2334
Langmuir 1995,11, 2334-2336
Second Harmonic Generation from Langmuir-Blodgett Films of Fullerene-ha-Crown Ethers and Their Potassium Ion Complexes David A. Leigh,*#+Andrew E. Moody,? Frances A. Wade,? Terry A. King,$ David West,$ and Gurmit S. Bahrag Department of Chemistry, University of Manchester Institute of Science and Technology, P. 0. Box 88, Manchester M60 1QD, United Kingdom, Department of Physics, University of Manchester, Manchester M13 9PL,United Kingdom, and Defence Research Agency, Fort Halstead, Sevenoaks, Kent TN14 7BF, United Kingdom Received February 10, 1995. I n Final Form: May 1, 199P Langmuir and Langmuir-Blodgett (LB) films of a family of amphiphilic fullerene derivatives, the fullerene-aza-crown ethers, have been produced. The use ofhigh concentrationsof K+ions in the subphase leads to Langmuir films (type 11)with significantly higher areaslmolecule than are obtained with no ions in the subphase (type I). The type I films were transferred onto hydrophilic glass substrates and were found t o exhibit second-ordernonlinear susceptibilities,~ ~ p of’ ~ 2.3-3.6 ) , pmN, the first xPJ2)values to be determined of LB films of fullerene derivatives. The physical’ and optical2 properties of buckminsterfullerene (c60)and the higher fullerenes have attracted unprecedented attention over the last 4 years. Less well studied and of significantly greater potential, however, are the characteristics of the growing number of welldefined, simple chemical derivatives of the parent f ~ l l e r e n e s .We ~ recently described4 the synthesis of a family of monoaza-crown ether adducts of (260; we now report the formation and nonlinear optical (NLO) properties of monolayer films of these amphiphilic fullerene derivatives, 1-3 and their K+ complexes.
n=l , C60-l- a m 1 2-crown-4 (CBO-lA1 2C4), 1 n=2, C60-i-aza-l5-crown-5 (C60-lA1 5C5), 2 n=3, Cso-l-aza-18-crown-6(C60-iA18C6), 3
of second harmonic generation from these films is uncertain, it seems likely that the phenomenon arises either from impurities in the films (fullerenes are notoriously difficult to obtain free from their oxides) or through magnetic dipole moment contribution^.^ Although fullerenes are not self-assembling amphiphilic molecules of the type generally used to form ordered films, Langmuir-Blodgett (LB)films of C60 have been described by several groups.* However, all have reported poor quality films. Calculated diameters for the C ~molecules O in these films are smaller than those expected from STMg and X-ray crystallographic datg,1° which give a value for the lattice spacing of around 10A and suggest the transfer of “clumps” or crystallites, rather than a monolayer, onto the substrate surface. The problems appear to arise from aggregation effects caused by using spreading solutionssa that are too concentrated or by overloading the water surface. Using dilute spreading solutions, Obeng and Bard obtained a radius of 5.6 f 0.7 A for c 6 0 in Langmuir films but they were unable to transfer the monolayer onto a substrate.ll Derivatization of C60 offers a route to amphiphilic molecules which have the potential to form more stable ~
Films of molecules containing extensive n-electron systems are among the elite of organic NLO material^,^ and fullerenes and their derivatives are a logical and promising target for investigations. Fullerenes have been shown to exhibit large third-order NLO activity6 and, despite their symmetry, second-order harmonic responses have also been measured for thin films of c60 formed by vapor d e p o ~ i t i o n ,the ~ ~highest ~ , ~ to date being a bulk order While the mechanism susceptibility, xPJ2),of 1.6
* To whom correspondence may be addressed: phone f44-161200-4448; fax +44-161-200-4539; e-mail David.leighOumist.ac.uk. University of Manchester Institute of Science and Technology. University of Manchester. 4 Defence Research Agency. Abstract published in Advance A C S Abstracts, J u n e 15,1995. (1)Hebard, A. F. Annu. Rev. Mater. Sci. 1993,23,159. (2)(a) Kafafi, Z.H. Photonics Spectra 1993,76. (b) Hamilton, B.; Rimmer, J. S.; Anderson, M.; Leigh, D. A.Adv. Mater. 1993,5, 583. (3)Taylor, R.; Walton, D. R. M. Nature 1993,363,685. (4)Davey, S. N.; Leigh, D. A.; Moody, A. E.; Tetler, L. W.; Wade, F. A. J. Chem. Soc., Chem. Commun. 1994,397. (5)Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991.
