Self-assembly of styryl naphthalene amphiphiles in aqueous dispersions and interfacial films: Aggregate structure, assembly properties, photochemistry, and photophysics

Article abstract of DOI:10.1021/jp990857m

A study of phospholipid and fatty acid amphiphiles containing the isomeric trans-α- and β-styrylnaphthalene chromophores is reported. On the basis of simulations for Langmuir films of fatty acids terminated with the two chromophores, it was anticipated that the β-styrylnaphthalene fatty acids should be able to pack into compact layers, having an extended glide or herringbone arrangement similar to those observed for corresponding stilbene, tolan, or azobenzene amphiphiles. In contrast, the simulations suggested that such an arrangement might be precluded on steric grounds for the isomeric α-styrylnaphthalene derivatives. The experimental studies carried out indicate that the β-styrylnaphthalene fatty acids and phospholipids do in fact exhibit assembly properties and photophysical behavior similar to that observed for the stilbene and tolan amphiphiles; thus we find evidence for aggregates having similar "signatures" to those observed previously for both films at the air-water interface and aqueous suspensions. The behavior of the α-styrylnaphthalene amphiphiles is much more complex. Whereas aqueous dispersions of the phospholipid and highly compressed films at the air-water interface suggest the presence of weak aggregates (possibly dimers) having a translation or face-to-face structure, studies of the fatty acid derivative at the air-water interface suggest that the α fatty acid may form a "pinwheel" or herringbone cluster before compression such that drastically different photochemistry and photophysics is observed as a function of compression.

Full text of DOI:10.1021/jp990857m

J. Phys. Chem. B 1999, 103, 9161-9167
9161
Self-Assembly of Styryl Naphthalene Amphiphiles in Aqueous Dispersions and Interfacial
Films: Aggregate Structure, Assembly Properties, Photochemistry, and Photophysics
Liaohai Chen,† Cristina Geiger,‡ Jerry Perlstein,‡ and David G. Whitten*,†
Chemical Science and Technology DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545,
and Department of Chemistry and NSF Center for Photoinduced Charge Transfer, UniVersity of Rochester,
Rochester, New York 14627
ReceiVed: March 11, 1999; In Final Form: May 23, 1999
A study of phospholipid and fatty acid amphiphiles containing the isomeric trans-R- and â-styrylnaphthalene
chromophores is reported. On the basis of simulations for Langmuir films of fatty acids terminated with the
two chromophores, it was anticipated that the â-styrylnaphthalene fatty acids should be able to pack into
compact layers, having an extended glide or herringbone arrangement similar to those observed for
corresponding stilbene, tolan, or azobenzene amphiphiles. In contrast, the simulations suggested that such an
arrangement might be precluded on steric grounds for the isomeric R-styrylnaphthalene derivatives. The
experimental studies carried out indicate that the â-styrylnaphthalene fatty acids and phospholipids do in fact
exhibit assembly properties and photophysical behavior similar to that observed for the stilbene and tolan
amphiphiles; thus we find evidence for aggregates having similar “signatures” to those observed previously
for both films at the air-water interface and aqueous suspensions. The behavior of the R-styrylnaphthalene
amphiphiles is much more complex. Whereas aqueous dispersions of the phospholipid and highly compressed
films at the air-water interface suggest the presence of weak aggregates (possibly dimers) having a translation
or face-to-face structure, studies of the fatty acid derivative at the air-water interface suggest that the R fatty
acid may form a “pinwheel” or herringbone cluster before compression such that drastically different
photochemistry and photophysics is observed as a function of compression.
