| Abstract
| - Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and anondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesizedand characterized in solution. Their dumbbell-shaped components are composed of polyether chainsinterrupted along their lengths by (i) two π-electron-rich stationstwo NP moieties or a MPTTF unit and aNP moietywith (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminatedby (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification wasinvestigated as a means to simplify both molecular structure and switching function previously observed inrelated bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes thetetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed(> 95 < 5), surprisingly across a wide temperature range (198−323 K), meaning that the CBPQT4+ ringcomponent resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. Asa consequence of these two effects, the assignment of NMR and UV−vis data is more simplified ascompared to previous donor−acceptor bistable [2]rotaxanes. This development has not only allowed formuch better control over the position of the ring component in the ground state but also for control over thelocation of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically(1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines anunambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. Thedegenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMRspectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from thesemeasurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+ring from one station to another as compared with previous systemsa finding that is thought to be aresult of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and arelatively unimpeded path between the two NP stations. This example augers well for exploiting rigidityduring the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excessesof structural conformations that have characterized the first generations of molecular switches based onthe donor−acceptor recognition motif.
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