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Langmuir-Blodgett films of crown ether containing biphenyl mesogenic groups
"[Tfin Solid PThlT,s',210/211 (1992) 628 630
628
Langmuir-Blodgett films of crown ether containing biphenyl mesogenic groups Xi Zhang, Zhihong Lin, Jiacong Shen and XinYi Tang Department ~[' Chemisto~, Jilin University, Changchun 130023 (People's Relmblic ~/" China)
Abstract We report here a new kind of Langmuir Blodgett (LB) film of crown ether with side chains containing mesogenic side chains of biphenyl. The fluorescence spectra of the LB film indicates that all the molecules exist in the "'excimer" state. X-Ray experiments of the LB film shows that the layered structure is well organized, and each monolayer has the same uniform thickness about 33/k.
I. Introduction The preparation of designed molecular assemblies is attracting considerable attention [1]. LB films, as controlled molecular assemblies, have resulted in increasing interest due to their potential utility in the areas of sensor, information storage, microelectronics, etc. [2]. Following the early works of Cram, Pederson and Lehn, many scientists have used synthetic crown ether to form stable functional LB multilayers [3, 4]. In present paper, we report a new kind of LB film of crown ether which contains biphenyl in the long side chains. The present findings provide a new kind of membrane material which is the outcome of the combination of macrocyclic polyether, liquid crystal and LB films.
studies with a polarizing microscope revealed that the compound is well ordered smectic liquid crystalline (T,, 104 C, Ti 156 C ) . Monolayers of the crown ether were spread from a chloroform solution onto a pure water subphase. The pressure-area isotherm was recorded at a compression rate of about 0.2nm 2mo1-1 min -I on a Langmuir trough. The multilayers of CEBB were obtained by the LB method at room temperature. The surface pressure of the monolayer during the deposition was maintained at 30 mN/m. The dipping speed of the substrate was chosen to be about 0.1 cm/min. Fluorescence spectra of solutions and LB films were measured using an RF-540 fluorescence spectrophotometer at an exciting wavelength of 265 nm. X-Ray diffraction was performed by means of a D ...... - r A rotated target (Cr target) diffractometer.
2. Experimental details 3. Results and discussion The compound bis(p-hexadecyloxy-p'-biphenyl)-dicarboxybenzo-18-crown-6 (CEBB) used in this study was synthesized by our group [5]. Its chemical formula is shown in Fig. 1. The yield of the reactions were quantitative and no impurities were detected by HPLC. 1H N M R (CDCls, TMS, 6): 0.87 (6 protons, CH3), 1.26 (56 protons, CH2), 3.89-4.38 (20 protons, OCH2R-), 6.87 7.85 (22 aromatic protons) ppm. IR v...... (KBr): 1727 (C-O), 1598 and 1515 (benzyl), 1138 and 1275 ( C - O in crown ring). DSC measurement and
o, _
9,
The pressure-area isotherm of the monolayer on a water subphase is shown in Fig. 2. The amphiphiles of the crown ether containing biphenyl mesogens form monolayers in the condensed phase at room temperature under pressure. The collapse pressure is about 42 mN/m. The limiting area is about 0.8 nm2/mol, nearly the same as the calculated result from a CPK model [6]. The fluorescence spectra of CEBB in chloroform show two overlapping peaks at 340 and 393 nm (Fig.
,-:,,--,
Fig. 1. Chemical formula of CEBB.