*
+
@
~~
(6) ,~ (a) . . Hoshi. H.: Nakamura. N.: Maruvama. Y.: Nakaeawa. T.: Suzuki, S.; Shiromaru, H.; Achiba, Y . Jpn.“ J. Appl.’Phys. i991,’301
L1397.(b) Kafafi, Z.H.; Lindle, J. R.; Pong, R. G. S.; Bartoli, F. J . L.; Lingg, J.; Milliken, J. Chem. Phys. Lett. 1992,188,492. (c) Kazjar, F.; Taliani, C.; Zamboni, R.; Rossini, S.; Danieli, R. Synth. Met. 1993,54, 0.
Zl.
(7) Wang, X. K.; Zhang, T. G.; Lin, W. P.; Liu, S.;Wong, G. K.; Kappes, M. M.; Chang, R. P. H.; Ketterson, J . B.App1. Phys. Lett. 1992,60,810. (8) (a) Williams, G.; Pearson, C.; Bryce, M. R.; Petty, M. C. Thin (b) Nakamura, T.;Tachibana, H.; Yumura, Solid Films 1992,209,150. M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Langmuir
1992,8,4.(c)Guo,J.;Xu,Y.;Li,Y.;Yang,C.;Yao,Y.;Zhu,D.;Bai,C. Chem. Phys. Lett. 1992,195,625. (d) Wing, P.; Shamsuzzoha, M.; Wu, X.; Lee, W.; Metzger, R. M. J. Phys. Chem. 1992,96,9025.(e) Wang, P.; Metzger, R. M.; Bandow, S.;Maruyama, Y. J. Phys. Chem. 1993,97, 2926.(0 Diederich, F.;Effing, J.; Jonas, U.; Jullien, L.; Plesnivy, T.; Ringsdorf, H.; Thilgen, C.; Weinstein, D. Angew. Chem., Int. Ed. Engl. 1992,31,1599.(g)Jehoulet,C.;Obeng,Y.S.;Kim,Y.T.;Zhou,F.;Bard, A. J. J . Am. Chem. Soc. 1992,114,4237. (9)(a) Wilson, J. R.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D.; de Vries, M. S.; Huziker, H. E.; Wendt, H. R. Nature 1991,348, 621. (b) Wragg, J. L.; Chamberlin, J. E.; White, H. W.; Kratschmer, W.; Huffman, D. R. Nature 1991,384,623. (10)(a) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990,347, 354. (b) Fleming, R. M.; Ramirez, A. P.; Rosseinsky, M. J.; Murphy, D. W.; Haddon, R. C.; Zahurak, S. M.; Makhija, A. V. Nature 1991,352,701. (11)Obeng, Y. S.;Bard, A. J. J. Am. Chem. SOC.1991,113,6279.
0743-746319512411-2334$09.0010 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 7,1995 2335
Letters
2ol 10
0 0
I
I
1
I
I
1
20
40
60
80
100
120
I
140
1
160
I
180
200
V
Area/moleculel(Al)
Figure 1. (a) Type I pressure-area (ll-A) isotherm film of CSO-l-aza-18-crown-6,3,on water (Milli-Q)at pH 5.0 and T = 15 "C. (b) Type I1 isotherm of 3 on a 1M KC1 subphase.
