Benzothiadiazole-containing heptapyrrin: modulation of diradical character and photothermal conversion via metal coordination

Conjugated polypyrroles have attracted intensive attention due to their unique properties, and they have emerged as potential platforms for stabilizing radicals. With the purpose to construct stable polypyrrole radicals, a benzothiadiazole unit has been incorporated into a heptapyrrin framework to afford 1. Thus, 1 exists as a singlet ground state diradical with a narrow singlet-triplet energy gap (ΔE_S-T) of −3.17 kcal mol⁻¹. On this basis, 1 has been coordinated with Pd(II) and Ni(II) salts to afford binuclear complexes 1-Pd and 1-Ni. Notably, 1-Ni retains the diradical character with a narrowed ΔE_S-T of −0.65 kcal mol⁻¹, whereas 1-Pd is a closed-shell species. As a result, 1, 1-Pd, and 1-Ni exhibit intense near-infrared-II (NIR-II) absorption bands at 1076, 1283, and 1309 nm, respectively, enabling them to function as NIR-II photothermal conversion materials. Thus, 1 and diradical species 1-Ni show relatively high photothermal conversion efficiencies (η) of 37.6% and 38.6%, respectively. Whereas, closed-shell derivatives of 1 such as the oxidized (1-ox), reduced (1-re) and coordination (1-Pd) species exhibit lowered efficiencies of 24.8%, 26.5%, and 30.9%, respectively. These results provide insight into developing stable open-shell oligopyrroles as efficient photothermal materials, with the efficiencies further enhanced by coordination with transition metal ions while retaining the open-shell character.