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| - Fast Excited-State Intramolecular Proton Transfer and Subnanosecond Dynamic StokesShift of Time-Resolved Fluorescence Spectra of the 5-Methoxysalicylic Acid/Diethyl etherComplex
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| - Excited-state intramolecular proton transfer (ESIPT) occurring in the salicylic acid (SA) derivative5-methoxysalicylic acid (5-MeOSA) in an apolar solvent (cyclohexane) and in the presence of the hydrogenbond accepting agent diethyl ether (DEE) is investigated. Analysis of the directly measured subnanosecondtime-resolved emission spectra (TRES) together with conventional steady-state fluorescence and time-correlatedsingle-photon-counting (TCSPC) decays indicates that ESIPT in this system occurs much faster thanfluorescence, and that the equilibrium between normal and tautomeric excited states is established before theemission from both states takes place. However, changes in time- and frequency-resolved fluorescence of the5-MeOSA/DEE complex are observed due to structural relaxation within the complex, which is reflected inthe dynamic Stokes shift of the tautomeric fluorescence band. The normal fluorescence band of 5-MeOSA/DEE does not exhibit marked changes within the investigated time range. A single-exponential relaxationtime of 460 ps was determined for the dynamic Stokes shift of the tautomeric band, and it is attributed to ageometric change within the 5-MeOSA/DEE complex upon excitation. Since both tautomeric and normalemission bands are well resolved and exhibit different time-dependent behaviors, a double-well potentialappears to be adequate to describe the excited state of the system studied.
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