Folding and Aggregation of Oligoviologens in Water and Cucurbit[n]uril (n=7, 8) Modulation

To study the conformations and aggregation of linear viologen radical cation-incorporated molecules in water, a series of water-soluble oligoviologens with different length and linkers were prepared. H-1 NMR experiments in deuteroxide showed that cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]) encapsulate their p-xylylene linkers instead of the bipyridiniurn units. UV-vis absorption experiments indicated that in the aqueous solution of sodium dithionite the viologen units of the oligomers were reduced to the radical cation species which were driven to dimerize through pleated conformations, leading to the formation of trisradical cation species. Dynamic light scattering experiments revealed that the foldamer dimers further aggregated to atiord nanoscale particles. Adding CB[7] decomposed all the aggregated foldamer nanoparticles through 1 : 1 encapsulation of the viologen radical cation units, whereas the addition of CB[8] broke the foldamers and their large aggregates through 1 : 2 encapsulation of the viologen radical cations. Introducing a rigid linker to the linear viologen molecule could further enhance their stacking in the radical cation state. Although the folding and dimerization of linear viologen radical cation molecules have been established in acetonitrile in the literature, this study demonstrates that in water such multiple viologen radical cation molecules not only undergo folding to form stable dimers, but also further aggregate to give rise to nanoscale particles.