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Studying the chirality of polymerized 10,12-tricosadynoic acid LB films using SHG polarized angle dependence and SHG-CD method
Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 424–429
Studying the chirality of polymerized 10,12-tricosadynoic acid LB films using SHG polarized angle dependence and SHG-CD method Gang Zou, Takaaki Manaka, Dai Taguchi, Mitsumasa Iwamoto ∗ Department of Physical electronics, Tokyo Institute of Technology, O-okayama 2-12-1-S3-33, Meguro-ku, Tokyo 152-8552, Japan Received 3 June 2005; received in revised form 13 November 2005; accepted 16 November 2005 Available online 4 January 2006
Abstract Using optical second harmonic generation (SHG) measurements, polymerized 10,12-tricosadynoic acid (PTDA) monolayer on quartz substrate were investigated. SHG polarized angle dependence measurements and optical second harmonic generation circular dichroism (SHG-CD) measurements were carried out to reveal the chirality of PTDA monolayer. It was revealed that the chiral PTDA films were formed when subphase solution contains Cu(II) ions, while the chirality for PTDA LB films were not formed when subphase solution is pure water or contains Cd(II) ions. The conformational change of the polydiacetylene chain due to metal ions was found the most probable origin. © 2005 Elsevier B.V. All rights reserved. Keywords: Tricosa-10,12-dynoic acid; Chirality; Photo-polymerization; Second harmonic generation measurements (SHG); Second harmonic generation circular dichroism (SHG-CD)
1. Introduction Chirality plays an important role in nature and biological systems [1]. Continuous interest has been devoted to the synthesis of chiral molecules through covalent bonds. Also much attention has been paid to the chirality of supramolecular systems over past decades. At a supramolecular level, the chirality of systems can be obtained through the non-covalent interaction between the component chiral molecules or even through a unique arrangement of achiral molecules [2,3]. The latter case is especially interesting since macroscopic chirality can be realized from achiral molecules without any chiral auxiliaries. The linear diacetylene, R1 C C C C R2 , and its derivatives exhibit interesting electronic [4,5] and optical [5,6] properties and have many potential applications in biosensors, pathogenic agents and so on. [7–9]. Although extensive attention had been paid to the polymerization of dynoic acid monolayer [10–15] and multilayer [16,17] due to their reactivity in the solid state and their ability to substitute for a length of alkyl chain without interfering in the molecular packing, few studies have been
∗
Corresponding author. Tel.: +81 3 5734 2191. E-mail address: [email protected] (M. Iwamoto).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.044
carried out on the chirality of polydiacetylene and its derivatives films [18,19]. The chiral molecule is distinguished from achiral molecules due to the lack of mirror plane. From the theoretical side, analyzing the nonlinear dielectric polarization related to the SHG, the vectorial formulae for specifying the dielectric polarization of the monolayer composed of chiral uniaxial molecules were derived using tensor components of the nonlinear optical susceptibility. The derived formulae clearly show the difference of the SHG between the monolayer composed of chiral molecules and achiral molecules. In a manner as predicted in this analysis, the chirality of the monolayer composed of chiral molecules, i.e., S-citronelloxy-cyanobiphenyl (S-CCB) was revealed by the polarized angle dependence SHG measurement [20]. Chiral molecules have an optical rotatory power, which can rotate the polarization plane of the incident light in the medium. Experimentally, the fact that circularly polarized light interacts differently with right-handed and left-handed molecules is applied to the measurements. In this case, the surface-specific nonlinear optical techniques such as the second-harmonic generation circular dichroism (SHG-CD) can be a powerful tool for studying the monolayer chirality, where differential SHG signals from chiral surface were recorded with right- and left- circularly polarized fundamental incidence. The SHG-CD technique was applied for a 2,2 -dihydroxy-1,1 -binaththyl (BN) solid/solution
G. Zou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 424–429
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interface by Patralli-Mallow et al. [21] and other researchers [22–25]. In this paper, the chirality of polymerized 10,12tricosadynoic acid (PTDA) monolayer and LB films on quartz substrate was studied by the polarized angle dependence SHG measurement and SHG-CD technique, respectively. Then the effect of metal ions in subphase solution on the conformation change of the PTDA monolayer and LB films on quartz substrate were discussed in detail.
cut filter to remove intense fundamental light and was detected by a photomultiplier tube (Hamamatsu photonics: R1463) after passing through a monochromator (Nikon: G-250). The signals obtained were averaged by a Boxcar integrator (Stanford Research: SR-250) and proceed by a PC. A light source ranging from 900 nm to 1200 nm was obtained by using an optical parametric oscillator (OPO: Continuum Surelite OPO) pumped by the third harmonic light of a Q-switched Nd-YAG Laser. All measurements were carried out in laboratory atmosphere.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Preparation of PTDA monolayer and LB films
10,12-Tricosadynoic acid (TDA), CH3 (CH)9 C C C C (CH)8 COOH, was purchased from Sigma–Aldrich Company and used without further purification. TDA was dissolved in AR grade chloroform and filtered in order to take off as few polymerized solids as possible. Subphase solution were prepared with Cu(NO3 ) 2 and CdCl2 (AR grade) and pure water (18.2 M) with the concentration of 0.01 M.
