Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO₂ Nanoparticles

Abstract

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V(1) and V(2), interconnected by a rigid terphenylene bridge. In their parent dication states, V(1)(2+) and V(2)(2+) can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side-to-side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO(2) nanoparticle, viologen V(2)(2+) is reduced at significantly more negative potentials (-0.3 V) than in flexible analogues that contain aliphatic bridges between V(1) and V(2). These overpotentials are analysed in terms of electron-transfer rates and a donor-bridge-acceptor (D-B-A) formalism, in which D is the doubly reduced viologen, V(1)(0), adjacent to the TiO(2) surface (TiO(2)-V(1)(0)), B is the terphenylene bridge and A is viologen V(2)(2+). We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V(2)(2+) by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution-based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO(2) surface.

Palladium-Catalysed Coupling Reactions En Route to Molecular Machines: Sterically Hindered Indenyl and Ferrocenyl Anthracenes and Triptycenes, and Biindenyls

“…Palladium-catalysed aryl-acetylene coupling reactions have been used by Stoddart's group to construct long rigid molecular spacers of the [2]rotaxane shuttle, 3, ( Figure 2) whereby low shuttling energy barriers were reported unless a "speed bump" moiety was attached to the naphthalene ring [6]. In a similar vein, we preceded these results with our non-degenerate tripodal two-station rotaxane shuttle, 4 (Figure 3), which was designed to be adsorbed in an oriented way on a titanium dioxide surface [7]. Unexpectedly, the synthesis of the rigid spacer, B, via Suzuki-type cross-coupling proved problematic, as no terphenyl product could be isolated for R = H, X = OH; however, when R = Me, X = OH, the terphenyl linker was isolable in 75% yield, and was fully characterised by X-ray crystallography [8].…”

“…The dynamic behaviour of the shuttle could, in principle, allow free rotation or function as a molecular brake, as in Figure 4. Palladium-catalysed formation of C-C bonds in the tripodal rigid non-degenerate shuttle, 4 [7,8], and the rigid ring-in-ring shuttle, 5 [9]. The planned approach was to prepare 9-(3-indenyl)anthracene, 6, and 9-(2-indenyl)anthracene, 7.…”

“…One could then envisage incorporation of a bulky organometallic fragment that could undergo a reversible η 6 Figure 3. Palladium-catalysed formation of C-C bonds in the tripodal rigid non-degenerate shuttle, 4 [7,8], and the rigid ring-in-ring shuttle, 5 [9].…”

From Molecular Machines to Stimuli‐Responsive Materials

“…Interestingly, a conformational change of the rotaxane occurs upon transfer of four electrons leading to the formation of its fully reduced form, which subsequently requires more positive potentials to reversibly oxidize it (Figure b‐ii). Few years later, the same group studied by cyclic voltammetry and spectroelectrochemistry the influence of the flexibility of the linker on the shuttling and electrochemical behavior of the rotaxane . Importantly, in contrast to flexible linkers, rigid ones such as triphenylene units precluded any molecular shuttling as explained by the formation of an electrical double layer at the interface.…”

Rotaxanes and pseudorotaxanes with threads containing viologen units

“…32 There should be mentioned here also rotaxanes and pseudorotaxanes containing cyclodextrins, [33][34][35] calixarenes 36,37 or cucurbiturils. 38,39 Today many works concerning viologen-based rotaxanes and pseudorotaxanes appear, [40][41][42][43][44][45] in these systems viologens are components of a thread [45][46][47][48][49] or are present as parts of the tetracationic cyclophane serving as a ring. [50][51][52][53] Moreover viologens find a variety of applications due to their redox properties, they are promising for design of molecular machines and switches and construction of new materials, the rapid development of viologen chemistry has its reflection in many reports concerning these compounds.…”