Introduction
tion spectra and photophysics for the monomer, yet very
different shape and potentially different possibilities for packing
in organized assemblies because of the nature geometry different
substitute position in the aromatic rings.16 In previous studies
it has been shown that trans isomers of simple styrylnaphthalene
derivatives exhibit similar photophysical behavior for the R-
and â-isomers and photoisomerization behavior not very dif-
ferent from that of trans-stilbene.17 From simple modeling
considerations we recognized that amphiphiles constructed by
attaching a fatty acid chain to the para position of the phenyl
ring of R- and â-trans-styrylnaphthalene should result in
molecules having very different shapes. Thus the â-isomer
would be anticipated to exist preferentially in the glide config-
uration in films and vesicles; as an “elongated” configuration
it might be anticipated to be very similar to the corresponding
stilbene and azobenzene amphiphiles such that edge-to-face
interactions should be easily attained and hence favored. In
contrast, the R-isomer would be anticipated to have a bent shape
in the “extended” amphiphile such that it would be expected to
fairly easily pack in a translation layer structure, permitting face-
to-face interactions, whereas an edge-to-face arrangement result-
ing in an extended glide or herringbone lattice should be
unfavorable because of the shape or steric effects.
In several recent studies we have shown that incorporation
of an aromatic group into the hydrocarbon chain of an
amphiphile such as a fatty acid or phospholipid results in
enhanced, and in some cases unusual, self-assembly processes,
as a consequence of strong noncovalent attraction between the
aromatic chromophores.1-4 These interactions are manifested
by changes in the absorption spectra, photophysics and photo-
chemistry of the aromatic chromophore, as well as by changes
in the structures or properties of the resulting assembly. In
several cases, for example for trans-stilbene derivatives,5-7
simple phenyl, biphenyl, and terphenyl derivatives,8 azoben-
zenes,9,10 tolans,11,12 and R, ω-diphenylpolyenes,13 indications
are strong that for both the “unit aggregate” and the extended
array in films and aqueous dispersions, the nearest neighbors
show an edge-to-face arrangement, which leads to an extended
glide or herringbone array. However, in other cases, most
notably the apparently structurally similar simple styrene and
styrylthiophene amphiphiles,14,15 there is good evidence through
experimental observations such as novel photoreactivity in the
aggregates and simulations that the preferred structures may
result from face-to-face interactions between neighboring
aromatic chromophores, resulting in an extended translation
layer array.
In the current paper we report a study of the self-assembly
and aggregation behavior of fatty acid and phospholipid
derivatives containing the styrylnaphthalene chromophore (Scheme
1). The synthesis of fatty acid derivatives 1 and 2 were
accomplished by techniques similar to those used for other
aromatic amphiphiles and from these the corresponding phos-
pholipids 3 and 4 were obtained.8 We have interrogated the
behavior of 1 and 2 in films at the air-water interface and in
supported multilayers by Langmuir-Blodgett technique and also
In an effort to probe the relative importance of different
factors that may control the self-assembly of aromatic am-
phiphiles, we considered the styrylnaphthalene chromophore as
potentially interesting for fruitful investigation because of the
existence of two isomeric chromophores having similar absorp-
† Los Alamos National Laboratory.
‡ University of Rochester.
10.1021/jp990857m CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/23/1999

9162 J. Phys. Chem. B, Vol. 103, No. 43, 1999
Chen et al.
SCHEME 1: Chemical Structures for Compounds Used
in This Study
SCHEME 2: Synthesis Route for 1 and 3
reaction between 1-naphthaldehyde and the corresponding Wittig
salt. Thus, dry bromoester (2.8 g, 8.5 mmol) and triphenylphos-
phine (3.3 g, 12.6 mmol) were dissolved in the dry xylene under
the nitrogen. The reaction mixture was refluxed overnight and
the resulting Wittig salt was filtered, recrystallized with
chloroform/benzene, and dried in a vacuum desiccator for 12
h. It was then dissolved (1.5 g, 2.52 mmol) in 50 mL of dry,
freshly distilled tetrahydrofuran (THF) and 40 mL of methylene
chloride, followed by the addition of K2CO3 (1.74 g, 12.6 mmol)
and 10 mg of 18-crown-6. After the mixture was stirred for a
few minutes, 1-naphthaldehyde (0.86 g, 5.54 mmol) was added
slowly. The reaction mixture was then refluxed overnight. The
crude product was purified by silica gel column chromatography
using hexane/ethyl acetate (10%) as eluent to yield 0.91 g (90%
yield) SNO6E, which was further purified by recrystallization
from THF/hexane as white crystals. Hydrolysis of this ester in
KOH/ethanol afforded the R-styrylnaphthalene fatty acid (1) in
examined aqueous dispersions of the phospholipids 3 and 4.
