Redox‐Switchable Calix[6]arene‐Based Isomeric Rotaxanes

Abstract

Operating molecular machines are based on switchable systems whose components can be set in motion in a controllable fashion. The presence of nonsymmetrical elements is a mandatory requirement to obtain and demonstrate the unidirectionality of motion. Calixarene-based macrocycles have proved to be very efficient hosts in the design of oriented rotaxanes and of pseudorotaxanes with strict control over the direction of complexation. A series of two-station rotaxanes based on bipyridinium-ammonium axles was synthesized and characterized. A recently reported supramolecularly assisted strategy for the synthesis of different orientational isomers was exploited, and the ammonium unit was identified as a proper secondary station for the calixarene. Displacement of the macrocycle was triggered by electrochemical reduction of the bipyridinium primary station, and it was shown that the shuttling is influenced both by the length of the chain of the axle component and by the position of the secondary station with respect to the calixarene rims.

New Geometries for Calix[6]arene‐Based Rotaxanes

“…S28–29, SI), these signals were assigned to the methylene groups of the DOV octyl chains labelled as ( & ) and ( § ) in Scheme . As seen for TPU , the appearance of these methylene resonances is usually ascribed to the formation of a pseudorotaxane complex.…”

“…Indeed, upon complexation, this thread might promote the rotaxanation reaction by the insertion of bulky diphenylacetyl chloride groups on its hydroxyl termini. Following a procedure verified in previous studies,, , the solid DDOV was mixed with a solution of DPU in CH 2 Cl 2 to promote the formation of pseudorotaxanes ( DPU⊃DDOV ). After 12 hours of mixing at room temperature, the undissolved salt was eliminated by filtration and the resulting homogeneous solution treated with diphenylacetyl chloride to complete the stoppering reaction (see Scheme ).…”

“…During the last two decades, we showed that, in apolar media, the tris‐( N ‐phenylureido)calix[6]arene TPU can take up 4,4′‐bipyridinium (viologen) salts and form pseudorotaxanes in a process in which the phenylurea groups at the upper rim of the calixarene play a crucial role in pivoting the unidirectional threading of the cationic viologen through the macrocycle upper rim . This process paved the way for the synthesis of oriented (pseudo)rotaxanes, characterized by the univocal orientation of the thread termini with respect to the two calix[6]arene rims …”

Tuning the Fluorescence Through Reorientation of the Axle in Calix[6]arene‐Based Pseudorotaxanes

“…On the contrary, when a pyridylpiridinium salt is used as the axle, both orientational pseudorotaxane isomers are formed . The different reactivity of the encapsulated axle was recently exploited for the synthesis of calix[6]arene‐based oriented rotaxanes . In this paper, we study the complexes formed by W and the stilbazolium tosylate SC18N as guest (Figure ).…”

“…[15,16] The differentr eactivity of the encapsulated axle was recently exploited for the synthesis of calix [6]arene-based oriented rotaxanes. [15][16][17][18] In this paper,w es tudy the complexes formed by W and the stilbazolium tosylate SC18N as guest ( Figure 1). Our results demonstrate that not only the confinement of ag uest but also its orientation inside the host can be employed as tools to tune the spectroscopic properties.…”