Synthesis, properties and highly selective mitochondria staining with novel, stable and superior benzothiadiazole fluorescent probes

The present manuscript describes the synthesis of two novel 2,1,3-benzothiadiazole (BTD) derivatives containing an excited state intramolecular proton transfer (ESIPT) site. Photophysical properties, X-ray analysis, ESIPT and intramolecular charge-transfer (ICT) of these novel fluorescent monosubstituted BTD derivatives were investigated. It is also shown that ESIPT and ICT can take place concomitantly. Theoretical calculations (ab initio and DFT) corroborate the high stability of these derivatives in the excited state due to efficient ESIPT and ICT processes. Also, the optimized calculated geometries of these new structures allowed a better understanding of the different behaviour of the dyes in a wide pH range (1–13). Finally, the new compounds exhibit impressive cellular selectivity and stain only mitochondria in different cell lines and are far better than the commercially available MitoTracker-red.