Light-controlled real time information transmitting systems based on nanosecond thermally-isomerising amino-azopyridinium salts

Article abstract of DOI:10.1039/c2cc17782j

Aminoazopyridines are valuable molecules for stable information transmitting systems as well as for light-controlled optical oscillators. Amino-substituted azopyridinium methyl iodide salts transmit optical information within the time scale of nanoseconds, and moreover, show oscillation frequencies up to 1 MHz at room temperature.

Full text of DOI:10.1039/c2cc17782j

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COMMUNICATION
Light-controlled real time information transmitting systems based on
nanosecond thermally-isomerising amino-azopyridinium salts
a
b
a
´
Jaume Garcia-Amoros, Santi Nonell and Dolores Velasco*
Received 12th December 2011, Accepted 30th January 2012
DOI: 10.1039/c2cc17782j
Aminoazopyridines are valuable molecules for stable information
transmitting systems as well as for light-controlled optical oscillators.
Amino-substituted azopyridinium methyl iodide salts transmit optical
information within the time scale of nanoseconds, and moreover,
show oscillation frequencies up to 1 MHz at room temperature.
with the capability to establish an azo-hydrazone tautomeric
equilibrium, have been reported last year as the fastest known
thermally-isomerising azo-dyes.11 These azoderivatives exhibited
relaxation times for their thermal back reaction in the micro-
second time scale.
Here we report on the thermal cis-to-trans isomerisation
kinetics of several push–pull amino-substituted azopyridinium
methyl iodide salts that exhibit thermal isomerisation kinetics
down to the nanosecond time scale at room temperature (Fig. 1).
These azoderivatives are able to reach oscillation frequencies of
their optical properties up to the MHz scale, which are comparable
to the AM radio waves (600 kHz to 1.6 MHz) and the ultrasounds
(1–20 MHz), and moreover, they are 103-fold higher than the
fastest systems reported previously. An additional and noteworthy
benefit is their high solubility in water, an environmentally friendly
solvent. This is an interesting feature for both biological and
medical applications such as photochromic ion channel blockers12
or the photo-control of neurotransmitters in the central nervous
system,13,14 which both operate in aqueous media.
The development of photo-actuators based on materials that
reversibly change some of their properties, e.g. mechanical,
magnetical, electrical, optical, etc., in response to external stimuli
is of great interest within materials science. A very attracting
possibility comes from light-responsive materials, which allow
remote operation without the need for direct contact to the
actuator. Specifically, light-driven optical oscillators are photo-
active materials that modify their optical properties periodically
and extremely fast upon exposure to light of the appropriate
wavelength. Oscillating materials have attracted a great deal of
attention over the last few years due to their potential application
in micropumps and autonomous valves that simulate the heart
beating, photo-active polymers that mimic the cilia movement,
artificial muscles for robotics, real-time optical information
processors, molecular rotary motors, photo-switchable optical
reflectors, among others.1–10
Azobenzene, a photochromic system, has been widely used
for obtaining light-driven actuators since it exhibits a totally
clean and reversible photo-isomerisation process between its
trans and cis isomers. Moreover, cis-to-trans conversion takes
place also thermally in the dark. For azobenzene-based ultrafast
optical switches to be used in real-time information transmitting
systems, it is essential that the return to the thermodynamically
stable trans form in the dark elapses very quickly, within the
sub-microsecond time scale. Push–pull derivatives are promising
chromophores for this purpose since they are endowed with a
very fast thermal cis-to-trans isomerisation kinetics at room
temperature.
Among all azobenzenes, hydroxy-substituted methyl azopyri-
dinium salts, which combine a strong push–pull configuration
a
`
`
Grup de Materials Organics, Institut de Nanociencia i
Nanotecnologia (IN2UB), Departament de Quı´mica Orga`nica,
´
`
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
b Grup d’Enginyeria Molecular, IQS School of Engineering,
´
Universitat Ramon Llull, via Augusta 390, E-08019, Barcelona,
Spain
Fig. 1 Chemical structure of azoderivatives 1–10.
Chem. Commun., 2012, 48, 3421–3423 3421
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The kinetics of the thermal cis-to-trans isomerisation process of
the different azoderivatives was studied by means of the nano-
second laser flash-photolysis technique. A population of cis isomers
was created by pulsed-laser irradiation of the trans isomer at
355 or 532 nm employing a Continuum Surelite I-10 Q-switched
Nd-YAG laser (5 ns pulse width, ca. 10 mJ per pulse). The
concomitant absorbance changes were monitored at 901 by a
white-light analysing beam produced by a Xe lamp (PTI, 75 W)
in combination with a dual-grating monochromator (PTI 101)
coupled to a Hamamatsu R928 photomultiplier for detection.
