Characterization of newly synthesized BODTA in Langmuir and Langmuir–Blodgett monolayers before and after UV irradiation

Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 419–427

Characterization of newly synthesized BODTA in Langmuir and Langmuir–Blodgett monolayers before and after UV irradiation Jin-Ho Park, Zhongzhe Yuan, Suck-Hyun Lee, Jae-Ho Kim∗ Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea Available online 25 November 2004

Abstract Polydiacetylenes (PDAs) are ordered and uniquely conjugated polymers synthesized from diacetylene monomers by UV-initiated photoreaction or thermally. The amphiphilic PDAs with controlled spacing and orientation often accomplished by Langmuir–Blodgett (LB) technique provide possible applications in the area of the photolithography, nonlinear optics and chemical sensors. In this study, we synthesized an amphiphilic of diacetylene monomer, 2,5-bis-octadeca-5,7-diynyloxy-terephthalic acid (BODTA), containing a diacetylene moiety in each of two symmetric side chains. Stable Langmuir monolayers of BODTA were constructed, and successfully transferred on the solid substrates as the monomeric form to investigate the changes of the molecular interaction, packing and orientation induced by UV-induced polymerization. In situ spectroscopic and scanning probe microscopic analyses indicate that at lower surface pressure, BODTA spontaneously forms micro-scale domains, and merges together as surface pressure increases. The molecular orientation and intermolecular interaction of BODTA were significantly altered by UV-induced polymerization. We found that two different polymerization mechanisms may be involved as the intermolecular distance and relative orientation to adjacent molecules changes by controlling the degree of compression of the monolayer at the interface. © 2004 Elsevier B.V. All rights reserved. Keywords: Diacetylene; Polydiacetylene; Langmuir monolayer; LB monolayer; AFM

1. Introduction Since the photochemical polymerization of crystalline derivatives of diacetylene (DA) was invented by Wegner in 1969 [1], polydiacetylenes (PDAs) have been studied for wide range of the applications in photolithography [2,3], nonlinear optical [4,5], organic solar cell [6], biomimetic membranes [7–11] and various sensor [12–16]. PDAs are ordered and uniquely conjugated polymers with repetitive alternating eneyne units from DA monomers by 1,4-addition photoreaction of UV (254 nm) or high-energy irradiation like ␥-rays without a need of radical initiators [4]. Polymerization behavior and properties of the PDA polymer were investigated in terms of the distance between the centers of the neighboring butadiyne moieties and the inclination angle between the butadiyne axis and the stack ∗

Corresponding author. Tel.: +82 31 219 2517; fax: +82 31 219 2516. E-mail address: [email protected] (J.-H. Kim).

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direction. Patel and Millier showed that the change in the effective conjugation length (lec ) of PDAs induces the color changes of PDAs [17]. The PDAs with 30 and more lec show the blue color with λmax in the region of 600–700 nm, while those with lec between 10 and 20 showed the red color with λmax in 500–600 nm region. PDAs with six and less lec appeared the λmax in the region of 400–500 nm. Lieser et al. studied the effect of spacer and tail lengths of alkyl groups connected with butadiyne moieties on polymer properties [18,19]. They showed that PDAs with 8 and more repeated units of alkyl chains form more stable and ordered monolayers on air–water interface and the chain length of spacer influenced the optical properties of PDAs. Tachibana et al. investigated the relationship between the chain length of the spacer and the absorbance of PDAs [20]. A PDA with two repeating units of methylene group shows the yellow phase, while with four units forms the red phase polymer. Furthermore, a polymer with six and more repeating units shows the bluish-red phase. Evans and co-workers fabricated

