Photophysical Properties of Trans-2-[4-(dimethylamino)styryl]-3-ethyl-1,3-benzothiazolium Perchlorate, a New Structural Analog of Thioflavin T

Spectral properties of a newly synthesized thiofl avin T (ThT) derivative, trans-2-[4-(dimethylamino)styryl]-3-ethyl-1,3-benzothiazolium perchlorate (DMASEBT) with absorption and fluorescence spectra shifted to longer wavelengths (than ThT), were studied. Quantum-chemical calculations established that DMASEBT is planar in the ground state. The energy minimum of the excited molecule corresponded to a twisted conformation (TICT-state) with a 90° angle between the planar fragments. Charge in the molecule redistributed and a non-fluorescing TICT-state was formed if the fragments rotated. Fluorescence occurred from the non-equilibrium excited state (LE-state). It was shown that limited torsional rotation of the molecular fragments and; therefore, a decreased probability of transitioning into the non-fluorescing TICT-state, were responsible for the significantly increased quantum yield and fluorescence lifetime of DMASEBT upon increasing the solvent viscosity and incorporating it into amyloid fibrils.