Mechanical Bond-Induced Radical Stabilization

Abstract

A homologous series of [2]rotaxanes, in which cyclobis(paraquat-p-phenylene) (CBPQT(4+)) serves as the ring component, while the dumbbell components all contain single 4,4'-bipyridinium (BIPY(2+)) units centrally located in the midst of oligomethylene chains of varying lengths, have been synthesized by taking advantage of radical templation and copper-free azide-alkyne 1,3-dipolar cycloadditions in the formation of their stoppers. Cyclic voltammetry, UV/vis spectroscopy, and mass spectrometry reveal that the BIPY(•+) radical cations in this series of [2]rotaxanes are stabilized against oxidation, both electrochemically and by atmospheric oxygen. The enforced proximity between the BIPY(2+) units in the ring and dumbbell components gives rise to enhanced Coulombic repulsion, destabilizing the ground-state co-conformations of the fully oxidized forms of these [2]rotaxanes. The smallest [2]rotaxane, with only three methylene groups on each side of its dumbbell component, is found to exist under ambient conditions in a monoradical state, a situation which does not persist in acetonitrile solution, at least in the case of its longer analogues. (1)H NMR spectroscopy reveals that the activation energy barriers to the shuttling of the CBPQT(4+) rings over the BIPY(2+) units in the dumbbells increase linearly with increasing oligomethylene chain lengths across the series of [2]rotaxanes. These findings provide a new way of producing highly stabilized BIPY(•+) radical cations and open up more opportunities to use stable organic radicals as building blocks for the construction of paramagnetic materials and conductive molecular electronic devices.

Radical-radical rotaxanes with tetracationic cyclophane ring, and quaternary azaaromatic rotaxanes with cage macrocycles

“…After exposure to air, the purple color of these solutions began to diminish, and it was observed that the color of solutions of the reduced shorter rotaxanes diminished slower than in the case of the longer ones. 8 The above presented results offer a new possibility to produce highly stabilized BIPY…”

“…As an example of such "molecular flask" may serve cucurbit [8]uril; when (BIPY •+ ) 2 is encapsulated in the cavity of CB8, the stability of this radical dimer is enhanced. 9,40 It was found that the ring W ring.…”

“…The influence of the mechanically interlocked nature of a homologous series of [2]rotaxanes Rn•6PF 6 (n=3-11) on their structural and electronic properties has been investigated (n represents the number of the methylene groups on each side of the BIPY 2+ unit situated in the middle of the thread. 8 Rotaxanes Rn consist of a ring W 4+ encircling the dumbbell which contains central BIPY 2+ unit serving as an electrostatic "speed bump"; they are "unlikely rotaxanes" built from two positively charged components. .…”

“…As examples, the rotaxanes containing one or more rings of the same kind, as well as rotaxanes containing different rings are presented, pointing out their application possibilities on various fields. Compounds described in both parts are promising for design of molecular electronic devices, 8 nonvolatile molecular memories 9 and switches, 10 molecular machines, 11,12 machines of an autonomous character 13,14 and materials of special properties. [15][16][17][18] One should point out that rotaxanes and pseudorotaxanes, widely investigated species belonging to mechanically interlocked molecules (MIMs) are of interest in construction of memory devices and switches, and in biological applications [19][20][21] Attention is paid today to polyrotaxanes, an intensively studied area of chemistry and related sciences; 22,23 supramolecular polymers are a topic of a wide research due to their self-assembled nature and valuable properties.…”

Donor-acceptor rotaxanes with tetracationic cyclophane ring

“…In investigations of rotaxanes [90][91][92][93][94][95] a growing attention is paid recently to radical-radical rotaxanes; 13,14,16,24,[96][97][98][99][100] one may hope that this area of radical-pairing interaction will become a topic of intense study. Owing to the constantly increasing amount of work concerning rotaxane chemistry, it would be impossible to cover all published reports; the present review is only a fragment of a wide research.…”