Steady-state and time-resolved emission studies of Thioflavin-T

Steady-state absorption and emission properties of 3,6-dimethyl-2(4-dimethylaminophenyl)-benzothiazolium cation (ThT+) were studied at room temperature in variety of solvents. The multifluorescence of this dye was observed in different environmental conditions. The predominant emission bands observed are around 400 nm (I band), 450 nm (II band) and 485 nm (III band). The fluorescence emission bands I and II observed in acidic, basic and aqueous solutions are for 350 nm excitation and band III for 390 nm excitation at low concentration (1 × 10−5 mol dm−3). However, the emission band at 520 nm was observed in all solvents at high concentration (1 × 10−2 mol dm−3). The emission band around 400 nm shifts from 400 nm to 470 nm (shift 70 nm) when the excitation wavelength is varied from 350 nm to 380 nm and the emission at 485 nm shifts from 485 nm to 520 nm (shift 35 nm) for excitation wavelength 390–420 nm. Fluorescence decays of this dye in various solvents monitored over each emissions showed a tri-exponential behavior and the analysis yielded three decay components, respectively, in the range 0.13–0.76 ns, 1.08–2.00 ns and 2.50–12 ns. Based on the steady-state and time-resolved emission measurements band I (400 nm) is assigned due to protonation, band II (450 nm) locally excited state (LE) and the band III (485 nm) due to dimerization via H-bonding. Time-resolved fluorescence anisotropy measurements as a function of viscosity were carried out over emissions 450 nm and 550 nm and were fitted to single exponential. The shorter reorientation times (56–332 ps) at emission maximum 450 nm were compared with the theoretical values estimated using the hydrodynamic model (SED model) and are attributed to free rotations of the molecule. However, longer reorientation times (188–726 ps) at emission maximum 550 nm were assigned to dimerization/higher aggregates of the solute molecule. The deviations of experimentally measured τr values at emission maxima 450 nm with those of theoretically estimated values were discussed.