Reconsider the fluorescence properties of 5-ASA and its sensing mechanism for iodide ion

5-ASA shows multiple fluorescence peaks at 408 nm, 480 nm and 500 nm in methanol solution, which can serve as ratiometric sensor for I^-. However, fluorescence mechanism of 5-ASA remains controversial and I^- detecting mechanism is unclear. With the aid of density functional theory and time-dependent density functional theory, this paper has carried out detailed investigations on both aspects at the molecular level. The origins of all the fluorescence peaks of 5-ASA have been assigned and a new fluorescence mechanism is proposed, which is convincing and intrinsically different from previously proposed mechanisms. Based on the new fluorescence mechanism, the I^- detecting mechanism is fully addressed. Two I^- detecting mechanisms have been proposed and are likely to coexist which lead to the sensitive detection of I^-.