There are two ways to prepare PTDA monolayer and LB films. One method is that: the monomer TDA molecules were dissolved into chloroform. After spreading the chloroform solution, the TDA monolayer were compressed to a designed surface pressure and deposited onto solid substrate by LB technique. Upon UV irradiation, the colorless TDA LB films turned into blue or red, depending on the polymerization conditions. These PTDA LB films were named PTDA-H2 O1, PTDA-Cu1 and PTDA-Cd1 according to the subphase solution, respectively. Fig. 1a and b shows the UV–vis spectra of PTDA LB films deposited at 10 mN/m from water and Cu(NO3 )2 subphase. The PTDA-H2 O1 films showed absorption peak at 598 nm, while PTDA-Cu1 films showed absorption peak at 537 nm. These results are similar to those reported in previous work, indicating the polymerization of TDA LB films [12–15,19]. And TDA monolayer could be polymerized first and then deposited. The second method is that: the monomer molecules were dissolved in chloroform. After spreading, the TDA monolayer was compressed to a designed surface pressure and then in situ polymerized first by UV irradiation. Finally the in situ polymerized TDA monolayer could be deposited onto solid substrate. And these PTDA LB films were named PTDA-H2 O2, PTDA-Cu2 and PTDA-Cd2 according to the subphase solution, respectively.
2.2. LB films deposition All LB deposition experiments were carried out at 20 ◦ C (±0.5 ◦ C) on a computer-controlled Langmuir film balance (600 mm × 150 mm in length and width, 10 mm in depth). The Langmuir film balance was placed in a dust-free box and protected against UV-light with opaque plastic sheets. The substrate for LB deposition was quartz. A 16 W UV lamp (λ = 254 nm) hung 10 cm above the LB films was used for UV-polymerization. The UV–vis absorption spectrums of PTDA LB films were measured after UV-polymerization using a spectrometer (Shimadzu: UV 2100). 2.3. SHG polarized angle dependence measurements PTDA monolayer on quartz substrate was characterized in detail by SHG polarized angle dependence measurements. For SHG measurement, s- or p-polarized fundamental light was focused onto the sample, with an incident angle of 45◦ . The SH light reflected from the sample was filtered by a fundamental cut filter to remove intense fundamental light and was detected by a photomultiplier tube (Hamamatsu photonics: R1463) after passing through a SH-pass filter. A light source (wavelength is 522 nm) was the third harmonic light of a Q-switched Nd-YAG Laser. The signals obtained were averaged by a Boxcar integrator (Stanford Research: SR-250) and proceed by a PC. All measurements were carried out in laboratory atmosphere. 2.4. SHG-CD measurements PTDA LB films on quartz substrate were characterized in detail by SHG-CD measurements. For the SHG-CD measurement, right- and left- circularly polarized fundamental light was focused onto the sample, with an incident angle of 45◦ . The SH light transmitted from the sample was filtered by a fundamental
Fig. 1. UV–visible spectra of polymerized TDA LB films on quartz plate: (a) PTDA-H2 O1, polymerized after deposition from water; (b) PTDA-Cu1, polymerized after deposition from Cu(NO3 )2 subphase; (c) PTDA-Cu2, deposited after polymerization on water surface; and (d) PTDA- H2 O2, deposited after polymerization on Cu(NO3 )2 subphase.
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Fig. 1c and d shows the UV–vis spectra of in situ polymerized TDA LB films deposited from water and Cu(NO3 )2 subphase. The PTDA-H2 O2 and PTDA-Cu2 films both showed absorption peak at 536 nm. 3.2. SHG polarized angle dependence characterization In order to confirm the chirality of the monolayer, we performed polarized angle dependence SHG measurement. In this measurement, the analyzer angle was fixed and the SH signals were measured as changing the input polarized angle [20]. Since the second-order NLO susceptibility χ(2) is a third-rank tensor, we can write the second-order NLO polarization in the form: (2)
Pi = χijk Ej Fk
(1)
where χijk is the second order susceptibility (SOS) tensor of monolayer, and Ej and Fk are external electric fields. While this susceptibility tensor can be expressed as a sum of symmetric and asymmetric part as: Pi =
1 {Sijk (Ej Fk + Ek Fj ) + Aijk (Ej Fk − Ek Fj )} 2
(2)
where Sijk and Aijk are components of SOS tensor related to symmetric and asymmetric parts, respectively. The symmetric part only appears under the SHG process. For the C∞v symmetry, the susceptibility tensors are expressed using contracted notation as ⎛ ⎞ 0 0 0 0 S15 0 ⎜ ⎟ 0 0 S15 0 0⎠ S = ⎝0 (3) S31
S31
S33
0
0
0
if the monolayer is composed of the chiral molecules, the mirror plane disappears and symmetry is reduced to the C∞ . As a result, the susceptibility tensors are expressed as ⎛ ⎞ 0 0 0 S14 S15 0 ⎜ ⎟ 0 0 S15 −S14 0 ⎠ S = ⎝0 (4) S31
S31
S33
0
0
0
here, the constructed components are expressed using χ(2) tensors components as S15 = χxxz + χxzx , S14 = χxyz + χxzy , S31 = χzxx , and S33 = χzzz . Assuming that s-polarized reflected SH is observed, we can obtain the SH intensity as I(2ω)s ∝ |S15 sin γi cos γi − S14 cos γi cos φ| 2
2
(5)
for the monolayer with C∞ symmetry, where γ i represents the input polarized angle, φ represents the incident angle [20]. On the other hand, the SH intensity is expressed as I(2ω)s ∝ |S15 sin γi cos γi |2
(6)
for the monolayer with C∞v symmetry. It is clear from Eq. (6) that the polarized angle dependence of the SH intensity becomes symmetric around γ i = π/4 for the monolayer with C∞v symmetry, whereas the polarized angle dependence of the SH intensity is not symmetry around γ i = π/4 for the monolayer with C∞ symmetry. Thus, symmetry of the monolayer can be determined
Fig. 2. The results for the polarized angle dependence measurement for in situ polymerized TDA monolayer deposited from: (a) water (PTDA-H2 O2); (b) Cu(NO3 )2 subphase (PTDA-Cu2); and (c) from CdCl2 subphase (PTDA-Cd2).
