Chemically modified thin organic films supported on polished silica substrates David S. Boyle and John M. Winfield* Department of Chemistry, University of Glasgow, Glasgow, UK G12 8QQ Organic films, of thickness up to ca. 1000 nm, derived from polycyclic aromatic hydrocarbons containing 3–15 rings, have been grown on highly polished silica glass supports by vacuum sublimation. The quality of the films, as judged by their electronic spectra, is very dependent on achieving a near subnanometre surface finish on the silica.Exposure of the films to MoF6 at room temperature results in irreversible adsorption of the latter, as demonstrated by electronic spectroscopy. The fluorides WF6 and AsF5 show similar behaviour, although in many cases their interactions with the films are less marked.The reactions of organic molecules have been studied tradition- Annealing processes observed at higher substrate temperature can lead to greater alignment of molecules with the sub- ally in solution but there is increasing emphasis on selectivity in syntheses and by the use of an ordered, structurally well strate12–14 and interactions with a polar substrate such as cleaved single-crystal KCl can be important in determining characterized environment it is often possible to obtain more information than would be the case in solution. Our interest the structure of the film, a notable example being films derived from a polymeric m-fluoroaluminium(III ) phthalocyanine.15 in the chemistry involved in the chemical–mechanical polishing of silica glass and silicon,1 led us to investigate the use of these materials as supports for organic films that could then be Experimental modified by interaction with volatile inorganic reagents, silica supports being particularly useful for investigations of reactions Wafers of silica (2×1 cm; Spectrosil B, Multilab Ltd) or silicon between the films and high oxidation state fluorides by elec- (2×1 cm; (100) p-type, MCP Wafer Technology Ltd), chemotronic spectroscopy.mechanically polished (surface roughness RA1 nm over Polycyclic aromatic hydrocarbons are potential p donors 250 mm) using a procedure described elsewhere,1 were degreand an obvious way of modifying their electronic properties is ased and cleaned carefully by ultrasonic agitation for 5 min in via the formation of charge-transfer complexes with volatile 68% nitric acid (GPR, Rho�ne Poulenc), Genklene (ICI) and Lewis-acid halides adsorbed on the surface of the film.isopropyl alcohol (GPR, M & B). High-purity (Gold Label, Interactions between p-donor aromatic hydrocarbons and high Aldrich) organic compounds were employed without further oxidation state metal halides have been studied in solution for purification wherever possible for the preparation of thin films, many years.2 In contrast to n-donor molecules, where the otherwise starting materials were purified by recrystallisation.formation of insoluble adducts with acceptor molecules is well Owing to the hygroscopic nature of many of the materials documented,3 studies involving p-donor aromatic hydro- employed, reactions were conducted in vacuo (10-4 Torr) using carbons indicate that the interactions are very much weaker, a flamed-out Pyrex glass vacuum system.Molybdenum hexafor example in systems exhibiting ‘contact charge transfer’ fluoride, tungsten hexafluoride, phosphorus pentafluoride (all behaviour.4 Fluorochem) and arsenic pentafluoride (Matheson) were puri- In this initial survey study, fourteen polycyclic aromatic fied by repeated trap-to-trap vacuum distillation over activated compounds possessing 3–15 aromatic rings and bis(ethylenedi- NaF at 77 K.They were transferred finally to Pyrex breakseal thio)tetrathiafulvalene (BEDT-TTF) were investigated. vessels containing