Thiazole Orange Fluoresces Freely: No Rigid Environment Required.

Twist motion is a common relaxation pathway in many fluorescent photoswitches, including the green fluorescent protein chromophore, thioflavins, and thiazole orange (TO); but the associated excited-state dynamics are highly sensitive to the local environment. In this work, we have studied the cationic dye TO that is virtually nonemissive in water but displays strong turn-on fluorescence upon DNA binding. We demonstrate that fluorescence is an intrinsic property of TO that does not require a rigid environment, as isolated TO cations in vacuo exhibit strong fluorescence with an unexpectedly long 11 ns lifetime, 3 orders of magnitude larger than its picosecond lifetime in water. Cryogenic fluorescence ion spectroscopy reveals that absorption and fluorescence spectra of isolated TO resemble those of the DNA-bound dye, which indicates that fluorescence from the gas-phase ions occurs from a near-planar geometry. We show that in the gas phase there is an intrinsic barrier on the excited-state potential energy surface along the twist coordinate preventing deexcitation through a conical intersection with the ground-state surface, as observed in water. Quantum-chemical calculations of Franck-Condon spectra and potential energy surfaces support our interpretation and challenge the prevailing assumption that a rigid environment is needed to turn on fluorescence in TO.