Synthesis, Photophysical and Redox Properties of Highly Stable Radicals of Nickel(II) Bis(2-aminophenyl)-azadipyrromethene

The development of stable organic dye radicals that exhibit intense and redox-switchable absorption in the short-wavelength infrared (SWIR) region is highly desirable but remains a significant challenge. Herein, we present two neutral nickel(II) radical complexes of tetradentate-type azadipyrromethene (ADP), which demonstrate exceptional stability under aerobic conditions and exhibit high photostability. The radical nature of the stable, newly synthesized <b>[Ni</b>^<b>II</b><b>(ADP</b>^.<b>)]</b> complexes was verified through electron paramagnetic resonance spectroscopy, which displayed a predominant organic radical character with a resonance at <i>g</i> = 2.003. These stable radicals <b>[Ni</b>^<b>II</b><b>(ADP</b>^.<b>)]</b> exhibit intense SWIR absorptions with a maximum wavelength (λ_max) around 1260 nm. These radicals can be readily oxidized to the corresponding stable cations or reduced to less stable anions, all of which maintain significant SWIR absorption characteristics. The electronic configuration, redox characteristics, and absorption spectra of the newly developed systems were systematically investigated and supported by theoretical calculations, indicating that the spin density is delocalized across the tetradentate ADP core and the nickel atom. Furthermore, the stable <b>[Ni</b>^<b>II</b><b>(ADP</b>^.<b>)]</b> radical exhibited a high photothermal conversion efficiency of 60.1% under the irradiation of 1064 nm laser, highlighting its potential for applications in SWIR photothermal therapy.