Computational Investigation of the Structural and Electronic Effects of Phenyl, Alkyl, and Halogen Fully Substituted Acenes

Acenes are a class of polycyclic aromatic hydrocarbons that may hold promise as organic semiconductors (OSCs) in solar cells and electronic devices. Their instability and poor solubility present challenges that may be improved by replacing the hydrogens with phenyl, halogen or alkyl substituents to sterically induce a helical twist to these otherwise planar molecules. This twisted structure also impacts the optical properties of these molecules. We employ time-dependent density functional theory (TD DFT) to investigate acenes spanning from naphthalene to heptacene. We focus on the structural and electronic effects of fully substituting these molecular backbones to create seven distinct substituent series, many of which have been previously synthesized. The end-to-end intramolecular twist increases linearly with acene length for all series, however the degree of twist varies significantly depending on the specific substituent. All series display similar trends of increasing red shifts in the estimated highest occupied molecular orbital-lowest-unoccupied molecular orbital (HOMO-LUMO), fundamental, and optical gaps as the number of fused rings along the polycyclic backbone increases. Despite the similarity of measured gaps, features distinguishing the series from one another are more apparent in their near ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectra. Furthermore, halogen and alkyl substituents display local minima for two other structural configurations in addition to the twisted structure. Gibbs free energy calculations show these three distinct configurations are likely energetically competitive at room temperature. One novel nonhelical geometry shows significant reductions in excitation energies, while the other displays similar values to the twisted acene structures. The structural and electronic trends of these series offer insights that can guide the use of these and similar acenes as functional materials.