Introduction to Femtochemistry: Excited-State Proton Transfer from Pyranine to Water Studied by Femtosecond Transient Absorption

In order to introduce students to the fascinating field of femtochemistry, we propose here a practical laboratory training course conceived for second-year master's students in chemistry. We describe the use of a broadband femtosecond transient absorption (pump-probe) experiment for monitoring a fast light-triggered chemical reaction in solution. The experiments are performed on the pyranine photoacid, which upon photoexcitation at 390 nm undergoes a proton transfer to the solvent in about 90 ps. While this practical course involves advanced equipment and techniques, the measured transient absorption data allow easy analysis and interpretation. The transient absorption spectra at a few selected delay times can be analyzed qualitatively in terms of bleach, induced absorption, and stimulated emission. Likewise, the transient absorption signals at a few chosen wavelengths can be quantitatively analyzed and explained with simple kinetic models to determine the time constant of the proton-transfer reaction. This training aims at giving the students the opportunity to face some of the current challenges in contemporary chemistry by learning the basics of ultrafast spectroscopy.