Induction of triplet-state emission of the transient proton-transfer keto form of methyl salicylate by heavy-atom perturbation

The rare (third) example of a triplet-state emission from a transient phototautomer is reported here for the case of the keto form of methyl salicylate. This molecule satisfies two of the three essential criteria predicted for observation of a triplet-singlet emission under conditions of excited-state intramolecular proton transfer: (1) published spectroscopic data confirm a keto tautomer triplet below the stable enol tautomer triplet, and (2) an easily accessible spectroscopic region. The third criterion, (3) noninterfering radiationless transitions, is not satisfied, and the T'(1) --> S'(0) transition is not observed. This difficulty is overcome by external heavy-atom perturbation using ethyl iodide matrices at 77 K, with time-resolved spectroscopy. The tautomer phosphorescence is observed at lambda(m) = 560 nm, with a lifetime in the microsecond range. Internal heavy-atom perturbation is demonstrated to be effective in 3,5-dibromo and -diiodo methyl salicylates, the enhanced T'(1) --> S'(0) emission occurring in the latter in both steady-state and time-resolved emission.