J. Phys. Chem. 1996, 100, 16629-16632
16629
Structural and Optical Nonlinear Characterizations of Langmuir-Blodgett Films of 1-Benzyl-9-hydrofullerene-60 Shihong Ma,*,† Xingze Lu,† Jian Chen,‡ Kui Han,† Liying Liu,† Zuen Huang,‡ Ruifang Cai,‡ Gongming Wang,† Wencheng Wang,† and Yufen Li† State Key Joint Laboratory for Materials Modification by Laser, Ion and Electron Beams, Department of Physics, Fudan UniVersity, Shanghai 200433, China, and Department of Chemistry, Fudan UniVersity, Shanghai 200433, China ReceiVed: March 26, 1996; In Final Form: June 4, 1996X
The structural and optical nonlinear features of the condensed layers at the air-water interface and LangmuirBlodgett (LB) films of the substituted fullerene 1-benzyl-9-hydrofullerene-60 (C60-Be) were investigated by π-A isotherm, small angle X-ray diffraction (SAXD), and optical measurements. Pure C60-Be molecules existed at the air-water interface in a bulk phase. A new type of two-chain amphiphilic molecule, 1,10bistearyl-4,6,13,15-tetraene-18-nitrogen-crown-6 (NC), was used as an inert material to construct mixed C60Be/NC LB films of much better quality than the pure C60-Be LB films. Our π-A, UV-visible absorption, and SAXD measurements show that the structural improvement in the mixed C60-Be/NC LB films was realized by insertion of the C60-Be molecules between the two hydrophobic chains of the NC molecules. The relatively large third-order nonlinear susceptibility χ(3)xxxx(-3ω;ω,ω,ω) ) 2.1 × 10-11 esu was deduced by measuring third-harmonic generation in the mixed C60-Be/NC LB films.
1. Introduction In recent years macroscopic quantities of fullerenes (C60) have been successfully synthesized from carbon soots.1 This type of highly symmetric compound has been attracting intense interest from the viewpoint of their physical, chemical, and electronic properties. Moreover, the observation of superconducting transitions in alkali-metal-doped fullerenes has accelerated investigations in this field.2,3 It was reported that LangmuirBlodgett (LB) multilayers of C60 doped with alkali metals exhibited superconductivity below 8.1 K.4 Also, C60 and its derivatives could have high optical nonlinearity and quick optical response due to their large-scale conjugated π-electron system. However, the second-order susceptibility would vanish because of the inversion symmetry of the C60 molecules. The LB technique offers a simple and attractive alternative for formation of organic thin films whose structures can be controlled at the molecular level. Earlier work concerning the deposition of pure C60 LB films has revealed great difficulties in the preparation of uniform C60 multilayers owing to their lack of amphiphilic features and the initial inhomogeneity in the Langmuir films at the air-water interface. In order to improve the LB film quality, mixtures of C60 and long-chain fatty acids or other amphiphilic spacer molecules have been used to construct C60-containing LB monolayers and multilayers.5-10 In this paper, we report the deposition parameters and optical characterizations of LB films of a substituted fullerene compound, 1-benzyl-9-hydrofullerene-60 (C60-Be), and its mixtures with an amphiphilic inert molecule, 1,10-bistearyl-4,6,13,15tetraene-18-nitrogen-crown-6 (NC). 2. Experimental Details The chemical structures, names, and acronyms of the substituted fullerene compound (C60-Be) and inert molecule NC †
Department of Physics. Department of Chemistry. X Abstract published in AdVance ACS Abstracts, September 1, 1996. ‡
S0022-3654(96)00911-2 CCC: $12.00
