Photosensitive vesicles from a cis-azobenzene gemini surfactant show high photoresponse

Article abstract of DOI:10.1039/b904390j

A zwitterionic gemini surfactant containing a photoactive azobenzene chromophore forms vesicles in aqueous solution only from a photostationary state containing a majority of the Z-isomer (26:74 E:Z). These vesicles are stable in the dark over several wee

Full text of DOI:10.1039/b904390j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Photosensitive vesicles from a cis-azobenzene gemini surfactant show
high photoresponse†‡
Rony Kiagus Ahmad,a Delphine Faure,a,b Pascale Goddard,b Reiko Oda*a and Dario M. Bassani*b
Received 4th March 2009, Accepted 6th May 2009
First published as an Advance Article on the web 8th June 2009
DOI: 10.1039/b904390j
A zwitterionic gemini surfactant containing a photoactive azobenzene chromophore forms vesicles in
aqueous solution only from a photostationary state containing a majority of the Z-isomer (26:74 E:Z).
These vesicles are stable in the dark over several weeks, trapping the azobenzene in the Z form.
Compared to an analogous system based on a cationic azobenzene gemini surfactant forming vesicles
with the E- but not the Z- isomer, the new system displays significantly enhanced photosensitivity
towards light-induced lysis of the vesicles.
Introduction
Amphiphilic molecules bearing chromophores capable of under-
going photoinduced isomerization, polymerization or lysis form
photoresponsive supramolecular systems1 in which properties
such as viscosity, micellization, sol-gel transition, or the disruption
Scheme 1 Cationic and zwitterionic gemini surfactants possessing an
azobenzene spacer.
of vesicles, can be controlled.2 Whilst the popularity of azobenzene
chromophores in the design of photochromic systems rests on their
proven reversible photochromic response and relative synthetic
accessibility, photocontrol based on the photoinduced scission of
the amphiphilic moiety tends to show better photoresponse.3–6
With some exceptions,7 examples of photocontrol of surfactant
aggregates based on photoisomerization generally show only
modest response upon irradiation. Furthermore, the structural
changes thus induced are generally tolerated by the dynamic
amphiphilic architecture unless the latter has strong local ordering
(e.g. crystals or organized in two dimensional films).3,1h,i,k
We recently reported an amphiphilic system based on a
cationic gemini surfactant incorporating an azobenzene scaffold.8
Compared to related stilbene chromophores, azobenzenes possess
accrued photostability and better resolved absorption bands of the
E and Z isomers, making them preferred in applications targeting
reversible photocontrol of chemical properties.§ Photoinduced
E → Z isomerization of 1 (Scheme 1) is proposed to alter the
occupational area of the head vs. tail groups and was shown to
provoke reversible light-induced variations in the surface pressure
of monolayers of 1 at the air-water interface. The system was
designed to exploit the photochemical stability of azobenzene
isomerization while maintaining the chromophore parallel to the
interface, thus ensuring sufficient space for isomerization. Despite
this, vesicles of 1 in water are stable to irradiation unless a
small amount of CTAB is present. The latter destabilizes the
vesicles and pre-disposes them to undergo light-induced lysis,
albeit inefficiently. In this respect, 1 behaves similarly to other
light-sensitive systems based on E, Z isomerization that have been
reported previously, requiring long irradiation times and/or high
power sources. We now report a zwitterionic system (compound
2, Scheme 1) in which the azobenzene chromophore is activated
by pre-irradiation to form the thermodynamically unfavorable Z
isomer. In the dark, the latter is relatively long-lived and allows
further transformations of the sample. Vesicles prepared from
these samples are stable over several weeks in the dark, but undergo
light-induces lysis even at low light intensities.
Results and discussion
The structures of 1 and 2 (Scheme 1) are based on the use of
a photoactive azobenzene chromophore as a rigid scaffold to
connect two alkylammonium amphiphiles. Molecular modeling of
aInstitut Europe´en de Chimie et Biologie, CNRS UMR CBMN-5248, 2 rue
Robert Escarpit, 33607, Pessac, France. E-mail: r.oda@iecb.u-bordeaux.fr;
Fax: 335 4000 3066; Tel: 335 4000 2229
=
the E and Z conformers suggests that isomerization of the N N
bInstitut des Sciences Mole´culaires, CNRS UMR 5255, 33405, Talence,
France. E-mail: d.bassani@ism.u-bordeaux1.fr.; Fax: 335 4000 6158;
Tel: 335 4000 2827
double bond will significantly alter the relative area occupied by
the polar head groups with respect to the non-polar alkyl chains.