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the PDA monolayers by self-assembled method using the disulfide derivatives with two DA moieties [21,22]. They investigated the effect of UV exposure time and the odd/even nature of the methylene spacer. It was found that the odd/even character of the spacer influences the degree of strain on the polymer backbone. They showed that the blue-phase polymer was formed from the monolayers with a long, odd-spaced conjugated spacer. As the length of the spacer and charge of terminal groups changes, the properties of polymer and polymerization behavior were also changed due to the difference in the packing density and distance [23–25]. As the consequence of sunlight absorption by the organic dye contained DA, the electric charge is generated from light-induced electron transfer and movement of the charge carriers [26–28]. The chromatic phase of PDAs was also changes by thermal energy [28], mechanical stress [12] and fabrication conditions like pH [16] and salts [29,30] of the subphase in air–water interface. Therefore, control of the molecular packing and distance is essential to fabrication of PDAs while maintaining designed chemical and physical properties for specific applications. One of the most intensely used techniques for this purpose is Langmuir–Blodgett (LB) monolayer technique [3–6,8–20,23–30]. In this study, we synthesized an amphiphilic derivative of DA monomer, 2,5-bis-octadeca-5,7-diynyloxy-terephthalic acid (BODTA) which contains the phthalic acid as amphiphilic terminal group with two DA moieties in each symmetric side chain. To fabricate organized molecular structures while controlling the molecular interaction, packing and orientation of molecules, Langmuir and LB monolayers of BODTA were constructed on various subphases and at different surface pressure. Langmuir and LB monolayers of BODTA were characterized by surface pressure–area (π–A) and surface potential–area (V–A) isotherms, in situ visible reflection absorption spectroscopy, Brewster angle microscopy (BAM), atomic force microscopy (AFM) and polarized FT-IR spectroscopy.

2. Experimental 2.1. Materials All reagents were used without further purification. 1Dodecyne, 5-hexyne-1-ol, p-toluenesulfonyl chloride, hydroxylamine hydrochloride, copper (I) chloride and 2,5diethyldihydroxytherephthlate (DEDHT) were purchased from the Aldrich Chemical Co. 10-Undecyn-1-ol was purchased from the Fluka Chemical Co. 2.2. Synthesis The DA derivative, BODTA, was synthesized as the scheme depicted in Fig. 1. The detailed synthetic procedures were described as below.

2.2.1. Iodination of a terminal alkyne n-Butyllithium was added to a solution of the terminal alkyne (3.0 × 10−2 mol) in 250 ml hexane cooled in an ice bath for 20 min. To the prepared solution, 1.5 equiv. of solid iodine was added and the mixture is stirred for 4 h. The solution was washed with a saturated aqueous sodium thiosulfate solution to remove residual iodine and the organic phase was collected and dried over sodium sulfate. The product (a) was used in the coupling reaction without further purification. Yield: 93.7%. 2.2.2. The Cadiot-Chodkiewicz coupling of the iodinated alkyne and 10-undecyn-1-ol Seventy-five milliliters of methanol was added to a mixture of 10-undecyn-1-ol (3.0 × 10−2 mol) and 30 ml 10% KOH until the solution becomes homogeneous solution. Hydroxylamine hydrochloride (0.1 equiv.) was added followed by addition of solution of copper (I) chloride (0.25 equiv.) in 10 ml ethylamine (70%). After cooling to −20 ◦ C, added a solution of iodinated alkyne in 20 ml THF. After allowing 4 h reaction, the product was purified by silica gel chromatography. The product was purified by column chromatography so as to obtain (b) as slightly yellow liquid. Yield: 55%. 1 H NMR (200 MHz, CDCl3 ): δ (ppm) 0.84 (CH3 , t, 3H), 1.22–1.63 (CH2 chain, m, 20H), 2.16–2.29 (CH2 , m, 4H), 2.38 (OH, s, 1H), 3.60 (CH2 O, t, 1H). 13 C NMR (200 MHz, CDCl3 ): δ (ppm) 13.99, 1.88, 19.06, 28.25, 28.76, 29.00, 29.21, 29.38, 31.58, 31.79, 61.99, 65.61, 76.78, 77.59. 2.2.3. Tosylation of the diacetylenic alcohol The above product was dissolved in 30 ml chloroform and cooled in an ice bath. 1.76 ml pyridine was added followed by addition of p-toluenesulfonyl chloride (2.2 × 10−2 mol) in a small portion. The solution was stirred for 8 h. The product (c) was collected by recrystallization from hexane and purified by silica gel chromatography. Yield: 53.4%. 1 H NMR (200 MHz, CDCl3 ): δ (ppm) 0.86 (CH3 , t, 3H), 1.24–1.70 (CH2 chain, m, 20H), 2.17–2.36 (CH2 , m, 4H), 2.43 (Ar CH3 , s, 3H), 4.02 (CH2 O, t, 2H), 7.31–7.78 (Ar H, AA BB , 4H). 13 C NMR (200 MHz, CDCl3 ): δ (ppm) 14.01, 18.41, 19.07, 21.52, 22.58, 24.09, 27.71, 28.20, 28.74, 28.99, 29.20, 29.37, 29.46, 31.79, 64.98, 66.01, 69.73, 75.90, 7.88, 127.74, 129.78, 132.95, 144.68. 2.2.4. Preparation of diacetylene(4,10)diester Tosylated diacetylene dissolved in 55 ml DMF, then 0.8 g K2 CO3 and 0.65 g DEDHT were added, and stirred for 28 h at 50 ◦ C. The product was purified by silica gel chromatography. Product (d) was purified by column chromatography using gel as column material and hexane/ethyl acetate (20:1) as the eluent. Yield: 83%. m.p.: 50–51 ◦ C. 1 H NMR (200 MHz, CDCl3 ): δ (ppm) 0.91 (CH3 , t, 3H), 1.22–2.45 (CH2 chain, m, 48H), 1.3 (CH3 of COOCH2 CH3 , 6H), 4.05 (CH2 O, t, 4H), 4.41 (COOCH2 , q, 4H) 7.37 (Ar H, s, 2H).

