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The covalent integration of fluorescent and photoswitchable components within the same molecular skeleton can be exploited to activate fluorescence under optical control. Specifically, a photoswitchable oxazine heterocycle can be connected to either a coumarin or a borondipyrromethene fluorophore. Illumination of the resulting molecular dyads at an appropriate activation wavelength either opens the heterocycle reversibly or cleaves it irreversibly, depending on the relative positions of its methylene and nitro substituents. These photochemical transformations shift bathochromically the main absorption band of the fluorophore and allow its selective excitation at a given wavelength. These hydrophobic molecular dyads can be entrapped within the hydrophobic interior of self-assembling nanoparticles of amphiphilic polymer. The supramolecular envelope around the switchable compounds enables their transfer into aqueous environments and their operation under these conditions with minimal influence on their photochemical and photophysical properties. The reversible fluorescence activation, possible in one instance, imposes intermittence on the detected emission and offers the opportunity to resolve closely-spaced nanocarriers in time to reconstruct images with subdiffraction resolution. The irreversible fluorescence activation, possible in the other, maintains emission on after the activation event and permits the monitoring of the diffusion of the activated nanocarriers in real time with the sequential acquisition of images. Thus, these operating principles to solubilize and operate photoswitchable fluorophores in aqueous environments with the aid of supramolecular nanocarriers can lead to valuable protocols to image specimens with subdiffraction resolution and to monitor dynamic events noninvasively.
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