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

Article abstract of DOI:10.1021/jp973421k

Fatty acids and phosphatidyl choline derivatives incorporating the photoreactive trans-styrylthiophene chromophore have been prepared and studied in Langmuir-Blodgett films and aqueous dispersions, respectively. Both absorption and fluorescence spectra in the assemblies show prominent shifts indicative of aggregation similar to that observed with similar trans-stilbene and trans-azobenzene derivatives previously investigated. Studies of aqueous dispersions of the pure styrylthiophene phospholipids indicate the formation of structures much larger than the typical small unilamellar bilayer vesicles formed from saturated phospholipids of comparable chain length. Studies of the disaggregation process for two of the phospholipids give aggregation numbers of approximately 2 and 6, corresponding to 4 and 12 styrylthiophene units per aggregate. The stability of these aggregates is very similar to those of corresponding stilbene aggregates, and on the basis of spectral similarities it seems reasonable to propose a "pinwheel" fraction of a glide or herringbone structure as the "unit aggregate" and predominate species present in the aqueous dispersions. However, the chief photoreaction observed for the aqueous dispersions of the phospholipids is dimerization to form the syn head-to-head dimer, consistent with topologically controlled reaction from a translation structure in which nearest-neighbor chromophores are aligned parallel. Simulations suggest that while a translation layer structure may be of lowest energy, glide layer structures that show good agreement between measured and predicted properties are also energetically accessible. A possible explanation for the observed reactivity and photophysics is that within the aqueous dispersions there may be equilibration between "glide" and "translation" structures. When mixtures of the styrylthiophene phospholipids and saturated phospholipids are codispersed, bilayer vesicles are formed that are capable of entrapping reagents such as carboxyfluorescein. Irradiation of the aggregated trans-styrylthiophene in these vesicles leads to release of the entrapped carboxyfluorescein concurrent with photodimerization of the styrylthiophene. The pattern of reagent release is similar to that observed upon photoisomerization of the corresponding azobenzene aggregates and suggests a "catastrophic" destruction of the vesicles.

Full text of DOI:10.1021/jp973421k

5440
J. Phys. Chem. A 1998, 102, 5440-5450
Self-Assembly of Styrylthiophene Amphiphiles in Aqueous Dispersions and Interfacial
Films: Aggregate Structure, Assembly Properties, Photochemistry, and Photophysics
Xuedong Song, Jerry Perlstein, and David G. Whitten*
Department of Chemistry and NSF Center for Photoinduced Charge Transfer, UniVersity of Rochester,
Rochester, New York 14627, and Chemical Science and Technology DiVision, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545
ReceiVed: October 22, 1997; In Final Form: December 2, 1997
Fatty acids and phosphatidyl choline derivatives incorporating the photoreactive trans-styrylthiophene
chromophore have been prepared and studied in Langmuir-Blodgett films and aqueous dispersions,
respectively. Both absorption and fluorescence spectra in the assemblies show prominent shifts indicative of
aggregation similar to that observed with similar trans-stilbene and trans-azobenzene derivatives previously
investigated. Studies of aqueous dispersions of the pure styrylthiophene phospholipids indicate the formation
of structures much larger than the typical small unilamellar bilayer vesicles formed from saturated phospholipids
of comparable chain length. Studies of the disaggregation process for two of the phospholipids give aggregation
numbers of approximately 2 and 6, corresponding to 4 and 12 styrylthiophene units per aggregate. The
stability of these aggregates is very similar to those of corresponding stilbene aggregates, and on the basis of
spectral similarities it seems reasonable to propose a “pinwheel” fraction of a glide or herringbone structure
as the “unit aggregate” and predominate species present in the aqueous dispersions. However, the chief
photoreaction observed for the aqueous dispersions of the phospholipids is dimerization to form the syn
head-to-head dimer, consistent with topologically controlled reaction from a translation structure in which
nearest-neighbor chromophores are aligned parallel. Simulations suggest that while a translation layer structure
may be of lowest energy, glide layer structures that show good agreement between measured and predicted
properties are also energetically accessible. A possible explanation for the observed reactivity and photophysics
is that within the aqueous dispersions there may be equilibration between “glide” and “translation” structures.
When mixtures of the styrylthiophene phospholipids and saturated phospholipids are codispersed, bilayer
vesicles are formed that are capable of entrapping reagents such as carboxyfluorescein. Irradiation of the
aggregated trans-styrylthiophene in these vesicles leads to release of the entrapped carboxyfluorescein
concurrent with photodimerization of the styrylthiophene. The pattern of reagent release is similar to that
observed upon photoisomerization of the corresponding azobenzene aggregates and suggests a “catastrophic”
destruction of the vesicles.
Introduction
rated alkyl chains.2,3 In several cases2-4 both experimental
studies and simulations support a structure for the aggregated
chromophores in which both small unit aggregates (small
limiting clusters) and extended structures have a glide or
herringbone arrangement of the chromophores characterized by
favorable noncovalent interactions that appear to be predomi-
nantly edge-to-face,9 similar to that found in both experimental
and theoretical studies of the benzene dimer.10 A unit aggregate
in the case of a “pinwheel” tetramer and its extended mosaic
glide layer are shown schematically in Figure 1. The strong
excitonic interactions observed in the absorption spectra of the
aggregated aromatics or dyes are well-modeled in calculations
using the extended dipole-extended dipole treatment of Kuhn
and co-workers.11 That these noncovalent interactions can be
a strong force in controlling self-assembly is also indicated by
the finding that films at the air-water interface often show the
aggregate as the predominant species, even before compression
results in close packing of the amphiphiles.7 While these
interactions manifest themselves most strongly in aqueous
dispersions and at the air-water interface, they can also be
observed as an influencing factor in other contexts in different
media. For example, dyads linking a cholesterol moiety with
A number of recent investigations have shown that the
“normal” tendency of amphiphiles to self-assemble to form
micelles or bilayer dispersions in water or interfacial films such
as Langmuir-Blodgett (LB) monolayers at the air-water
interface can be modified or enhanced when aromatic groups
or other substituents possessing extended π-conjugation are
incorporated into the amphiphile structure.1-8 Studies with fatty
acids and phospholipids containing trans-stilbene,2 trans-
azobenzene,3 or diphenylacetylene units5 have shown enhanced
tendencies for self-association that manifest themselves in terms
of tight aggregation of the chromophores with pronounced shifts
in absorption and emission spectra, higher phase transition
temperatures for the phospholipid suspensions in water (as well
as persistence in the spectral shifts associated with the aggrega-
tion above the phase transition temperature), resistance of the
aggregated amphiphiles to dispersion upon dilution with satu-
rated fatty acid or phospholipid hosts in both films and aqueous
bilayers, and a tendency for several of the modified phospho-
lipids to form much larger macroscopic structures than the small
unilamellar vesicles generally obtained on dispersion of saturated
phospholipids and other double-chain surfactants having satu-
S1089-5639(97)03421-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/14/1998

Self-Assembly of Styrylthiophene Amphiphiles
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5441
CHART 1
Figure 1. Schematic representation of (A) “pinwheel” tetramer of
aromatic amphiphile (as seen from above) showing edge-face interac-
tions between neighboring π-systems and (B) a mosaic of these
fragments into a growing “glide” or herringbone array.
