Excited state enol-keto tautomerization in salicylic acid: A supersonic free jet study

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Abstract
Excited state enol‐keto isomerization in salicylic acid (SA) monomer and dimer has been studied in a supersonic free jet expansion. Two carboxylic group rotamers of SA with significantly different photophysical properties are found in the expansion. Rotamer I, the major form of SA in the expansion, has an intramolecular hydrogen bond and can undergo excited state tautomerization reaction. Its S 1 origin is at 335.34 nm. Single vibronic level emission spectra are dominated by progressions in OH stretching (3230 cm−1), and in‐plane bending of the carboxylic group (240 cm−1). The spectra appear to consist of two components, normal (UV) and tautomer (BLUE) emissions, even at the origin. The intensity of the BLUE relative to the UV emission depends on the vibronic state, rather than the excess vibrational energy between the origin and 1100 cm−1. The fluorescence decay time profiles for both the emission components of rotamer I are identical within ∼1 ns experimental time resolution. A nonradiative decay process with an activation energy of ∼1100 cm−1 is deduced from an abrupt decrease in fluorescence lifetimes above this energy. The rotamer II cannot undergo excited state tautomerization. Its electronic origin is at 311.52 nm and emits only UVfluorescence. Upon increasing the concentration of the SA sample, a new spectrum is observed. Due to a nonlinear concentration dependence of the intensity and the propensity of SA to form dimers in solution, it is assigned to the SA dimer. This spectrum shows possible evidence of double proton transfer in the S 1 state.

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