Results of these investigations indicate markedly different self-
assembly and aggregation behavior for the two styrylnaphthalene
isomers. As anticipated from the above discussion the two
amphiphiles incorporating a â-styrylnaphthalene chromophore
exhibit aggregation very similar to that found for the stilbenes
and related aromatics. The behavior of the R-styrylnaphthalene
amphiphiles is, in contrast, much more complex and provides
a remarkable demonstration of the role of competing interactions
in the self-assembly process and in resultant assembly properties.
Experimental Section
1
Reagents were purchased from Aldrich and were used as
received. All solvents for spectroscopic studies were spectro-
scopic grade either from Aldrich or Fisher. Milli-Q water,
obtained by passing house-deionized water through a Millipore-
RO/UF water purification system, was used for all measurements
in an aqueous medium. L-R-Glycerophosphorylcholine as the
cadmium chloride complex, lipophilic Sephadex LH-20 (25-
100-µm beads), Sephadex G-50, and L-R-dimyristoylphospha-
tidylcholine (DMPC, 99+%) were purchased from Sigma
Chemical Co.
a 70% yield. H NMR (CDCl3) δ: 8.3-6.9 (m, 13H); 4.0 (t,
2H); 2.4 (t, 2H); 1.85 (m, 2H); 1.75 (m, 2H); 1.55 (m, 2H).
Elemental Analysis: calculated for C24H24O3, C 79.97, H 6.71;
found, C 80.23, H 6.66.
The corresponding phospholipid derivative (3) was prepared
according to the routes previously described in the literature.8
1H NMR (CDCl3) δ: 8.2-6.8 (m, 24H); 7.06-7.02 (d,2H); 5.2
(m, 1H); 4.4-3.7 (m, 10H); 3.4-3.3 (s, 9H); 2.4 (t, 4H); 1.85
(m, 4H); 1.75 (m, 4H); 1.55 (m,4H). FAB: C
56H64NO10P m/z
calcd for (M + H) ) 942.4, found 942.4.
The R-styrylnaphthalene fatty acid derivative (1) was syn-
thesized in a multistep synthesis depicted in Scheme 2. A
mixture of 450 mL of methylene chloride, 450 mL of water,
ethyl 6-bromohexanoate (40.0 g, 0.179 mol), p-cresol (9.68 g,
0.09 mol), sodium hydroxide (5.38 g, 0.13 mol), and quaternary
ammonium bromide (17.34 g, 0.05 mol) was stirred vigorously
at room temperature for 12 h. After working up, the crude
product was purified by vacuum distillation and 14.75 g (66%
yield) of pure product (TO6E) was obtained at 117-120 °C
(0.6 mmHg). Bromination of TO6E with NBS in carbon
tetrachloride afforded the bromo derivative in a 69% yield. The
styrylnaphthalene chromophore was synthesized by the Wittig
The â-styrylnaphthalene fatty acid and its corresponding
phospholipid derivative were synthesized on the basis of very
similar procedures to those for the R-styrylnaphthalene deriva-
tives but starting with a 2-naphthaldehyde. â-Styrylnaphthalene
fatty acid (2): 1H NMR (CDCl3) δ: 7.7-6.8 (m, 13H); 4.0 (t,
2H); 2.4-2.3 (t, 2H); 1.75 (m, 2H); 1.64 (m, 2H); 1.44 (m,
2H). Elemental Analysis: calculated for C24H24O3, C 79.97, H
6.71; found, C 79.78, H 6.57. The â-styrylnaphthalene phos-
pholipid (4): 1H NMR (CDCl3) δ: 8.0-6.8 (m, 24H); 7.15-
7.02 (d, 2H); 5.2 (m, 1H); 4.0-3.4 (m, 10H); 3.2 (s, 9H); 2.4
(t, 4H); 1.80 (m, 4H); 1.70 (m, 4H); 1.55 (m,4H). FAB: C56H64-
NO10P m/z calcd for (M + H) ) 942.4, found 942.4.