by measuring polarized angle dependence of the SH intensity and this method could be a very sensitive to detect the chirality of the thin films. The result for the polarized angle dependence SHG measurements for in situ polymerized TDA monolayer deposited from water, Cu(NO3 )2 and CdCl2 subphase were shown in Fig. 2. As shown in Fig. 2a and b, a significant difference was actually observed for PTDA-H2 O2 and PTDA-Cu2 monolayer. As a result, the relationship between two tensor components S14 , S15 can be obtained as S14 /S15 ≈ 1/7 for PTDA-H2 O2 monolayer and S14 /S15 ≈ 1/5 for PTDA-Cu2 monolayer, respectively. These results indicated that S14 is nonzero for PTDA-H2 O2 and PTDA-Cu2 monolayer. However, S14 is negligibly small for PTDA-Cd2 monolayer, which can be interpreted from Fig. 2c.
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All above results indicated that PTDA-Cd2 monolayer remains the C∞v symmetry after polymerization. We have repeated this measurement many times, and could always obtain similar experimental results, i.e., the reproducibility of the asymmetrical shape of the polarized angle dependence measurement shown in Fig. 2a and b were quite well. Furthermore, we have done a similar measurement for PTDA-H2 O1 and PTDA-Cu1, the similar reproducibility of the asymmetrical shape of the polarized angle dependence could be obtained, which indicated that PTDA monolayer showed C∞ symmetry under these conditions, whereas PTDA-Cd1 monolayer remains the C∞v symmetry after polymerization. Since the tensor component S14 actually exhibits the character of monolayer chirality [20], it is interesting to note that PTDA-H2 O1 and PTDA-Cu1 monolayer may show chiral although the monomer TDA molecules are achiral; whereas PTDA-Cd1 monolayer showed achiral. However, we must note that achiral surface which is anisotropic in the surface plane may also exhibit such difference effect in the polarized angle dependence SHG measurement. So we used SHG-CD measurement to further confirm the chirality of PTDA LB films. 3.3. SHG-CD characterization SHG-CD spectrum for PTDA LB films on quartz were examined in detail by using a circularly polarized light. The difference between left- and right-circularly SH spectra as a function of wavelength is represented as SHG-CD spectrum. The left circularly polarized light gave a larger SHG spectrum than that for right circularly polarized light. SHG-CD measurement is powerful to probe the presence of the chirality. The intensity of SHG-CD spectrum is defined as ISHG-CD =
(Ilcp − Ircp ) 2(Ilcp − Ircp ) = , Iaverage Ilcp + Ircp
(7)
where subscripts lcp and rcp are the left- and right-circularly polarized light, respectively. In Fig. 3, SHG-CD spectrum of the polymerized TDA LB films deposited from Cu(NO3 )2 subphase (PTDA-Cu1 LB films) showed obvious Cotton effect. Negative and positive Cotton effects were observed at 1004 and 1044 nm, respectively, with a crossover at 1024 nm. This result indicated that chiral PTDA LB films formed although the monomer TDA molecules were achiral. And the SHG-CD spectrum for PTDAH2 O1 and PTDA-Cd1 LB films were also studied, however, no obvious Cotton effects for PTDA-H2 O1 and PTDA-Cd1 LB films could be observed (shown in Figs. 4 and 5), indicating that chiral PTDA films could not form under present conditions. Furthermore, we have done a similar measurement for the in situ polymerized TDA monolayer (PTDA-H2 O2, PTDA-Cu2 and PTDA-Cd2 films); similar results for SHG-CD spectrum could be obtained. Moreover, it could be confirmed that the asymmetrical shape of the polarized angle dependence measurement for PTDA-H2 O2 monolayer came not from the chirality, but from monolayer anisotropic in the surface plane. All above results suggested that chiral PTDA films can form under certain conditions although the monomer TDA molecules are achiral; the metal ions in the subphase solution would play an important role in the formation of the chiral PTDA films.
Fig. 3. (a) SHG spectrum and (b) SHG-CD spectrum for PTDA LB films on quartz plate that were deposited from Cu(NO3 )2 subphase and then polymerized (PTDA-Cu1).
