control of two-dimensional aggregates of a long-chain merocyanine by change of molecular...
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Chemical Physics Letters 405 (2005) 416–421
Control of two-dimensional aggregates of a long-chainmerocyanine by change of molecular environments
in monolayer assemblies
Michio Murata, Masumi Villeneuve, Hiroo Nakahara *
Department of Chemistry, Faculty of Science, Saitama University Shimo-okubo 255, Sakura-ku, Saitama 338-8570, Japan
Received 16 November 2004; in final form 17 January 2005
Available online 16 March 2005
Abstract
An amphiphilic unsymmetrical merocyanine forms J- and H-aggregates by changing molecular environments in the binary and
the ternary mixed monolayer assemblies, which have been characterized by visible absorption spectra films having red-shifted and
blue-shifted bands in comparison with the solution spectrum, respectively. From the surface pressure–area isotherms of the mixed
monolayers, the film structures were studied at the air/water interface. The in-plane X-ray diffraction and the atomic force micros-
copy measurements of their LB films were done to observe the fine lattice structure. The aggregate structures were well correspond-
ing to the dye arrangements speculated by calculation with an extended dipole model for the spectral shifts.
� 2005 Elsevier B.V. All rights reserved.
1. Introduction
Much attention has been attracted to the J-aggregates
of some symmetrical cyanine dyes and porphyrin deriv-atives in two-dimensional molecular organized films and
polymer films from the view points of their photosensi-
tizing and photoreactive functions [1]. The J-aggregates
of the amphiphilic long-chain merocyanine dye (McC18)
having an unsymmetrical structure of the acceptor and
the donor moieties combined with the p-conjugatedpolymethine were reported previously in the mixed mon-
olayers and Langmuir–Blodgett films with Cd2+ salts offatty acids [2–5]. In addition to the red-shifted J-band,
the blue-shifted band of the transient state through the
compression (at 15 mN/m) on the ternary mixed mono-
layers of McC18–methylarachidate–hexadecane at the
air/water interface has been reported [6]. Hirano and
coworkers have found the blue shifted band assigned
to the H-aggregate in the ternary mixed LB films of
0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.02.055
* Corresponding author. Fax: +81 48 858 3700.
E-mail address: [email protected] (H. Nakahara).
McC18 cadmium salt, Cd2+ stearate and octadecane
[4,7].
It is necessary for the different optical behavior of the
merocyanine dye aggregates to clarify their two-dimen-sional micro- and nano-structures in the organized
molecular films mixed with molecular matrix compo-
nents. In our early work [2], the aggregation number
and the two-dimensional structural parameters were
estimated by using the extended dipole model [8]. Fur-
thermore, several studies by the atomic force micros-
copy (AFM) on the J-aggregation structure have been
reported [9–11]. On the other hand, the direct observa-tion of two-dimensional structures of the H-aggregates
has been scarcely carried out. Recently, the grazing
incidence in-plane X-ray diffraction (GIXD) for the or-
ganized molecular films has provided valuable informa-
tion on the two-dimensional lattice structures in
monolayer assemblies of hydrogenated and fluorinated
carboxylates [12–14] as well as the grazing incidence of
X-ray reflection [15,16].In this Letter, structures of the binary and the ternary
mixed monolayers of McC18 cadmium salt with
M. Murata et al. / Chemical Physics Letters 405 (2005) 416–421 417
cadmium arachidate and octadecane and their Lang-
muir–Blodgett films are investigated by the GIXD meth-
od, the AFM observation as well as the optical behavior
in addition to the examination with the surface pres-
sure–area (p–A) isotherms for the mixed monolayers
on water. We discuss the relation of the spectral featuresand the structures of the LB films.
2. Experimental
A long-chain merocyanine, 3-carboxymethyl-5-[2-
(3-octadecylbenzothiazolin-2-ylidene) ethylidene]rhoda-
nine (abbreviated as McC18, and whose structure isdemonstrated in the inset of Fig. 1) was purchased from
Japanese Research Institute for Photosensitizing Dyes,
Co. (Okayama, Japan). Arachidic acid (AA) and octa-
decane (OD) were purchased from Tokyo Chemical
Industry Ltd., and purified by recrystallization from
hexane. The monolayers were spread from the chloro-
form solution on distilled pure water (pH 5.8) or the
aqueous buffer solution containing Cd2+ ion(3 · 10�4 M CdCl2 and 3 · 10�5 M KHCO3 (pH 6.8)).