The relaxation time of the cis isomers, t (t = 1/k), was
determined by fitting a monoexponential function to the data.
The maximum oscillation frequency of the different azo-dyes,
which is a direct measurement of their information transmitting
capability, was determined as nmax = 1/(3t).
4-(4-Hydroxyphenylazo)pyridine (1) shows a fast relaxation
time of 14 ms for its thermal isomerisation in ethanol at 298 K
(Table 1). This fact is indicative that its thermal cis-to-trans
isomerisation process in the dark takes place through a rotation
around the N–N bond (rotational mechanism). The replace-
ment of the phenol group in compound 1 by an aniline
(compound 2) causes a slight increase of the relaxation time
up to 24 ms (Table 1 and Fig. 2a). This differential behaviour
between azo-dyes 1 and 2 arises from the fact that although the
amino function is a better electron-donating group than the
hydroxyl one, azophenols establish the azo-hydrazone tautomeric
equilibrium that facilitate the rotation around the N–N bond.15,16
This was clearly evidenced when azophenol 1 was alkylated to
obtain 4-(4-(5-hexenyloxy)phenylazo)pyridine (3), which exhibited
a slow thermal relaxation kinetics beyond the second time scale
since its capability to establish the tautomeric equilibrium was
cancelled. This variation in the isomerisation kinetics was not
observed for the N,N-dimethylamino-substituted azocompounds
4 and 5, with thermal relaxation times of 19 ms and 27 ms in
ethanol (Fig. 2b), respectively, comparable with that obtained
for its non-alkylated counterpart 2 (24 ms, Table 1). As a whole,
all the compounds 1–5 exhibited low oscillation frequencies of
their optical properties ranging them from 1 to 54 Hz.
In order to diminish the thermal relaxation time of the azo-
chromophore beyond the millisecond time scale, the pyridinic
nitrogen of azo-dyes 1–5 was methylated to obtain their
corresponding methyl iodide azopyridinium salts (compounds
6–10) thereby creating a strong push–pull system.w The alkoxylated
azopyridinium 8 exhibited a 1000-fold faster thermal isomerisation
process (2.8 ms) in comparison with its non-methylated counter-
part 3 (t 4 1 s in ethanol at 298 K). In addition, the hydroxy-
substituted azopyridinium salt 6 exhibited a relaxation time of
150 ms, which is 100-fold faster than that registered for the
non-ionic hydroxyazopyridine 1 (14 ms, Table 1). The kinetic
results arise from the strong electron-withdrawing character of
the pyridinium salt, which induces a strong push–pull configuration
in the azo-system that facilitates the rotation around the N–N bond
of the cis isomer to recover the most stable trans form.
Extending this concept further, we hypothesized that combining
the strong electron-withdrawing character of the pyridinium salt
with the high electron-donating behaviour of the amino group
which, moreover, is capable to establish an amino-imino
tautomeric equilibrium,17,18 we would obtain very short thermal
relaxation times, perhaps under a microsecond. All the amino-
and N,N-dimethylamino-substituted azopyridinium salts 7, 9
and 10 exhibited very fast thermal relaxation kinetics in ethanol
at 298 K ranging from 281 to 368 ns (Table 1 and Fig. 2c and d).
The relaxation time of these azoderivatives was up to 104-times
slower in the non-polar solvent 1,2-dimethoxyethane (DME),
thereby registering relaxation times for their thermal isomerisa-
tion process ranging from 240 ms to 10 ms. To the best of our
knowledge, azo-dyes 7, 9 and 10 are the fastest thermally-
isomerising azo-dyes reported heretofore in the literature.
