Langmuir 1997, 13, 3187-3190
3187
Photoinduced Molecular Reorientation in Optical Nonlinear Langmuir-Blodgett Films Jianhua Xu, Xingze Lu,* Kui Han, Guangpeng Zhou, and Zhiming Zhang State Key Joint Laboratory for Materials Modification by Laser, Ion and Electron Beams, Department of Physics, Fudan University, Shanghai 200433, China Received September 17, 1996X
Photoinduced optical anisotropy due to molecular reorientation in hemicyanine Langmuir-Blodgett multilayers has been investigated by polarized UV-visible absorption and rotation-angle second harmonic generation measurements. Under polarized UV nanosecond laser illumination, the hemicyanine molecules reorient and align their chromophore axes along the direction of the UV beam polarization. Annealing of the films leads to randomization of the molecules and possibly dissociation of aggregate species.
1. Introduction Langmuir-Blodgett (LB) films have been receiving considerable attention because of the scientific and technological significance.1-3 Amphiphilic hemicyanine dyes are promising organic optical nonlinear materials because of their high hyperpolarizability β (10-28-10-27 esu or 3.7 × 10-49 to 3.7 × 10-48 C m3 V-2 in SI units; 1 C m3 V-2 ) 27 × 1019 esu) and capability of forming stable LB multilayers. For a given type of molecule with specific functions, the macroscopic physical and chemical properties of LB films are determined by molecular assembly. Thus, molecular orientation, alignment, and their control in LB films are significant in their applications to microelectronics and photonics. Molecular alignment along the flow direction of Langmuir films at the air-water interface during deposition by the conventional vertical dipping method has potentially wide applications in liquid crystal aligning agents4 and waveguide frequency doublers,5 which require a high degree of azimuthal alignment and effective control of molecular reorientation based on better understanding of the mechanisms involved. One could control the inplane orientation of LB films by selection of deposition parameters such as dipping speed6 or by application of shearing by a rotating disk.7 In this paper, the effect of polarized nanosecond UVlaser illumination and thermal annealing on the in-plane molecular orientation in Y-type interleaving multilayer samples of a hemicyanine dye and inert material arachidic acid was investigated by optical linear (polarized UVvisible absorption) and nonlinear (rotation-angle second harmonic generation, SHG) techniques. The phenomenon would provide a possible technique to fabricate macroscopically polar, stable LB films for various purposes and would provide potential applications in optical recording and information storage. X
Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Tredgold, R. H. Rep. Prog. Phys. 1987, 50, 1609. (2) Khanarian, G. Thin Solid Films 1987, 152, 265. (3) Prasad, P. N. Thin Solid Films 1987, 152, 275. (4) Zhu, Y. M.; Lu, Z. H.; Jia, X. B. Phys. Rev. Lett. 1994, 72, 2573. (5) Bosshard, C.; Florsheimer, M.; Kupfer, M.; Gunter, P. Opt. Commun. 1991, 85, 247. (6) Lu, Xingze; Han, Kui; Ma, Shihong; Zhang, Zhiming J. Phys. D.: Appl. Phys. 1996, 29, 1576. (7) Mingotaud, C.; Agricole, B.; Jego, C. J. Phys. Chem. 1995, 99, 17068.
S0743-7463(96)00904-3 CCC: $14.00
Figure 1. Molecular structure of the hemicyanine dye (DAEP).
