Charge transfer process determines ultrafast excited state deactivation of thioflavin T in low-viscosity solvents.

Here we provide first direct experimental results about photoinduced TICT-state formation for Thioflavin T (ThT). In this work, femtosecond transient absorption spectra dynamics for ThT, dissolved in low-viscosity solvents (water, ethanol, 2-propanol, butanol) was investigated. It was found that decay lifetime of fluorescent LE-state for ThT in low-viscous solvents does not exceed 12 ps, and its value correlates well with rising time of the absorption band at 470 nm. It indicates that LE-state of ThT initially formed upon photoexcitation is quite rapidly converted to a transient state characterized by absorption at 470 nm. We associate this emerging intermediate state with nonfluorescent TICT-state of the dye. Rate of LE → TICT process significantly depends on viscosity and is comparable to the rate of solvent relaxation resulting in time-dependent Stokes shift of ThT stimulated emission band. TICT-state deactivation was found to be also viscosity dependent and its lifetime changed from 3.8 ± 0.1 ps (in H...