Excited-state proton transfer (ESPT) in aqueous solution is of fundamental interest in understanding chemical reactivity. The photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) provides a paradigm to study ESPT using the newly developed femtosecond stimulated Raman spectroscopy (FSRS) with unprecedented spectral and temporal resolution. The S0 Raman spectra of HPTS-photoacid (PA) and photobase (PB) differ significantly between 1000�1600 cm-1 (e.g. the C�O stretch, 1285/1325 cm-1 in PA/PB; and ring-H rocking, 1150/1140 cm-1 in PA/PB), showing HPTS structural rearrangement after ESPT occurs. The time-dependent excited-state (S1) spectra reveal ultrafast conformational dynamics from PA to PB as the phenolic proton transfers to water-acetate complex, evident from the appearance of the acetic acid peak at 860�880 cm-1 on picosecond timescale. The onset of strong low-frequency modes within 250�650 cm-1 (acetate concentration dependent) following photoexcitation indicates the loss of coplanarity and increase of torsional freedom of the PA conjugated ring system before ESPT, mediated by water.

Excited state proton transfer from pyranine to acetate in water: Multidimensional reaction coordinate revealed by femtosecond stimulated Raman spectroscopy

243rd National Meeting (2012)

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