From the results of polarized angle dependence SHG measurement combined with SHG-CD measurement, it is interesting to note that PTDA LB films can form chirality under certain conditions although the monomer TDA molecules are achiral. This should be ascribed to the formation of polydiacetylene chains. It is well known that polymerized TDA is formed through a typical topochemical reaction, the main chains of the polymer are -conjugated backbones consisting of alternating double and triple bonds. The model of the polydiacetylene chains should be a “Wormlike” chain (Porod–Kratky chain [14]) presents a continuous curvature of the chain skeleton shown in Fig. 6. And the polydiacetylene chains could form a helical structure under certain condition and this helical structure could be observed by TEM technology [19]. Thus, this helical structure will cause macroscopic chirality of polymerized TDA in the interfacial films. However, the formation of the conjugated backbone is very sensitive to the environment. The optical properties of the backbone depend sensitively on strain. In particular, rotation about C C bond of the polymer backbone, which changes the planarity of the backbone, is critical. Rotation about this bond necessitates changes in the conformation of the pendant side groups, and thus there exists a sensitive interplay between them. During the polymerization, the extended backbones will form
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Fig. 4. (a) SHG spectrum and (b) SHG-CD spectrum for PTDA LB films on quartz plate that were deposited from water and then polymerized (PTDAH2 O1).
Fig. 5. (a) SHG spectrum and (b) SHG-CD spectrum for PTDA LB films on quartz plate that were deposited from CdCl2 subphase and then polymerized (PTDA-Cd1).
in a strained configuration because of the geometric restriction imposed by the side group arrangement. The subsequent application of heat, stress, or other stimuli leads to side chain fluctuations or reconfigurations and allows the backbone to adopt a more relaxed conformation that involves rotation about the C C bonds, and thus leads to changes in the optical properties. Thus, the arrangement of side group will play an important role in the configuration of polymer backbone. When the arrangement of side group is suitable, every unit will act cooperatively with regular rotation for polymer backbone, the helical structure will form and that will cause a macroscopic chirality. In present system, the introduction of Cu(II) ions will cause the suitable arrangement of side group, and that will cause a macroscopic chirality. However, in the case of water and CdCl2 subphase, macroscopic chirality will not be obtained under present conditions. Amphiphilic diacetylene derivative with a carboxylic group were reported to form salts with metal ions such as Cd(II), etc., whilst coordinating with metal ions such as Cu(II), etc. The tetra-coordinated Cu(II) ions is in a planar square structure, which will cause regular arrangement of side group and the formation of helical structure for the polymer backbone. Thus, it will cause a macroscopic chirality of PTDA LB films. Since the
arrangement of side group will play an important role in the configuration of polymer backbone, applying extra heat, stress, or other stimuli will change the arrangement of side group of the polymer. Thus, it is possible to expect the formation of the chiral PTDA LB films deposited from water, when applied suitable extra heat, stress, or other stimuli during polymerization. Thus, the formation of the chirality could be controlled when applied suitable stimuli during polymerization.
Fig. 6. Schematic representation of the shape of the polydiacetylene chains.
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4. Conclusions Using optical second harmonic generation measurements, PTDA monolayer on quartz substrate were investigated in detail. SHG polarized angle dependence measurement was carried out to reveal the chirality of in situ polymerized TDA monolayer. A significant difference was actually observed for PTDACu2 monolayer, which indicated that PTDA-Cu2 monolayer were with C∞ symmetry. Furthermore, the chirality of PTDA monolayer was studied by the optical second harmonic generation circular dichroism (SHG-CD) measurements. The SHGCD spectrum of PTDA-Cu1 LB films showed obvious Cotton effect, which indicated that the polymerized TDA LB films deposited from Cu(NO3 )2 subphase can form chirality although the monomer TDA molecules are achiral. All above experiments indicated that the chiral PTDA film could be formed when subphase solution contains Cu(II) ions, while the chirality of PTDA film could not be observed when subphase is pure water or contains Cd(II) ions under the same conditions. The explanation based on conformation change has been presented. And the formation of the chiral PTDA films were expect to be controlled. Acknowledgement Support from the Japan Society for the Promotion of Science (P04347) is gratefully acknowledged. References [1] N. Berova, K. Nakanishi, R.W. Woody, Circular Dichroism Principles and Applications, second ed., Whiley-VCH, 2000. [2] J.M. Rib´o, J. Crusats, F. Sagu´es, J. Claret, R. Rubires, Science 292 (2001) 20.
