An approach to insulated molecular wires: synthesis of water-soluble conjugated rotaxanes

Abstract

Hydrophobic self-assembly has been used to direct the synthesis of conjugated [2] and [3]rotaxanes (20 and 21) in aqueous solution, by Glaser coupling a water-soluble alkyne 3 in the presence of a cyclophane 6. No rotaxanes were formed when cyclodextrins were used instead of the cyclophane. NMR and electrospray mass spectrometry were used to probe the binding properties of the stopper unit 3. NMR ring current shifts and NOEs show that the cyclophane 6 binds mainly to the terminal phenylene unit of 3. In solution cyclodextrins bind less strongly than the cyclophane, whereas in the gas-phase cyclodextrins bind more strongly. The water-soluble rotaxanes are fully characterised by electrospray mass spectrometry, NMR and UV–VIS emission/absorption. Both rotaxanes tend to fragment, by unthreading and by dumbbell-cleavage, during electrospray ionisation, particularly at high cone voltages. The insulation of the conjugated dumbbell inside the [3]rotaxane results in increased fluorescence efficiency. Time-resolved fluorescence measurements show that these rotaxanes decompose during photolysis to give products with longer fluorescence lifetimes; the rate of this photodecomposition is slower for the [3]rotaxane than for the naked dumbbell. The extension of this synthetic approach to larger polyrotaxanes was explored by coupling alkyne 3 and diethynylbenzene 2 in the presence of cyclophane 6; this gives some longer [2] and [3]rotaxanes but higher polyrotaxanes are not formed.