trans-stilbene,12 trans-azobenzene,13 or squaraine dyes14 exhibit
“supergelating” abilities in a variety of organic solvents;
formation of the gel is accompanied by spectral shifts from
monomer in solution, in some cases, to spectra similar to those
observed in the aqueous dispersions and interfacial films.
Similar molecules containing cholesterol, but no aromatic
chromophore, do not show comparable gelating tendencies.
When the monomer aromatic is photoreactive (e.g., trans-
stilbene), photoreactivity is often modified or eliminated in
aggregate assemblies. However, for the assemblies of the
azobenzene-derivatized amphiphiles3 and gelators13 irradiation
does result in trans-cis isomerization, although with reduced
efficiencies, and in many cases with strong disruption of the
structured assemblies. We have found, for example, that mixed
vesicles containing azobenzene-derivatized phospholipids un-
dergo release of entrapped reagents upon photolysis, concurrent
with photoisomerization.13 The large structures formed by
extended arrays of azobenzene phospholipids are completely
destroyed as photoisomerization occurs, and indications are that
the cis-azobenzene phospholipids must form much smaller and
less organized structures.
While the glide or herringbone structure is often found both
experimentally and in simulations to be the likely structure for
these aggregates,2-4,15 simulations frequently suggest that
translation layer structures may be close in energy. Since these
are gas-phase simulations, which do not permit the estimation
of solvent-solute interactions, any one of several low-energy
minima from the gas-phase calculations may be considered to
be of comparable probability for the structure in the actual
systems studied in which solvent-solute interactions play a
critical role. For example, the styrene amphiphiles 1 and 2
(Chart 1) form LB films and vesicles, respectively, in which
there are small spectral shifts compared to the above-described
systems and the present work.16 The aggregation and conse-
quent reactivity seem most consistent with a translation-layer
structure in which there are noncovalent interactions that are
predominantly face-to-face or “π-stacking”. Both of these
assemblies yield a single cyclobutane photodimer upon irradia-
tion, which has been shown to be the â or syn head-to-head
product. Simulations clearly predict that the lowest energy
configurations should be translation layers and the structure that
most closely predicts experimentally measurable properties such
as the exciton splitting, “tilt angle” of the chomophore and area/
molecule in the condensed film is a translation layer array in
which the two double bonds of nearest-neighbors are aligned
and separated by 4.10 Å, which is within the “magic distance”
of 3.5-4.2 Å for photodimerization of several olefins such as
cinnamic acid derivatives in the crystalline state.17
The styrylthiophene chromophore is formally similar to
stilbene and might be expected to exhibit similar photophysical
and photochemical behavior. trans-Styrylthiophene shows a
similar intense absorption spectrum in the ultraviolet, with the
long wavelength band red-shifted compared to trans-stilbene
but with a B band at nearly the same energy. Irradiation of
trans-styrylthiophene has been reported to result in isomerization
but to little dimerization.18 In the crystal trans-styrylthiophene
is photostable, as predicted from the crystal structure that shows
unfavorable packing for topologically controlled dimerization.19
Substituted trans-styrylthiophene derivatives have been found
to exhibit both photoisomerization and photodimerization in
solution as well as photodimerization in the solid state in some
cases.19a In the present paper we report the synthesis and
investigation of fatty acid and phospholipid derivatives incor-
porating the styrylthiophene chromophore at the end of a
hydrocarbon chain. We find that these amphiphiles exhibit
similar behavior to the above-mentioned aromatic derivatized
amphipihles in terms of exhibiting aggregate formation, concur-
rent with pronounced changes in absorption spectra and pho-
tophysical behavior. Although many aspects of the observed
behavior suggest the predominance of aggregates quite similar
to those observed for stilbene, azobenzene, and squaraine
derivatives,2-4 we find the chief photoreaction that occurs for
suspensions of the phospholipids in water is photodimerization
to give a major product that appears to derive from a topological
control in the bilayer similar to that observed for styrene 2.16
We find that mixtures of the styrylthiophene with saturated
phospholipids result in the formation of vesicles that can entrap
reagents such as carboxyfluorescein. Interestingly, we find that
irradiation of these vesicles under conditions producing pho-
todimerization results in rapid and nearly complete release of
the entrapped reagents.
Experimental Section
Materials and General Techniques. Synthetic reagents
were purchased from Aldrich Chemical Co. and used as received

5442 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Song et al.
CHART 2
unless otherwise stated. R-, â-, and γ-cyclodextrins (CDNs,
99+%) were purchased from Aldrich. L-R-glycero-3-phospho-
rylcholine as the cadmium chloride complex, L-R-dimyristoyl
phosphatidylcholine (DMPC, 99+%), and L-R-dipalmitoyl
phosphatidylcholine (DPPC, 99%) were obtained from Sigma.
All solvents for spectroscopic studies were spectroscopic grade
from Fisher or Aldrich. Milli-Q water was obtained by passing
in-house distilled water through a Millipore-RO/UF water
purification system. Deuterated solvents were purchased from
MSD Isotopes or Cambridge Isotope laboratories.
