The Journal of
Physical Chemistry
0 Copyright I994 by the American Chemical Society
VOLUME 98, NUMBER 48, DECEMBER 1,1994
LETTERS Langmuir-Blodgett Film and Second Harmonic Generation of Cso(C4Hfl2) L. B. Gan, D. J. Zhou, C. P. Luo, and C. H. Huang* State Key Laboratory of Rare Earth Material Chemistry and Applications, Peking University, Beijing 100871, China
T. K. Li and J. Bai Institute of Photographic Chemistry, Academia Sinica, Beijing 100101, China
X. S. Zhao and X. H. Xia Department of Chemistly, Peking University, Beijing 100871, China Received: June 29, 1994; In Final Form: September 11, 1994@
Both the Langmuir and Langmuir-Blodgett films of a Cm derivative Cm(C4HgN2) at the air-water interface have been prepared. The second-order molecular hyperpolarizability is determined to be (3.6 f 1.2) x esu.
The discovery of an efficient synthesis method for fullerenes' has led to intense study on these close-caged molecules2-6due to their fascinating physical and chemical Several groups have investigated the LB films of fullerenes and their derivatives. 12-21 In most cases only multilayers are obtained; it is difficult to transfer monolayers onto solid substrates. Heiney et al. observed that the fullerene epoxide (2600 forms monolayers more easily than c60 or CmH2.14 We recently prepared the known fullerene piperazine Cm(C4HgN2) through the reaction of C ~ and O 2-aminoethyl hydrogen sulfate.22z23The two hydrophilic nitrogen atoms in the molecule should greatly enhance the stability and the transferability of its Langmuir film. In this Letter, we report the first observation of Langmuir and Langmuir-Blodgett films of Cm(CJIgN2) and its second harmonic generation study. The compound C~O(CJI~NZ) was synthesized by the reaction between C a and 2-aminoethyl hydrogen sulfate in a multisolvent system toluene/ethanol/H20.22 Spectroscopic data show that it is the same as the Cm piperazine monoadduct described by @Abstract published in Advance ACS Abstracts, November 15, 1994.
Kampe et al.23 Two solutions were prepared as spreading solutions: (A) 0.84 g/L in 1:2 CS2/ benzene; (B) 0.061 g/L in 1:lO CS2/CH2C12. The substrates were kept in a chromic acid solution for 10 h, then treated with 50% sulfuric acid at 70 "C for 10 min, and finally cleaned with distilled water. The Langmuir films were formed by depositing solution A or B on a pure water subphase (20 f 1 "C, pH = 5.60) on a NIMA Langmuir trough. The solvents were allowed to evaporate over a period of 15-20 min, after which the floating films were compressed at a rate of 20 " i n , and the surface pressure/area (n-A) isotherms were recorded. The monolayer was transferred in Z type onto hydrophilic pretreated fused quartz plates at a rate of 10 d m i n . During the film transferring procedure, the surface pressure was maintained within a few percent of 25 "/m. n-A Isotherm. The n-A isotherms are different for different solutions (Figure 1). A 80 p L sample of solution A gave curve a with a plateau around 8 mN/m; the measured molecular area is about 1.3 nm2 per molecule. A 120 pL sample of solution A gave curve b with limiting area of 0.27 nm2 per
0022-3654/94/2098-12459$04.50/0 0 1994 American Chemical Society
Letters
12460 J. Phys. Chem., Vol. 98, No. 48, 1994
TABLE 1: Monolayer Comparison of Fullerenes and Their Derivatives
compd C60 c70
c70
CmHt-Bu CmHz
cmo
limiting area, nm2/molecule 0.958 1.76 1.60 0.88 0.94 0.96
collapse press., "/m '65 '30 '50
>30 -73 -73
model vertical vertical
transfer condition treatment transferability X either X either
vertical vertical vertical horizontal vertical
either hydrophobic hydrophobic hydrophobic hydrophilic
-
-
molecule, and the film can sustain more than 40 mN/m, typical for multilayer fullerenes and their derivatives.1z-21 This multilayer film could be transferred onto hydrophilic quartz substrates; however, there were visible patchy domains in the films both on the substrates and at the air-water interface, indicating aggregation of the molecule^.^^*^^ A 500 p L sample of solution B spread very well on a pure water subphase and yielded an excellent monolayer at the air-water interface (curve c). The monolayer can sustain more than 40 mN/m. The limiting area is 0.89 nmzper molecule, which is slightly smaller than that of Cm and its other derivatives (Table 1). The surface pressure/area isotherm of solution B is fairly reversible. Expansion and recompression of the monolayer was repeated twice. It showed a slight hysteresis in the first process, but there is hardly any further change which occurred in the second process. Assuming that the substituent is attracted toward the water and that the projection of the molecules onto the water surface forms a triangular lattice of circular objects of radius R, the calculated area per molecule is 21/3R2, which yields a nearestneighbor distances of 10.1 f 0.5 A, in good agreement with the distances found in Cm and its other derivative^.'