Enhanced Aggreagtion of Derivatized Tolan Surfactants through Donor-Acceptor Interactions at the Air-Water Interface and in Langmuir-Blodgett Films

Article abstract of DOI:10.1021/jp960579g

Control of aggregation behavior of derivatized surfactants by moderately polar functional groups as well as the role of charge-transfer interactions governing photophysical phenomena was studied using surfactants incorporating the diphenylacetylene (tolan

Full text of DOI:10.1021/jp960579g

12616
J. Phys. Chem. 1996, 100, 12616-12623
Enhanced Aggregation of Derivatized Tolan Surfactants through Donor-Acceptor
Interactions at the Air-Water Interface and in Langmuir-Blodgett Films
Catie Weiss Farahat,† Thomas L. Penner,*,‡ Abraham Ulman,*,§ and David G. Whitten*,†
Center for Photoinduced Charge Transfer, Hutchison Hall, UniVersity of Rochester,
Rochester, New York 14627
ReceiVed: February 26, 1996; In Final Form: April 25, 1996X
Control of aggregation behavior of derivatized surfactants by moderately polar functional groups as well as
the role of charge-transfer interactions governing photophysical phenomena was studied using surfactants
incorporating the diphenylacetylene (tolan) chromophore (TFAs) modified with either a sulfone or an ether
group (sTA or oTA, respectively). H aggregation is prevalent in these systems as monitored by the spectral
shifts of absorption and emission. Packing of surfactants and aggregation were found to be strongly influenced
by the presence of bivalent metal cations, Cd2+, in the subphase, as demonstrated by molecular area and
chain orientation measurements. The nature of the aggregates formed from sTA surfactants is directly affected
by complexation of the acid headgroup; in contrast, the associated oTA chromophore aggregation is unaffected
by subphase modifications, despite the altered packing of surfactant chains. The photophysical behavior of
aggregated TFAs in monolayer systems was followed at the air-water interface as well as with supported
films on quartz, and conclusions can be made of the surfactant arrangements’ stability. A unique mixed
aggregate was formed when both the ether- and sulfone-derivatized TFAs were incorporated into a single
phosphatidylcholine molecule (mixed PC) and deposited as an LB film. This monolayer exhibits additional
enhancement in aggregation properties attributed to donor-acceptor interactions between the tolan chro-
mophores that are distinct from a statistically mixed LB sample prepared from a mixture of TFAs. The
current results are viewed in terms of general principles governing aggregation for a family of chromophore-
derivatized surfactants in LB film.
Introduction
formation of a soap monolayer at the air-water interface, and
many studies have focused on the role played by the surfactant
headgroup in the packing of molecules in LB assemblies.11
Experimental techniques such as surface reflectance,12 FTIR-
ATR spectroscopy,11,13 and linear dichroism,14 as well as
computer simulation9 methods are increasingly used to probe
the organization of a large family of derivatized surfactants.
The assembly of derivatized surfactants in two-dimensional
matrices using Langmuir-Blodgett techniques has been of great
interest in recent years.1-5 A number of systems have been
designed and synthesized in which a chromophore is embedded
within an amphiphilic chain in order to study the aggregation
of the extended π systems in conjunction with the assembly
properties of the surfactant itself. Chromophore aggregation
is frequently characterized by distinct photophysical behavior
associated with the coupling of the π systems in particular
molecular arrangements governed by hydrophobic, hydrophilic,
and van der Waals forces. The wide range of applications of
this two-dimensional ordering of molecular systems includes
studies of energy transfer,6 long-range electron transfer,7 and
nonlinear optical devices,8 as well as a determination of
fundamental forces controlling molecular aggregate formation
under specific boundary constraints.9
Kuhn and co-workers have studied aggregation for a number
of configurations of clustered dye molecules, using an extended
dipole model which relates spectral energy shifts between
aggregate and monomer to the orientation and distance between
dye molecules.3,10 In an extension of this model, contributions
due to the angular distribution of chromophores about the surface
normal have been incorporated in order to predict spectral shifts
upon aggregation.5 The orientation of chain axes and chro-
mophore transition moments have been found to be further
directed by the presence of bivalent cations necessary for the
Incorporation of trans-stilbene within fatty acid chains (SFAs)
has been studied extensively in micelles, bilayer vesicles, and
LB films.4 The observed changes in the absorption and
fluorescence upon aggregation in such systems can be explained
by a modified “card-pack” exciton model in which the excitation
energy is delocalized across an array of transition moments of
the individual stilbene molecules.4,10 Further investigations have
revealed the structure of spectroscopically similar H aggregates
in vesicles formed from trans-stilbene-derivatized phospholipids,
and recent results suggest there is a unit size for the aggregate
from which the extended assembly is built.15 Indeed, the
tendency to form small aggregates of a specific size (e.g., four
chromophore units) strongly governs the packing behavior of
the entire surfactant system and may be considered a general
model for the behavior of several types of chromophores.15c,16
In general, H aggregation appears to predominate over other
types of aggregation in a wide variety of modified stilbene
systems, even those specifically designed to exhibit altered (i.e.,
J aggregation) behavior.4b,17
Directional energy transfer has been demonstrated between
types of aggregate clusters in heterogeneous media.4,17 Exten-
sive studies performed in our lab have involved mixtures of
surfactants derivatized with stilbene units at different positions
along the hydrophobic chain that show self-aggregation does
occur, resulting in discrete domains within LB film samples.
