Bending versus Twisting Acenes - A Computational Study.

Area of Science:

• Organic electronics
• Materials science
• Computational chemistry

Background:

• Polycyclic aromatic hydrocarbons (PAHs) are crucial in organic electronics.
• Modifying PAH structures, like introducing twists or bends, tunes their electronic, magnetic, and optical properties.
• Isolating and controlling the impact of these structural distortions remains challenging.

Purpose of the Study:

• To investigate how backbone twisting and bending influence the electronic properties of acenes, serving as model compounds for larger PAHs.
• To understand the fundamental effects of non-planar geometries on aromatic character and orbital delocalization.

Main Methods:

• Computational modeling of acene molecules with varying degrees of twisting and bending.
• Analysis of electronic structure, including frontier orbital energies (HOMO-LUMO gap) and spin distribution.
• Assessment of aromaticity and π-orbital delocalization under distorted geometries.

Main Results:

• Acenes retain aromatic character and π-orbital delocalization even with significant distortions (up to 30° per ring), showing only minor σ-π orbital mixing.
• Increasing backbone twist leads to a decrease in the HOMO-LUMO energy gap.
• Bending has a negligible effect on the HOMO-LUMO gap, as both orbital energies increase similarly.
• In the triplet state, bending localizes spin, while twisting promotes spin delocalization.

Conclusions:

• Backbone distortions in PAHs can be used to tune electronic properties without losing aromaticity.
• Twisting is an effective strategy for reducing the energy gap, while bending has less impact on it.
• The observed spin localization/delocalization effects provide insights for designing PAHs with specific magnetic properties for applications in organic electronics.