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Chemical Physics Letters 405 (2005) 32–38

Effects of an amphiphilic perfluoro-carboxylate on the J-aggregatesof a long-chain merocyanine with chlorine substituent in themixed monolayers and their mixed Langmuir–Blodgett films

Michio Murata, Kazuhiko Mori, Akira Sakamoto, Masumi Villeneuve, Hiroo Nakahara *

Department of Chemistry, Faculty of Science, Saitama University, Shimo-okubo 255, Sakura-ku, Saitama 338-8570 Japan

Received 30 November 2004; in final form 25 January 2005

Abstract

A long-chain merocyanine derivative with the chlorine substituent at the 5-position of the benzothiazol moiety has been studied

for formation of the stable J-aggregates in the mixed monolayers and their LB films with Cd perfluorooctadecanoate (PFC18) and

octadecane (OD) in comparison with the one without the substituent, which forms the H-aggregates in the mixed films with Cd

arachidate and OD. In addition to the monolayer p–A isotherms, visible absorption spectra, the grazing angle in-plane X-ray dif-

fraction and the AFM observation indicate that PFC18 promotes J-aggregation of the dye due to the phase separation in the mixed

films.

� 2005 Elsevier B.V. All rights reserved.

1. Introduction

J-aggregates of cyanine dyes are interesting for the

particular photophysical behavior [1–6], i.e., the intensenarrow absorption band shifted to longer wavelength

relative to the monomer band and an almost Stokes-

shift free fluorescence band [5]. An unsymmetrical mer-

ocyanine with the long alkyl chain (McC18) also forms

the J-aggregates in the mixed monolayers and the Lang-

muir–Blodgett (LB) films with Cd2+ salts of fatty acids

[7–10], and the aggregation number and the two-dimen-

sional structure were estimated by using the extended di-pole model for the spectral shifts [7,11]. Recently, the H-

aggregates of McC18 dye were reported in the ternary

mixed LB films with cadmium stearate (or arachidate)

and octadecane (OD) [9–11], which has been found as

the transient state in the compression process in the ter-

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2005.01.123

* Corresponding author. Fax: +81 48 858 3700.

E-mail address: nakahara@chem.saitama-u.ac.jp (H. Nakahara).

nary mixed monolayer of McC18 with methyl arachi-

date and hexadecane [12].

In this Letter, using another long-chain merocyanine

derivative substituted with chlorine at the 5-position ofbenzothiazol ring (5-ClMcC18), the mixed monolayers

and LB films of 5-ClMcC18 with perfluorooctadecano-

ate (PFC18) instead of cadmium arachidate and octade-

cane (OD) have been investigated on the formation of

the J-aggregates. Perfluorinated amphiphiles are known

for the strong surface activity at the air/water interface

which can be attained in the lower concentration

[13,14], since the van der Waals attraction of fluorocar-bons is very weak [15]. And further, phase separation of-

ten occurs in the mixed films with amphiphilic

hydrocarbons because of the large coherent force among

fluorocarbon chains [16–18]. The effects of these fluoro-

carbon behaviors on the J-aggregate of the chlorine

substituted merocyanine (5-ClMcC18) in the organized

films were studied by the surface pressure–area iso-

therms, visible absorption spectra, the in-plane X-raydiffraction and the atomic force microscopy (AFM).

M. Murata et al. / Chemical Physics Letters 405 (2005) 32–38 33

2. Experimental

The structural formula of the 5-ClMcC18 pur-

chased from Japanese Research Institute for Photo-

sensitizing Dyes, Co. (Okayama, Japan) is given in the

inset of Fig. 2. Perfluorooctadecanoic acid (PFC18),C18F37COOH, octadecane (OD), and C18H38, were pur-

chased from Fluorochem Ltd. and Tokyo Chemical

Industry Co. Ltd. and purified by recrystallization from

the chloroform and hexane solutions, respectively. The

mixed monolayers were spread from the chloroform

solution onto the aqueous subphase containing buffer

[3 · 10�4 M CdCl2 and 3 · 10�5 M KHCO3 (pH 6.8)],

or the distilled water (pH 5.8). Their surface pressureversus molecular area (p–A) isotherms were measured

using the Langmuir-type film balance (Lauda) and in

situ visible absorption spectra of the monolayers were

obtained by a multichannel photodetected spectroscopy

(Otsuka electro.) at 12.5 �C. The mixed monolayers at

the air/water interface were transferred by the LB tech-

nique (up-trip) at 25 mN/m and 12.5 �C onto solid sub-

strates which were preliminarily coated with cadmiumarachidate (AA, 1–4 layers) to obtain the hydrophilic

