biostudies-literatureWu FEuropean Research Councile202202170https://www.ebi.ac.uk/biostudies/studies/S-EPMC9324097biostudies-literatureUnknown61(23)Open-shell non-alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene-based diradicaloids (1-3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (E_g ^opt =0.52-0.69 eV) and persistent stability under ambient conditions (t_1/2 =11.7-33.3 h). More importantly, this new type of diradicaloids possess a low-lying triplet state with an extremely small singlet-triplet energy gap, as low as 0.002 kcal mol^-1 , with a clear dependence on the molecAngewandte Chemie (International ed. in English)Benzo-Extended Cyclohepta[def]fluorene Derivatives with Very Low-Lying Triplet States.PMC9324097819698Bogani LLiu JFeng XFu YLiu RPopov AALombardi FWu FKomber HDmitrieva EHuang ZLohr TGAlexandropoulos DIMa JLiu FIsrael NfalseBenzo-Extended Cyclohepta[def]fluorene Derivatives with Very Low-Lying Triplet States.Open-shell non-alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene-based diradicaloids (1-3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (E_g ^opt =0.52-0.69 eV) and persistent stability under ambient conditions (t_1/2 =11.7-33.3 h). More importantly, this new type of diradicaloids possess a low-lying triplet state with an extremely small singlet-triplet energy gap, as low as 0.002 kcal mol^-1 , with a clear dependence on the molec2022-01-01T00:00:00Z2022 Jun2025-04-27T01:08:44.693Z2025-04-06T18:08:59.81ZS-EPMC93240973529069910.1002/anie.202202170