Time-resolved spectroscopy of thioflavin T solutions: Asynchronous optical sampling method with two...

Area of Science:

• Photochemistry
• Molecular Spectroscopy
• Physical Chemistry

Background:

• Thioflavin T (ThT) is a fluorescent dye used to study amyloid formation.
• Understanding the photophysics of ThT in different environments is crucial for its applications.
• Intramolecular charge transfer (ICT) processes significantly influence molecular photochemistry.

Purpose of the Study:

• To investigate the ultrafast photochemistry of Thioflavin T (ThT) in various solvents.
• To study the influence of solvent polarity on the twisted intramolecular charge transfer (TICT) process in photoexcited ThT.
• To demonstrate the utility of near-UV asynchronous optical sampling (ASOPS) transient absorption spectroscopy for studying condensed-phase reactions.

Main Methods:

• Employed asynchronous optical sampling (ASOPS) with synchronized femtosecond lasers in the near-UV (NUV) region (380-430 nm).
• Performed single-color NUV-NUV and two-color NUV-near-IR (NIR) pump-probe spectroscopy.
• Measured interferometrically detected time and frequency-resolved pump-probe signals of ThT in different solvents.

Main Results:

• Observed reduced vibronic coupling strengths (214 and 526 cm⁻¹) in the excited state of ThT when dissolved in chloroform.
• Theoretically confirmed that these vibrational modes possess significant electric dipole moments in the excited state.
• Found that the ICT process in ThT is less efficient in chloroform, leading to an increased fluorescence quantum yield.

Conclusions:

• Solvent properties, particularly polarity, significantly influence the photophysical behavior of ThT.
• Chlorine-containing solvents like chloroform can suppress the ICT process in ThT by altering vibronic coupling.
• NUV-NUV and NUV-NIR ASOPS-transient absorption spectroscopy is a powerful technique for probing ultrafast photochemical reactions.