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

Poly(4,4′-diphenylenevinylene) rotaxanes and [2]rotaxanes with α-, β-, γ-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 α- or β-cyclodextrins, are the least easily quenched, with Stern–Volmer constants about two orders of magnitude smaller than from the wider γ-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 α-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.