Self-Assembly of Styryl Naphthalene Amphiphiles
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9163
Absorption spectra were recorded on a Hewlett-Packard
8452A diode array spectrophotometer. Steady-state fluorescence
was measured using a SPEX Fluorolog spectrofluorometer. All
emission and excitation spectra were corrected for instrumental
response. The irradiation of the samples was carried out at room
temperature with a 200-W high-pressure mercury lamp (Oriel
model 68700) with a 7-60 Corning glass filter (300 < λ < 400
nm). The Langmuir-Blodgett films of 1 and 2 were prepared
by spreading a chloroform solution of material (1 × 10-3 M)
onto an aqueous surface containing cadmium chloride (3 × 10-4
M) buffered with 1 × 10-5 M NaHCO3 (pH ) 6.8) on a KSV
5000 Langmuir-Blodgett trough. The temperature of the trough
was controlled by a water bath. The monolayers were transferred
onto quartz substrates that were previously treated by piranha
solution in a vertical deposition protocol. Bilayer vesicles of 2
and 4 with DMPC were prepared on the basis of the literature
procedure,18 and an ultrasonic liquid processor sonicator, model
XL2015, from Misonic Inc. was used for probe sonication. The
crystal structures of 1 and 3 were obtained by mounting the
single crystals, which were grown from THF/CHCl3 and
methanol respectively, on a glass fiber under Paratone-8277 and
immediately placed in a cold nitrogen stream at -80 °C on the
X-ray diffractometer. The X-ray intensity data were collected
on a standard Siemens SMART CCD area detector system
equipped with a normal focus molybdenum-target X-ray tube
operated at 2.0 kW (50 kV, 40 mA). Monte Carlo simulated
annealing experiments were carried out using PACK software19
to generate the details of the packing of the lowest-energy gas-
phase monolayers for the fatty acid derivatives 1 and 3.
Molecular structures were generated using the CHEMX/CHEM-
LIB molecular modeling package.20 Bond lengths and bond
angles were optimized using the MM2 force field. All single-
bond torsion angles were allowed to vary during the course of
the simulation. Monolayers with either simple translation or
glide layer symmetry were generated by the PACK routine.
Approximately 700 low-energy monolayers were generated for
each symmetry type for which surface area/molecule was
computed to compare with the experimental results.
Figure 1. Normalized absorption spectra of 2 (A) and 1 (B) in different
media. Solid line, in chloroform; dashed line, Langmuir-Blodgett
monolayer film.
per molecule is 26.9 Å2, which is also similar to that obtained
for the corresponding trans-stilbene fatty acid. This area-per-
molecule value derived from the isotherm is in good agreement
with the one obtained from PACK simulation (26.7 Å2). The
relatively small area per molecule is consistent with the
styrylnaphthalene chromophore existing in an extended con-
figuration in the films. The absorption spectrum for 2 in
Langmuir-Blodgett multilayers transferred to quartz supports
shows a pronounced blue shift (∼70 nm) compared to solution
as indicated in Figure 1A. Accordingly, the fluorescence
spectrum also exhibits similar red shift, reminiscent of those
obtained for the stilbene and azobenzene amphiphiles in LB
films. In fact the shape and wavelength of the shifted absorption
spectrum is remarkably similar to those for stilbene, tolan, and
R,ω-diphenylpolyenes in the same assemblies, even though the
absorption spectrum for the monomer is at relatively longer
wavelengths. The corresponding phospholipid, 4, forms clear
dispersions upon sonication in aqueous solution. The absorption
and fluorescence spectra of these dispersions are virtually
identical to those of the LB films of 2. Dilution of an aqueous
dispersion of 4 with a dispersion containing an excess of the
saturated phospholipid DMPC results in a shift to longer
wavelengths. These dispersions are resistant to dilution as
indicated by the finding that no changes can be detected in the
absorption and emission spectra until the ratio of DMPC/â-
styrylnaphthalene reaches 36:1 (Figure 2A). It should be pointed
