control of two-dimensional aggregates of a long-chain merocyanine by change of molecular...

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Control of two-dimensional aggregates of a long-chain merocyanine 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. Ó 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 (McC 18 ) having an unsymmetrical structure of the acceptor and the donor moieties combined with the p-conjugated polymethine were reported previously in the mixed mon- olayers and Langmuir–Blodgett films with Cd 2+ salts of fatty 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 McC 18 –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 McC 18 cadmium salt, Cd 2+ 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 McC 18 cadmium salt with 0009-2614/$ - see front matter Ó 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). www.elsevier.com/locate/cplett Chemical Physics Letters 405 (2005) 416–421

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www.elsevier.com/locate/cplett

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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