Chapter 3

Synthesis and Deposition of Electron Donors, Acceptors, and Insulators as Components

of Zirconium Diphosphonate Multilayer Films

Η. E. Katz, Μ. L. Schilling, S. Ungashe, T. M. Putvinski, and C. E. Chidsey

AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

The deposition of three organodiphosphonates as multilayered zirconium salts is described. The organic cores of the compounds include quaterthienyl, dicyanodipyridylquinodimethane, and biphenyl, which are electron donating, accepting, and insulating, respectively. Structures of the multilayers are proposed based on ellipsometric and absorbance data. Preliminary electrical measurements indicate that the films are nonconductive. The compounds are potential components of heterostructures in which photoactivated charge transfer may be observed.

Current and anticipated uses of organic thin films include those that may be classed as "passive", such as for electrical insulation or protection against corrosion, and others in which the films "actively" contribute to an electrical or optical response. The effectiveness of these films depends upon achieving dense substrate coverage with layers of defined thicknesses of tens to hundreds of angstroms. Many of the latter, "active," applications rely additionally upon the spacing and directional sequence or orientation of multiple chemical components comprising the film.

Several options are available for producing films with the required architectural control. For example, the sequence of components in a multilayer may be established by subliming compounds onto a substrate in a particular order.1

Unfortunately, control of orientation and thickness with this method is difficult The Langmuir-Blodgett technique2 offers firmer control of layer thickness and orientation, as does organotrichlorosilane-based self-assembled multilayer deposition.3 However, all of these procedures lead to films that are held together primarily by ordinary covalent bonds or van der Waals interactions, and therefore lack the desirable attributes of strength and temperature stability that are associated with other material classes such as inorganic crystals and ceramics.

Mallouk and coworkers recentiy demonstrated the layer-by-layer deposition of zirconium a,ω-alkylidenediphosphonates on Au and Si substrates, forming multilayers whose schematic structure is illustrated in Figure L.4 The resulting films

0097-6156/92/0499-0024$06.00/0 © 1992 American Chemical Society

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3. KATZ ET AL. Zirconium Diphosphonate Multilayer Films 25

Zr Zr

0 V 0 ^ Zr Zr Λ

° o Ο ? Zr

Zr ο n

Z r

I 0

ο ο ο ο - Zr Zr

\?\ 0 / Zr

Figure 1. Schematic diagram of Z r organodiphosphonate multilayer structure.

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26 SUPRAMOLECULAR ARCHITECTURE

are markedly stable because the interlayers consist of Z r + 4 ions strongly and multiply coordinated to phosphonate groups, analogously to high-melting, chemically robust zirconium phosphate and phosphonate bulk layered solids. We wished to take advantage of this new multilayer deposition method and the resulting architecture to incorporate conjugated moieties with well-defined conformations and electronic properties. These new molecular subunits may allow us to explore the bulk passivating properties of the multilayers, as well as in observing microscopic electron transfer phenomena in a rigorously defined arrangement

This paper focuses on three organodiphosphonates that have been synthesized and deposited as zirconium phosphonate multilayers. Compounds 1 and 2 (Figure 2) comprise an electron donor-acceptor pair, and were designed to possess minimal conformational freedom, two antiparallel C-P bonds at each end, and no ionic groups in the middle of the molecules that might interfere with multilayer formation. These structural features are shared by 3, which is envisioned as a spacer or barrier molecule separating electroactive layers, and as a building block for insulating multilayers.

Syntheses and Electrochemical Properties

The reaction sequences leading to the three compounds are shown in Figure 2. While the syntheses of 1 and 3 are straightforward, that of 2 required three steps for which there was little precedent in the literature. The reaction in which 2 is actually formed may be viewed as a dehydrogenative Swern oxidation in which the dimethylsulfoxide (DMSO) oxidant is activated by the trimethylsilyl phosphonate groups also present in the substrate.