Additionally, whereas conformer E is expected to be near-planar,
the Z isomer is significantly distorted because of non-bonded
repulsions between the alkoxy substituents. It is therefore antici-
pated that upon photo- or thermal E, Z isomerization, the ensuing
structural variation will induce an important reorganization of the
surfactant aggregates.
† Dedicated to Pr. Shinkai on the occasion of his 65th birthday.
‡ Electronic supplementary information (ESI) available: 1H and 13C NMR
of 1 and 2, ms of photolyzed samples of 2, UV-VIS absorption spectra
of 1 and 2 in dichloromethane solutions at different irradiation times. See
DOI: 10.1039/b904390j
§ Surfactants containing photo-isomerisable stilbene units have been re-
ported to undergo micelle to vesicle transition,3 but stilbene photoisomeri-
sation exhibits significant photochemical fatigue because of competitive
photocyclisation from the Z-isomer to afford dihydrophenanthrene.
Compounds
1 and 2 were prepared from 2-methyl-4-
nitrophenol and 2-amino-4-nitrophenol, respectively, by reductive
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coupling according to Scheme 2. Alkylation of the phenol
group under mildly basic conditions smoothly introduced the
hydrocarbon chains. Following reductive coupling using LiAlH4
in THF, quaternization of the corresponding tertiary amines was
attempted as an entryway to the corresponding bis-cationic azo-
amphiphiles. In contrast to 1, where bis-cationization is achieved
from the corresponding bromomethyl derivative 5 using an excess
of trimethylamine, reaction of 10 with an excess of iodomethane
only affords the mono-cationic intermediate. More vigorous
conditions or alkylating reagents (e.g. dimethylsulfate) lead to
decomposition. Fortunately, oxidation of the aromatic amines in
10 with m-CPBA proved successful and led to the amine-oxide
amphiphile 2. Both 1 (with bromide counterions) and 2 are soluble
in chloroform, but form aggregates in aqueous environments.
Fig. 1 Electronic absorption spectra of the E (solid lines) and Z isomers
(dashed lines) of 1 (A) and 2 (B) in dichloromethane.
spectra of the Z isomers reported in Fig. 1. The composition
of the PSS at 365 nm and 450 nm, and the calculated extinction
coefficients at lmax for the E- and Z-isomers of 1 and 2 are given in
Table 1. Overall, the compounds behave similarly with the notable
exception that the rate of thermal Z → E isomerization of 2 is
considerably slower (by a factor of three) than that of 1 (t1/2 = 4.8
and 14.8 hrs. for Z-1 and Z-2, respectively). As expected, the return
of the Z isomer to the E form can be accelerated by irradiation
with 254 or 450 nm light.
The critical micelle concentrations (CMC) obtained by con-
ductivity and tensiometric measurements are very low for both
E-1 and E-2 (CMC = 2.5 ¥ 10-6 M for E-1, no value could be
determined for E-2 due to its low water solubility). In the case of
1, previous results showed that at concentrations above the CMC,
E-1 readily forms vesicular aggregates in aqueous solutions. Vesi-
cles of E-1 containing 10% CTAB were shown to undergo light-
induced collapse to a suspension of small crystallites.8a Irradiation
of monolayers of 1 formed at the air-water interface indicates
that an increase in the molar surface area (determined from the
minimum area before collapse in p-A isotherms) accompanies
E → Z isomerization, consistent with the hypothesis that the
Z-isomer is non-planar and may not pack as well as the E
isomer. The situation is apparently similar for 2, which also shows
Scheme 2 Syntheses of E-1 and E-2. i, C16H33I, K2CO3 18-crown-6, THF,
80 ◦C (95%); ii, LiAlH4, THF, 80 ◦C (33%); iii, NBS, CCl4, reflux (50%); iv,
Me3N, CH3CN, 20 ◦C (90%); v, C16H33I, K2CO3 18-crown-6, THF, 80 ◦
C
(95%); vi, MeI, DMF, 90 ◦C, (40%); vii, LiAlH4, THF, 80 ◦C (30%); viii,
m-CPBA, CHCl3, -5 ◦C (20%).
