Article
pubs.acs.org/JPCC
Reversible Transition between Excimer and J‑Aggregate of
Indocarbocyanine Dye in Langmuir−Blodgett (LB) Films
Pintu Debnath,† Santanu Chakraborty,† Subrata Deb,‡ Jayasree Nath,§ D. Bhattacharjee,†
and Syed Arshad Hussain*,†
†
Thin Film and Nanoscience Laboratory Department of Physics and §Department of Chemistry, Tripura University, Suryamaninagar
− 799022, Tripura, India
‡
Department of Physics, Women’s College, Agartala − 799001, Tripura, India
S Supporting Information
*
ABSTRACT: In the present Article, a reversible transition behavior from Jaggregates to excimer of an indocarbocyanine dye 1,1′-dioctadecyl-3,3,3′,3′tetramethylindocarbocyanine perchlorate (DiI) in Langmuir−Blodgett (LB) films
was reported. Surface pressure−area (π−A) isotherms, UV−vis, and fluorescence
spectroscopies as well as atomic force microscopy (AFM) were used for
characterizations of the films. π−A isotherms suggest a balance of interactions
between DiI and fatty acids in the mixed monolayer at DiI mole fraction XDiI =
0.4, resulting in a stable and ideally mixed monolayer. It has been observed that
pure DiI formed excimer in LB films, whereas both J aggregates and excimer were formed in LB films when DiI was mixed with
long chain fatty acids, viz., stearic acid or arachidic acid. In fatty acid matrix at XDiI = 0.4, only J aggregates were formed in the LB
films. This has been confirmed using deconvolution of spectroscopic results as well as using excitation spectroscopy. The
coherent size of the J aggregate was found to be a maximum for the mixed film at the mole fraction 0.4 of DiI in fatty acid matrix.
The J-aggregate domain in the LB film contains approximately (20 ± 5) coherent sizes. However, J aggregates were totally absent
when DiI was mixed with cationic surfactant, polymer, or nanoclay.
■
developed.15 The J-aggregates are candidates for highly
integrated optical data processing for instance optical
computers. An excimer is a molecular complex of two, usually
identical, molecules that is stable only when one of them is in
an excited state.16 The excimer results in a broad and
structureless longer wavelength band in the fluorescence
spectrum and is important in excimer laser technologies.17
Over the past few decades, cyanine dyes have drawn
considerable interest due to their strong self-associating
property, leading them to the formation of different types of
technologically suitable aggregating species. Most of the work
on cyanine derivatives concentrated on the formation of single
types of aggregating species for a particular derivative. For
example, some derivatives are found to form J-aggregates,13,18,19
some form H-aggregate,20−22 while others form excimers in the
ultrathin films.23 The extent of such aggregation can be
controlled by mixing these dyes with different fatty acid
matrices/polymer/surfactants/clay, which may result in the
formation of dye aggregates with different functionality.24−27
However, there are very few reports regarding the formation of
different types of aggregated species in the ultrathin film of a
single derivative at different fabrication conditions.28 Therefore,
emphasis must be given to the efforts to quantify the optimized
condition to fabricate different types of technologically friendly
INTRODUCTION
Nowadays, intensive studies on dye aggregation have attracted
considerable attention due to their potential role in various
fields such as energy transfer in photosynthesis,1 light
harvesting,2 optical memory,3 organic solar cells,4 organic
light emitting diodes,5 photodynamic therapy,6 photographic
process,7 and laser technologies.8 The Langmuir−Blodgett
(LB) technique is one of the most promising candidates for
fabricating nanoscale aggregating systems in ultrathin films. It is
done by arranging various kinds of molecules onto uniform
monolayer assemblies, which may be suitable for development
of different-sized molecular aggregates with variety of
functionality.9−12
Depending on the molecular orientation in the aggregate,
different types of aggregated species are formed in the ultrathin
films. Dye aggregates with a narrow absorption band shifted to
a longer wavelength region with respect to the monomer
absorption band and with almost zero Stokes shift are generally
referred to as J-aggregates. J-aggregates of organic dyes are now
being extensively studied for their use as materials for spectral
sensitization, optical storage, and ultrafast optical switching.13
Through J-aggregation, the first and second optical hyperpolarizability are strongly enhanced as compared to monomer
due to the excitonic energy delocalization,14 thus allowing
second and third harmonic generation (SHG, THG)
applications in minimal sizes. Also, optoelectronic devices
such as organic light emitting diodes, lasers, Q-switches, and
optical switching devices based on J-aggregates have been
© 2015 American Chemical Society
Received: March 4, 2015
Revised: April 7, 2015
Published: April 8, 2015
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Article
■
RESULTS
Monolayer Characteristics. The thermodynamic behavior
of mixed monolayer at the air−water interface and also the
existing nature of interactions among binary components can
be highlighted from the surface pressure−molecular area (π−
A) isotherms. Figure 1 shows the surface pressure−area per
aggregated species from a particular dye, so that one can switch
from one species to another according to the need. Here we
report the reversible transition between different kinds of
aggregated species using an indocarbocyanine dye in LB films.
Considering the wide application of nanoscale aggregates, this
result is very important from an application point of view.
■
EXPERIMENTAL SECTION
DiI, SA, AA, octadecylammine (ODA), and isotactic poly(methyl methacrylate) (PMMA), purity >99%, were purchased
from Sigma-Aldrich chemical company and used as received.
Working solutions were prepared by dissolving them in
spectroscopic grade chloroform (SRL). The clay mineral used
in the present work was synthetic Laponite, obtained from
Laponite Inorganics, UK and used as received.
A commercially available Langmuir−Blodgett (LB) film
deposition instrument (Apex 2000C, Apex Instruments Co.,
India) was used for surface pressure−area isotherm measurements as well as for mono- and multilayer film preparation.
Ultrapure Milli-Q water of resistivity 18.2 MΩ−cm was used as
subphase. The concentration of the stock solutions for both SA
and DiI was 0.5 mg/mL. DiI and SA were mixed at different
mole fractions using the stock solutions. In order to measure
the isotherm and film preparation, 60 μL of either pure or
mixed solutions was spread onto the subphase with a
microsyringe. After complete evaporation of volatile solvent,
the barrier was compressed at a rate of 5 mm/min to record the
surface pressure−area per molecule isotherms. The surface
pressure (π) versus average area available for one molecule (A)
was measured by a Wilhelmy plate arrangement.12 Each
isotherm was repeated a number of times and data for surface
pressure−area per molecule isotherms were obtained by a
computer interfaced with the LB instrument. Smooth
fluorescence grade quartz plates (for spectroscopy), ZnSe
single crystal (for FTIR studies), and Si-wafer (for AFM
studies) were used as solid substrates. Y-type deposition at a
particular surface pressure was followed to transfer Langmuir
film onto solid substrates at a deposition speed of 5 mm/min.
