Photoinduced morphism of gemini surfactant aggregates

Article abstract of DOI:10.1039/b416287k

The photochemical behaviour of an azobenzene chromophore inserted in a gemini surfactant imparts photocontrol to the resulting amphiphile assemblies, including the collapse, upon irradiation, of the multi lamellar vesicles formed in aqueous solution. The

Full text of DOI:10.1039/b416287k

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Photoinduced morphism of gemini surfactant aggregates
Delphine Faure,a Julien Gravier,b Thomas Labrot,b Bernard Desbat,c Reiko Oda*b and Dario M. Bassani*a
Received (in Cambridge, UK) 22nd October 2004, Accepted 30th November 2004
First published as an Advance Article on the web 17th January 2005
DOI: 10.1039/b416287k
near-planar, the Z isomer is significantly distorted because of
non-bonded repulsion between the alkoxy substituents. It is
therefore anticipated that upon photo or thermal E, Z isomerisa-
tion, the ensuing structural variation will induce an important
reorganisation of the surfactant aggregates. In particular, in the
case of vesicular aggregates, a sudden increase in the surface
curvature of the surfactant aggregates can induce a transition to a
micellar structure.1 Although such vesicle to micelle transitions are
not uncommon upon thermal activation, the use of light as an
external stimulus generally requires a drastic change in the
molecular structure (e.g. bond-breaking).
The photochemical behaviour of an azobenzene chromophore
inserted in a gemini surfactant imparts photocontrol to the
resulting amphiphile assemblies, including the collapse, upon
irradiation, of the multi lamellar vesicles formed in aqueous
solution.
Amphiphilic molecules containing photochromic elements can
undergo conformational and electronic changes upon irradiation,
offering an attractive alternative for morphology modulation of
amphiphile assemblies using an external stimulus. Compared to
the use of temperature, pH variation, or the addition of salt or co-
surfactants, the use of photo-isomerisable groups in principle
allows switching between morphologies associated to each of the
two isomers.1 However, the ensuing structural change is generally
insufficient to provoke the collapse of vesicular aggregates. This
can be brought about through the use of photo-cleavable groups,
where irradiation leads to the irreversible destruction of the
assemblies.2 Gemini surfactants containing a photo-isomerisable
stilbene spacer have been reported to undergo a micelle to vesicle
transition,3,4 but stilbene photoisomerisation exhibits significant
fatigue because of competitive photocyclisation from the Z isomer
to afford dihydrophenanthrene. This is not the case for the
azobenzene-derived surfactant reported herein, which was
designed to undergo a large structural variation upon E, Z
isomerisation.
The synthesis of the desired symmetrical gemini surfactant was
achieved by reductive coupling of the corresponding 3-alkoxy-4-
methylnitrobenzene, followed by bromination and subsequent
amination with trimethylamine to afford E-16azo16.{ Its absorp-
tion spectrum (Fig. 1) is typical of azobenzene derivatives,
comprised of a strongly allowed pp* transition centred at 350 nm
(lmax 5 325 nm) and a weakly-allowed np* transition at ca.
460 nm. Upon irradiation with UV light (365 nm), a photo-
stationary state enriched in the Z isomer (80 : 20 Z : E in
CDCl3, as determined by 1H NMR) is quickly reached.
Thermal reversion to the E isomer in solution is readily
followed by UV-vis spectroscopy, and is characterized by a
rate constant of kD 5 7.32 6 1025 s21 (t1/2 5 160 min.).
The high photosensitivity of 16azo16 is in agreement with
efficient photoinduced E, Z isomerisation (WE,Z 5 0.25 and
WZ,E 5 0.43).
The structure of 16azo16 (Scheme 1) is based on the use of a
photoactive azobenzene chromophore as a rigid scaffold to
connect two cationic alkylammonium amphiphiles. Molecular
modelling of the E and Z conformers suggests that isomerisation
of the NLN 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
The aggregation behaviour of 16azo16 and its response to UV
irradiation was studied at the air–water interface by p-A isotherms
which confirmed that the light-induced changes in the molecular
structure have an important effect on the 2-D packing behaviour
Scheme 1 Synthesis and photoisomerisation of 16azo16. i, RI, K2CO3
18-crown-6, THF, 80 uC (95%); ii, LiAlH4, THF, 80 uC (30%); iii, NBS,
CCl4, reflux (50%); iv, Me3N, CH3CN, 20 uC (90%).
Fig. 1 Absorption spectra of E-16azo16 in chloroform (solid line). The
spectrum of Z-16azo16 (dashed line) was deduced by subtracting the
contribution of the E isomer at the photostationary state.
*r.oda@iecb.u-bordeaux.fr (Reiko Oda)
d.bassani@lcoo.u-bordeaux1.fr (Dario M. Bassani)
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Fig. 3 DIC optical microscope images of vesicles formed from a 10 : 1
mixture of 16azo16 and CTAB before (a) and after (b) irradiation. Scale
bar 100 mm.
hundred nm to y10 mm. Irradiation of the solution was performed
using the medium pressure Hg lamp associated to the epifluores-
cence microscope (330–380 nm bandpass filter), which presents the
advantage of allowing in situ monitoring of the sample during
irradiation. The vesicles remained unchanged under and after
irradiation of the sample, and their integrity was further confirmed
by FFTEM.
Fig. 2 Effect of 365 nm irradiation on p-A isotherms of 16azo16 at the
air–water interface. Solid line: E-16azo16. Dashed line: 1 : 1 mixture of E-
and Z-16azo16 obtained by pre-irradiation of the sample prior to
deposition. Inset shows the pressure response of the balance held at
2
constant area (ca. 110 A /molecule) upon irradiation with a multiband