The UV-Vis electronic absorption spectra of E-1 and E-2 in
chloroform are typical of azobenzene derivatives, exhibiting strong
p–p* absorption bands centered around 300 nm and a weaker
n–p* transition located at ca. 450 nm (Fig. 1). To elucidate the
origin of the shoulder at 350 nm in the absorption spectra of
E-1, we performed INDO/S SCF-CI calculations (ZINDO) on
the PM3-optimized geometry. The results show that in 1, the p–p*
HOMO–LUMO transition typical of azobenzenes is accompanied
by an additional p–p* transition originating from mixing of the
HOMO-2-LUMO transition with the HOMO–LUMO transi-
tion. Compared to 1, 2 possesses electronic transitions that are
shifted bathochromically, a consequence of the inductive electron
withdrawing vs. donating effect of the ammonium vs. methylene
substituents.
Table 1 Spectral and photochemical data for 1 and 2a
lmax (e)b
PSS (Z:E)c
As typically observed for azobenzene derivatives, irradiation of
E-1 and E-2 at 365 nm leads to the isomerization of the azo bond
to afford photostationary states (PSS) that are enriched in the
Z isomers. In dichloromethane, the isomerization occurs cleanly,
giving rise to isosbestic points at 410 nm and 394 nm for 1 and 2,
respectively. The composition of the PSS can be readily determined
by 1H-NMR spectroscopy, as the two isomers exhibit well-resolved
spectra that are significantly shifted (see experimental section).
This information can then be used to deduce the absorption
E
Z
365 nm 450 nm kr (s-1)d
1
2
325 (18780) 460 (1490) 425 (2760) 80:20
316 (13160)
430 (1560)
21:79
16:84
4.0 ¥ 10-5
1.3 ¥ 10-5
2.3 ¥ 10-6e
277 (7250) 74:26
430 (4280)
a In dichloromethane. b Absorption maxima (nm) and molar extinction
coefficient (L mol-1 cm-1). c Composition of photostationar◦y state. d Rate
constant for thermal Z → E isomerization in the dark at 20 C. e Thermal
isomerization in aqueous vesicles determined from Fig. 4.
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Table 2 Krafft temperature of 2a
an increase in molar surface area upon photoinduced E → Z
isomerization (Fig. 2), although the light-induced increase in the
molar surface area is significantly smaller than for 1. Indeed, as
the PPS is reached upon irradiation with 365 nm light, the molar
surface area at surface pressure 41 mN/m is observed to increase
100 mM
600 mM
E-2
PSS (365 nm)
55 ◦
32 ◦
C
C
98 ◦
40 ◦
C
C
2
2
˚
˚
from 50 or 56 A /molecule to 70 or 64 A /molecule, for 1 or 2,
respectively. A possible explanation for the smaller variation in
the case of 2 is that, compared to the di-cationic nature of 1, the
zwitterionic nature of 2 reduces the Coulombic repulsion between
molecules.
a In aqueous solutions.
(DIC). The images are very similar to those previously obtained
for E-1, for which the presence of vesicles that are polydisperse
in size, ranging from a few hundred nm to ~10 mm was evidenced
by freeze fracture transmission electron microscopy (FFTEM).
Fig. 3 shows the difference between the globular, polycrystalline
aggregates formed by E-2 vs. the vesicular aggregates obtained by
solubilization of the PSS(365 nm) mixture of 2. In this respect,
2 behaves contrary to 1, which only forms vesicles from the E
isomer, and for which irradiation of the aggregates in water leads
to precipitation. The specific reasons behind this contradictory
behavior presumably lie in the subtle balance between the solid-
state crystal packing forces and the hydrophilicity of the polar
head groups in the E and Z isomers of 1 and 2. Differences in
solubility between the E and Z isomers of azobenzene-containing
polymers are common, and have been attributed to differences in
the dipole moment between the isomers.9
Fig. 2 Effect of 365 nm irradiation on p-A isotherms of 2 at the air-water
interface. Solid line: E-2. Dashed line: PSS(365 nm) mixture of E- and Z-2
obtained by pre-irradiation of the sample prior to deposition.
