B/N-type organic diradicaloids with enhanced diradical character and near-infrared absorption for photothermal therapy

Organic near-infrared (NIR) absorbing materials may convert NIR light energy into thermal energy, and have been applied as photothermal agents for photothermal therapy. In this study, we disclose the rational construction of organic NIR diradicaloids via B/N-heterocycle fusion and enhancing quinoidal conjugation for photothermal therapy efficacy. Two novel B/N-doped diradicaloid molecules featuring the fusion of two B/N-heterocycle motifs and two para-quinodimethane moieties have been designed and synthesized. Their B/N-heterocycles and quinoidal structural features bestow increased open-shell characteristics and red-shifted NIR light absorption, along with excellent ambient stability. Notably, these two molecules can be assembled into the spherical nanoparticles, which show desirable photothermal conversion capability and stability. More importantly, in vivo experiments reveal that the NPs can completely eliminate tumor cells under laser irradiation at 880 nm, accompanied by the high biosafety. This work thus not only illustrates the rational design of organic diradicaloids with enhanced open-shell nature for NIR absorption π-systems, but also provides a new kind of diradicaloid photothermal materials for in vivo therapeutic applications.