Table 1 Wavelength of maximum absorption of the trans form in ethanol, relaxation time for the thermal cis-to-trans isomerisation and
oscillation frequency for azocompounds 1–10 in different solvents at 298 K
Non-ionic azopyridines
Ionic azopyridines
lEtOH
tEtOH
nEtOH
lEtOH
tEtOH
nEtOH
twater
nwater
tACN
nACN
tDME
nDME
a
a
a
a
a
a
1
2
3
4
5
363 nm
420 nm
358 nm
444 nm
433 nm
14 ms
24 ms
41 s
19 ms
27 ms
24 Hz
14 Hz
o1 Hz
53 Hz
12 Hz
6
7
8
9
10
413 nm
530 nm
415 nm
552 nm
560 nm
150 ms
281 ns
2.8 ms
329 ns
368 ns
2.2 kHz
1.2 MHz
120 Hz
1.0 MHz
0.9 MHz
—
—
1.2 MHz
44 Hz
—
—
1.1 MHz
79 Hz
—
248 ms
—
—
1.3 kHz
287 ns
7.5 ms
310 ns
302 ns
316 ns
4.2 ms
333 ns
305 ns
a
a
—
175 Hz
34 Hz
1.1 MHz
1.1 MHz
1.0 MHz
1.1 MHz
1.9 ms
9.7 ms
a
Not determined.
Fig. 2 Transient absorption change photo-induced by laser pulse irradiation (lirradiation = 355 nm for 2 and 4 and 532 nm for 7 and 9) of azo-dyes
2 (a), 4 (b), 7 (c) and 9 (d) in ethanol at 298 K ([AZO] = 20 mM, lobs = 375 nm for 2 and 4 and 545 nm for 7 and 9).
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3422 Chem. Commun., 2012, 48, 3421–3423
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Fig. 3 Oscillation of the optical density generated by green light irradiation (lirradiation = 532 nm) (a) and photostability after 2500 laser pulses (b)
of an ethanol solution of azo-dye 10 at 298 K ([AZO] = 20 mM, lobs = 545 nm).
Taking into account the very fast thermal isomerisation
kinetics exhibited by the amino-substituted azopyridinium
salts 7, 9 and 10, they are the best candidates thus far for
light-driven real time information processing and optical
oscillating systems. Fig. 3(a) shows the oscillation of the optical
density of azo-dye 10 in ethanol solution at room temperature
with the time. The maximum oscillation frequency in the optical
properties for azocompounds 7, 9 and 10 ranges from 0.9 MHz
to 1.2 MHz at 298 K (Table 1).
atmosphere, and then, CH3I (3 cm3) was added. The solution was
stirred at room temperature for 2 hours. After, the product was
precipitated by adding hexanes, isolated by vacuum filtration and
dried. 7 was obtained as a reddish crystalline solid (160 mg, 63%).
1H NMR (400 MHz, d6-acetone) d: 9.05 (2H, d, arH, J = 7.1 Hz), 8.24
(2H, d, arH, J = 7.1 Hz), 7.93 (2H, d, arH, J = 9.0 Hz), 6.93 (2H, d,
arH, J = 9.0 Hz), 4.58 (3H, s, CH3) ppm. FT-IR (ATR) n: 3343 and
3272 (N–H st), 3170 (QC–H st), 1600 (CQC st), 1474 (NQN st)
+
cmÀ1. HR-MS (ESI-MS): m/z Calcd. for C12H13N4 [M+] 213.1135;
found 213.1140.
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Both the repeatability and photostability of our prototypes
were checked by submitting them to several pulsed green light
(532 nm)–dark cycles. Fig. 3(b) evidences the high photo-
stability exhibited by the amino-substituted azopyridinium
salt 10 in ethanol solution at 298 K thereby observing that after
2500 cycles neither the absorbance change nor the relaxation time
of the molecular oscillator was altered by the continuous work of
the system. A similar behavior was registered for azo-dyes 7, 9
and 10 in the different solvents analyzed.
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In summary, the bistable nature, short excited life time
(down to 281 ns at 298 K) and high photostability of the
aminoazopyridinium salts reported herein make them ideal
photo-active molecules for real time information transmitting
systems and optical oscillators. In fact, the information trans-
mission capability of these azopyridinium salts elapse 105-fold
faster than that by means of the neuronal synapses (0.5 ms to
10 ms). The oscillation frequency of the systems reported herein
is around 1 MHz at room temperature and they show no fatigue
upon continuous work. On the other hand, their high solubility
in aqueous media makes them valuable chromophores for
photo-controlled biological and medical applications.
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Financial support for this research was obtained from the
Ministerio de Ciencia e Innovacion (Spain) through grant no.
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Notes and references
w Synthesis of 4-[(4-aminophenyl)azo]-1-methylpyridinium iodide (7):
2 (150 mg) was dissolved in anhydrous CH2Cl2 (10 cm3) under an inert
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3421–3423 3423