A striking difference between 1 and 2 is the relative dispersibility
of the Z and E isomers, which leads to widely divergent behavior
of the aggregates towards irradiation. For 1, the E isomer can
easily form aggregates in water, and irradiation of these aggregates
in water leads to the formation of a precipitate. In the case
of 2, the low water solubility of the E isomer prevents it from
forming solutions of dispersed aggregates under conditions that
are conducive for the formation of vesicles from E-1 (sonication
and gentle heating). However, the significant thermal stability of
the Z isomer of 2 allowed us to envisage formation of vesicles
from the E, Z mixture of isomers present in the PSS obtained
by irradiation of solutions of E-2 in dichloromethane. Thus,
irradiation with 365 nm light followed by evaporation of the
organic solvent affords a powder that is readily solubilized in
water. The composition of the sample thus obtained is close
to the proportion of Z and E isomers characteristic of the
initial PSS (Z/E = 74/26, Table 1), as verified by its absorption
spectra. To confirm and better quantify the higher solubility of the
irradiated sample, the Krafft temperatures of the irradiated and
non-irradiated samples were determined at two concentrations
(Table 2). In both cases, the Krafft temperature of the irradiated
sample is lower than that of the non-irradiated sample (E-2).
Importantly, at concentrations of 0.6 mM, E-2 is insoluble in
water up to temperatures of ca. 100 ◦C, whereas samples of 2
Fig. 3 Optical microscope images of aggregates formed by 5mM disper-
sions of compound 2 in water. a) E-2; b) PSS(365 nm) mixture of 2.
As mentioned in the introduction, the vesicular aggregates
formed by the solubilization of PSS(365 nm) mixtures of 2 in water
are stable over several weeks in the dark. To investigate this point
further, we proceeded to follow the evolution of solutions of the
vesicles formed from PSS(365 nm) mixtures of 2 (5 mM in water)
by optical microscopy and by UV-Vis absorption spectroscopy.
The latter allows evaluation of the composition in E and Z isomers
based on the strong absorbance of the transition at 315 nm present
in E isomer. To improve reproducibility and to minimize scattering
from inhomogeneous or highly absorbing samples, aliquots of
the 5 mM aqueous solution of the aggregates were diluted to
200 mM just prior to recording the absorption spectra. At t = 0, the
absorption spectra (Fig. 4a) is similar to that of the PSS mixture
in dichloromethane. Its slow evolution over several days in the
dark is characteristic of thermal Z → E isomerization. From the
time-dependence of the peak at 316 nm (Fig. 4b), a lifetime for Z-2
in the aggregates of 120 hrs (5 days) can be estimated. However,
even after 35 days, the system has not fully reverted to the E
form, as attested by its UV-absorption spectrum.¶ This implies
◦
enriched in the Z isomer are solubilized at only 40 C. This has
a profound impact on the aggregation properties of 2, as only
samples prepared from the PPS(365 nm) mixture of Z and E
isomers lead to the formation of solubilized aggregates in water.
The nature of the aggregates formed upon solubilization of the
PSS(365 nm) mixture of irradiated samples of 2 was explored
using optical microscopy with differential interferential contrast
¶ The exact composition of the mixture cannot be accurately determined
from the absorption spectra.
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Fig. 4 Evolution of aggregates formed by dispersion of PSS(365 nm)
mixtures of 2 (5mM) in water over time in the absence of light. a) UV-Vis
absorption spectra (aliquots diluted to 200 mM); inset: Absorbance at
316 nm, solid line represents best fit to monoexponential decay (k = 2.3 ¥
10-6 s-1). Optical microscope images at t = 0 (b), 6 (c), and 24 (d) days.
Fig. 5 Vesicles formed from a PSS(365 nm) mixture of 2 (5 mM in water)
were irradiated with a low-pressure Hg (254 nm, 7 W). Variation of UV
absorption spectra of 2 (a) and optical microscope images of the aggregates
following irradiation for 30 (b), 90 (c), and 150 (d) min.
that only a portion of the Z-2 in the aggregates can undergo
thermal return to the E isomer, whereas the remainder is blocked
in the Z configuration. The portion that does undergo thermal
return does so much more slowly than in fluid solution, where the
half-life of the Z-2 isomer is less than one day. The decay of the
vesicle population, as judged from the optical microscope images
(Fig. 4b-d) is much slower than the thermal isomerization. After
6 days, when the composition of 2 begins to plateau, the samples
are still almost exclusively composed of vesicles. Indeed, vesicles
persist on timescales of the order of 2–3 weeks, implying that the Z
and E isomers of 2 are trapped in the bilayers of the vesicles. Thus,
although E-2 does not possess sufficient solubility to allow the
direct formation of vesicles in water, it is possible to circumvent
this limitation by irradiation to form a mixture enriched in the
more soluble Z-2 isomer.
