Influence of cyclodextrin size on fluorescence quenching in conjugated polyrotaxanes by methyl viologen in aqueous solution

Poly(4,4'-diphenylenevinylene) rotaxanes and [2] rotaxanes with alpha-, beta-, gamma-cyclodextrin macrocycles were synthesised and their sensitivities to fluorescence quenching by methyl viologen in aqueous solution were determined, relative to uninsulated analogues. Stern-Volmer analysis revealed that the fluorescence quenching response of polyrotaxanes is strongly dependent on the diameter of the cyclodextrins. Polyrotaxanes, composed of the smaller diameter alpha- or beta- cyclodextrins, are the least easily quenched, with Stern-Volmer constants about two orders of magnitude smaller than from the wider gamma-cyclodextrin polyrotaxane and the uninsulated polymer. Time-resolved photoluminescence results demonstrate the crucial role of interchain aggregation on the sensitivity to fluorescence quenchers. The materials with the highest Stern-Volmer constants exhibit the most biexponential photoluminescence decay, which is indicative of aggregation, and the emission spectra of solutions containing methyl viologen resemble the early-time emission spectra (0-3 ns after excitation) of the unquenched samples. The results show that the threaded alpha-cyclodextrin is effective in preventing aggregation, and in hindering fluorescence quenching, even when only a small fraction of the conjugated polymer is encapsulated. This conclusion is relevant to the application of these materials in optoelectonic devices, such as light-emitting diodes, where it is essential to prevent luminescence quenching without hindering charge transport.