For AFM measurement, a single layer was deposited. The
transfer ratio was estimated by calculating the ratio of decrease
in the subphase area to the actual area on the substrate coated
by the layer and was found to be 0.98 ± 0.02.
UV−vis absorption and fluorescence of pure solutions, as
well as those of mixed LB films, were recorded using absorption
spectrophotometer (PerkinElmer, Lambda 25) and fluorescence spectrophotometer (PerkinElmer, LS 55), respectively.
The absorption spectra were recorded at 90° incidence using a
clean quartz slide as reference.
The atomic force microscopic (AFM) image of monolayer
film was taken with a commercial AFM (Innova AFM
system,Bruker AXS Pte Ltd.) by using silicon cantilevers with
a sharp, high-apex-ratio tip (Veeco Instruments). The AFM
images presented here were obtained in intermittent-contact
(“tapping”) mode. Typical scan areas were 1 × 1 μm2.
The ATR-FTIR spectra of pure SA, SA-DiI mixed (XDiI =
0.4) LB films deposited onto ZnSe single crystal were recorded
using an FTIR spectrophotometer (PerkinElmer, Model No.
Spectrum 100, USA).
The excess Gibbs free energy of mixing (ΔGexc) for the
mixed monolayer system has been calculated using software
MATLAB 2k9b.
Figure 1. Surface pressure (π) vs area per molecule (A) isotherms of
DiI in SA matrix at different mole fractions along with pure DiI and SA
isotherm. Inset shows the molecular structure of DiI.
molecule isotherms of DiI mixed with SA at different molar
ratio along with those of pure DiI and SA for comparison. Each
isotherm is a compilation of three compressions, each from
three independent sets of experiments. The error limits for each
mole fraction have been estimated using the corresponding
isotherms for each mole fraction with the help of Origin 6.0 and
are listed in Table 1.
Table 1. Error Limits of the Isotherms of DiI, SA, and Their
Mixtures at Air−Water Interface
DiI mole fraction (XDiI)
error limits (%)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0−0.33
0.33−0.88
0.33−1.0
0−1.2
0.33−1.6
0.33−1.4
0−1.4
0.33−1.2
1.4−2
0−1.4
1.2−2.6
Upon compression of the monolayer, the DiI and SA both
showed a smooth rise of surface pressure. The lift-off area of SA
was found to be 0.276 nm2 determined by the method
described by Ras et al.29 The molecular areas of 0.23 nm2 and
0.21 nm2 at surface pressures 15 and 25 mN/m of SA isotherm
are in good agreement with the reported results.30 The shape of
the SA isotherm is steeper in nature. The DiI isotherm showed
a sharp phase transition at π = 32.5 mN/m, probably from
liquid expanded state to a mixture of liquid and condensed
states with initial lift-off area 1.95 nm2.
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The isotherms corresponding to DiI−SA mixed monolayers
at different molar ratios spaced themselves between those of
pure components with lift-off areas ranging between those of
individual components indicating the presence of interaction
among constituent components in the mixed isotherms. With
increasing mole fractions of DiI, the area per molecule of DiI−
SA mixed isotherms was found to increase systematically.
Interestingly, for DiI mole fractions in the range 0.5−1.0,
isotherms were DiI-like, whereas they were SA-like for DiI mole
fractions <0.5. Also, it is clear from the isotherms that at lower
mole fractions of DiI, the phase transition occurred at relatively
lower surface pressures.
In the quest of more information about the molecular
interactions between DiI and SA in the Langmuir monolayer,
additivity and surface phase rule were employed.31The
miscibility or the phase separation of these two binary
components in the mixed monolayer can be determined on
the basis of the shape of the π−A isotherms.32 The miscibility
of mixed monolayer was determined quantitatively by analyzing
the excess area AE of the mixed monolayer at the air−water
interface. It is given by AE = A12 − Aid, with Aid = A1X1 + A2X2,
where Aid is the ideal area per molecule, A1 and A2 are the areas
occupied by the monomers of SA and DiI, respectively, and X1
and X2 are the mole fractions of the components in the
mixtures. A12 is the experimentally observed area per molecule.
In the ideal case, the plot of AE versus X2 should be a straight
line. Any deviation from it (AE = A12 − Aid ≠ 0) indicates
partial miscibility and nonideality.33,34 If attractive intermolecular forces are dominant, AE will be negative. On the other
hand, positive value (AE > 0) indicates a repulsive interaction
between the constituent components of the mixed monolayer.
Figure 2a shows the plot of AE versus mole fractions of DiI at
different surface pressures, viz., 5, 10, 15, 20, 25, and 30 mN/m.
From the figure, a noticeable deviation from the additivity rule
is observed, which is an indication of interaction among the
constituent molecules in the mixed monolayer. The deviations
are strongly dependent on surface pressure and mole fractions
of mixing. At lower mole fractions (XDiI = 0.2 and 0.3), a
negative deviation showing an attractive interaction among the
binary components in the monolayer is observed. At XDiI = 0.1,
the interaction is strongly pressure dependent. Here the
interaction is repulsive in nature at lower surface pressure
and approaches ideal mixing at relatively higher surface
pressures. However, at mole fractions XDiI > 0.4, an appreciable
positive deviation is observed, which clearly demonstrates the
existence of strong repulsive interaction among the constituent
molecules of mixed monolayers. These two types of
interactions balance each other in the case of ideally mixed
monolayers. In the present case, experimental observations
reveal the formation of almost ideally mixed monolayer at DiI
mole fraction of 0.4.
Thermodynamic stability of the mixed monolayers, as
compared to that of pure monolayers, was determined by
analysis of the excess Gibbs free energy.35 The excess free
energy of mixing (ΔGexc) has been calculated by using the
relation36
ΔGexc = N
∫0
Figure 2. Plot of (a) excess area per molecule (AE); (b) excess Gibbs
free energy of mixing (ΔGexc) vs the mole fraction of DiI in the DiI-SA
mixed monolayer at surface pressures of 5, 10, 15, 20, 25, and 30 mN/
m; and (c) compressibility (Cs) as a function of surface pressure for
mixtures of DiI and SA obtained from (π−A) isotherms.