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[3] M. Wang, G.L. Siliva, B.A. Armitage, J. Am. Chem. Soc. 122 (2000) 9977. [4] M. Sugi, J. Mol. Electron 1 (1985) 3. [5] P.S. He, in: L.H. Shi, D.B. Zhu (Eds.), Polymer and Organic Solid, Science Press, Beijing, 1997, p. 25. [6] G. Khanarian, Thin Solid Films 152 (1987) 265. [7] D.H. Charych, J.O. Nagy, W. Spevak, M.D. Bednarski, Science 261 (1993) 585. [8] Z. Ma, J. Li, M. Liu, J. Cao, Z. Zou, J. Tu, L. Jiang, J. Am. Chem. Soc. 120 (1998) 12678. [9] Y. Lu, Y. Yang, A. Sellinger, M. Lu, J. Huang, H. Fan, R. Haddad, G. Lopez, A.R. Burns, D.Y. Sasaki, J. Shelnutt, C.J. Brinker, Nature 410 (2001) 913. [10] N. Mino, H. Tamura, K. Ogawa, Langmuir 1 (1991) 2336. [11] C. Ohe, H. Ando, N. Sato, Y. Urai, M. Yamamoto, K. Itoh, J. Phys. Chem. B 103 (1999) 435. [12] H.L. Zhou, W.X. Lu, S.F. Yu, P.S. He, Langmuir 16 (2000) 2797. [13] P.S. He, H.L. Zhou, G. Zou, Polymer 44 (11) (2003) 3235. [14] G. Zou, K. Fang, P.S. He, Langmuir 19 (11) (2003) 4633. [15] G. Zou, K. Fang, P.S. He, Langmuir 18 (17) (2002) 6602. [16] S.F. Yu, H.L. Zhou, P.S. He, J. Mater. Sci. 34 (1999) 3149. [17] R.W. Carpick, T.M. Mayer, D.Y. Sasaki, A.R. Burns, Langmuir 16 (2000) 4639. [18] A.F. Drake, P. Udvarhelyi, D.J. Ando, D. Bloor, J.S. Obhi, S. Mann, Polymer 30 (1989) 1063. [19] X. Huang, M. Liu, Chem. Commun. 1 (2003) 66. [20] H. Fujimaki, T. Manaka, H. Ohtake, A. Tojima, M. Iwamoto, J. Chem. Phys. 119 (2003) 7427. [21] T. Patralli-Mallow, T.M. Wong, J.D. Byers, H.I. Lee, J.M. Hicks, J. Phys. Chem. 97 (1993) 1383. [22] J.M. Hicks, T. Patralli-Mallow, Appl. Phys. B: Lasers Opt. 68 (1999) 589. [23] G.J. Simpson, J. Chem. Phys. 117 (7) (2002) 3398. [24] T. Patralli-Mallow, A.L. Plant, M.L. Lewis, J.M. Hicks, Langmuir 16 (2000) 5960. [25] K. Fujiwara, H. Monjushiro, H. Watarai, Chem. Phys. Lett. 394 (2004) 349.
Studying the chirality of polymerized 10,12-tricosadynoic acid LB films using SHG polarized angle dependence and SHG-CD method Gang Zou, Takaaki Manaka, Dai Taguchi, Mitsumasa Iwamoto ∗ Department of Physical electronics, Tokyo Institute of Technology, O-okayama 2-12-1-S3-33, Meguro-ku, Tokyo 152-8552, Japan Received 3 June 2005; received in revised form 13 November 2005; accepted 16 November 2005 Available online 4 January 2006
Abstract Using optical second harmonic generation (SHG) measurements, polymerized 10,12-tricosadynoic acid (PTDA) monolayer on quartz substrate were investigated. SHG polarized angle dependence measurements and optical second harmonic generation circular dichroism (SHG-CD) measurements were carried out to reveal the chirality of PTDA monolayer. It was revealed that the chiral PTDA films were formed when subphase solution contains Cu(II) ions, while the chirality for PTDA LB films were not formed when subphase solution is pure water or contains Cd(II) ions. The conformational change of the polydiacetylene chain due to metal ions was found the most probable origin. © 2005 Elsevier B.V. All rights reserved. Keywords: Tricosa-10,12-dynoic acid; Chirality; Photo-polymerization; Second harmonic generation measurements (SHG); Second harmonic generation circular dichroism (SHG-CD)
1. Introduction Chirality plays an important role in nature and biological systems [1]. Continuous interest has been devoted to the synthesis of chiral molecules through covalent bonds. Also much attention has been paid to the chirality of supramolecular systems over past decades. At a supramolecular level, the chirality of systems can be obtained through the non-covalent interaction between the component chiral molecules or even through a unique arrangement of achiral molecules [2,3]. The latter case is especially interesting since macroscopic chirality can be realized from achiral molecules without any chiral auxiliaries. The linear diacetylene, R1 C C C C R2 , and its derivatives exhibit interesting electronic [4,5] and optical [5,6] properties and have many potential applications in biosensors, pathogenic agents and so on. [7–9]. Although extensive attention had been paid to the polymerization of dynoic acid monolayer [10–15] and multilayer [16,17] due to their reactivity in the solid state and their ability to substitute for a length of alkyl chain without interfering in the molecular packing, few studies have been
∗
Corresponding author. Tel.: +81 3 5734 2191. E-mail address: [email protected] (M. Iwamoto).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.044
carried out on the chirality of polydiacetylene and its derivatives films [18,19]. The chiral molecule is distinguished from achiral molecules due to the lack of mirror plane. From the theoretical side, analyzing the nonlinear dielectric polarization related to the SHG, the vectorial formulae for specifying the dielectric polarization of the monolayer composed of chiral uniaxial molecules were derived using tensor components of the nonlinear optical susceptibility. The derived formulae clearly show the difference of the SHG between the monolayer composed of chiral molecules and achiral molecules. In a manner as predicted in this analysis, the chirality of the monolayer composed of chiral molecules, i.e., S-citronelloxy-cyanobiphenyl (S-CCB) was revealed by the polarized angle dependence SHG measurement [20]. Chiral molecules have an optical rotatory power, which can rotate the polarization plane of the incident light in the medium. Experimentally, the fact that circularly polarized light interacts differently with right-handed and left-handed molecules is applied to the measurements. In this case, the surface-specific nonlinear optical techniques such as the second-harmonic generation circular dichroism (SHG-CD) can be a powerful tool for studying the monolayer chirality, where differential SHG signals from chiral surface were recorded with right- and left- circularly polarized fundamental incidence. The SHG-CD technique was applied for a 2,2 -dihydroxy-1,1 -binaththyl (BN) solid/solution
G. Zou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 424–429
425
interface by Patralli-Mallow et al. [21] and other researchers [22–25]. In this paper, the chirality of polymerized 10,12tricosadynoic acid (PTDA) monolayer and LB films on quartz substrate was studied by the polarized angle dependence SHG measurement and SHG-CD technique, respectively. Then the effect of metal ions in subphase solution on the conformation change of the PTDA monolayer and LB films on quartz substrate were discussed in detail.