activated NaF and stored at 77 K until Although intrinsically related, there are important diVerences required.Instrumentation was as follows: electronic spectra between series of structurally comparable compounds, for PE Lambda 9, IR Nicolet 5DXC with a SpectraTech collector example the cata- and peri-condensed systems. In the former, for diVuse reflectance and an MTEC 100 cell for PAS, IR all the carbon atoms participate in a conjugated system in microscopy Nicolet Nic-Plan with ZnSe ATR accessory.contrast to the latter, which often possess individual discrete cycles of p-electron conjugation, with peri bonds having bond Preparation of organic thin films lengths approaching those of a C–C single bond.5 The eVect Since this was a survey study, films were synthesised using a of substituents on thin film formation has been examined in, straightforward thermal evaporation procedure in vacuo based for example, rubrene, which is a non-planar cata-condensed on previous work in this Department.9,12,15 The apparatus hydrocarbon by virtue of the four phenyl groups attached to consisted of an Edwards 306 coating unit, capable of 10-7 Torr the tetracene backbone, decacyclene, a non-planar peri-conwith a liquid-nitrogen cooled cold trap.A copper–constantan densed hydrocarbon and coronene tetracarboxylic acid. thermocouple was employed to measure the substrate tempera- The structures and optical properties of thin films derived ture. Cooling the silica support below ambient temperature from polycyclic hydrocarbons and their derivatives on various was not possible with the equipment used. A measured quantity supports have been widely reported.6–15 Structural information (ca. 10 mg) of material was placed in a cleaned molybdenum has been obtained usually by transmission electron or stainless-steel boat at a measured distance from the sub- microscopy9,10,12,15 or by electronic spectra recorded at low strate. The substrate was heated to 493 K at 10-5–10-6 Torr temperatures.6–8,11 Particularly relevant to the present work and allowed to equilibrate for 30 min.This procedure ensured are studies of tetracene and pentacene films supported on clean and dry substrates were obtained. The substrate was glass6–8,11 or carbon10 from which it has been concluded that allowed to cool to the required temperature, whereupon the amorphous structures are formed when films of these hydroorganic material was evaporated by heating resistively the carbons are formed by condensation of the vapour at low boat for 1 min.The substrate was allowed to cool to ambient temperature. Although long-range order is absent, short-range order, to the extent of a few lattice parameters, is present. temperature before being removed for spectroscopic analysis J.Mater. Chem., 1997, 7(10), 2039–2042 2039at room temperature. Film thicknesses were estimated annealing process being significant only above a temperature of half the mp of the compound being evaporated.12 The ratios, assuming a sticking coeYcient of unity, which has been demonstrated to yield values correct to within an order of substrate temperature5mp for the low-melting pyrene and fluoranthrene were somewhat larger than the optimal (Table 1) magnitude.16 owing to experimental limitations.FTIR spectra of the silica and silicon supported films, Preparation of chemically modified thin organic films although restricted in the information available using silica, A thin film supported on silica was loaded into an evacuable were indistinguishable from the spectra of the parent hydro- Pyrex cylindrical cell fitted with Spectrosil B windows and a carbons, strongly suggesting that no chemical change, for Pyrex/PTFE (J.Young) stopcock. Its electronic spectra was example decomposition or