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Fig. 1. Schematics of the synthesis of BODTA monomer.

NMR (200 MHz, CDCl3 ): δ (ppm) 14.52, 14.71, 19.28, 19.57, 23.07, 25.09, 28.71, 29.21, 29.47, 29.68, 29.86, 29.94, 32.26, 61.65, 65.45, 66.14, 69.26, 76.92, 77.99, 116.67, 124.79, 151.70, 168.64. IR (KBr, cm−1 ): 2957, 2918, 2847, 1697, 1427, 1213, 1057, 785, 725. EI: calculated mass for C48 H70 O6 : 743.1, measured: 743.2. 13 C

2.2.5. BODTA from diacetylene(4,10)diester 1.6 × 10−3 mol KOH was added to a solution of diesters (0.24 g) dissolved in 30 ml EtOH, the mixture was stirred for 24 h at 50 ◦ C. The mixture was added 35% HCl until pH to 2–3. Final pure product (e) was obtained by recrystallization and filtration from a cosolvent (MeOH/EA/MC) and dried under reduced pressure. Yield: 84%. m.p.: 138–139 ◦ C. 1 H NMR (200 MHz, CDCl ): δ (ppm) 0.88 (CH , t, 3H), 3 3 1.26–2.43 (CH2 chain, m, 48H), 4.33 (CH2 O, t, 4H), 7.88 (Ar H, s, 2H), 10.96 (COOH, s, 2H). 13 C NMR (200 MHz, CDCl3 ): δ (ppm) 14.12, 18.85, 19.19, 22.67, 24.51, 27.93, 28.27, 28.86, 29.08, 29.29, 29.47, 29.57, 31.89, 64.89, 66.53, 70.59, 75.53, 78.39, 117.51, 122.74, 151.63, 163.84. IR (KBr, cm−1 ): 3261, 2957, 2918, 2849, 1697, 1427, 1213, 1057, 785, 725. EI: calculated mass for C44 H62 O6 : 686.67, measured: 687.4.

