Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible (Pd6L4)-L-II coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the (Pd6L4)-L-II cage disassembles to afford (Pd2L2)-L-II species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments. Water-soluble nanocontainers that can encapsulate and modulate the properties of synthetic fluorescent probes could expand the versatility of such probes in biologically relevant environments. Here, the authors encapsulate the resazurin/resorufin redox pair within a flexible palladium-based coordination cage and find that confinement drastically alters the optical and redox properties of the two dyes.

Automated summary

Confining the resazurin/resorufin redox pair within a flexible palladium-based coordination cage drastically alters their optical and redox properties.