Melting points were taken on an El-Temp II melting point
apparatus and are uncorrected. Proton NMR spectra were
recorded on a General Electric QE300 MHz spectrometer using
deuterated solvent locks. FAB mass spectra were measured at
the Midwest Center for Mass Spectrometry. Absorption spectra
were obtained on a Hewlett-Packard 8452A diode array spec-
trophotometer. The circular dichroism (CD) study was carried
out on a JASCO J-710 spectropolarimeter. Fluorescence spectra
were recorded on a SPEX Flurolog-2 spectrofluorimeter and
were uncorrected. Differential scanning calorimetry (DSC)
measurements were carried out on a MC-2 Ultrasensitive
Scanning Calorimeter from MicroCal, Inc. Size extrusion
experiments for the particles in aqueous dispersions of am-
phiphiles were performed with an extruder through CoStar
Nucleopore Polycarbonate Filters. Dynamic light scattering
measurements were carried out at the Eastman Kodak research
laboratories. All samples were routinely filtered through a 1.2-
µm nylon syringe filter before data acquisition. A 200-W
Mercury lamp (Oriel) or a photochemical reactor (Rayonet, The
Southern New England Ultraviolet Company) was used for
irradiation. The 365-nm line was separated through an interfer-
ence filter (365 BP 10, T ) 217).
derivatized fatty acids or phospholipids were prepared by
spreading a chloroform solution of material to be studied onto
an aqueous surface containing cadmium chloride (2.5 × 10-4
M) and sodium bicarbonate (3 × 10-5 M, pH ) 6.6-6.8) on a
KSV 5000 automatic film balance at room temperature (23 °C).
The monolayers were then transferred onto quartz substrates.
Quartz glass slides were cleaned sequentially in chloroform and
detergent and followed by “piranha” solution (30% H2O2/H2-
SO4) cleaning for 30 min. The quartz substrates were rinsed
throughly in Milli-Q water and dried before use in the LB
assemblies. Reflectance spectra for the monolayers at the air/
water interface were recorded with a SD1000 fiber optics
spectrometer (Ocean Optics, Inc.) equipped with optical fibers,
an LS-1 minature tungsten halogen lamp, and a CCD detector.
Bilayer vesicles were prepared according to established proto-
cols.21 A Cell Disrupter W220F from Heat Systems Ultrasonics,
Inc. (setting 6.5, 35w) was used for probe-sonication. Aggregate
size measurements are similar to the procedures used for
azobenzene-derivatized phospholipid (APLs)3 and stilbene-
derivatized phospholipids (SPLs).2 The fluorescence lifetime
measurements are described elsewhere.5
Identification of Photoproducts for Aggregates. An aque-
ous dispersion of TS4EPC (11 mg, see Chart 2 for its structure)
was irradiated in a photochemical reactor using ultraviolet light
until the blue-shifted absorption band of the aggregates disap-
peared. Sodium hydroxide (5 equiv) was added to the reaction
mixture followed by stirring at 45 °C for 24 h. The mixture
was acidified by 5% hydrochloric acid to pH ) 2, and the
products were extracted with chloroform. Chloroform was
removed, and the residue was dissolved in methanol containing
a drop of concentrated sulfuric acid followed by refluxing for
2 h. The sulfuric acid was neutralized by sodium bicarbonate,
and the solvent was removed. TLC (chloroform/methanol )
95/5) showed one major spot and one minor spot for the residue.
No product was observed in the absence of irradiation. The
The general methods used for preparing Langmuir-Blodgett
(LB) films and self-assemblies are based on techniques described
by Kuhn and co-workers.20 Monolayers of chromophore-

Self-Assembly of Styrylthiophene Amphiphiles
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5443
TABLE 1: Absorption and Fluorescence Data for TPLs and
TS8A in Different Media
major photoproduct was separated by silica gel chromotagraphy
to give an oil. 1H NMR (500 MHz, CDCl3), δ: 6.8-7.3(m,
14H), 4.55(dd, 4H), 3.70(s, 6H), 2.55(t, 4H), 2.25(t, 4H), 1.90-
(m, 4H). FAB: calcd for [M + H]+, 573.2; found, 573.2
Synthesis of Styrylthiophene Derivatives. The synthesis
of the styrylthiophene derivatives investigated in this study with
their acronyms is outlined in Chart 2.
BrTK8E: Methyl suberyl chloride (2.1 g) in 10 mL of toluene
was added dropwise to a mixture of AlCl3 (9 g) and dry toluene
(40 mL) at 0 °C. The reaction mixture was allowed to warm
to room temperature over 3 h and then decanted into ice water.
Ether was used to extract the product, and the ether layer was
then washed with 5% NaHCO3 aqueous solution. After the
ether was removed, a raw product (9.1 g) was obtained, which
was pure enough for bromination.
The raw product (5.3 g) obtained above together with
N-bromosuccinimide (NBS, 4.8 g) in CCl4 (100 mL) was gently
refluxed for 5 min, followed by irradiation with a sunlamp to
initiate the reaction. The reaction was complete in 10 min, and
the product, a precipitate, was separated by filtration. After
the solvent was removed, the residue was recrystallized in THF/
hexane to give white needle crystals (3.4 g, 49%). 1H NMR
(300 MHz, CDCl3), δ: 7.95 (d, 2H), 7.51(d, 2H), 4.53(s, 2H),
3.70(s, 3H), 2.95(t, 2H), 2.35(t, 2H), 1.4-1.8(m, 10H).
TSK8E: A mixture consisting of BrTK8E (2.4 g) and
triphenylphosphine (3.1 g) in dry xylene (100 mL) was refluxed
for 4 h and then cooled to room temperature. The precipitate
(3.2 g) that was produced was collected and dissolved in a
solvent mixture consisting of methylene chloride (25 mL) and
tetrahydrofuran (50 mL). To this solution was added potassium
carbonate (3.6 g), 18-crown-6 (20 mg), and 2-thiophenecar-
boxaldehyde (2 mL). The mixture was refluxed for 12 h, and
a dark solid was separated by filtration. The solvent was
removed, and the residue was purified by a silica gel column
using CH2Cl2/hexane ) 1/1 as eluent to give TSK8E (0.8 g,
32%). 1H NMR (300 MHz, CDCl3), δ: 7.97(d, 2H), 7.58(d,
2H), 7.39(d, 1H), 7.35-7.05(m, 2H), 7.09(t, 1H), 6.97(d, 1H),
3.69(s, 3H), 2.98(t, 2H), 2.35(t, 2H), 1.8-1.4(m, 10H).
abs
max
(ꢀmax
em
λ
λ
∆λabs b ∆λem b
max
c
compounds
TS8EPC
media
CHCl3
H2O
LB film
DMPC
CHCl3
H2O
)
(QYa)
(nm)
(nm)
332(5.04) 388(0.041)
290(3.74) 426(0.33)
42
46
40
41
286
427
334(4.91)
TS4EPC
TS8A
332(5.39) 388(0.032)
306(3.93) 426(0.13)
334(5.13)
26
40
41
DMPC
CHCl3
LB film
332
286
388
427
46
46
monolayerd 286
a Quantum yield, stilbene in methyl cyclohexane as reference.
b Spectral shifts relative to those for monomer. c Extinction coefficient
(×10-4). d Monolayer film at the air-water interface.