decarboxylation, occurred during then obtained. The cell was attached to a reaction manifold evaporation. Well resolved transmission electronic spectra which incorporated a breakseal vessel containing the fluoride which resembled closely those obtained from solutions, were and the entire system evacuated, pumped out for 24 h and obtained from silica-supported films derived from tetracene repeatedly flamed out thoroughly to remove residual moisture.and perylene over a range of thickness ca. 30–800 nm. All the The breakseal vessel was cracked open, the fluoride transferd bands expected in the 600–200 nm region of the spectra were by vacuum distillation to a second vessel containing activated resolved easily.However there was no evidence for solid-state NaF and vapour allowed to fill the cell and manifold. When splittings which might have been expected from previous the reaction cell containing the thin film was supplied with work,6,7,11 possibly because spectra were recorded only at suYcient fluoride to achieve a stoichiometry fluoride5 room temperature.The situation for ovalene films was similar film>151, the cell was isolated from the manifold and allowed except that thick films, ca. 900 nm, showed some loss of to equilibrate for ca. 30 s. Volatile material was removed and resolution particularly below 300 nm.Of the other cata-conthe cell pumped for 5 min to remove any volatile material densed hydrocarbon films examined, anthracene gave well remaining. The evacuated cell was removed from the line and resolved spectra from films up to ca. 100 nm thickness, thick the electronic spectrum of the chemically modified film pentacene films gave rather poorly resolved spectra at waverecorded at room temperature. lengths 400 nm, chrysene spectra were virtually unaVected by increasing the film thickness while the information obtained from rubrene films was limited by interference eVects.Similar Results practical limitations were apparent in the spectra of peri- Determination of experimental conditions for film growth hydrocarbon films.Reasonably resolved bands from violanthrene, decacyclene and fluoranthrene were obtained only The room-temperature electronic spectra of films deposited on from very thin films, thickness ca. 50 nm. Perylene tetracar- commercially polished silica wafers were characterised by very boxylic dianhydride produced well resolved spectra at all film broad absorption bands from which little information could thicknesses whereas for coronene tetracarboxylic dianhydride, be extracted, since the new bands formed on the subsequent spectra were satisfactory only for thin films.The situation was addition of a high-oxidation state fluoride could hardly be similar for coronene tetracarboxylic acid. discerned. However using wafers polished by the method Despite their limited nature, the electronic spectra enabled described previously,1 satisfactory electronic spectra were the experimental suitability of a supported film for further obtained in all cases with the exceptions of pyrene and to study to be assessed, the operational criterion being a compari- some extent, rubrene where the spectra, irrespective of film son with the appropriate solution electronic spectrum.thickness, were dominated by interference patterns. Films, up to an estimated thickness of ca. 1000 nm were stable in vacuo over long time periods apart from anthracene and fluor- Chemically modified supported hydrocarbon films anthrene films which fractured readily. In all cases films were annealled at the temperature at which the substrate was heated Exposure of films derived from the hydrocarbons listed in during evaporation, the most satisfactory temperatures being Table 1 to either boron trifluoride or phosphorus pentafluoride