LB films than the simple hydrophobic parent hllerenes. l2 Furthermore, the introduction of a single functional group into c60 generates a dipole across the molecule which should result in an effective increase in the second-order susceptibility of the fullerene films. Consequently, we sought to prepare LB films of monofunctionalized fullerene-aza-crown ether derivatives formed from the selective monoamination of the parent fullerenes in toluene s ~ l u t i o n .These ~ particular fullerene derivatives are also of interest because of their ability to bind to alkalimetal ions;4it was anticipated that doping of the subphase with alkali-metal salts could result in increased stabilization of the films of 1-3. We first attempted to prepare Langmuir films of 1-3 in the absence of metal ions in the subphase. To avoid the problems of aggregation that occur when using concentrated spreading solutions, very dilute solutions were employed (0.06mM). To obtain a typical isotherm (Figure la), 0.8 mL of a freshly prepared13 0.06 mg/mL solution of the fullerene-aza-crown (1-3)in dichloromethanewas used. The material was deposited dropwise onto the water surface (500cm2)of a Joyce-Loebl dual A-B trough using a microsyringe. After the spreading solvent had evaporated (5-10 min), the material was compressed a t a rate of 1cm2/sto the maximum surface pressure of 50 mN/m, and the surface pressure was measured using a Wilhelmy piate.14 From the II-A isotherms generated (type I.,Figure 1) the areas/molecule were foynd to be 91 f 7 A2 for 1,94 f 6 A2 for 2,and 90 f 8 A2 for 3.15 Calculation of the correspon$ing radii produced values of 5.1 f 0.4A for 1, 5.2f 0.3 A for 2, and 5.1 f0.4 for 3. These values are similar to those determined for c60 from STM (10.0f 1.0
A
(12) (a) Maliszewskyj, N. C.; Heiney, P. A.; Jones, D.
R.;Strongin,
R. M.; Cichy,M. A.; Smith, A. B.Langmuir 1993,9,1439. (b)Goldenberg, L. M.; Williams, G.; Bryce, M. R.;Monkman, A. P.; Petty, M. C.; Hirsch,
A.; Soi, A. J. Chem. SOC.,Chem. Commun. 1993, 1310. (c) Diederich, F.; Jonas, U.; Gramlich, V.;Herrmann, A.; Ringsdorf, H.; Thilgen, C. Helv. Chim. Acta 1993, 76, 2445. (d) Maggini, M.; Karlsson, A.; Pasimenti, L.; Scorrano, G.; Prato, M.;Valli, V. Tetrahedron Lett. 1994, 35,2985. (e) Hawker, C. J.; Saville, P. M.; White, J. W. J. Org. Chem. 1994,59, 3503. (13) Freshly prepared solutions were used in order to prepare the LB
films since the isotherms obtained from solutions made up one or more days in advance were different from those obtained from freshly made solutions. They contained an additional transition region probably indicating the presence of other species in the solution, namely oxidation products. (14) To our knowledge, this is the first time that a Wilhelmy plate has been used to measure fullerene isotherms. Previous examples have all reported the use of a Langmuir float. (15) These values were obtained by examining five reproducible isotherms for each of the fullerene-aza-crown ethers.
io A
Figure 2. Semiempiricalminimum energystructurecalculated for CM- l-aza-l8-crown-6,3, showing the approximate dimensions of the molecule (similar vqlues were obtained for 1 and 2 with heights of -15 and 16 A, respectively). Calculations
were carried out on a Tektronics CAChe workstation using MOPAC 6.0. Type I film
Fullerene
-----air
Azacrown ether
~ ~ c 3 6 $ water
Type I1 fllm
or
Figure 3. A schematic representation of the different morphologies possessed by type I and type I1 Langmuir films.
A for
the intermolec@ar s p a ~ i n g )and ~ X-ray crystallographic data (10.02A),1° consistent with the presence of a monolayer on the surface with little or no aggregation of the dipped species. However, when the monolayers are expanded and recompressed, appreciable hysteresis was observed, implying that some molecular aggregation occurs on compression even with these amphiphilic fullerene derivatives. Molecular modeling shows that the C6o-aza-crown ethers are roughly cylindrical, with a radius determincd by the c60 moiety (i.e., -5 A) and a height of 15-16 A (Figure 2),so the small radius values obtained for the type I isotherms suggest that the c60aza-crown ethers line up with their axes perpendicular to the aidwater interface (Figure 3). The effect of adding potassium ions to the subphase was then investigated. At small K+ ion concentrations (~0.1 M) no significant changes in the isotherms were detected,sf but using a 1M KC1 subphase the isotherms changed to the type I1 kind shown in Figure l b and the following areadmolecule were obtained: 110 f 7 A2for 1,111 f 6.A2for 2,108f 6 A2 for 315(the corresponding
Letters
2336 Langmuir, Vol. 11, No. 7, 1995 "I
0.8
I
I
rk I -
00 190
290
390
490
590
WAVELENGTH [nm)
Figure 4. UV/vis absorption spectra of (a) a solution of 1and (b) a monolayer of 1 on hydrophilic glass.
