Switchable protection and exposure of a sensitive squaraine dye within a redox active rotaxane

Abstract

In nature, molecular environments in proteins can sterically protect and stabilize reactive species such as organic radicals through non-covalent interactions. Here, we report a near-infrared fluorescent rotaxane in which the stabilization of a chemically labile squaraine fluorophore by the coordination of a tetralactam macrocycle can be controlled chemically and electrochemically. The rotaxane can be switched between two co-conformations in which the wheel either stabilizes or exposes the fluorophore. Coordination by the wheel affects the squaraine’s stability across four redox states and renders the radical anion significantly more stable—by a factor of 6.7—than without protection by a mechanically bonded wheel. Furthermore, the fluorescence properties can be tuned by the redox reactions in a stepwise manner. Mechanically interlocked molecules provide an excellent scaffold to stabilize and selectively expose reactive species in a co-conformational switching process controlled by external stimuli. Incorporating sensitive chromophores into interlocked systems that are mechanically responsive to external stimuli is attractive for tuning of the dye’s optical properties and stability. Here, the authors report the mechanically controlled protection and deprotection of a squaraine dye within a [2]rotaxane, governed by macrocycle shielding/de-shielding upon chloride addition or oxidation.