Characterization of Langmuir–Blodgett films of N,N-dioctadecyl-p-nitrophenylaniline mixed with arachidic acid

Thin Solid Films 327–329 (1998) 607–611

Characterization of Langmuir–Blodgett films of N,N-dioctadecyl-pnitrophenylaniline mixed with arachidic acid Dong-Myung Shin*, Mi-Kyoung Park, Sung-Taek Lim Department of Chemical Engineering, Hong-Ik University, 72-1, Sangsu-Dong, Mapo-Gu, Seoul 121-791, South Korea

Abstract Optical and spectroscopic properties of Langmuir–Blodgett (LB) films of N,N-dioctadecyl-p-nitrophenylaniline (DONPA) mixed with cadmium arachidate (CdA) were measured to characterize the orientation of molecular and structural effect on the second harmonic generation (SHG) intensity of the films. The LB films were fabricated with various molar ratios of DONPA and CdA to investigate the orientation effect of DONPA on CdA in the mixed LB films. The molecular orientations of molecules in LB films were examined using the polarized transmission and reflection–absorption FT-IR and the UV-visible absorption technique. The SHG intensities of LB films were measured. Chromophores of DONPA formed the J-aggregates. DONPA (in a 1:1 mixture of DONPA:CdA) LB film was oriented with a polar angle of 61 ± 3° from the surface normal, and has azimuthal orientation which is parallel to the dipping direction.  1998 Elsevier Science S.A. All rights reserved Keywords: Langmuir–Blodgett films; Second harmonic generation; Fourier transform infrared spectroscopy (FT-IR)

1. Introduction The Langmuir–Blodgett (LB) technique offers an efficient method for preparing well-ordered organic ultra-thin films on suitable substrates. In recent years, the non-linear optical (NLO) properties of LB films have received much attention. Suitable derivatives must retain the optical characteristics of the parent NLO molecules but yield ordered, homogeneous, non-centrosymmetric LB films [1]. Organic materials that have a nitroaniline group can be applied to non-linear optics, such as second harmonic generation (SHG), wave-guide, etc. Nitroaniline derivatives do not form stable monolayer films at the air–water interface by themselves [1,2]. However, nitroaniline derivatives mixed with fatty acid form a stable monolayer at the air–water interface and generate homogeneous multilayers on a solid substrate. In many cases, for the formation of more ordered LB films, divalent cations such as Cd2 + and Mn2 + were added to the subphase [3]. There have been many investigations into the effect of fatty acid salts on the quality of LB films [4,5]. To investigate molecular orientation in mixed LB films, the transmission and reflection–absorption FT-IR technique * Corresponding author.

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has recently been used. The orientation can be obtained by measuring the spectrum of a film in both the transmission and the reflection modes, and then comparing the intensities of the absorption bands of key functional groups [6]. In this work, we synthesized N,N-dioctadecyl-p-nitrophenylaniline (DONPA) for non-linear optical material. We investigated SHG from non-centrosymmtric LB films of DONPA mixed with cadmium arachidate. The transmission and reflection–absorption FT-IR technique for LB films was carried out to investigate an influence of the cadmium ion and the molecular orientation of DONPA in mixed LB films (Table 1).

2. Experimental details N,N-dioctadecyl-p-nitrophenylaniline (DONPA) was synthesized with p-nitroaniline (Aldrich 99%) and 1-Bromooctadecane and was purified by recrystallization from methanol. Arachidic acid (Aldrich 99%) and cadmium chloride (Aldrich 99%) were used with no further purification. Fig. 1 shows the molecular structure of DONPA. The p-A isotherm and deposition of non-centrosymmetric (Z-type) films were performed using LB trough

 1998 Elsevier Science S.A. All rights reserved

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Table 1 Band assignments of DONPA mixed CdA LB films Transmission n (cm − 1)

Reflection n (cm − 1)