it out that the spectrum of the most diluted solutions is not the
Results and Discussion
As shown in Figure 1 for the fatty acids, both styrylnaph-
thalene isomers show similar absorption spectra when dissolved
at low concentrations (1 × 10-5 M) in organic solvents such as
ethanol, acetonitrile, and chloroform, etc. under conditions where
only the monomer should be present. Trans-to-cis photoisomer-
ization of styrylnaphthalene derivative has been well studied
in the literature,21-25 with an average measured quantum
efficiency of 0.1. Thus, irradiation of either isomer in organic
solvents under these conditions results in spectral changes
consistent with photoisomerization to produce the corresponding
cis isomers. This was confirmed by proton NMR studies in
which the appearance of two set of the doublet peaks with
chemical shifts of 6.54-6.52 ppm (J ) 8.7) and 6.96-6.94
ppm (J ) 8.7) was assigned to the alkene protons of the cis
styrylnaphthalene isomer. The photoisomerization is reversible
in the early phases for both the R- and â-fatty acids and appears
to be qualitatively and quantitatively similar to the photoisomer-
ization processes observed for the stilbenes and the simple
styrylnaphthalene derivatives previously studied. For the trans-
â-styrylnaphthalene fatty acid 2, deposition of a film from a
chloroform solution followed by compression results in a
relatively sharp isotherm, very similar to those obtained from
the corresponding trans-stilbene fatty acid derivatives or for
saturated fatty acids such as arachidic acid. The limiting area

9164 J. Phys. Chem. B, Vol. 103, No. 43, 1999
Chen et al.
ferred to a quartz support, the absorption spectrum of this film
(Figure 1B) is quite different from that of 2, showing only a
small blue shift of 18 nm compared with the solution spectrum,
whereas the fluorescence shows a small red shift. Similar
absorption and fluorescence spectra are also observed for
sonicated aqueous dispersions of 3. In contrast to the results
for the â-styrylnaphthalene derivatives, dilution of an aqueous
dispersion of 3 with a dispersion containing an excess as small
as five-fold of the saturated phospholipid DMPC results in a
shift to longer wavelengths closely corresponding to the
monomer of the R-styrylnaphthalene (Figure 2B), suggesting
that the aggregates of 3 may be much less stable than those of
4 or the corresponding stilbene phospholipids. Irradiation of
films of 1, prepared by compression to 20 mN/m and transferred
to quartz slides, or aqueous dispersions of 3 with light in the
range 300-400 nm results in a clean bleaching of the long-
wavelength transition in each case. In contrast to the photo-
isomerization of the monomer in solution, the bleaching
observed here is not reversed by short-wavelength irradiation,
and the spectral shape is also quite different from that for
photoisomerization. The photoproduct of irradiated aqueous
dispersions of R-styrylnaphthalene phospholipid was tentatively
identified by NMR analysis. Thus, the aqueous dispersions of
3 was irradiated with 300 nm wavelength light in a Rayonet
photochemical reactor for a period of 18 h, and the resulting
solution was hydrolyzed and then extracted with chloroform.
Because of the poor solubility of 3 in the aqueous solution, only
a small amount of photoproduct was isolated. A relatively noisy
NMR spectrum for the photoproduct was obtained, yet there
was a distinguishable new peak appearing at 3.6 ppm. Because
the chemical shift for cyclobutane protons is usually between
3.3 ppm and 3.7 ppm, and no protons from the styrylnaphthalene
phospholipid have a peak in this region, the signal maybe most
reasonably assigned to the protons associated with the cyclobu-
tane hydrogens for the corresponding styrylnaphthalene pho-
todimers. Thus we may tentatively assign the photoproduct to
a dimer formed by the cycloaddition of two R-styrylnaphthalene
units. On the basis of the photoreactivity as well as the spectral
shift, it seems reasonable that both assemblies of the R-styryl-
naphthalene amphiphiles pack into transition layer structures
where photodimerization is topologically allowed.