˚
These results indicate that E, Z isomerisation is insufficient to
inducing a morphology change of the 16azo16 vesicles. One way to
approach the phase transition is to ‘‘destabilise’’ the bilayers
through the addition of molecules capable of interacting with the
surfactant aggregate surface. This was achieved by the addition of
a small amount of micelle-forming surfactant, cetylammonium
bromide (CTAB). Vesicles formed from a mixture of 16azo16 and
CTAB (10 : 1) are similar in shape to those previously obtained
using 16azo16 alone. However, as shown by the DIC images in
Fig. 3, upon irradiation the vesicles rapidly deform (within y30 s)
and rupture to transform into much smaller birefringent objects
(Fig. 4). The exact nature of the latter could not be unambiguously
determined as they were not resolved either by optical microscopy
or by cryofracture TEM. It is likely that the vesicles are
transformed into a mixture of micelles and small crystallites. The
vesicles are not spontaneously re-formed after irradiation is halted,
despite the reversible nature of the E, Z isomerisation.
UV-254/365 nm lamp (8W, up arrows: 365 nm, down arrows: 254 nm).
(Fig. 2). In the case of the E isomer, the detectable increase of
the surface pressure from a quasi-zero value is observed at
2
135 A /molecule, which is followed by the collapse of the film at
˚
2
50 A /molecule. The solution of 16azo16 was then pre-irradiated at
˚
365 nm before deposition at the water surface ([Z] : [E] 5 1 : 1),
yielding a monolayer with a compression isotherm that is
substantially displaced: the first detectable increase is observed at
2
around 230 A /molecule (a 70% increase), and the collapse at
˚
2
70 A /molecule (a 40% increase). This behaviour contrasts with
˚
what has been previously observed with a related anionic gemini
surfactant containing a stilbene chromophore, in which E, Z
isomerisation induced a decrease in molecular area.3 In the latter
case, however, the E isomer is expected to be less planar than the Z
isomer due to repulsion between the polar head groups located in
the ortho positions relative to the exocyclic double bond. Together,
these results suggest that the propensity of aromatic gemini
surfactants to successfully pack into a dense monolayer is directed
by the overall shape (globular vs. planar) of the molecules rather
than the relative areas occupied by the polar head groups and the
non-polar alkyl chains.
In the absence of UV irradiation, the vesicular aggregates
formed by 16azo16 in the presence of CTAB display exceptional
thermal stability. Indeed, heating a suspension of the vesicles in
water to 80 uC did not provoke any change in the aggregate
morphology or size distribution, as evidenced by optical micro-
scopy. We can therefore attribute the light-induced collapse of the
The change in molecular area at the air–water interface upon
irradiation can be used to construct a light-to-mechanical force
transducer. By simply irradiating a monolayer of 16azo16 using a
hand-held UV lamp, it is possible to provoke changes in the
pressure monitored at constant surface area. The result is shown in
Fig. 2 (inset) where 365 nm light induces an increase in the surface
pressure as a result of the E A Z isomerisation. The process is
thermally reversible and 254 nm light (or visible light) can be used
to accelerate the return to the initial state. This sequence can be
repeated several times without loss in performance, except for a
drift in baseline resulting from slow evaporation of the aqueous
subphase.
In aqueous solution, 16azo16 spontaneously forms vesicles upon
sonication and gentle heating, as evidenced by optical microscopy
with differential interferential contrast (DIC) technique and by
freeze fracture transmission electron microscopy (FFTEM)
(Cryo-electron Microscope FEI EM120 (120 kV)). The vesicles
are polydisperse with a size distribution ranging from a few
Fig. 4 Rupturing of vesicles under irradiation. Images are taken every
10 s. Scale bar 15 mm.
1168 | Chem. Commun., 2005, 1167–1169
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vesicles to the destabilisation of the lamellar structure due to a
change in the surface curvature induced by E ,Z isomerisation.
In conclusion, we have unambiguously shown that a cationic
gemini surfactant having azobenzene spacer can reversibly change
its conformation at the air–water interface, allowing us to
modulate the surface pressure using a remote external stimulus,
namely UV irradiation. In solution, 16azo16 spontaneously forms
vesicles which, when prepared in the presence of a small amount of
CTAB, readily rupture upon UV irradiation. Current effort is
oriented towards rendering the system completely reversible and to
investigate its potential in the controlled release of substances in
aqueous environments.
Notes and references
{ All compounds exhibited satisfactory 1H, 13C and MS spectra. Full
experimental details of the synthesis will be published elsewhere.
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Delphine Faure,a Julien Gravier,b Thomas Labrot,b Bernard Desbat,c
Reiko Oda*b and Dario M. Bassani*a
aCentre de Recherche en Chimie Mole´culaire, LCOO CNRS UMR
5802, 33405 Talence, France. E-mail: d.bassani@lcoo.u-bordeaux1.fr;
Fax: 335 4000 6158; Tel: 335 4000 2827
bInstitut Europe´en de Chimie et Biologie MOBIOS CNRS UMR 5144,
33607 Pessac, France. E-mail: r.oda@iecb.u-bordeaux.fr;
Fax: 335 400 3066; Tel: 335 4000 2229
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cCentre de Recherche en Chimie Mole´culaire, LCPM, CNRS UMR
5803, 33405, Talence, France
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Chem. Commun., 2005, 1167–1169 | 1169