π
[A12 − (A1X1 + A 2 X 2)] dπ
the interactions among the components of a binary monolayer
system. The value of ΔGexc provides information on whether
the particular interaction is energetically favorable (ΔGexc < 0)
or not (ΔGexc > 0), while for ΔGexc = 0, ideal mixing takes
place, i.e., absence of interactions between components.
where A12 is the experimentally observed area per molecule, A1
and A2 are the areas per molecule of the individual components
at mole fractions X1 and X2, and N is Avogadro’s number. The
excess Gibbs free energy ΔGexc reveals the information about
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clearly shows that the interaction between DiI and SA in the
mixed film for the DiI mole fraction 0.4 is completely different
than that in the other films. Our later studies confirm that the
organizational behavior of constituent molecules in the DiI−SA
mixed films of XDiI = 0.4 is totally different than those of other
mole fractions.
From the plot of the collapse pressure of the DiI−SA mixed
isotherm versus mole fraction of DiI (XDiI) (Figure S1 of
Supporting Information) it has been observed that the collapse
pressure of the mixed films is largely dependent on mole
fraction or the composition of the mixed films. This indicates a
certain degree of miscibility or mixing of the sample molecule
DiI and the matrix SA in the DiI−SA mixed films. Our previous
studies of Gibbs free energy of mixing and excess area per
molecule also support this.
Spectroscopic Characterization. Formation of Excimer
in Conventional LB Film. Figure 3a,b shows the normalized
UV−vis absorption and steady state fluorescence spectra of DiI
solution and LB film (10 layered) lifted at surface pressure 15
mN/m. The pure DiI solution spectrum shows a distinct band
Figure 2b shows the plot of excess Gibbs free energy of
mixing (ΔGexc) with mole fraction (XDiI) for surface pressures
5, 10, 15, 20, 25, 30, and 35 mN/m. The negative values of the
Gibbs free energy for mole fractions XDiI = 0.2 and 0.3) indicate
that the interaction between constituents of the monolayer is
favorable and that these molecules closely interact with each
other forming stable complexes. The negative value of ΔGexc
also confirms that the interactions between unlike molecules
(SA and DiI) is more attractive than those in between like
molecules (viz., SA and SA or DiI and DiI). Also, the values of
ΔGexc are found to be decreasing with surface pressure. The
minimum value of ΔGexc suggests the formation of a highly
stable system. This result is also in good agreement with that
obtained from the excess area vs mole fraction plot. The
positive Gibb’s excess free energy for XDiI > 0.4 as evidenced
from the figure is suggestive of formation of thermodynamically
unstable monolayers, which also found to increase with surface
pressure, and finally attains a maximum at 35 mN/m. ΔGexc > 0
indicates a tendency of individual component molecules to
interact preferentially with molecules of the same kind. This
results in the formation of bidimensional monolayer aggregates
by the monolayer components and a thermodynamically
unstable monolayer, as explained by using the schematic
diagram (shown in a later part of the manuscript).
At mole fraction XDiI = 0.4, ΔGexc = 0 indicating almost ideal
mixing between binary constituents of the mixed monolayer. In
this case, some organizational changes may take place in SA−
DiI mixed LB film, which subsequently leads to the formation
of aggregates.
It is interesting to note that in case of mixing ratio 3:7, DiI−
SA interaction dominates over SA−SA interaction. On the
other hand, for mixing ratio 7:3, DiI molecules being more in
solution, DiI−DiI interaction as well as also hydrophobic
interactions between long chains predominates over the DiI−
SA interaction. This difference in interaction may be
responsible for the observed opposite behavior at lower and
higher mole fraction in the DiI−SA mixed monolayer.
The study of compressibility (C) of the monolayer at the
air−water interface is another very important tool to gain
information about the inherent interactions involved in the
system. Compressibility (C) is calculated using the following
standard thermodynamic relation in two dimensions36,37 C =
−(1/A)(dA/dπ), where A is the area per molecule at the
indicated surface pressure π.
The states of Langmuir monolayer are reflected as peaks in
the C−π curve; the presence of a peak in the curve represents
the inherent phase transitions involved in the system. The
asymmetry of the peaks in the different C−π curves clearly
indicates that phase transitions of different DiI−SA mixed
monolayers are dependent on the extent of interaction among
DiI and SA. This again depends on the mixing ratio of DiI and
SA. From Figure 2c, for the film at DiI mole fraction of 0.4 a
clear difference in the value of highest compressibility and its
position in the C−π curve are observed with respect to all the
other monolayer films. At higher mole fractions of DiI, the
compressibility values are almost the same except at DiI mole
fraction 0.9, which is very close to the nature of pure DiI film,
indicating almost DiI-like behavior in the mixed film. At lower
mole fractions of DiI (0.1−0.3), the compressibility values are
slightly greater than that at higher mole fractions, but less than
that at 0.4 mole fraction. Moreover, the positions of the peaks
of the C−π curves in the mixed films lie in between that of pure
DiI and SA, except for the film at DiI mole fraction 0.4. This
Figure 3. Normalized (a) UV−vis and (b) steady state fluorescence
spectra of DiI in solution and 10 layer LB film lifted at surface pressure
15 mN/m. Inset of (b) shows the excitation spectra of DiI in solution
(λex = 574 nm) and in LB film (λex = 706 nm).
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explains the presence of a monomeric band in the absorption
spectrum.
It is well-known that the presence of excimeric species in the
self-assembled structure is unwanted for certain technological
applications. So, it is important to control the extent of excimer
formation or convert this kind of aggregated species into some
other type of technologically friendly aggregated species such as
J-aggregates. This will serve a dual purpose both for J-aggregate
and for excimer according to the need of the application. So,
emphasis must be given to the efforts to quantify the optimized
conditions for fabricating such species so that one can
transform from one species to another as per requirement. In
the quest of doing so we have prepared the DiI LB films under
various conditions, viz., mixing with fatty acids, cationic
surfactants, polymers, incorporating clay minerals, varying the
deposited surface pressure and number of layers, and so forth.
This will be discussed in the following sections.