radii being 5.6 f 0.3 A,5.7 f 0.3 A,and 5.6 f 0.3 A). Figure l b shows a typical type I1 isotherm obtained on the doped subphase. These values for the molecular areas are significantly larger than those for c60, suggesting that complexation causes either tilting of the fullerene azacrown ether molecular axis (Figure 3(i)) or a conformational change in the crown ether moiety which results in an effective increase in the molecular surface area (Figure 3(ii)). Whichever mechanism is operating, doping the subphase with potassium ions clearly produces a change in the morphology of the Langmuir films when high concentrations of dopant are used. Deposition was then attempted using hydrophilic glass slides. Hydrophilic glass was used because of the reported difficulties in depositing CSOonto hydrophobic substrates' and it was anticipated that the crown moiety of 1-3 could adhere to the hydrophilic surface. Deposition was attempted both with and without ions in the subphase using the same conditions employed to generate the isotherms. A low value of surface pressure was chosen, n = 10 mN/ m, to discourage aggregation.sf Successful depositions of both type I and type I1monolayers were achieved, although the type I1 films proved more problematical because a t high dopant concentrations the substrate acts as a crystallization site for the added salt and interferes with deposition. A transfer ratio close to unity was observed in all cases, with UV/vis absorption spectra (Figure 4) and nonlinear optical measurements confirming transfer of the material onto the substrate.
The films were examined for second-order NLO activity a t room temperature by polarized transmission second harmonic generation from the output of a Q-switched Nd:YAG laser a t wavelength 1.064pm)with resolution of the incidence angle dependence of the harmonic signal as described elsewhere.16 Second-order NLO activity was measured, giving conversionefficienciesfor the type I films of 0.2 x for 1, 0.5 x for 2, and 0.4 x for 3 relative to the peak harmonic intensity from quartz. These correspond to susceptibilities xpJ2)equal to 2.3, 3.6, and 3.2 pmN, respectively, for the monolayers, which to our knowledge are the first NLO measurements obtained from a n LB film of a fullerene or fullerene derivative. The differences in susceptibilities between the films of the fullerene-aza-crowns probably arise from variations in film quality, rather than any dependence on the size of the attached crowns. All the values obtained for the type I films are reproducibly higher than the previous value of 1.6 p m N found for thin films of c 6 0 by Kazjar et aZ.6c Interestingly, the type I1 films all gave susceptibilities xpJ2)of-1 pmN, the lower susceptibilities arise from either tilting of the magnetic dipoles (Figure 3(i)), the lower packing density of the molecules because of KC1 intercalation (Figure 3(ii)), or the general poorer film quality of the type I1 films. The incorporation of the aza-crown ether moiety clearly has a significant effect on both the LB film forming abilities of fullerene materials and their nonlinear optical properties. Furthermore, potassium ion complexationalters both the film morphology and its optical properties, although a t this stage it is not clear by which mechanism. We are currently attempting to deposit multilayer5 (of both type I and type I1 films) of 1-3 and other aza-crown ether derivatives using the LB technique and investigating further optoelectronic characteristics of these fascinating materials.
Acknowledgment. This work has been carried out with the support of the Defence Research Agency, Fort Halstead, and the SERC Molecular Electronics Committee Grant No. GWJ41390. LA9501039 (16)Hodge, P.;Mi-Adib, Z.;King, T.A,; West, D.Macromolecules 1993,26,1789.
