Band Structure Engineering by Substitutional Doping in Solid-State Solutions of [5-Me-PLY(O,O)]2B(1-x)Be(x) Radical...

Area of Science:

• Solid-state chemistry
• Materials science
• Organic radical chemistry

Background:

• Spiro-bis(5-methyl-1,9-oxido-phenalenyl)boron radical ([2]2B) is a solid-state radical with distinct crystal packing.
• Spiro-bis(5-methyl-1,9-oxido-phenalenyl)beryllium ([2]2Be) is a spin-free analog with a different crystal structure.
• Controlling molecular packing is crucial for tuning electronic properties in organic materials.

Purpose of the Study:

• To achieve substitutional doping of the [2]2B radical by co-crystallization with [2]2Be.
• To investigate the impact of doping on crystal structure, phase transitions, and electronic properties.
• To understand the relationship between molecular packing and conductivity in these solid solutions.

Main Methods:

• Co-crystallization of [2]2B and [2]2Be to form solid solutions ([2]2B(1-x)Be(x)).
• X-ray crystallography to determine crystal structures and space groups (P1̅ and P2₁/c).
• Electrical conductivity measurements and analysis of activation energies.
• Extended Hückel theory calculations to model band structures.

Main Results:

• Isolated solid solutions [2]2B(1-x)Be(x) with x up to 0.59.
• Observed a phase transition from P1̅ to P2₁/c at x = 0.1.
• Enhanced conductivity and reduced activation energy for x = 0-0.25.
• Demonstrated that molecular packing is concentration-dependent.
• Showed significant narrowing of bandwidth upon [2]2Be incorporation, from 1.4 eV to 0.3 eV.

Conclusions:

• Substitutional doping of [2]2B with [2]2Be is feasible, leading to tunable solid-state properties.
• The concentration of spin-free molecules dictates crystal packing and electronic bandwidth.
• This work provides a model system for studying structure-property relationships in organic radical materials.