The excited state isomerization of thioflavin T (ThT) is responsible for the quenching of its fluorescence in a nonrestricted environment. The fluorescence quantum yield increases substantially upon binding to amyloid fibers. Simulations reveal that the variation of the twisting angle between benzothiazole and benzene groups (theta(1)) is responsible for the subpicosecond fluorescence quenching. The evolution of the twisting process can be directly probed by photoelectron emission with energies epsilon >= 1.0 eV before the molecule reaches the phi(1)-twisted configuration (similar to 300 fs).