Therefore, the monolayers on the buffer solution consist
of all Cd2+ salts of AA and McC18. These cadmium salts
of the long-chain merocyanine and arachidic acid are
abbreviated as McC18(Cd2+) and AA(Cd2+), respec-
tively. On the other hand, the long-chain merocyanine
and arachidic acid on distilled pure water are abbrevi-
ated as McC18 and AA, respectively. Surface pressureversus mean molecular area (p–A) isotherms were mea-
sured by a Langmuir-type film balance (Lauda) and vis-
ible spectra of the monolayers at the air/water interface
80
70
60
50
40
30
20
10
0
Surf
ace
Pres
sure
(m
N/m
)
706050403020100
Area per molecule (Å2)
S
NC18H37
HC
HC
S
NO
S
CH2COOH(e)(c)
(d)
(b)(a)
Fig. 1. Surface pressure–area isotherms of the mixed monolayers
on the aqueous subphase (5 · 10�4 M CdCl2 and 3 · 10�5 M KHCO3,
pH 6.8): (a) AA, (b) McC18(Cd2+):AA(Cd2+) = 1:2, (c) McC18
(Cd2+):AA(Cd2+):OD = 1:1:1 and (d) McC18(Cd2+) together with the
dot-dashed line (e) for the ternary monolayer of
McC18:AA:OD = 1:1:1 spread on the distilled water (pH 5.8) at
12.5 �C. The dotted lines indicate the calculated curves by assuming
the corresponding ideal mixtures.
were obtained in situ by a multi-channel spectroscopy
(Otsuka electro.) at 12.5 �C. The J-aggregates morphol-
ogy of the mixed monolayers of McC18 on the water sur-
face were obtained directly by the fluorescence
microscope (Olympus: IMT-2 type) equipped with a di-
chroic mirror and a trough on the stage. The J-aggre-gates were excited at 550 nm and observed at
590 � nm by the silicon intensify target (SIT) camera.
The single monolayers were transferred by the Lang-
muir–Blodgett (LB) method at 25 mM/m and 12.5 �Conto the hydrophobic solid plate precoated with
AA(Cd2+). The absorption and fluorescence spectra of
the LB films were recorded on Hitachi U-3210 and
MPF-3 fluorescence spectrophotometers, respectively.The lateral packing of long hydrocarbons in the orga-
nized molecular films was examined by the grazing angle
incident X-ray diffractometer (Bruker: MXP-BX, 40 kV,
40 mA) and the surface morphology was observed by
the atomic force microscopy (Seiko: SPA300, Si3N4can-
tilevers: k = 0.09 N/m).
