However, underlying full microscopic explanations for the photoacidity phenomenon have remained sparse. With ultrafast X-ray spectroscopy, locally probing the electronic structure of a proton donating group of an amine aromatic photoacid has now provided direct insight in the changes of electronic structure.

Major electronic structure changes occur on the base side

The long standing open question for photoacidity has now finally been resolved: major electronic structure changes occur on the base side of the so-called Förster cycle, whereas the acid side plays a minor role.  
Photoacids have been known for more than 70 years. Theodor Förster has been the first to correctly describe the observations of absorption and fluorescence spectra of photoacids, and connect positions of the electronic transitions giving rise to optical absorption bands to the increased acidity properties of photoacids in the electronic excited state.

Pronounced effects on the conjugate photobase side

Many research activities have been pursued in the following decades, but apart from quantum chemical calculations of photoacid molecules of medium size, focussing on the intramolecular electronic charge distribution changes of the proton donating moieties of photoacids, microscopic insight have remained limited. Some of these studies have indicated – in line with previous suggestions based on physical organic principles – that the effects of electronic excitation are much more pronounced on the conjugate photobase side of the Förster cycle (Fig. 1)

Fig. 2 Estimated charge distribution changes on the APTS photoacid and conjugate photobase forms, showing major changes in Mulliken charges and in the electric dipole moment upon electronic excitation.

The K-edge X-ray absorption spectra of nitrogen atoms in the molecular structure were measured

Scientists from the Max-Born-Institute in Berlin, Stockholm University, the University of Hamburg, Helmholtz-Zentrum Berlin, Ben-Gurion University of the Negev in Beersheva and Uppsala University, have now successfully pursued a novel combined experimental and theoretical approach to study the electronic charge distributions of photoacids along the four stages of photoacids provide direct microscopic insight into the electronic structural changes of the proton donating amine group of an aminopyrene derivative in aqueous solution. The K-edge X-ray absorption spectra of nitrogen atoms in the molecular structure were measured at the synchrotron BESSY II in transmission mode to locally probe electronic structure on ultrafast time scales. Together with quantum chemical calculations, such results provide a consistent picture of photoacid behaviour (Fig. 2): electronic charge distributions of the proton donating group are only minor on the photoacid side, but substantial on the conjugate photobase side

Yet the overall dipole moment change of the whole molecule is as important as the local charge distribution changes, hence solvation dynamics by the solvent water is the second important factor governing photoacidity.

Fig. 1: Förster cycle of an amine photoacid, showing electronic ground states S0 and the first excited states S1 of the acidic (left) and basic (right) species. There are the four stages of photoacid behaviour in aqueous solution, as shown by the cartoons. In the centre transient soft-X-ray spectra are shown, measured on 8-aminopyrene-1,3,6-trisulfonate (APTS).

More information

• Electronic Structure Changes of an Aromatic Amine Photoacid along the Förster Cycle
  Sebastian Eckert, Marc-Oliver Winghart, Carlo Kleine, Ambar Banerjee, Maria Ekimova, Jan Ludwig, Jessica Harich,  Mattis Fondell, Rolf Mitzner, Ehud Pines, Nils Huse, Philippe Wernet, Michael Odelius, and Erik T. J. Nibbering
  Angewandte Chemie International Edition (24 March, 2022)
  DOI: 10.1002/anie.202200709
• Contact Michael Odelius