Symposium on Molecular Architecture
Designer Solids and Surfaces Thomas E. Mallwk and Haiwon Lee The University of Texas at Austin, Austin, TX 78712
In the past few years many advances have been made in the oreoaration of suoramolecular solid materials. Such solids,khIcb usually contain one or more distinctly molecular comoonents. are now being investigated hecause of their potehtial applications in he&rogene& catalysis, molecular electronics, integrated optical systems, and solar energy conversion. The interesting chemical and physical properties that make these applications possible derive from "molecu- lar recognition"-i.e., specific molecule juxtaposition and reactivity in the solid state. The periodicity of crystalline solids also plays an important role in determining the properties of some of these new materials. For example, lattice oeriodicitv can he used to oosition molecules . orooerlv - .for polymerization reactions (I). Cooperative electronic effects whichderive from the Deriodic arrangement of molecules are responsible for the proberries of organic metals (2),semironductors (2). ~ u ~ e r r o n d u c t o(3). r s and ferromagnets ( 4 ) . Despite the practical advantages that supramolecular solids offer, our ability to synthesize such materials according to rational designs is still rather primitive. Organic chemists have a t their disposal hundreds of functional group-specific reactions that allow them to prepare molecules of precisely controlled architecture and function; the elegant host-guest chemistrv described hv Francois Diederich and Andrew Hamilton in this syrnp&ium exemplifies the level of design that is now attainable to the creative and persistent synthetic organic chemist. On the other hand, solid state reactions are most often carried out under conditions where only the most stable products are obtained (51, and the solid state analorme of organic functional group chemistry is only now beginbing to dkvelop. This me& that rationilly designed target structures may he synthetically unattainable; likewise, the t b e r m ~ d ~ n a m i c afavored l l ~ products of a seemingly rational solid state synthesis are often completely unexpected and are sometimes much more interesting than the original target. An excellent examole of the latter ohenomenon is the family of high criticai temperature ( ~ , cuprate j superconductors. which followed Bednorz and Miiller's discoven, of superconductivity in Lan-,Sr,CuO< in 1986 (6).This compound, with x = 0.16, has a T, of 35 K and adopts the relatively simple KnNiFa structure. Cbu and co-workers (7) reasoned that substituting the smaller Y3+ for La3+ would compress the CuOz sheets found in this structure, thereby increasing T,. They prepared a mixed oxide containing Y, Ba, and Cu in the appropriate ratio and found that only a small fraction of the sample underwent a superconducting transition but with the amazingly high T,of 91 K. Subsequent experiments (8.9) showed that the superconducting ohase was an unexoected new com~ound.YBa?Cu2O.i-,, . . . ... and that no K2NiF&pe composition was stable in the Y-BaCu-0 system. Many experiments followed in laboratories around the world, and even higher T,materials (e.g., in the Bi-Sr-Ca-Cu-0 and T1-Ba-Ca-Cu-0 systems) were discovered. A recent compilation of structural data from these new materials shows that the T,value depends not only on
the degree of oxidation of the CuOn sheets but also on the details of the three-dimensional structure around them (10). The highest T,materials invariably have complex structures (such as T1Ba2Ca3Cua0,,, .. T, . = 122 K (11))and are the unexpected products of reacting the constituent oxides and peroxides-TlsOa, BaOz, CaO?, CuO-at elevated temperaLures. At the temperatures of these "shake and bake" reactions (800-900 "C), there is little hope of doing what the organic chemist does-assembling a compound of predictable structure from two or more nieces. while retainine the bonding relations between most of the donstituent a t o i s . The activg tion energy needed to sustain intergrain diffusion between most ionic solids is comparable to the bond energies of the product compound. Therefore, rationally designed supramolecular solids cannot be made in high-temperature, solid phase reactions from refractory precursors. Fortunately, several low-temperature reaction strategies are available to solid state chemists. A wide variety of interesting, metastable materials have been prepaied from hydrothermal and sol-gel syntheses a t temperatures ranging from ambient to about 300 "C. Other routes which are the subiect of the present article include reacting a refractory hut microporous "host" with a suitable molecular "guest," and assembling a metastable solid a t low temperature from two or more soluble (or volatile) molecular orecursors. These techniaues are now being used extensivel; to prepare "designer" solids of predictable structure and properties. ~