2.3. Preparation of Langmuir monolayers and LB monolayers of BODTA The measurements of the π–A and V–A isotherms and the deposition of LB monolayers of BODTA were used a computer-controlled KSV 5000 Langmuir trough (KSV Instruments Ltd., Finland) with a symmetric compression. A chloroform solution of BODTA was prepared with a concentration of 2.0 × 10−4 M. Langmuir monolayer was prepared by dropwise spreading the solution by micro-syringe, allowed to evaporate the solvent for 15 min at 15 ◦ C on MilliQ water or aqueous subphase containing one of the following salts of CaCl2 , CdCl2 and CuCl2 with the concentration of 5.0 × 10−4 M. The monolayer was compressed to the target pressures of 5, 20, 30 and 40 mN/m and transferred by vertical pull-up methods on various substrates depending on the analytical tools. 2.3.1. Surface potential measurement The V–A isotherm of BODTA monolayer at the air–water interface were measured by a standalone surface potential measuring system (KSV1000SPA, KSV Instruments Ltd., Finland). The vibrating plate with a frequency

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of about 120 Hz was placed ca. 2 mm above the water surface and the reference electrode made from platinum foil was submerged in the subphase. 2.3.2. Brewster angle microscopy To investigate the morphology of the BODTA monolayer at the air–water interface, Brewster angle microscopic measurements were carried out with a MiniBAM (Nanofilm Technologie GmbH, Germany) with lateral resolution of approximately 2 ␮m. 2.3.3. UV–vis spectroscopy UV–vis measurements were carried out on JASCO V-570 spectrophotometer (JASCO, Japan) in the range 250–800 nm with a resolution 0.5 nm. All spectra were recorded after baseline correction. 2.3.4. In situ visible reflectance absorption spectroscopy A multi-channel photodiode array detector (MCPD 2000, Otsuka electronics Co., Japan) was used for the in situ reflectance measurement equipped by Y-type optical fiber connected to I2 lamp as light source. 2.3.5. Polarized Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra were measured by Magma-IRTM 550 spectrometer (GRASEBY SPACE, Nicolet Magna-IRTM 500 spectrometer, Madison, USA) with liquid nitrogen-cooled mercury cadmium tellurium (MCT-A) detector. Rotational infrared polarizer was placed just before the sample stage. The sample was set on the angle-controlled rotational stage. 2.3.6. Atomic force microscopy (AFM) All AFM images were acquired with LS (Park Scientific Instruments, CA, USA) using contact mode with V-shaped Si3 N4 micro-levers and 5 ␮m scanner at scan rate of 1 Hz. 2.4. Photo-polymerization of BODTA monolayers Polymerization of BODTA monolayer was performed with 255 ±5 nm UV irradiation from a low-intensity lamp (model VL-4, Vilber Lourmat, Inc.) with average power of 4 W at distance of 1 cm above the sample surface.

3. Results and discussion Prior to investigate the polymerized BODTA monolayers, the behavior of BODTA molecules and the optimal condition for preparation of BODTA monolayers at air–water interface were investigated. Fig. 2 presents the π–A isotherms of BODTA on various subphases. The subphase of either pure water or containing the metal ion of 5.0 × 10−4 M was maintained at 15 ◦ C to improve the stability of the monolayer. On pure water subphase, the

Fig. 2. π–A isotherms of BODTA on various subphases. The concentration of the salts was 5.0 × 10−4 M.

