Characterization of Benzo[a]naphtho[2,3‐f]pentalene: Interrelation between Open‐shell and Antiaromatic Characters Governed by Mode of the Quinoidal Subunit and Molecular Symmetry

Abstract The singlet open‐shell character and antiaromaticity are intriguing features in π‐conjugated carbocycles. These two exhibit similar chemical and physical properties. However, they rarely coexist in the same molecule. Understanding the interrelation between the open‐shell and antiaromatic characteristics in the same molecule is crucial to control the electronic properties. Herein we describe the synthesis and characterization of a new member of diareno[ a , f ]pentalene, benzo[ a ]naphtho[2,3‐ f ]pentalene 6 . Unlike its isomer 5 with a closed‐shell ground state, 6 exhibits an appreciable open‐shell character and a moderate antiaromatic feature. The behaviors of the open‐shell index ( y 0 ) against the difference of the proton chemical signal (Δ δ (H 1 )) between pentalenide dianions/neutral pentalenes for our reported pentalenes 1 , 4 , 5 , and 6 give a thought‐provoking conclusion about the interrelation between open‐shell and antiaromatic characteristics in this series. The mode of the incorporated quinoidal moiety and the formal molecular symmetry are critical to balance these two characteristics.