Host-Guest Chemistry Intercalation (or insertion) reactions of ions and molecules with layered solids such as graphite, and microporous solids such as zeolites, have been known for centuries'. These reactions proceed a t room temperature and are topochemical, in the sense that covalent bonds within both the host and guest are preserved (12). Host-guest compounds in which one molecular component, such as water or thiourea, forms a hydrogen-bonded cage structure around a nonpolar euest molecule have also been known for manv" "vears (13). . . These "old" techniques are of particular contemporary interest in the search for designer - materials with s~ecificallv tailored properties. One approach to making solids capable of optical second harmonic generation (SHG)-an important property for applications in integrated electronic and optical devices-is to use host-guest chemistry to rause alignment of large ensern. bles of hyperpolarieable mulecules. Mulecules such as p -
' Graphite intercalation chemistry dates back nearly 150 years, to
Schauffautl's preparation of graphite sulfate (Schauffaijtl.P. J. hakt. Chem. 1841, 21, 155).The term "zeolite" (coined by Cronste*, A. F. Akad. Hand1 Stockholm 1758, 1Z 120) derives from Greek words meaning "boiling stone" and refers to the release upon heating of large quantities of water that are entrained in the channel networks of natural zeolites. Volume 67
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AlP04- 5
4 1 p
%a u -
-
Figure I. TWO views of Me polar AIPOA-5 channel (horn Smith. J. V. them. Rev. 1988, 88, 149. Copyright 1988. American Chemical saclety: reprinted with permission).
nitroaniline can be more easily polarized by an electric field that forces electrons from an electron-rich (-NHd to an electron-poor (-NOz) substituent than by the opposite electric field. In the oscillating field of a light wave such compounds generate light at the doubled frequency if the electron donor-acceptor vectors of individual molecules can he arranged to reinforce each other. In centrosymmetric media (homogeneous solutions, polymers, and crystals containing inversion centers) these vectors cancel and no SHG is observed (14). Unfortunately, most organic molecules, including p-nitroaniline, do crystallize in centric space groups, precluding the use of the pure compounds in SHG-based devices. Cox et al. have shown that the noncentric, microporous aluminophosphate AIPOa-5 strongly absorbs p-nitroaniline within its 7.5-A diameter pores (Fig. 1). The polar host c~ystalforces alignment of the guest molecules; at a loading of 13%~-nitroaniline by weight, the pores are maximally filled and the SHG signal is highest (15). Similar effects have been demonstrated by Eaton and coworkers (161, who have made inclusion complexes of hyperpolarizable molecules such as benzenechromium tricarbonyl and D-nitroaniline with hvdroaen-bonded hosts (e.n., thio. .. urea, &cyclodextrin). These solids nre simply precipitated f r o ~ a s o l u t i o n othe f host andruest,andexhihit wbstantial . SHG when the inclusion complex crystallizes in a polar space group. Surprisingly, while only 30% of all organic and organometallic compounds crystallize in acentric groups, 85% of the compounds studied by Eaton et al. were acentric and polar. This result suggests that dipolar molecules that are interesting from the point of view of nonlinear optics
Figure 2. Zealitetramework structures. Clockwise horn upper left Faujasite(ZeoiitesX and Y). Zeolite A. ZSM-5. Zeolite L. (from Newsam. J. M. Science 1988,231, 1093 and Dyer, A. Chem. Ind. 1984, 7.237. Copyright 1986, American Association fwlhe Advancement of Science, and copyright 1984. Society of Chemical Indusby; reprinted with permission).
