Langmuir and Langmuir-Blodgett films of C60 derivatives

ELSEVIER

Thin Solid Films 284-285

( 1996) 76-79

Langmuir and Langmuir-Blodgett

films of ChOderivatives

S. Ravaine, C. Mingotaud, P. Delhab * Centre de Recherche Paul Pascal, CNRS, Avenue Albert Schweitzer, 33600 Pessac, France

Abstract Several methanofullerenes

functionalized

with either polar or fluorinated groups have been synthesized

and their ability to form Langmuir

films have been investigated. In both cases, monolayers have been obtained at the gas-water interface. Mixing one of the Cm derivatives grafted by CF, groups with an amphiphilic fluorinated compound allows the formation of high-quality Langmuir-Blodgett films, with a well defined intrinsic repeat distance along the normal to the substrate. Keywords: Cm derivative; Langmuir film; Langmuir-Blodgett films; Fluorinated derivatives

1. Introduction

The physical properties of fullerenes, essentially due to their unique geometry and electronic structure, have become the focus of considerable interest in monomeric or aggregated forms [ 11. Numerous efforts have been devoted to the elaboration of well-ordered thin films containing Cm or C70, in particular using the Langmuir-Blodgett technique [ 2-51. In fact, since the highly controversial initial report of Obeng and Bard [ 21, many studies aimed at producing monomolecular films of fullerenes have revealed that these compounds are highly hydrophobic and exhibit a strong tendency to form three-dimensional aggregates at the gas-water interface [ 351. One of the strategies which have been developed to avoid such aggregation consists of functionalizing Cho with polar groups to obtain more stable amphiphilic derivatives [ 6-101. Then, well organized monolayers of the epoxide Cm0 171, fulleropyridines [ 81 and a &,, derivative containing an acid group [ 91 have been recently obtained at the water surface. Here we report a comparative study by the Langmuir technique of the ability of several Cm derivatives functionalized with either polar or fluorinated groups to form stable monomolecular films at the gas-water interface. The formation of well-defined LB films of various mixtures of a perfluoro methanofullerene and a fluorinated tetrathiafulvalene derivative is also described. Such a result could be explained by possible intermolecular interactions between neighboring CF, groups. * Corresponding author. 0040-6090/96/$15,00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)08275-l

2. Experimental

details

C& (99%) was purchased fro Technocarbo (France) and its purity was confirmed by high-pressure liquid chromatography (HPLC) . Compounds l-4 (see Fig. 1) were synthesized according to previously described procedures [ 11,121. Products 5-9 were obtained by condensation of 4 with the corresponding alcohols in the presence of NJ+/’ dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP). lH,lH,2H,2H, perfluorododecan-l-01 and ldocosanol were respectively purchased from ABCR and Aldrich and were used without any further purification. The syntheses of the other used fluorinated alcohols have been already published [ 131. The fluorinated ethylenedithiotetrathiafulvalene 10 was obtained as previously described [ 141. The purity of the methanofullenes was determined by ‘H, 13C and “F NMR and by elemental analysis. NMR and WVis spectra were respectively recorded on an ARX 300 and a PU 8800 Philips spectrometers. HPLC analyses were carried out with Waters 3000E HPLC system and a C,,-silica column (ultrasep from ICS). A Waters 484 W detector monitored fullerene absorbance at 340 nm. The compression isotherm curves were recorded using a commercial KSV 5000 Teflon through at 293 K and under a continuous nitrogen flow saturated with water. The surface pressure was measured by a Wilhelmy balance. The subphase was Millipore Q-grade water with a resistivity higher than 18 MR cm. Chloroform and carbon disulfide were used as spreading solvents. All solutions (concentration ca. 3. low4 mol 1-i) were stored at - 18 “C between experiments to avoid solvent evaporation. Compression speeds were ranged between 1 and 3 A’ molecule- ’ min- ‘. The deposition onto

II

S. Ravaine et al. /Thin Solid Films 284-285 (19%) 76-79

a Langmuir balance are similar to the one recorded with the classical Wilhelmy method in the low surface pressure range (below 20 mN m-l). For higher surface pressures, large differences are found between the two measurements due to the high rigidity of the films. The limiting molecular areas A0 (obtained by extrapolation of the linear part of the GA curves to zero pressure) are 34, 32, 59 and 82 A’ for l-4 respectively. As the theoretical value expected for a single fullerene molecule is around 80 A’ [ 151, multilayer organizations are clearly present at the water surface in the case of 1,2 and 3. On the opposite side, the methanofullerene 4 could form a monolayer at the gas-water interface. Then, it seems that the methoxy and ester groups of 1 and 2 are not sufficiently polar to give measurable effect on aggregation, when the acid and the phenol functions of 3 and 4 respectively allow one to partially or totally counterbalance the T-Tinteractions which assure the cohesion of fullerene 3d aggregates