Assignment

2917 2849 1596 1552 – 1469 – 1301 1198 1116

2918 2849 1596 – 1487 1468 1434 1303 1199 1116

nas(CH2) ns(CH2) n(benzene) nas(COO-) nas(NO2) d(CH2) ns(COO-) ns(NO2) n(C-N) n(CR-N)

n, stretching vibrational mode; d, deformation mode; a, asymmetric mode; s, symmetric mode.

with a wilhelmy plate (NIMA 611). The DONPA and AA of the various molar ratio (2:1, 1:1 and 1:2 mixtures) were spread from a 1 × 10 − 3 M solution in chloroform on ultrapure water subphase containing 2.0 × 10 − 4 M CdCl2. The water used as the subphase was purified with a Mili-Q purification system (18.3 MW-cm, pH 5.6). The seven layers of LB films were transferred onto substrate at a constant pressure of 21 mN/m. Either silicon wafer, quartz or aluminum was used as substrate for the spectroscopic measurement. The SHG of DONPA in LB films was measured using a Q-switched Nd:YAG laser operating at 1064 nm with a 10ns pulse width. The p- and s-polarized harmonic signals were detected in transmission as a function of angle of incidence. The UV-visible absorption spectra were measured with the diode array type spectrophotometer (Hewlett–Packard, HP8452A). The refractive index, extinction coefficient and thickness of LB films deposited on an Si wafer were obtained from ellipsometer with a He-Ne laser operating at 632.8 nm wavelength (Plasmos, SD2100). The infrared spectra were obtained from the Nicolet Magna IR 560 FT-IR spectrophotometer equipped with a DTGS-KBr detector with resolution of 4 cm − 1. For the reflection–absorption measurement, the Harrick Model versatile reflection attachment (VRA) was used at the angle of incidence of 82°, together with a double-diamond polarizer.

Fig. 2. Pressure area isotherm of mixtures of DONPA and CdA.

DONPA:CdA are 3.4 nm2 and 2.8 nm2, respectively. The gradual decrease in limiting area is due to the smaller area per molecule of cadmium arachidate than that of DONPA itself [7]. We expected a larger area per molecule for 2:1 DONPA: CdA. However, the area per molecule of 2:1 DONPA:CdA was smaller than that of 1:2 DONPA:CdA, and the shape of p-A isotherms of the 2:1 DONPA:CdA was very similar in shape to that of pure CdA. The small surface area may result from premature aggregation of DONPA at the surface pressure of 0 mN/m. The premature aggregation of DONPA was observed when pure DONPA was spread onto water subphase. The aggregates are easily observed as an isolated island which has clear contrast to the water subphase (S.T. Lim, M.K. Park, O. Kwon, D.M. Shin, unpublished data). When the preformed aggregate was formed, the CdA mainly affected the surface pressure. Fig. 3 shows the UV-visible absorption spectrum of the 1:1 mixture DONPA:CdA that layered on a fused quartz plate and that of DONPA in CHCl3 solution. The absorption maximum of DONPA obtained from CHCl3 was at 400 nm and 445 nm, which represented Jaggregate absorption for LB film [8]. The J-aggregate formation in LB films is possible when molecules form aggregates whose molecular plane is parallel to the substrate

3. Results and discussion Fig. 2 shows a pressure–area isotherm of DONPA mixed with CdA with the molar ratios of 2:1, 1:1 and 1:2. The area per molecule at 21 mN/m of 1:1 DONPA:CdA and 1:2

Fig. 1. Molecular structure of DONPA.

Fig. 3. UV-visible absorption spectrum of DONPA in chloroform solution and the mixed LB film.

