Per Hedegård

Nonbenzenoid graphene nanoribbons provide a versatile platform for tailoring electronic structures beyond conventional benzenoid π-conjugated systems, yet the impact of their topology on quantum transport remains largely unexplored. Here, we investigate electrical conductance through suspended ladder phenylene nanoribbons containing antiaromatic four-membered rings using scanning tunneling microscope lifting spectroscopy. As the nanoribbon is progressively detached from the substrate, differential conductance measurements reveal successive charge-state transitions accompanied by alternating Kondo resonances, demonstrating the dynamic generation of localized spins in an intrinsically closed-shell nanoribbon. Density functional theory calculations show that the antiaromatic topology reduces the HOMO–LUMO gap and induces a pronounced length dependence of the occupied frontier orbitals, which shift progressively toward and eventually across the electrode Fermi level with increasing suspended length, driving successive charge-state transitions. The transport behavior is quantitatively described within a double-barrier tunneling framework incorporating charge quantization and many-body interactions. These results suggest an antiaromaticity-driven mechanism for charge-state evolution and propose antiaromaticity as a molecular design principle for controlling charge and spin states in nonbenzenoid carbon nanostructures.

For more than two millennia, Archimedean screws have been used for water pumping and, more recently, as turbines in hydroelectric power stations[1]. Their helical-blade structure, essential to their function, is also found in chiral molecules and nanostructures, where injected electric current has been proposed to generate torque-mediated rotation[2,3]. Here, we present a torque-driven molecular machine with a rotor actuated by electric current and conductance enhanced by its rotation. Suspending a helicene molecule between two electrode tips, we show that an applied voltage induces cyclic molecular motion with 100% directional preference that can be inverted by the voltage’s polarity. Above a voltage threshold, the current through the rotating molecule is significantly enhanced, and the enhancement onset is tuned by stretching or compressing the helical molecule. Such effects were absent in previous helical molecular junctions, likely due to structural and operational differences[4–6]. Calculations show that molecular rotation by current-induced forces explains both the observed cyclic motion and current enhancement. This mechanism predicts a transition from stepwise to continuous rotation at a threshold voltage, yielding a current increase as rotation allows adopting highly conducting configurations not accessible at rest. Our work introduces a prototype for a new class of torque-driven molecular machines with perfect rotor unidirectionality and significant current modulation.

Spin coupling between magnetic metal atoms and organic radicals plays a pivotal role in high-performance magnetic materials. The complex interaction involving multispin centers in bulk materials makes it challenging to study spin coupling at the atomic scale. Here, we investigate the d-π-d spin interaction in well-defined metal-organic coordinated structures composed of two iron (Fe) atoms and four all-trans retinoic acid (ReA) molecules, using low-temperature scanning tunneling microscopy and atomic force microscopy. The ReA molecule is turned into a spin-1/2 radical state by dehydrogenation, facilitating strong magnetic coupling with the coordinated Fe atoms. Comprehensive theoretical analysis, based on density functional theory and valence bond theory, further elucidates the intrinsic mechanism of ferrimagnetic spin coupling in the coordination structure. Specifically, simultaneous antiferromagnetic coupling of an Fe dimer to ReA radicals parallelizes the dimer spin orientation. This work contributes to the fundamental understanding of the spin interaction in metal-organic coordination structures and provides microscopic insights for designing advanced magnetic materials.

Spin coupling between magnetic metal atoms and organic radicals plays a pivotal role in high-performance magnetic materials. The complex interaction involving multi-spin centers in bulk materials makes it challenging to study spin coupling at the atomic scale. Here, we investigate the d-π-d spin interaction in well-defined metal-organic coordinated structures composed of two iron (Fe) atoms and four all-trans retinoic acid (ReA) molecules, using low-temperature scanning tunneling microscopy and atomic force microscopy. The ReA molecule is turned into a spin-1/2 radical state by dehydrogenation, facilitating strong magnetic coupling with the coordinated Fe atoms. Comprehensive theoretical analysis, based on density functional theory and valence bond theory, further elucidates the intrinsic mechanism of ferrimagnetic spin coupling in the coordination structure. Specifically, simultaneous antiferromagnetic coupling of Fe dimer to ReA radicals parallelizes the dimer spin orientation. This work contributes to the fundamental understanding of spin interaction in metal-organic coordination structures and provides microscopic insights for designing advanced magnetic materials.

Proteases are essential enzymes for a plethora of biological processes and biotechnological applications, e.g., within the dairy, pharmaceutical, and detergent industries. Decoding the molecular level mechanisms that drive protease performance is key to designing improved biosolutions. However, direct dynamic assessment of the fundamental partial reactions of substrate binding and activity has proven a challenge with conventional ensemble approaches. We developed a single-molecule (SM) assay for the direct and parallel recording of the stochastic binding interaction of Savinase, a serine-type protease broadly employed in biotechnology, with casein synchronously with monitoring proteolytic degradation of the substrate. SM recordings enabled us to determine how the overall activity of Savinase and two mutants relies on binding efficiency, enzymatic turnover and activity per binding event. Analysis of residence times revealed three characteristic binding states. Mutations were found to dominantly alter the likelihood of sampling the long lived state, with lifetimes longer than 30 seconds, indicating this state contributes to overall activity and supporting a level of processivity for Savinase. This observation challenges conventional expectations, as the protease has no characterized substrate binding site, or binding domain, aside from the active site. These insights, inaccessible through conventional assays, offer new perspectives for engineering proteases with improved hydrolytic performance.

By now, it is well known that chiral molecules can affect the spin of electrons passing through. In addition, the magnetization of, e.g., nanomagnets covered by chiral molecules can be affected by the presence of molecules. We are studying the mechanisms that explain various observations involving combinations of magnets and chiral molecules. We find that there exists a molecule induced contribution to the magnetic anisotropy of the magnets. Out of equilibrium, when electrons are actually being transported through a nano-magnet covered with chiral molecules, a molecule induced torque acting on the magnetization is emerging. It is of the spin-transfer-torque kind, already discussed in other parts of spintronics. This current induced torque can help explain the observed breaking of the Onsager reciprocity principle in experiments involving magnets and chiral molecules.