J-Aggregate Formation in the DiI−SA Mixed LB Film.
The preceding section of the manuscript reveals the formation
of DiI excimer in LB films. It would be very interesting if this
could be controlled. There are few reports regarding the
control of the size of aggregates by diluting the dyes with fatty
acid matrix.43 Also, the aggregate forming dyes have been mixed
with non-aggregating dyes in order to control the extent of
aggregation.44−46 In one of our earlier works, we tried to
control the J-aggregate formation of a thiacyanine dye in LB
film by diluting with octadecyl trimethylammonium bromide
(OTAB).47 It has been observed that the J-aggregate formation
can be minimized in the LB film when the thiacyanine dye was
mixed with OTAB at the ratio 10:90.
Here, at first we verify this probability by mixing DiI with SA
in different mole fractions. Monolayer characteristics were
confirmed by π−A isotherms in all fabrication conditions
(Figure 1). Figure 4a shows the UV−vis absorption spectra of
pure DiI and DiI in SA matrix mixed at different mole fractions.
From the figure, it is observed that red shifting of the monomer
band (563 nm) of pure DiI occurred when mixed with SA;
however, the position of the high energy shoulder (524 nm)
remains almost the same. As the mole fraction of DiI in SA
increases, the monomer band shifts toward the longer
wavelength and is found to be maximum at DiI mole fractions
of 0.4 and 0.5 (597 nm). The interesting thing is that if we
increase the mole fraction further, the monomer band at 563
nm appears again, along with this longer wavelength band (597
nm) up to XDiI = 0.8. A closer look to the absorption spectra at
XDiI = 0.4 and 0.5 (Figure 4a) reveals that the monomer band is
almost indistinguishable in these two cases and the longer
wavelength band is very sharp, distinct, and prominent. This
means that at these mole fractions all the monomers form
aggregates. On further increasing the DiI mole fractions (XDiI >
0.5) a weak hump is observed in the position of the monomer
band at XDiI = 0.6, and this monomer band goes on increasing
with the increase in the DiI mole fraction, and correspondingly,
the longer wavelength band (597 nm) decreases. However, at
XDiI = 0.9 only a prominent monomer band at 563 nm is found
to be present. In the present case this red-shifted prominent
longer wavelength band at 597 nm may be due to the formation
of J-aggregates in the DiI−SA mixed LB film.
It is relevant to mention here that, according to Kasha
exciton theory, J-aggregates are formed when interaction
between transition dipoles of two or more chromophores are
arranged head-to-tail.48,49 J-Aggregates are characterized with a
narrow absorption band that is shifted to longer wavelengths,
system within the 450−600 nm region with a prominent 0−0
band at around 564 nm and a small high energy shoulder at 524
nm, which may be the vibronic component of the monomer
band. The absorption spectrum of pure DiI LB film shows an
overall broadening of peaks along with a red shift of about 8 nm
(in the monomeric band) in comparison to pure solution
spectrum. This red shift and broadening may be due to the
change in the microenvironment in the LB films together with
the closer association of a large number of DiI molecules in LB
films resulting in the formation of aggregates. In order to
substantiate whether the aggregation takes place at the
monolayer or due to multilayer films, we have also prepared
monolayer DiI−SA LB films at different mole fractions (Figure
S2 of Supporting Information). However, the spectral profile
was found to be exactly identical except for a slight change in
intensity. This suggests that aggregation behavior also exists in a
monolayer system.
The steady state fluorescence spectrum of DiI solution
(Figure 3b) shows a distinct prominent monomer peak at 574
nm in the 550−650 nm region. The shift of the monomer band
of about 18 nm in the solution fluorescence spectrum in
comparison to the solution absorption spectrum may be due to
the deformation of electronic states of DiI molecules. However,
the fluorescence spectrum of DiI in LB film is very interesting.
It shows a broad band profile in the 600−800 nm region with
indistinguishable monomer band and a prominent broad
structureless band at 706 nm. The origin of this longer
wavelength broad structureless band in the LB films may be due
to the formation of strong excimeric emission that may
originate due to the formation of organized structures of
molecular stacking in the LB films. It is worthwhile to mention
that such a structureless broad band appeared in the longer
wavelength region of the emission spectrum when excimer is
formed in the mixed LB films.17
To have any idea about the origin of the longer wavelength
band at 706 nm in the LB film fluorescence spectra, we
measured the excitation spectra of the DiI solution and LB film
with monitoring wavelength 574 and 706 nm, respectively, i.e.,
their corresponding emission maxima (inset of Figure 3b). The
spectral profile of the solution excitation spectrum is in good
agreement with the corresponding solution absorption
spectrum. The LB film excitation spectrum possesses similar
a spectral profile. The sharp resemblance of excitation spectra
for both solution and that of LB film definitely leads us to the
conclusion that the broad spectral profile of the emission
spectrum at longer wavelength of around 706 nm originates
from the formation of DiI excimer in the LB films. It is
worthwhile to mention in this context that other cyanine dyes
have also been observed to form excimer in certain
conditions.38 There are some other molecules such as perylene
derivatives,39,40 pyrene,41 anthracene derivative,42 and oxazole
derivative30 that are found to form excimers in LB films.
A closer look at Figure 3b reveals that a monomeric band is
scarcely observed in the LB film fluorescence spectrum,
indicating almost all of the dye molecules are in the excimeric
state. Then, what accounts for the presence of prominent
monomeric band in the DiI LB film absorption spectrum? It is
well-known that excimers are excited state dimers, which exist
only in the excited state, not in the ground state. On the other
hand, absorption is a ground state phenomenon whereas
fluorescence is an excited state one. So the presence of
excimeric species is only confirmed by fluorescence. This
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that the 597 nm band is due to the formation of J-aggregate in
the mixed LB film. Interestingly, at mole fractions XDiI = 0.4
and 0.5, the longer wavelength excimer band totally disappears,
and only the 597 nm band is present. This indicates that at
these mole fractions of DiI only J-aggregate is present in the
mixed film and the excimeric band is totally absent. On further
increasing the DiI mole fraction, the excimer band appears and
becomes prominent with increasing mole fraction of DiI in the
mixed films. In the DiI−SA mixed LB film for XDiI = 0.9, only
the excimeric band is present and the J-band is totally absent.
