Fast isomerizing methyl iodide azopyridinium salts for molecular switches

Article abstract of DOI:10.1021/ol1013679

(Equation Presented). The usefulness of azopyridinium methyl iodide salts for designing new promising light-controlled molecular switches is presented. Large absorbance changes have been produced in the samples by irradiation with light at λ = 355 nm. The thermal recovery of the initial state took place completely within 130-450 ms, which is much faster than that reported previously for other push-pull azobenzene-doped nematic mixtures.

Full text of DOI:10.1021/ol1013679

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3514-3517
Fast Isomerizing Methyl Iodide
Azopyridinium Salts for Molecular
Switches
Jaume Garcia-Amoro´s,† Walter A. Massad,‡ Santi Nonell,‡ and Dolores Velasco*,†
Grup de Materials Orga`nics, Institut de Nanocie`ncia i Nanotecnologia,
Departament de Qu´ımica Orga`nica, Facultat de Qu´ımica, UniVersitat de Barcelona,
Mart´ı i Franque`s 1-11, E-08028 Barcelona, Spain, and Grup d’Enginyeria Molecular,
Institut Qu´ımic de Sarria`, UniVersitat Ramon Llull, Via Augusta 390,
E-08017 Barcelona, Spain
dVelasco@ub.edu
Received June 14, 2010
ABSTRACT
The usefulness of azopyridinium methyl iodide salts for designing new promising light-controlled molecular switches is presented. Large
absorbance changes have been produced in the samples by irradiation with light at λ ) 355 nm. The thermal recovery of the initial state took
place completely within 130-450 ms, which is much faster than that reported previously for other push-pull azobenzene-doped nematic
mixtures.
Liquid crystals (LCs) are very attractive materials because
they combine the fluidity from the liquid phase and the
molecular order typical of the solid state. The doping of
nematic liquid crystalline matrixes with photochromic mol-
ecules, such as azobenzenes, offers a cheap and clean
wireless control of the host mesophase properties, which
depend mainly on the molecular alignment of the me-
sogens.1,2 Irradiation of azobenzene-doped LC mixtures with
UV light at a constant temperature slightly below that of
the nematic-to-isotropic phase transition produces a huge
decrease of the nematic order parameter due to the presence
of the bent cis isomer in the mixture.3,4 As a consequence,
the optical properties of the whole sample suffer a dramatic
change due to a cooperative motion between the mesogen
molecules and those of the azo-dye (domino effect). Optical
switches benefit from this phenomenon and emerge as a new
type of promising light-controlled material.5
† Universitat de Barcelona.
(3) Sung, J.-H.; Hirano, S.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.;
‡ Universitat Ramon Llull.
Ikeda, T. Chem. Mater. 2002, 14, 385
(4) Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warner, M. Phys.
ReV. Lett. 2001, 87, 0155011
.
(1) (a) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.;
Shelley, M. Nat. Mater. 2004, 3, 307. (b) Palffy-Muhoray, P. Nat. Mater.
.
2009, 8, 614
.
(5) (a) Tong, X.; Pelletier, M.; Lasia, A.; Zhao, Y. Angew. Chem., Int.
Ed. 2008, 47, 3596. (b) Alam, Z.; Yoshioka, T.; Ogata, T.; Nonaka, T.;
Kurihara, S. Chem.sEur. J. 2007, 13, 2641. (c) Ma, J.; Li, Y.; White, T.;
Urbas, A.; Li, Q. Chem. Commun. 2010, 46, 3463. (d) Hrozhyk, U. A.;
Serak, S. V.; Tabiryan, N. V.; Hoke, L.; Steeves, D. M.; Kimball, B. R.
Opt. Express 2010, 18, 8697.
(2) (a) Yu, Y.; Ikeda, T. J. Photochem. Photobiol. C: ReV. 2004, 5,
247. (b) Barrett, C. J.; Mamiya, J.-I.; Yager, K. G.; Ikeda, T. Soft Matter
2007, 3, 1249. (c) Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol., A
2006, 182, 250. (d) Ikeda, T.; Mamiya, J.-I.; Yu, Y. Angew. Chem., Int.
Ed. 2007, 46, 506. (e) Ikeda, T. J. Mater. Chem. 2003, 13, 2037
.
10.1021/ol1013679  2010 American Chemical Society
Published on Web 07/14/2010