Energy transfer is observed between aggregated clusters across
* To whom correspondence should be addressed.
† Present address: Department of Chemistry, University of Rochester.
‡ Present address: Eastman Kodak Company, Rochester, NY 14650.
§ Present address: Department of Chemistry, Polytechnic University,
Brooklyn, NY 11201.
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00579-5 CCC: $12.00 © 1996 American Chemical Society

Enhanced Aggregation of Derivatized Tolan Surfactants
J. Phys. Chem., Vol. 100, No. 30, 1996 12617
layers to low-energy aggregate traps in the system. Additional
studies have involved the arrangement of donor-acceptor-
substituted stilbene surfactants in a supported monolayer and
the effect of substitution on aggregation behavior.4 Both charge-
transfer interactions between modified chromophore units4 as
well as trapping of the excitation energy of aggregate systems
by a small residual population of monomers5 have been
proposed as mechanisms that explain the complex spectral
observations when polar chromophores have been studied in
LB films. A broader knowledge of the influences of polar
substituents on the photophysical properties and aggregation
behavior of various chromophore-derivatized amphiphiles is
necessary to understand the general controlling influences of
chromophore-substituted surfactants in different heterogeneous
media.18
SCHEME 1
In the present work, representative derivatized surfactants
incorporating the rodlike diphenylacetylenic (tolan) chro-
mophore (TFAs) were studied. The tolan moiety in this series
was modified with either a sulfone or an ether group (sTA or
oTA, respectively) between the aromatic ring and a 12-carbon
described by Radhakrisman et al. and Bligh and Dyer from the
corresponding TFAs.21 The product mixed PC was purified by
preparative TLC. The TFAs and mixed PC derivative were
1
found to be pure by TLC and H NMR, and the fast atom
bombardment (FAB) mass spectra obtained show the M+ ions
associated with the parent compounds. Lignoceryl acid was
purchased from Avanti Polar Lipids Inc. and used as received.
Chloroform and methylene chloride used in all spectroscopic
and LB work were HPLC-grade, pentene-stabilized solvents
(Fisher). Milli-Q filtered house-deionized water was used for
all measurements in an aqueous medium. Quartz glass slides,
1 × 5 cm were cleaned sequentially in chloroform and detergent
and followed by “piranha” solution cleaning (30% H2O2/H2SO4)
for 30 min.22 The quartz substrates were rinsed thoroughly in
Milli-Q water and dried before use in the Langmuir-Blodgett
assembly.
Langmuir-Blodgett Film Preparation. An aliquot of tolan
fatty acid or phospholipid in CHCl3 (∼1 mM stock solution)
was spread onto an aqueous subphase at 20 °C; unless otherwise
noted, the subphase was composed of CdCl2, 3.0 × 10-4 M,
and buffered with 5.2 × 10-4 M NaHCO3 (pH 6.8). The organic
solvent was allowed to evaporate from the surface for at least
15 min before a compression/expansion cycle was carried out
to a maximum surface pressure of 40 mN/m. A film compres-
sion in which the isotherm was recorded preceded deposition
of the monolayer onto quartz in the upstroke of the substrate
through the subphase. The transfer ratios obtained for single
monolayers deposited were 1.0 ( 0.1 in all samples prepared.
Steady-state measurements were the result of averaged 2-3
separate slide samples.
hydrophobic chain in order to study the role of moderately polar
functional groups on the aggregation and photophysical behavior
of tolan-modified surfactants in LB film. The tolan-derivatized
fatty acids oTA and sTA were found to exhibit blue-shifted
absorption and red-shifted fluorescence emission upon assembly
in Langmuir film at the air-water interface and in deposited
LB films, spectral behavior that is characteristic of H aggregation
of chromophores in restricted media similar to that of stilbene-
modified surfactants (SFAs). Unlike the SFAs,19 however, the
packing of surfactants was found to be strongly dependent on
the presence of the bivalent metal cation Cd2+ in the subphase;
thus, for TFAs, the complexation of the acid headgroup appears
to exert direct influence on the nature of aggregates formed in
some cases. A unique mixed aggregate was formed when both
ether- and sulfone-derivatized TFAs were incorporated into a
single phosphatidylcholine molecule (mixed PC) and deposited
as an LB film. This monolayer sample exhibits photophysical
properties distinct from a statistically mixed LB film sample
prepared from oTA and sTA, in a ratio of 1:1.
Experimental Section
Materials. The derivatized fatty acids, oTa and sTA, were
synthesized20 in a multistep synthesis by palladium-catalyzed
coupling of the corresponding acetylene and iodobenzene
derivatives as described in Scheme 1 and crystallized from
isopropyl alcohol/heptane before use. The derivatized mixed
phosphatidylcholine (mixed PC) was synthesized by the method
Absorption and Fluorescence. Routine absorption spectra
were recorded on a Hewlett-Packard 8452A diode array spec-
trophotometer. Monolayer samples were oriented perpendicular
to the incident light source, and no variation in absorption values
was observed upon rotation of the sample ( 45°. Steady-state
emission was measured using a SPEX Fluorolog spectrofluo-

12618 J. Phys. Chem., Vol. 100, No. 30, 1996
Farahat et al.