So, based on the study of Figure 4 we can assume the existence
of both types of aggregated species (excimer and J-aggregate) in
the DiI−SA mixed film. At DiI mole fractions, i.e., at XDiI = 0.4
and 0.9, these two types of aggregates transform in between
them. At mole fraction XDiI =0.4, transition occurs from excimer
to J-aggregate, and at XDiI = 0.9, the phenomenon is reversed.
Except for these mole fractions (0.4, 0.5, and 0.9) of DiI in SA,
in all other mole fractions of mixing, both types of aggregated
species coexist in the mixed films.
In order to have clear idea about the existence of different
types of aggregated species in mixed film, we have investigated
the excitation spectra of these films by exciting them at their
corresponding emission maxima (Figure 5). The resemblance
Figure 4. Normalized (a) UV−vis absorption and (b) steady state
fluorescence spectra of pure DiI and DiI−SA mixed LB films. All the
films are 10 layered. The number denotes the corresponding mole
fractions of DiI in SA.
with respect to the monomer absorption band, with a nearly
resonant fluorescence.
The study based on the absorption characteristics of the
binary system of DiI and SA (Figure 4a) clearly indicates
certain transition behavior between the different types of
aggregated species present in the system. The previous section
of this manuscript clearly shows the existence of excimeric
species in the pure LB film of DiI. Now, to confirm the
existence of different types of aggregated species in the binary
system we have measured the fluorescence spectrum of pure
DiI and different binary mixtures of DiI and SA. Figure 4b
shows the fluorescence spectra of pure DiI and DiI in SA matrix
mixed at different mole fractions. The spectral profile of pure
DiI and that of DiI−SA mixed LB film at XDiI = 0.1 are almost
the same; only the presence of a longer wavelength band at 706
nm (excimeric band) is present. With the increase in the DiI
mole fraction, the longer wavelength excimeric band (706 nm)
gradually diminishes with the corresponding increase in shorter
wavelength band at 598 nm in the mixed LB film. A closer
inspection of the absorption and fluorescence spectra of DiI−
SA mixed LB films reveal that both the absorption and
fluorescence maxima of the 597 nm band are sharp and have
almost overlapping peak positions with almost zero Stokes shift.
These are the characteristics of J-aggregates and hence confirm
Figure 5. Excitation spectra of (curve 1) DiI in solution (λem = 574
nm), (curve 2) pure DiI LB film (λem = 706 nm), (curve 3) XDiI = 0.4
of DiI in DiI−SA mixed LB film (λem = 597 nm), and XDiI = 0.8 of DiI
in DiI−SA mixed LB film (λem = 706 (curve 4) and 597 nm (curve 5)).
between curves 1 (excitation spectrum monitored at emission
maxima of DiI solution) and 2 (excitation spectrum monitored
at emission maxima of pure DiI LB film) confirms the existence
of excimeric species in the pure LB film. The fluorescence
spectroscopic study based on Figure 4b suggests the presence
of J-aggregated species in the mixed LB film at XDiI = 0.4 and
0.5. This study also suggests the coexistence of both excimer
and J-aggregate in all other mole fractions (except XDiI = 0.9
where only excimer is present). The excitation spectrum (curve
3) monitored at emission maxima (597 nm) for XDiI = 0.4 is
red-shifted with respect to the excitation spectrum of DiI
solution (curve 1) and pure LB film (curve 2) and very similar
to the corresponding absorption spectrum. This definitely
confirms that the origin of the 597 nm band in the fluorescence
spectrum is due to the presence of J-aggregates. On the other
hand, for the mixed film fabricated at XDiI = 0.8 two emission
maxima are there, one corresponding to the J-band at 597 nm
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Figure 6. Deconvolution of the absorption spectra of DiI mixed with SA at different mole fractions and pure DiI LB film absorption spectra. The
number denotes the corresponding mole fractions of DiI in SA.
existence of J-aggregated species in the mixed film fabricated at
XDiI = 0.8. So, the study of excitation spectra strongly confirms
our assumption about the existence of different types of
aggregated species in the mixed film under various fabrication
conditions as mentioned earlier.
Deconvolution of Spectra. The process of deconvolution
of spectra into several Gaussian curves allows the adequate
and the other to the excimer band at 706 nm. So, we have
measured two excitation spectra by monitoring the two
emission maxima: 706 nm (curve 4) and 597 nm (curve 5).
The spectral profile of curve 4 replicates the profiles of curves 1
and 2, indicating the existence of excimeric species in the mixed
LB film (for mole fraction XDiI = 0.8). On the other hand, the
spectral profile of curve 5 resembles that of curve 3 ensuring the
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corresponding to J-aggregation; only excimeric species is
present. This has been confirmed by the spectroscopic
measurement as depicted in Figure S5(a−e) of the Supporting
Information. At this stage, failing to achieve J-aggregation under
the circumstances mentioned above, next we tried by
incorporating nanoclay Laponite, thereby forming hybrid film.
The reason for choosing the clay mineral Laponite was its high
cation exchange capacity and layered structure,50,51 which
makes it an ideal host material for preparing organoclay hybrid
films.52 Enhanced mechanical stability of the hybrid LB film
with respect to the conventional LB film is another reason for
trying to investigate the optimized condition for making Jaggregates in the hybrid LB films. It is important to mention in
this context that in one of our earlier works we controlled the
formation of excimer of a coumarin derivative in LB films by
incorporating nanoclay Laponite.53 In the present case also, we
tried the same but failed; excimer remains present in the hybrid
film and J-aggregate is absent. This is evidenced from Figure
S6(a-d) of the Supporting Information. However, the reason
for the absence of J-aggregation in the polymer mixed LB film
and in the organoclay hybrid film is unclear to us. A study to
explore this is on-going in our laboratory. We have also tried to
induce J-aggregation in the pure DiI LB film by varying the
deposited surface pressure and also by varying the number of
deposited layers, but failed (not shown). Now at this stage,
failing to achieve J-aggregation in the LB film, next we tried by
employing another fatty acid matrix (AA) other than SA.
Interestingly, in this case J-aggregate was formed in the DiI−AA
mixed LB film and the spectral profile is like that of DiI−SA LB
film (Figure S7(a-e) of Supporting Information).
So from the above study, it is clear that DiI has a greater
tendency to form J-aggregate in the LB film when mixed with
fatty acid matrices at a particular mole fraction. Other than that,
in all other conditions mentioned in the manuscript DiI forms
excimers in the LB film.