D.-M. Shin et al. / Thin Solid Films 327–329 (1998) 607–611

[9,10]. The absorption spectra of LB films of 2:1 and 1:2 DONPA: CdA showed the same peaks as that of 1:1 DONPA:CdA LB film. The measurement of transmission and reflection–absorption (RA) FT-IR spectrum was carried out to investigate the molecular orientation of DONPA and AA in the mixed LB films. The z-type LB film of pure CdA was deposited on substrate to compare with the spectrum of mixed films. The LB films with seven layers were deposited onto Si wafers for the transmission spectrum, and onto Al film that was coated onto glass slides by evaporation for the RA spectrum. Fig. 4 displays the FT-IR spectra of LB films of CdA and the 1:1 mixture of DONPA and CdA. In the case of pure CdA LB film, asymmetric (2917 cm − 1) and symmetric (2849 cm − 1) CH2 stretching bands were observed in the transmission spectra. The absorption intensity was stronger for pure CdA with transmission mode than those obtained with RA mode. Such a polarization dependence is known to occur when the fully-extended hydrocarbon tails are oriented close to the normal of the substrate surface [6]. In the fingerprint region of IR spectrum, an asymmetric COO − stretching band (1545 cm − 1) appears strongly in the transmission spectrum and a symmetric COO − stretching band (1430 cm − 1) strongly appears in the RA spectrum. This indicates that the head group of CdA, carboxylate group is also oriented close to the surface normal. A number of methods have been proposed to obtain more quantitative evaluation of molecular orientation of LB films. Umemura and colleagues [11] obtained the molecular orientation based on an analysis of the dichroic ratio, which was calculated from measurements of the transmission and the RA absorbance. The ratio of the transmission absorbance, AT, to the RA absorbance, AR, is related to the orientation angle, f, that a transition dipole moment makes with the surface normal in the following form 2

AT tan f = AR 2mz

(1)

The parameter mz is the intensity-enhancement factor that arises due to the metal surface use in the RA measurement, and depends on five parameters: the frequency of the absorption band (n), the angle of incidence (v), the film thickness (h) and the refractive indexes of LB film and substrate (n2 and n3). The appropriate values of mz can be determined using the approach outlined in Umemura’s paper. The tilt angle, g, of the hydrocarbon chain axis from the surface normal, can be evaluated by the orthogonal relation among a, b and g cos2 a + cos2 b + cos2 g = 1

(2)

In this equation, a and b are the angles between the surface normal and the transition moments of the asymmetric and the symmetric CH2 stretching, respectively. In our case, the thickness and refractive index of pure CdA LB film were h = 22 nm and n2 = 1.47, respectively. The refractive indices of Si wafer and aluminum were

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Fig. 4. Transmission (T) and reflection (R) FT-IR spectra of LB films of pure CdA and 1:1 DONPA:CdA.

n3 = 3.429 and = 5.24 + 34.80i, respectively [11]. Using these equations, we find that the transition dipole moments of the asymmetric and symmetric CH2 vibrations orient at angles of 83° and 83.5° to the surface normal, respectively, and CdA chains tilt with an angle of 9° from the surface normal in LB film. All of these values compare favorably with the results of Umemura et al., who reported a 7° tilt angle for the hydrocarbon chain axis of cadmium stearate LB film [11]. Similar values have also been obtained by other researchers [5,6]. The molecular orientation of the 1:1 mixture of DONPA:CdA in LB film was investigated. In contrast with pure CdA, both CH2 stretching bands in the RA spectrum exhibited stronger intensities than those in the transmission spectrum. This indicated that hydrocarbon tails in mixed LB film were oriented at a wide angle from the surface normal, and were calculated to be oriented at an angle of 29° from the surface normal. The tilt angle is an average value of the hydrocarbon tail of DONPA and CdA in the mixed film. In the fingerprint region of the IR spectrum, the vibration bands mainly associated with the head groups of DONPA can give information about head group orientation. The NO2