Irradiation of the vesicles with 450 or 254 nm light accelerates
the return to the E isomer. Contrary to thermal Z→E isomer-
ization, photoinduced isomerization leads to the rapid rupture
of the vesicles. This is illustrated in Fig. 5, where the lysis of
the vesicles is complete after just 2.5 hrs irradiation with a small
(6 W) low-pressure Hg lamp (254 nm) on the solutions in 1 cm
quartz cell. For comparison, vesicles formed by related E-1 do
not undergo photoinduced lysis unless a small amount of CTAB
is added to destabilize them and, even so, require the focused
output of a 100 W high-pressure Hg lamp on the solution confined
between a slide glass and a cover glass (sample thickness of
~100 mm). As discussed above, the changes in the absorption
spectra recorded following irradiation of the aggregates do not
match the spectra of E-2 in fluid solution. In fact, the deviation
is even more apparent in Fig. 5a, where the decrease in the long
wavelength transition is not accompanied by an increase of the
band at 320 nm typical of the E isomer. Instead, the band at
430 nm undergoes a bathochromic shift of ca. 40 nm before
decreasing monotonously with an isosbestic point at 410 nm.
The absence of an isosbestic point at early irradiation times
is synonymous with the occurrence of processes that cannot
be modelled by a simple transition between two states. This is
presumably due to the aggregation properties of 2, as mass-
spectrometric analysis of the samples after irradiation confirmed
that 2 did not undergo decomposition (see ESI‡). The accrued
photoresponse of 2 vs. 1 may lie in the fact that the vesicles are
initially formed from the less stable Z isomer, whose return to the
more stable E form is thermodynamically favored. Additionally,
while all vesicles are inherently meta-stable, it is possible that the
presence of a high proportion of the insoluble E-2 isomer may
further de-stabilize vesicles formed from 2. In this case, the low
cmc of 2 can be an important factor in controlling the stability
of the vesicles in that it will slow equilibration via the aqueous
phase.
Experimental
Reagents and solvents were used as received unless otherwise
noted. Tetrahydrofuran was dried over Na/benzophenone and
distilled immediately prior to use. Water (18.2 MX.cm) was
purified on Purelab Prima Elga apparatus. Merck silica gel 60
(70–230 mesh) was used for chromatography. Lyophilization was
performed on Heto DryWinner 6–85. Merck silica gel 60 (70–
230 mesh) was used for chromatography. 1H NMR and 13C NMR
spectra were recorded on a Bruker DPS-200 FT, AC-250 FT or
DPX-300 FT spectrometer using deuterium solvent as internal
reference unless otherwise indicated. Chemical shifts are given
in ppm (d) with respect to tetramethylsilane. Abbreviations used
ares = singlet, d = doublet, t = triplet, q = quartet, and br = broad.
Melting points are uncorrected. Mass spectra were recorded on a
VG Autospec-Q by EI (70 eV); m/z (%).
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Optical Microscopy with Differential Interferential Contrast
(DIC): Samples sealed between slide glass and cover glass were
observed with a NIKON Eclipse PhysioStation E600FN with
adequate condensers and prism for DIC observations.
3,3¢-Bis(hexadecyloxy)-4,4¢-bis(methyl)azobenzene (5)
A dry round-bottom flask equipped with a magnetic stirrer and
a reflux condenser in which were placed LiAlH4 (5 equiv.) and
dry THF was cooled to -80 ◦C under a nitrogen atmosphere.
A solution of 4 in dry THF was added drop-wise, and the
mixture was allowed to return to room temperature, then heated
to reflux. After 3 days at reflux, the reaction mixture was
cooled, carefully quenched by the addition of water, and extracted
with dichloromethane. The combined organic fractions were
successively washed with NH4Cl (10%) and water, and then dried
over MgSO4. The volatile components were removed on a rotary
evaporator and the residual solid purified by flash chromatography
(diethylether:pentane 1:9) to afford 5 as a yellow solid in 33% yield.
1H NMR (CDCl3): 7.47 (dd, 1H, J1 = 1.89 Hz, J2 = 7.91 Hz); 7.38
(d. 1H, J = 1.89 Hz); 7.27 (d, J = 7.91 Hz); 4.08 (t, 2H, J = 6.41
Hz); 2.3 (s, 3H); 1.84 (q, 2H, J = 6.71 Hz); 1.26 (br, 26H); 0.85 (t,
3H, J = 7.02 Hz1). RMN 13C (CDCl3):159.8; 150.1; 125.6; 130.1;
114.2; 108.3; 72.6; 29.35; 23.1; 16.63; 11.
