Optical observation of different conformational isomers in rubrene ultra-thin molecular films on epitaxial graphene

•Optical properties of ultra-thin rubrene films on various substrates obtained•Two molecular species which differ in their optical absorption were identified.•Different molecular conformations contribute to the optical absorption behavior.

Optical differential reflectance spectroscopy in combination with low-energy electron diffraction and scanning tunneling microscopy is used to investigate (ultra-)thin rubrene films grown on epitaxial graphene. The optical absorption behavior is compared to thin films on graphite(0001), muscovite mica(0001), and amorphous glass. All the optical spectra can be explained by the presence of two different spectral components, namely a high-energy and a low-energy component. We assign these two optical species to two different conformations of the rubrene molecule.