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Journal of Chemical Education
have a strong tendency t o direct their inclusion compounds into polar structures. Solid state hoseguest chemistry has also been used to generate catalytically interesting supramolecular structures. Zeolites (molecular sieves) are well known as shape-selective strong acid catalysts for hydrocarbon cracking and reforming reactions (17). They also make excellent templates for the assembly of multicomponent molecular catalysts. The well-defined pore networks of zeolites such as types A, Y, L, and ZSM-5 (Fig. 2) direct the siting of catalyst molecules, allowing one to induce cooperative effects or to isolate reactive intermediates spatially. The pore structure also controls access of substrates to the "active site", in close analogy to the function of proteins in enzymatically catalyzed reactions. Herron and co-workers have exploited tbese properties to make biomimetic oxygen carrier and monooxygenase supramolecular assemhlies (18).
1 Co(salen), 1, encapsulated by a "ship-in-a-bottle" synthesis within the supercages of zeolite Y, hinds dioxygen, mimicking the action of hemoglobin. The zeolite supercage accommodates only one molecule of 1, so irreversible bimolecular disproportionation to form p-0x0 bridged dimers, which is rapid in fluid solution, cannot occur (19). Zeolite A containing both Fe(I1) and Pd(0) centers shows regioselective monooxygenase activity in the presence of straight-chain hydrocarbons and a mixture of HZand 0 2 . The palladium centers of this bifunctional catalyst combine hydrogen and oxygen tomake H202,which then oxidizes alkane to l-alkanolat the iron sites. Pore blocking by the reaction products limits the utility of this reaction with zeolite A. However, Fe(I1)-exchanged ZSM-5, which has slightly larger pores, allows the products to escape; also, this high-silica, hydrophobiczeolite preferentially sorbs the nonpolar hydrocarbon reactants within its channel network, allowing for higher turnover numbers and excellent product selectivity (20). Our group has used large-pore zeolites such as L and Y as templates for the self-assembly of biomimetic photosynthetic systems (21). By simultaneously exploiting the steric (size 'exclusion) and ion-exchange properties of tbese hosts one can cause as many as four electroactive species to fall into line a t the zeolite/aqueous solution interface. Figure 3 shows the generic four-suhunit case involving anion A, which is excluded from the anionic pore structure on the basis of its charge, cation B-C, which straddles the interface because subunit B is size-excluded, and cation D, which is not sizeexcluded and occupies sites within the zeolite. An A-B-D triad can be assembled a t an electrode b s interposing a rationic polymer (whirh bindsanions A) betbeen the zeoiite and the electrode surface. The integrity of self-assembling
Water
Figure 3. Self-assemblyof a interlace.
Zeolite L or Y
four-subunitredox chain at the zeoiltelwater
-
triads such as Fe(CN)s4-Os(bpy)32+-(trimethylamin~)metbvl ferrocene+ (bov 2.2'-binwidine) can he checked e l e c t ~ o ~ h e m i c a(215; l l ~ this' partilu~ara;semblyacts as a current rectifier because the (trimethvlamino)rnethvlferrocenefa (-Fc+Io) couple within the zeokte cannot communicate directly with the electrode. The Os(bpy)32+/3+couple is more oxidizing than Fc+Ioand can pass electrons from Fc+lO to the electrode, but not the other way. Similar experiments with R ~ ( b p y ) , ~ + and / + cobaltocene/cobalticenium derivatives show current rectification in the opposite sense (22a), since the ruthenium couple is the better reducing agent than the cobalt couple. One can progress from simple electrochemically driven molecular rectifiers (the analogue of a semiconductor p-n junction) t o photodiodes (the analogue of a semiconductorbased solar cell) by incorporating a suitable light absorbing molecule into the chain.
