Bimodal intermolecular proton transfer in water: photoacid-base pairs studied with ultrafast infrared spectroscopy

Exploring the mechanism of proton transfer between acid and bases is of fundamental importance for the understanding of the autoionization of water and the associated von Grotthuss mechanism, acid-base neutralizations, enzyme catalysis, and proton pumps through membranes. Photo-acids have been the subject of intense investigations since the landmark discovery of Forster that the extreme fluorescence Stokes shift observed for several classes of aromatic dyes can be explained to be due to excited state proton transfer. Photo-acids can be used as a means of light triggered proton transfer, thus enabling time-resolved studies. The photo-acidity, with typically a decrease in pKa of 5-10 units upon excitation, is a research subject on its own, where traditionally a charge transfer from the oxygen atom of the OH group to the aromatic ring is thought to be responsible for the increased acidity of the photoacid. A more detailed explanation invokes the fact that for aromatic systems electronic states excited along the short and long axes are often energetically close, with a possible occurrence of state inversion due to polar interactions with the solvent. In the case of pyranine (8-hydroxy-l,3,6-trisulfonate-pyrene), a widely used dye stain, it is argued that the initially excited state cannot be the one responsible for the fluorescence emission, and thus a state inversion should occur. The chapter discusses pyranine with femtosecond mid-infrared spectroscopy in the fingerprint region in H2O (between 850-1350 cm-1), where vibrational bands of the sulfonate-groups are located, and D2O (1250-1800 cm-1) where aromatic ring and C-O modes have their resonances.