Fig. 1: Non-benzenoid non-alternant polycyclic conjugated hydrocarbons.
From: Bistability between π-diradical open-shell and closed-shell states in indeno[1,2-a]fluorene
a, Classical non-benzenoid non-alternant PCHs: pentalene, azulene and heptalene. b, Generation of indacenes and indenoindenes through benzinterposition and benzannelation of pentalene, respectively. Grey filled rings represent Clar sextets. c, Closed-shell Kekulé (left) and open-shell non-Kekulé (right) resonance structures of QDMs. Note that meta-QDM is a non-Kekulé molecule. All indenofluorene isomers, being derived through benzannelation of indacenes, contain a central QDM moiety. d, Closed-shell Kekulé (top) and open-shell non-Kekulé (bottom) resonance structures of indenofluorenes. Compared to their closed-shell structures, 1 and 5 gain two Clar sextets in the open-shell structure, and 2–4 gain only one Clar sextet in the open-shell structure. The coloured bonds in d highlight the ortho- and para-QDM moieties in the two closed-shell Kekulé structures of 5. e, Scheme of the on-surface generation of 5 (C20H12) by voltage pulse-induced dehydrogenation of 6 (C20H14). Structures 7 and 8 represent the two monoradical species (C20H13).
