Abstract
By studying the low-frequency phonon bands of a series of crystalline acenes, this article lays the foundation for the development of structure–property relationships for phonons in organic semiconductors. Combining state-of-the art quantum–mechanical simulations with simple classical models, we explain how and why phonon frequencies and group velocities do or do not change when varying the molecular and crystal structures of the materials.
Highlights
A precise understanding of phonons in OSCs is key to understanding the above-mentioned properties
The choice of the order is such that a1 (a2) denotes the longer distance of symmetry equivalent molecules, while the a3 axis is roughly parallel to the long molecular axis
At room temperature polymorph 5A-II is more stable than 5A, we will primarily focus on 5A as again its structure is more consistent with the structures of the shorter acenes, as explained in Section S3 (ESI†)
Summary
A precise understanding of phonons in OSCs is key to understanding the above-mentioned properties. There are only very few, scattered, examples of crystals consisting of conjugated molecules for which phonon band structures have been measured. These comprise, for example, deuterated naphthalene[32,33] and anthracene,[34] for which the dependence of the bands on temperature,[32,35,36,37] pressure,[38] and on the anharmonic nature of the phonons[39] have been determined. For our discussion we will rely on suitably benchmarked dispersion-corrected density-functional-theory (DFT) calculations These have the additional benefit of providing direct access to the atomistic motions that describe specific phonon modes.
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