Dibenzononazethrene Isomers: Stable Singlet Diradicaloids with Efficient Photothermal Conversion

Organic singlet diradicaloids are promising photothermal agents owing to their exceptional light-harvesting capabilities and efficient light-to-heat conversion. In this study, two stable dibenzononazethrene isomers, DBNZ1 and DBNZ2, are synthesized through a concise method and isolated in crystalline form. An open-shell singlet diradical ground state, with diradical indices of 0.67 and 0.69, is confirmed through a combination of theoretical and experimental approaches. Steady-state and time-resolved absorption spectroscopy demonstrated efficient light absorption in the far-red region and excellent light-to-heat conversion, attributed to a rapid nonradiative process. Encapsulation of DBNZ1 with an amphiphilic polymer produced water-dispersible nanoparticles (DBNZ1-NPs) with enhanced stability, achieving an impressive photothermal conversion efficiency of 72.9%. An <i>in vitro</i> photothermal therapy study demonstrated that DBNZ1-NPs functioned as biocompatible tumor ablation agents when activated by an 808 nm laser, highlighting their potential application in cancer photothermal therapy.