cut filter to remove intense fundamental light and was detected by a photomultiplier tube (Hamamatsu photonics: R1463) after passing through a monochromator (Nikon: G-250). The signals obtained were averaged by a Boxcar integrator (Stanford Research: SR-250) and proceed by a PC. A light source ranging from 900 nm to 1200 nm was obtained by using an optical parametric oscillator (OPO: Continuum Surelite OPO) pumped by the third harmonic light of a Q-switched Nd-YAG Laser. All measurements were carried out in laboratory atmosphere.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Preparation of PTDA monolayer and LB films
10,12-Tricosadynoic acid (TDA), CH3 (CH)9 C C C C (CH)8 COOH, was purchased from Sigma–Aldrich Company and used without further purification. TDA was dissolved in AR grade chloroform and filtered in order to take off as few polymerized solids as possible. Subphase solution were prepared with Cu(NO3 ) 2 and CdCl2 (AR grade) and pure water (18.2 M) with the concentration of 0.01 M.
There are two ways to prepare PTDA monolayer and LB films. One method is that: the monomer TDA molecules were dissolved into chloroform. After spreading the chloroform solution, the TDA monolayer were compressed to a designed surface pressure and deposited onto solid substrate by LB technique. Upon UV irradiation, the colorless TDA LB films turned into blue or red, depending on the polymerization conditions. These PTDA LB films were named PTDA-H2 O1, PTDA-Cu1 and PTDA-Cd1 according to the subphase solution, respectively. Fig. 1a and b shows the UV–vis spectra of PTDA LB films deposited at 10 mN/m from water and Cu(NO3 )2 subphase. The PTDA-H2 O1 films showed absorption peak at 598 nm, while PTDA-Cu1 films showed absorption peak at 537 nm. These results are similar to those reported in previous work, indicating the polymerization of TDA LB films [12–15,19]. And TDA monolayer could be polymerized first and then deposited. The second method is that: the monomer molecules were dissolved in chloroform. After spreading, the TDA monolayer was compressed to a designed surface pressure and then in situ polymerized first by UV irradiation. Finally the in situ polymerized TDA monolayer could be deposited onto solid substrate. And these PTDA LB films were named PTDA-H2 O2, PTDA-Cu2 and PTDA-Cd2 according to the subphase solution, respectively.
2.2. LB films deposition All LB deposition experiments were carried out at 20 ◦ C (±0.5 ◦ C) on a computer-controlled Langmuir film balance (600 mm × 150 mm in length and width, 10 mm in depth). The Langmuir film balance was placed in a dust-free box and protected against UV-light with opaque plastic sheets. The substrate for LB deposition was quartz. A 16 W UV lamp (λ = 254 nm) hung 10 cm above the LB films was used for UV-polymerization. The UV–vis absorption spectrums of PTDA LB films were measured after UV-polymerization using a spectrometer (Shimadzu: UV 2100). 2.3. SHG polarized angle dependence measurements PTDA monolayer on quartz substrate was characterized in detail by SHG polarized angle dependence measurements. For SHG measurement, s- or p-polarized fundamental light was focused onto the sample, with an incident angle of 45◦ . The SH light reflected from the sample was filtered by a fundamental cut filter to remove intense fundamental light and was detected by a photomultiplier tube (Hamamatsu photonics: R1463) after passing through a SH-pass filter. A light source (wavelength is 522 nm) was the third harmonic light of a Q-switched Nd-YAG Laser. The signals obtained were averaged by a Boxcar integrator (Stanford Research: SR-250) and proceed by a PC. All measurements were carried out in laboratory atmosphere. 2.4. SHG-CD measurements PTDA LB films on quartz substrate were characterized in detail by SHG-CD measurements. For the SHG-CD measurement, right- and left- circularly polarized fundamental light was focused onto the sample, with an incident angle of 45◦ . The SH light transmitted from the sample was filtered by a fundamental
Fig. 1. UV–visible spectra of polymerized TDA LB films on quartz plate: (a) PTDA-H2 O1, polymerized after deposition from water; (b) PTDA-Cu1, polymerized after deposition from Cu(NO3 )2 subphase; (c) PTDA-Cu2, deposited after polymerization on water surface; and (d) PTDA- H2 O2, deposited after polymerization on Cu(NO3 )2 subphase.
