Substitution effects on the excited-state intramolecular proton transfer of salicylic acid: an infrared spectroscopic study on the OH stretching vibrations of jet-cooled 5-methoxysalicylic acid

Infrared spectra of jet-cooled 5-methoxysalicylic acid in the 3-μm region were measured for both the electronic ground (S0) and first excited (S1) states. Effects of the methoxy-substitution were investigated in relation to the excited-state intramolecular proton transfer of salicylic acid. Though the electronic absorption–emission spectra exhibit the mirror-image relation, which has been regarded as evidence for the suppression of excited-state intramolecular proton transfer, the infrared spectra showed an extremely large elongation of the phenolic O–H bond upon photoexcitation. The results represent that the significant enhancement of the intramolecular hydrogen bond does occur in S1 of 5-methoxysalicylic.