Figure 2. Normalized absorption spectra of 4 (A) and 2 (B) aqueous
dispersion upon dilution by DMPC. (A) solid line, pure 4 aqueous
dispersion; dash-dot line, DMPC:4 ) 5:1; dashed line, DMPC:4 )
36:1; dotted line, in chloroform. (B) Solid line, pure 2 aqueous
dispersion; dash-dot line, DMPC:2 ) 5:1; dashed line, DMPC:2 )
20:1; dotted line, in chloroform.
same as the one in chloroform where only monomer is present;
it is still blue-shifted compared to the monomer, but red-shifted
compared to the original aggregated form, suggesting that an
intermediate product may be formed with a smaller aggregation
number relative to the aqueous dispersion of 4. Even though
both 2 and 4 undergo photoisomerization as the predominant
photoreaction when irradiated in dilute solutions as the mono-
mer, the LB films of 2 and the aqueous dispersions of 4 show
little change upon irradiation at 300-400 nm. The photostability
of 2 and 4 in films and aqueous bilayer assemblies, respectively,
as well as the pronounced spectral shifts suggest that the
arrangement of the fatty acid 2 in LB films and of the
phospholipid 4 in aqueous dispersions is very similar to that of
the stilbene and azobenzene amphiphiles in corresponding
media.4 Thus the observed behavior is consistent with self-
assembly of these amphiphiles to form similar aggregates, most
likely with an extended glide or herringbone structure in which
the primary interaction is an edge-to-face interaction between
neighboring aromatics. In these arrangements both photoisomer-
ization and photodimerization from â-styrylnaphthalene chro-
mophores are precluded.
The contrast in behavior between R-isomers 1 and 3 and
â-isomers 2 and 4 upon self-assembly is consistent with different
aggregation behavior or different extended structures for the
two types of amphiphiles. On the basis of their spectral
similarities, previously obtained crystal structure data for the
stilbenes, and the lack of photoisomerization or photodimer-
ization (despite reasonably long fluorescent lifetimes), it is
reasonable to assign a herringbone or glide structure with edge-
to-face interactions between neighbors to the â-styrylnaphthalene
amphiphiles. In fact, from the remarkable spectral similarity of
aggregates formed from the stilbenes, tolans, R,ω-diphenylpoly-
enes, and now â-styrylnaphthalenes, it appears that we can
associate this type of aggregate with a characteristic large
spectral shift and “signature” as shown in Figure 1. The smaller
spectral blue shifts in absorption and apparent photodimerization
for the R-styrylnaphthalene amphiphiles are similar to the
previously observed behavior of styrylthiophenes and styrene
derivatives and seem most consistently atttributable to a
translation layer structure extended array that is characterized
by face-to-face interactions.
For the R-styrylnaphthalene amphiphiles quite different
behavior is observed. When the fatty acid 1 is compressed to a
pressure of 35 mN/m, a film with a limiting area of 24 Å2 is
obtained, which is again very similar to the value obtained from
PACK simulations. Not surprisingly, after the film was trans-
Additional support for these assignments may be obtained
from the crystal structures of 1 and 2 (Figure 3). Single crystals
of 1 and 2 were grown from THF/CHCl3 and methanol,

Self-Assembly of Styryl Naphthalene Amphiphiles
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9165
Figure 3. Single-crystal structures of 2 (A) and 1 (B).
respectively. As it has been found for several other aromatic-
functionalized amphiphiles, we find that the crystals of 1
resemble an LB multilayer (y-deposition) array. However, unlike
the stilbenes, for 1 the styrylnaphthalene chromophores within
a layer are in a face-to-face or translation array. Although there
is clearly a face-to-face arrangement between neighboring
molecules, the separation between the olefin bonds of nearest
neighbors is 7.7 Å, which is clearly greater than the “magic
distance” required for a topologically controlled dimerization
in the crystal. Not surprisingly, these crystals of 1 are photo-
stable, unlike the compressed monolayer films or aqueous
dispersions. For the crystals of 2, the arrangement is also

9166 J. Phys. Chem. B, Vol. 103, No. 43, 1999
Chen et al.
