Thiazole Orange Fluoresces Freely: No Rigid Environment Required

Area of Science:

• Photophysics
• Chemical Physics
• Spectroscopy

Background:

• Twist motion is a key relaxation pathway for fluorescent photoswitches.
• Thiazole orange (TO) is typically nonemissive in water but fluoresces strongly when bound to DNA.

Purpose of the Study:

• Investigate the intrinsic fluorescence properties of thiazole orange (TO).
• Determine the environmental factors influencing TO's excited-state dynamics and fluorescence.
• Challenge the assumption that a rigid environment is necessary for TO fluorescence.

Main Methods:

• Cryogenic fluorescence ion spectroscopy of isolated TO cations.
• Quantum-chemical calculations of Franck-Condon spectra and potential energy surfaces.
• Comparative analysis of TO in water, DNA-bound, and gas-phase.

Main Results:

• Isolated TO cations in a vacuum exhibit strong fluorescence with a long 11 ns lifetime.
• Gas-phase TO spectra resemble those of DNA-bound TO, indicating a near-planar geometry.
• An intrinsic barrier on the excited-state potential energy surface prevents deexcitation in the gas phase.

Conclusions:

• Fluorescence is an intrinsic property of TO, not solely dependent on environmental rigidity.
• The excited-state dynamics of TO are governed by an internal barrier to twisting in the absence of environmental constraints.
• This study reframes the understanding of fluorescence mechanisms in photoswitchable dyes.