surface. The absorption and fluorescence spectra of the

mixed LB films were recorded on the absorption

(Hitachi U-3210) and fluorescence (MPF-3) spectropho-

tometers, respectively. The in-plane spacing of the two-

dimensional lattice of the single layer LB film deposited

on glass was determined by the grazing angle incident

Fig. 1. Structures optimized for the merocyanine skeletons substituted with (

The values on atoms in molecules indicate Mulliken charges calculated by p

ly = 1.18 D and lz = �1.25 D) for (a) and 8.7 D (lx = 8.7 D, ly = �0.21 D an

show x,y-axis, and z-axis is along the direction from to the behind of the pa

X-ray diffractometer (GIDX, at 0.2� and a scanning rate

at 0.05�/50 s, Bruker, MxP-BX, 40 kV, 40 mA) [19]. The

atomic force microscopy (AFM, Seiko Instruments

SPA300, Si3N4 cantilevers: the spring constant

k = 0.09 N/m) was used for observation of the surface

morphology of the mixed LB films.The preliminary information on the electronic struc-

tures of the merocyanine skeleton of dyes substituted

at the 5-position of the benzothiazol ring was obtained

for the optimized molecular geometries by calculation

using the density functional theory at the B3LYP

(Becke�s three-parameter hybrid method [20] with the

Lee–Yang–Parr correlation function [21]) level in com-

bination with the 6-31G* basis set performed by theGAUSSIAN 98 program [22].

3. Results and discussion

With respect to the effect of the substituent (X) at the

5-position of the benzothiazol of the unsymmetrical

merocyanines on the J-aggregation, dipole momentsand Mulliken charges based on electronic structures of

those chromophores were calculated by the density func-

tional theory at the level of B3LYP/6-31G*, though the

geometrical optimization of McC18 for the analysis of

the vibration spectra was performed by GAUSSIAN

[23,24]. In this typical case of X = �Cl, the results of

the calculation are shown in Fig. 1a, in comparison with

a) X = �Cl and (b) X = �H at the 5-position of the benzothiazol ring.

opulation analysis. Estimated dipole moments were 6.5 D (lx = 6.3 D,

d lz = �0.89 D) for (b). Molecular axes fixed on the plane of the paper

per.

34 M. Murata et al. / Chemical Physics Letters 405 (2005) 32–38

that of X = �H in b. The dipole moments in the p-elec-tron system from the rhodanine to the benzothiazol are

8.7 D for X = �H and 6.5 D for X = �Cl, respectively,

whereas the dipole moments was enlarged by

X = �CH3 to be 9.4 D. In the case of large polarization

of the chromophore plane, the molecular interactionsseem to be limited in the short range to form fairly small

clusters such as dimmers bound strongly. The relatively

small dipole moments seem to be favorable for forma-

tion of the J-aggregate having a large domain with the

long coherent exciton [8].

3.1. Characteristics of the mixed monolayers of

5-ClMcC18 with fluorinated long-chain carboxylate

and octadecane

Fig. 2 shows the p–A isotherms for the binary and

the ternary mixed monolayers of: (a) 5-

ClMcC18:PFC18 = 1:2 (5-ClMcC18-PFC18) and (b)

5-ClMcC18:PFC18:OD = 1:1:1 (5-ClMcC18 - PFC18-

OD) together with (c) PFC18 and (d) 5-ClMcC18 on

the buffer solution containing Cd2+ ion. Furthermore,the PFC18 monolayer on the distilled water surface is

indicated in the inset of Fig. 2 for comparison with that

on the buffer solution. For the PFC18 monolayers on

the buffer solution and the distilled water surface, the

molecular areas were 33 and 36 A2/molecule, respec-

tively, of which the latter is near to the value previously

reported [15], though the collapse pressures were almost

unchanged. For the 5-ClMcC18 - PFC18 monolayer, theobserved isotherm is remarkably expanded, as com-

pared with the calculated curve (dotted line, a 0) assum-

ing the ideal mixing of Cd salts of 5-ClMcC18 and

Fig. 2. Surface pressure–area isotherms for the monolayers of: (a) 5-

ClMcC18:PFC18 = 1:2 and (b) 5-ClMcC18:PFC18:OD = 1:1:1

together with (c) PFC18 and (d) 5-ClMcC18 on the buffer solution

containing Cd2+ ion (the chemical structure of 5-ClMcC18 is shown in

the inset). The calculated curves for the ideal mixing of (a 0) 5-

ClMcC18:PFC18 = 1:2 and (b 0) 5-ClMcC18:PFC18:OD = 1:1:1 are

indicated by dotted lines. The PFC18 monolayer on the distilled water

surface is shown in the inset.

