Rational design of ZnSalen as a single and two photon activatable fluorophore in living cells

Rational design of effective photoactivatable/photoswitchable fluorophores, by introducing appropriate photoreactions to tune the electron transfer processes of the ground or/and excited states and switch fluorescence off/on, is crucial to achieve high temporal and spatial resolution in live cell (organism) imaging. Besides one photon activatable fluorophores, it is highly desirable to develop two photon activatable fluorophores using light in the NIR or IR region, which reduces photodamage and allows deep penetration into cells or tissues. In this work, we describe the design of one and two photon activatable ZnSalen by incorporating thioether moieties in the 3,3′-positions which quenches the fluorescence as a result of a PET process. Through one or two photon irradiation, the thioethers can be oxidized to sulfoxides and the fluorescence of ZnSalen "switched on", due to the electron-withdrawing sulfoxides, which perturbs the PET process. We further demonstrate the application of this ZnSalen as a photoactivatable fluorophore in living cells using one and two photon fluorescence microscopies. In two photon microscopy, a high signal to noise contrast was achieved by irradiation with an 840 nm laser. Moreover, this photoactivatable ZnSalen was successfully applied in bioimaging in a model living organism Caenorhabditis elegans (C. elegans), and ca. 5 times fluorescence intensity increase was observed after one and two photon irradiation. This paradigm by modulation of the PET process in ZnSalen provides a promising methodology for the design of photoactivatable fluorophores in further applications in super resolution molecular imaging.