Observing Optically Dark States in the Photoswitchable Azobenzene with Resonant Inelastic X-ray Scattering

The trans-cis isomerization in azobenzene is a reversible reaction that has strong application potential ranging from photo-stimulating molecular motors to molecular electronics. Though azobenzene has been intensively and thoroughly investigated in optical spectroscopy, the S0 -> S1 transition is symmetry forbidden. Here, we report on measurements of resonant inelastic X-ray scattering (RIXS) at the nitrogen K-edge of the trans- and cis-forms of azobenzene dissolved in ethanol. In RIXS, we observe isomer-specifi�c spectral features, which contain a distinct signature of the S1 state and further information on the electronic properties of the isomers of azobenzene. For the cis-isomer an additional peak arises, revealing a transition suppressed in the trans-conformation due to diferent orbital hybridization, thereby giving distinct spectral finn=gerprints to the trans- and cis-isomers of azobenzene in RIXS, containing information about valence excitations and their nitrogen character.