those approaching or slightly greater than, half the melting at room temperature resulted in no observable changes in point of the hydrocarbon, Table 1.The observations are con- electronic spectra.In contrast, spectral changes were substansistent with those made previously using this procedure, the tial in many cases on exposure to molybdenum or tungsten hexafluoride vapours or to arsenic pentafluoride under similar conditions. The most detailed studies were undertaken for Table 1 Optimum temperature range for thin film growth on polished tetracene, perylene and ovalene silica-supported films since silica glass high quality spectra could be obtained readily from a range of substrate temperature films of diVerent thicknesses, ca. 50–1000 nm. compound TS/K TS/mp anthracene 288–298 0.6 Modified tetracene, perylene and ovalene films. Admission of tetracene 333–353 0.54 MoF6 or AsF5 to tetracene films at room temperature led to pentacene 333–353 0.60 immediate colour changes, orange–yellow to blue–green with chrysene 293–303 0.56 rubrene 293–303 0.49 MoF6 and to emerald-green in the case of AsF5.No colour perylene 313–333 0.60 change was observed using WF6. The electronic spectra of the ovalene 333–353 0.46 three modified films, recorded after removal of any unchanged decacyclene 313–333 0.46 volatile materials by pumping at room temperature, are compyrene 288–298 0.68 pared with the unmodified film spectrum in Table 2.In all fluoranthene 288–298 0.76 cases high-energy bands due to tetracene were observed and PTCDAa 313–333 0.56 CTCDAb 333–353 0.48 in the WF6-treated film there was no evidence for any signifi- CTCAc 333–353 0.48 cant change throughout. In the spectrum of AsF5-treated violanthrone 333–353 0.49 tetracene additional weak bands at low energy, lmax>600 nm BEDT-TTFd 293–303 0.59 were apparent.The striking features of the spectrum obtained from MoF6-treated tetracene films were strong bands, lmax 350 aPTCDA=perylene tetracarboxylic dianhydride. bCTCDA=coronene and 670 nm, and its general appearance is reminiscent of the tetracarboxylic dianhydride. cCTCA=coronene tetracarboxylic acid.dBEDT-TTF=bis(ethylenedithio)tetrathiafulvalene. spectrum of tetracene in dimethyl sulfate after oxidation with 2040 J. Mater. Chem., 1997, 7(10), 2039–2042Table 2 Electronic spectra, lmax/nm (absorption coeYcient/cm-1),a of Table 4 Electronic spectra, lmax/nm (absorption coeYcient/cm-1),a of supported ovalene films before and after exposure to MoF6, WF6 supported tetracene films before and after exposure to MoF6, WF6 or AsF5 or AsF5 lmax/nm(a/cm-1) after exposure to lmax/nm(a/cm-1) after exposure to lmax/nm(a/cm-1) lmax/nm(a/cm-1) before exposure MoF6 WF6 AsF5 before exposure MoF6 WF6 AsF5 225(sh) 235(sh) 206(22.7) 205(sh) 213(45.9) 215(45.9) 255(sh) 260(sh) 285(sh) 285(sh) 225(sh) 225(sh) 230(sh) 225(sh) 275(11.6) 280(10.6) 275(11.7) 275(sh) 310(6.5) 350(49.5) 325(sh) 337(sh) 350(sh) 305(sh) 305(sh) 305(sh) 338(sh) 385(8.3) 385(13.2) 413(sh) 413(6.2) 350(26.8) 387(4.1) 387(4.7) 385(sh) 385(5.7) 438(31.2) 437(11.7) 475(sh) 462(sh) 465(6.9) 465(12.0) 412(sh) 412(5.9) 412(sh) 438(7.4) 438(7.2) 438(3.8) 505(30.3) 495(sh) 500(11.7) 525(sh) 530(10.5) 460(4.4) 472(1.1) 478(sh) 472(11.0) 472(3.4) 550(5.5) 550(10.2) 620(sh) 503(14.0) 503(13.9) 505(2.5) 525(7.3) 520(13.6) 525(2.4) 670(3.9) 750(2.6) 750(9.1) 605(sh) 625(0.8) 670(25.0) 675(1.0) 850(2.9) 962(3.6) 1000(0.9) 1000(9.1) 725(sh) 775(4.1) 765(1.8) 1010(3.1) aSee footnote to Table 2.aDefined by: absorbance=104a×film thickness(cm); sh=shoulder. Table 5 New, low-energy bands, lmax/nm (absorption coef- ficient/cm-1),a of some cata-condensed hydrocarbons after exposure to MoF6, WF6 or AsF5 Table 3 Electronic spectra, lmax/nm (absorption coeYcient/cm-1),a of supported