(s, 1H), 7.16(d, 2H), 7.10(d, 1H), 7.02(t, 1H), 2.66(t, 2H), 2.34-
(t, 2H), 1.61(m, 4H), 1.30(m, 6H).
TS8EPC: The synthesis is similar to the reported proce-
dures.5,22 Yield: 44%. 1H NMR (300 MHz, CDCl3), δ: 7.40-
(d, 4H), 7.26-7.10(m, 8H), 7.08(d, 2H), 7.02(t, 2H), 6.90(d,
2H), 5.24(m, 1H), 4.40(m, 3H), 4.16(m, 1H), 4.00(m, 2H), 3.88-
(m, 2H), 3.37(s, 9H), 2.60(t, 4H), 2.32(m, 4H), 1.60(m, 8H),
1.30(m, 12H). FAB: m/z calcd for [M + H]+, 878.3; found,
878.3.
TS4A: The preparation procedure is identical with that for
TS8A. Yield, 59%; mp, 126-128 °C. 1H NMR (300 MHz,
CD3OD), δ: 7.48(d, 2H), 7.30(m, 2H),7.20(d, 2H), 7.17(m, 1H),
6.99(m, 1H), 6.93(d, 2H), 2.67(t, 2H), 2.33(t, 2H), 1.91(m, 2H).
TS4EPC: The synthesis is similar to that for TS8EPC.
Yield: 67%. 1H NMR (300 MHz, CDCl3), δ: 7.38(d, 4H),
7.15-7.10(m, 8H), 7.07(d, 2H), 7.02(t, 2H), 6.90(d, 2H), 5.28-
(m, 1H), 4.42(m, 3H), 4.20(m, 1H), 4.08(m, 2H), 3.90(m, 2H),
3.37(s, 9H), 2.66(m, 4H), 2.35(m, 4H), 1.95(t, 4H). FAB: calcd
for [M + H]+, 766.2; found, 766.2.
Results
TS8A: Two grams of zinc dust was washed with 5 mL of
10% hydrochloric acid for 1 min followed by three washings
of distilled water. To the zinc was added 5 mL of amalgamating
solution (containing 5 mL of concentrated hydrochloric acid,
50 mL of water, and 5 g of mercuric chloride). The mixture
was agitated for 1 min, followed by three washings of distilled
water. To the amalgamated zinc was added 10 mL of 25%
hydrochloric acid, 20 mL of toluene, and 800 mg of TSK8E.
The mixture was refluxed with fast stirring. TLC (hexane/
acetone ) 4/1 as eluent) showed complete disappearance of
the starting material after 20 min. After the mixture cooled to
room tempertaure, the toluene layer was collected and the
solvent was removed. The residue was purified by a silica gel
column using hexane/acetone ) 4/1 as eluent to afford 300 mg
of white crystals (41%). 1H NMR (300 MHz, CD3OD), δ: 7.4-
(d, 2H), 7.29(d, 1H), 7.25(s, 1H), 7.16(d, 2H), 7.10(d, 1H), 7.02-
(t, 1H), 6.92(d, 1H), 3.65(s, 3H), 2.62(t, 2H), 2.34(t, 2H),
1.61(m, 4H), 1.35(m, 6H).
A mixture containing the white crystals (300 mg) obtained
above, potassium hydroxide (0.5 g), water (5 mL), and acetone
(10 mL) was refluxed for 0.5 h, and the acetone was then
removed. The aqueous phase was acidified using 10% hydro-
chloric acid, and the precipitate was collected. The raw product
was recrystallized from a mixed solvent of acetone and water
to give white crystals (250 mg, 87%); mp, 124-125 °C. 1H
NMR (300 MHz, CD3OD), δ: 7.41(d, 2H), 7.29(d, 1H), 7.24-
Preliminary Characterization of Styrylthiphene Am-
phiphiles in Dilute Solution, Aqueous Dispersions, and
Langmuir-Blodgett Films. The structures of the fatty acid
and phospholipid derivatives used in this study are shown in
Chart 2 together with their acronyms. Both the fatty acids and
phospholipids are soluble in organic solvents such as chloroform
or methylene chloride. For both sets of compounds dilute
solutions show an intense absorption in chloroform (A band)
at 332 nm and fluorescence with a maximum at 388 nm, which
are indistinguishable from those of the alkyl-substituted trans-
styrylthiophene monomer. Compared to that of trans-stilbene,
these spectra are similar but with less structure and with red-
shifts of 17 and 32 nm for absorption and fluorescence,
respectively. Aqueous dispersions of the phospholipids TS4-
EPC and TS8EPC, prepared by sonication, give clear solutions
that scatter light and that exhibit blue-shifted absorption and
red-shifted fluorescence spectra relative to those for the same
compounds in chloroform. Dispersions of TS4EPC and TS8-
EPC in water with an excess of saturated phospholipid such as
dimyristoyl phosphatidyl choline (DMPC) give absorption
spectra slightly broader than those obtained in chloroform, while
the fluorescence appears nearly identical with that in organic
solvents. Data for absorption and fluorescence spectra in
different media are listed in Table 1. Figure 2 compares
absorption and fluorescence spectra of TS4EPC in different
media.

5444 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Song et al.
Figure 2. Absorption and fluorescence spectra of TS4EPC in different
media: s in chloroform, -×- in water, -‚‚- in DMPC.
Figure 3. Normalized absorption and fluorescence spectra of TS8A
in cyclodextrins: s R-cyclodextrin, - - - â-cyclodextrin, ‚‚‚ γ-cyclo-
dextrin.
Both TS8A and TS8EPC form insoluble monolayer films upon
dispersion from chloroform at the air-water interface. TS4A,
however, is too soluble in water to form an LB film. The films
of TS8A exhibit pressure-area isotherms similar to those of
the trans-stilbene fatty acids and saturated fatty acids with a
limiting area/molecule of 22-24 Å.2 This suggests close packing
into a highly ordered, near-crystalline array. The films are easily
transferred to rigid supports such as quartz with qualitatively
similar spectra to those for the aqueous dispersions of TS8EPC.
In contrast, films of TS8EPC however exhibit a much broader
isotherm, and the films are not easily transferred to quartz slides.
For compression of the films of TS8EPC, an early phase change
is observed followed by a moderately steep rise to a limiting
area of 30 Å2/molecule; at this point it appears a bilayer may
be forming since a limiting area/molecule for a two-chain
amphiphile would be expected to be signficantly larger. Both
films show the same blue-shifted absorption. As was previously
observed for the stilbene2 and azobenzene3 amphiphiles, studies
of the reflectance spectra of films of TS8A and TS8EPC at the
air-water interface show essentially constant (blue-shifted)
spectra before, during, and after compression suggesting that
whatever interaction exists between chromophores in the
amphiphiles does not occur as a consequence of the compres-
sion.
