Photoelectron Spectroscopy of Oppositely Charged Molecular Switches in the Aqueous Phase: Theory and Experiment

XUV photoelectron spectroscopy (XPS) is a powerful methodfor investigatingthe electronic structures of molecules. However, the correct interpretationof results in the condensed phase requires theoretical models thataccount for solvation. Here we present experimental aqueous-phaseXPS of two organic biomimetic molecular switches, NAIP and p-HDIOP. These switches are structurally similar, but haveopposite charges and thus present a stringent benchmark for solvationmodels which need to reproduce the observed & UDelta;eBE = 1.1 eV differencein electron binding energy compared to the 8 eV difference predictedin the gas phase. We present calculations using implicit and explicitsolvent models. The latter employs the average solvent electrostaticconfiguration and free energy gradient (ASEC-FEG) approach. Both nonequilibriumpolarizable continuum models and ASEC-FEG calculations give verticalbinding energies in good agreement with the experiment for three differentcomputational protocols. Counterions, explicitly accounted for inASEC-FEG, contribute to the stabilization of molecular states andreduction of & UDelta;eBE upon solvation.