Atomically Precise Synthesis and Characterization of Heptauthrene with Triplet Ground State

Abstract

By virtue of multitunable spin structures upon designing the π-electron topologies, phenalenyl-based nanographenes are of substantial interest in fundamental science and for potential applications in spintronics. Heptauthrene, as one of the well-known phenalenyl diradicals, is composed of one benzene-fused bisphenalenyls in mirror symmetry and expected to have a triplet ground state. However, the synthesis of unsubstituted heptauthrene remains very challenging due to the high reactivity of triplet diradicals. Here, we report a combined in-solution and on-surface synthesis of unsubstituted heptauthrene, whose chemical structure is characterized through bond-resolved atomic force microscopy. Combined with mean-field Hubbard model calculations, its triplet ground state is unambiguously confirmed by the underscreened Kondo resonance in response to the magnetic field, as well as the engineered spin-state switching upon extra hydrogen atom addition and dissociation on the radical site. Our results provide access to phenalenyl-based nanographenes with high-spin ground state, potentially useful in constructing high-spin networks.

Renormalization of spin excitations and Kondo effect in open-shell nanographenes

“…Our results show a Kondo peak. We note that Kondo peak measurements have been reported in at least two spin-1 NGs on metallic substrates [19,21], but not on triangulenes. However, so far STM spectra of single triangulene molecules have only been measured for nonmetallic Xe(111) as substrate, and no Kondo effect was found in this case.…”

“…This situation has changed dramatically with the advent of on-surface synthesis techniques [9][10][11] combined with surface scanning probe techniques, such as atomic force and scanning tunneling microscopes (AFM and STM). These techniques make it possible now to synthesize an increasing number of open-shell NGs, such as ribbons with zigzag edges [12], the Clar's goblet [13], rhombenes [14], triangulenes [15][16][17][18], heptauthrene [19], and others [20][21][22][23][24][25], and to probe their electronic properties with atomic scale resolution.…”

“…In the case of a (spin) degenerate ground state (GS), the coupling to the substrate gives rise to the Kondo effect, screening the magnetic moment of the adsorbate [42]. This certainly occurs in NGs whose GS has S = 1/2, but it has also been observed in NGs predicted to have S = 1 [19,21]. Note that the NG hosts the localized electrons, while the conduction electrons are in the substrate.…”

Renormalization of spin excitations and Kondo effect in open shell nanographenes

“…This situation has changed dramatically with the advent of on-surface synthesis techniques [9][10][11] combined with surface scanning probes techniques, such as atomic force and scanning tunneling microscopes (AFM and STM). These techniques make it now possible to synthetize an increasing number of open-shell NGs, such as ribbons with zigzag edges [12], the Clar's goblet [13], rhombenes [14], triangulenes [15][16][17], heptauthrene [18] and others [19][20][21][22][23], and to probe their electronic properties with atomic scale resolution.…”

“…In the case of a (spin) degenerate ground state (GS), the coupling to the substrate gives rise to the Kondo effect, screening the magnetic moment of the adsorbate [40]. This certainly occurs in NGs whose GS has S = 1/2, but it has also been observed in NGs predicted to have S = 1 [18,20]. Note that the NG hosts the localized electrons, while the conduction electrons are in the substrate.…”