Both TS4A and TS8A are rendered more water soluble upon
addition of R-, â-, or γ-cyclodextrins; the effect is particularly
noticeable in the case of TS8A. The complex of TS8A with
â-cyclodextrin shows similar absorption and fluorescence to the
spectra obtained in chloroform; however, the spectra for TS8A
in the presence of R-cyclodextrin are noticeably sharper, and
the fluorescence is much more intense (see below). In contrast,
the complex of TS8A with γ-cyclodextrin gives a slightly blue-
shifted absorption, which is similar to those of the TS phos-
pholipids in water with excess DMPC. The fluorescence of
TS8A in γ-cyclodextrin is broadened and red-shifted compared
to the monomer in the other cyclodextrins and similar to that
for the TS phospholipids in water. Figure 3 compares absorp-
tion and emission spectra of TS8A in the presence of the
different cyclodextrins. Figure 4 compares absorption and
fluorescence spectra of TS8A, TS8EPC, and TS4EPC in the
several different environments discussed above. While TS8A
exists in cyclodextrin solutions almost exclusively as the
complex, solutions of TS4A in the presence of the different
cyclodextrins show only a trace amount of complex formation.
Figure 4. Absorption and fluorescence spectra of TS8A, TS8EPC, and
TS4EPC in different media: s TS8EPC in water, ‚‚‚ TS4EPC in water,
-×- TS8A monolayer, - - - TS8A in γ-CD.
On the basis of the general similarities between the TS
amphiphiles and the well-studied trans-stilbene derivatives, it
appears reasonable to assign the shifted absorption and fluo-
rescence spectra obtained in the pure phospholipid aqueous
dispersions and Langmuir-Blodgett films of TS8A, TS8EPC,
and TS4EPC to “H” aggregates in which the TS chromophores
are packed with their long-axis polarized transitions parallel.
The slightly broadened absorption spectra obtained for the TS
phospholipid dispersions in the presence of excess DMPC can
also be assigned to a dimer by analogy to the results obtained
for the stilbene derivatives2 and from the similar spectrum
obtained for TS8A in the presence of γ-cyclodextrin. This
suggests that the aggregates obtained in the pure dispersions
must be larger than dimers. As shown in Figure 4, the
absorption spectrum for TS4EPC in water is broader and
significantly less blue-shifted than that for TS8EPC in water
while the monolayer of TS8A shows the most pronounced blue-
shift. It has previously been shown that the extent of the blue-
shift in the absorption spectra can be roughly correlated with
the aggregate size.2 The larger blue-shifts observed for the
monolayers and LB films of TS8EPC and TS8A compared to

Self-Assembly of Styrylthiophene Amphiphiles
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5445
TABLE 2: Equilibrium and Rate Constants for
Deaggregation Process
T (°C)
k (×103 min-1
)
K (×102)
TS8EPC
TS4EPC
50
55
60
32
35
40
45
50
55
1.0
1.2 × 10-5
7.0
2.1
1.78
2.26
4.34
7.62
11.8
Figure 6. Benesi-Hildebrand plot of ln Aagg as a function of ln Adi
for TPLs in DMPC vesicles: 9 TS4EPC at 35 °C, + TS8EPC at 55
°C.
Figure 5. Time evolution of absorption spectra of TS4EPC (upper)
and TS8EPC (lower) assemblies in water when treated with DMPC
vesicles.
for TS4EPC over the temperature range 35-55° C shows good
linearity and enables an estimate of ∆H ) 19.5 kcal/mol and
∆S ) 55 cal/mol K for the disaggregation. The results for TS4-
EPC may be compared with the corresponding trans-stilbene
phospholipid S4EPC,2 which also forms relatively small ag-
gregates. Both the enthalpy (ca. 5 kcal/mol/chromphore) and
entropy (ca. 15 cal/mol K/molecule) as well as the rate constants
for disaggregation (2 × 10-2 min-1 for S4EPC at 30 °C and
2.1 × 10-2 min-1 for TS4EPC at 32 °C) are similar even though
the aggregation numbers are different.
TS4EPC aqueous dispersions suggest relatively extended ag-
gregates in the former and smaller aggregates in the latter.
Studies of Details of Aggregation of Styrylthiophenes in
Aqueous Dispersions. As has been found with azobenzene-
and stilbene-derivatized phospholipids, mixing of pure aqueous
dispersions of the substituted phospholipid with aqueous disper-
sions of pure saturated phospholipids, in large excess, leads to
disaggregation of the “H” aggregates to dimer (or monomer
for phospholipids containing only a single chromophore).2,3 The
presence of isosbestic points during the conversion has been
taken as evidence that clean evolution from an aggregate of
definite size to the dimer occurs during the process. As shown
in Figure 5, similar spectral changes occur for both TS8EPC
and TS4EPC with DMPC. For the previously studied modified
phospholipids,2,3 it was found that this sort of “membrane
exchange” process occurs only above the phase transition
temperature, Tc, for both of the phospholipids undergoing the
mixing. In the present studies it was found that the exchange
could be observed only at or above 60° C for TS8EPC and above
30° C for TS4EPC, implying that the two phospholipids exhibit
the chain-melting transition at these temperatures. The disag-
gregation process follows first-order kinetics (independent of
[DMPC]), suggesting extrusion of an individual phospholipid
from the aggregate is the rate-limiting step. Table 2 gives
equilibrium and rate constants for the disaggregation process.
From the equilibrium constants, a modified Benesi-Hildebrand
relationship2,23 (Figure 6) was used to determine the aggregation
numbers; in terms of phospholipid molecules, these were found
to be 2.02 and 5.96 for TS4EPC and TS8EPC, respectively. In
terms of TS chromophores these correspond to 4 and 12,
respectively, for the two phospholipids. A plot of ln K vs 1/T
As was also the case with the pure dispersions of azobenzene-
and stilbene-phosphatidyl choline derivatives,2,3 the styrylthio-
phene phospholipid pure aqueous dispersions give evidence of
forming structures that may be much larger than those for simple
saturated phospholipids and perhaps of quite different shapes.