Molecules 2 and 3 occupy positions B-C and D (Fig. 31, respectively, when ion-exchanged onto the surface of zeolite L particles. 2 is a photochemically active donor-acceptor diad that rapidly undergoes internal electron transfer (from the R ~ ( b p y ) to ~ ~the + diquaternary 22-bipgridine moiety) when excited by visible light. 3 is a better electron acceptor than the acceptor end of 2, and captures the elektron before reverse electron transfer can occur within 2 toregenerate the ground state (21e). The result is a long-lived light-induced state in which an electron has been transferred from 2 to 3. This system is reminiscent of the donor-acceptor-acceptor triad in the photosynthetic reaction center of purple bacteria. in which extremelv efficient and irreversible lieht-induced charge separation occurs. Regeneration of the ground state orobablv ocrurs bv "uohill" electron transfer from 3 to the aEceptor knd of 2, followed by acceptor to donor charge recombination within 2. Lea0 - Chemistw2
Another versatile approach to the synthesis of supramolecular solids is to snar, together soluble or volatile molecular precursors, taking &vantage of the structure-directing character of ionic bonds where the subunits are joined. This method has been most widely used with lamellar solids, e.g., the amine-metal halide salts (RNH&MXa (so-called layer perovskite salts, which actually have a structure closely related, Figure 4, t o K%NiF4),the vanadyl phosphonates VO(RP03) H 2 0 . R'OH, and the zirconium phosphonates Z I ( R P O ~ )These ~. compounds are made by combining suitable molecular precursors (an organic amine or phosphonic acid) with a suitable soluble metal salt. The layered solid precipitates in the desired structure as an insoluble salt. The interlayer spaces of these lamellar metallorganic compounds
.
LEW Is a registered trademark of lnterlego Ag. The authors are indebted to GeoffreyOzin for using this term to describe someof their work.
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Figure 4. Struchlres of layered metal-organic compounds: (RNH&MX4 (from ref lb), and proposed structures for VMRPO.). HsO. R'OH and Zr (RP0.b.
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can he reearded as microsconic reaction vessels in which the organic molecules are held, by the periodic inorganic lattice, in the DroDer orientation for a s ~ e c i f i chemical c reaction. For example, hiacetylene molecul~s(R) can he held a t just the right angle and separation distance from each other in (RNH3)zMXa salts ( I b , c) so that high polymers are formed. Positioning of the R groups is crucial because chain termination steps occur in competition with free radical polymerization. Well-polymerized polydiacetylene layers are interesting for potential optical computing applications hecause of their high third-order nonlinear optical susceptibility. Catalytic oxidation of alkanes and related compounds in layered solids like those shown in Figure 4 relies on molecule-specific recognition and reactivity. In the catalytic air oxidation of butane to maleic anhydride, the layered compound (V0)2P207 is thought to hind the substrate in a concerted fashion a t neighboring electrophilic vanadium and nucleophilic phosphate oxygen sites (22). In the metallorganic version of this catalyst, VO(RP03) .Hz0 .R'OH, the alcohol oxygen occupies the potentially active vanadium coordination site. Interestingly, this layered solid loses R'OH(R = alkyl, R' = henzyl) when heated in vacuo. The compound from which R'OH has been removed "recognizes" primary alcohols in preference to secondary and tertiary alcohols, since the structure has formed around the R'OH template (23). Substrate-specific recognition of this kind is very desirable if one wishes to design highly selective oxidation catalysts. A creative approach to three-dimensional LEG0 chemistry has recently been described by Fagan, Ward, and coworkers (24). They prepared cationic organometallic huilding blocks in various shapes from organic arenes and CP*RU(CH~CN)~+ (Cp* = pentamethylcyclopentadienyl) according to reaction 1. nCp*Ru(CH,CN),'
+ (CsHJ,R
-
(Cp*RuCsH5+),R+ 3nCH,CN
Figure 5. Hypothetical three-dimensional network formed horn a tetrahedral tetracation and a linear dianion (from ref 24, copyright 1989, American Chemical Society; reprinted with permission). In the (CU~[C(C~H&N),]~,,* hamework, adjacent tetrahedra are derived from Cu' and quaternary carbon atoms (25).