Coherent Size of J-Aggregates. The coherent size of the
J-aggregate is very important, as the nonlinear application of Jaggregates is largely influenced by the spectroscopic aggregation number.54 The coherent size or spectroscopic aggregation
number of J-aggregates can be easily estimated from the full
width at half-maximum (fwhm) of the absorption band of the
monomer (Δν1/2(M)) and the J-aggregate (Δν1/2(J)) by using
the following equation54,55
determination of different species present in the spectra.
Deconvolution of absorption spectra of pure DiI LB film along
with the different DiI−SA mixed LB films fabricated at different
DiI mole fractions in SA is shown in Figure 6. Deconvolution of
pure DiI absorption spectrum (Figure 6a) reveals two Gaussian
curves corresponding to the monomer and the vibronic
component of monomer placed at higher energy. Deconvolution spectra of DiI−SA mixed LB film absorption spectrum at
XDiI = 0.2 possess three Gaussian curves with identical peaks
corresponding to vibronic component, monomer, and Jaggregate (Figure 6b). The corresponding absorption spectrum
possess two peaks, one corresponding to the higher energy
band and the other to the red-shifted monomer band. This
indicates that the red-shifted absorption band is an overlap of
monomer and J-aggregate bands. The intensity of the longer
wavelength band goes on increasing with the increase in mole
fraction up to XDiI = 0.4, indicating the transition toward Jaggregation (Figure S3(a) of Supporting Information).
Deconvolution of the corresponding fluorescence spectra
gives two Gaussian curves corresponding to the peak positions
of J-band and excimeric band. Here also, the intensity of the
Gaussian curve corresponding to the J-band goes on increasing
with the increase in DiI mole fractions in SA (Figure S3(b) and
(c) of Supporting Information). Deconvolution of the
absorption spectrum for the mixed films fabricated at mole
fraction XDiI = 0.4 possesses two Gaussian curves corresponding
to the J-band and the higher energy vibronic component of the
monomer band (Figure 6(c)). The curve corresponding to the
monomer band is absent indicating the presence of only Jaggregate in the film. Now, the absence of monomer band in
the absorption spectra essentially rules out the possibility of
formation of excimer in the LB film. Deconvolution of the
corresponding fluorescence spectrum also confirms the same
(Figure S3(d) of Supporting Information). The existence of
different types of aggregated species in the mixed film
fabricated at XDiI = 0.8 is confirmed by the presence of three
distinct Gaussian curves in the deconvoluted absorption
spectrum depicted by Figure 6d. Deconvolution of the
corresponding fluorescence spectrum also possesses Gaussian
curves corresponding to the J-band and the excimer band
(Figure S3(e) of Supporting Information). Finally, the
transition from J-aggregation to excimer takes place for the
film fabricated at XDiI = 0.9 as evidenced from deconvolution of
the corresponding absorption (Figure 6e) and fluorescence
spectra (Figure S3(f) of Supporting Information), where only
excimer band is present.
Effect of Other Film Forming Parameters on the JAggregate Formation of DiI in the Mixed LB Film.
Previous sections of this manuscript clearly demonstrate the
condition for the formation of J-aggregate in the DiI−SA mixed
LB film. Considering the technological importance of this type
of aggregated species, emphasis must be given to the effort to
quantify various conditions to fabricate such type of aggregated
species in the LB film. In the quest of doing so we have first
tried by employing a cationic amphiphile ODA, fabricating
DiI−ODA mixed LB films at different mixing ratios, but failed
to achieve any trace of J-band. Excimeric species prevailed as
the prominent aggregated species in all the fabricated films
(Figure S4(a−e) of Supporting Information). This may be due
to the predominance of repulsive interaction between DiI and
ODA as both of these two molecules are cationic in nature.
Thereafter, we tried by mixing with a neutral polymer matrix
PMMA. Here also, we did not get any trace of J-band
N1/2 = Δν1/2(M)/Δν1/2(J)
Table 2 shows the measured aggregation number (N) of the
mixed LB films fabricated at different DiI mole fractions.
Clearly, the coherent size of the J-aggregate increases with the
increase in DiI mole fraction and becomes maximum at DiI
mole fraction XDiI = 0.4. Upon further increasing the DiI mole
fraction, the aggregation number is found to decrease, implying
Table 2. Calculated Aggregation Number at Different Mole
Fractions of DiI Mixed with SA
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mole fractions of DiI
aggregation number (N)
0.1
0.2
0.3
0.4
0.7
0.8
2.64
2.81
3.10
3.36
2.45
2.52
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Figure 7. AFM image of monolayer LB films of DiI mixed with SA onto silicon substrate at mole fractions (a) XDiI = 0.9 and (b) XDiI = 0.4 along
with line analysis spectrum.
Here, in the DiI−SA mixed (XDiI= 0.4) film, almost the entire
surface was covered with these individual nanodisk-like
structures. These disks are attributed to single domains, i.e.,
islands of J-aggregates on the silicon substrate. Topographic
cross-sectional analysis of a single island of J-aggregate clearly
demonstrates that the lateral size of a single J-aggregate domain
is in the range of 100 nm and height ranges from 1 to 1.2 nm.
However, actual lateral diameter of a single J-aggregate domain
should be smaller than the measured value due to limiting
radius of the cantilever tip, approximately 8 nm. The lateral size
of a J-aggregate is considered as its physical size consists of a
number of closely spaced coherent sizes in the single domain of
J-aggregates. Estimation of coherent sizes based on their
topographic characterization in a single domain is limited
because their sizes are in the range of few nanometers as
measured from absorption spectrum. Considering the geometry
and finite size effects of the tip, estimation of real diameter of
individual J-aggregate is possible using the following equation59
that the maximum J-aggregation occurs at XDiI = 0.4. Analysis of
deconvolution spectra also confirms the same. These values of
N are almost within the error limit of that in an LB film (N = 6
± 3).56
Atomic Force Microscopy. To get visual confirmation and
ideas about the structure of J-aggregates in the monolayer film,
DiI−SA mixed monolayer LB films were studied by atomic
force microscope (AFM).
