Forcing Twisted 1,7-Dibromoperylene Diimides to Flatten in the Solid State: What a Difference an Atom Makes

Area of Science:

• Organic electronics
• Materials science
• Supramolecular chemistry

Background:

• Perylene diimides (PDIs) are crucial n-type semiconductors in organic electronics.
• Molecular conformation significantly impacts PDI solid-state properties and electronic performance.
• Achieving planar PDI cores is challenging due to inherent molecular twisting.

Purpose of the Study:

• To develop a strategy for flattening twisted 1,7-dibromoperylene diimide (PDI(H2Br2)) cores in the solid state.
• To investigate the influence of N-substituents on PDI conformation and solid-state packing.
• To enable the design of PDIs with improved electronic properties through controlled planarity.

Main Methods:

• Synthesis of four novel 1,7-dibromoperylene diimide derivatives with varying N-substituents.
• X-ray powder diffraction to analyze solid-state structures.
• Density Functional Theory (DFT) calculations to study conformational preferences and energy barriers.

Main Results:

• 1,7-dibromoperylene diimides with low-stereochemical-hindrance N-substituents (CH2/NH spacers) readily flatten in the solid state.
• The energy barrier for interconversion between twisted and flat atropisomers is low (26.5 kJ/mol).
• Achieved planarization results in centrosymmetric molecules and facilitates π-π stacking.

Conclusions:

• Flexible N-substituents can effectively overcome the rotational barrier, inducing planarity in PDI cores.
• This approach offers a general strategy for flattening twisted polyaromatic cores in various organic semiconductors.
• Controlled molecular planarity is key to optimizing crystal packing and electronic interactions for enhanced device performance.