Chemical Science

Correlation between Molecular Arrangement andEmission Mechanism of Melem in Langmuir-Blodgett Filmsof Lanthanide(III) Complex with Stearic Acid
optical behavior, that is, a basic structural model of theLB film is important for the description of emission
phenomena. In order to get structural information
about the thin-films, we have carried out X-ray
diffraction measurements on beamline BL02B2 [4].
Because of the low emittance and high brilliance ofSPring-8, we succeeded in reliable and precise datacollection even at lower angle diffraction from LB filmsapprox. 25 nm thick. The XRD data were interpreted
to indicate that the lanthanides (Ln(III)) are arranged
in a plane coordinated to stearic acid (SA,CH3(CH2)16COOH), whereas melem is intercalated
into the film without complexation to Pr(III) (Fig. 1(a)).
In the PrSA film, melem is located between alkyl
chains of SA at intervals of 54.4 Å, which is different
from melem in the SA film without Pr(III). From theBragg peaks the metal-metal interlayer distance wascalculated as 48.6 Å, regardless of the presence ofmelem. However, EXAFS spectroscopy at the Pr(III)L3-edge, carried out at beamline BL01B1 [5], which
provides information about the coordination number,showed that the coordination structure around Pr(III)is changed by the presence of melem in the film
(Fig. 1(b)). In the case of the film with melem, thecoordination structure around Pr(III) is almost thesame as that of the Pr(III) coordinated to SA in thesolid state.
The emission spectra of the films were also studied
in detail. In the PrSA film, two distinct fluorescencebands of melem with different polarity were observed

6(a)(b)|FT| (arb units)(008) Pr(008) Pr(006) Pr

5(003) ML(002) ML

Intensity (arb units)

Control of molecular arrangements, which havebeen extended to 2 and 3 dimensions, is one of thekey approaches to functional progress on moleculardevices at the nanoscale. Research has been
concentrated on metal-organic coordination
compounds because their various coordination modes
lead to multi-functional properties. However, in thecase of the solid state or in solution, it is difficult tocontrol their proper arrangements. For thesemolecular systems, we have proposed an innovative
application of an existing film technology to make amolecular base functional control. A technique using
Langmuir-Blodgett (LB) films [1] that consist of layersof organic molecules is susceptible to controlling theorientation and density of the molecules. The films
are built-up one layer at a time until the desired
number of layers is obtained. For this system, weused two different layers. One layer contains theorganic compound melem, which is capable ofemitting light at specific wavelengths. The other layercontains a metal ion of the lanthanide series, namely
praseodymium (Pr(III)), complexed with long-chain
fatty acids [2]. Since the first application of LB film
techniques in the 1930s, our investigation into metal
complexes and molecular interactions will uncover
new optical properties for the first time. Consequently,
a new possibility has been found by combing anestablished method and coordination chemistry [3].
Knowing the exact position of these molecules isessential to understanding a profound effect on their

4321023452 (degree)

60712345Interatomic Distance (Å)

Fig. 1. (a) Synchrotron X-ray diffraction patterns of the PrSA-melem film (green), the SAmelem film (red), and the PrSA film (blue), together with assignments of the Bragg reflections.
(b) Fourier transforms of EXAFS spectra of Pr L3-edge for the PrSA-melem film (green) and thePrSA film (blue) compared with that for [Pr(SA)3(H2O)] in the solid state (dotted line).

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6

Emission
Excitation
SampleIntensity (arb. units)

h350

h 'Polarizer

400
450
Wavelength (nm)500

Fig. 2. Fluorescence (dotted line) and the polarized fluorescence spectra ofmelem in the PrSA film (λ ex = 280 nm) with the vertical or the horizontal
emission polarizer, which can be resolved into two components (blue and green).

at 375 nm and 405 nm (Fig. 2 and Fig. 3), polarized at
an angle of 0° and 30°, respectively. Interestingly, thedual polarized emissions of melem obtained from thethin film have not been observed in a mixture of thetwo compounds in the solid state nor in solution. Thatis, the arrangement and orientation of the molecules
using the LB film technique has led to novel emission
properties. The present finding may lead to theapplication of the development of duplex fiber-optic
communication systems using this film.

PrH2NhN

NNN

N

H2NN
N

Melem0°N H2

F1= 375 nm

30°
54.4 Å48.6 Å

F2= 405 nm

Fig. 3. Illustration of dual polarized emissions of melem arranged in the PrSA film.

References[1] I. Langmuir: J. Am. Chem. 39 (1917) 1848; K.B.
Blodgett: J. Am. Chem. 57 (1935) 1007.
[2] A. Ishii, K. Habu, S. Kishi, H. Ohtsu, T. Komatsu, K.
Osaka, K. Kato, S. Kimura, M. Takata, M. Hasegawa andY. Shigesato: Photochem. Photobiol. Sci. 6 (2007) 804.
[3] M. Hasegawa et al.: J. Photochem. Photobiol. A 178(2006) 220; M. Hasegawa et al.: Sci. Technol. Adv. Mater.
7 (2006) 72; A. Ishii et al.: Chem. Phys. Chem. 8 (2007)
1345.
[4] H. Ohashi et al.: Appl. Phys. Lett. 84 (2004) 520; M.
Takata et al.: Adv. X-ray Anal. 45 (2002) 377.
[5] T. Uruga et al.: J. Synchrotron Rad. 6 (1999) 143.

Ayumi Ishii a and Miki Hasegawa a,b,*a College of Science and Engineering,

Aoyama-Gakuin University

b Research Center for Advanced Science and Technology,

The University of Tokyo*E-mail: hasemiki@chem.aoyama.ac.jp

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