Figure 7a shows the AFM image of monolayer LB film of DiI
mixed with SA at XDiI = 0.9. Almost the entire film surface was
covered by DiI−SA mixed film with almost uniform film
deposition. Any kind of definite structure formation was not
seen in the AFM image, except for the presence of one or two
aggregated sites. The AFM image of monolayer LB film of pure
DiI is similar to that fabricated at XDiI = 0.9 (not shown). On
the other hand, disklike objects were observed on the DiI−SA
film fabricated at XDiI = 0.4 (Figure 7b). It may be mentioned in
this regard that absorption and fluorescence spectroscopic
measurements have already confirmed the existence of Jaggregate in the LB film at this mole fraction of DiI. So, the
presence of these nanodisk-like structures may be due to the
formation of J-aggregates in the monolayer LB film. Similar
circularly shaped J-aggregates of other cyanine dyes in LB films
have already been reported.57,58 The observed difference
between the domain structures for excimer in pure DiI LB
film and for J aggregates in DiI−SA mixed LB film gives
compelling visual evidence of the formation of two types of
aggregates in LB films.
Wi = 2(2RZi − Zi 2)1/2 + Li
Here, Wi, Zi, and Li are, respectively, the apparent lateral size,
height, and real diameter of the particle and R is the radius of
the tip. The real diameter of individual J-aggregated domains is
estimated to be 92.3 nm when an apparent lateral size of 100
nm and a height of 1 nm were taken. The radius of the tip used
for the calculation was 8 nm. This suggests that each Jaggregate domain in the LB film contains approximately (20 ±
5) coherent sizes. Here, the estimated value has some
limitations as it accounts for only the geometric effect of the
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antisymmetric COO−1 stretching vibration of the carbonylate
group.66
On the other hand, a close look at the FTIR spectrum for
DiI−SA mixed LB film shows that the relative band positions
are changed compared to that in pure SA LB film spectrum.
This indicates that there may be some orientational effects on
the DiI−SA mixed LB film due to interaction between SA and
DiI. Notable change is occurred in the CO stretching region.
It has been observed that in the DiI−SA mixed LB film
spectrum the band position at 1710 cm−1 is shifted toward the
lower wavenumber region, indicating interaction between
carboxylic acid (−COOH) group of SA and N of DiI. At the
same time, the band positions 2853 and 2920 cm−1 are also
shifted toward the higher frequency region and become very
weak, which also indicates there may be some interaction
(mainly hydrophobic−hydrophobic) occurring between the
hydrocarbon chains of SA and DiI molecule. On the other
hand, the presence of the 1559 cm−1 band in the SA−DiI mixed
LB film spectrum confirms the presence of carboxylate anion in
the mixed films. This indicates that the interaction between the
N atom of DiI with the −COO− is electrostatic in nature in the
mixed LB films.
DiI contains two nitrogen atoms labeled (i) and (ii).
Structures 1 and 2 are two canonical forms of DiI (Figure 9).
tip and does not take into account the interactions between the
tip and the sample. However, there is always some differences
between the calculated and estimated size of the aggregates.
The diameter of the J-aggregate estimated from AFM study is
substantially larger than the coherent size approximated from
the absorption spectrum. It may be mentioned in this context
that the spectroscopic aggregation number for the J-aggregate
has been calculated considering that the spectral width of Jband is due to only a motional/exchange mechanism. However,
this number is not the exact aggregation number of J-aggregate,
as some mechanisms of line broadening such as temperature
and static/dynamic disorder were not taken into account. If
these effects were considered, then it is possible that the
aggregation number and coherence length will be much higher
than the data presented here.
■
DISCUSSION
The formation of two types of aggregated species of DiI in LB
films when mixed with SA at different mole fractions is clearly
demonstrated in the preceding section. Spectroscopic study
clearly exhibits a transition behavior between excimer and Jaggregate in the DiI−SA mixed LB films. Conjugating πelectron structure of DiI is mainly responsible for this behavior.
To have a greater idea about the interaction between SA and
DiI, we measured the ATR-FTIR spectra of pure SA, SA-DiI
mixed (XDiI = 0.4) LB films (Figure 8) deposited onto a ZnSe
single crystal. All the films were deposited onto ZnSe substrate
at a surface pressure of 15 mN/m.
Figure 8. ATR-FTIR spectra of pure SA, SA-DiI mixed (XDiI = 0.4) LB
films deposited onto ZnSe single crystal. All the films were deposited
onto ZnSe substrate at a surface pressure of 15 mN/m.
Figure 9. Two resonating forms of DiI molecule. Case 1:
Transformation of lone pair from N atom labeled (i) to N atom
labeled (ii). Case 2: Transformation of lone pair from N atom labeled
(ii) to N atom labeled (i).
ATR-FTIR spectrum of pure SA LB film shows strong
prominent characteristic bands at 2920, 2853, 1710, and 1558
cm−1. The first two bands at 2920 (υa) and 2853 (υb) cm−1 are
identified as the stretching vibrations of the CH2 group within
the long alkyl chain of SA.60−62 The band at 1710 cm−1 is due
to the stretching vibration of carbonyl group.63 This band is
indicative of the acid form of SA. It is generally observed that
for the free CO (carbonyl group) stretching vibration this
band appears at around 1760 cm−1. However, a shift toward
lower wavenumber is observed when H-bonds are formed or
association occurs.64,65 The SA LB film spectrum also shows a
peak at around 1558 cm−1, which is assigned to the
From structure 1, it is observed that the N atom labeled (ii) is
positively charged, hence SA molecules interact electrostatically
with this N atom. On the other hand, the N atom labeled (i)
contains a lone pair. From structure 2, it is obvious that the
lone pair of N atom labeled (i) participates in resonance. As a
result in structure 2, N atom with label (i) gets positively
charged. This ensures that SA molecules can interact with the
N atom, labeled either (i) or (ii). When SA molecule gets
attached to N (ii) and the complex comes in contact with
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another resonating form of DiI with N (i) positive, SA will have
a tendency to form a complex with the molecule having
positively charged N (i). At lower mole fraction of DiI (i.e., XDiI
< 0.4), the number of DiI molecules was less than SA molecules
in the mixture. As a result, there is a possibility that all the DiI
molecules will form complexes with SA. But due to the
presence of an excess amount of SA molecules in the mixture
and the resonating structure of DiI, the DiI molecules may
come out of the complex and attach to the neighboring free SA.