rometer with a 300-nm cutoff filter. All emission and excitation
spectra were corrected for instrumental response. Film samples
were oriented 45° relative to the incident light and detected at
a 30° angle from incidence. Fluorescence quantum yields for
solutions were determined by comparing with a standard trans-
stilbene in n-pentane (φf ) 0.0435).23
Fluorescence Lifetime Measurements. Time-correlated
single-photon-counting (TCSPC) experiments were carried out
on an instrument consisting of a mode-locked Nd:YLF laser
(Quantronix 4000 Series) operating at 76 MHz as the primary
laser source. The second harmonic (KTP crystal) of the Nd:
YLF laser was used to synchronously pump a dye laser
(Coherent 700) circulating Rhodamine 6G in ethylene glycol
as the gain medium. The pulse width of the dye laser was
typically 8 ps, as determined by autocorrelation, and was cavity
dumped at a rate of 1.9 MHz. The dye laser was tuned to the
desired wavelength and frequency doubled in a â-barium borate
(BBO) crystal for sample excitation (∼295 nm). Emission from
the sample was collected by two convex lenses and focused at
the entrance slit of a Spex 1681 monochromator (0.22 m) and
was detected by a red-sensitive multichannel plate (MCP)
detector (Hamamatsu R3809U-01). The single-photon pulses
from the MCP detector were amplified and used as the stop
signal for a time-to-amplitude converter (TAC, EG&G Ortec),
while the signal from a photodiode, detecting a small fraction
of the dye laser output, was used as the start signal for the TAC.
The start and stop signals for the TAC were conditioned before
entering the TAC by passing through two separate channels of
a constant fraction discriminator (CFD, Tennelec TC 454). The
output of the TAC was connected to a multichannel analyzer
(MCA) interface board (Norland 5000) installed inside a
486DX2 personal computer. The MCA was controlled by
software from Edinburgh Instruments (Edinburgh, UK). The
same software was used to carry out the deconvolution of the
data and exponential fitting using the nonlinear least-squares
method. The nonexponential fluorescence decays were analyzed
using the distribution of lifetimes method provided as part of a
commercial software package by Edinburgh Instruments.24
Polarization Spectroscopy. Absorption spectra for polariza-
tion measurements were recorded on a Perkin-Elmer Lambda
19 DM scanning spectrometer. Supported monolayers on quartz
substrate (one side) were investigated in terms of preferential
absorption of the p and s linear polarized light components using
a film polarizer. Evaluation of molecular orientation was
accomplished by considering a four-phase system and assuming
uniaxial orientation of the transition moment from the surface
normal with an angle θ,25 Figure 1. The ratio of p- and
s-polarized intensities are related to θ as follows:
Figure 1. Absorption and fluorescence spectra for (a) oTA and (b)
sTA in CHCl3 and LB film. Excitation wavelengths for emission
spectra were 290 nm (CHCl3) and 260 nm (LB film).
enter the horizontal plane of the optical fibers. Reference spectra
were obtained from the pure subphase surface without a spread
film. Relative reflectivity (Rf) was calculated from intensity
measurements in the dark, ID, and on the subphase surface, IR:
IRf - ID
Rf ) - log
(2)
(
)
[
]
IR - ID
Results and Discussion
Pure TFAs in Organic Solution and LB Film. The tolan-
derivatized acids oTA and sTA are soluble in relatively nonpolar
organic solvents (CHCl3, CH2Cl2), and the observed absorption
and emission spectra are attributed to the monomeric form of
the chromophores in dilute solution.27c The complex structure
of the derivatized tolan absorption spectra can be better
understood in relation to the photophysical properties of tolan
in solution. In brief, the absorption of tolan is due to three
energetically close-lying excited singlet states. Emission occurs
from both the lowest excited singlet associated with the ground-
state equilibrium geometry and S2, associated with the equilib-
rium geometry of the excited state.27c The absorption and
emission of the supported monolayers of pure oTA or sTA Cd2+
salts on quartz are compared with those from CHCl3 solutions
in Figure 1a,b. LB films of oTA and sTA exhibit altered
absorption spectra (blue-shifted ∼ 4700 cm-1 relative to CHCl3
solutions), and upon excitation, emission is red-shifted 1000-
3000 cm-1 from that observed for the monomers in CHCl3. The
spectral behavior of TFAs, like that of stilbene fatty acids, is
attributed to the formation of H aggregates of the tolan
chromophores in LB films in which the transition moments
along the long tolan axis are roughly aligned in a head-head
arrangement such that a splitting of exciton levels occurs in
the excited state. The photophysics associated with the exciton
model are depicted in Figure 2. Similarly, dilution of the TFAs
with saturated fatty acids of the same extended length (e.g.,
lignoceryl acid, C24) in LB films up to a molar ratio of 1:10
results in aggregated chromophore systems that display photo-
physical properties essentially identical to the pure samples; this
3
Ap n1 cos i + n3 cos r
2n1 n3 sin2 i
)
cos i cos r +
(1)
n24 tan2 θ
(
)
As n1 cos r + n3 cos i
where n1 ) 1.00 (air), n2 ) 1.50 (assumed for LB film), n3 )
1.49 (300 nm) (quartz),26 i ) 45° (angle of incidence at the
air-LB film interface), and r ) angle of refraction at the LB
film-substrate interface:
n
r ) sin-1 1 sin i ) 28°
[
]
n3
Surface Reflectance Spectroscopy. Reflectance spectra of
spread monolayers on the surface of the subphase were recorded
using an Ocean Optics spectrometer equipped with optical fibers
and detector. Both excitation (D2 lamp source) and detection
fibers were situated above the surface so that the reflected beams

Enhanced Aggregation of Derivatized Tolan Surfactants
J. Phys. Chem., Vol. 100, No. 30, 1996 12619
Figure 2. Depiction of exciton splitting of energy levels upon
aggregation and associated photophysics.
