  In order to investigate the optical properties of rubrene we study the
vibronic progression of the first absorption band (lowest \pi -> \pi^*
transition). We analyze the dielectric function of rubrene in solution and thin
films using the displaced harmonic oscillator model and derive all relevant
parameters of the vibronic progression. The findings are supplemented by
density functional calculations using B3LYP hybrid functionals. Our theoretical
results for the molecule in two different conformations, i.e. with a twisted or
planar tetracene backbone, are in very good agreement with the experimental
data obtained for rubrene in solution and thin films. Moreover, a simulation
based on the monomer spectrum and the calculated transition energies of the two
conformations indicates that the thin film spectrum of rubrene is dominated by
the twisted isomer.
