Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift ΔE<i>_m</i> is mainly caused by dispersion effects and depends sensitively on the molecule’s specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide ΔE_<i>m</i>= −<i>QW</i>_<i>m</i> in two factors, where <i>Q</i> depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while <i>W</i>_m is a geometry factor expressing the site dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)