2. Experiment 2.1. Material Synthesis. The hemicyanine dye E-N-docosyl4-[2-(4-diethylaminophenyl)ethenyl]pyridinium bromide (abbreviated as DAEP) was synthesized with a procedure similar to that of Girling et al.;8 4-picoline and 1-bromodocosane were reacted to give docosylpicolinium bromide. The latter was then reacted with N-diethylaminobenzaldehyde in the presence of piperidine to give DAEP. Its molecular structure is shown in Figure 1. Spectroscopic-grade arachidic acid C19H39COOH (abbreviated as AA) was purchased directly from Shanghai No. 1 Reagent Plant and used without further purification. 2.2. Sample Preparation. DAEP and AA were spread from 10-3 mol L-1 (0.64 mg mL-1 and 0.31 mg mL-1, respectively) chloroform solutions onto an aqueous subphase in two separate compartments of a KSV5000 Langmuir trough. The subphase was deionized, doubly distilled water at 20 °C with CdCl2 of 3 × 10-4 mol L-1 and a pH value of 5.8-6.0. The surface pressurearea (π-A) isotherms were recorded at a compression rate of 3 mm min-1. Y-type interleaving multilayers of DAEP/AA were deposited on hydrophilically treated quartz plates of a dimension 30 mm × 18 mm × 2 mm at a constant pressure of 30 mN m-1. The DAEP layers were deposited during upstrokes with a dipping speed of 3 mm min-1 and AA during downstrokes with a dipping speed of 2 mm min-1. Insertion of the AA layers not only ensured the noncentrosymmetric structure of the hemicyanine multilayers, which is a prerequisite for nonvanishing macroscopic second-order optical nonlinearity, but also improved the degree of order and stability of the films.9 We prepared four Y-type DAEP/AA interleaving samples A, B, C, and D of 24 bilayers (for sufficient signal-to-noise ratio in optical measurements) under identical conditions. The transfer ratio during deposition of all samples could be kept at 1 ( 0.05. The LB multilayers on one side of all samples were removed prior to optical characterizations. 2.3. Polarized Absorption Measurement. Polarized UVvisible absorption spectra of the samples were recorded on a Shimadzu UV-365 spectrophotometer using a bare quartz plate as a reference. The incident light was normal to the sample surface and linearly polarized by insertion of a polarizer. 2.4. Rotation-Angle SHG Measurement. The experimental setup used for SHG measurements is shown in Figure 2. The fundamental beam of 50 ps pulse width, 10 Hz repetition rate, 1 mJ pulse-1 at 1.064 µm from an active mode-locked Nd:YAG (8) Girling, I. R.; Cade, N. A.; Kolinsky, P. V.; Earls, J. D.; Cross, G. H.; Peterson, I. R. Thin Solid Films 1985, 132, 101. (9) Liu, Liying; Zheng, Jiabiao; Wang, Wencheng Opt. Commun. 1992, 93, 207.
© 1997 American Chemical Society
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Figure 2. Experimental geometry for the rotation-angle SHG measurement.
Figure 4. Angular patterns of the p-in/p-out SHG intensity: (a) sample A, fresh; (b) sample B with parallel illumination; (c) sample C with perpendicular illumination; (d) sample D, annealed.
Figure 3. Polarized absorption spectra: (a) sample A, fresh; (b) sample B with parallel illumination; (c) sample C with perpendicular illumination; (d) sample D, annealed. laser was directed onto the samples at a fixed incident angle of 45° through a long-pass filter F1. The SHG signal at 532 nm was detected in transmission by a photomultiplier tube (PMT) cooled to -20 °C and a boxcar averager, the output of which was displayed on an xy recorder. The SHG intensities from the samples were ratioed to those from a z-cut quartz plate reference to eliminate the measurement errors caused by laser power fluctuations. An infrared-blocking filter and a 532 nm interference filter F2 were inserted to ensure that only the second harmonic radiation was detected. The dependence of SHG intensity on the azimuthal angle φ between the dipping direction on the sample and the y-axis (in the incident plane) was measured by rotating the samples around the surface normal (z-axis), and φ ) 0° corresponds to a geometry with the dipping direction in the plane of incidence. Special attention was paid to ensure that the same area was measured when the sample was rotated. 2.5. Illumination and Annealing. UV light illumination of the samples at normal incidence was carried out by using the tripled (355 nm) output of a Q-switched Nd:YAG laser with 10 ns pulse width, 10 Hz repetition rate, and 1 mJ/pulse energy. An 8 mm diameter spot was directed onto the sample after passing through a Glan-type polarizer. The peak power density was estimated at 5 × 108 W m-2 and the electric field was 6 × 105 V m-1 approximately. Samples B and C were illuminated for 30 min with their dipping direction parallel or perpendicular to the UV beam polarization, respectively. After this treatment, SHG and absorption measurements were performed on samples B and C with the measuring spot being completely overlapped by the UV illuminated area. Annealing of sample D was performed in a temperaturecontrolled heater. The film was heated to 60 °C and kept there for 20 min and was then freely cooled to room temperature.