perylene films before and after exposure to MoF6, WF6 new bands lmax/nm(a/cm-1) or AsF5 after exposure to (lmax/nm(a/cm-1) after exposure to hydrocarbon MoF6 WF6 AsF5 lmax/nm(a/cm-1) before exposure MoF6 WF6 AsF5 anthracene 920(0.5)b 460(2.3) 430(2.5) pentacene 785(4.9) —c —c 225(sh) 975(0.8) 260(13.8) 270(sh) 270(sh) chrysene 455(sh) 433(sh), 480(sh) 330(8.9) 330(sh) 533(sh) 500(1.4) 560(0.7) 340(8.4) 360(sh) 358(sh) 365(sh) 680(1.5) 680(1.1) 435(sh) 407(17.7) rubrene 615(10.1) —c —d 470(sh) 495(5.6) 479(13.3) 490(4.8) 505(sh) aSee footnote to Table 2.bLoss of resolution at higher energies. cNo 660(4.4) 623(13.3) 650(4.4) significant change. dNot examined. 1000(3.8) 1230(5.5) aSee footnote to Table 2. Table 6 New, low-energy bands, lmax/nm (absorption coef- ficient/cm-1),a of some peri-hydrocarbon derivatives after exposure to MoF6 SO3. The latter spectrum has been attributed to a mixture of mono- and di-positive tetracene cations.17 hydrocarbon derivative lmax/nm(a/cm-1) Perylene films darkened immediately on exposure to MoF6, perylene tetracarboxylic dianhydride 620(sh) WF6 or AsF5 at room temperature, the colour change being 770(4.4) most marked with MoF6 where the sequence golden– coronene tetracarboxylic dianhydride 500(14.5) yellow�red�crimson�violet�black was observed over 1 h.violanthroneb 725(6.3) The electronic spectra obtained after removal of volatile mate- 920(12.4) rial (Table 3) were all similar and, as in the tetracene–AsF5 decacyclene 770(3.2) case (Table 2) new bands at low energies were observed, lmax aSee footnote to Table 2.bExposure to AsF5 resulted in a new weak ca. 650 and 1000 nm with MoF6 and AsF5 and lmax band at lmax 690(3.7). ca. 623 nm with WF6. The behaviour of supported ovalene films was similar. Blue– green colouration of the orange films was immediately apparent a blue colouration on exposure to MoF6 and the spectra of modified films had two prominent new features, lmax=775 and on their exposure to MoF6 or AsF5 and although no change in colour was apparent when WF6 was admitted, the electronic 895 nm.Fluoranthrene films showed no change in their spectra after MoF6 treatment and those derived from coronene tetra- spectra in all cases showed several new bands at lmax>500 nm (Table 4).carboxylic acid (CTCA) lost spectral resolution. Other modified films. New spectral bands observed after Discussion modification of four cata-condensed hydrocarbon films and four films derived from peri-hydrocarbon derivatives are sum- The most striking finding from this work is the strong adsorption of molybdenum and tungsten hexafluorides and of arsenic marised in Tables 5 and 6.New low-energy bands after MoF6 treatment were observed in all cases, after AsF5 treatment in pentafluoride on many of the supported organic films as demonstrated by the appearance of new bands at low energies three cases but only for anthracene and chrysene was there any evidence for modification by WF6.Films derived from that may generally be described as being due to charge transfer between the organic molecules and the adsorbed fluorides. bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) developed J. Mater. Chem., 1997, 7(10), 2039–2042 2041Monocyclic p donors such as benzene or hexafluorobenzene, cases and by AsF5 in a substantial number. Adsorption of BF3 and PF5 on the films, if it occurs, does not result in any change interact only weakly with high oxidation state halides and in these latter cases isolable complexes are not formed.2 Apart in electronic properties of the hydrocarbons, while WF6 occupies an intermediate position.A more detailed consideration from the reaction between tetracene and MoF6, where the spectrum of the modified film (Table 2) provides some evidence requires