used in this work are shown in Figure 1. The C60-Be with a purity of 99.5% was synthesized11 by the chemistry department of our university, and NC with a purity of 99.8% was obtained from Lanzhou Institute of Chemical Physics, Academia Sinica. According to flash chromatography, C60-Be contains only trace amounts of C60 (less than 0.4%). No multiadducts C60(CH2Ph)n (n g 2) were found in the reacting products on the basis of FAB-MS and 1H NMR spectra. The C60-Be powder was dissolved in benzene at concentrations of 0.12 g L-1 (1.48 × 10-4 mol L-1, solution A) and 0.25 g L-1 (3.08 × 10-4 mol L-1, solution B), respectively. The powder could be completely dissolved in the solvent. Surface pressure-area (π-A) isotherm measurements and LB film deposition were all carried out on a KSV 5000 two-compartment Langmuir trough made in Finland. The accuracy for our surface pressure measurement (by a Wilhelmy plate) was about 0.05 mN/m. The surface area of each compartment could be changed in the range of 157-707 cm2. The fullerene derivative (C60Be) and its mixtures with NC dissolved in benzene were spread onto deionized, doubly distilled water at 22 °C. After 50 min of solvent evaporation time, the floating film was then compressed at a rate of 5 × 10-3 nm2 molecule-1 min-1 and transferred to substrates at a constant pressure of 15 mN m-1 (C60-Be) or 20 mN m-1 (NC) using the conventional vertical dipping method. The substrates (glass, silicon, and quartz) were treated to obtain hydrophilic surfaces. UV-visible absorption spectra of the LB films (or solutions) were recorded on a Shimadzu UV-365 spectrophotometer using a bare fused quartz plate as a reference. Small angle X-ray diffraction (SAXD) measurements were performed on a Rigaku D/max-RB X-ray diffractometer using the KR radiation of Cu. When the samples were deposited for SAXD measurements, 10-4 mol L-1 CdCl2 was added in the subphase to form scattering centers (Cd2+ ions) in LB multilayers for promotion of SAXD intensity. The setup for measurements of transmitted second-harmonic generation (SHG) and third-harmonic generation (THG) is shown in Figure 2. An IR beam (mode-locked Nd:YAG, 1.064 © 1996 American Chemical Society
16630 J. Phys. Chem., Vol. 100, No. 41, 1996
Ma et al.
Figure 3. Compressional isotherms of C60-Be at different spread quantities: (a) 300 µL of solution A (0.12 g L-1); (b) 200 µL of solution B (0.25 g L-1); (c) 300 µL of solution B (0.25 g L-1); (d) 400 µL of solution B (0.25 g L-1); (e) 300 µL of solution B (0.25 g L-1), following the first three compression-expansion cycles up to 25 mN m-1.
Figure 1. Molecular structures of (a) 1,10-bistearyl-4,6,13,15-tetraene18- nitrogen-crown-6 (NC) and (b) 1-benzyl-9-hydrofullerene-60 (C60Be). In b, the large circles represent carbon atoms, and small circles represent hydrogen atoms.
Figure 2. Experimental setup for transmitted SHG (THG) measurements.
µm, 40 ps full-width at half-maximum, repetition rate 10 Hz, energy 1 mJ/pulse) polarized either parallel (p) or perpendicular (s) to the plane of incidence was directed onto the vertically mounted samples with a variable incident angle θ. The SHG (or THG) signals were detected by a photomultiplier (PMT) tube and a boxcar averager and displayed on an X-Y recorder. An IR blocking filter and 532 nm (or 355 nm) interference filter were inserted to ensure that only SHG (or THG) signals were detected. Nonlinear susceptibilities χ(2) (or χ(3)) were obtained by fitting the envelope of θ-dependent SHG (or THG) intensity to the theoretical curve. A Z-cut quartz wedge (or CS2 solution) was used as reference. 3. Results and Discussion 3.1 π-A Isotherms of C60-Be, NC, and Their Mixtures. The measured isotherms of C60-Be with different spread
Figure 4. Compressional isotherms of C60-Be/NC mixtures at different molar ratios: (a) 1:1; (b) 1:2; (c) 1:4.2; (d) 0:1.