TABLE 1: Fluorescence Quantum Yields and Lifetimes in
CHCl3
φf
τ,a ns
Figure 4. Absorption and fluorescence spectra for oTA, sTA, and
mixed PC solutions in CHCl3 (2 × 10-5 M); λexc ) 290 nm, 25 °C.
oTA
0.0033
0.16
<0.02
0.35
sTA
mixed PC
chromophore PCs investigated (e.g., bis(stilbene)PC), where no
association was detected in the ground state or excited state for
dilute organic solutions, the moderately strong electron donor
and acceptor functionalities of oTA and sTA, respectively, were
expected to alter the observed photophysics of the system in
restricted media through increased charge-transfer interactions.4b
In Figure 4, the absorption and emission of the mixed PC in
CHCl3 are compared with those of the individual TFAs in
solution, as well as a dilute (1:1) acid mixture, revealing the
altered mixed PC absorption that is distinct from the simple
spectral addition of the two acid samples. At first glance, the
dominance of emission by the feature virtually identical with
that of sTA emission alone is surprising. However, the emission
spectrum of mixed PC can be understood upon consideration
of the 50-fold difference in quantum yields, φf, between oTA
and sTA in CHCl3 (Table 1). If the two chromophores in the
mixed PC are in fact remote from one another, then the emission
is expected to be dominated by sTA fluorescence. On the other
hand, if there is some close interaction between the ether- and
sulfone-tolans of the mixed PC in solution, as suggested by the
distinctive absorption spectrum (Figure 4b), then another
explanation for the emission features is plausible: possibly an
equilibrium exists between an open (remote chromophores) form
and a closed (interacting chromophores) form such that the
fraction of the population adopting the open form acts as a
Fo¨rster energy trap for the entire system, giving rise to emission
associated with monomeric sTA, Figure 2.
0.018
0.05, 0.28
a Lifetimes obtained from the fit of the data to mono- or biexponential
decay kinetics.
Figure 3. Fluorescence decay kinetics (λexc ) 296 nm, λmon ) 390
nm) for pure oTA and sTA LB film samples. Inset: Comparison of
TFA decay kinetics during the first 5 ns after pulse, linear scale.
suggests that the mosaic-like clusters of aggregates cannot be
readily disrupted upon “dilution” by surfactant molecules.28
Simple monoexponential decay was measured for the fluores-
cence of oTA and sTA surfactants in CHCl3, Table 1. The
decay kinetics associated with the LB film samples are
significantly longer than for the organic solutions (Figure 3);
the highly nonexponential fluorescence decay of the LB films
persists over nanoseconds, as compared with the picosecond
measurements obtained for CHCl3 solutions of TFAs, Table 1.
The natural lifetimes of the monomers in solution are calculated
to be ca. 1 ns for the individual chromophores; therefore, the
observed photophysics of the LB films are attributed to emission
from the lower forbidden state, as depicted in Figure 2 for the
excitonic splitting of energy levels for an H-aggregated system.
closed
closed
open
sssf
hv
sssf
open
*closed
ET
*closed
+
sssf
flr
sssf
closed + *open
open + hν
*open
From a biexponential fit of the decay kinetics for the mixed
PC in CHCl3, two lifetime components were obtained, 50 and
300 ps, suggesting two remote TFAs in solution. However,
the value of φf for the mixed PC is not an average of those
associated with the individual oTA and sTA fluorophores.
Clearly, some pathway for depopulating the excited state(s) of
the mixed PC is available, giving rise to a quantum yield
Mixed PC in Organic Solution. The tendency to form a
mixed aggregate composed of ether- and sulfone-derivatized
tolans was explored using the mixed PC. Unlike other bis-

12620 J. Phys. Chem., Vol. 100, No. 30, 1996
Farahat et al.
Figure 5. Absorption and fluorescence for LB films of (a) mixed PC
compared with pure oTA and sTA films; (b) mixed PC compared with
films composed of mixtures of TFAs in various ratios, as noted.
significantly lower than the one expected for remote fluoro-
phores in organic solution.29
Figure 6. (a) Fluorescence decay kinetics of LB film samples compared
with representative kinetics for CHCl3 solution, as noted; (b) distribution
of lifetimes associated with kinetics.
Mixed PC in LB Film. When an LB film is prepared from
the mixed PC on quartz substrate, the absorption and emission
are characteristic of an H-aggregated chromophore system,
Figure 5a. Absorption and emission are shifted to the blue and
red, respectively, similar to the behavior of stilbene-derivatized
surfactants. However, the spectral features of the mixed PC
LB film are distinct from pure TFA LB films as observed from
the narrow absorption absorption and emission of the mixed
PC appearing at lower energies relative to the pure TFAs’ LB
film bands. Accordingly, LB films of mixtures of oTA and
sTA in various ratios were prepared, and their spectra are
compared with the mixed PC in Figure 5b. These results
demonstrate that the steady-state spectra for the mixed PC are
not additive with respect to the individual chromophores in
restricted media, but rather, a unique aggregate is formed
between the derivatized tolans in the mixed PC that is not readily
adopted in the mixed TFA samples. It must be noted that some
narrowing of emission bands occurs in the mixture LB films
that is distinct from pure TFA LB films, indicative of some
degree of enhanced interactions between chromophores. The
red-shifted absorption spectrum of the mixed PC (relative to
pure TFAs) is then attributed to a population of “heterodimers”
as opposed to the mixed TFAs in LB films which appear to
contain predominantly zones of like aggregates with some
residual interactions between different chromophores. No
modification of spectral features was observed with dilute CHCl3
solutions of TFA mixtures; Beer’s law is obeyed, and absorption
and emission appear to represent simple mixtures of unassoci-
ated TFAs. Absorption and fluorescence spectra for samples
of mixed PC diluted with lignoceroyl acid (LA) (1:7) display
the same spectral shifts associated with aggregate formation,
suggesting the heterodimer is responsible for these observations.