PFC18. From the fact that the individual collapse pres-

sures are not observed, it is considered that 5-ClMcC18

and PFC18 appear to be miscible in the monolayer with-

out any phase separation. The net molecular interaction

is expected to be repulsive due to the difference in the

cohesive force and the rigidity of the fluorocarbon andthe hydrocarbon chains, leading to different aggregated

structures from the one in the mixed 5-ClMcC18 mono-

layer with Cd arachidate (AA) [25]. For the 5-

ClMcC18–PFC18–OD monolayer, the observed p–Aisotherm is remarkably condensed with the smaller

molecular area, as compared with the calculated curve

(dotted line, b 0) for the ideal mixing, assuming that the

molecular occupied area of OD were comparable toAA. The larger collapse pressure appears to be an evi-

dence for the well organized films containing 5-

ClMcC18, PFC18 and nonpolar OD. The deviation of

the isotherm from the calculated curve suggests a possi-

ble molecular arrangement similar to the one in

McC18:AA:n-hexadecane = 1:1:1 monolayer [12].

In addition, the in situ absorption spectra of the mon-

olayers on the aqueous buffer solution were examined tostudy the two-dimensional aggregation of the dye mole-

cules. Fig. 3 shows the surface pressure dependence of

the absorption spectra of: (a) the 5-ClMcC18–PFC18

and (b) the 5-ClMcC18–PFC18–OD monolayers from

5 to 40 mN/m and at 12.5 �C, in comparison to the spec-

tra of (a) the pure monolayer on the buffer solution at

25 mN/m and (b) the chloroform solution of 5-

ClMcC18. In both the binary and ternary systems, an in-tense narrow absorption band shifted to the longer

wavelength at around 610 nm relative to the monomer

band at 520 nm of the solution is characteristic of the

J-aggregate [12]. From the J-band with the narrower

half band width (FWHM) in the mixed monolayers than

the pure 5-ClMcC18 monolayer, it is found that the Cd

salt of PFC18 promotes the formation of the more

homogeneous J-aggregate with large domains. This sug-gests that the close packing of the chromophores is as-

sisted by the molecular interaction of the

fluorocarbons itself and that of the hydrocarbons itself.

In the surface pressure dependence of the absorption

spectra for the 5-ClMcC18–PFC18 monolayer, the

broad shoulder around 576 nm shifts to the shorter

wavelength at 565 nm with the increase of the surface

pressure. The absorbance of the J-band is enhanced withthe increase of the surface pressure until 25 mN/m, then

it decreases over 25 mN/m. From these results, it is con-

sidered that a part of PFC18 contributes to the forma-

tion of the J-aggregate by mixing with 5-ClMcC18,

and the rest segregates the 5-ClMcC18 J-aggregate by

the self-aggregation of fluorocarbons in the monolayers,

resulting in the shoulder band around 576 nm. On the

other hand, for the 5-ClMcC18–PFC18–OD monolayer,the narrow intensified band of the J-aggregate is ob-

served at 610 nm without the shoulder irrespective of

0.06

0.05

0.04

0.03

0.02

0.01

0.00750700650600550500450

absorption fluorescence

750700650600550500450

absorption fluorescence

0.10

0.08

0.06

0.04

0.02

0.002220181614

Source spectrum

B1

0.05

0.04

0.03

0.02

0.01

0.002220181614

Source spectrum

A1

0.20

0.15

0.10

0.05

0.00750700650600550500450

5 mN/m 10 25 30 40 5-ClMcC18

(solution)

0.06

0.05

0.04

0.03

0.02

0.01

0.00750700650600550500450

5 mN/m 25 30 35 40 5-ClMcC18

(monolayer)

Fluorescence Intensity(a. u.)

Fluorescence Intensity(a. u.)

