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 H2O) 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).