Shining Light on Benzothiadiazoles and Indolines : Photophysical Properties and Cancer Cell Imaging

Abstract: Fluorescent molecules, also called fluorophores, play an important role in life sciences. As crucial tools in fluorescence microscopy, they can be used to study biological processes on a cellular level, which is a fundamental aspect of cancer research. Depending on the specific research question, the requirements for fluorophores can be very diverse. Expanding the toolbox of fluorescent probes can therefore contribute to the possibilities of fluorescence imaging in general, especially since no single fluorophore is perfect for all applications.In this thesis, donor-acceptor fluorophores based on two different core structures, 2,1,3-benzothiadiazole and indoline, were investigated. The compounds were photophysically studied by steady-state absorption and fluorescence spectroscopy and examined in fluorescence cell microscopy.Monosubstituted benzothiadiazole derivatives with nitrogen-based substituents were synthesized and photophysically studied in various solvents. Fluorescence microscopy experiments revealed three promising new compounds that specifically stain lipid droplets in cancer cells. Lipid droplets are interesting cellular targets for imaging because of their high relevance in the energy metabolism of cancer cells. Furthermore, two fluorescent indoline derivatives, which differ only by one functional group (imine vs ketone) were compared for their photophysical properties and cell imaging behavior. The indolin-3-imine derivative showed pH sensitivity due to protonation of the imine moiety, which was significantly enhanced in the excited state because of photobasicity. Fluorescence microscopy experiments showed ubiquitous cell staining with a tendency towards lysosomes, which are acidic cell organelles. In contrast, the indolin-3-one analogue featured lipid droplet-specific staining, although with rapid photobleaching. Spectroscopic studies in solution revealed photoisomerization through ring-opening upon irradiation, which explained the emission bleaching during microscopy. While this behavior is not beneficial for imaging applications, it was found that the indolin-3-one exhibited photoactivated cytotoxicity, which likely arises from singlet oxygen generation.