An architectural rationale and an experimental program aimed at the development of molecular electronics switching devices for memory and computing applications are discussed. Two-terminal molecular switch tunnel junctions are identified as the critical device components of molecular electronics-based circuitry. They can be tiled in two dimensions and are tolerant of manufacturing defects. Singly and multiply configurable solid-state switching devices that are based upon electrochemically switchable molecular and supramolecular systems are discussed in terms of both the synthesis of the molecular components and the fabrication and performance of the devices.

*To whom correspondence should be addressed. E-Mail: stoddart@chem.ucla.eduOne sentence summary -The realization of the Borromean link as a molecular compound has been accomplished synthetically by exploiting the virtues of coordination, supramolecular, and dynamic covalent chemistry in a cooperative manner.

The reversible nature of the imine bond formation in CDCl(3) solution has been exploited to exchange substituted for unsubstituted m-phenylenediamine (MPD) units in hemicarcerand octaimines. Moreover, acid-catalyzed imine exchange has been shown to provide a novel mechanism whereby ferrocene (Fc) can be released as an entrapped guest from the hemicarceplex C(2)B(4)&crcldt;Fc dissolved in CDCl(3) to give the hemicarcerand C(2)B(4) when excess of both MPD and trifluoroacetic acid are present.

A range of secondary dialkylammonium (R 2 NH 2 ϩ ) ions has been shown to thread through the cavities of appropriately-sized crown ether compounds to afford interwoven complexes. X-Ray crystallographic investigations to probe the solid-state properties of these supermolecules have revealed that many subtle factors-e.g., solvent of crystallisation, crown ether conformation and anion interactions-can influence the nature of the overall three-dimensional superstructures. Nonetheless, a family of building blocks-namely R 2 NH 2 ϩ ions and crown ethers-can be generated, which constitute a molecular meccano kit. By mixing and matching these modules in different ways, intricate interwoven supramolecular architectures can be constructed. From relatively simple beginnings-where one R 2 NH 2 ϩ ion is threaded through one monotopic crown ether (dibenzo[24]crown-8)-the designed evolution of the building blocks in the molecular meccano kit has led to more elaborate multiply encircled and/or multiply threaded superstructures. The effects of † Based on the presentation given at Dalton Discussion No. 3, 9-11th September 2000, University of Bologna, Italy. Electronic supplementary information (ESI) available: rotatable 3-D crystal structure diagram in CHIME format. See http://www.rsc.org/ suppdata/dt/b0/b003769i/ This review is dedicated to Professor David J. Williams on the occasion of his 60th birthday. crown ether constitution, macroring size, and both crown ether as well as R 2 NH 2 ϩ ion substitution, upon the solidstate behaviour of these interwoven complexes have also been examined. A statistical analysis of the hydrogen bonding interactions observed in these systems has been carried out. It reveals that a reasonable correlation exists between N ϩ -H ؒ ؒ ؒ O bond angle and H ؒ ؒ ؒ O bond length. The correlation between C-H ؒ ؒ ؒ O bond angle and H ؒ ؒ ؒ O distance is poor, suggesting that C-H ؒ ؒ ؒ O hydrogen bonding interactions are of secondary importance in determining the co-conformations adopted by these supermolecules.

Over the past two decades a vast array of interlocked and intertwined molecular compounds-specifically, catenanes, rotaxanes, and knots [1] -have been assembled using supramolecular assistance (templation [2] ) as the key element in their synthesis [3] under either kinetic [4] or thermodynamic [5] control. Although these synthetic protocols have been implemented more recently for the construction of more intricate variants, such as oligocatenanes, [6] molecular necklaces, [7] and cyclic daisy chains [8] to name but a few, the topological challenge of the de novo synthesis of Borromean ring compounds [9] still presents a considerable hurdle to be overcome. Our way of addressing this challenge is to employ recognition motifs to create initially stable ring-in-ring superstructures [10] which could serve as templates for subsequent catenation. Here, we report 1) the design and noncovalent synthesis of a prototype, mutually orthogonal, partially preorganized, ring-in-ring complex, together with 2) the solid-state characterization of the 1:1 complex in the context of the X-ray crystal structures of its two separate ring components, and 3) the solution-state behavior of the 1:1 complex.Previously, we have shown [11] that bisparaphenylene[34]crown-10 (BPP34C10) can encircle two dibenzylammonium (DBA ) ions simultaneously by locating the two NH 2 centers 6.9 ä apart in the polyether loops of BPP34C10 and relying upon N ÀH ¥¥¥ O hydrogen bonds to form a stable 1:2 (DBA & BPP34C10 ' DBA ) complex. Thus, it seemed not unreasonable to us that BPP34C10 might also be able to complex (Figure 1) with a suitably proportioned dicationic Figure 1. Employing precedent in the design and synthesis of a proposed 1:1 complex formed between BPP34C10 and 1-2 H 2 , together with an overlaid set of 70 co-conformations of BPP34C10 ¥ 1-2 H 2 sampled from the last 70 ps of a 100-ps MD experiment. macrocycle containing two NH 2 centers. After inspection of the solid-state superstructure [11] of the 1:2 complex formed between BPP34C10 and DBA , and construction of CPK space-filling molecular models, we concluded that the macrocycle 1-2 H 2 was a suitable candidate for further investigation. Our intuition was given a considerable boost by the results of the molecular dynamics (MD) calculations. [12] They predicted the existence of a stable co-conformation [13] in which the BPP34C10 ring encircles the dicationic macrocycle, such that the polyether loops surround the NH 2 centers, namely, the 1:1 (BPP34C10 ' 1-2 H 2 ) complex (illustrated in Figure 1) might constitute a discrete supermolecule.The synthesis (Scheme 1) of the dicationic macrocycle 1-2 H ¥ 2 PF 6 was achieved by employing a bis-Wittig reaction in the key cyclization step. The bisphosphonium salt [14] 2-H ¥ 3 PF 6 was protected (Boc 2 O/Et 3 N/MeOH) to give 3 ¥ 2 PF 6[15] M.Abstracts of Papers, Part 1, 222nd National Meeting of the American Chemical Society (Chicago, IL) 2001, MEDI 307.[16] Female Rhesus monkeys (body weight 4.0 ± 7.0 kg) were sedated with 10 mg kg À1 of ketamine ...

Dipyrido[24]crown-8 (DP24C8) has been synthesized and shown to form [2]pseudorotaxanes spontaneously with dibenzylammonium ions. These complexes, which have been demonstrated by (1)H NMR spectroscopy to form faster in solution than when the macrocyclic polyether is dibenzo[24]crown-8 (DB24C8), are also stronger than their DB24C8 counterparts. One of the [2]pseudorotaxanes has been used to construct a [2]rotaxane (see above) comprising a dumbbell-shaped component based on a dibenzylammonium ion which is encircled by a DP24C8 macrocycle and terminated by (triphenylphosphonium)methyl stoppers.

Redox‐controllable molecular switches composed of two interlocking rings are incorporated here into Langmuir films starting from two different redox states. The morphologies of the resulting monolayers and their current–voltage characteristics are then investigated. These switches are considered to be ideal candidates for the fabrication of molecule‐based electronic devices.

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