Rational Design of a Metal-Free Organic Radical Cation Exhibiting Mid-IR π-π* Transition Based on the Exchange...

Area of Science:

• Organic Chemistry
• Materials Science
• Spectroscopy

Background:

• Mid-infrared (mid-IR) responsive dyes are crucial for various applications, but existing highly π-extended chromophores are synthetically challenging and difficult to handle.
• Developing smaller, practical chromophores that absorb in the near- and mid-IR region is a significant challenge in materials science.

Purpose of the Study:

• To introduce a novel molecular design strategy for creating small, stable chromophores with optical band gaps in the near- and mid-IR region.
• To demonstrate the feasibility of using small organic molecules for mid-IR applications, overcoming limitations of larger systems.

Main Methods:

• A phenanthrene-based Blatter-type diradical was synthesized and oxidized to form a stable radical cation.
• The optical properties of the radical cation were investigated using UV-Vis spectroscopy, revealing a π-π* electronic transition extending into the mid-IR region (ca. 2000 cm^-1 or 5000 nm).
• Electron Paramagnetic Resonance (EPR) spectroscopy was employed to confirm the delocalized charge and spin characteristics of the radical cation.

Main Results:

• A novel radical cation, weighing less than 600 Da, was generated exhibiting mid-IR absorption (ca. 2000 cm^-1).
• The resulting mid-IR responsive chromophore demonstrated excellent stability in solution under ambient conditions for one week.
• EPR spectroscopy confirmed the delocalized nature of the SOMO and SUMO orbitals, supporting the assignment of the mid-IR band to a π-π* transition.

Conclusions:

• The