Photo-controllable electronic switches based on azopyridine derivatives

Article abstract of DOI:10.1039/c2cc34457b

Stable photo-controllable electronic switches based on new light-sensitive azopyridines are reported herein. Such systems produce notable variations in the cathodic current density on working at low reduction potentials when UV light falls on them. The ap

Full text of DOI:10.1039/c2cc34457b

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Photo-controllable electronic switches based on azopyridine derivativesw
a
b
b
a
´ ´ ´
Jaume Garcia-Amoros, Elvira Gomez, Elisa Valles and Dolores Velasco*
Received 21st June 2012, Accepted 18th July 2012
DOI: 10.1039/c2cc34457b
Stable photo-controllable electronic switches based on new light-
sensitive azopyridines are reported herein. Such systems produce
notable variations in the cathodic current density on working at low
reduction potentials when UV light falls on them. The appropriate
design of the azopyridine chromophore allows modulating the
response time of the final opto-electronic switch.
are valuable target molecules for this aim since they exhibit
completely reversible isomerisation and reduction processes.
Indeed, this type of azo derivatives has been already used
recently for obtaining fast light-driven optical switches and
photo-oscillators.18,19 However, the ability of the azopyridine
chromophore as light-induced electro-switches remains still
unexplored. As far as we know, there is only one previous
study which deals with the photo-electrochemical properties of
azobenzenes.20
Molecular switching materials have attracted much attention
in the last few years because of the very rapid development of
modern technology. This phenomenon arises from the great
usefulness of such materials in data-elaboration, -storage
and -communication elements for many devices. Specifically,
molecular photo-switches are materials than operate reversibly
between two different states just by applying light.1–3
Here we report on the molecular design and opto-electronic
switching of several azopyridine derivatives (Fig. 1). Such
compounds induce notable changes in the cathodic current
density up to 7 mA cmꢀ2 when UV light falls on them with the
added benefit that they work at low voltages down to ꢀ0.67 V
referenced versus the Ag/AgCl/KCl (3 M) electrode. The
rational molecular design of the azopyridine core allows a
proper tuning of the response time of the light-controlled
electro-switch which will determine its final applicability.
Light is a very convenient trigger for switching purposes
since it is a cheap and environmentally friendly energy source
which can be wireless manipulated both locally and quickly.
As a consequence of the molecular variations produced upon
illumination of the material, a significant modification of some
of its properties is induced. Modifying the conductance of molecules
by applying light, that is, opto-electronic switching, is nowadays
a very active area of research due to its high impact on nano-
electronics.4–8 Moreover, opto-electronic switches are also of great
interest within biology since they are able to mimic photoreceptor
cells, which convert the incoming light into electrical impulses that
are transmitted to the brain via nerve fibers.9
Azobenzenes have been widely used for obtaining a great
variety of light-controlled materials such as artificial muscles,10–13
logic gates,14,15 the photo-control of ion channel blockers and
enzymes activity in living organisms,16,17 among others. Azo-dyes
isomerise to their metastable cis form upon UV-light irradiation;
the initial state is restored by irradiation with visible light or by
simply thermal relaxation.
Fig. 1 Chemical structure of azopyridines 1–4.
For opto-electronic switching, the design and preparation of
new molecules that exhibit both a chromogenic group and an
electro-active function is required. In this way, azopyridines
a
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`
Grup de Materials Organics, Institut de Nanociencia i
Nanotecnologia (IN UB), Departament de Quımica Organica,
2
´
`
´
Universitat de Barcelona, Martı i Franques 1- E-08028, Barcelona,
Spain. E-mail: dvelasco@ub.edu; Fax: +34 93 339 78 78;
`
Tel: +34 93 403 92 60
Electrodep, Institut de Nanociencia i Nanotecnologia (IN UB),
b
2
`
Departament de Quımica Fısica, Universitat de Barcelona,
´
Martı i Franques 1- E-08028, Barcelona, Spain
´
´
`
Fig. 2 First reduction process (O1/R1) for the trans isomer of
azopyridines 1–4 in anhydrous DMF + 0.1 M TBAP at T = 315 K
([AZO] = 1 mM, v = 100 mV sꢀ1).
w Electronic supplementary information (ESI) available: Experimental
procedures, full spectroscopic data for the novel azo-dyes 3 and 4 and
additional voltammetric experiments. See DOI: 10.1039/c2cc34457b
c
9080 Chem. Commun., 2012, 48, 9080–9082
This journal is The Royal Society of Chemistry 2012

