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

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO-LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.