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(2)O) to 360 +/- 60 ps (in butanol). It was proposed that a nonradiative deactivation process proceeds through a conical intersection between TICT(S(1)') and S(0) energy levels. The results obtained confirm the earlier proposed model that twisted internal charge transfer process takes place in the excited state of the dye and that ThT behaves as a molecular rotor (Stsiapura, V. I.; Maskevich, A. A.; Kuzmitsky, V. A.; Uversky, V. N.; Kuznetsova, I. M.; Turoverov, K. K. J. Phys. Chem. B 2008, 112, 15893-15902).