Donor–π–acceptor Benzothiadiazole molecular rotor fluorophores: photophysical characterisation and amyloid beta binding

In this study, two novel Benzothiadiazole-based donor–π–acceptor fluorescent probes (BTD1 and BTD2) were designed and synthesised to evaluate their photophysical behaviour and sensing potential. The probes BTD1 and BTD2 displayed emission wavelengths of 635 nm and 637 nm, respectively in PBS pH 7.4, Both the probes displayed molecular rotor behaviour and distinct spectral changes at strongly basic pH values due to a structural transformation. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations supported the donor–acceptor electronic distribution.. Molecular docking studies revealed moderate binding affinity towards amyloid-β1–42 aggregates, which was further supported by fluorescence enhancement of probes upon interaction with the aggregates. The binding constants for BTD1 and BTD2 were calculated to be 1.59 × 105 M−1 and 8.47 × 105 M−1 respectively, using the fluorescence titration experiments towards Aβ1–42 aggregates.