3. Results and Discussion 3.1. Dipping-Induced Orientation. Parts a-d of Figrue 3 show the polarized absorption spectra of the samples A, B, C, and D, respectively, where the broken
and solid lines represent the absorbance with the incident electric field parallel and perpendicular to the dipping direction on the samples, respectively. Since the quartz substrate and AA are transparent in the spectral range 350-650 nm, DAEP layers were the only contributor to the observed absorption spectra. The linear anisotropy parameter (dichroic ratio) was defined as rL t A|/A⊥, where A| and A⊥ were the areas below the respective absorption curves. For the fresh sample A, Figure 3a gives rL ) 1.6 ( 0.1 > 1, demonstrating that the transition dipole moment of the DAEP chromophores in the sample was preferentially oriented in the line of the dipping direction (φ ) 0° and 180°). In order to obtain not only the line of molecular orientation but also information on the sense of chromophore direction (φ ) 0° or 180°) and azimuthal symmetry, which could not be extracted from the polarized absorption spectra, we performed rotation-angle SHG measurements. The resultant angular patterns of the p-in/p-out SHG intensity from the four samples are shown in parts a-d of Figure 4, respectively. The radial length between the center and a data point represents intensity in that direction. The data were reproducible. Since the hyperpolarizability β of AA was at least 2 orders of magnitude smaller than that of DAEP, those patterns directly reflected the in-plane symmetry of DAEP molecular arrangement in the interleaving multilayer samples. The angular pattern I(φ) of the fresh sample A in Figure 4a shows roughly an asymmetric “oval” with a maximum at φ ) 0° and a mirror symmetry about the dipping line, which could be explained by using a classical nonlinear oscillator model.6 The DAEP chromophore axes were not only parallel to each other but also had the same sense of direction as the dipping, leading to a nonvanishing in-plane polarization. The inversion asymmetry [I(φ ) 0°) > I(φ ) 180°)] observed in Figure 4a was a result of different superposition configurations (constructive or destructive) between the in-plane and normal polarization components, as shown by eq 3 and the insert of Figure 2 in ref 6. We also defined the nonlinear anisotropy ratio rNL t [I(0°) + I(180°)]/(I(90°) + I(270°)] and measured rNL ) 1.8 ( 0.1 from Figure 4a for the fresh sample A. This value
Molecular Reorientation
is fairly close to the linear result (rL ) 1.6), which verified the agreement between two characterizations, as we expected. 3.2. Photoinduced Molecular Reorientation. The polarized absorption spectra given in Figure 3b for sample B after being illuminated by the UV nanosecond laser beam polarized in the dipping direction show a larger degree of anisotropy (rL ) 2.0) compared with that of the fresh sample A (rL ) 1.6) given in Figure 3a. The SHG intensity pattern of sample B also displays an enhanced alignment along the dipping direction (rNL ) 2.4), as shown in Figure 4b. An increase in the relative magnitudes of I(0°) and I(180°), a decrease in that of I(90°) and I(270°), and a reduced inversion asymmetry confirmed a partial molecular reorientation from the perpendicular (to the dipping line) to the parallel line (dipping line or illuminating field polarization line), in both senses of direction. In another words, the molecules tend to rotate their chromophore long axes toward the field polarization direction. The UV pulses induced an additional electric dipole, which interacted with the optical field and created a torque leading to reorientation of the DAEP molecular axes along the field polarization direction against the thermal randomization10 to minimize the interaction energy -p‚E between the induced dipole p and the external field E. This mechanism also played an important role in the optical Kerr effect.11 When the illuminating UV beam is polarized perpendicular to the dipping direction on the sample, the DAEP molecular axes should switch from the dipping direction to the field polarization direction (perpendicular to the dipping direction). This was also proved by our linear and nonlinear data. In Figure 3c, the polarized absorption spectra of the illuminated sample C gives rL ) 0.8 < 1, representing an anisotropy in the opposite sense, i.e., the long axis of the optical absorption lies in the direction perpendicular to the dipping direction. Figure 4c further displays an oval SHG pattern of sample B elongated in the directions of φ ) 90° and 270° and gives rNL ) 0.8, identical to the rL value of the same sample. Our result is different from the recent work by Aktsipetrov et al.,12 which concluded that molecules in LB monolayers would reorient their chromophore axes toward a direction perpendicular to the field polarization because of an increase of interaction of photoexcited molecules under illumination of a visible polarized CW radiation (1.5 × 103 W m-2, much weaker than ours). We attributed our observed molecular reorientation to the dipole interaction between hemicyanine molecules and the strong external field (6 × 105 V m-1 or peak power density of 5 × 108 W m-2). This was further verified by the fact that such an effect was insensitive to the wavelength of the external field in our recent work (unpublished). In LB films, since amphiphilic molecules are essentially linked together by van der Waals interaction instead of strong chemical bonds, molecular reorientation could be realized more easily than in most solid films. For instance, the molecular tilt angle increased from 33° to 36° in hemicyanine LB films when the temperature was raised from 30 to 70 °C.13 On the other hand, the anisotropic features in the reoriented samples B and C were found to (10) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984; p 196. (11) Mayer, G.; Gires, F. Compt. End. Acad. Sci. (Paris) 1964, 258, 2039. (12) Aktsipetrov, O. A.; Mishina, E. D.; Murzina, T. V.; Akhmediev, N. N.; Novak, V. R. Thin Solid Films 1995, 256, 176. (13) Kajikawa, K.; Shirota, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1990, 29, 913.