^-^^ This monolayer can be readily transferred onto hydrophilic quartz plate with no visible domains. This behavior is quite different from Cm and its other derivatives, whose monolayers could not be easily transferred.12J4 The two hydrophilic nitrogen atoms play an important role in enhancing the stability and transferability of the Langmuir films. UV-vis Spectra. The UV-vis spectra of the LB films on quartz plates were recorded on a Shimadazu W - 3 1 0 0 spectrophotometer. All the films show three characteristic bands of Cm(C&IxN2) centered at 220, 270, and 328 nm, (Figure 2), the latter two bands red-shifted about 12 nm compared with those of Cm(CdHxN2) in CHCl3 solution,22as in the case of Cm LB f i l m ~ . ' ~ 9A ' ~plot of the absorbance at 220 nm against the layer number results in a linear line (Figure 3), indicating effective stacking of the the monolayers.
12 17 13 18 14 14 14
X X X
X
Yes Yes
300
200
Figure 1. Surface pressudarea (n-A) isotherms: (a) 80 p L of A, (b) 120 p L of A, and (c) 500 p L of B.
ref
4M)
this work
500
h n lm
Figure 2. UV-vis spectra of the LB films: (a) one layer, (b) three layers, and (c) five layers.
1
2
3
4
layer number
Figure 3. Plot of absorbance at 220 nm against the layer number.
SHG Study. Cm should have a large nonlinear optical response because of the existence of n electrons in the cage molecular structure, and its third-order molecular susceptibility x(3)and hyperpolarizability y have been reported to be 3.3 x and 1.6 x esu, 24 respectively. However, it does not show second harmonic generation (SHG) due to its central symmetry structure. The monofunctionized derivative Cm(C&IgNZ) has no central ~ y m m e t r y ; ~ it ~ .should *~ have SHG response in highly ordered LB films. The SHG was measured in transmission with a Y-cut quartz plate as reference and with a Nd:YAG laser beam (11 =lo64 nm) at an angle of 45" to the film surface.25 We assume that the refractive index is similar to that of Cm26 (1.90), and the film thickness is 1.5 nm (which is slightly larger than the nearest-neighbor distance 1.0 nm; the film thickness does not affect the value of x(2)and /3 significantly). By using the method in refs 27 and 28, the second-order molecular hyperpolarizability /3 and susceptibility $2) are calculated to be (3.6 f 1.2) x esu and (1.8 f 0.8) x esu, respectively. The values are relatively small compared with other SHG material.z7-29 The relative SH intensity are 1, 1.2, and 2.0 for 1, 3, and 5 layers, respectively. The subquadratic dependence of intensity on layer
Letters number may be the result of several factors such as local field effects and absorbance of light at the SH frequency by the molecules. In summary, the piperazine c 6 0 monoadduct CaoCJ-IsN2 in CH2C12/CS2 forms Langmuir film with a limiting molecular area of 89 A2 at the aidwater interface. Unlike c60 and its many other derivatives, the Langmuir film can be readily deposited onto quartz plate. The Z-type LB films of C60CfigNz are also obtained, and UV-vis spectra indicate good stacking of the monolayers. The films exhibit weak SHG. The second-order molecular hyperpolarizability /?and susceptibility have been determined to be (3.6 f 1.2) x esu and (1.8 k 0.8) x esu, respectively. Acknowledgment. The authors thank Climbing Program (A National Fundamental Research Key Project) and the National Natural Science Foundation of China for financial support. References and Notes (1) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (2) Haddon, R. C. Acc. Chem. Res. 1992, 25, 127. (3) Hawkins, J. M. Acc. Chem. Res. 1992, 25, 150. (4) Fagan, P. J.; Calabrese, J. C.; Malone, B. Acc. Chem. Res. 1992, 25, 134. (5) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. Rev. 1991,91, 1213. (6) Tayer, R.; Walton, D. R. M. Nature 1993, 363, 685. (7) Stephens, P. W.; Cox, D.; Lauher, J. W.; Mihaly, L.; Wiley, J. B.; Allemand, P. M.; Hirsch, A.; Holczer, K.; Li, Q.; Thompson, J. D.; Wudl, F. Nature 1992, 355, 331. (8) Holczer, K.; Klein, 0.; Huang, S. M.; Kaner, R. B.; Fu, K. J.; Whetten, R. L.; Diederich, F. Science 1991, 252, 1154. (9) Ruoff, R. S.; Malhotra, D. L.; Huestis, D. L.; Tse, D. S.; Lorents, D. C. Nature 1993, 362, 140. (10) Sijbensma, R.; Srdanov, G.; Wudl, F.; Castoro, J. A.; Wilkins, C.; Friedman, S. H.; Decamp, D. L.; Kenyon, G. L. J. Am. Chem. SOC. 1993, 115, 6510.