Thus while dipalmitoyl phosphatidyl choline has an approximate
size (diameter, for small unilamellar vesicles) of 9.3 nm,24a the
stilbene phospholipids have sizes estimated by light scattering
or membrane filtration in the range 200-500 nm. Similarly,
membrane extrusion experiments indicate sizes of approximately
150 and 100 nm for dispersions of TS4EPC and TS8EPC,
respectively. As indicated above, phase transition temperatures
may be roughly indicated by the temperatures at which mixing
occurs readily with DMPC (Tc ) 20.9 °C);24b for TS8EPC there
is also a sharp discontinuity in a plot of fluorescence efficiency
vs T at 60°, which supports a chain-melting occurrence in this
range.
Although the TS phospholipids are chiral (prepared from
R-glyceryl phosphatidyl choline), there is no induced circular
dichroism (ICD) in the region of the TS chromophore absorp-
tions when the phospholipids are dissolved in organic solvents
such as chloroform. However, for the aqueous dispersions of
both TS4EPC and TS8EPC, there is a strong biphasic (excitonic)
ICD in the regions of the aggregate absorption with a crossover

5446 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Song et al.
point coinciding with the absorption maximum. The apparent
molar ellipticity of the TS4EPC dispersion is more than one-
third higher than that of the TS8EPC dispersion, which may
reflect either a larger asymmetry of the smaller aggregate for
the former or a shorter distance between the chromophore and
the chiral headgroup or a combination of both factors.
Photophysics and Photochemistry of Styrylthiophene
Phospholipids in Solution and Aqueous Dispersions. As
shown in Table 1, the fluorescence quantum yields are relatively
low (a few percent) in organic solutions but increase to the range
10-30% in aqueous dispersions. The fluorescence decays in
chloroform are found to be monoexponential with very short
lifetimes (23 ps for TS4EPC and 29 ps for TS8EPC, close to
the limit of instrument resolution). For the cyclodextrin
complexes of TS8A, it was found that the monomer in
R-cyclodextrin has a fluorescence efficiency of 0.24, while for
â-cyclodextrin, the fluorescence efficiency is 0.03, similar to
the chloroform solution, while the “dimer” in γ-cyclodextrin
shows a fluorescence quantum yield of 0.2. As has been found
for other aqueous dispersions of aggregated fluorescent chro-
mophores,2,5 the fluorescence decay of dispersions of the
styrylthiophene phospholipids are complicated; in each case the
fluorescence can be fit into three lifetime distributions. The
measured distributions are similar; in each case there is an
extremely short lifetime distribution (20-100 ps), which is of
negligible percentage and could be attributed to either monomer
or an artifact. A second distribution, 0.34 ns and 25% for TS4-
EPC and 0.2 ns and 35% for TS8EPC, and third distribution,
2.7 ns for TS4EPC (75%) and 4.8 ns for T8EPC (65%), comprise
most of the decay and can be tentatively assigned to aggregate
and excimer (see below), respectively, in accord with the similar
assignments for the stilbene phospholipids.2
Figure 7. (upper) Absorption and (lower) ICD spectra of TS4EPC
aqueous dispersion upon irradiation at 365 nm.
CHART 3
The styrylthiophenes are photoactive in both organic solutions
and aqueous dispersions. Relatively efficient photoisomerization
from trans to cis occurs upon irradiation of chloroform solutions
at 365 nm, which was confirmed by the appearance of signals
attributed to the new cis protons in 1H NMR. This is consistent
with the reported results for irradiation of simple styrylthio-
phenes in benzene.18 The cis isomers are thermally stable at
room temperature. Both of the styrylthiophene phospholipids
undergo photobleaching upon irradiation of the aggregate band
in aqueous dispersions. As shown in Figure 7 for TS4EPC
aqueous dispersions, the photobleaching coincides with a
disappearance of the ICD bands; the net efficiency of the
photobleaching is about one-sixth that of the photoisomerization
in chloroform. The product from irradiation of aqueous
dispersions of TS4EPC was isolated and analyzed. The process
involved hydrolyzing the crude product to cleave the phos-
phatidylcholine residue and reesterification of the product acid.
The major photoproduct was studied by 1H NMR and FAB mass
spectroscopy and found to be a photodimer, which can be
assigned as either 1 or 3 of the four possible dimers resulting
from cyclodimerization of two trans-styrylthiophenes. (Chart
3) This structural assignment was based upon the symmetrical
1
cyclobutane AA′BB′ multiplets centered at δ 4.55 for the H
NMR of the major product and literature reports for structurally
related photodimer sets.25 The presence of only one major
photodimer from the aqueous dispersions clearly attests to a
well-ordered packing structure in the aggregates.16 The excita-
tion and fluorescence spectra of the TS phospholipid aqeuous
dispersions show interesting changes as the irradiation proceeds.
They remain effectively unchanged through the major portion
of the irradiation (except for a decrease in intensity; the products
do not fluoresce). However, at late stages (∼90% reaction) the
fluorescence spectra become broader and are similar to that of
the excimer observed for TS8A in γ-cyclodextrin. The excita-
tion spectra also shift to the red and could be attributed to either
the monomer or dimer (Figure 8). Another interesting change
that accompanies photodimerization of the aqueous dispersions
of pure TS4EPC or TS8EPC is a reduction in size of the
assemblies. Thus while no TS8EPC would pass through a

Self-Assembly of Styrylthiophene Amphiphiles
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5447
TABLE 3: Simulated Two-Dimensional Layer Structures
for TS8A
d
d
rank typea ab
cb
âb
areac λ1 λ2 ratiod energy
0
1
2
3
4
5
6
7
8
9
10
exptl
t
t
t
t
t
t
g
g
g
g
g
5.27 4.12 83.70 21.58 298
5.25 4.42 96.95 23.03 290
5.61 4.34 94.79 24.26 306
5.27 4.44 98.47 23.14 290
4.28 6.06 71.03 24.53 304
6.75 4.12 78.29 27.23 311
6.55 8.23 90.00 26.95 308 338 0.04 -24.52
6.46 8.23 90.00 26.38 305 338 0.02 -24.52
7.75 9.55 90.00 37.01 342 345 0.00 -22.82
6.83 10.79 90.00 36.85 340 338 22.6 -22.63
6.15 7.39 90.00 22.72 295 339 0.02 -22.20
23.0 290
-31.02
-30.15
-30.06
-29.03
-28.20
-27.41
a t ) translation layer structure, g ) glide layer structure. b a, c, â
are predicted unit cell dimensions in Å and angle in deg between a
and c, respectively. The value of 90° for glide layers is assumed (not
predicted) because the glide layer space group requires it. See ref 28
for details of the unit cell symmetries for glide and translation layers
and ref 15 for pictorials. c Surface area in Å2 per molecule. d λ1 and λ2
are computed spectral peaks from exciton coupling and the ratio ïf
the long-wavelength predicted intensity to the short-wavelength pre-
dicted intensity using the extended dipole approximation of Kuhn et
al. (ref 11).
