  The influence of solubilizing substitutional groups on the electronic and
optical properties of functionalized pentacene molecules and crystals have been
investigated. Density functional theory is used to calculate the electronic and
optical properties of pentacene, TIBS-CF$_{3}$-pentacene, and TIPS-pentacene.
The results are compared with experimental absorption spectra of solutions and
the complex dielectric function of thin films in the 1 eV to 3 eV energy range.
In all cases, the band gaps of the isolated molecules are found to be smaller
than those of the crystals. The absorption spectra and dielectric function are
interpreted in terms of the transitions between the highest occupied molecular
orbitals and lowest unoccupied molecular orbitals. The bands associated to C
and Si atoms connecting the functional side group to the pentacene in the
(6,13) positions are found to be the main contributors to the optical
transitions. The calculated dielectric functions of thin films agree with the
experimental results. A redshift is observed in crystals as compared to
molecules in experiment and theory both, where the amplitude depends on the
packing structure.