BODTA monolayer was unstable and collapsed lower surface pressure of ca. 10 mN/m as shown in Fig. 2(d). The estimated area/molecules of BODTA on pure water was ca. ˚ 2 at 10 mN/m. This is significantly smaller than that of 24 A ˚ 2 reported in previous study of the similar structure ca. 70 A of DA [31]. To improve the stability of BODTA monolayer at air–water interface, metal ions were added in the subphase because metal ions form the stable coordinated complex with BODTA through carboxylic group of the molecule [32]. On the subphase containing Cu2+ ion, the monolayer showed significantly larger value of the area/molecule and collapsed at higher pressure as shown Fig. 2(a). On Cu2+ -contained sub˚2 phase, the estimated area/molecule of BODTA was ca. 68 A and the monolayer was collapsed near 50 mN/m indicating significant improvement of the stability due to effective complexation of BODTA with Cu2+ ions at the interface. However, the similar effect was not observed from the subphase containing either Cd2+ or Ca2+ ions. The monolayers on Cd2+ or Ca2+ ions contained subphase were collapsed at 5 mN/m and the area/molecule was unreasonably small as shown in Fig. 2(b) and (c). This is presumably due to inefficient complexation between the metal ions and carboxylic group of the molecule. The stability and morphological analysis of BODTA monolayer on Cu2+ -contained subphase were also investigated by BAM during the compression. As shown in Fig. 3(a), the monolayer shows the smooth morphology without any noticeable aggregates or defects up to 50 mN/m. Further compression to 55 mN/m and higher induced collapse of the monolayer as indicated BAM image in Fig. 3(b). To investigate conformational changes of BODTA in the monolayer on Cu2+ subphase during the compression, V–A isotherm was measured as depicted in Fig. 4(b). The surface potential measurement has provided critical information for determining monolayer properties such as phase transition [33] and molecular orientation [34,35]. Change of V sensitively reflects the conformation change of the BODTA during compression. As shown in Fig. 4(b), the surface potential was

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423

Fig. 3. BAM images of BODTA monolayers on the subphase containing Cu2+ ion (a) before and (b) after monolayer collapsed above 50 mN/m.

continuously increased even before the surface pressure develops above 0 mN/m. It may suggest that the orientation of the BODTA molecule changes as soon as the compression starts or it forms the self-organized structure as soon as floating at the air–water interface. The later was found what really happened by additional analysis of the monolayer by AFM and FT-IR measurements. As the compression proceeds, the self-organized structures get closer to each other without significant conformational changes of the molecule until the surface pressure reaches near 30 mN/m. Further compression beyond 30 mN/m, V–A isotherm reflects relatively rapid conformational change of BODTA and sharp increase of surface concentration of the molecule as smaller decrease of the area per molecule. This implies that BODTA form a selforganized domain before the compression and the orientation of the molecule is titled to the air–water interface with relatively higher angle when the monolayer was compressed until the surface pressure reached ca. 30 mN/m. 3.1. AFM images before and after UV irradiation The morphology of BODTA LB monolayers transferred on mica at different surface pressures of 5, 20, 30 and 40 mN/m was investigated by AFM as shown in Fig. 5. The

Fig. 4. π–A and V–A isotherms of BODTA on the subphase containing Cu2+ ions.

AFM image of LB monolayer transferred at 5 mN/m shows the domains with average size of 300–500 nm with surface coverage of around 67%. The height difference between the ˚ The size of domain and surrounding lower area is about 12 A. the domain and the height difference between the domain and surrounding of the monolayers transferred at different surface pressure were summarized in Table 1. As increasing the transfer pressure, the size of domains remains relatively constant while the surface coverage of the domains increases and the height difference of domains to the surrounding decreases. It reflects that BODTA molecules form the organized domains on the subphase at lower surface pressure without assistance of the external force. During the compression, the distance between adjacent domains decreases without significant variation of the relative orientation and distance of the BODTA molecule in the domain while rapid change in the orientation and distance of the BODTA molecules, which were located at the periphery of the domain. This observation is consistent with the interpretation of the π–A and V–A isotherms. Therefore, rapid increase V observed in V–A isotherm largely stems from the increased surface concentration of the domain at the air–water interface. To investigate the structural change of BODTA molecule in the monolayer by UV-induced polymerization, the BODTA LB monolayers prepared by transferring at different surface pressure were subjected to UV irradiation for 10 min. AFM images of the resulting polymerized LB monolayer are shown in Fig. 6 and summarized the coverage and relative height of the monolayer in Table 1. Comparing with the AFM images of the monomeric BODTA in Fig. 5, the AFM images after UV irradiation do not show any feature of the domains. On the other hand, the surface coverage was increased and the height difference between the densely packed area and relatively lower coverage area was decreased. Apparently those domains were destroyed by polymerization of BODTA molecules and the inclination angle of BODTA molecules in the monolayer were significantly reduced. Careful evaluation of the AFM images, coverage and relative height, BODTA molecules in the LB monolayer suggests that BODTA was polymerized more effectively when it was compressed to the surface pressure at 5 and 40 mN/m. The difference in the effectiveness of the polymerization may stems from a slight difference in the orientation of BODTA and inter-molecular