In Figure 6a, fluorescence decay kinetics for LB film samples
of the mixed PC and pure TFAs as well as the 1:1 mixed acid
sample are compared, demonstrating the long-lived nonexpo-
nential fluorescence decay of the mixed PC in LB film. For
qualitative comparison of the fluorescence decay kinetics for
LB film samples presented in Figure 6a, a distribution analysis
was utilized to show the overall relative contributions of various
fluorescent components. It must be mentioned that reasonable
analyses are expected from decay kinetics of 10 000 counts for
systems in the absence of any energy transfer or spectral shifts
during the lifetime of the excited state,24 and therefore,
interpretation of the individual lifetime clusters must be done
with care. Nevertheless, some comparison between sample
profiles is useful. Figure 6b depicts the relative intensities of
lifetimes components for the four LB film samples subjected
to this analysis. On inspection of the lifetime distribution
comparison, one finds that a significant portion of fluorescence
emission of the mixed PC LB film system is due to very long-
lived (>20 ns) emissive species that are virtually absent in pure
TFA samples and present to a much smaller degree in the 1:1
TFA mixture LB sample.
Influence of Cd2+ Complexation on TFAs’ Aggregation
Behavior. It is common practice to prepare Langmuir films
on an aqueous subphase that contains bivalent metal cations,
such as Cd2+, in order to form stable monolayer soaps of the
complexed surfactants that are insoluble in the aqueous phase.2
In the previously discussed studies, Cd2+ was consistently
employed in this manner to prepare monolayers for investigation.
Monolayers formed upon aqueous solutions containing no metal
cations are expected to be of comparable overall stability but
composed of surfactants (and associated chromophores) arranged
differently, due to the larger molecular area adopted by the
surfactants under these conditions. Cd2+ complexation is known
to alter the molecular area of arachidic acid, for example, at
the subphase surface, in addition to causing a shift in the
molecular orientation of the fatty acid chains from 25°, relative
to the surface normal, to a near-perpendicular orientation.25
Nevertheless, we were surprised to find that some aggregation
and photophysical behavior of the TFAs is dramatically altered
when monolayers are prepared in the absence of Cd2+ cations.
In the present studies, UV surface absorption (relative reflec-

Enhanced Aggregation of Derivatized Tolan Surfactants
J. Phys. Chem., Vol. 100, No. 30, 1996 12621
TABLE 2: Molecular Areas from Isotherms and Tilt
Angles from Polarization Spectra
tolan/subphase
molecular area,a Å2
θ, deg
sTA/CdCl2
oTA/water
oTA/CdCl2
sTA/water
22
26
30
34
12
15
16
18
a Measured from surface pressure-area isotherm extrapolated to 0
compression ((5% molecular area). b θ calculated from polarized
absorption spectra, as described in the text ((10% angle).
Figure 8. Absorption spectra for LB films on quartz transferred from
subphase as noted.
Figure 7. Absorption spectra (relative reflectance) for TFA monolayers
at the air-water interface, within 30 min of film compression to 40
mN/m; subphase as noted.
tance) was used to probe the aggregation behavior of tolan-
functionalized fatty acids at the air-water interface in the
presence and absence of Cd2+ ions in buffered (pH 6.8) aqueous
subphase. While surface pressure-area isotherms monitored
during compression for oTA and sTA on either buffered water
or buffered Cd2+-containing subphase display similar shapes,
the molecular areas obtained for the TFAs at 40 mN/m were
found to be significantly different between oTA and sTA, as
well as between those obtained on different subphases, Table
2. Indeed, a trend reversal was observed, for oTA exhibits a
slightly larger molecular area when complexed with Cd2+ than
in water and the sTA/Cd2+ system exhibits a smaller molecular
area than does sTA on water. Notably, reflectance spectra
obtained at the air-water interface compared for the four
systems reveal the marked difference between sTA on water
(λabs ) 300-330 nm) and on Cd2+-containing subphase (λabs
< 240 nm) (Figure 7), while oTA and the mixed PC absorption
spectra are virtually indistinguishable in both subphase composi-
tions and when monitored as a function of time. In order to
determine more precisely the arrangement of tolan chro-
mophores that are aggregated under the two subphase conditions
discussed, the two-dimensional density (molecular area) and the
photophysical properties are considered.30
Figure 9. Absorption and fluorescence spectra for sTA on quartz,
transferred from subphase, compared with CHCl3 solution as noted;
λexc ) 260 nm (CdCl2), λexc ) 300 nm (water, CHCl3).
deposited from different subphase conditions. All fluorescence
spectra of the supported monolayers prepared from Cd2+
subphase show broad emission at 380-420 nm; however,
emission at lower energy (λem ) 430 nm) resulted only upon
excitation of sTA deposited from the water subphase. An
important comparison is made of the absorption and emission
of sTA in CHCl3 solution and the LB films of sTA prepared
on both water and Cd2+-containing subphase, revealing absorp-
tion of the LB film from water that is broadened and more
structured than the CHCl3 solution, yet only slightly perturbed
in energy, Figure 9, in contrast with the blue-shifted absorption
of sTA/Cd2+. The red-shifted emission from the sTA/water LB
film is also compared with fluorescence associated with other
samples. These results strongly support a model in which an
arrangement is adopted of compressed sTA chromophores on
the water subphase that may not form an extended H-aggregated
array to the same extent as with Cd2+ complexation.
