Phenalenyl-Based Neutral Radical Molecular Conductors: Substituent Effects on Solid-State Structures and Properties

We report the preparation, crystallization, and solid-state characterization of cycloheptyl and
cyclooctyl-substituted spirobiphenalenyl radicals and the corresponding σ-dimer of the cyclooctyl derivative.
The crystal structure shows that the cycloheptyl radical (9) is monomeric in the solid state, with the molecules
packed in an unusual one-dimensional (1-D) fashion that we refer to as a π-chain structure, whereas the
cyclooctyl variant exists both as π-dimer 10 and σ-dimer 10d. The neutral radical 9 shows the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility, χp ≈ 4.5 × 10-4
emu/mol and is assigned a resonating valence bond (RVB) ground state. We highlight the relationship
between the magnetic properties of the Heisenberg antiferromagnet and the RVB ground state in 1-D and
further elucidate the electronic structure of this new class of compounds. Magnetic susceptibility
measurements show that 10 is a diamagnetic π-dimer, whereas 10d is a diamagnetic σ-dimer. Extended
Hückel calculations for 9 indicate that the solid is a one-dimensional organic metal with a bandwidth of
about 0.4 eV. Pressed pellet conductivity measurements indicate values of σRT = 1.5 × 10-3 S/cm for
compound 9 and σRT = 1.0 × 10-6 S/cm for compound 10. The structural results and transport properties
are discussed in the light of extended Hückel theory band structure calculations and DFT investigations of
the electronic structure of related compounds.