Synthesis, photophysical and spectroscopic characterization of amphiphilic D-A-π-A benzothiadiazoles

Two positively charged amphiphilic benzothiadiazole derivatives with a Donor-Acceptor-π spacer-Acceptor structure (D-A-π-A) were developed, based on a methoxy-phenyl-benzothiadiazole model compound, by adopting methoxy-phenyl as the electron donor and 2,1,3-benzothiadiazole and 2,3-dimethylbenzothiazol-3-ium or 2,3-dimethylnaphtothiazol-3-ium iodide as combined electron acceptors. The aim of the study was to investigate the photophysical properties of the benzothiadiazole derivatives in detail, focusing on substituent and solvent polarity effect, together with their biocompatibility. From a comprehensive study involving steady-state fluorescence and ultra-fast time-resolved spectroscopic techniques (fs-TA and fs-UC) and TDDFT electronic structure calculations, it is shown that intramolecular excited state charge transfer is present in the investigated D-A-π-A benzothiadiazole derivatives. The differential Stokes shift of the two emission bands with a change in the solvent polarity and excitation wavelength discriminates the locally excited (LE) emission from the charge transfer (CT) emission. The introduction of the 2,3-dimethylbenzothiazol-3-ium or 2,3-dimethylnaphtothiazol-3-ium iodides on the 2,1,3-benzothiadiazole group inhibits excited-state intersystem crossing process and prompts the predominance of the internal conversion as the main excited-state deactivation channel. Quantum chemical calculations corroborated the occurrence of ICT through depiction of the differential electronic distributions in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The synthesized amphiphilic D-A-π-A conjugated benzothiadiazoles present good biocompatibility and rapid cellular uptake, distributing in the cytoplasm. • Synthesis of amphiphilic benzothiadiazole derivatives with a Donor-Acceptor-π spacer-Acceptor structure (D-A-π-A). • Comprehensive investigation of the excited state decay mechanisms of amphiphilic benzothiadiazole derivatives. • The synthesized benzothiadiazoles present good biocompatibility and rapid cellular uptake, distributing in the cytoplasm.