Picosecond Studies of Unimolecular Processes

Abstract

FÖRSTER first postulated that a number of functional groups may undergo large pK changes as a consequence of electronic excitation [1]. For example aromatic carbonyl oxygens become more basic, while phenolic hydrogens become more acidic in the excited state [2,3]. WELLER applied this theory to explain the large Stokes shift seen in the fluorescence of salicylic acid and its methyl ester [4]. Upon absorption of a photon, a change of +2 in the pK of the carbonyl oxygen and of -6 in the phenol group drives the proton across the intramolecular bond [2,3]. The resulting zwitterion fluoresces at 450 nm. The ultraviolet emission at 340 nm of salicylic acid (and methyl salicylate) was attributed to molecules in which the protons were still bound to the oxygen of the phenol group.