Figure 4. Normalized absorption spectra of LB films of 1 obtained at different compression stages: a, first compression; b; second compression;
c; in chloroform. Inset I: Isotherms of 1 (1); first compression; (2); subsequent compression; inset II: schematic representation of chromophore
orientation for 1: a; glide or pinwheel aggregates; b; translation aggregates.
somewhat like an “LB multilayer arrangement”; however, here
the extended molecules are tilted 68° from the layer normal, in
contrast to the much closer to “perpendicular” arrangements
found for the stilbene crystals and for 1. Individual molecules
are in the “extended” structure. It should be pointed out that
the arrangement among molecules in the “layer” is a glide or
herringbone arrangement; however, the edge-to-face interactions
that occur between adjacent molecules in this structure are
somewhat different from those observed for crystals of stilbene
derivatives26 or in simulations on the layer structures in that
the closest approach is between phenyl rings of one molecule
and a naphthyl group of its neighbor instead of the simple
phenyl-phenyl contact that exists for the stilbene derivative.
Indications that the self-assembly of 1 may be much more
complex than for other aromatic fatty acids examined thus far
come from a study of its behavior in films at the air-water
interface and the corresponding LB films obtained at different
stages of compression. Thus when films of 1 are slowly
compressed [Figure 4 I(1)], there is a sharp rise at a relatively
large area/molecule, followed by a decrease in pressure after
an early maximum of ca. 30-35 mN/m is reached and then a
second increase. This “unusual” compression behavior is
observed only on the initial compression. Decompression and
subsequent recompression result in a single rise in the isotherm
at much smaller areas per molecule [Figure 4 I (2)]. This
behavior contrasts to that for the films of 2, which exhibit
constant isotherms with a single rise (no hysteresis) upon
successive compression-decompression cycles. The films of 1
are stable enough to be transferred to quartz supports during
the first rise in pressure, as well as during the second.
Interestingly, the spectra of the LB films of 1 obtained during
the two compression stages are quite different (Figure 4). The
film of 1 obtained upon the second compression shows
absorption and fluorescence spectra identical to those obtained
for the aqueous dispersions as shown in Figure 1, which we
have attributed to a translation layer arrangement. As described
above, these LB films are susceptible to rapid photobleaching.
In contrast, the films of 1 obtained from the first compression
show spectra nearly identical to those of 2, which are character-
ized by a pronounced blue shift and the “signature” attributed
to the “glide” or pinwheel aggregates above (Figure 4II). As
would be anticipated for a glide aggregate with edge-to-face
interactions, the films of 1 from this “first” compression are
photostable.
The simplest interpretation of the observed behavior is that
before to or during the first compression of 1 at the air-water
interface, the molecules aggregate in small clusters or “unit
aggregates” in which the favored noncovalent interactions are
edge-to-face. As these clusters begin to pack together (first rise),
an extended array or mosaic is formed, but the mismatch
between the relatively small area occupied by the headgroups
at the surface and the larger surface area of the pinwheel
chromophore aggregates caused by the “bent” shape of the
R-styrylnaphthalene renders this array metastable. Thus it
collapses or rearranges to a more compact and stable translation
layer array on further compression. To determine whether the
interconversion of the two aggregates is reversible, we have
examined the isotherms as a function of temperature. As shown
in Figure 5, the collapse pressure for the first compression
decreases as the temperature increases, suggesting a thermal
instability of the edge-to-face arrays for R-styrylnaphthalenes.
It also has been found that over a range of temperatures, once
the “second” aggregate is formed, no return to the first is
detected, reinforcing the idea that face-to-face interaction is the
favored interaction for the bulky and “bent”-shape R-styryl-
naphthalene chromophore in the amphiphile structures of 1 and
3. The presence of two distinctly different aggregates in the
films of 1 shows that the local and extended structure produced
by self-assembly of functionalized amphiphiles may be strongly
affected by competing forces or interactions. In this case favored
noncovalent interactions control the local structure of small
clusters but result in a metastable mosaic as the structure is

Self-Assembly of Styryl Naphthalene Amphiphiles
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9167
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Acknowledgment. We are grateful to the Center for Material
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