First, a comparison was made between UV absorption spectra
of quartz-supported monolayers and air-water interface films,
to monitor if the integrity of film aggregation was maintained
upon transfer to solid support, Figure 8. The broad absorption
(300-330 nm) of sTA monolayer formed at the air-water
interface of a water subphase was observed for the supported
film from the same subphase conditions; however, one notable
difference was observed between absorption for sTA/Cd2+ at
the air-water interface (λ < 240 nm) and the supported film
(λ ) 260 nm). Some explanation for the sTA spectral variations
observed can be made in light of the UV polarization measure-
ments and the present experimental technique (Vide infra). No
distinguishable spectral differences were observed between oTA
In a second investigation, an attempt was made to shift the
arrangement of sTA on water that gives rise to the unique
photophysical features compared in Figure 9 to that of sTA on
Cd2+-containing subphase by simply injecting an appropriate
volume of CdCl2 stock solution into the subphase to reproduce
the Cd2+-containing subphase conditions. Upon diffusion of
Cd2+ ions throughout the bulk subphase under the formed film,
the spectral feature of the film was expected to shift toward
that associated with sTA/Cd2+ at the air-water interface.
Surprisingly, no such spectral shift took place after 24 h, even
after repeated expansion and compression cycles of the film in
addition to thorough mixing of the subphase below the formed

12622 J. Phys. Chem., Vol. 100, No. 30, 1996
Farahat et al.
as those representing an aVerage tilt adopted by the chro-
mophore population. With the orientation of chromophores in
mind, it is of particular interest to compare the absorption
spectrum for sTA/Cd2+ on quartz with that of the monolayer
on the subphase surface. The lowest energy absorption band
at 260 nm is virtually absent in the spectrum taken at the air-
water interface, while it is present for the quartz-supported
system. It is proposed that because of the small average tilt
angle associated with the sTA/Cd2+ system in the LB film, a
similar angle of chains (perpendicular to the subphase surface)
is adopted and the interaction of the p-polarized light with the
molecules’ transition moments is minimized due to the orienta-
tion of the horizontal plane of the fiber optics parallel with the
surface plane.31 In contrast, the relatively large average tilt angle
of sTA/water is expected to allow interaction of both p and s
components of the electric field with the chromophores to occur.
No oblique reflection setup is possible with the present system
in order to probe further this phenomenon.
Figure 10. Comparison of absorption spectra for sTA film at the air-
water interface, in subphase as noted, 1 and 24 h after compression at
40 mN/m.
Conclusions
Novel ground-state aggregation is observed for amphiphilic
tolan derivatives in spread monolayers at the air-water interface
and in LB films. In agreement with our understanding of the
aggregation properties of stilbene-derivatized surfactants, a
model of H aggregation serves to explain some of the photo-
physical observations for the LB films of tolan derivatives,
prepared on Cd2+-containing subphase, depicted by the exciton
splitting model (Figure 2). However, the simple H-aggregate
model for arrangement of aggregated chromophores is not
sufficient to completely explain all of the current results. The
behavior of sTA on a water subphase suggests a more expanded
matrix of surfactants is formed, in which both chain packing
and tolan aggregation occur to a smaller degree than with Cd2+
complexation. Nevertheless, the appearance of red-shifted
fluorescence emission is indicative of an aggregated sTA sample
in which emission occurs from the forbidden energy level. The
comparable ground-state absorption spectra of oTA, independent
of subphase composition, suggest the extended H aggregation
of chromophores occurs to a similar extent under both subphase
conditions.
The unusual photophysical behavior of the sTA/water system
may be understood by considering the following plausible
models: (1) The emission that is observed for sTA/water LB
films is excimer-like, implying that the sTA molecules have
no significant ground-state interactions that would significantly
alter their absorption spectra, yet the emission is clearly shifted
from that associated with the monomer. The likelihood for
formation of a complex between one excited sTA molecule and
a ground-state sTA in a restricted medium such as an LB film
during the lifetime of the excited state is small, and therefore,
other explanations are sought. (2) Only residual emissive
aggregates may exist (<1%) amidst nonaggregated sTA, and
these aggregates act as energy traps (Vide supra).5,32 The
microheterogeneous nature of the aggregated tolans is estab-
lished by the wavelength dependence of the steady-state
fluorescence and excitation spectra, as well as the highly
nonexponential nature of the fluorescence intensity decays. Red-
shifted fluorescence spectra for tolan derivatives exhibit excited-
state lifetimes that are 40-500 times longer than those of
monomeric species in solution. The observed lifetimes are also
several times longer than the natural radiative lifetime of the
monomeric species and can be attributed to emission from the
lowest (forbidden) excitonic level of particular aggregates.
Moreover, the degree of aggregation in LB films appears
dependent on both tolan substituents and subphase composition.
