On the fluorescence of methyl salicylate: the significance of its IMHB

The two forms of methyl salicylate bearing an intramolecular hydrogen bond (IMHB) are responsible for the three fluorescence emissions produced by this compound on electronic excitation in inert media. Whereas the form possessing an IMHB between its hydroxyl group and ether oxygen undergoes no excited state intramolecular proton transfer (ESIPT) in its first excited electronic state, that with an IMHB involving the carbonyl oxygen exhibits ESIPT with near-unity efficiency. Whereas the former species exhibits standard photophysical behaviour, the latter species exhibits two fluorescence emissions from the same electronic excited state; a photophysical scheme is proposed, which brings together all the available photophysical evidence for methyl salicylate in inert media.