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Abstract:
By solving the first-principles many-body Bethe-Salpeter equation, we compare the optical properties of two
prototype and technological relevant organic molecular crystals: picene and pentacene. Albeit very similar for the
structural and electronic properties, picene and pentacene show remarkable differences in their optical spectra.
While for pentacene the absorption onset is due to a charge-transfer exciton, in picene it is related to a strongly
localized Frenkel exciton. The detailed comparison between the two materials allows us to discuss, on general
grounds, how the interplay between the electronic band dispersion and the exchange electron-hole interaction
plays a fundamental role in setting the nature of the exciton. It represents a clear example of the relevance of the
competition between localization and delocalization in the description of two-particle electronic correlation.