Krafft Temperatures (Tk): The Krafft temperature is the
minimum temperature at which the hydrated surfactant becomes
soluble. Below Tk, a precipitate is formed. Since the solubilization
kinetics is strongly influenced by the presence of any metastable or
kinetically controlled aggregates, care was taken so that all samples
were treated in the same manner. The powder of surfactant was
first solubilized in water to obtain the solution of about 1% w/w,
and then it was freeze-dried to get a very fine airy powder. This
powder was again dispersed in water to obtain 3 mM solutions,
much above the cmc of all the investigated surfactants. To make
sure that all the samples formed well-hydrated precipitate, the
solutions were plunged into liquid nitrogen to get fast precipitation
and freezing, and then they were kept at 2 ◦C for several hours so
that they melted. The temperature of the solutions was increased
between 2 and 80 ◦C with a temperature increase rate of 1 ◦C
per 10 min and the macroscopic aspect was observed. Because
2 is zwitterionic, commonly used conductivity measurements to
determine Tk could not be used.
Kinetics of Vesicles solutions in water without irradiation in
dark: Vesicles of the mixture of irradiated samples of 2 were
spontaneously formed by dispersing the irradiated (in CH2Cl2)
and dried powder in water followed by weak sonication (20 seconds
in bath type sonicator). Since it was difficult to identify vesicles
in the solutions with too low concentration, 5 mM solutions
were chosen for kinetics measurements. These solutions were
diluted to 200 mM just before UV measurements. Irradiations
were conducted with a low pressure mercury lamp (6 W) or with
a 365 nm fluorescent lamp (Vilber-Lormat T-6L, 6 W). Actual
radiated power was measured with a calibrated NIST-traceable
radiometer/photometer to be 2.6 and 4.8 mW/cm2 at 254 and
365 nm, respectively.
4,4¢-Bis(bromomethyl)-3,3¢-bis(hexadecyloxy)azobenzene (6)
In a two-necked round-bottom flask equipped with a mag-
netic stirrer and a reflux condenser were placed 5 (1 equiv.),
N-bromosuccinimide (2.5 equiv.), and a catalytic amount of
(PhCO2)2. Carbon tetrachloride was added, and the mixture
refluxed under nitrogen for 8 hrs. After cooling, the precipitate was
filtered off and washed with CCl4. The solution was concentrated
on a rotary evaporator and the residual solid obtained purified by
flash chromatography (diethylether:pentane 1:9) to afford 6 as a
1
yellow solid in 54% yield. H NMR (CDCl3): 7.49 (m, 3H); 4.61
(s, 2H); 4.14 (t, 2H, J = 6.4 Hz); 1.84 (q, 2H, J = 6.71 Hz); 1.26 (s,
26H); 0.85 (t, 3H, J = 7.02 Hz). RMN 13C (CDCl3): 157.6; 154.1;
131.8; 129.7; 117.7; 104.2; 69.2; 29.7; 28.4; 22.4; 14.1. MS (FAB+)
m/z (%): 688.5 (100).
The absorption spectra of 1 and 2 were calculated using
INDO/SCF-CI as parametrized by Zerner (ZINDO) at the PM3
energy-minimized geometry.10
3,3¢-Bis(hexadecyloxy)-4,4¢-bis(trimethylammonium-
methyl)azobenzene dibromide (1)
A solution of 6 (1 equiv.) was dissolved in dry THF in a two-
necked round-bottom flask equipped with a magnetic stirrer and
a nitrogen inlet. Dry trimethylamine was bubbled trough the
solution, which was allowed to stir at rt for 24 hrs. The solvent
was removed on a rotary evaporator and the solid dissolved in a
minimum amount of chloroform and precipitated by the addition
of acetone. The solid was collected, dissolved in pure water, filtered,
and lyophilized to afford 1 as a bright yellow solid (mp = 180–
2-Hexadecyloxy-4-nitrotoluene (4)
In a dry round-bottom flask equipped with a magnetic stirrer and
a reflux condenser were placed 2-methyl-5-nitrophenol (1 equiv.),
K2CO3 (5 equiv.), and a catalytic amount of 18-crown-6. Dry
THF was added, followed by 1-iodohexadecane (1.1 equiv.) in dry
THF. The mixture was refluxed overnight, cooled, poured into
water, and extracted with dichloromethane. The combined organic
fractions were successively washed with NH4Cl (aq), water, brine,
and dried over MgSO4. The volatile components were separated
on a rotary evaporator and the residual solid purified by flash
chromatography (diethylether:pentane 1:9) to afford 4 as a pink
solid in quantitative yield. 1H NMR (CDCl3): 7.74 (dd, 1H, J1 =
2.14 Hz, J2 = 8.24 Hz); 7.63 (d, 1H, J = 2.14 Hz); 7.26 (d, 1H,
J = 8.24 Hz); 4.04 (t, 2H, J = 6.41 Hz); 2.3 (s, 3H); 1.84 (q,
2H, J = 6.71 Hz); 1.26 (br, 26H); 0.85 (t, 3H, J = 7.02 Hz).