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The voltammetric study of azopyridines 1–4 allows determining
the voltage of operation for each opto-electronic switch, E(O1/R1).
All voltammetric experiments were performed in anhydrous
DMF + 0.1 M of tetrabutylammonium perchlorate (TBAP)
as a supporting electrolyte at 315 K (Fig. 2).
a p-deficient pyridine ring produced a clear decrease in the first
reduction potential registering values of ꢀ1.28 V and ꢀ1.14 V
for the ortho- (1) and para-substituted (2) azopyridines,
respectively (Table 1). This first reversible reduction process
corresponds to the gain of one electron affording radical
anionic species. The reduction potential was notably positively-
shifted when the strong electron-withdrawing nitro group was
introduced at a conjugated position of the azobenzene core
since it provides a more efficient electronic delocalisation of
the lone electron. Accordingly, the first reduction process took
place at ꢀ0.68 V and ꢀ0.72 V for the nitro-substituted
azopyridines, 3 and 4, respectively (Table 1). Both 3 and 4
showed lower first reduction potentials than their non-pyridine-
containing counterpart, 4-(5-hexenyloxy)-40-nitroazobenzene
(ꢀ0.85 V, see Fig. S1 (ESIw)).
Parent trans-4-(5-hexenyloxy)azobenzene shows its first
reversible reduction (O1/R1) at ꢀ1.42 V (Fig. S1, ESIw) similarly
to trans-azobenzene (ꢀ1.33 V). This result evidences that the
introduction of alkoxyl chains in the azobenzene core does not
overly disturb the electrochemical behaviour of the azo molecule.
The substitution of one of the benzene rings of the azo-dye by
Table 1 Cathodic reduction potentials of the first, E(O1/R1), and second
reduction processes, E(O2/R2), for the trans isomer of azopyridines 1–4.
Relative, Dj, and absolute, Djmax, current density change at E(O1/R1) upon
irradiation with UV-light at T = 315 K ([AZO] = 1 mM, anhydrous
DMF + 0.1 M TBAP, v = 100 mV sꢀ1
)
Enlarging the reduction scan to more negative potentials, a
second irreversible process (R2) was detected for azopyridines
1–2 at ꢀ2.09 V and ꢀ1.95 V, respectively (Table 1 and Fig. S1,
ESIw), associated with the gain of a second electron forming
the dianionic species. Contrarily, this reduction process was
reversible (O2/R2) for the nitro-substituted azopyridines, 3
and 4. Furthermore, it took place at a more positive potential,
ꢀ1.21 V and ꢀ1.39 V, respectively. The differential electro-
chemical behaviour indicates a higher stability of the reduced
species formed from the nitro-substituted azo-dyes, 3 and 4,
due to the efficient resonance established between the azo
function and the nitro electron-withdrawing group. The scan
to positive potentials evidenced that azo compounds 1–4 are
stable to oxidation processes.
E(O1/R1)/V
E(O2/R2)/V
Dj/%
Djmax/mA cmꢀ2
1
2
3
4
ꢀ1.28
ꢀ1.14
ꢀ0.68
ꢀ0.72
ꢀ2.09
ꢀ1.95
ꢀ1.21
ꢀ1.39
1.53
3.97
4.16
3.02
2.43
6.81
4.86
3.24
Photo-voltammetric experiments showed that the isothermal
irradiation of the electrochemical bath with UV light at 315 K
produced a clear decrease in the cathodic current density (see both
Fig. 3 and 4a). This feature can be associated with a lower diffusion
coefficient for the cis form of the electro-active species.
The dropping of the cathodic current density due to the
trans-to-cis photo-isomerisation of the azo-dye, that is, the
absolute efficiency of the opto-electronic switch, Djmax, was
quantified by subtracting the cathodic current density value
at a constant reduction potential equal to E(O1/R1) before
UV-irradiation, j0, from the one once the photo-stationary
state has been reached, jirrad (Djmax = jirrad ꢀ j0). The relative
photo-electronic efficiency, Dj, which is independent of the
Fig. 3 Photo-voltammetric experiment for azopyridine 2 in anhydrous
DMF + 0.1 M TBAP at T = 315 K ꢁ 0.1 K ([AZO] = 1 mM, v =
100 mV sꢀ1). Inset: evolution of the cathodic current density at E(O1/R1) =
ꢀ1.14 V with the time upon irradiation with UV light (320 o l o 390 nm,
high pressure mercury lamp 500 W; luminous emittance of the irradiation
set-up = 1400 lux) and in the dark isothermally.
Fig. 4 Photo-voltammetric experiment (a), evolution of the cathodic current density at E(O1/R1) = ꢀ0.72 V with the time: increase in the
cathodic current density by keeping in the dark a previously UV-irradiated solution (first part of the curve) and decrease in the cathodic current
density upon UV-light irradiation of the sample (second part of the curve) (b) and cyclic photo-voltammetric experiment (c) for the nitro-
substituted azopyridine 4 in anhydrous DMF + 0.1 M TBAP at T = 315 K ([4] = 1 mM, v = 100 mV sꢀ1, UV irradiation was performed with a
high pressure mercury lamp 500 W; luminous emittance of the irradiation set-up = 1400 lux).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9080–9082 9081