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Fig. 1c and d shows the UV–vis spectra of in situ polymerized TDA LB films deposited from water and Cu(NO3 )2 subphase. The PTDA-H2 O2 and PTDA-Cu2 films both showed absorption peak at 536 nm. 3.2. SHG polarized angle dependence characterization In order to confirm the chirality of the monolayer, we performed polarized angle dependence SHG measurement. In this measurement, the analyzer angle was fixed and the SH signals were measured as changing the input polarized angle [20]. Since the second-order NLO susceptibility χ(2) is a third-rank tensor, we can write the second-order NLO polarization in the form: (2)
Pi = χijk Ej Fk
(1)
where χijk is the second order susceptibility (SOS) tensor of monolayer, and Ej and Fk are external electric fields. While this susceptibility tensor can be expressed as a sum of symmetric and asymmetric part as: Pi =
1 {Sijk (Ej Fk + Ek Fj ) + Aijk (Ej Fk − Ek Fj )} 2
(2)
where Sijk and Aijk are components of SOS tensor related to symmetric and asymmetric parts, respectively. The symmetric part only appears under the SHG process. For the C∞v symmetry, the susceptibility tensors are expressed using contracted notation as ⎛ ⎞ 0 0 0 0 S15 0 ⎜ ⎟ 0 0 S15 0 0⎠ S = ⎝0 (3) S31
S31
S33
0
0
0
if the monolayer is composed of the chiral molecules, the mirror plane disappears and symmetry is reduced to the C∞ . As a result, the susceptibility tensors are expressed as ⎛ ⎞ 0 0 0 S14 S15 0 ⎜ ⎟ 0 0 S15 −S14 0 ⎠ S = ⎝0 (4) S31
S31
S33
0
0
0
here, the constructed components are expressed using χ(2) tensors components as S15 = χxxz + χxzx , S14 = χxyz + χxzy , S31 = χzxx , and S33 = χzzz . Assuming that s-polarized reflected SH is observed, we can obtain the SH intensity as I(2ω)s ∝ |S15 sin γi cos γi − S14 cos γi cos φ| 2
2
(5)
for the monolayer with C∞ symmetry, where γ i represents the input polarized angle, φ represents the incident angle [20]. On the other hand, the SH intensity is expressed as I(2ω)s ∝ |S15 sin γi cos γi |2
(6)
for the monolayer with C∞v symmetry. It is clear from Eq. (6) that the polarized angle dependence of the SH intensity becomes symmetric around γ i = π/4 for the monolayer with C∞v symmetry, whereas the polarized angle dependence of the SH intensity is not symmetry around γ i = π/4 for the monolayer with C∞ symmetry. Thus, symmetry of the monolayer can be determined
Fig. 2. The results for the polarized angle dependence measurement for in situ polymerized TDA monolayer deposited from: (a) water (PTDA-H2 O2); (b) Cu(NO3 )2 subphase (PTDA-Cu2); and (c) from CdCl2 subphase (PTDA-Cd2).
by measuring polarized angle dependence of the SH intensity and this method could be a very sensitive to detect the chirality of the thin films. The result for the polarized angle dependence SHG measurements for in situ polymerized TDA monolayer deposited from water, Cu(NO3 )2 and CdCl2 subphase were shown in Fig. 2. As shown in Fig. 2a and b, a significant difference was actually observed for PTDA-H2 O2 and PTDA-Cu2 monolayer. As a result, the relationship between two tensor components S14 , S15 can be obtained as S14 /S15 ≈ 1/7 for PTDA-H2 O2 monolayer and S14 /S15 ≈ 1/5 for PTDA-Cu2 monolayer, respectively. These results indicated that S14 is nonzero for PTDA-H2 O2 and PTDA-Cu2 monolayer. However, S14 is negligibly small for PTDA-Cd2 monolayer, which can be interpreted from Fig. 2c.
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All above results indicated that PTDA-Cd2 monolayer remains the C∞v symmetry after polymerization. We have repeated this measurement many times, and could always obtain similar experimental results, i.e., the reproducibility of the asymmetrical shape of the polarized angle dependence measurement shown in Fig. 2a and b were quite well. Furthermore, we have done a similar measurement for PTDA-H2 O1 and PTDA-Cu1, the similar reproducibility of the asymmetrical shape of the polarized angle dependence could be obtained, which indicated that PTDA monolayer showed C∞ symmetry under these conditions, whereas PTDA-Cd1 monolayer remains the C∞v symmetry after polymerization. Since the tensor component S14 actually exhibits the character of monolayer chirality [20], it is interesting to note that PTDA-H2 O1 and PTDA-Cu1 monolayer may show chiral although the monomer TDA molecules are achiral; whereas PTDA-Cd1 monolayer showed achiral. However, we must note that achiral surface which is anisotropic in the surface plane may also exhibit such difference effect in the polarized angle dependence SHG measurement. So we used SHG-CD measurement to further confirm the chirality of PTDA LB films. 3.3. SHG-CD characterization SHG-CD spectrum for PTDA LB films on quartz were examined in detail by using a circularly polarized light. The difference between left- and right-circularly SH spectra as a function of wavelength is represented as SHG-CD spectrum. The left circularly polarized light gave a larger SHG spectrum than that for right circularly polarized light. SHG-CD measurement is powerful to probe the presence of the chirality. The intensity of SHG-CD spectrum is defined as ISHG-CD =
(Ilcp − Ircp ) 2(Ilcp − Ircp ) = , Iaverage Ilcp + Ircp
(7)
where subscripts lcp and rcp are the left- and right-circularly polarized light, respectively. In Fig. 3, SHG-CD spectrum of the polymerized TDA LB films deposited from Cu(NO3 )2 subphase (PTDA-Cu1 LB films) showed obvious Cotton effect. Negative and positive Cotton effects were observed at 1004 and 1044 nm, respectively, with a crossover at 1024 nm. This result indicated that chiral PTDA LB films formed although the monomer TDA molecules were achiral. And the SHG-CD spectrum for PTDAH2 O1 and PTDA-Cd1 LB films were also studied, however, no obvious Cotton effects for PTDA-H2 O1 and PTDA-Cd1 LB films could be observed (shown in Figs. 4 and 5), indicating that chiral PTDA films could not form under present conditions. Furthermore, we have done a similar measurement for the in situ polymerized TDA monolayer (PTDA-H2 O2, PTDA-Cu2 and PTDA-Cd2 films); similar results for SHG-CD spectrum could be obtained. Moreover, it could be confirmed that the asymmetrical shape of the polarized angle dependence measurement for PTDA-H2 O2 monolayer came not from the chirality, but from monolayer anisotropic in the surface plane. All above results suggested that chiral PTDA films can form under certain conditions although the monomer TDA molecules are achiral; the metal ions in the subphase solution would play an important role in the formation of the chiral PTDA films.