Scheme 1. Synthetic Route for Azocompounds 1-6
The response time of the photoswitch is a key feature in
their overall performance. Fast systems, working on the time
scale of milliseconds, have been reported previously by Ikeda
et al. using push-pull azoderivatives.6 Unfortunately, they
require temperatures substantially above 298 K, which limits
the usefulness of the final device.
The response time of the switch depends mainly on the
isomerization rate of the azo-dye. We report herein two new
azopyridinium methyl iodide salts with a very fast thermal
cis-to-trans relaxation process, on the time scale of a few
milliseconds at room temperature. Azopyridinium salts are
excellent photoactive guests able to induce great optical
changes in host nematic mesophases with a short response
time.
Azocompound 1 was obtained by the reaction between
2-hydrazinopyridine and 1,4-benzoquinone in acidic media.7
The synthesis of azocompound 2 was carried out Via the
coupling of the 4-aminopyridine diazonium salt with phenol
in basic media at 0-5 °C.8 The subsequent alkylation of
the free hydroxyl group of azocompounds 1 and 2 via the
Williamson and Mitsunobu methodologies with 1-bromo-
10-undecene and 5-hexen-1-ol yielded 3 and 4, respectively.
Further methylation of the pyridinic nitrogen atom of 3 and
4 with methyl iodide in anhydrous THF provided the methyl
iodide azopyridinium salts 5 and 6, respectively (Scheme
1).
The electronic spectra of the stable trans isomers of
azocompounds 5 and 6 exhibit an intense peak at 405 and
410 nm, respectively, which corresponds to the allowed
π-π* transition. It should be highlighted that this band is
red-shifted by 50-60 nm relatively to that of non-push-pull
azobenzenes at ca. 350 nm (Figure S1 in Supporting
Information). This notable red-shift is related to a strong
charge transfer from the alkoxy group to the positively
charged nitrogen atom.
The thermal cis-to-trans isomerization of azobenzenes 5
and 6 was studied by laser flash photolysis in two solvents:
ethanol and acetonitrile. Figure 1 shows the transient
bleaching of the 405-nm band of azocompound 5 upon
pulsed laser irradiation in ethanol (Continuum Surelite I-10
Q-switched Nd:YAG laser, 355 nm, 5 ns pulse width, 1-10
mJ per pulse). The relaxation time for cis-5 and cis-6 was
4.6 and 4.2 ms in acetonitrile and 570 µs and 2.8 ms in
ethanol, respectively (Table 1). It can be stated that the para-
Table 1. Wavelength of Maximum Absorption for the trans
Isomer, λmax, Relaxation Time at 298 K, τ, and Enthalpies and
Entropies of Activation for the Thermal cis-to-trans
Isomerization Process, ∆Hq and ∆Sq, of Azo-dyes 5 and 6 in
Ethanol and Acetonitrile ([AZO] ) 0.02 mM)
∆Hq
∆Sq
(J K-1 mol-1
)
solvent
λmax (nm) τ (ms) (kJ mol-1
)
5
6
ethanol
acetonitrile
ethanol
407
399
415
409
0.57
4.6
2.8
48 ( 2
54 ( 1
53 ( 1
53 ( 2
-22 ( 5
-20 ( 1
-19 ( 4
-22 ( 6
acetonitrile
4.2
substituted azo dye 5 is always fastser than the corresponding
ortho counterpart 6. However, no relation has been found
between the relaxation time and the solvent polarity.
The activation parameters (∆Hq and ∆Sq) for the back
reaction of azo-dyes 5 and 6 were determined in both solvents
(Table 1). The enthalpy of activation for 5 and 6 was ca. 50
kJ mol-1 lower than that registered for non-push-pull azo-
dyes,9 although the entropies of activation for both types of
azoderivatives were comparable (around -20 J K-1 mol-1).
Figure 1. Transient absorption generated by UV irradiation (λ )
355 nm) of 5 in ethanol at 298 K ([5] ) 0.02 mM, λobs ) 405 nm).
The continuous curve was determined by fitting of a monoexpo-
nential growth function to the data.
(6) (a) Okano, K.; Shishido, A.; Ikeda, T. Macromolecules 2006, 39,
145. (b) Tsutsumi, O.; Ikeda, T. Mol. Cryst. Liq. Cryst. 2001, 368, 411.
Org. Lett., Vol. 12, No. 15, 2010
3515

