Porphyrin-decorated bistable rotaxanes: Molecular folding governs switchable singlet oxygen photoproduction

Bistable rotaxanes were used to develop pH-switchable systems for singlet dioxygen photoproduction. Based on the combination of a Zn(II)-tetraphenylporphyrin photosensitizer, which was attached to the macrocycle, and a black-hole-quencher, which was used as one of the rotaxane-stoppers, singlet oxygen production could be switched on/off ((Δ= 0.15/0.03) by the addition of base/acid. However, we found that only a sufficiently long linker between both stations on the thread enabled switchability, and that the direction of switching was inversed with regard to the original design. This unexpected behavior was attributed to intramolecular folding of the rotaxanes, as indicated by extensive theoretical calculations. This evidences the importance to take into account the conformational flexibility of large molecular structures when designing functional switchable systems.