Photo-driven optical oscillators in the kHz range based on push-pull hydroxyazopyridines

Article abstract of DOI:10.1039/c1cc10302d

Push-pull azophenols are valuable target molecules for stable photo-driven optical oscillators. Hydroxyazopyridinium methyl iodide salts show oscillation frequencies up to 10 kHz with no signs of fatigue upon continuous work. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1cc10302d

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COMMUNICATION
Photo-driven optical oscillators in the kHz range based on
push–pull hydroxyazopyridines
a
b
a
´
Jaume Garcia-Amoros, Santi Nonell and Dolores Velasco*
Received 17th January 2011, Accepted 10th February 2011
DOI: 10.1039/c1cc10302d
Push–pull azophenols are valuable target molecules for stable
photo-driven optical oscillators. Hydroxyazopyridinium methyl
iodide salts show oscillation frequencies up to 10 kHz with no
signs of fatigue upon continuous work.
However, they are still too slow for their use as photo-driven
optical oscillators. We hypothesized that the introduction of
strong electron-withdrawing groups in the azobenzene core
resulting in a push–pull electronic distribution could decrease
considerably the thermal cis-to-trans relaxation time in these
azo-dyes and would therefore further increase the oscillation
frequency of the optical oscillator. Hence, new substituted
azophenols with powerful electron-withdrawing groups were
designed for identifying fast photo-driven oscillators.
Photo-driven oscillators modify dramatically some of their
properties, e.g. mechanical, optical, etc., quickly and in a
periodic fashion upon illumination. Such devices are currently
of considerable interest in materials science for their prospective
use in, e.g., nanomechanical devices.1,2
Here we report on the synthesis and the thermal cis-to-trans
isomerisation kinetics of several push–pull azophenols, which
show relaxation times ranging from 27 ms to 33 ms at room
temperature. The very fast relaxation kinetics exhibited by
these azo-dyes makes them ideal candidates for applications as
light-controlled photo-oscillators. The oscillation frequency of
their optical properties spans the range from 10 Hz to 10 kHz,
which is comparable to the radio waves (1 Hz to 105 Hz) and
is also more than 100-fold higher than the hummingbird
wingbeat (20–80 Hz). Moreover, the oscillating behaviour of
the systems reported herein shows no fatigue over several
working cycles. The bistable nature and short excited state
lifetime of these azoderivatives are challenging features for
their further incorporation as guests in photo-active liquid-
crystalline materials9,10 as well as for obtaining fast photo-
controllable polymers and elastomers.11
Azobenzenes are very useful chromophores for designing
light-controlled materials because of their reversible photo-
isomerisation between their two trans and cis isomers of
different stabilities. Moreover, cis-to-trans back conversion
takes place also thermally in the dark.
Among all azobenzenes, 4-N,N-dimethylamino-40-nitro-
azobenzene (DNAB) is the fastest known azo-dye, with relaxation
times for thermal isomerisation ranging from 21 ms to 122 ms
in alcoholic solutions.3–5
Hydroxyazobenzenes on the other hand show comparable
thermal isomerisation kinetics (e.g., 205 ms for 1, Table 1).6–8
Table 1 Relaxation time, t, for the thermal cis-to-trans isomerisation
of the azo-dyes 1–11 in ethanol, effect of 1 eq. phenol added, and
maximum oscillation frequencies, nmax, for the corresponding optical
oscillators at 298 K
The kinetics of the thermal cis-to-trans isomerisation process
of the different azophenols (Fig. 1)w were studied by means of
the laser flash-photolysis technique. cis-Azobenzenes were
generated by UV-photolysis of the corresponding trans isomer
using a Q-switched Nd-YAG laser (355 nm, 5 ns pulse-width,
1–10 mJ per pulse) and the recovery of the trans isomer was
monitored by means of a CW Xe lamp. The relaxation time of
the cis isomers, t (t = 1/k), was determined by fitting a
tEtOH
n
EtOH/Hz
tphenol
nphenol/Hz
1
2
3
4
5
6
7
8
205 ms
27 ms
4.6 ms
399 ms
50 ms
49 ms
2.9 ms
14 ms
1.3 ms
33 ms
1.63
12.3
72.5
0.83
6.67
6.80
115
23.8
256
10 100
2220
204 ms
—
—
—
—
11 ms
1.1 ms
1.4 ms
644 ms
—
1.63
—
—
—
—
30.3
303
238
518
—
9
10
11
150 ms
—
—
a
`
`
Grup de Materials Organics, Institut de Nanociencia i
Nanotecnologia, Departament de Quı´mica Orga`nica,
´
Universitat de Barcelona, Martı i Franques 1-11, 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, Institut Quımic de Sarria,
Universitat Ramon Llull, via Augusta 390, E-08019,
`
´
Barcelona, Spain
Fig. 1 Chemical structure of azoderivatives 1–11.
