol fall the Co - Co process at Epa = 0.02 V as well. It should also be
1
0
within the range of values reported for CPD derivatives. The mentioned that the electrolysis at 0.2 V does not affect the CPD
half-life time of 5 hours at 318 K, found for both compounds, is core since the electrochemically induced ring closing process is
conversely quite lower than that of 52 h reported for the 2,7-di- induced at much more positive potential values. The conversion
III
5+
2
tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene reference of the CPD isomer Co (L
o
)
into its corresponding DHP form
(state ‘‘0,1’’ in Fig. 3) has been subsequently
are less achieved by thermal relaxation at 318 K overnight. The spectro-
thermally stable than the simple DHP, which is most probably scopic and electrochemical signatures of the resulting solution
14
III
5+
c 2
compound in cyclohexane. Such a large difference suggests that Co (L )
+
II
4+
the free ligand L
o o 2
and its metal complex Co (L )
III
5+
c 2
due to the presence of the electron withdrawing pyridinium support the conclusion that Co (L ) is produced quantitatively.
substituent decreasing the electron density on the photochromic No residual anodic peaks attributed to the oxidation of the Co(II)
1
0
core and stabilizing the radical intermediate.
centre are detected on the CV curve and the absorption bands
III
5+
The above-described findings are consistent with the idea that a characteristic of the closed form Co (L
)
c 2
increase gradually
combination of optical, thermal and electrochemical stimuli could during the thermal relaxation. Finally, an exhaustive electrolysis
be used to selectively address the reversible inter-conversion (1 electron per molecule) carried out at ꢀ0.3 V restores the
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II
4+
2
II
4+
2
III
5+
2
III
5+
Fig. 3 UV-vis spectrum and electrochemical signature (inset) of the four stable forms: Co (L
c
)
(0,0), Co (L
CN + 0.1 M TBAP, UV-vis spectroscopy: l = 1 cm, CV: 0.1 V s , and WE = glassy carbon; from (0,0) to (1,0) irradiation at l 4 630 nm,
00 W, 273 K, t = 5 min; from (1,0) to (1,1) electrolysis at +0.2 V; from (1,1) to (0,1) thermal relaxation at 318 K overnight; from (0,1) to (0,0) electrolysis at
o
)
(1,0), Co (L
o
)
c 2
(1,1), and Co (L )
ꢀ1
(0,1), 1 mM in CH
3
5
ꢀ
1 2
0.3 V. Above: logic circuit combining NOR and AND gates; input I : irradiation (‘‘1’’: open-ring form and ‘‘0’’: closed-ring form); input I : electrolysis
II
III
(
‘‘0’’: Co form and ‘‘1’’: Co form); output: ratio of absorbance at 337 and 311 nm (‘‘0’’: ratio 4 1.1; ‘‘1’’: ratio o 1.1).
II
4+
2
initial state Co (L
c
)
2
(state ‘‘0,0’’ in Fig. 3) as judged from the is designated as input I . The output is the ratio of absorbance at 337
comparison of the spectroscopic and electrochemical features and 311 nm, A337/A311. If this ratio is higher than 1.1, the output is
of the final and initial solution. The logic circuit with NOR and considered as ‘‘1’’, otherwise it is considered as ‘‘0’’. Indeed, among
II
4+
III
AND gates of Fig. 3 was constructed with the four distinct forms the four distinct states derived from Co (L ) , the Co redox state
c 2
II
4+
derived from Co (L ) , utilizing irradiation and electrolysis as or the DHP form is characterized by an enhanced absorption band
c 2
inputs. The irradiation which inter-converts the closed-ring (state at ca. 330 nm due to the p–p* transition band of a tolyl-terpy
‘
‘0’’) and the open-ring (state ‘‘1’’) forms is designated as input I
1
.
0 4
coordinating to a trivalent cation or to the S –S transition band
The electrolysis which allows us to control the redox-state of the involving the DHP core. The combination of both inputs with the
II
III
cobalt centre (states ‘‘0’’ and ‘‘1’’ for the Co and Co forms, resp.) above-defined output leads to the truth table shown in Fig. 3.
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Conclusions
6 (a) D. Roldan, V. Kaliginedi, S. Cobo, V. Kolivoska, C. Bucher,
W. Hong, G. Royal and T. Wandlowski, J. Am. Chem. Soc.,
We have reported the synthesis and the characterization of a
new terpyridine ligand functionalized with a dimethyldihydro-
pyrene subunit through an electron withdrawing pyridinium
linker. Complexation of this ligand with Co(II) does not affect
significantly the kinetics and the efficiency of the ring-opening
photo-process undergone by the photochromic core under
visible irradiation. By combining the two distinguishable forms
of the photochromic core which can be addressed photo-
chemically and the two distinct redox states of the complexed
metal centre which can be addressed electrochemically, it is
shown that it is possible to reach a quadri-state switch from
which a molecular logic circuit with NOR and AND gates can be
constructed.
2
2
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4
1
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