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symmetric stretching, CyC (in benzene) stretching, C–N stretching, and CR –N stretching bands appear to be stronger in the RA spectrum. The transition dipole moment of these stretching bands was found to be oriented at angle of 61 ± 3° from the surface normal. The orientation was calculated with h = 31 nm and n2 = 1.52. It is interesting to observe that the NO2 asymmetric stretching band only appears in the RA spectrum. This indicates that the molecular plane of -NO2 is perpendicular to the substrate. The orientation of the CdA head group in mixed LB film is equal to that of pure CdA LB film. Both the 61 ± 3° orientation of long molecular axis and the perpendicular orientation of short molecular axis of DONPA prove the J-aggregation model in Fig. 5. The thickness of one layer of 1:1 DONPA:CdA LB film calculated from the tilt angle was about 2.2 nm, but the experimental value was 4.4 nm per layer. This result suggests that a bilayer was transferred onto substrate from the air–water interface. The FT-IR spectrum of 1:2 DONPA:CdA LB film was the same as that of 1:1 DONPA:CdA LB film. The analysis of the FT-IR spectrum for 2:1 DONPA:CdA LB film proves that the hydrocarbon chain and head group had a larger tilt angle to the surface normal, 31° and 77 ± 2°, respectively. The angle difference can be explained by the tetrahedral structure of nitrogen. The SHG experiments were performed with z-type 7layer LB films of 2:1 1:1, and 1:2 DONPA:CdA. Fig. 6 shows fringing patterns in which the polarized incidence light parallel (filled circle) or perpendicular (open circle) to the dipping direction. The fringes are convex and asymmetric for 1:1 DONPA:CdA LB film. The SH intensity is much larger for a polarized light parallel to the dipping direction than that obtained with perpendicular direction. These indicate that the long molecular axis of DONPA orients with a wide angle from the surface normal, and with a small angle from the dipping direction in the surface plane, while the orientation of the short axis of DONPA retains a perpendicular direction from the substrate plane. The molecular orientations are consistent with those obtained with the transmission and RA FT-IR technique; the long axis of the head group oriented with an angle of 61 ± 3° from the surface normal. On the other hand, in the fringing pat-

Fig. 6. SH intensities as a function of incident angle for z-type 7-layer LB films. The polarized incidence light parallel (filled circle) or perpendicular (open circle) to the dipping direction. (a) 1:1 DONPA:CdA (b) 2:1 DONPA:CdA (c) 1:2 DONPA:CdA.

Fig. 5. The orientation of DONPA in mixed LB film.

terns obtained from DONPA:CdA (mole ratio of 2:1, 1:2) LB film, the SH intensities generated with the parallelpolarization light to the dipping direction are smaller than that obtained with perpendicularly-polarized light, and

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changes little as a function of incidence angle. The low SH intensity obtained with parallel polarization light indicates that DONPA orients with a wide angle from the dipping direction in the surface plane when DONPA is solubilized in 2:1 and 1:2 DONPA:CdA LB films. The SH intensity obtained from the 1:1 mixture DONPA:CdA LB film is much larger than those obtained from LB films of 2:1 and 1:2 DONPA:CdA. This result supports the ordered orientation of 1:1 DONPA:CdA in LB film than the other mixture.

4. Conclusions Optical and spectroscopic properties of LB films of DONPA mixed with CdA were studied. Chromophores of DONPA formed the J-aggregates. The head group of DONPA in 1:1 mixture DONPA:CdA LB film was found to be oriented with an angle of 61 ± 3° from the surface normal with calculations of the polarized transmission and RA FT-IR absorption intensity difference. The fringing patterns obtained with the polarized incidence light parallel or perpendicular to the dipping direction showed that the long molecular axis of DONPA in 1:1 DONPA:CdA LB films was oriented with a wide angle from the surface normal, and a parallel orientation to the dipping direction. The DONPA in 2:1 and 1:2 DONPA:CdA LB films orients with a wide angle from the dipping direction in the surface plane. These results indicate that the DONPA organized better when it was mixed with CdA in 1:1 mixture LB film.

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Acknowledgements This research was supported by KOSEF (grant number: 951-0301-038-2). The authors would like to thank Dr. Y.H. Min and Professor C.S. Youn in KAIST for supporting the SHG measurements.

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