Dimeric phenalenyl-based neutral radical molecular conductors.

Area of Science:

• Organic chemistry
• Materials science
• Solid-state physics

Background:

• Spiro-biphenalenyl radicals are complex organic molecules with potential electronic applications.
• Understanding their solid-state behavior is crucial for designing novel conductive materials.

Purpose of the Study:

• To synthesize and characterize ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals.
• To investigate their solid-state structure, magnetic properties, and electrical conductivity.
• To explore the unprecedented conductivity enhancement during phase transitions.

Main Methods:

• Single crystal X-ray diffraction for structural analysis.
• Magnetic susceptibility measurements to determine magnetic ground states.
• Electrical resistivity measurements to assess conductivity.

Main Results:

• Both ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals form conducting pi-dimers in the solid state.
• Compound 4 is diamagnetic, while compound 3 is paramagnetic at room temperature.
• Phase transitions between paramagnetic and diamagnetic forms were observed for both compounds.
• Conductivity increased by two orders of magnitude upon transition to the diamagnetic state, contrary to Peierls dimerization predictions.

Conclusions:

• The observed conductivity enhancement is unprecedented and challenges existing models of Peierls dimerization.
• The behavior may be attributed to a decrease in on-site Coulombic correlation energy (U) in the dimeric structures.
• These findings open new avenues for designing organic conductors with tunable electronic properties.