Mixed-vs-Segregated Stack Polymorphism in the N,N,N',N'-Tetramethylbenzidine-TCNQF₄ Charge Transfer Complex

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Organic charge transfer (CT) cocrystals are 1-D materials formed by donor (D) and acceptor (A) molecules.
• These materials typically exhibit segregated (separate D/A columns) or mixed (alternating D-A) stack motifs.
• Polymorphism in CT crystals is common, but coexisting mixed and segregated stack phases are rare.

Purpose of the Study:

• To synthesize and characterize a new organic CT cocrystal exhibiting both mixed and segregated stack phases.
• To investigate the structural and optical properties of these coexisting phases.
• To explore potential stack distortions in the segregated phase at low temperatures.

Main Methods:

• Synthesis of a novel CT complex using N-TMB (donor) and TCNQF4 (acceptor).
• Single-crystal X-ray diffraction for structural analysis.
• Polarized infrared (IR) and Raman spectroscopy for optical characterization.

Main Results:

• A new CT complex of N-TMB and TCNQF4 was successfully synthesized, displaying both mixed and segregated stack motifs.
• Common structural and optical features were identified between the mixed and segregated phases using spectroscopy and X-ray diffraction.
• Low-temperature data indicated a stack distortion within the segregated stack phase.

Conclusions:

• This study presents a rare example of an organic CT cocrystal exhibiting both mixed and segregated stack phases.
• The findings highlight the potential for designing materials with tunable structural motifs.
• The observed stack distortion in the segregated phase warrants further investigation into its electronic implications.