Quantitative Conformational Study of Redox‐Active [2]Rotaxanes, Part 2: Switching in Flexible and Rigid Bistable [2]Rotaxanes

Abstract

Translational movement of the macrocycle in two structurally similar bistable [2]rotaxanes, which is induced by a four-step electrochemical process in solution, has been investigated by using a methodology developed in the preceding article (Chem. Eur. J. 2008, 14, 1107-1116). Both [2]rotaxanes contain a crown ether that can be accommodated by either of two interconnected viologen recognition sites. These sites are substantially different in terms of their affinity towards the crown ether and they possess considerably different electrochemical reduction potentials. The two [2]rotaxanes differ in the length and the rigidity of a bridge that links these sites. A combination of molecular mechanics modelling and NOE spectroscopy data provides information about the conformations of both [2]rotaxanes in the parent oxidation state when the crown ether exclusively populates the strong recognition site. To determine the population of the recognition sites at subsequent stages of reduction, a paramagnetic NMR technique and cyclic voltammetry were used. The key finding is that the flexibility of the connecting bridge element between the recognition sites interferes with shuttling of the crown ether in [2]rotaxanes. It can be demonstrated that the more flexible trimethylene bridge is folded, thus limiting the propensity of the crown ether to shuttle. Consequently, the crown ether populates the original site even in the second reduced state of the flexible [2]rotaxane. On the contrary, in the [2]rotaxane in which two viologen sites are connected by a larger and more rigid p-terphenylene bridge, the predominant location of the crown ether at the weak recognition site is achieved after just one single electron reduction.

Rotaxanes and pseudorotaxanes with threads containing viologen units

“…In rotaxane 10 the viologen units V 1 and V 2 are bound by a flexible trimethylene chain L 1 , while in rotaxane 11 the viologen units V 1 and V 2 are linked by a rigid terphenylene chain L 2 (Scheme 5). 74 It should be pointed out that the flexibility of the chains L 1 and L 2 influences the shuttling of the ring. In 10 the trimethylene link L 1 is flexible and may be folded, therefore the shuttling of the ring is limited, while in 11 the terphenylene link L 2 is rigid and the shuttling proceeds without limitations.…”

“…for 11•PF 6 ) the ring always encircles V 2 The above investigation of tripodal rotaxanes 10 and 11 shows that the nature of the link L between stations V 1 and V 2 is crucial for their switching ability. 74 In order to explain how electrostatic attraction may help to control the shuttling process, [2]rotaxanes and related [2]pseudorotaxanes containing oppositely charged threads and rings have been investigated. 75 In these systems, referred to as ion-paired [2]rotaxanes and ion-paired [2]pseudorotaxanes, the positive and negative charges are compensated and the mutual molecular recognition is considerably increased by electrostatic attraction between components, which are not only mechanically interlocked, but also strongly electrostatically attached.…”

“…containing two viologen units. 74 The investigation of both rotaxanes 10 and 11 in their parent and reduced states was made using the same electrochemical and NMR spectroscopic methods as in the former experiment.…”

Redox-Dependent Conformational Switching of Diphenylacetylenes

“…The most well known of these involve the use of redox active catenanes, [5][6][7][8] metal coordination compounds, [9][10][11][12][13][14][15] rotaxanes, [11,12,14,[16][17][18][19][20][21][22] π-systems, [23][24][25][26][27][28] and crowded alkenes [29]. These systems are finding increasing application in solid-state electronic devices.…”

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

“…Herein, we extend these solution-phase studies [48] by adsorbing a [2]rotaxane with a rigid bridge between the stations, 1, onto a TiO 2 NP surface to form hetero [2]rotaxane TiO 2 ·1 (see Scheme 1.) The terminal groups of the tripodal linker have been modified to enable strong binding to TiO 2 , but otherwise the molecules are the same as those previously reported.…”