Abs

orba

nce

Abs

orba

nce

Wavenumber (cm-1 × 103)Wavenumber (cm-1 × 103)

Wavelength (nm) Wavelength (nm)

Wavelength (nm) Wavelength (nm)

Abs

orba

nce

Abs

orba

nce

(a) (b)

(c) (d)

(e) (f)

Abs

orba

nce

Abs

orba

nce

Fig. 3. The surface pressure dependence of in situ absorption spectra for the mixed mono-layers of (a) 5-ClMcC18:PFC18 = 1:2 as compared with

the pure 5-ClMcC18 monolayer (25 mN/m, dotted line) and (b) 5-ClMcC18:PFC18:OD = 1:1:1 on the aqueous buffer solution, together with the

chloroform solution spectrum of 5-ClMcC18 (dotted line). The corresponding absorption spectra (solid lines) of (c) the binary and (d) the ternary

mixed LB films (transferred at 25 mN/m and 12.5 �C) together with the fluorescence spectra (dashed lines). The results of deconvolution absorption

spectra for the binary and the ternary mixed LB films are indicated in (e) and (f), respectively.

M. Murata et al. / Chemical Physics Letters 405 (2005) 32–38 35

the surface pressure. The OD molecules are considered

to help the packing of the alkyl chains and adjusting

the space of 5-ClMcC18. Additionally, in this case any

blue shifted band due to the H-aggregate formed in

another ternary (McC18:AA:OD = 1:1:1) monolayer[9–11] was not observed. From this fact, it is considered

that the PFC18 as a matrix component is suitable for the

J-aggregate formation.

3.2. Spectroscopic studies of the mixed LB films

As for the mixed monolayers of 5-ClMcC18 with

PFC18 and OD only a single layer was transferred onto

hydrophilic solid substrate by the LBmethod at 25 mN/m

and 12.5 �C. Fig. 3c and d shows the absorption

spectra (solid lines) for the 5-ClMcC18–PFC18 and

5-ClMcC18–PFC18–OD LB films, respectively. The J-

aggregate at 609 nm is observed with a broad shoulderat the shorter wavelength in the binary LB film and

the FWHM of the J-band is wider, as compared with

the spectrum of the mixed monolayer on the buffer solu-

tion. These facts suggest the change of the molecular

arrangements through the transfer process. A plausible

explanation may be the following: Cd salt of PFC18

aggregates by itself, and then segregates the J-aggregates

and converts them into smaller ones in the LB films. In

36 M. Murata et al. / Chemical Physics Letters 405 (2005) 32–38

the absorption spectrum of the ternary LB film, the nar-

rower J-band at 606 nm is observed with a little broad-

ened base, as compared with that of the ternary

monolayer. As shown in the dotted lines of Fig. 3c

and d, the fluorescent bands of the binary and the ter-

nary LB films excited by 540 nm, are observed at 620and 610 nm, respectively. Furthermore, in order to com-

pare main aggregate species in Fig. 3c and d, the unsym-

meterical absorption spectra were deconvoluted with the

Lorentzian functions [10,26], while the absorption spec-

trum of the H-aggregate for a squarylium dye was

deconvoluted by Gaussian function [27]. The results of

the deconvolution are shown in Fig. 3e and f for the

5-ClMcC18–PFC18 and the 5-ClMcC18–PFC18–ODLB films, respectively. The main bands are determined

to be A1 at 609 nm for the former and B1 at 606 nm

for the latter. The fact that the content of B1 is larger

and the FWHM is narrower than those of A1, indicates

that the size of J-aggregate is larger and the size distribu-

tion is more homogeneous in the latter system than the

former.

3.3. Structural studies of the mixed LB films

Fig. 4 shows the lattice spacing of the two-dimen-

sional mixed LB films determined by the GIXD for (a)

5-ClMcC18–PFC18 and (b) 5-ClMcC18–PFC18–OD

LB films, in comparison with (c) pure 5-ClMcC18 and

(d) PFC18 LB films transferred onto hydrophilic glass

plates, preliminarily coated with (e) the Cd stearate filmtransferred at the down-trip, at 25 mN/m and 12.5 �C.

Fig. 4. In-plane X-ray diffraction for the mixed LB films of (a) 5-

ClMcC18:PFC18 = 1:2 and (b) 5-ClMcC18:PFC18:OD = 1:1:1 in

comparison with (c) pure 5-ClMcC18, (d) PFC18 and (e) Cd stearate

LB films, together with the corresponding molecular packing estimated

for (A) PFC18 and (B) Cd stearate LB films in the right side.