Figure 11. Depiction of transition moment orientation with respect to
quartz surface normal and s- and p-polarized light components.
film. This behavior suggests that the original structure is ordered
and that a sort of annealing process takes place that starts from
grain boundaries and penetrates into the cluster. That repeated
expansion-compression did not affect the order is not surprising
since the expansion probably gives small ordered clusters. A
control experiment revealed the stability and preference for the
expanded orientation of an sTA film formed on either subphase;
when sTA is spread on a Cd2+-containing subphase, initially,
the spectral features include absorption below 240 nm, which
appears stable over ∼8 h. Maintenance of the film at a stable
surface pressure (40 mN/m) over 24 h results in a surface
absorption that has shifted to 300-320 nm and is significantly
broadened, Figure 10. This resultant spectrum is similar to that
of sTA on the water surface and appears to represent the stable
arrangement of aggregated sTA in the monolayer.
Linear Dichroism of TFA-Supported Monolayers. Polar-
ized absorption spectra of single monolayer films of TFAs,
prepared from aqueous or Cd2+-containing subphase supported
on quartz slides, were compared in the UV region. From an
analysis of the ratio of p- and s-linearly polarized components
of a spectrum, a value for the tilt angle, θ, of the long axis
associated with the π-π* transition was estimated from eq 1
(Figure 11). These tilt angles are compared in Table 2, along
with the molecular areas obtained from the isotherms of spread
monolayers immediately after compression. The trend of
molecular tilt angles of the TFAs from the surface normal is
consistent with that observed for molecular areas obtained from
surface pressure-area isotherms. It is important to note that
because of the inhomogeneity of chromophore arrangements
in the supported LB films, the tilt angles of an assumed uniaxial
sample of long-chain molecules need to be broadly interpreted

Enhanced Aggregation of Derivatized Tolan Surfactants
J. Phys. Chem., Vol. 100, No. 30, 1996 12623
(5) (a) Evans, C. E.; Song, Q.; Bohn, P. W. J. Phys. Chem. 1993, 97,
12302. (b) Song, Q.; Evans, C. E.; Bohn, P. W. J. Phys. Chem. 1993, 97,
13736. (c) Evans, C. E.; Bohn, P. W. J. Am. Chem. Soc. 1993, 115, 3306.
(6) (a) Bohn, P. W. Annu. ReV. Phys. Chem. 1993, 44, 37. (b) Penner,
T. L. Thin Solid Films 1988, 160, 241.
(7) (a) Kuhn, H. J. Photochem. 1979, 10, 111. (b) Hsu, Y.; Penner,
T. L.; Whitten, D. G. J. Phys. Chem. 1992, 96, 2790.
Given the nonexponential nature of the fluorescence decay
kinetics, which do not easily fit into a simple model with few
components, the existence of both aggregates and monomers
in varying percentages is likely. While any fluorescence rise
for this system cannot be detected within 20 ps, the limit of the
instrumentation, energy transfer cannot be ruled out as a
mechanism for the trapping of excitation energy in a small
percentage of aggregates. (3) Ground state H dimers of sTA
on water may be formed in which the absorption appears close
to that of the organic solution. Conceivably, there may be both
blue-shifted absorption occurring associated with H aggregation
as well as red-shifted absorption due to charge-transfer behavior,
resulting in the absorption that is close in energy to that of the
monomer. Furthermore, the emission expected from such a
system should be red-shifted relative to the monomeric spectrum,
as is presently observed. For a related system in which charge
transfer plays a crucial role in the photophysics of polar-
substituted stilbene surfactants, see ref 4b. Current work in our
laboratory is continuing to address the issues involved with the
aggregation behavior of the tolan series.
(8) Penner, T. L.; Willand, C. S.; Robello, D. R.; Schildkraut, J. S.;
Ulman, A. Proc. SPIE 1991, 1436, 169.
(9) Perlstein, J. J. Am. Chem. Soc. 1994, 116, 11420.
(10) (a) Kuhn, H.; Mo¨bius, D.; Bucher, H. In Techniques of Chemistry;
Weissberger, A., Rositer, B. W., Eds.; Wiley: New York, 1973; Vol. 1,
Part 3B. (b) Kuhn, H.; Mo¨bius, D. Angew. Chem. 1971, 83, 672. (c) Kuhn,
H.; Mo¨bius, D. Angew. Chem., Int. Ed. Engl. 1972, 10, 620.
(11) Ahn, D. J.; Franses, E. I. J. Phys. Chem. 1992, 96, 9952.
(12) Gruniger, H.; Mo¨bius, D.; Meyer, H. J. J. Chem. Phys. 1983, 79,
3701.
(13) Takenaka, T.; Nogami, K.; Gotoh, H.; Gotoh, R. J. Colloid Interface
Sci. 1971, 35, 395.
(14) (a) Chollet, P.-A. Thin Solid Films 1978, 52, 343. (b) Vandevyver,
M.; Barraud, A.; Daudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid
Interface Sci. 1982, 85, 571.
(15) (a) Chen, H.; Farahat, M. S.; Law, K.-Y.; Whitten, D. G. J. Am.
Chem. Soc., in press. (b) Song, X.; Geiger, C.; Furman, I.; Whitten, D. G.
J. Am. Chem. Soc. 1994, 116, 4103. (c) Song, X.; Geiger, C.; Leinhos, U.;
Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 10340.
