AN APPROACH TO INSULATED MOLECULAR WIRES - SYNTHESIS OF WATER-SOLUBLECONJUGATED ROTAXANES

Hydrophobic self-assembly has been used to direct the synthesis of con jugated [2] and [3]rotaxanes (20 and 21) in aqueous solution, by Glase r 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 pheny lene 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 m ass spectrometry, NMR and W-VIS emission/absorption, Both rotaxanes te nd to fragment, by unthreading and by dumbbell-cleavage, during electr ospray 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 wit h longer fluorescence lifetimes; the rate of this photodecomposition i s slower for the [3]rotaxane than for the naked dumbbell. The extensio n of this synthetic approach to larger polyrotaxanes was explored by c oupling alkyne 3 and diethynylbenzene 2 in the presence of cyclophane 6; this gives some longer [2] and [3]rotaxanes but higher polyrotaxane s are not formed.