Probing Ultrafast Structural Dynamics in Proton Transfer Reactions in Solution with Time-Resolved Infrared and X-Ray Spectroscopies

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Photoacids are molecular systems that exhibit a pronounced increase in acidity upon electronic excitation, enabling proton transfer dynamics to be studied in real time using time-resolved spectroscopy. The underlying mechanism for photoacidity is, however, still poorly understood. The goal of the project is to combine the structure resolving potential of time-resolved IR and x-ray spectroscopies to determine hydrogen bond geometries of photoacid-base complexes in liquid solution, and the changes of these hydrogen bond geometries upon electronic excitation of the photoacid chromophores. With femtosecond IR spectroscopy we will investigate the response of OH vibrations of naphthols and NH stretching vibrations of aminonaphthalenes, as well as aminopyrenes. Contributions of the photoacid-base hydrogen bond and of the solvent to the observed frequency shifts in both the electronic ground and excited states will be compared with quantum chemical calculations. Dynamics of hydrogen bonds of the photoacid-base complexes in both electronic states will be studied with transient two-dimensional IR photon echo spectroscopy. Photoacid behaviour has been ascribed to intramolecular charge transfer. The key role played by the intramolecular charge transfer will be studied by investigating a family of naphthol molecules with different substituents, understood to affect the intramolecular charge transfer capability. Time-resolved x-ray spectroscopy will applied to study the N K-edge of a cationic photoacids of the proton donating amino-group, with which we will have direct access to the electronic charge distribution changes of the proton donating group upon UV excitation. The results obtained with time-resolved x-ray absorption will be combined with transient IR absorbance of NH stretching modes of the cationic photoacid, thereby linking the two structurally resolving techniques.