One of the key goals of supramolecular chemistry is to assemble structural building blocks into arrays with new properties that emerge only in supramolecular architectures. These new properties may improve our understanding of non-covalent interactions, or they may endow useful functionality. Catalysts, sensors and molecular machines or computers are all possible applications.
In the past, we have developed effective approaches for the preparation of pseudorotaxanes, rotaxanes and catenanes based on π-donor/π-acceptor interactions.[1,2] Our approach has been to bring together neutral electron-rich components such as crown ether 1 (the "donor") and electron-poor components such as pyromellitic diimide 2 (the "acceptor") to create highly-coloured donor-acceptor systems that are neutral, chemically robust and capable of post-synthetic modification. Catenanes 4 and 5 are formed from 2 and 3a-b by alkene metathesis, but we have used a range of other bond-forming reactions with related building blocks.
Recently, we discovered that lithium salts affect the colour of a chloroform solution of 1 and 2 to give a bright orange solution (Figure 1). Further studies confirmed the formation of a highly symmetrical pseudo-rotaxane consisting of one crown and one pyromellitic diimide held together by two lithium cations, through coordination to oxygen atoms of both organic components (Figure 2) [3,4]
The pseudorotaxane formation of naphthyl diimide 3a was also investigated in presence of lithium. Surprisingly, almost no lithium effect was observed in this case. Taking advantage of this observation, the switchable nature of the pseudorotaxane system was investigated. The starting point is a pale yellow solution of the pyromellitic diimide 2 with the crown ether 1 due to the weakness of the donor-acceptor complex. After the addition of the naphthalene diimide 3a, the solution changes colour to light purple (Figure 3) characteristic of the naphthalene diimide pseudorotaxanes complex. When lithium bromide was added, the solution changes to bright orange, because the pyromellitic diimide 2 replaces the naphthalene diimide 3a. Further NMR experiments confirmed this process demonstrating that switching between the naphthalene diimide and the pyromellitic diimide pseudorotaxane can be induced by lithium cations.
From these observations, we have, in partnership with the group of Prof J. F. Stoddart (UCLA), designed and produced molecular switches based on rotaxanes containing the two "acceptor" units (the naphthalene diimide and the diimide pyromellitic) and crown ether 1 as "donor" (See, for example, Figure 4). These molecules were designed to be inserted in a Langmuir-Blodgett type device. At the end of the rotaxanes, a hydrophilic stopper will permit the formation of a monolayer on silica; for our preliminary studies, two hydrophobic "stoppers" were used. [5,6]