Electrochemical experiments were performed on the soluble octamethyltetraamide and tetraethyl ester precursors to 1 and 2, respectively. The quaterthiophene displayed a partially reversible oxidation at +1.1 V vs SCE in 0.05 M B u 4 N P F 6 - T H F . The quinodimethane exhibited two fully reversible reductions at -0.32 and -0.71 V under the same conditions. Thus, the energy required to transfer an electron from 1 to 2 is expected to be ca. 1.4 eV (11,000 c m " 1 ), considerably less than the energy available from photoexcitation of the compounds. Compound 2 as a multilayer on Au displayed a hint of the predicted reversible electrochemistry, but much less than the theoretical current was observed due to the insulating nature of the system (vide infra).

Deposition and Characterization of Multilayers

The protocol described by Mallouk for long chain aliphatic diphosphonic acids was modified to enable the deposition of more rigid diphosphonic acids with Z r + 4 . Surfaces with high densities of phosphate/phosphonate functionality were prepared by immersing gold-coated silicon or mica in 1.5 m M ethanolic 8-mercaptooctylphosphonic acid, or by treating silicon oxide surfaces with (3-aminopropyl)trimethoxysilane in octane at reflux, followed by P O C l 3 and a tertiary amine in C H 3 C N . These POf-terminated surfaces were then zirconated with 5 mM aqueous Z r O C l 2 . Using 1 as the substrate for optimization of conditions, aqueous DMSO was judged to be the most suitable solvent from which to deposit layers of the diacids. It was necessary to maintain a temperature of >80 °C and a pH of 3-4 to ensure complete coverage of the zirconated surfaces by the aromatic compounds in a reasonable time.

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KATZ ET AL. Zirconium Diphosphonate Multilayer Films

' - ^ S H O ) - ' — Η ϊ Ο , * - / ^ - © _ Ρ Ο , Η ϊ

N — / N — / 2) TMSBr N — / N — /

3 Figure 2. Syntheses of organodiphosphonates.

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28 SUPRAMOLECULAR ARCHITECTURE

Layers of 1 and 3, which are constrained to pack as extended molecular structures, were completely formed in 5-20 minutes, giving the expected ellipsometric thicknesses, 21 and 15 A , respectively. An idealized structure for the multilayer array of 1 is shown in Figure 3. Slight defects in this structure could arise from C-C bond rotation or tilting of the long axes of the molecules.

On the other hand, 2, despite possessing only two conformationally significant rotational degrees of freedom, may exist in conformations where the C-P bonds are not parallel to the long axis of the molecule, and may have particularly electron-deficient phosphonate groups. This species required 1 hour for each deposition and gave layers of only half the 20 A thickness expected if the molecules were fully extended. A possible structure for this array appears in Figure 4; the upper limit of the layer thickness for this particular arrangement is 14 A. Twisting the pyridyl rings away from normality to the surface would further reduce the thickness. The layer-to-layer reproducibility of the monolayer depositions in the construction of multilayers is illustrated in the plots of Figure 5, in which ellipsometric thickness as a function of layer number defines nearly straight lines.

Multilayers of 1 and 2 were further characterized by UV-vis spectroscopy on glass; Figure 6 shows absorbance vs layer number for these systems. From the slope of the plot for 1, it was determined that each layer contained 20 A of material with extinction coefficient 3.4 χ 1 0 4 M " 1 c m " 1 (derived from solution spectra) and number density corresponding to an appropriate crystallographic5 concentration of 2.7 M . A similar treatment of the absorbance data from 5 layers of 2 (deposited atop 5 layers of 1), with extinction coefficient 4.8 χ 1 0 4 M _ 1 c m " 1 , £ave a concentration of 2 of only 1.2 M , assuming the ellipsometric thickness of 11 A/layer. Considering that the molecules are of similar molecular weight and allowing for the fact that the density of 2 is less than that of 1 due to the absence of sulfur atoms, the low calculated concentration for layers of 2 is indicative of poorer packing, more severe tilting, and/or a greater number of discontinuities than is reflected in Figure 4. The possibility that the extinction coefficients of the molecules in solution do not exactly correspond to the extinction coefficients in the films adds a further degree of uncertainty to the above analyses.