Also, the hydrophobic repulsion between the hydrocarbon
chains of SA and DiI influence the DiI to come out from the
DiI−SA complex. Thus, the DiI−SA complex is not stable, but
rather remains an unstable intermediate complex. This situation
has been shown in case 1 and case 2 of Figure 9, where SA
interacted with two different N atoms of DiI. As a result, there
exist two competing aggregated species, but none is prominent
at lower mole fraction. This situation is schematically depicted
in Figure 10a. It is evidenced from their absorption spectra
(Figure 4a, XDiI = 0.1−0.3), in which an overlapping band
position of these two types of aggregated species was observed.
This was confirmed from the deconvolution of the corresponding absorption spectrum (Figure 6b,c). At intermediate mole
fractions, viz., at XDiI = 0.4 and 0.5, the concentrations of both
DiI and SA in the mixture are almost same. The delocalized
positive charge of DiI interacts electrostatically with SA and the
formed complex. Due to the unavailability of excess DiI or SA
molecules in the systems, this complex is stable and forms the
corresponding stable aggregated species in the system. Here
due to the stable complex formation, the π-electron conjugation
gets restricted and hence the corresponding peak due to
conjugation disappears from the absorption spectrum (Figure
4a). Only the J-aggregate remains present as shown in Figure
10b. There also exists a hydrophobic−hydrophobic interaction
which may be responsible for some structural deformation and
helps in the formation of J-aggregate in the system.
Deconvolution of the absorption spectrum for the film at XDiI
= 0.4 also confirms the presence of only J-aggregated species in
the LB films (Figure 6d). Thereafter, with the increase in the
DiI mole fraction (XDiI = 0.6−0.8) in the DiI−SA mixture the
peak corresponding to the conjugation of DiI (558 nm) again
becomes prominent with the increase in DiI mole fraction. At
these mole fractions, the number of DiI molecules in the
mixture is larger than that of SA, and as such even after the
formation of stable complex, some free DiI molecules remain
present in the system. These free DiI molecules along with the
DiI−SA complex account for the peaks positioned at 558 and
597 nm, respectively (Figure 4a). This situation has been
schematically shown in Figure 10c. However, if we increase the
DiI mole fraction further, i.e., at XDiI = 0.9, only excimer is
present in the system (schematically shown in Figure 10d) as
evidenced from spectroscopic measurements. This is because at
this mole fraction the number of DiI molecules with respect to
SA molecules in the system is so large that the mixed film
behaved like pure DiI LB film. So from the above discussion, it
is clear that SA molecules play an important role for structural
deformation and molecular arrangement in the DiI−SA mixed
film and are also responsible for the transition behavior in the
mixed LB films which is reversible in nature.
Figure 10. Schematic representation of excimer-J-aggregate reversible
transition mechanism. (a) At DiI mole fraction XDiI = 0.2 in SA matrix,
more SA molecules are present in the system than DiI. As a result,
almost all the DiI molecules form complexes with SA, but due to the
resonating structure of DiI and the presence of an excess amount of SA
in the system, an unstable intermediate complex is formed, the
stacking of which formed the J-aggregate. This complex being unstable
goes through an intermediate state thereby releasing free DiI
molecules in the system for a short time which accounts for the
excimeric species in the system. As a result, at this mole fraction of DiI
two competing aggregated species exist in the system, but none is
prominent. (b) At mole fraction XDiI = 0.4 in the SA matrix, the
numbers of DiI and SA molecules are almost the same in the system.
As a result, a stable complex is formed, the stacking of which results in
the formation of J-aggregate in the system. (c) At higher mole fraction
of DiI (XDiI = 0.8), due to the presence of excess amount of DiI in the
system, both J-aggregate (due to stacking of stable complex) and
excimer (due to presence of free DiI) are present in the system. (d) At
mole fraction XDiI = 0.9, the situation is almost like pure DiI and forms
only excimer.
attractive as well as repulsive interactions between the
components depending on the mole fraction of mixing as
well as surface pressure. However, stable and ideally mixed
monolayers formed at DiI mole fraction of 0.4 in SA due to the
balance of interactions between DiI and fatty acids in the mixed
monolayer. It has been observed that pure DiI forms excimers
in LB films, whereas both J-aggregate and excimer were formed
in LB films when DiI was mixed with long chain fatty acids, viz.,
SA or AA. At XDiI = 0.4 in fatty acid matrix only J-aggregates
were formed in the LB films, although both J-aggregate and
excimer were present at higher as well as lower mole fraction of
DiI. Deconvolution of spectroscopic results as well as excitation
spectroscopy measurements confirmed this. AFM study gave
compelling visual evidence of the formation of disklike Jaggregate domains in DiI−SA mixed LB films. The coherent
size of J-aggregates was found to be maximum for the mixed
film at XDiI = 0.4 in fatty acid matrix. It was found that
approximately 20 ± 5 coherent sizes exist in each J-aggregate
■
CONCLUSION
Langmuir and Langmuir−Blodgett (LB) films of an indocarbocyanine dye DiI were investigated. Surface pressure−area
(π−A) isotherms of DiI and its mixture with SA suggest both
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domain in the mixed LB films. However, J-aggregates were
totally absent when DiI was mixed with cationic surfactant,
polymer, or nanoclay. Therefore, we have quantified the
optimized condition to fabricate different types of technologically friendly aggregated species, i.e., J-aggregate and excimer
using a particular indocarbocyanine dye DiI. This is very
important as one can transform from one species to another
according to the needs of applications.
■
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ASSOCIATED CONTENT
S Supporting Information
*
Variation of collapse pressure with mole fractions of DiI in
mixed LB films, UV−vis absorption spectra of DiI−SA mixed
monolayer LB films, variation of excimer and J-band intensities
with mole fractions of DiI in mixed LB films, deconvolution of
the absorption and fluorescence spectra of different LB films,
absorption and fluorescence spectra of DiI-ODA, DiI-PMMA,
DiI-Clay, and DiI-AA mixed LB films. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Email: [email protected], [email protected]. Ph:
+919862804849 (M), +91381 2375317 (O). Fax:
+913812374802.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors are grateful to DST, for financial support to carry
out this research work through FIST DST project ref. SR/FST/
PSI-191/2014. The author Pintu Debnath acknowledges the
financial support through INSPIRE fellowship of DST, Govt. of
India. The authors are also grateful to Dr. Utpal De,
Department of Chemistry, Tripura University for discussion
about the results. Authors also acknowledge Dr. Atikul Islam
for helping Gibbs free energy calculations using Matlab.
■
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