The Cd salt of PFC18 and Cd stearate films take the

in-plane structures of the isotropic hexagonal sub-cell

packing with the spacings of 4.9 and 4.1 A, respectively

[19]. Assuming the two-dimensional close hexagonal

packing of fluorocarbons or hydrocarbons in the LB

films, as schematically illustrated in the right hand sideof Fig. 4, the occupied molecular areas are estimated

to be about 33.6 and 20.5 A2/molecule for (A) PFC18

and (B) Cd stearate, respectively. These values corre-

spond well to the results of the p–A isotherms. In the

GIXD of the 5-ClMcC18–PFC18 LB film, both the dif-

fraction of the close hexagonal fluorocarbons (4.9 A)

and hydrocarbons (4.1 A) were observed simultaneously

in addition to other small diffractions such as 4.4 A,though no clear diffraction was observed for (c) the pure

5-ClMcC18 LB film. These facts suggest that the homo-

geneous J-aggregate formation of 5-ClMcC18 with

dense chromophore packing is enhanced by the sur-

rounding domain of PFC18 with the large coherent

force among fluorocarbons. On the other hand, in the

case of 5-ClMcC18–PFC18–OD LB films, the main dif-

fractions at 4.0 and 4.2 A are ascribed to the distortedhexagonal packing of hydrocarbons. Several weak

diffraction peaks are also observed at 3.9, 3.6 and

4.5 A. It is considered that the diffractions of the

5-ClMcC18–PFC18–OD LB film indicate two main in-

plane structures of distorted hexagonal forms of hydro-

carbons with the lattice spacings of 4.2 and 3.9 A

together with 4.0 and 3.6 A. Thus, the J-aggregates of

5-ClMcC18 are formed in the mixed LB films with thebrickstone-like arrangement of the chromophores.

To observe the domain structures in the relatively

mesoscopic scale of the J-aggregates, the AFM on

the mixed LB films was performed. Fig. 5 shows the

AFM images of the mesoscopic surface structures

(4 lm2) for the single layers of: (a) 5-ClMcC18–

PFC18 and (b) 5-ClMcC18–PFC18–OD LB films on

the mica substrate which was preliminarily hydrophil-ized with four monomolecular layers of Cd stearate.

In the 5-ClMcC18–PFC18 LB film, various sizes of

the small domains distributed inhomogeneously were

observed. The AFM images of the 5-ClMcC18–

PFC18 LB film indicate the phase separation structures

consisting of domains of the 5-ClMcC18 aggregates

surrounded by the PFC18 molecules [17]. In the 5-

ClMcC18–PFC18–OD LB film, the higher �flat hill�surface extends remarkably in comparison with the 5-

ClMcC18–PFC18 LB film. It is considered that the 5-

ClMcC18 aggregates are separated from the PFC18

region and assembled through the transfer process of

the mixed monolayer onto the substrate from the water

surface, and formed the J-aggregate along the transfer

direction, as indicated by the arrow in Fig. 5b [28,29].

The more detailed discussion on the phase separationusing AFM requires further experiments about various

mixture ratios.

Fig. 5. AFM images (4 lm2) for the single layers of the mixed LB films (a) 5-ClMcC18:PFC18 = 1:2 and (b) 5-ClMcC18:PFC18:OD = 1:1:1. The

transfer direction is indicated by the arrow in (b).

M. Murata et al. / Chemical Physics Letters 405 (2005) 32–38 37

4. Conclusion

The fluorinated long-chain carboxylate instead of the

hydrogenated one was used to control the J-aggregates

structure in the mixed monolayers and LB films of the

5-ClMcC18. From the in situ absorption spectra, the

J-band was observed with the shoulder band in the 5-

ClMcC18:PFC18 = 1:2 but the clear J-band only in the5-ClMcC18:PFC18:OD = 1:1:1 monolayers, indicating

the different arrangements of the chromophores in the

mixed monolayers. The J-aggregate of 5-ClMcC18 was

affected by the transfer process of the mixed monolayers

to segregate the J-aggregates of 5-ClMcC18 forming

various smaller ones in the mixed LB films. These facts

have been supported by the analyses of the homogeneity

from the deconvolution of the absorption spectra. Fromthe GIXD of the binary and the ternary LB films con-

taining the J-aggregate of 5-ClMcC18, the two-dimen-

sional lattice structures have the hexagonal and

somewhat distorted hexagonal forms, respectively. The

phase separation structures in the LB films are suggested

by the AFM images.

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