(1)
By attaching Cp*Rut units to organic templates Rcontaining phenyl groups disposed a t the corners of triangles, tetrahedra, octahedra, etc., they obtained multiply charged cations with positive charges localized a t the polyhedral vertices. By combining these with polyhedral anions (assuming that strong ionic interactions will direct the structure), it should he possible to construct a t will a desired three-dimensional solid network. Figure 5 shows an example of the kind of structure that might result from the combination of a tetrahedral tetracation with a linear dianion. Electrochemi832
Journal of Chemical Education
cally generated solids which represent the simplest case, dimer cations combined with planar monomeric anions such as tetracyanoquinodimethane. (= TCNQ-), show interesting stacked structures in which both ionic interactions and intrastack *-a interactions are important. Interestingly, a zeolitic solid with the topology shown in Figure 5 has recently been synthesized by another route (25). In the compound Cur[C(CsH4CN)4]BF4.xCtiH5N02,both the copper and quaternary carbon atoms are the basis of tetrahedral building blocks, and the cyanophenyl groups act
as linear bridges between them. An open framework structure is thus formed with disordered, essentially "liquid" nitrobenzene and BFa- ions filling the interstitial space. Deslgner Surfaces Well-ordered molecular surface assemblies are often easier to prepare in a rational and predictable way than are three-dimensional solids. They are usually synthesized by sinele- or multi~le-stepadsorntion techniaues: invariablv thesynthesis makes use of the'self-assembling character df the solid state structure oroduced. In this way desiener surfaces are closely related to the designer solids &cussed above. Surface-sensitive measurements such as infrared
ation and recognition. The two-component film (Fig. 6) contains 22-thiobisethylacetoacetate (TBEA) which binds CuZ+ions specifically, and octadecylmercaptan (OM). The Cu2+ion permeates the film where the OM molecules are not wellpacked (i.e., a t t h e TBEAsites), are bound by the TBEA ligand and sensed electrochemically. Other ions, such as Fe3+,do not bind efficiently in the tetradentate TBEA sites; the electrochemical response for Fe3+ is completely suppressed because thev cannot access the remainder of the gold surface, which iscovered by the well-ordered OM layer. Geddes et al. have shown that asineleLanemuir-Blodeett monolayer can act as a molecular reciifier (35). They assembled a monolayer of the amphiphilic donor-acceptor molecule 4 NC.
spectroscopy, ellipsometry, low-energy electron diffraction, and X-rav ~hotoelectronsoectroscoDv, along with newer methods &h as scanning tunneling m&ioscopj (STM), surface Haman spectroscopy (26). surface NEXAFS (27). and x-ray standing-wave techniques (28) can now provide avery detailed picture of the structure of molecular assemblies on surfaces. These surface "nanostmctures" offer a variety of potential applications in chemical sensing, optoelectronics, molecular-scale electronics, microlithography, and catalysis. The growth from solution of well-ordered monolayers is usually accomplished by incorporating a surface-specific functional group (e.g., a thiolfor gold surfaces or a silanolfor oxide surfaces) into the adsorbate molecule. Coo~erative interactions between adsorbates, such as alkyl chaii stacking and hvdroeen . . bonding, can eive rise to very well-ordered t&dimmsional arrays. o n e creative use of this adsorption chemistrv recentlv reported bv Rubinsteinet al. (29)was the preparation of th:n films cabable of selective ionic permeOM
TBEA
cu2+-TBEA
Figure 6 Bltunavmal rurtacs assembly composed 01 a d s o r b TBEA and OM 101 se1ec1 ve elechocnamlcal detect on of C P sons ifrom ref 29. caprlghl 1968. Macrnlllan Magaanes Ltd reprinted with permlss1on)
.