The activation parameters reveal that the transition state is
easily reached and the system does not experiment a strong
molecular reorganization on achieving it from the initial cis
state, thereby pointing to a rotational isomerization mecha-
nism.
Figure 3. Transient absorption generated by UV laser pulse
irradiation (λ ) 355 nm) of the nematic mixture 5-5CB at 306 K
(xazo ) 0.003, λobs ) 405 nm).
Figure 2. Mechanism proposed for the thermal cis-to-trans isomer-
neous processes occur: the trans-to-cis isomerization of the
azo-dye and the photoinduced nematic-to-isotropic phase
transition. The absorbance of the AZO-LC mixtures in the
isotropic phase is higher than in the nematic one. Thus, an
increase in the probe absorbance is observed when the
photoinduced phase transition takes place. When the irradia-
tion is ceased, the initial absorbance value of the probe is
recovered isothermally in the dark as a result of the thermal
back-isomerization of the azo-dye and also the restoring of
the nematic order of the host mesophase.
ization process for the azopyridinium methyl iodide salt 5.
Figure 2 shows the proposed mechanism for the thermal
back isomerization of cis-5 and cis-6. Electron transfer from
the alkoxy group to the pyridinium salt produces a partial
breaking of the double N-N bond of the azo moiety, thereby
facilitating the rotation around this bond to recover the more
stable initial trans configuration. The rotational mechanism
has been demonstrated previously for other push-pull
azobenzenes such as N,N-dialkylaminoazobenzenes.10
The very fast thermal relaxation process exhibited by
azocompounds 5 and 6 is exploited herein for the design of
new molecular switches. Two host nematic liquid crystals,
5CB and 6CB (4-cyano-4-n-alkylbiphenyls), were doped
with the corresponding azo-dyes at two different molar
fractions, 0.003 and 0.01. Both mesogens were chosen for
their clarification temperatures, TN-I (308 and 300 K
respectively), conveniently close to room temperature, which
is appropriate for their application as photoswitches.
Trans isomers, with a rod-like shape, tend to align
themselves along the director direction. Therefore, they can
be introduced into nematic mesophases at low concentrations
without destroying the LC order because of their positive
contribution to the nematic potential of the mesophase.
However, when trans-to-cis photoisomerization occurs, the
nematic potential decreases.
The kinetics of the thermal cis-to-trans isomerization
deviated from a first-order profile and were fitted to a
biexponential decay function (eq 1).
∆A ) A·e-t/τ + B·e-t/τ
(1)
1
2
The first term of the function corresponds to the back-
isomerization of the chromophore. The second one can be
related to the reorientation of the surrounding mesogen
molecules toward the director direction.11 Four different
parameters have been used to quantify properly the actuation
of the optical switch: the absorbance change produced upon
UV light irradiation, ∆A, the relaxation times of the fast and
slow terms, τ1 and τ2, and the response time of the switch,
tR, defined as the time required to restore the absorbance to
90% of its preirradiation value (Table 2).
The cis-to-trans isomerization of both chromophores took
place in around 15-35 ms. The reorientation of the mesogen
molecules that are close to that of the azo-dye occurred in
around 78 and 205 ms, depending on the host mesogen and
on the azo-dye concentration. Finally, the response times of
the different optical switches ranged between 135 and 454
ms at room temperature, which are much shorter than those
previously reported. The increase of the azo-dye concentra-
tion produced higher absorbance changes upon UV irradia-
tion of the nematic mixture as well as an increase in the rate
of the thermal isomerization process. At higher molar fraction
Figure 3 shows that the absorbance of all of the AZO-LC
mixtures increased considerably within the time scale of
nanoseconds upon irradiation with a laser light pulse (λ )
355 nm) at the corresponding observation wavelength, λobs
.
When the sample is irradiated with UV light, two simulta-
(7) Dougan, S. J.; Melchart, M.; Habtemariam, A.; Parsons, S.; Sadler,
P. J. Inorg. Chem. 2006, 45, 10882.
(8) Song, X.; Li, J.; Zhang, S. Liq. Cryst. 2003, 30, 331.
(9) Garcia-Amoro´s, J.; Mart´ınez, M.; Finkelmann, H.; Velasco, D. J.
Phys. Chem. B 2010, 114, 1287.
(10) (a) Baba, K.; Ono, H.; Itoh, E.; Itoh, S.; Noda, K.; Usui, T.; Ishihara,
K.; Inamo, M.; Takagi, H. D.; Asano, T. Chem.sEur. J. 2006, 12, 5328.
(b) Talaty, E. R.; Fargo, J. C. Chem. Commun. 1967, 65. (c) Asano, T.;
Okada, T. J. Org. Chem. 1986, 51, 4454.
(11) Tsutsumi, O.; Shiono, T.; Ikeda, T.; Galli, G. J. Phys. Chem. B
1997, 101, 1332.
3516
Org. Lett., Vol. 12, No. 15, 2010