c
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character of the pyridinic nitrogen in the hydroxyazopyridine
dye, the introduction of additional nitro groups in the
convenient positions was carried out (compounds 7 and 9).
As anticipated, these two azoderivatives showed relaxation
times of 2.9 ms and 1.3 ms in ethanol, respectively, which
decreased further down to 1.1 ms and 644 ms in the presence of
phenol (Table 1). The corresponding oscillation frequencies
ranged from 30 Hz to 520 Hz. This demonstrates that the
generation of push–pull systems by the establishment of
hydrogen bonds between phenol groups and the pyridinic
nitrogen, with the subsequent generation of a deficient electron
density in the later, is a versatile option towards the generation
of fast photo-driven oscillators.
Fig. 2 Transient absorptions generated by UV irradiation (l = 355 nm)
of azo-dyes 1–3 in ethanol at 298 K ([AZO] = 20 mM, lobs = 370 nm).
Extending the concept even further, we reasoned that
azopyridines with a permanent positively charged nitrogen
should increase the kinetics and the stability of the final photo-
driven oscillator. For this purpose, methylation of the pyridine
nitrogen of azo-dyes 6 and 8 was done to afford the corres-
ponding pyridinium methyl iodide salts 10 and 11. 10 and 11
presented relaxation times of 33 ms and 150 ms at room
temperature in ethanol, respectively (Fig. 3 and Table 1). To
the best of our knowledge, these are the fastest thermally
isomerising azo-dyes heretofore reported in the literature.
Due to their very fast thermal isomerisation rate, 10 and 11
are the best candidates to be applied as light-controlled optical
monoexponential function to the data. The maximum oscilla-
tion frequency of the photo-driven oscillator, nmax, is defined
as nmax = 1/(3t). Table 1 collects both t and nmax values for all
the photo-driven oscillators studied.
The parent azophenol 1 showed a relaxation time of 205 ms
for its thermal isomerisation in ethanol at 298 K. In a first
attempt to decrease the relaxation time of the azo-dye, cyano-
and nitro- electron-withdrawing groups were introduced at
position 40 of the azophenol (compounds 2 and 3). The
relaxation time of the 40-cyanoazophenol 2 was 10-fold lower
than that of the parent azophenol 1 (27 ms vs. 205 ms,
Table 1). The relaxation time decreased even further for the
40-nitroazophenol 3, as expected, showing a t value of 4.6 ms
(Fig. 2). Thus, on going from the azophenol 1 to its push–pull
nitro counterpart 3, a clear increase of the oscillation frequency
from 1.6 to 72.5 Hz was observed.
The placement of the hydroxyl group at position 2 was
also considered due to the possible acceleration of the iso-
merisation kinetics via intramolecular hydrogen bonding
(compounds 4 and 5). However, a notable increase of the
relaxation time was detected for both azo-dyes, showing
t values of 399 ms and 50 ms in ethanol at 298 K, substantially
larger than those of 1 and 3, respectively.
The results observed for the nitro-substituted compound 3
encouraged us to introduce stronger electron-withdrawing
groups at position 40 of the azophenol core. For this purpose,
one of the benzene rings of the azo-dye was substituted
by a pyridine ring as hydrogen bonding between ethanol and
the pyridine moiety should increase the push–pull electronic
distribution of the azo-dye (compounds 6 and 8). As shown in
Table 1, the relaxation time of these two azo compounds was
49 ms and 14 ms in ethanol, respectively, substantially shorter
than that of the parent compound 1, but too large compared
to that of 3. In an attempt to accelerate the thermal isomeri-
sation kinetics phenol was added to the ethanol solution.