0040-6090/92/$5.00
~" 1 9 9 2
Elsevier Sequoia. All rights reserved
Xi Zhang et al. / LB.films of crown ether containing biphenyl mesogonic groups
-
629
I 60
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m-
ZO
~'
40
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2
0
. 0.3
l.f
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1.6
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AREA PER MOLECULE
Fig. 2. Surface pressure area isotherm of the CEBB monolayer on water subphase at 20 'C. 3). It was found that the fluorescence intensity at 393 nm become weaker with decreasing CEBB concentration. In general, this indicates that there exist intramolecular excimers. The CEBB gives a good Y-type multilayer with a fluorescence peak at 400 nm (Fig. 4). The spectra are almost identical to that of a dense solution in chloroform, but different from that of a dilute solution (normal monomer fluorescence peak 340 nm). This is the reason that most of the molecules in solid LB films or dense solution exist in the "excimer" state. Furthermore, in solid LB films, the distance between two chains linked to the same crown ring is too long to form an intermolecular excimer, so we deduce that the fluorescence spectra at 400 nm attribute to the intramolecular excimer state. A plot of the fluorescence intensity of LB films as a function of the number of monolayers shows a linear behavior, which indicates the thickness of each layer to be the same in the films. In order to study the microstructure of the LB films, we show the X-ray diffraction pattern of the CEBB LB film with 11 monolayers in Fig. 5. Two Bragg peaks
3OO
1+O0
500
wavelength
/
nm
Fig. 4. Fluorescencespectra of a different number of CEBB LB films (1) 3, (2) 5, (3) 9, (4) 11 layers. were observed in the 20 angle at 1-10 ° when using Cr target ( K s 2.29 A), and the sharpness of these peaks indicates a regular layer structure in the film. According to the Bragg equation, a layer spacing of 66/~ was calculated from the first reflection peak. It corresponds to the d-spacing of the CEBB bilayer, thus the thickness of one layer is about 33/k. When using a Cu target ( K s 1.54 A), two peaks were also observed; one was stronger and sharper, the other much smaller and weaker. It was found that the intensity of the first reflection peak decayed with radiation time, and in about 30 min it decayed to a constant level. At the same time, its corresponding layer spacing also decreased several angstroms, whereas the small peak did not
20'
~o
t~o
5o0
Fig. 3. Fluorescence spectra of CEBB in chloroform solution. (1) 3.5 × 10 9 M; (2) 3.5 × 10-8 M; (3) 1.1 x 10 6 M ; ( 4 ) 3.5 X 10-6 M.
21 o
-
.
Fig. 5. Low angle X-ray diffraction peaks from the CEBB LB multilayer on silicon (11 layers total, Cr target).
630
.¥i Zhang et al. / LB fihns ~[ crown ether conta&m~ biphenyl mesogonic groups
change during radiation. This perhaps means that CEBB multilayers exist in a metastable state. References 1 H. Ringsdorf, B. Schlarb and J. Venzmer, Angew. Chem. Int. Ed. Engl., 27(1988) 113.
2 G. G. Goberts, Adv. Phys., 34 (1985)475. 3 S. Palacin, Thin Solid Films, 178 (1989) 327. 4 M. Sugawara, M. Kataoka, H. Sazawa, S. Nagase, R. Naganawa and Y. Umezawa, 4th h~t. Con[~ LB Films, Tsukuba, Japan, 1989. 5 X. Zhang, Z. Lin, Z. Wang and J. Shen, Chem. J. Chin. Uniz~., 12 (1991) 1262 (in Chinese). 6 J. Cram, Angew. Chem. Int. Ed. Engl., 27(1988) 1009.
628
Langmuir-Blodgett films of crown ether containing biphenyl mesogenic groups Xi Zhang, Zhihong Lin, Jiacong Shen and XinYi Tang Department ~[' Chemisto~, Jilin University, Changchun 130023 (People's Relmblic ~/" China)
Abstract We report here a new kind of Langmuir Blodgett (LB) film of crown ether with side chains containing mesogenic side chains of biphenyl. The fluorescence spectra of the LB film indicates that all the molecules exist in the "'excimer" state. X-Ray experiments of the LB film shows that the layered structure is well organized, and each monolayer has the same uniform thickness about 33/k.
I. Introduction The preparation of designed molecular assemblies is attracting considerable attention [1]. LB films, as controlled molecular assemblies, have resulted in increasing interest due to their potential utility in the areas of sensor, information storage, microelectronics, etc. [2]. Following the early works of Cram, Pederson and Lehn, many scientists have used synthetic crown ether to form stable functional LB multilayers [3, 4]. In present paper, we report a new kind of LB film of crown ether which contains biphenyl in the long side chains. The present findings provide a new kind of membrane material which is the outcome of the combination of macrocyclic polyether, liquid crystal and LB films.
studies with a polarizing microscope revealed that the compound is well ordered smectic liquid crystalline (T,, 104 C, Ti 156 C ) . Monolayers of the crown ether were spread from a chloroform solution onto a pure water subphase. The pressure-area isotherm was recorded at a compression rate of about 0.2nm 2mo1-1 min -I on a Langmuir trough. The multilayers of CEBB were obtained by the LB method at room temperature. The surface pressure of the monolayer during the deposition was maintained at 30 mN/m. The dipping speed of the substrate was chosen to be about 0.1 cm/min. Fluorescence spectra of solutions and LB films were measured using an RF-540 fluorescence spectrophotometer at an exciting wavelength of 265 nm. X-Ray diffraction was performed by means of a D ...... - r A rotated target (Cr target) diffractometer.