3. Results and discussion
Fig. 1 shows the p–A isotherms for the binary and the
ternary monolayers of (b) McC18(Cd2+):AA(Cd2+) = 1:2
and (c) McC18(Cd2+):AA(Cd2+):OD = 1:1:1 in the molar
ratios, respectively, together with the calculated curves
(dotted line) assuming the ideal mixture. In the case of
the ternary mixed monolayers, the molecular occupiedarea of OD was assumed to be comparable to
AA(Cd2+). The isotherm for the ternary mixed mono-
layer of (e) McC18:AA:OD = 1:1:1 on the distilled water
is also shown in Fig. 1. By comparing (c) and (e), it is
clear that the Cd2+ ion rigidifies the monolayer. The
pure dye McC18(Cd2+) forms an expanded monolayer
with the limiting area (Ap! 0: obtained from extrapolat-
ing the linear part to zero pressure) of about 55 A2/mol-ecule, which suggests the orientation of the merocyanine
chromophore with its long-axis parallel to the water sur-
face by taking account of the result of the X-ray crystal
analyses of the planar merocyanine dye analog [17,18],
as later described. The values of compressibility at
25 mN/m of the isotherms were (a) 0.00075, (b) 0.002,
(c) 0.005 and (d) 0.006 m/mN. With any matrix molecu-
lar component, the mixed monolayers of McC18(Cd2+)
were stabilized to form the condensed film with less
compressibility and higher collapsed pressures. By mix-
ing AA(Cd2+) in two moles of the dye McC18(Cd2+) the
isotherm of the mixed monolayer was almost consistent
with the calculated curve, assuming ideal mixing or com-
plete phase separation. By further mixing OD partially
instead of with half of AA molecules of the binary
monolayer, the molecular area is slightly reduced atthe collapse pressure (McC18(Cd
2+):AA(Cd2+):OD =
1:1:1) on the buffer aqueous subphase and the somewhat
418 M. Murata et al. / Chemical Physics Letters 405 (2005) 416–421
expanded film (McC18:AA:OD = 1:1:1) on the distilled
pure water could be obtained, indicating nonpolar OD
molecules inserted partially between the alkyl chains of
the mixed monolayers and possibility the remainders
placed on top of the monolayers. The OD molecules
are considered to play the role which helps the lateralpacking of alkyl chains and adjust the nano-space
through formation of the two-dimensional lattice of
McC18 monolayer by compression, resulting in a clearly
different orientation and packing of the chromophores,
as suggested later in the fluorescence images which were
McC18 (pure) McC18 (solution) McC18 : AA = 1 : 2
McC18 : AA : OD = 1 : 1 : 1 5mN/m 10mN/m 25mN/m 40mN/m collapse
Abs
orba
nce
(arb
.uni
ts)
Wavelength (nm)
750700650600550500450
McC18 : AA = 1 : 2 McC18 : AA : OD = 1 : 1 : 1
(a)
(b)
(c)
Fig. 2. In situ absorption spectra of (a) the mixed monolayers of
McC18(Cd2+):AA(Cd2+) = 1:2, together with those of the pure
McC18(Cd2+) monolayer and the chloroform solution of McC18, and
(b) surface pressure dependence of in situ absorption spectra of the
mixed monolayer of McC18(Cd2+):AA(Cd2+):OD = 1:1:1. The absorp-
tion spectra of those binary and ternary mixed LB films are shown in
(c).
dependent on the spreading condition of the monolayers
on the aqueous subphase.
Fig. 2a shows the visible absorption spectra for the
pure McC18(Cd2+) and the binary monolayer of
McC18(Cd2+):AA(Cd2+) = 1:2 on the buffer aqueous
subphase at 25 mN/m, in comparison with the chloro-form solution spectrum of McC18. The monolayer spec-
tra have the red-shifted absorption maxima at around
609 nm to compare with the solution spectrum at
530 nm for the McC18(Cd2+) monomer, which could
be characterized due to the J-aggregate formation [2].
The mixed monolayer of McC18(Cd2+) with AA(Cd2+)
gave sharpened J-band at 610 nm with less the half band
width and simultaneously accompanying with a broadshoulder around 555 nm, which suggests various num-
bers of the J-like small aggregates with a head-to-tail
structure of the chromophores. It has been found that
the matrix molecules of AA(Cd2+) promote the J-aggre-
gate formation with more homogeneous and larger do-
mains in the organized monolayer, in comparison with
the pure McC18(Cd2+) monolayer. In this case the
Cd2+ salts of fatty acids seemed to assist a closer packingof hydrocarbons as well as that of the chromophores.
Furthermore, the spectra of the ternary monolayer of
McC18(Cd2+):AA(Cd2+):OD = 1:1:1 is shown in Fig.
2b at various surface pressures from 5 to 40 mN/m.