13C NMR(CDCl3):157.39; 147.17; 135.08; 130.35; 115.42; 105.35;
68.57; 29.35; 23.1; 16.63; 11. MS (FAB+) m/z (%): 378 (M + 1,
100), 400 (M + Na, 57.77).
◦
1
182 C) in 80% yield. H NMR (CDCl3): E-1: 8.05 (d, 1H, J =
8.25 Hz); 7.61 (d, 1H, J = 7.92 Hz); 7.48 (s, 1H); 5.03 (s, 2H);
4.15 (t, 2H, J = 6.39 Hz); 3.47 (s, 9H); 1.84 (q, 2H, J = 6.71 Hz);
1.26 (s, 26H); 0.85 (t, 3H, J = 7.02 Hz), Z-1: 7.84 (d, 1H, J =
8.25 Hz); 6.93 (s, 1H); 5.78 (d, 1H, J = 7.92 Hz); 4.04 (t, 2H, J =
6.39 Hz); 3.31 (s, 9H); 1.84 (q, 2H, J = 6.71 Hz); 1.26 (s, 26H);
0.85 (t, 3H, J = 7.02 Hz). 13C NMR (CDCl3): 158.9; 155.0; 136.5;
136.5; 119.0; 117.8; 104.6; 63.5; 53.4; 29.7; 22.7; 14.2. IR (cm-1,
KBr): 871, 892, 1105, 1262, 1419, 1469, 1604, 2851, 2920. MS-ESI
m/z (%): 887 (20, (M - Br-)+), 403 (100, (M - 2Br-)2+). Elemental
analysis: Calculated for C52H94N4O2Br2·H2O (M = 984 g mol-1):C,
63.41; H, 9.75; N, 5.69; O, 4.87. Found: C, 63.63; H, 9.65; N, 4.73;
O, 5.71%.
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4-Amino-3-hexadecyloxynitrobenzene (8)
125; 118; 104; 68,9; 43.0; 30.3; 23.1; 14.0. IR (cm-1, KBr): 537,
660, 863, 997, 1012, 1270, 1405, 1472, 1601, 2852, 2918. MS-TOF
m/z (%): 781 (100), 360 (50). Elemental analysis: Calculated for
C48H84N4O4·2H20 (M = 817.2 g mol-1): C, 70.50; H, 10.77; N, 6.85;
O, 11.75. Found: C, 71.13; H, 10.41; N, 6.82; O, 9.66%.
This compound was prepared in 95% yield starting from 2-amino-
5-nitrophenol analogously to 4. 1H-NMR (CDCl3):7.80 (dd, 1H,
J1 = 2.28 Hz, J2 = 8.67 Hz); 7.67 (d, 1H, J= 2.28 Hz); 6.65 (d, 1H,
J = 8.67 Hz); 4.09 (t, 2H, J = 6.39 Hz); 1.87 (q, 2H, J = 6.7 Hz);
1.26 (s, 26H); 0.88 (t, 3H, J = 0.2 Hz). 13C-NMR(CDCl3):146.3;
139.0; 138.5; 118.9; 111.7; 106.61; 68.9; 31.9; 22.7; 14.1. ESI-MS
m/z (%) 379 (100, M + H+).
Conclusion
We have prepared a zwitterionic gemini surfactant based on a
photosensitive azobenzene scaffold that exhibits slow thermally-
activated Z-E isomerization. The latter property has allowed us
to prepare vesicles from photostationary mixtures enriched in the
more soluble Z isomer. In these aggregates, thermal isomerization
to the E isomer is slow compared to fluid solution. In the absence
of light, the vesicles are stable over several weeks, but can be
ruptured by irradiation with a low-power lamp.