[W.

e

-Gz%

Fig. 1.ChemicalStructures of compounds l-10. optically polished slides of glass or calcium fluoride ( CaF2) was achieved by the vertical lifting method at a constant surface pressure of 4 mN m-l, at 293 K and under a continuous dried nitrogen flow. A waiting time of 5 min was spent after each deposition cycle to dry the substrate. X-ray diffraction patterns of LB films were recorded by an INEL curve detector (0-20) associated with an IBM computer for peak assignments.

To well characterize the formation of Langmuir films of 3, the nature of the water subphase has been modified. Fig. 2 shows the &A curves of this derivative recorded at 293 K on hydrochloric acid or sodium hydroxide aqueous solutions. As expected, raising the proton concentration in the subphase does not affect the behavior of 3. However, working with a very basic subphase involves a large increase of its limiting areas (A, E 102 A*). This result is surely due to the formation of the corresponding salt of 3. One should note that a similar experiment carried out onto a 10-l M NaCl aqueous solution did not involve any modification of the D-A curve recorded on pure water [ 171. This indicates that the observed effect on the behavior of 3 at the gas-water interface is not due to changes in the ionic strength of the subphase. The second part of our work has consisted of the study by the Langmuir technique of fluorinated Cso derivatives, with the aim of determining if such functionalization can involve the formation of a monomolecular film of fullerene at the gas-water interface as it has been previously reported in the literature [ 81. 25-

3. Results and discussion

z 1

20-

+m

0

0

+ClV

0

Cl

A

0

A

+a

s.

0

A

3.1. hngmuirfilms I

The initial part of our work consisted of the study by the Langmuir technique of several C&derivatives functionalized with more or less polar groups in order to relate their amphiphilic character with their ability to form stable monolayers at the gas-water interface. Fig. 2 shows the surface pressure (n>-molecular area (A) isotherm curves of compounds 14. I7 values above 22 mN m - ’ have been deliberately suppressed because the floating films became too rigid and the Wilhelmy plate began to be pushed out of the subphase. Indeed, isotherms measured with

&

lo-

0

0

+U

0

0

+u

8 g

+a

0

+I3

5-

+

z

+ 0

, 0

20

A 0

40

,

,

60

A A

0

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,G,F,

A 0

l

u ;

A

0

0 u

A

,

p,

,

7,;

80 100 120140

Area

,

,

160

(A*)

Fig.2. Surface. pressure-molecular areaisothermsat 293K of 1.2.4and9 onpurewater,andof3onan8X10-2MNaOHaqueoussubphase(A),on purewaterandona5X10-2MHClaqueoussubphase(@).

S.Ravaine et al. /Thin SolidFilms284-285 (1956)76-79

78

2H rA

2

0 0 0

. .

70

110

90

Area

130

150

(A*)

Fig. 3. Surface pressure-molecular area isotherms of S-fl at the gas-water interface at 293 K.

Fig. 3 shows the &A isotherms of compounds 5-8 recorded at 293 K. ZI values above 14 mN m- ’ have been deliberately suppressed for the same reason as above. The A0 values of 79,90,91 and 88 A’ respectively obtained from the curves of 5-g clearly indicate that these methanofullerenes form initially monolayers from which some fullerenes are progressively pushed out during compression leading to multiplanar monolayers [ 171. The length of the fluorinated chain has no significant effect on their behavior on the water surface. Nevertheless, one can argue about their amphiphilic character, as the only hydrophilic part of them consists of non-highly polar carboxylic ester groups, which are surrounded by strong hydrophobic moieties. However, stable monomolecular films of non-amphiphilic perfluoro compounds have already been obtained at the air-water interface [ 18,191. The layer packing of such compounds seems to be due to strong intermolecular interactions between neighbouring CF2 groups within the layer. Then, if one assume that the Van der Waals diameters of a perfluoroalkyl chain and a Cm molecule are 6 and 10 8, respectively [ 151, it appears that even if an average distance of 4 8, separates the fluoro groups from each other, they can force the fullerene molecules to adopt a monolayer packing on the water subphase. To confirm such an interpretation, we recorded at 293 K the Ii-A curve of the Cm derivative 9 (see Fig. 2). In fact, this methanofullerene differs from compounds 5-g only by the fact that its functional alkyl chain is completely hydrogenated. The A0 value of 18 A’ obtained from its isotherm undoubtedly shows that 3d aggregation occurs at the air-water interface. One may conclude that intermolecular interactions between CF2 groups really contribute to the formation of quasi-monomolecular films of compounds 5-g at the gas-water interface, and that no specific effect due to the ester groups of these molecules can be detected. 3.2. Langmuir-Blodgettfilm elaboration Attempts to transfer monolayers of pure derivatives l-9 onto solid substrates by the vertical lifting method were unsuccessful because of the too high rigidity of the Langmuir