Some preliminary electrical measurements have been performed on multilayers of 1 and 3. A coating of as few as 3 layers of 1 on indium-tin oxide-coated glass formed an insulating barrier to a four-point probe. Multilayers of 1 on glass, when exposed to B r 2 vapor, displayed a weak absorbance at 680 nm, roughly in the range expected for quaterthiophene radical cation,6 but did not show increased sheet conductivity. An Hg drop electrode resting on 10 or 20 layers of 3 on Au formed a capacitor. The dielectric constant of the multilayer was estimated to be 4, in rough agreement with bulk layered Zr phosphonates,7 and the resistivity was ca. 10 1 3ohm cm. Further experiments are in progress to determine these values more precisely. Understanding the mechanisms of conductivity and dielectric response in these materials will aid in the interpretation of data that may be acquired in the future on photoactivated systems.

Conclusions

Fully conjugated diphosphonic acids containing electron-rich, electron-poor, and electrochemically unreactive chromophores have been synthesized and shown to form

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KATZ ET AL. Zirconium Diphosphonate Multilayer Films

Zr Zr

Figure 4. Layer arrangement of 2-Zr

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30 SUPRAMOLECULAR ARCHITECTURE

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3. KATZ ET AL. Zirconium Diphosphonate Multilayer Films 31

Ο 5 10 15 20 layer number

0.3 r

0.1

Ο 2 4 6 8 10 12

layer number

Figure 6. Absorbance vs layer number for (a) 1 on glass, and (b) 5 layers of 2 (layers 6-10) on 5 layers of 1 (layers 1-5) on glass. Absorbances for layers of 1 are recorded at its λ max of 390 nm, and absorbances for layers of 2 are cumulated based on readings at its ^max» 440 nm. Note that the second sample constitutes an acceptor-donor heterostructure.

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32 S U P R A M O L E C U L A R A R C H I T E C T U R E

stable, densely packed Zr-based self-assembled multilayer assemblies. The films electrically and chemically passivate underlying layers. We plan to investigate optically induced charge transfer in heterogeneous multilayers containing various arrangements of compounds such as 1-3, with particular attention to the environmental stability, kinetics, decay, and mechanism (conduction vs tunneling, distance dependence) of the charge transfer, and the possible utilization of the charge separation and recombination events to produce useful optical and electrical signals.

Literature Cited

1. So, F.F.; Forrest, S.R.; Shi, Y.Q.; Steir, W.H. Appl. Phys. Lett. 1990, 56, 674-676.

2. Popovitz-Biro, R.; Hill, K.; Landau, E.M. Lahav, M.; Leiserowitz, L.; Sagiv, J.; Hsuing, H.; Meredith, G.R.; Vanherzeele H. J. Am. Chem. Soc. 1988, 110, 2672-2674.

3. Maoz, R.; Netzer, L.; Gun, J.; Sagiv, J. J. Chim. Phys. 1988, 85, 1059-1065; Tillman, N.; Ulman, Α.; Penner, T.L. Langmuir 1989, 5, 101-111.

4. Lee, H.; Kepley, L.J.; Hong, H.-G.; Mallouk, T.E. J. Am. Chem. Soc. 1988, 110, 618-620.

5. Van Bolhuis, F.; Wynberg, H.; Havinga, E.E.; Meijer, E.W.; Staring, E.G.J. Synth Met. 1989, 30, 381-389; Visser, G.J.; Heeres, G.J.; Wolters, J.; Vos, A. Acta Cryst. 1968, 24B, 467-473.

6. Fichou, D.; Horowitz, G.; Garnier, F. Synth. Met. 1991, 39, 125-131; Caspar, J.V.; Ramamurthy, V.; Corbin, D.R. J. Am. Chem. Soc. 1991, 113, 600-610.

7. Casciola, M.; Costantino, U.; Fazzini, S.; Tosoratti, G. Solid State Ionics 1983, 8, 27-34.

RECEIVED January 16, 1992

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