structural information to be obtained on the precise nature of the surface layer but the importance of the oxidizing for formation of tetracene radical cations,17 there is no compelling evidence that electron transfer is complete.The presence ability of the fluoride is clear. of multiple charge-transfer transitions in the spectra of many of the modified films is not unexpected and has been reported We thank Dr.T. Baird and Dr. J. R. Fryer (both University for numerous solid complexes.3 of Glasgow) and Dr. R. Richardson (Glasgow Caledonian There have been many studies of charge-transfer complexes University) for practical assistance and provision of facilities in which attempts have been made to correlate the energy of and are grateful to EPSRC for financial support of this work. the charge-transfer absorption band(s) with intrinsic properties of the donor or acceptor species.18,19 However the results References obtained suggest that in the systems studied here the appearance or otherwise of charge-transfer absorption bands cannot 1 D.S. Boyle and J. M. Winfield, J.Mater.Chem., 1996, 6, 227. be correlated with any single property of a given molecule. 2 P. R. Hammond and R. R. Lake, J. Chem. Soc. A, 1971, 3800; 3806; This may reflect the diversity of polycyclic aromatic com- 3819; P. R. Hammond and W. S. McEwan, J. Chem. Soc. A, 1971, 3812; R. R. McLean, D. W. A. Sharp and J. M. Winfield, J. Chem. pounds investigated; diVerences in chemical behaviour and Soc., Dalton T rans., 1972, 676.spectroscopic characteristics between series of structurally 3 R. Foster, Organic Charge T ransfer Complexes, Academic Press, related polycyclic aromatic hydrocarbons (PAHs) have been London, 1969, and references therein. widely reported.20 Correlations within any given series, for 4 R. S. Mulliken, Recl. T rav. Chim. Pays-Bas., 1956, 75, 845.example cata-condensed PAHs, are expected and often found.21 5 E. Clar, Ber. Dtsch. Chem. Ges., 1936, 69, 607. For example, the linear acene series exhibits most clearly the 6 W. Hofberger, Phys. Status Solidi A, 1975, 30, 271. 7 R. Hesse, W. Hofberger and H. Ba�ssler, Chem. Phys., 1980, 49, 201. eVect of annelation, the three main absorption bands a, para 8 L. Sebastian, G.Weiser and H. Ba�ssler, Chem. Phys., 1981, 61, 125. and b, (characteristic to all PAHs) are progressively red-shifted 9 J. R. Fryer and D. J. Smith, Proc. R. Soc. L ondon Ser. A, 1982, with each additional aromatic ring, representing a dilution of 381, 225. the aromaticity; hence anthracene is colourless, tetracene is 10 R. Elermann, G. M. Parkinson, H. Ba�ssler and J. M. Thomas, golden–yellow and pentacene violet–blue. These compounds J.Phys. Chem., 1983, 87, 544. (D2h symmetry) provide a sample group for which it would be 11 R. Jankowiak, K. D. Rockwitz and H. Ba� ssler, J. Phys. Chem., 1983, 87, 552. expected that for a series of donor–acceptor complexes, the 12 J. R. Fryer, Mol. Cryst. L iq. Cryst., 1983, 96, 275. intermolecular repulsion and relative orientations of donor 13 M.Migita, Y. Taniguchi, H. Ishihara, M. Akagi, T. Ishiba and and acceptor molecules would be similar. However, no simple H. Tamura, J. Appl. Phys., 1985, 58, 1187. conclusions can be drawn from a comparison of the lowest 14 W. Dietsche, Th. Rapp and H. C. Basso, J. Appl. Phys., 1986, energy CT band observed after exposure to MoF6 with the 59, 1431.MO coeYcient xi19 or Ei22 of the donor PAH for the acene 15 J. R. Fryer and M. E. Kennedy, Macromolecules, 1988, 21, 259; J. R. Fryer, MSA Bull., 1994, 24, 521 and references therein. series anthracene, tetracene and pentacene. 