Bending versus Twisting Acenes - A Computational Study

Polycyclic aromatic hydrocarbons (PAHs) are widely used in organic electronic devices. The electronic, magnetic, and optical properties of PAHs can be tuned by structural modifications to the aromatic backbone to introduce an inherent distortion from planarity, such as bending or twisting. However, it remains difficult to isolate and control the effects of such distortions. Here, we sought to understand how backbone twisting and bending affect the electronic properties of acenes, as models for larger PAHs. We found that, even when highly distorted from planarity (30 degrees per ring), acenes maintain their aromatic character and pi orbital delocalization with minor mixing of the sigma and pi orbitals. In addition, the energy gap between the HOMO and LUMO decreases with increasing twist, while the gap is hardly affected by bending, since the energy of both orbitals increase to a similar extent. For bent acenes in the triplet state, the spin becomes more localized with increasing bend, whereas twisting produces an evenly distributed spin delocalization. These findings can guide the synthesis of PAHs with tailored properties.

Automated summary

The study found that even when acenes are highly distorted, they still maintain their aromatic character and pi orbital delocalization, with a decrease in energy gap between HOMO and LUMO as twist increases, while bending has little effect on the gap. Spin becomes more localized with increasing bend for bent acenes in the triplet state, whereas twisting produces evenly distributed spin delocalization. These findings can guide the synthesis of PAHs with tailored properties.