Dual relaxation channel in thioflavin-T: An ultrafast spectroscopic study

• Fluorescence quantum yield of thioflavin-T in chloroform is ∼30 times higher compared to in methanol, although both having similar viscosity.
• Observed slow rise time in the fluorescence transient of ThT in chloroform.
• Experimental data indicate the involvement of a new TICT state in the excited state manifold of the dye.
• Upon excitation to the LE state, ThT undergoes two different amplitude motions leading to either TICT-1 state or TICT-2 state.
• LE to TICT-1 pathway is barrier-less in nature, but LE to TICT-2 has an intrinsic barrier, which is higher in chloroform than in methanol.

In the present contribution, the ultrafast excited state dynamics of thioflavin T (ThT) has been investigated in methanol and chloroform. The first hand data of 30 times higher fluorescence quantum yield and observation of slow rise time in fluorescence intensity of ThT in chloroform compared to methanol indicate the complicated photophysics of the molecule. Time resolved fluorescence data along with temperature dependence on fluorescence quantum yield, TD-DFT calculations and femtosecond transient absorption study vividly suggest the involvement of one more twisted intramolecular charge transfer state (TICT-2) along with the TICT-1 state, in the excited state manifold of ThT, which was not identified earlier. It was also established that the depletion of the molecule from the locally excited state to the TICT-1 state probably barrierless in nature, however, an inherent activation energy barrier is present between LE and TICT-2 states. This activation energy barrier in methanol (0.59 kcal mol−1) was found to be exactly same as that available at room temperature, whereas the same in chloroform is found to be quite high (1.58 kcal mol−1). This barrier is proposed to be responsible for the high quantum yield of ThT in chlorinated solvents and we demand a revisit to the mechanism of fibril sensing by ThT.

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