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

• Published in: Journal of physical chemistry. C Vol. 119; no. 10; pp. 5747 - 5751
• Main Authors: Megow, Jörg, Körzdörfer, Thomas, Renger, Thomas, Sparenberg, Mino, Blumstengel, Sylke, Henneberger, Fritz, May, Volkhard
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 12.03.2015
• Subjects: Absorption spectra; Approximation; Crystal structure; Diimide; Dispersions; Electronics; Mathematical analysis; Nanomaterials
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.5b01587

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 Δ E m 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 Δ E m = −QW m in two factors, where Q depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while W 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).