Robustly coherent spin centers that can be integrated into devices are a key ingredient of quantum technologies. Vacancies in semiconductors are excellent candidates, and theory predicts that defects in conjugated carbon materials should also display long coherence times. However, the quantum performance of carbon nanostructures has remained stunted by an inability to alter the sp2-carbon lattice with atomic precision. Here, we demonstrate that topological tailoring leads to superior quantum performance in molecular graphene nanostructures. We unravel the decoherence mechanisms, quantify nuclear and environmental effects, and observe spin-coherence times that outclass most nanomaterials. These results validate long-standing assumptions on the coherent behavior of topological defects in graphene and open up the possibility of introducing controlled quantum-coherent centers in the upcoming generation of carbon-based optoelectronic, electronic, and bioactive systems.

Open‐shell non‐alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene‐based diradicaloids (1–3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (Egopt=0.52–0.69 eV) and persistent stability under ambient conditions (t1/2=11.7–33.3 h). More importantly, this new type of diradicaloids possess a low‐lying triplet state with an extremely small singlet–triplet energy gap, as low as 0.002 kcal mol−1, with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non‐alternant open‐shell PHs with high‐spin ground states, and opens up novel possibilities and insights into understanding the structure–property relationships.

Vanadium(IV) magnetic centers are prime candidates as molecular quantum information units. One of the longstanding problems is to obtain an extendable scaffold that transmits the magnetic interaction to a degree usable for quantum processing, and allows upscaling to multiple centers, while preserving a sufficiently long coherence time. Here, we show that fused porphyrins allow tailored scaffolding of vanadyl quantum units, with an almost flat conjugated π-system that offers substantial advantages for communication between vanadyl ions, leading to the long spin-lattice (T1 = 30 ms) and coherence (Tm = 5.5 µs) times. The antiferromagnetic exchange coupling in these vanadyl dimers (J = 1 GHz) is stronger than the hyperfine interaction, resulting in complex EPR spectra in which both unpaired electrons couple equally to both I = 7/2 51V nuclei. Isolation of the syn- and anti-isomers, with vanadyls on the same or opposite sides of the con-jugated channel, showcases the sensitivity of quantum units to different configurational environments, and offers a way to tune the inter-action in poly-porphyrin systems by controlling the stereochemistry.

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We investigate the mechanisms of nuclear decoupling in synthetically-tailored graphenenoids, where the electron spin state is introduced by topological manipulation of the lattice. We compare molecular graphenoids containing one and two spin centres, introduced by pentagonal rings in the honeycomb lattice. Exploiting the molecular nature of the systems, we investigate the role of different nuclear species and environments. Variations on the Carr-Purcell-Meiboom-Gill pulse trains are used to prolong the coherence time of the electron spin of the radicaloids, leading to substantial improvements in performance and coherence times up to 300 µs at liquid nitrogen temperature. The investigation of electron spin coherence as a function of inter-pulse spacing, with times close to the inverse of the nuclear precession frequency, reveals that a train of pulses in-phase with the nuclear precession maximises the nuclear decoupling. At room temperature the limits imposed by the sample treatment and environment are reached, indicating what amelioration is necessary to further enhance the quantum performance.