quantities of C60-Be are shown in Figure 3. The averaged molecular area of C60-Be, as obtained by extrapolating the steeply rising part of the curves to zero pressure, depended on the spread quantity of C60-Be and varied gradually from 0.58 nm2 molecule-1 [spread volume: 300 µL of solution A; Figure 3a], to 0.42 nm2 molecule-1 [200 µL, solution B; Figure 3b], 0.39 nm2 molecule-1 [300 µL, solution B; Figure 3c], and 0.32 nm2 molecule-1 [400 µL, solution B; Figure 3d]. The dependence of the averaged molecular area of C60-Be on the spread quantity was probably due to collapse of the Langmuir monolayers at the water-Teflon rim and formation of the bulk phase.6,12 The isotherms in the second compression process following the initial compression-expansion cycle in the case of curve c showed a slightly steeper and more condensed behavior. After four successive cycles, a repeatable isotherm was recorded, as shown in curve e in Figure 3. The molecular area was found to be about 0.32 nm2, suggesting a bilayer structure at the air-water interface (0.58 nm2/0.32 nm2 ≈ 2). This result agrees with those previously reported for pure C60Ht-Bu.13 Figure 4 shows the surface pressure-area behavior of pure NC and the mixture of C60-Be with NC at different molar ratios. Since the molecular area of C60-Be was 0.58 nm2, as deduced from the curve in Figure 3a, and that of NC was 0.50 nm2, from the curve in Figure 3d, the averaged molecular area of the C60-Be/NC mixture at a molar ratio 1:1 should be about 0.54 nm2 [(0.58 + 0.50) nm2/2] if the two kinds of molecules were miscible and still formed monolayers at the air-water interface. But our data, as deduced from the curve in Figure 4a, gave 0.27 nm2, which is only half of the expected value, suggesting that each C60-Be and NC molecule could possibly form an overlapping configuration. When the percentage of C60-Be molecules in the mixtures is reduced, the average
LB Films of 1-Benzyl-9-hydrofullerene-60
Figure 5. UV-visible absorption spectra of pure C60-Be (a) in a hexane solution and (b) in a Z-type LB film with 18 layers on each side of a quartz slide.
molecular area increased due to the reduction in the probability of such overlapping, as seen in Figure 4b,c,d. 3.2 Deposition and Structural Characterizations of the C60-Be LB Films. We found that LB film deposition was difficult for pure C60-Be (transfer ratio ∼0.80) and much easier for NC and C60-Be/NC mixtures (transfer ratio ∼1.00 ( 0.05). The UV-visible absorption spectra of pure C60-Be in a Z-type LB film with 18 layers on each side of a quartz substrate and in a hexane solution are shown in Figure 5. The absorption spectra of the LB film and solution all exhibit three major absorption bands at 225, 275, 340 nm and 232, 260, 328 nm, respectively, in the 200-800 nm range, corresponding to the three spectral absorption peaks at 213, 257, and 329 nm in C60 solution.8 The red-shift of the bands at 275 and 340 nm in the LB film absorption spectrum with respect to those in the solution (260 and 328 nm) offers preliminary evidence for the existence of the J-aggregation in the pure C60-Be LB films. Figure 6 shows the UV-visible absorption spectra from LB multilayers of NC, C60-Be, interleaving C60-Be/NC, and mixed C60-Be/NC. Notice that a shoulder appears only in the absorption spectrum of the mixed C60-Be/NC multilayer. We attribute this new feature to the interactions between the conjugated π-electron systems of C60-Be and the NC molecules upon insertion of the C60-Be between two tails of the NC molecules. This interaction seems weak or missing in the interleaving C60Be/NC sample (transfer ratio was 1.00 ( 0.05 for NC in upstrokes and 0.80 ( 0.05 for C60-Be in downstrokes) due to the separate depositions of the two types of molecules. Small angle X-ray diffraction (SAXD) was employed to characterize the periodic structure of the films. Figure 7 shows the SAXD patterns obtained from the Z-type LB multilayer films of mixed C60-Be/NC and pure NC molecules. The sharp Bragg lines in both patterns confirm the well-ordered periodic structure of the films. Notice that the averaged thickness of each mixed C60-Be/NC layer, 5.3 nm as deduced from Figure 7a, is only slightly larger than 5.1 nm, the thickness of each NC layer as deduced from Figure 7b. This fact suggests that each C60-Be and NC molecule not only forms an overlapping geometry but also constructs a compact configuration by insertion of the C60Be between two chains of the NC molecules in the mixed films.
J. Phys. Chem., Vol. 100, No. 41, 1996 16631
Figure 6. UV-visible absorption spectra of (a) pure C60-Be (Z-type, 18 layers on each side), (b) pure NC (Z-type, 38 layers), (c) pure C60Be interleaved with NC (Y-type, 16 bilayers on each side), and (d) 1:1 mixture of C60-Be and NC (Z-type, 36 layers on each side).
Figure 7. Small angle X-ray diffraction patterns of (a) 1:3 C60-Be/ NC mixture in a 28-layer Z-type LB film and (b) pure NC in a 35layer Z-type LB film.
3.3 Optical Nonlinear Susceptibilities of C60-Be in LB Films. Since C60-Be molecules have nearly inversion symmetry leading to a small second-order susceptibility χ(2) and the thickness of our sample films were relatively small (