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Fig. 5. AFM images of BODTA LB monolayers transferred on mica from the subphase containing Cu2+ ions at different surface pressure: (a) 5, (b) 20, (c) 30 and (d) 40 mN/m.

distance in the monolayer. At the surface pressure at 5 mN/m, it may have relatively larger volume for free rotation of DA moiety, which contributes to the effective polymerization. In the case of 40 mN/m, BODTA molecules has slightly higher tilted angle and were packed much closer than those cases of lower surface pressure such as 10, 15 and 20 mN/m. This close proximity and optimal orientation of BODTA presumably induces the effective polymerization. Therefore, it would be possible to follow different polymerization mechanisms. In the case of the monolayer prepared at 5 mN/m, the polymerization reaction would involve 1,2 addition, whereas for 40 mN/m it is likely by 1,4 addition mechanism. Details of the structural difference of two polymerized monolayers are currently under investigation in our laboratory. In the case of the LB monolayers prepared from surface pressures between 20 and 30 mN/m, despite highly tiled orientation of the molecule, the estimated intermolecular distance is larger ˚ for polymerization than the required distance of 4.7–5.2 A to occur [21,22]. The optimal intermolecular distance plays critical factor for determining effectiveness of the polymerization.

3.2. Polarized Fourier transform infrared (FT-IR) spectroscopy Polarized FT-IR spectra were measured to calculate the tilted angle of C C axis in DA moiety of BODTA LB monolayers on CaF2 prepared at several different surface pressures. Prior to measure the polarized FT-IR spectra, FT-IR spectrum of BODTA (Fig. 7) was acquired in KBr pellet and the vibrational modes of the orientation-sensitive bands were assigned as summarized in Table 2. By using Eqs. (1)–(4) in the previous report [36], the tilted angle of the hydrocarbon chain was calculated. The value of α in Eq. (1), the ratio of the absorbance (A) of the band from sand p-polarized radiation, was used to calculate ω in Eq. (2). To calculate the tilted angle, the known values of reflective indexes of the each component and the angle of the incident (i) were used. The angle of the reflective radiation (r) in Eq. (3) was obtained by the known values of reflective indexes of the each component. The reflective indexes of each component are as follow: n1 (air) = 1, n2 (LB monolayers) = 1.5, n3 (CaF2 ) = 1.415. By substituting calculated and known values

Table 1 Surface coverage of the domain and the height difference between the domain and surrounding surface Before UV irradiation

Surface coverage (%) ˚ Height difference (A)

After UV irradiation

5 mN/m

20 mN/m

30 mN/m

40 mN/m

5 mN/m

20 mN/m

30 mN/m

40 mN/m

67 12 ± 2

72 10 ± 2

75 10 ± 2

88 9±1

92 2±1

89 3±1

88 3±1

91 5±2

These results were obtained from AFM image analysis.