Phenol forms a stable hydrogen bond with the nitrogen atom
of pyridine. Indeed, addition of 1 eq. phenol increased the
isomerisation rate of azo-dyes 6 and 8 (Table 1) by a factor
of 4.5 and 10, respectively. The differential effect can be
associated to the difficulty of the pyridinic nitrogen to establish
a hydrogen bond with phenol at position 2 of the azobenzene
core due to steric hindrance factors.
Fig. 3 Transient absorptions generated by UV irradiation (l = 355 nm)
of azo-dyes 10 and 11 in ethanol at 298 K ([AZO] = 20 mM,
lobs = 420 nm).
Fig. 4 Oscillation of the optical density of an ethanol solution of
azo-dye 11 generated by UV light irradiation (l = 355 nm, 5 ns
pulse-width) at 298 K ([AZO] = 20 mm, lobs = 420 nm).
In order to increase further the strength of the push–pull
electronic distribution as well as the hydrogen-bond acceptor
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Chem. Commun., 2011, 47, 4022–4024 4023

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9: 1H NMR (400 MHz, d6-acetone): 9.23 (1H, s, arH), 8.96
(1H, d, arH, 3J = 5.3 Hz), 8.14 (1H, br s, -OH), 7.91 (2H, d,
arH, 3J = 8.9 Hz), 7.62 (1H, d, arH, 3J = 5.3 Hz), 7.08 (2H, d, arH,
3J = 8.9 Hz) ppm. HR-MS ESI, m/z: Calc. for C11H8N4O3: 244.0596;
found: 245.0676 (M+ + H).
oscillators. Fig. 4 shows the oscillation of the optical density of
azo-dye 11 with time. The oscillation frequencies of azo-dyes
10 and 11 in ethanol solution are of 10 kHz (10) and 2.2 kHz
(11), respectively, at room temperature (Table 1). It can be
also seen in Fig. 4 that the optical oscillators reported show
no fatigue after several cycles (less than 1% of the signal after
3 pulses with UV-light for all the azo-dyes).
10: 1H NMR (400 MHz, d6-acetone): 9.07 (1H, m), 8.68 (1H, m),
8.22 (1H, m), 8.12 (1H, m), 8.05 (2H, d, arH, 3J = 9.0 Hz), 7.06
(2H, d, arH, 3J = 9.0 Hz), 4.73 (3H, s, CH3-N) ppm. HR-MS ESI, m/z:
Calc. for C12H12N3O+: 214.0975; found: 214.0981 (M+).
In summary, stable photo-driven optical oscillators based
on push–pull azophenols have been reported. The most promising
oscillators are those based on hydroxyazopyridinium methyl
iodide salts, due to the strong push–pull electronic distribution
of the azo-dye which allows a fast thermal isomerisation
of the cis isomer within the time scale of microseconds. The
oscillation frequency of these devices ranges within 2 and
10 kHz at room temperature and they show no fatigue upon
continuous work.
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Notes and references
w Azocompounds 1–5 and 8 were prepared by coupling the diazonium
salts of aniline, 4-cyanoaniline, 4-nitroaniline or 4-aminopyridine with
phenol (1–3 and 8) or p-cresol (4 and 5) in basic media at 0–5 1C.12,13
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pyridine, 2-hydrazino-5-nitropyridine or 4-hydrazino-3-nitropyridine
and 1,4-benzoquinone in acidic media.10,14 2-Hydrazino-5-nitropyridine
and 4-hydrazino-3-nitropyridine were obtained from 2-chloro-5-nitro-
pyridine and 4-chloro-3-nitropyridine, respectively, by means of an
SNAr reaction with N2H4ꢀH2O in 1,4-dioxane at room temperature
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respectively. First, both azo-dyes were acetylated by using AcCl and
Et3N in dry THF. Next, the methylation of their pyridinic nitrogen was
carried out using CH3I in THF at reflux for 1 h. Finally, the acetyl
group was cleaved with a catalytic amount of AcCl in MeOH to afford
the methyl iodide azopyridinium salts 10 and 11.16 The spectroscopic
characterization of the novel azo-dyes 9 and 10 is given below.
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4024 Chem. Commun., 2011, 47, 4022–4024
This journal is The Royal Society of Chemistry 2011