A Strategy for Polar Crystals with Dipolar Heterohelicenes

Area of Science:

• Organic chemistry
• Materials science
• Crystallography

Background:

• Polar crystals are crucial for applications like piezoelectricity and nonlinear optics.
• Developing strategies for ordered polar organic materials, particularly helicenes, remains a challenge.
• Flexible polar materials are highly sought after for advanced technological applications.

Purpose of the Study:

• To develop a method for creating ordered polar crystals using unsymmetrically substituted azahelicenes.
• To investigate the structural and electronic properties of these novel polar materials.
• To demonstrate the potential of helicene-based organic molecules for polar material applications.

Main Methods:

• Synthesis of unsymmetrical aza[5]helicenes via regioselective mono-protiodesilylations.
• Crystal structure determination using single-crystal X-ray diffraction.
• Characterization using optical experiments and theoretical calculations (ab initio, Hirshfeld surface analysis).

Main Results:

• Successfully synthesized unsymmetrical aza[5]helicenes and formed 1D chain columnar packings, particularly with triisopropylsilyl substituents.
• Enantiopure crystals exhibited spontaneous polarization, confirmed by experimental and theoretical analyses.
• CH-π interactions were identified as key to regulating polar assembly in the 1D chain columns.
• A polarizability value of 4.53 μC/cm² was calculated for the nitro derivative, confirming spontaneous polarization.

Conclusions:

• The study presents the first example of a spontaneously polar crystal of helicenes, achieved through substituent control in azahelicenes.
• The findings highlight the importance of specific substituents (triisopropylsilyl, electron-withdrawing aryl) and CH-π interactions for forming polar columnar structures.
• This work contributes to the development of novel polar materials derived from organic molecules for advanced applications.