16 L. Holland, Vacuum Deposition of T hin Films, Chapman and Hall, In principle CT bands might be correlated with the Lewis- London, 1956. acid strengths or oxidizing abilities of the binary fluorides.The 17 W. Ij. Aalbersberg, G. J. Hoijtink, E. L. Mackor and former appear not to be relevant, at least using gas-phase F- W. P.Weijland, J. Chem. Soc., 1959, 3049. aYnities as the indicator of Lewis acidity. Thermochemical 18 R. S. Mulliken and W. D. Person, Annu. Rev. Phys. Chem., 1962, estimates of gas-phase F- aYnities are in the order 13, 107. 19 M. J. S. Dewar and H. Rogers, J. Am. Chem. Soc., 1962, 84, 395; AsF5>PF5>BF3,23 however gas-phase F- transfer reactions A. R. Lepley and C. C. Thompson, Jr., J. Am. Chem. Soc., 1967, observed by ICR24 and F- ion displacement reactions in 89, 5523. MeCN25 both point to WF6 being a weaker Lewis acid than 20 C. A. Coulson, B. O’Leary and R. B. Mallion, Hu�ckel T heory for BF3.The existence of the [MF7]-, M=Mo and W, anions in Organic Chemists, Academic Press, London, 1978, and references equilibrium with the hexafluorides MF6 in MeCN at room therein; E. Clar, T etrahedron, 1959, 5, 98; 1959, 6, 355; 1960, 9, 202. temperature, suggests that WF6 and MoF6 have comparable 21 Z. H. Zaidi and B. N. Khanna, J. Chem. Phys., 1969, 50, 3291; Z. H. Khan and B.N. Khanna, J. Chem. Phys., 1973, 59, 3015; F- ion aYnities.26 It appears therefore that although Lewis E. Clar and W. Schmidt, T etrahedron, 1975, 31, 2263; Z. H. Khan, acidity may be a necessary criterion, it is not a suYcient Can. J. Spectrosc., 1978, 23, 8; 1984, 29, 63; Z. Naturforsch., T eil A, criterion for modification of PAH films observed here. 1984, 39, 668; 1987, 42, 91; Spectrochim.Acta, Part A., 1988, 44, The superior one-electron oxidizing ability of MoF6 com- 313; 1125. pared with WF6 is well established both in the gasmidt, J. Chem. Phys., 1977, 66, 828. and in solution, anhydrous HF28 or MeCN,29 and studies of 23 T. E. Mallouk, G. L. Rosenthal, G. Mu� ller, R. Brusasco and N. Bartlett, Inorg. Chem., 1984, 23, 3167.the behaviour of these and other fluorides with respect to the 24 P. M. George and J. L. Beauchamp, Chem. Phys., 1979, 36, 345. oxidative intercalation of graphite30–33 make an interesting 25 M. F. Ghorab and J. M. Winfield, J. Fluorine Chem., 1990, 49, 367. comparison with the present work. Bartlett and co-workers 26 M. F. Ghorab and J. M. Winfield, J. Fluorine Chem., 1993, 62, 101. have established that a thermodynamic threshold exists for the 27 N. Bartlett, Angew Chem., Int. Ed. Engl., 1968, 7, 433. intercalation of fluoroanions derived from binary fluorides, 28 A. M. Bond, I. Irvine and T. A. O’Donnell, Inorg. Chem., 1975, 14, MFx.30 The fluorides MoF6 and AsF5 are suYciently oxidizing 2408; 1977, 16, 841. 29 G. M. Anderson, J. Iqbal, D. W. A. Sharp, J. M. Winfield, for spontaneous intercalation to occur at room tempera- J. H. Cameron and A. G. McLeod, J. Fluorine Chem., 1984, 24, 303. ture,30–32 the estimated changes in DG2980 for 30 N. Bartlett and B. W. McQuillan, in Intercalation Compounds, ed. MoF6(g)+e-�[MoF6]-(g) and for 3 2 M. S. Whittingham and A. J. Jacobson, Academic Press, New York AsF5(g)+e-�[AsF6]-(g)+DAsF3(g) being similar. Tungsten and London, 1982, ch. 2, p. 19. hexafluoride and phosphorus pentafluoride are insuYciently 31 D. Vaknin, D. Davidov and H. Selig, J. Fluorine Chem., 1986, strong oxidizing agents to initiate spontaneous intercalation 32, 345. 32 M. Lerner, R. Hagiwara and N. Bartlett, J. Fluorine Chem., 1992, of [WF6]- or [PF6]-,30 although PF5 or BF3 do react with 57, 1. graphite in the presence of HF and with Cl2 as the oxidizing 33 F. Okino and N. Bartlett, J. Chem. Soc., Dalton T rans., 1993, 2081. agent.32 In the present work modification of the supported polycyclic hydrocarbon films by MoF6 has been demonstrated in all Paper 7/01893B; Received 18th March, 1997 2042 J. Mater. Chem., 1997, 7(10), 2039&ndas