Photoelectron spectroscopy of the model GFP chromophore anion.

A photoelectron spectroscopy study of the anionic model chromophore of the green fluorescent protein is presented. From the photoelectron spectra taken at 3.496 eV, 4.62 eV, and 6.15 eV the vertical and adiabatic detachment energies are determined to be 2.8 ± 0.1 eV and 2.6 ± 0.2 eV, respectively. The vertical detachment energy is higher than the S1← S0 absorption maximum (2.57 eV) and indicates that the S1 state is bound with respect to electron detachment in the Franck-Condon region. The photoelectron spectrum taken at 6.15 eV, together with TD-DFT calculations, are used to assign a number of excited states in the neutral radical that correspond to electron loss from occupied orbitals in the anion. The photoelectron spectrum at 2.58 eV shows evidence for electrons formed by thermionic emission, suggesting that internal conversion is the dominant relaxation pathway following S1← S0 excitation.