4-Dimethylamino-3-hexadecyloxynitrobenzene (9)
A round-bottom flask equipped with a magnetic stirrer and fitted
with a reflux condenser and a septum is loaded under nitrogen
with 8 (0.25 g, 0.66 mmol), K2CO3 (0.45 g, 3.3 mmol), a catalytic
amount of 18-crown-6, and 15 mL of dry DMF. Methyl iodide
(41 mL, 4.0 mmol) is added through the septum and the mixture
is heated to 90 ◦C with stirring for 24 hrs. The DMF is then
removed under vacuum and the oily residue is taken up in
dichloromethane, washed with water, and dried over MgSO4. The
solvents are evaporated on a rotary evaporator and the waxy solid
thus obtained is carefully purified by column chromatography
(diethylether:pentane 1:9) to afford 9 (0.1 g) as a yellow solid in
Acknowledgements
Financial support from the CNRS, the French Ministry of Re-
search and Asian development bank is gratefully acknowledged.
1
Notes and references
40% yield. H-NMR (CDCl3): 7.83 (dd, 1H, J1 = 2.15 Hz, J2 =
8.85 Hz); 7.67 (d, 1H, J = 2.15 Hz); 6.75 (d, 1H, J = 8.85 Hz);
4.05 (t, 2H, J = 6.40 Hz); 1.87 (q, 2H, J = 6.7 Hz); 1.26 (s, 26H);
0.88 (t, 3H, J = 0.2 Hz). 13C-NMR (CDCl3):144.3; 139.0; 138.5;
118.9; 111.7; 106.6; 68.9; 43.9; 31.9; 22.7; 14.1. SM (FAB+) m/z
(%): 407 (M + 1, 100).
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Compound 10 was obtained analogously to 5 in 30% yield from
9. 1H-NMR (CDCl3): 7.50 (dd, 1H, J1 = 1.85 Hz, J2 = 8.25 Hz);
7.43 (d, 1H, J = 1.85 Hz); 6.95 (d, 1H, J = 8.25 Hz); 4.11 (t, 2H,
J = 6.70 Hz); 1.87 (q, 2H, J = 6.7 Hz); 1.26 (s, 26H); 0.88 (t, 3H,
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4,4¢-Bis(dimethylamineoxide)-3,3¢-bis(hexadecyloxy)-azobenzene
(2)
In a two-necked round-bottomed flask fitted with a septum and
a nitrogen inlet is placed 10 (1 equiv.) dissolved in CHCl3. The
solution is cooled to -5 ◦C and a solution of m-CPBA (2 equiv.)
◦
in CHCl3 (also cooled to -5 C) is added drop-wise through the
septum. The reaction is allowed to warm to rt and is stirred is
continued for 12 hrs. The volatile components are removed on a
rotary evaporator and the residue is taken up in dichloromethane,
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concentrated on a rotary evaporator and triturated with pentane.
The solid thus obtained is filtered and dried under vacuum to
give 2 as a red-brown solid in 20% yield (mp = 158–160 ◦C). 1H-
NMR (CDCl3): E-2: 9.02 (d, 1H, J = 8.55 Hz); 7.73 (dd, 1H, J1 =
8.55 Hz, J2 = 1.82 Hz); 7.51 (d, 1H, J = 1.82 Hz); 4.25 (t, 2H,
J = 5 Hz); 3.73 (s, 6H); 1.94 (q, 2H, J = 7.3 Hz); 1.26 (s, 13H);
0.88 (t, 3H, J = 0.2 Hz), Z-2: 8.68 (d, 1H, J = 8.55 Hz); 6.66 (d,
1H, J = 1.82 Hz); 6.29 (dd, 1H, J1 = 8.55 Hz, J2 = 1.82 Hz); 4.00
(t, 2H, J = 5 Hz); 3.61 (s, 6H); 1.94 (q, 2H, J = 7.3 Hz); 1.26 (s,
13H); 0.88 (t, 3H, J = 0.2 Hz). 13C-NMR (CDCl3): 153; 150; 140;
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3178 | Org. Biomol. Chem., 2009, 7, 3173–3178
This journal is
The Royal Society of Chemistry 2009
©