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scan rate but concentration-dependent,21 was determined to be
Dj = |(jirrad ꢀ j0)/j0| ꢂ 100.
Indeed, both photo- and electrochemical properties of the
reported opto-electronic switches remained unaltered after
more than 20 working cycles.
All azopyridines showed notable changes in the cathodic
current density upon UV-irradiation thereby showing absolute
In summary, azopyridine derivatives are stable photo- and
electro-active molecules thereby being valuable materials to be
applied further in micro- and nano-light-controllable electronic
switches. These molecules induce notable changes in the current
density up to 7 mA cmꢀ2 when they are irradiated with UV-light.
Specially remarkable are those electro-switches based on nitro-
substituted azopyridines 3 and 4 which, besides their low voltage
of operation (down to ꢀ670 mV versus the Ag/AgCl/KCl (3 M)
electrode), they are able to switch back and forth between both
electric states within the minute time scale.
efficiencies ranging from 2.0 mA cmꢀ2 to 5.4 mA cmꢀ2
.
According to both their absolute and relative efficiency, both
azopyridines 2 and 3 were proved as the most appropriate
chromophores to be applied in light-driven electronic switches
since they exhibited the highest Dj and Djmax values (Dj = 3.97%
and Dj = 4.16% and Djmax = 6.81 mA cmꢀ2 and Djmax
=
4.86 mA cmꢀ2 for 2 and 3, respectively).
Besides both the absolute and relative efficiency of the opto-
electronic switch, the response time is also a crucial parameter
to consider in its overall performance and usefulness. Upon
turning off the irradiation, the initial value of the cathodic
current density was restored in all the cases analysed (Fig. 3
(inset) and Fig. 4b).
Financial support for this research was obtained from the
Ministerio de Ciencia e Innovacion (Spain) through grant no.
´
CTQ2009-13797.
Parent azopyridines, 1 and 2, exhibited slow thermal back
reactions needing several hours to recover their stable trans form.
Hence, the system operates between two very stable states. This
feature can be further exploited for the fabrication of light-
controlled electronic memories. The return to the initial state was
dramatically faster for the nitro-substituted azopyridines, 3 and 4,
due to their push–pull electronic distribution. In this case, the opto-
electronic switch shifts in a quick fashion between both states
thereby registering relaxation times within a few minutes.
Time-resolved photo-voltammetric experiments were performed
with both nitro-substituted azopyridines 3 and 4 in order to
determine the characteristic times of the corresponding light-driven
electronic system. Fig. 4b shows the evolution of the cathodic
current density in a photo-voltammetric experiment carried
out with the nitro-substituted azopyridine 4.
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The variation of the cathodic current density in the dark was
fitted to eqn (1), whereas the current density change under
UV-irradiation was described by eqn (2):
jt ꢀ j0 = Djmax ꢂ exp(ꢀktht)
jt ꢀ j0 = Djmax ꢂ [1 ꢀ exp(ꢀkirradt)]
(1)
(2)
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c
9082 Chem. Commun., 2012, 48, 9080–9082
This journal is The Royal Society of Chemistry 2012