Bistability between π-diradical open-shell and closed-shell states in indeno[1,2-a]fluorene

Abstract

Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received great interest owing to their unusual electronic structure and potential applications in nonlinear optics and photovoltaics. Here we report the generation of unsubstituted indeno[1,2-a]fluorene on various surfaces by the cleavage of two C–H bonds in 7,12-dihydroindeno[1,2-a]fluorene through voltage pulses applied by the tip of a combined scanning tunnelling microscope and atomic force microscope. On bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state, but it exhibits charge bistability between neutral and anionic states on the lower-workfunction surfaces of bilayer NaCl on Ag(111) and Cu(111). In the neutral state, indeno[1,2-a]fluorene exhibits one of two ground states: an open-shell π-diradical state, predicted to be a triplet by density functional and multireference many-body perturbation theory calculations, or a closed-shell state with a para-quinodimethane moiety in the as-indacene core. We observe switching between open- and closed-shell states of a single molecule by changing its adsorption site on NaCl.

Observation of SOMO-HOMO Inversion in a Neutral Polycyclic Conjugated Hydrocarbon

“…We generated a nonbenzenoid polycyclic conjugated hydro- 47 or molecular conformation, 48 through proximity to adsorbates, 49 or by means of external fields. 50…”

“…Electronic characterization of 1 by STM-based orbital-density imaging reveals an open-shell biradical ground state, with manifestation of the unusual phenomenon of SHI, wherein the SOMOs are lower in energy than the HOMO, which is in contrast to the majority of reported open-shell molecules. The demonstrated ability to detect SHI at the single-molecule level facilitates future experiments and applications of inducing or switching (on and off) SHI by changes in the adsorption site 47 or molecular conformation, 48 through proximity to adsorbates, 49 or by means of external fields. 50 …”

Prediction of the ground state for indenofluorene-type systems with Clar's π-sextet model

“…[12][13][14][15] Among such CPHs conjugated indenofluorenes (IFs) possess one of the most interesting topologies of delocalized -electrons with different types of behavior (aromatic/antiaromatic) and ground state electronic structure (biradical/quinoidal). [16,17] They exhibit a 6-5-6-5-6 ring architecture, which is obtained by a fusion of the indene unit to various positions of the fluorene unit. Since 2010 all five possible IF regioisomers have been intensively studied by Haley's [18][19][20] and Tobe's [21,22] group (Structures 1-5 in, Figure 1).…”

Redox-Switchable Aromaticity in a Helically Extended Indeno[2,1-c]fluorene

“…25 Similarly, switching between the open-and closed-shell states of an indacene core has recently been observed by changing the adsorption site of an on-surface synthesized unsubstituted indeno[1,2-a]fluorene. 26 Alternatively, antiaromatic (IF) motifs can be generated by oxidation or reduction of more stable and readily accessible aromatic precursors, and is, in principle reversible. 27−34 However, this almost always proceeds via intermediate radical states that are typically unstable, potentially leading to undesired side reactions.…”

“…Modulation of various molecular properties, including, in principle, (anti)­aromaticity can be achieved using molecular switches driven by various stimuli. − For example, it was shown that photoswitching − of a biphenylene-diarylethene allows to induce significant and reversible changes in the antiaromatic character of the central biphenylene motif by modulation of the conjugation pattern . Similarly, switching between the open- and closed-shell states of an indacene core has recently been observed by changing the adsorption site of an on-surface synthesized unsubstituted indeno­[1,2- a ]­fluorene …”