Practice question

Fluorescence originates from a spin-allowed singlet-singlet transition that occurs extremely rapidly after photon absorption. The excited S1 state deactivates through radiative and competing non-radiative pathways within picoseconds to nanoseconds, yielding lifetimes typically between one to ten nanoseconds for biomolecules like tryptophan, fluorescein and GFP. Phosphorescence, involving spin-forbidden triplet to singlet conversion, persists milliseconds to seconds and is distinct. Lifetimes of microseconds, milliseconds or seconds therefore cannot represent prompt fluorescence, while around ten nanoseconds accurately captures its intrinsic photophysical timescale in aqueous solution.