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Squaraine-based [2]rotaxanes that function as visibly active molecular switches.

Molecular switches undergo reversible (co)conformational shifts or molecular transformations, under the influence of pH, photons, cations, anions, heat, and/or electrons, between two or more different stable states. Although many external stimuli (inputs) are available to operate molecular switches, the most important outputs are generally mechanical forces and optical signals: the former allows the construction of artificial molecular machines and the latter provides the possibility for molecular sensing. Thanks to burgeoning growth in the synthesis of interlocked molecules, several rotaxaneand catenane-based molecular switches have been developed in which detectable optical signals accompany the migration of interlocked macrocycles between different recognition sites; such materials are potentially useful for constructing complicated molecular logic and optical devices. Light-active molecular switches providing fluorescence outputs at long wavelengths, approaching the near-infrared (NIR) region, have potential practical importance in biomedicine and photodynamic therapy for sensing and signaling in living tissues and cells. Squaraine-based fluorescence dyes generally exhibit low quantum yields in polar solvents; in their host-encapsulated forms, however, they can experience less-polar local environments and, therefore, exhibit significantly increased quantum yields, even in polar solvents. This behavior suggests a straightforward approach toward the design of squaraine-based optical switches: if the solvent-exposed and encapsulated forms of a squaraine unit can be generated as two distinguishable stable states of a switchable rotaxane in a polar solvent, variations in the optical signals in the longwavelength-fluorescence region should be produced within each switching cycle. Herein, we report the synthesis and operation of squaraine-based optical molecular switches, which display striking changes in their fluorescence signals that are visible to the naked eye. Previously, we reported that the molecular cage 1 forms complexes with the squaraine derivatives 2 under the assistance of Na ions as templates. Thus, we anticipated that the [2]rotaxane[4-long·Na2]ACHTUNGTRENNUNG[4 ClO4] (Scheme 1) would function as a molecular switch in CD3CN if we could remove the templating Na ions and, thereby, move the molecular cage from the squaraine station to the bipyridinium one; this process should have the effect of decreasing the intensity of the long-wavelength-fluorescence emission of the [2]rotaxane.