Figure 8. Emission and excitation spectra of TS4EPC aqueous
dispersion upon photobleaching: s 0 min, - - - 16 min, -‚‚- 26 min,
‚‚‚ 36 min.
polycarbonate membrane with 100-nm pores, the photoproduct
from the dispersion could pass completely through the same
membrane.
TABLE 4: Simulated Two-Dimensional Layer Structures
for TS4A
Reagent Entrapment and Photorelease from Vesicles
Containing Styrylthiophene Phospholipids. While the pure
dispersions of TS4EPC and TS8EPC are not able to entrap water
soluble reagents such as carboxyfluorescein (CF), presumably
because they do not exist as simple bilayer vesicle structures,
mixtures of DPPC with small amounts of the styrylthiophene
phospholipid TS8EPC do form evidently closed structures that
can entrap carboxyfluorescein for prolonged periods (more than
one week) in the dark. Particularly interesting results were
observed for incorporation of TS8A and TS8EPC in DPPC to
form vesicles with carboxyfluorescein entrapped on the inside
at a concentration high enough to ensure efficient self-
quenching.26 Only a small percentage (<3 mol %, but primarily
aggregate) of TS8EPC in the DPPC vesicle is needed to produce
almost total release of the entrapped CF upon photolysis at 365
nm. In contrast, incorporation of TS8A to a level of >10 mol
% (mostly monomer) results in only a small amount of release
(<10%) upon photolysis under the same conditions. The pattern
of release for the TS8EPC-DPPC dispersions is similar to that
observed for photorelease of entrapped CF from azobenzene
phospholipid aggregate-DPPC dispersions and suggests a
“catastrophic” mechanism for destruction of the vesicle struc-
ture.3
Monte Carlo Simulations of the Structure of Styrylthio-
phene Fatty Acid Clusters: Relevance to the Structure of
the Aggregate in LB Films and Phospholipid Dispersions.
Additional information concerning possible aggregate structures
in the assemblies of the styrylthiophene amphiphiles can be
obtained by examining possible packing geometries using Monte
Carlo cooling simulations. The detailed procedures for the
structural predictions and the technique have been reported
elsewhere.4b,27 The simulated packing of TS4A and TS8A into
monolayers was accomplished by applying Kitaigorodskii’s
Aufbau principle (KAP)28 to a Monte Carlo simulation using
the MM2 force field. The predicted spectral shifts resulting
from exciton interaction for the simulated structures were
calculated using the extended dipole-extended dipole method
developed by Kuhn and co-workers.11 For this computation a
transition moment of 7.07 D was estimated from the oscillator
strength of the styrylthiophene monomer in chloroform at 332
nm with an approximate classical dipole length of 8.9 Å,
rank typea ab
cb
5.59 4.31 94.86 24.01 300
6.89 6.49 90.00 22.36 297 338 0.18 -28.31
âb
areac λ1 λ2 ratiod energy
d
d
0
1
2
t
g
t
-29.39
5.90 4.25 95.09 24.98 303
5.52 4.06 92.11 22.40 294
5.56 4.34 100.23 23.75 293
6.70 7.12 90.00 23.85 298 338 0.10 -27.21
6.14 4.31 85.08 26.37 307
6.81 7.18 90.00 24.45 296 339 0.01 -26.64
6.41 8.07 90.00 25.86 304 338 0.06 -26.59
6.12 4.07 79.66 24.50 305
4.96 4.66 86.15 23.06 300
-27.82
-27.34
-27.29
3
4
t
t
5
6
g
t
-26.72
7
8
9
g
g
t
-26.30
-26.29
10
exptl
t
23
290
a t ) translation layer structure, g ) glide layer structure. b a, c, â
are unit cell in Å and deg. c Surface area in Å2 per molecule. d λ1 and
λ2 are computed spectral peaks from exciton coupling and the ratio ïf
the long-wavelength predicted intensity to the short-wavelength pre-
dicted intensity using the extended dipole approximation of Kuhn et
al. (ref 11).
estimated from the carbon-carbon end-to-end distance for trans-
styrylthiophene. The dielectric constant was assumed to be 2.5.
The exciton splitting was computed by summing the interaction
energy of the dipoles over the monolayer lattice. Convergence
was achieved by the time 14 × 14 unit cells were included in
the computation. Table 3 summarizes data for the lowest energy
translation and glide structures predicted for TS8A, while Table
4 gives data for the lowest energy structures predicted for TS4A.
The predictions are somewhat different for the two fatty acids.
Thus for TS8A the lowest energy structures are all translation
layers; of those the ones of rank 1 and 3 show good agreement
in terms of the predicted absorption spectra for the aggregate
and the measured area/molecule. The glide layer structure of
rank 10 also shows reasonable agreement with the measured
properties. Figure 9 compares overhead views of the simulated
structures of rank 1 (translation) and 10 (glide). For the
simulated structures of TS4A the lowest energy structures are a
mix of glide and translation layers. Although TS4A is too water
soluble to study as a film at the air-water interface, a reasonable
assumption is that the area/molecule in the compressed mono-
layer would be approximately the same as that for TS8A.

5448 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Song et al.
Figure 9. (a, left) Rank 1 translation layer of TS8A. View is looking down on the layer from the thiophene side showing the packing of the
thiphene groups (sulfur in yellow, carbon in green). Cell dimensions of the layer in angstroms, angles in degrees. (b, right) TS8A glide layer rank
10. View is also from the thiphene side. The glide direction is along the c-axis.
Several low-energy structures (all within 3 kcal/mol of each
other in energy) are either glide or translation; however, in terms
of agreement between experimental and predicted parameters
for TS8A, there is little basis to favor any specific structure.
The molecular simulations suggest that glide and translation
layer structures may be relatively close in energy for the
corresponding fatty acid monolayers and that the spectral shifts
observed and other properties might be accommodated by either
a translation or glide structure. Since these simulations are for
gas-phase structures and do not take into account possible
interactions between water and the headgroups, it is quite
possible that solvent interactions may alter the relative energies;
thus, it is reasonable to consider any of the structures listed in
Tables 3 and 4 as potentially accessible or even global minimum
energy configurations. In fact, given the anticipated somewhat
more dynamic nature of the aqueous phospholipid bilayer
suspensions in water compared to crystals or condensed LB
films or supported multilayers, it seems possible that for the
former oscillation or equilibration between differing assembly
minima may take place. Thus it is reasonable that the “true”
structure of the aqueous phospholipid dispersions might consist
of equilibrating glide and translation structures or oscillating
translation layer structures or both or a more complex array.
