Absorption and photoluminescence spectroscopy of rubrene single crystals

We report the intrinsic absorption and photoluminescence spectra of rubrene single crystals, deriving them from a series of experiments performed at different excitation wavelengths and in different experimental geometries. We describe the absorption spectra for all three light polarizations in the crystal, and discuss how anisotropic wavelength-dependent absorption and emission affect the characteristics of observed photoluminescence spectra. We identify vibronic progressions both in absorption and emission and discuss their parameters and the main vibrational modes that are responsible for them. We propose that the most commonly measured absorption and emission in rubrene, the one with light polarization perpendicular to the $c$ axis of the crystal, is caused by vibronic-induced depolarization of the $c$-polarized electronic transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).