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Fig. 6. AFM images of polymerized BODTA LB monolayers on mica transferred at different surface pressure: (a) 5, (b) 20, (c) 30 and (d) 40 mN/m.

from Eqs. (1)–(3) and measured value of Ap (i), absorbance of band by p-polarized radiation with incident angle i, the value of ϕ can be calculated, which is either tilted angles of hydrocarbon (α) or (β) for symmetric and anti-symmetric stretching mode of –CH2 , respectively. Finally by plugging in the calculated values of α and β in Eq. (5), we obtained the tilted angle of long alkyl chain axis (γ) to surface normal. The tilted angle of long alkyl chain axis (γ) indicates the orientation of C C axis to the surface normal of the substrate. The tilted angles of the C C axis of BODTA in LB monolayers before and after UV irradiation are shown in Table 3. The LB monolayers for the inclination angle calculation were prepared at two different surface pressures. As indicated in the isotherms and spectroscopic data, the orientation of C C axis changes to

higher angle to the surface of the substrate as the transferred pressure increased. This is true for both LB monolayer samples of before and after UV irradiation. By UV irradiation, the tilted angle of C C axis is significantly reduced as shown in Table 3. This indicates that UV-induced polymerization significantly disturbed the monomeric arrangement by formation of two-dimensional network structure. The changes of the inclination angle calculated from FT-IR spectra are consistent with the variation of the morphology and height Table 2 Assignments of vibrational modes of FT-IR spectrum of BODTA in KBr pallet Wavenumber (cm−1 )

Assignments

3267 3062 2956 2924 2848 1722 1456 1300, 1059 922, 852

ν(O H)acid ν( C H)benzene ring νas (CH3 ) νas (CH2 ) νs (CH2 ) ν(C O) δ(CH2 ) ν(C O C) δ(O H)oop

Table 3 The tilted angle changes of C C axis of BODTA by UV-induced polymerization Transferred surface pressure (mN/m) Before irradiation (◦ ) After irradiation (◦ ) Fig. 7. FT-IR spectrum of BODTA in KBr pellet.

5 45.5 62.9

30 5.3 31.8

The calculated angle indicates the orientation of C C axis to the surface normal of the substrate.

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difference in the AFM image of UV-irradiated LB monolayer of BODTA. Two different analytical results suggest that BODTA molecules in the monolayer tend to lower the inclination angle by UV-induced polymerization and maintain less organized structure. Furthermore, much larger decrease in the tilted angle changes was observed from the monolayer transferred at higher surface pressure. This indicates that polymerization mechanism and polymerized structure of BODTA may be different at different compression state. FT-IR results support the molecular behavior of UV-induced polymerization at different compression state, which was observed AFM studies. Therefore, the mechanism and degree of polymerization were affected by the molecular orientation and distance in the monolayer, which can be precisely controlled by LB technique: α=

As (0) Ap (0)

(1)

1−α = cos 2ω

1+α

(2)

n1 sin i = n3 sin r

(3)

Ap (i) (n1 + n3 )n1 n3 = As (0) (n3 cos i + n1 cos r) 
n21 sin2 i cos2 φ

cos i cos r × + n1 n3 n42 sin2 φ cos2 ω

cos2 α + cos2 β + cos2 γ = 1

(4)

(5)

4. Conclusions In this paper, an amphiphilic DA derivative with phthalic acid, BODTA was synthesized and investigated various aspects of the molecular behavior in the monolayer structure including structural changes by UV-induced polymerization. BODTA was fabricated as a stable monolayer on the subphase containing Cu2+ ions and spontaneously. During compression of the monolayer, the surface coverage of the domains increased while maintaining almost constant orientation of the BODTA. Before BODTA polymerized, the inclination angle of the molecule in the LB monolayer was near perpendicular to the substrate at full compression, and initial domain formation was preserved. By UV-induced polymerization, the inclination angle was significantly reduced and the domain structure was perturbed. By controlling intermolecular distance and the inclination angle of BODTA, two difference polymerization mechanisms were suggested. This study shows that by utilizing LB technique it is possible to precisely control intermolecular interaction and the reaction pathway, which provides the tool for customizing properties of the materials.

Acknowledgements The authors thank Mr. Jun-Hyun Park for the measurement of the isotherms and preparation of LB films of BODTA. This work was supported by a grant from Center for Nanoscale Mechatronics and Manufacturing of 21st Century Frontier Research Program.

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