Fig. 3. (a) SHG spectrum and (b) SHG-CD spectrum for PTDA LB films on quartz plate that were deposited from Cu(NO3 )2 subphase and then polymerized (PTDA-Cu1).
From the results of polarized angle dependence SHG measurement combined with SHG-CD measurement, it is interesting to note that PTDA LB films can form chirality under certain conditions although the monomer TDA molecules are achiral. This should be ascribed to the formation of polydiacetylene chains. It is well known that polymerized TDA is formed through a typical topochemical reaction, the main chains of the polymer are -conjugated backbones consisting of alternating double and triple bonds. The model of the polydiacetylene chains should be a “Wormlike” chain (Porod–Kratky chain [14]) presents a continuous curvature of the chain skeleton shown in Fig. 6. And the polydiacetylene chains could form a helical structure under certain condition and this helical structure could be observed by TEM technology [19]. Thus, this helical structure will cause macroscopic chirality of polymerized TDA in the interfacial films. However, the formation of the conjugated backbone is very sensitive to the environment. The optical properties of the backbone depend sensitively on strain. In particular, rotation about C C bond of the polymer backbone, which changes the planarity of the backbone, is critical. Rotation about this bond necessitates changes in the conformation of the pendant side groups, and thus there exists a sensitive interplay between them. During the polymerization, the extended backbones will form
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Fig. 4. (a) SHG spectrum and (b) SHG-CD spectrum for PTDA LB films on quartz plate that were deposited from water and then polymerized (PTDAH2 O1).
Fig. 5. (a) SHG spectrum and (b) SHG-CD spectrum for PTDA LB films on quartz plate that were deposited from CdCl2 subphase and then polymerized (PTDA-Cd1).
in a strained configuration because of the geometric restriction imposed by the side group arrangement. The subsequent application of heat, stress, or other stimuli leads to side chain fluctuations or reconfigurations and allows the backbone to adopt a more relaxed conformation that involves rotation about the C C bonds, and thus leads to changes in the optical properties. Thus, the arrangement of side group will play an important role in the configuration of polymer backbone. When the arrangement of side group is suitable, every unit will act cooperatively with regular rotation for polymer backbone, the helical structure will form and that will cause a macroscopic chirality. In present system, the introduction of Cu(II) ions will cause the suitable arrangement of side group, and that will cause a macroscopic chirality. However, in the case of water and CdCl2 subphase, macroscopic chirality will not be obtained under present conditions. Amphiphilic diacetylene derivative with a carboxylic group were reported to form salts with metal ions such as Cd(II), etc., whilst coordinating with metal ions such as Cu(II), etc. The tetra-coordinated Cu(II) ions is in a planar square structure, which will cause regular arrangement of side group and the formation of helical structure for the polymer backbone. Thus, it will cause a macroscopic chirality of PTDA LB films. Since the
arrangement of side group will play an important role in the configuration of polymer backbone, applying extra heat, stress, or other stimuli will change the arrangement of side group of the polymer. Thus, it is possible to expect the formation of the chiral PTDA LB films deposited from water, when applied suitable extra heat, stress, or other stimuli during polymerization. Thus, the formation of the chirality could be controlled when applied suitable stimuli during polymerization.
Fig. 6. Schematic representation of the shape of the polydiacetylene chains.
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4. Conclusions Using optical second harmonic generation measurements, PTDA monolayer on quartz substrate were investigated in detail. SHG polarized angle dependence measurement was carried out to reveal the chirality of in situ polymerized TDA monolayer. A significant difference was actually observed for PTDACu2 monolayer, which indicated that PTDA-Cu2 monolayer were with C∞ symmetry. Furthermore, the chirality of PTDA monolayer was studied by the optical second harmonic generation circular dichroism (SHG-CD) measurements. The SHGCD spectrum of PTDA-Cu1 LB films showed obvious Cotton effect, which indicated that the polymerized TDA LB films deposited from Cu(NO3 )2 subphase can form chirality although the monomer TDA molecules are achiral. All above experiments indicated that the chiral PTDA film could be formed when subphase solution contains Cu(II) ions, while the chirality of PTDA film could not be observed when subphase is pure water or contains Cd(II) ions under the same conditions. The explanation based on conformation change has been presented. And the formation of the chiral PTDA films were expect to be controlled. Acknowledgement Support from the Japan Society for the Promotion of Science (P04347) is gratefully acknowledged. References [1] N. Berova, K. Nakanishi, R.W. Woody, Circular Dichroism Principles and Applications, second ed., Whiley-VCH, 2000. [2] J.M. Rib´o, J. Crusats, F. Sagu´es, J. Claret, R. Rubires, Science 292 (2001) 20.
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