A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin

Area of Science:

• Organic Chemistry
• Materials Science
• Photochemistry

Background:

• Activatable near-infrared (NIR) dyes are crucial for various applications, including medical diagnostics.
• Existing NIR dyes often lack stability or require complex activation mechanisms.
• Benzitetraazaporphyrins (BzTAPs) are a class of porphyrinoids with potential for tunable electronic properties.

Purpose of the Study:

• To design and synthesize novel benzitetraazaporphyrins (BzTAPs) with redox-switchable NIR properties.
• To investigate the relationship between electronic structure (18π vs. 20π electrons) and NIR absorption characteristics.
• To explore the potential of these BzTAPs as activatable NIR probes for reductive environments.

Main Methods:

• Synthesis of 18π- and 20π-electron BzTAPs.
• Characterization using X-ray diffraction, Nuclear Magnetic Resonance (NMR), and UV/Visible-NIR spectroscopy.
• Computational analysis including molecular orbital calculations.

Main Results:

• The 20π-electron BzTAP (1) exhibited strong NIR absorption and antiaromatic character with a paratropic ring-current.
• The 18π-electron BzTAP (2) displayed weak aromaticity and was NIR-silent.
• NIR-silent BzTAP 2 was efficiently converted to NIR-active BzTAP 1 using mild reducing agents like amines.
• Molecular orbital analysis indicated that a symmetry-lowering perturbation in the 20π-electron BzTAP skeleton allows for an electric-dipole-allowed HOMO-LUMO transition, responsible for intense NIR absorption.

Conclusions:

• Redox state significantly influences the NIR optical properties of BzTAPs.
• The synthesized BzTAPs offer a platform for developing activatable NIR probes.
• These probes are particularly suitable for applications in reductive biological environments.