The identification of operating principles to activate fluorescence under the influence of external stimulations is essential to enable the implementation of imaging strategies requiring the spatiotemporal control of emission. In this context, our laboratories designed mechanisms to switch fluorescence with either light or pH based on the unique photochemical and photophysical properties of either photoresponsive or halochromic oxazines respectively. These heterocycles can be connected covalently to fluorescent chromophores and opened with either light or pH to impose a significant bathochromic shift on the main absorption of the emissive appendage. Such a spectral change allows the selective excitation of the resulting species to activate bright fluorescence with infinite contrast and spatiotemporal control. Indeed, these mechanisms for fluorescence activation enable the acquisition of images with subdiffraction resolution, the selective signaling of cancer cells and the monitoring of translocating species in real time. Thus, our structural designs for fluorescence switching under external control can evolve into invaluable probes for the implementation of bioimaging strategies that would be impossible to perform with conventional fluorophores.
A mechanism to photoactivate far-red/near-infrared fluorescence with infinite contrast and under mild visible illumination was designed around the photophysical properties of borondipyrromethene (BODIPY) dyes and the photochemical behavior of oxazine heterocycles. Specifically, the photoinduced and irreversible cleavage of an oxazine ring with a laser line at 405 nm extends the electronic conjugation of a BODIPY chromophore over a 3H-indole auxochrome with a 2-(4-methoxyphenyl)ethenyl substituent in position 5. This structural transformation shifts bathochromically the main absorption band of the BODIPY component to allow the selective excitation of the photochemical product with a laser line of 633 nm and produce fluorescence between 600 and 850 nm. This combination of activation, excitation, and emission wavelengths permits the visualization of the cellular blastoderm of developing Drosophila melanogaster embryos with optimal contrast and essentially no autofluorescence from the biological specimen. Furthermore, the sequential acquisition of images, after the photoactivation event, enables the tracking of individual cells within the embryos in real time. Thus, our structural design and operating principles for the photoactivation of far-red/near-infrared fluorescence can evolve into invaluable probes to monitor cellular dynamics in vivo.
This study reports the synthesis of a photoactivatable fluorophore with optimal photochemical and photophysical properties for the real-time tracking of motion in vivo. The photoactivation mechanism designed into this particular compound permits the conversion of an emissive reactant into an emissive product with resolved fluorescence, under mild illumination conditions that are impossible to replicate with conventional switching schemes based on bleaching. Indeed, the supramolecular delivery of these photoswitchable probes into the cellular blastoderm of Drosophila melanogaster embryos allows the real-time visualization of translocating molecules with no detrimental effects on the developing organisms. Thus, this innovative mechanism for fluorescence photoactivation can evolve into a general chemical tool to monitor dynamic processes in living biological specimens.
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