Picosecond transient absorption spectra of aminosalicylates in confirmation of the triple excitation mechanism

Using picosecond transient absorption studies, it is shown that the twisted intramolecular charge-transfer state (TICT) and the excited intramolecular proton-transfer state (ESIPT or PT) of specific aminosalicylates both contribute to the long-wavelength fluorescence band, F{sub 2}, observed as an unresolved band to the red of the normal fluorescence band, F{sub 1}. The transient absorption band for 2-hydroxy-4-(dimethylamino)benzoic acid methyl ester (PDASE) is shown to be a composite of the two excited-state absorption modes: S{sub n}{double prime}(TICT) {l arrow} S{sub 1}{double prime} (TICT) and S{sub n}{prime}(PT){l arrow} S{sub 1}{prime}(PT). This corroborates previous steady-state fluorescence studies of the unresolved F{sub 2} band. The assignments are based on comparison with the excited-state absorption spectra of various substituted aminosalicylates in polar and nonpolar solvents.