,CN
on a platinum surface and measured i-V curves between the platinum and magnesium contacts evaporated on top of the film. 4 orients in such a way that the hydrophobic donor portion of the molecule (dodecyloxyphenylcarbamate) is bound to the platinum, while the acceptor group (5-bromotetracyanoquinodimethane) is in contact with the evaporated magnesium overlayer. When the platinum side is held negative, a dc current flows, presumably because negative charee is transferred from the donor to the acceDtor of . Darts . the molecule; the resultingdiradical might beexpected t o be a mod electrical conductor. Consistent with this interpretation, no current flows when the polarity is reversed: This impressive demonstration of diodelike behavior with a single molecular layer suggests that more complex devices, such as transistors, and even molecular integrated circuitry might be achieved with the proper supramolecular surface assemblies. I t is tempting to speculate that three-dimensionally well-ordered multilayer films composed of such molecules might be able t o perform these and other interesting functions. The most venerable and still most versatile method for preparingmultilayer surface films of controlled architecture is the Langmuir-Blodgett monolayer transfer technique (31). Film growth by this method involves the coherent transfer of a compressed amphiphilic monolayer, wh~chis assembled a t an air-water interface, to a suitably prepared substrate. Hundreds of layers can be grown sequentially on a substrate, in a precisely controlled manner, by repeatedly dipping the substrate through the air-water interface. Excellent reviews of this technique and its many applications are available (3lb-e). While the Langmuir-Blodgett technique is both effective and versatile, the films produced are metastable and can be easily disrupted, e.g., by a dust particle. Several groups have attempted t o prepare more stable multilayers using sequential adsorption/reaction strategies. Some of these newer films are not only more stable than Langmuir-Blodgett multilayers, but have the added advantage of being applicable to nonplanar (e.g., high surface area) substrates. Netzer and Sagiv a d s o r b e d hexadecenyltrichlorosilane, C13Si (CH2),,CH=CH2, onto silicon, to produce well-ordered monolayers that were bound to the surface via Si-0-Si bonds (32). The terminal olefin groups were then exposed to chemical activatine" aeents (diborane. followed bv. hvdroeen - peroxide and aqueous base), to convert them into terminal hvdroxvl erouDs. The hvdroxvl erouDs were then available to bind a second iayer of the ads"o;bate; which could be activat-
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Volume 67 Number 10 October 1990
833
ed to bind a third layer, and so on. While only two or three layers could be grown before structural defects in the film became important, this work laid the conceptual foundation for later, more successful systems. Tillman et al. recently used an analogous technique in which the terminal group was a protected carboxylic acid rather than an olefin (33). Films of excellent quality, up to 30 layers thick, could be produced by sequential deprotection/adsorption steps. Our approach to this prohlem has been to use LEG0 chemistrv on surfaces to eenerate stable. lamellar solid state - - - ---~ structures layer by laye; By anchoring a phosphonic acid monolaver to a silicon or eold surface and then seouentiallv adsorbing Zr4+and decan;diylbisphosphonic acid ~~
We eratefullv acknowledee the efforts of our co-workers. listed in the references. This work was supported by who grants from the National Science Foundation (PYI Award CHE-8657729) and the Robert A. Welch Foundation. TEM also acknowledees support from an Alfred P. Sloan Foundation fellowship.
.
131
+ -+
121. ete.
multilayer film
an ordered multilayer film which adopts the well-known Zr(03PR)~ structure (Fig. 4) is produced (34). These films can he grown to precisely controlled thicknesses and are good insulators, even when very thin. For example, a single 24-A layer synthesized on gold via the sequence of steps 1-23 in Scheme 1comoletelv blocks electron transfer between the metal and redo; coupies such as Fe(CN)63-14-in solution. Interestinelv. the zirconium ~hosnhonatestructure is ouite flexible incomposition; mixed phosphonate/inorganic p'hoshate materials adoot the same structure. but with small acidic-OH groups id place of some of theinterlamellar-R groups (35). These mixedphases, when grown bythesequential adsorption technique on electrode surfaces act as thin film molecular sieves, "recognizing" electroactive cations and neutrals that are small enough to fit into the open interlamellar spaces and excluding larger molecules (36). Electroactive species (e.g., ferrocene derivatives) can also he bound covalently into these films. Since the Zr(03PR)~ structure can accommodate manv different kinds of R groups, we are optimistic that surface structures can be svnthesized that will have other interestine su~ramolecular properties, such as chiral molecular recognition, substratesnecific heteroeeneous catalvsis, vectorial . . and lieht-induced eiectron transcr. Conclusions
While manv excitina advances have been made in recent years in the design of kpramolecular solids and surfaces, i t is clear that only the tip of an apparently substantial iceberg has been uncov&ed. o u r synthetic tools for preparing these materials and surface microstructures are still rudimentary, although they are developing rapidly thanks to the interest in solid state prohlems of many talented chemists from more traditional discinlines. Even a t this earlv stage of develooment, the s u p r ~ o l e c u l a chemistry r of iolidsrs limited less bv the availabilitv of svnthetic methods than i t is bv our ability to imagine what "designer" materials we could now create.
834
Acknowledgment
Journal of Chemical Education
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