The fact that the absorption spectra show relatively little
variation with temperature might be taken as evidence either
that there are not dramatic differences in chromophore-
chromophore interactions in differing contributing ensembles
or that the composition does not vary substantially over the
temperature range examined. The observation that photodimer-
ization occurs in the aqueous phospholipid dispersions may also
be attributed to dynamic possibilities not available for crystals
or even LB multilayers. Thus for TS8A and TS4A only one of
the simulated low-energy translation layer structures having
reasonable agreement with measured parameters (structure of
rank 9 for TS4A in Table 4) has double bonds in adjacent
molecules within the “magic distance” limit of 4.2 Å.17,30 The
necessity for motion within the ensemble to bring adjacent
molecules close enough to dimerize may be one reason for the
Discussion
From the experimental results presented above, it would seem
reasonable to infer that the styrylthiophene phospholipids and
fatty acids form aggregates of similar strength and structure to
those observed previously for trans-stilbene and azobenzene
amphiphiles.2,3 In particular, the aggregation numbers observed
(4 and 12), enthalpy and entropy of dissociation, absorption
spectral shifts, and structured fluorescence are all consistent with
a “supramolecular” unit aggregate. A reasonable structure might
be a “pinwheel” tetramer2-4 characterized by edge-face as-
sociation of the chromophores (Figure 1) similar to those
deduced for the structurally similar stilbene and azobenzenes
and for the somewhat different squaraines. The packing together
of “pinwheel” unit aggregates readily generates an extended
glide or herringbone lattice, as has been pointed out previously.
We have found that crystals of several amphiphilic trans-stilbene
derivatives have structure of this type, which closely resemble
Langmuir-Blodgett multilayers.15 However, the observation
of clean photochemical reaction in the pure aqueous suspensions
of the styrylthiophene phospholipids to produce what is most
reasonably assigned as the syn head-to-head photodimer appears
much more consistent with a topologically controlled process,
which can be envisioned to occur most readily from a translation
layer arrangement of the chromophores in which there is a
parallel or face-face arrangement of the chromophores,29 such
as in the simulated translation layer structure shown in Figure
9a. In this regard the photodimerization appears closely related
to that observed for crystals of molecules such as the cinnamic
acid derivatives30 or monolayer films and phospholipid suspen-
sions of amphiphilic styrene derivatives.16

Self-Assembly of Styrylthiophene Amphiphiles
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5449
Figure 10. (a, left) Simulated structure of TS8A from rank 1 translation layer (Table 3); (b, right) simulated structure of resultant syn-head-head
photodimer.
relatively low quantum yield for dimerization compared to the
solution-phase isomerization yield.
photodimer. Figure 10 compares simulated structures of a pair
of TS8A molecules in the rank 1 translation layer before and
after formation of the syn head-to-head photodimer. Although
the area of the photodimer is slightly smaller than that of the
pair of monomers, the shape is quite different as is the “π”-
surface exposed to adjacent molecules. Both of these changes
that occur upon photodimerization might be expected to play
important roles in destabilizing the aggregate structure. The
observation that “excimer” fluorescence becomes prominent as
high conversion to photodimer occurs in pure dispersions
suggests the possibility that the “defect” introduced by conver-
sion of two monomers to a photodimer may facilitate the
collapse of “aggregate” excited states to “excimer” and actually
accelerate photodimerization at moderate-to-high conversion.
Since the assessment of quantum yields for reaction at moderate
to high conversion cannot be carried out with any precision, it
is very hard to test this possibility. A perhaps more interesting
possibility is that the formation of photodimers in a mosaic of
aggregate results in weakened noncovalent interactions between
the π-systems and at some stage to a complete collapse of the
aggregate “mosaic”. This would be consistent with the pho-
todimerization producing a similar “catastrophic” destruction
One of the more interesting findings in this study is that
photolysis of mixed bilayer vesicles (TS8EPC:DPPC) under
conditions where the photolysis should convert aggregates
styrylthiophene monomer to photodimer results in release of
reagents entrapped within the vesicles. If photodimerization
were a simple topologically controlled process that resulted in
little change of shape and size of the reacting pair, it would be
anticipated that little disruption of the vesicle macrostructure
would result. Clearly this is not the case, since the photoinduced
release that occurs is quite comparable in both extent and speed
to that produced by irradiation of trans-azobenzene aggregates.3
Further evidence that photodimerization of the aggregated
styrylthiophene results in major structural disruption is found
by examining the effective size of the phospholipid dispersions,
prior and subsequent to photodimerization. The relatively large
structures retained upon filtration through a 100-nm membrane
are clearly disrupted as the photodimerization occurs. The
change in the residual absorption and emission spectra as
photodimerization proceeds also supports the idea that major
structural rearrangments occur as monomer is converted to

5450 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Song et al.
(12) Geiger, H. C.; Saeva, F. D.; Whitten, D. G. Manuscript in
preparation.
of the vesicle structure somewhat similar to that when ag-
gregated azobenzene is photoisomerized but not when the
photoisomerization involves an isolated azobenzene monomer.
One of the interesting questions that is raised by the finding
of relatively strong attractive noncovalent interactions between
the chromophores in the modified amphiphiles examined in this
and related studies is whether there are specific π-interactions
that govern the strength and structure of the assemblies or if it
is much more shape and topology that play decisive roles. We
are currently seeking to address these questions so as to be able
to develop and control more closely the coupling of monomer
structure with ensemble properties.
(13) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.;
Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994,
116, 6664. (b) Murata, K.; Aoki, M.; Nishi, R.; Ikeda, A.; Shinkai, S. J.
Chem. Soc., Chem. Commun. 1991, 1715.
(14) Stanescu, M. Unpublished results.
(15) Vaday, S.; Geiger, H. C.; Cleary, B.; Perlstein, J.; Whitten, D. G.
J. Phys. Chem. 1997, 101, 321.
(16) Zhao, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994,
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(17) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647. Kaupp, G.
CRC Handbook of Organic Photochemistry and Photobiology; Horspool,
W. M., Song, P.-S., Eds.; CRC Press: Boca Raton, FL, 1995; p 50.
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Acknowledgment. We thank the U.S. National Science
Foundation (Grant Number CHE-9521048) for generous support
of this research.
(19) (a) Robertson, J. M.; Woodward, I. Proc. R. Soc. London, Ser. A
1937, 162, 568. (b) Schmidt, G. M. PhotoreactiVity of the Photoexcited
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