40

50

A

A

0

a

00 50

1 .i .

4

60

A

70

80

00

100

Area (A21 Fig. 4. Surface pressure-molecular area isotherms of 5.10 and their molar 1:4 mixture. The theoretical curve for immiscible components or ideal mixture of 5 and 10is also given for the molar ratio 1:4.

films. However, to elaborate well-ordered LB films containing fullerenes, we have mixed compound 5 with the amphiphilic perfluoro ethylenedithiotetrathiafulvalene 10,which forms high-quality LB films [ 201. In fact, we assumed that interactions between perlluoroalkyl chains of 5 and 10were strong enough to “fluidify” the monolayer, allowing one to transfer mixtures of them onto solid substrates by the LB technique. After spreading dilute chloroform solutions (ca. 10-4mo11-‘) of 1:2and1:4mixturesof5andlOattheairwater interface, IT-A isotherm curves were recorded at 293 K following a continuous compression process. In the case of immiscible or ideally miscible Langmuir films, the average molecular area of the mixtures should be given by the following additivity relation [ 211: A =NsAs +NAo

where NS and N,, are the molar fractions of 5 and 10while A5 and Alo are the corresponding molecular area. Then, one

can compare the theoretical curve deduced from this equation with the experimental one. In the case of the 1:4 mixture, these two curves are given in Fig. 4. From these data, it is clear that the two compounds 5 and 10 aremiscible at the air-water interface with an experimental area expansion. This result was expected for such compounds bearing both perfluorinated chains, which can easily interact together. Similar result was found for the 1:2 mixture. This behavior is in favor with the formation of an uniform monolayer at a given surface pressure. These mixed monolayers were successfully transferred onto hydrophilic substrates using the vertical lifting method. The transfer ratio values were zero during the immersion of the substrate and close to one during its emersion (Z-type deposition). Then, interactions between perlluoroalkyl chains of 5 and 10allow the formation of LB films of mixtures of them. 3.3. LB film characterizations In order to well characterize the structural organization of the multilayer films, W-Vis absorption spectra of a 1:2

S. Ravaine et al. /Thin Solid Films 284-285 (1996) 76-79

mixture of 5 and 10 in the forms of a 50-layer LB film and a 0.6 X 10m4M chloroform solution were recorded. In the latter case, the characteristic absorption bands of fullerenes derivatives (around 250,320 and 425 nm) were clearly observed. In the LB film spectrum, the first two peaks were broadened and shifted by 15-20 nm to longer wavelengths. These modifications of absorption have been already observed for evaporated films of C, and for several fullerene derivatives LB films [ 8,9]. More interestingly, the absorption band (around 450 nm) characteristic of the aggregated state of fullerene molecules [ 17,221 does not appear in the LB film spectrum. One can conclude that 5 has been transferred in a monolayer form. The layer structure of the LB films was analyzed by X-ray diffraction experiments: an intense Bragg peak at an angle of ca. 3.9” was observed. If we assume that this value corresponds to the (001) diffraction, it can be assimilated to a repeat distance of 2.5 nm, which is close to the molecular length of 5. This is consistent with the Ztype deposition of these LB films. 4. conclusions We have studied by the Langmuir technique the ability of several new methanofullerenes functionalized by hydrophilic or fluorinated groups to form monomolecular films at the gas-water interface. Concerning the first series of compounds, monolayers have been obtained when the polar groups grafted on the Cm molecule were strong enough to counterbalance the high hydrophobicity of the fullerene. However, this type of functionalization has not still allowed the elaboration of high-quality LB films. This aim has been achieved working with the second series of derivatives, which exhibited intermolecular interactions between neighbouring CF, groups within a given layer. Acknowledgements

We thank Pr. J. Cousseau for giving us compound 10 and Dr. H.T. Nguyen for the fluorinated alcohols which allow the syntheses of methanofullerenes 6-8.

79

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