2. Experimental details 3. Results and discussion The compound bis(p-hexadecyloxy-p'-biphenyl)-dicarboxybenzo-18-crown-6 (CEBB) used in this study was synthesized by our group [5]. Its chemical formula is shown in Fig. 1. The yield of the reactions were quantitative and no impurities were detected by HPLC. 1H N M R (CDCls, TMS, 6): 0.87 (6 protons, CH3), 1.26 (56 protons, CH2), 3.89-4.38 (20 protons, OCH2R-), 6.87 7.85 (22 aromatic protons) ppm. IR v...... (KBr): 1727 (C-O), 1598 and 1515 (benzyl), 1138 and 1275 ( C - O in crown ring). DSC measurement and
o, _
9,
The pressure-area isotherm of the monolayer on a water subphase is shown in Fig. 2. The amphiphiles of the crown ether containing biphenyl mesogens form monolayers in the condensed phase at room temperature under pressure. The collapse pressure is about 42 mN/m. The limiting area is about 0.8 nm2/mol, nearly the same as the calculated result from a CPK model [6]. The fluorescence spectra of CEBB in chloroform show two overlapping peaks at 340 and 393 nm (Fig.
,-:,,--,
Fig. 1. Chemical formula of CEBB.
0040-6090/92/$5.00
~" 1 9 9 2
Elsevier Sequoia. All rights reserved
Xi Zhang et al. / LB.films of crown ether containing biphenyl mesogonic groups
-
629
I 60
¢0
m-
ZO
~'
40
¢J m,r
2
0
. 0.3
l.f
o.T
b
1.6
n m~
AREA PER MOLECULE
Fig. 2. Surface pressure area isotherm of the CEBB monolayer on water subphase at 20 'C. 3). It was found that the fluorescence intensity at 393 nm become weaker with decreasing CEBB concentration. In general, this indicates that there exist intramolecular excimers. The CEBB gives a good Y-type multilayer with a fluorescence peak at 400 nm (Fig. 4). The spectra are almost identical to that of a dense solution in chloroform, but different from that of a dilute solution (normal monomer fluorescence peak 340 nm). This is the reason that most of the molecules in solid LB films or dense solution exist in the "excimer" state. Furthermore, in solid LB films, the distance between two chains linked to the same crown ring is too long to form an intermolecular excimer, so we deduce that the fluorescence spectra at 400 nm attribute to the intramolecular excimer state. A plot of the fluorescence intensity of LB films as a function of the number of monolayers shows a linear behavior, which indicates the thickness of each layer to be the same in the films. In order to study the microstructure of the LB films, we show the X-ray diffraction pattern of the CEBB LB film with 11 monolayers in Fig. 5. Two Bragg peaks
3OO
1+O0
500
wavelength
/
nm
Fig. 4. Fluorescencespectra of a different number of CEBB LB films (1) 3, (2) 5, (3) 9, (4) 11 layers. were observed in the 20 angle at 1-10 ° when using Cr target ( K s 2.29 A), and the sharpness of these peaks indicates a regular layer structure in the film. According to the Bragg equation, a layer spacing of 66/~ was calculated from the first reflection peak. It corresponds to the d-spacing of the CEBB bilayer, thus the thickness of one layer is about 33/k. When using a Cu target ( K s 1.54 A), two peaks were also observed; one was stronger and sharper, the other much smaller and weaker. It was found that the intensity of the first reflection peak decayed with radiation time, and in about 30 min it decayed to a constant level. At the same time, its corresponding layer spacing also decreased several angstroms, whereas the small peak did not
20'
~o
t~o
5o0
Fig. 3. Fluorescence spectra of CEBB in chloroform solution. (1) 3.5 × 10 9 M; (2) 3.5 × 10-8 M; (3) 1.1 x 10 6 M ; ( 4 ) 3.5 X 10-6 M.
21 o
-
.
Fig. 5. Low angle X-ray diffraction peaks from the CEBB LB multilayer on silicon (11 layers total, Cr target).
630
.¥i Zhang et al. / LB fihns ~[ crown ether conta&m~ biphenyl mesogonic groups
change during radiation. This perhaps means that CEBB multilayers exist in a metastable state. References 1 H. Ringsdorf, B. Schlarb and J. Venzmer, Angew. Chem. Int. Ed. Engl., 27(1988) 113.
2 G. G. Goberts, Adv. Phys., 34 (1985)475. 3 S. Palacin, Thin Solid Films, 178 (1989) 327. 4 M. Sugawara, M. Kataoka, H. Sazawa, S. Nagase, R. Naganawa and Y. Umezawa, 4th h~t. Con[~ LB Films, Tsukuba, Japan, 1989. 5 X. Zhang, Z. Lin, Z. Wang and J. Shen, Chem. J. Chin. Uniz~., 12 (1991) 1262 (in Chinese). 6 J. Cram, Angew. Chem. Int. Ed. Engl., 27(1988) 1009.