Table 2. Absorbance Change, ∆A, Produced upon UV
Irradiation of Nematic Samples, Characteristic Relaxation
Times, τ1 and τ2, and Response Time for the Thermal
Relaxation Process, tR, for the Different AZO-LC Mixturesa
mixture
xazo
∆A
τ1, ms
τ2, ms
tR, ms
5-5CB
5-6CB
0.003
0.003
0.01
0.003
0.01
0.287
0.550
0.760
0.326
0.361
0.430
17
35
21
28
15
19
111
205
160
170
78
239
454
251
352
145
135
6-5CB
6-6CB
0.003
109
a The measurement temperatures were 298 and 306 K for the 6CB and
5CB samples, respectively.
Figure 4. Repeatability of the absorbance change, ∆A (crosses),
characteristic relaxation times, τ1 and τ2 (empty and filled circles,
respectively), and response time for the thermal isomerization
process, tR (half-filled circles), for a 6-5CB mixture at 306 K (xazo
) 0.01, λobs ) 420 nm).
of the chromophore, the larger cis isomer population created
upon photoisomerization produces a more pronounced
decrease of the nematic potential. Accordingly, the more
stable initial state, where all rod-like molecules point into
the director direction, will be more quickly recovered after
turning off the irradiation.
The temperature range in which the switches work depends
on the TN-I of the host mesogen used. This range was
294-300 K for the 6CB mixtures and 305-308 K for the
5CB ones, which are suitable ranges for further applications
in optical devices. Particularly noteworthy is the combination
of the azo-dye 6 and the mesogen 6CB, whose response time
of 135 ms outperforms by a factor better than 2 the earlier
systems,6 with the added benefit of the lower operation
temperature.
The switching repeatability of our prototypes was checked
by submitting them to several UV light-dark cycles. Neither
the change of the optical properties nor the relaxation time
of the molecular switch was altered by the continuous work
of the system (Figure 4).
In summary, finding new azoderivatives with fast cis-to-
trans isomerization kinetics at room temperature is a huge
challenge in obtaining quicker molecular switches. Our
results provide evidence that azopyridinium methyl iodide
salts are excellent guests in this respect, enabling the
modification of the optical properties of host nematic
mesophases at temperatures close to 300 K. These fast
responding AZO-LC systems are attractive systems from the
point of view of applications of dye-doped LCs to photonic
devices.
Acknowledgment. This article is dedicated to Prof. Dr.
Heino Finkelmann on occasion of his 65th birthday. Financial
support from the Ministerio de Educacio´n y Ciencia (CTQ-
2009-13797) is gratefully acknowledged. J.G.-A. is grateful
for the award of a doctoral grant from the Universitat de
Barcelona.
Supporting Information Available: Experimental pro-
cedures and full spectroscopic data for all new compounds
(3-6). First-order rate constants for the thermal cis-to-trans
isomerization process of azocompounds 5 and 6 in ethanol
and acetonitrile at different temperatures. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL1013679
Org. Lett., Vol. 12, No. 15, 2010
3517