(16) Preliminary results involving phospholipids of tolan derivatives in
aqueous solution suggest a unit size composed of six chromophores:
Farahat, C. W. Unpublished results.
(17) Spooner, S. P.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 1240.
(18) Chen, H.; Farahat, C. W.; Farahat, M. S.; Geiger, H. C.; Leinhos,
U. W.; Liang, K.; Song, X.; Penner, T. L.; Ulman, A.; Perlman, J.; Law,
K.-Y.; Whitten, D. G. Mater. Res. Bull. 1995, 6, 39.
H-type aggregation is observed for the covalently linked ether/
sulfone-derivatized tolan phospholipid, mixed PC. Indeed, some
interaction between the covalently linked ether and sulfone is
even observed in dilute organic solutions of the mixed PC. Upon
consideration of φf values for oTA and sTA, the spectral shape
of the emission is not expected to vary significantly from that
of one close to pure sTA in CHCl3, yet the low fluorescence
quantum yield of the mixed PC in CHCl3 can be explained by
a mechanism of an enhanced nonradiative decay pathway
available to the molecule in solution due to the close proximity
of ether and sulfone tolans in a single molecule. Furthermore,
while the pure sTA or oTA LB films exhibit distributions of
several lifetime components, which point to the inhomogeneous
nature of TFA aggregates in the supported film, both the degree
of red-shifted emission associated with the mixed PC aggregate
in LB film and the very long fluorescence decay time indicate
a unique aggregated form of the ether- and sulfone-modified
tolans is achieved when they are constrained in the PC, in
contrast to a simple 1:1 distribution of the individual acids in
LB film. This effect is attributed to an enhancement of H
aggregation between ether- and sulfone-derivatized tolans due
to charge-transfer interactions that are favorable when donor
and acceptor species are closely aligned.
(19) Farahat, C. W. Unpublished results.
(20) Ulman, A. Unpublished results.
(21) (a) Radhakrishman, R.; Robson, R. J.; Takagaki, Y.; Khorana, H.
G. Methods in Enzymology; Academic: New York, 1982; Vol. 72, Chapter
29. (b) Bligh, D.; Dyer, W. J. Can. J. Biochem. Physiol. 1959, 37, 911.
(22) Pintchovski, F.; Pricw, J. B.; Tobin, P. J.; Peavey, J.; Kobold, K.
J. Electrochem. Soc. 1979, 126, 1428.
(23) Saltiel, J.; Marinari, A.; Chang, D. W.-L.; Mitchener, J.; Megarity,
E. D. J. Am. Chem. Soc. 1979, 101, 2982.
(24) Gakamsky, D. M.; Goldin, A. A.; Petrov, E. P.; Rubinov, A. N.
Biophys. Chem. 1992, 44, 47.
(25) (a) Chollet, P.-A. Thin Solid Films 1978, 52, 343. (b) Vandevyver,
M.; Barraud, A.; Daudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid
Interface Sci. 1982, 85, 571.
(26) CRC Handbook of Chemistry and Physics, 71st ed.; CRC: Boca
Raton, FL, 1990.
(27) (a) Tanizaki, Y.; Inoue, H.; Hoshi, T.; Shiraishi, J. Z. Phys. Chem.
(Munich) 1971, 74, 45. (b) Hirata, Y.; Okada, T.; Mataga, N.; Nomoto, T.
J. Phys. Chem. 1992, 96, 6559. (c) Ferrante, C.; Kensy, U.; Dick, B. J.
Phys. Chem. 1993, 97, 13457.
(28) This phenomenon of aggregate clusters forming amidst a high
concentration of fatty acid “dilution” molecules in LB film occurs commonly
with a variety of chromophores such as stilbene- and squaraine-derivatized
surfactants; see refs 4b and 15.
(29) The fluorescence “tail” observed for the mixed PC in the red region
of the spectrum (410-500 nm) was analyzed by distribution of lifetimes
with identical results, compared with the 360-nm emission maximum. This
spectral feature may be due to emission from the *closed state of the system,
referred to in the text.
Acknowledgment. We gratefully acknowledge the National
Science Foundation for a Science and Technology Center Grant
(CHE-9120001).
(30) The values of molecular areas obtained from isotherms compare
well with those obtained for saturated fatty acids under the same subphase
and surface pressure conditions, between 20 and 35 Å2 (ref 2). While the
formation of multilayers or collapsed domains at the air-water interface
cannot be ruled out before microscopy techniques are utilized (e.g., Brewster
angle, fluorescence), the stability of films under the present conditions
suggest only monolayers are formed.
(31) Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys.
1986, 85, 4966.
(32) This mechanism of energy trapping is being further probed with
trans-stilbene-derivatized surfactant systems: Farahat, M. S. Unpublished
results.
References and Notes
(1) Ulman, A. An Introduction to Ultrathin Organic Films; Aca-
demic: Boston, 1991.
(2) Roberts, G. Langmuir Blodgett Films; Plenum: New York, 1990
and references therein.
(3) (a) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett.
1970, 6, 11. (b) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys.
Lett. 1970, 6, 207.
(4) (a) Mooney, W. F., III; Brown, P. E.; Russell, J. C.; Costa, S. B.;
Pederson, L. G.; Whitten, D. G. J. Am. Chem. Soc. 1984, 106, 5659. (b)
Furman, I.; Geiger, H. C.; Whitten, D. G.; Penner, T. L.; Ulman, A.
Langmuir 1994, 10, 837.
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