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remain at least a few months at room temperature, which shows that such alignment and reorientation are stable against thermal relaxation. From the effective control of optical anisotropy in our samples by 10 ns pulses, we suggest that the response time of molecular reorientation in LB films should not be much longer than 10 ns. The relatively fast switching character and reversibility12 of this phenomenon offers a potential application background in optical recording and information storage. A significant reduction in the average SHG intensity from samples B and C after UV illumination was clearly observed in parts b and c of Figures 4 compared to Figure 4a. Hemicyanine molecules form H-aggregates in LB films, leading to a relatively sharp absorption band at the high-energy side of the monomer band13,14 or an absorption peak blue shift leading to SHG intensity reduction due to the β off-resonant effect in hemicyanine LB films.15 There was also evidence for an increase in the degree of aggregation in LB films upon UV illumination.16 Thus, the slight blue shift (5 nm) observed in the absorption spectra in parts b and c of Figure 3 compared with Figure 3a due to an increase in the degree of the H-aggregate under UV illumination would cause a reduction in SHG intensity. On the other hand, possible photodegradation or photobleaching would also reduce the SHG intensity. However, this effect should not play a dominant role because of the polarization-switching behavior observed in parts b and c of Figure 4. 3.3. Thermal Randomization. The polarized absorption spectra given in Figure 3d for the annealed sample D displayed much smaller anisotropy (rL ) 1.1) compared with that of the fresh sample A given in Figure 3a because the stronger thermal motions made the in-plane molecular orientation tend toward a random distribution at higher temperatures. This is also verified by the nearly isotropic SHG pattern shown in Figure 4d with rNL ≈ 1.0. By performing the reflected SHG, absorption, and smallangle X-ray diffraction experiments, Kajikawa et al.13 confirmed that the hemicyanine and arachidic acid molecules in LB films would not have any drastic structural changes when the temperature was raised to 70 °C. Thus, the annealing process would not destroy our sample, which was also verified by the nearly identical color, appearance, and absorption band shape of the sample before and after annealing. Notice that the absorption peak in Figure 3d (485 nm) was considerably red-shifted with respect to that of the fresh sample A in Figure 3a (475 nm), which demonstrated possible dissociation of the H-aggregate in hemicyanine LB multilayers. The average SHG intensity reduction by a factor of 2 on annealing was observed from a comparison of parts a and d of Figure 4. First, an azimuthal randomization of the molecular chromophore axes on annealing would make the SHG component originating from the in-plane polarization vanish.17 Second, the structural deterioration such as partial disorder of the normal component of molecular chromophores on annealing also reduce the SHG intensity. Third, an enhancement of the SHG intensity could also happen when the temperature was raised and Haggregated hemicyanine molecules were dissociated.13,15 The first two effects should overtake the third one, leading to the observed SHG reduction on annealing. (14) Carpenter, M. A.; Willand, C. S.; Penner, T. L. J. Phys. Chem. 1992, 96, 2801. (15) Han, K.; Ma, S. H.; Lu, X. Z. Opt. Commun. 1995, 118, 74. (16) Unuma, Y.; Miyata, A. Thin Solid Films 1989, 179, 497. (17) Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30, L1525.
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4. Conclusions Photoinduced optical anisotropy in Y-type interleaving multilayers of DAEP/AA was characterized by linear and nonlinear spectroscopies quantitatively and consistently. A polarized UV nanosecond beam could be used to control molecular reorientation effectively by aligning the chromophore axes along the beam polarization direction.
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Thermal annealing would substantially reduce the anisotropy and the average SHG intensity. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant Number 19434011 and by the National Postdoctoral Foundation of China. LA960904R