The addition of OD to the binary mixture induced the
blue-shifted band at 510 nm from the monomer band
at 530 nm even at the low surface pressure of 5 mN/m,
which is considered to arise from the side-by-side inter-action of the transition moments along the long-axis
of the chromophore, indicating the presence of the
H-aggregate. As the surface pressure increases up to
25 mN/m, the sharp J-aggregate band at 610 nm grows
significantly and also the H-aggregate bands are en-
hanced a little and further shifts to the shorter wave-
length near 500 nm. Above 25 mN/m to the collapse
pressure over 40 mN/m, the absorption spectra changedcounterwise. In the ternary monolayer both the blue-
shifted H- and the red-shifted J-aggregate bands were
observed in every surface pressure, while the intermedi-
ate band around 555 nm disappeared. In the mixed
monolayers containing hexadecane or octadecane, these
long hydrocarbons are considered to play the role of
two-dimensional lubricants through spreading and com-
pression, leading to different two-dimensional crystal-lines or aggregates. Fig. 3 shows the in situ
fluorescence microscope images for the ternary mono-
layers. When the ternary monolayer was spread on the
aqueous subphase containing Cd2+ ions, a lot of fine
microcrystallites with the emitting J-aggregates were ob-
served as shown in Fig. 3a, whereas the feather-like do-
mains were seen in the mixed monolayer when spread on
distilled pure water in Fig. 3c. With an increase in sur-face pressure, these microcrystallites and feather-like do-
mains assembled as shown in Fig. 3b and d, respectively.
Fig. 3. In situ fluorescence images of the ternary monolayer, (a) just as spread (zero surface pressure) and (b) at 15 mN/m, on the aqueous subphase
containing Cd2+ ions and the buffer (pH 6.8); those corresponding images (c) at zero and (d) at 15 mN/m for the mixed monolayer spread on the
distilled water (pH 5.8).
Fig. 4. In-plane X-ray diffraction of LB films of (a)
McC18(Cd2+):AA(Cd2+) = 1:2, (b) McC18(Cd
2+):AA(Cd2+):OD =
1:1:1, and (c) McC18(Cd2+), deposited on glass plates at 25 mN/m,
together with the corresponding molecular packing estimated in the
right side.
M. Murata et al. / Chemical Physics Letters 405 (2005) 416–421 419
From these facts, it is suggested that crystallization ofthe ternary monolayers can be controlled by Cd2+ ions
in the subphase, and these observation correspond well
to the result of the p–A isotherms.
The single layer of the binary and the ternary
monolayers of McC18(Cd2+):AA(Cd2+) = 1:2 and
McC18(Cd2+):AA(Cd2+):OD = 1:1:1, respectively, well
transferred onto the hydrophobic quartz plates by the
Langmuir–Blodgett method at 25 mN/m. The absorp-tion spectra of the LB films are shown in Fig. 2c. The
J-band at 605 nm was observed mainly in the binary
LB film, which slightly shifted to the shorter wavelength
in comparison with the monolayer spectrum at 610 nm.
Whereas in the ternary LB film, the J-band at 605 nm
was relatively reduced and the H-band at 502 nm was
enhanced together with the broad shoulder around
555 nm. The change of absorption spectra of the mixedLB film from the monolayer on the buffer solution sur-
face, suggests that the molecular rearrangement oc-
curred through the transfer process as reported in the
flow orientation model [19]. From deconvolution of
the absorption spectra for the mixed LB films by the
Lorentzian [5,20], the content of the J-band (605 nm)
was about 47% for the binary system, while that for
the ternary system was 11%, which is comparable to14% of the H-band (around 505 nm) content.
The in-plane spacings of the two-dimensional lattice
of the binary and ternary LB films were determined by
X-ray diffraction of the grazing incidence (GIXD) at
0.2� and a scanning rate at 0.05�/50 s. Fig. 4 shows
GIXD patterns for the binary and the ternary LB films
in comparison with the pure McC18(Cd2+) LB film (two
layers in the Y-type, deposited at 25 mN/m, 12.5 �C).Although a clear diffraction was not observed for the
pure McC18(Cd2+) film, the single diffraction at 4.2 A
appeared in the binary LB film, which suggests an iso-
tropic hexagonal sub-cell packing of the rotational
Fig. 5. AFM images (5 · 5 lm) of two layers (Y-type) of the mixed LB films deposited at 25 mN/m, for (a) McC18(Cd2+):AA(Cd2+) = 1:2 and
(b) McC18(Cd2+):AA(Cd2+):OD = 1:1:1.
