Spin-carrying benzoquinone derivatives.

Area of Science:

• Organic Chemistry
• Materials Science
• Crystallography

Background:

• 1,4-benzoquinone derivatives are versatile organic molecules with diverse electronic properties.
• Stable organic radicals, such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), are of interest for developing novel electronic materials.
• Charge-transfer (CT) complexes involve the interaction between electron donor and acceptor molecules.

Purpose of the Study:

• To investigate the formation of single-component charge-transfer (CT) complexes in 1,4-benzoquinone derivatives functionalized with oxy-TEMPO radicals.
• To elucidate the structural basis and electronic interactions within these novel CT complexes.
• To explore the influence of radical positioning on CT complex formation and magnetic properties.

Main Methods:

• Synthesis of 1,4-benzoquinone derivatives substituted with oxy-TEMPO radicals at various positions (2-, 2,5-, and 2,6-).
• Recrystallization of synthesized compounds from suitable solvents to obtain crystalline materials.
• Single-crystal X-ray diffraction analysis to determine molecular and crystal structures.
• Analysis of intermolecular interactions, including charge-transfer pathways and radical-radical contacts.

Main Results:

• Recrystallization of 1,4-benzoquinone derivatives with oxy-TEMPO radicals at the 2- or 2,5-positions yielded black crystals, indicative of CT complex formation.
• Crystal structure analysis revealed the formation of unusual single-component CT complexes where the nitroxide moiety acts as the electron donor and the 1,4-benzoquinone core as the electron acceptor within the same molecule.
• Derivatives with oxy-TEMPO radicals at the 2,6-positions did not form intramolecular CT complexes.
• A specific derivative with amino-TEMPO radicals at the 2,5-positions showed one TEMPO group participating in CT formation, while the other exhibited a significant oxygen-to-oxygen contact with a neighboring molecule, leading to a large exchange coupling (J/k(B) = -154 K).

Conclusions:

• The positional arrangement of oxy-TEMPO radicals on the 1,4-benzoquinone core critically dictates the formation of single-component CT complexes.
• Intramolecular charge transfer is favored when radicals are positioned at the 2- or 2,5-positions, leading to unique donor-acceptor interactions within the molecule.
• The study highlights the potential for designing novel organic materials with tunable electronic and magnetic properties through strategic functionalization of benzoquinone systems with stable radicals.