J. Phys. Chem., Vol. 98, No. 48, 1994 12461 (11) Friedman, S. H.; Decamp, D. L.; Sijbesma, R. P.; Srdanov, G.; Wudl, F.; Kenyon, G. L. J. Am. Chem. SOC. 1993, 115, 6506. (12) Obeng, Y. S.; Bard, A. J. J. Am. Chem. SOC. 1991, 113, 6279. (13) Jehoulet, C.; Obeng, Y. S.; Kim, Y. T.; Zhou, F.; Bard, A. J. J. Am. Chem. SOC. 1992, 114, 4237. (14) Maliszewskyj, N. C.; Heiney, P. A,; Jones, D. R.; Strongin, R. M.; Cichy, M. A.; Smith, A. B., I11 Langmuir 1993, 9, 1439. (15) Nakamura, T.; Tachibana, H.; Yumura, M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Langmuir 1992, 8, 4. (16) Williams, G.; Pearson, C.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1992, 209, 150. (17) Williams, G.; Moore, A. J.; Bryce, M. R.; Lvov, Y. M.; Petty, M. C. Synth. Met. 1993, 55, 2955. (18) William, G.; Soi, A.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1993, 230, 73. (19) Long, C. F.; Xu, Y.; Guo, F. X.; Li, Y. L.; Xu, D. F.; Yao, Y. X.; Zhu, D. B. Solid State Commun. 1992, 82, 381. (20) Back, R.; Lennox, R. B. J. Phys. Chem. 1992, 96, 8149. (21) Guo, J.; Xu, Y.; Li, Y. L.; Yang, C.; Yao, Y. X.; Zhu, D. B.; Bai, C. L. Chem. Phys. Lett. 1992, 195, 625. (22) Gan, L. B.; Zhou, D. J.; Luo, C. P.; Huang, C. H.; Pan, J. Q.; Lu, M. J.; Wu, Y. Manuscript in preparation. (23) Kampe, K. D.; Egger, N.; Vogel, M. Angew. Chem., Int. Ed. Engl. 1993, 32, 1174. (24) Gong, Q. H.; Sun, Y.X.; Xia, Z. J.; Zou, Y. H.; Gu, Z. N.; Zhou, X. H.; Qiang, D. J. Appl. Phys. 1992, 71, 3025. (25) Zhou, D. J.; Huang, C. H.; Wang, K. 2.; Xu, G. X.; Zhao, X. S.; Xie, X. M.; Xu, L. G.; Li, T. K. Langmuir 1994, 10, 1910. (26) Ren, S. L.; Wang, Y.; Rao, A. M.; McRae, E.; Holden, J. M.; Hager, T.; Wang, K. A.; Lee, W. T.; Ni, H. F.; Selegue, J.; Eklund, P. C. Appl. Phys. Lett. 1991, 59, 2678. (27) Ashwell, G. J.; Hargreaves, R. C.; Baldwin, C. E.; Bahra, G. S.; Brown, C. R. Nature 1992, 357, 393. (28) Lupo, D.; Prass, W.; Scheunemann, U.; Laschewsky, A.; Ringsdorf, R.; Ledoux, I. J. Opt. SOC. Am. B 1988, 5, 300. (29) Ashwell, G. J.; Jackson, P. D.; Crossland, W. A. Nature 1994,368, 438.
