Topological Engineering of a Frustrated Antiferromagnetic Triradical in Aza-Triangulene Architectures

Abstract

Open-shell nanographenes provide a versatile platform to host unconventional magnetic states within their π-conjugated networks. Particularly appealing are graphene architectures that incorporate spatially separated radicals and tunable interactions, offering a scalable route toward spin-based quantum architectures. Triangulenes are ideal for this purpose, as their radical count scales with size, although strong hybridization prevents individual spin control. Here, we realize a radical reconfiguration strategy that transforms a single-radical aza-triangulene into a frustrated antiferromagnetic triradical by covalently extending it with armchair anthene moieties of increasing length. Scanning tunnelling spectroscopy reveals edge-localized Kondo resonances and a doublet-to-quartet spin excitation, evidencing the emergence of correlated spins. Multi-reference electronic-structure calculations trace the progressive increase in polyradical character with anthene length, driven by the clustering of frontier states within a narrow energy window. Consequently, the initial single-radical doublet reorganizes into a frustrated triradical with weakly coupled edge spins, a molecular analog of a three-qubit quantum register.

Figures (4)

• Figure 1: Construction of the extended aza-triangulene platforms through anthene addition. Progression of open-shell states in aza-[3]-triangulene architectures by incorporation of anthene units with increasing length. The red and blue shadows indicate radical regions. The resulting 7AGNR segments gain open-shell character as their length increases due to a topological transition, forcing the AT core to reorganize its electrons in zero-energy modes. The aromatic Clar sextets are colored in light blue. A schematic representation of active energy states and their electron/spin redistribution found in this work is added at the bottom.
• Figure 2: Synthesis route of extended AT platforms. a) Chemical reactions performed in solution to yield the targeted precursors 1 (top) and 2 (bottom). b) Reactions on-surface of cyclodehydrogenation (CDH) and dehydrogenation (DH) leading to the desired planar nanostructures. c) nc-AFM and d) STM images of extended aza platforms AAT (top) and BAT (bottom) performed with a CO-functionalized tip at constant height with $V=0\,\mathrm{mV}$ and $2\,\mathrm{mV}$, respectively (scalebars $4\,\text{\angstrom}$). The additional bright features in the bay regions of the molecules visible in the nc-AFM images are CO molecules adsorbed next to the nanographenes.
• Figure 3: Detection of $\pi$-magnetism. Low-energy $\mathop{}\!\mathrm{d} I/\mathop{}\!\mathrm{d} V$ spectra measured over a zigzag edge of a) AAT and b) BAT with a CO-functionalized tip at 5.4 K. Spectra over all three edges are qualitatively equivalent and featureless on the bare Au (see Fig. S5). Grey plots are the absolute value of the numerically calculated derivative of $\mathop{}\!\mathrm{d} I/\mathop{}\!\mathrm{d} V$; dashed lines in a) are Gaussian fits to the inelastic spin excitation signal. Steps at $\pm35\,\mathrm{mV}$ correspond to external vibrations of the CO molecule at the tip. c,d) High-resolution $\mathop{}\!\mathrm{d} I/\mathop{}\!\mathrm{d} V$ plots of the Kondo resonances at 1.3 K of AAT and BAT, respectively, with (purple) and without (blue) 2.8 T magnetic field applied. The red-dashed lines are Hurwitz-Fano fits JACOB_Temperature_2023TURCO_Demonstrating_2024 providing intrinsic Kondo linewidths $\Gamma_K=(0.61\pm0.03)\,\mathrm{mV}$ and $\Gamma_K=(0.79\pm0.03)\,\mathrm{mV}$, for the zero field case, which corresponds to $T_K=(2.49\pm0.05)\,\mathrm{K}$ and $T_K=(2.84\pm0.07)\,\mathrm{K}$ for AAT and BAT, respectively. e,f) Constant-height current Kondo maps of AAT and BAT, respectively (with a CO-functionalized tip, $V=2\,\mathrm{mV}$, scalebars $4\,\text{\angstrom}$, see also Fig. S7), compared with simulated $\mathop{}\!\mathrm{d} I/\mathop{}\!\mathrm{d} V$ maps from the molecular Kondo orbitals (see Methods and CALVO-FERNANDEZ_Theoretical_2024)
• Figure 4: Spin states and transition from mono to polyradical. Many-body states diagram obtained from multi-reference calculations for a) AAT and b) BAT. It shows the most important spin multiplets of the three lowest energy states. The triradial spin multiplets appear highlighted by dashed lines. c) Scheme of the energy reduction and occupancy increase of the NO3 as the size of the aza-nanographene increases, extracted from CASSCF(9,11). d) Energy difference between two degenerate doublet states and the excited quartet state as a function of the hopping parameter $t$, obtained from the Hubbard toy model depicted as an inset (see also Supplementary Fig. 16) for parameters $\varepsilon=0$, $\varepsilon_0=-3$ eV, and $U=2.5$ eV for each of the four orbitals.