420 M. Murata et al. / Chemical Physics Letters 405 (2005) 416–421
hydrocarbon chains of McC18(Cd2+) and AA(Cd2+)
[13]. Taking account of the molecular size (15.8 A long,
7.7 A wide and 3.6 A thick) of the chromophore for
McC18, three basic cross-sectional areas are assumed,namely 22.7 A2 along the short axis, 56.7 A2 along the
long axis and 122 A2 in the plane of the p-electron sys-
tem. The hexagonal packing of the alkyl chain is realized
when the McC18 molecules are embedded among the
AA molecules with the brickstone-like arrangement
(head-to-tail alignment) of the chromophores at the slip
angle a of approximately 30�. As schematically illus-
trated in Fig. 4a, half of AA(Cd2+) molecules is put onthe chromophores of McC18(Cd
2+), tilted to the sub-
strate surface. On the other hand, the GIXD pattern
for the ternary LB film exhibits the coupling of diffrac-
tions at 4.2 and 3.9 A, suggesting the formation of an
orthorhombic molecular packing [13], where OD mole-
cules inclined a little or lay nearly vertical on the chro-
mophores packed with AA(Cd2+) and the alkyl
substituents of McC18(Cd2+), as illustrated in Fig. 4b.
It suggests side-by-side arrangement with a = 65� ratherthan a brickstone-like of the chromophores.
Fig. 5 shows the AFM images of the mesoscopic sur-
face structures (5 · 5 lm) of two layers of (a) the binary
and (b) the ternary LB films. The height information
suggests that the domain has an extremely flat surface
(double layers) and single layered defects were observed
for the Y-type binary mixed LB films. The flat defect
and the projecting domain correspond to the monolayer
and bilayers, providing the hydrophilic and the hydro-phobic surfaces, respectively. From the GIXD measure-
ment, it is considered that the outer layer of the mixed
LB film was peeled off through the up draw transfer pro-
cess, resulting in the inclination of the chromophore of
McC18(Cd2+). In contrast to this, fewer defects were ob-
tained in the ternary LB film, where the hydrophobic
M. Murata et al. / Chemical Physics Letters 405 (2005) 416–421 421
groups of the two layers of the mixed LB film were ex-
posed. The film appears as if many aggregates which
are distributed uniformly are connected to each other.
OD affects the monolayer to be more homogeneous.
By selecting the mixing components suitably added to
McC18, the J- and the H-aggregates formation can becontrolled in organized molecular films. As previously
reported [2], the aggregation structure of the chromo-
phore in the two-dimensional lattice were estimated to
be the side-by-side dimer for the H-aggregate and the
head-to-tail brickwork of nearly ten transition moments
for the J-aggregate by applying the point-dipole [21] and
the extended dipole models [8], respectively.
4. Conclusion
The binary monolayer of McC18(Cd2+):AA(Cd2+) =
1:2 provides the characteristic J-aggregates when spread
on the aqueous subphase containing Cd2+ ions, having
the red-shifted absorption with the shoulder and fluores-
cence spectra. When the ternary monolayer ofMcC18(Cd
2+):AA(Cd2+):OD = 1:1:1 was spread on the
buffer aqueous subphase, both the J- and the H-aggre-
gates having the red-shifted and the blue-shifted bands,
respectively, were formed, irrespective of the surface
pressures. The fluorescence microscopic images of the
J-aggregate in the ternary monolayer are clearly differ-
ent, depending on whether there are Cd2+ ions in the
aqueous subphase or not. The in-plane GIXD gave asingle hexagonal pattern at 4.2 A for the binary LB film,
whereas it changed to the distorted hexagonal with the
lattice spacings of 4.2 and 3.9 A for the ternary LB film.
In the AFM images, relatively large and homogeneous
domains were found in the ternary LB films. The differ-
ent packing of the hydrocarbons of AA(Cd2+) and the
long-chain substituent of McC18(Cd2+) in the two-
dimensional lattice are ascribed to the brickworkarrangement of the chromophores for the J-aggregate
and the side-by-side one for the H-aggregate, taking ac-
count of the extended-dipole and the point-dipole mod-
els, respectively.
Acknowledgments
The authors thank Mr. H. Uchimi for his help in the
observation of the dye mixed monolayers on the water
surface by fluorescence microscopy.
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