Designing effective single-molecule electromagnets with radially π-conjugated carbon structures

When charge flows through a molecular circuit, it induces a magnetic field that allows the circuit to behave as a nanoscale electromagnet. However, in single-molecule circuits this magnetic field is usually weak. Here we show that radially π-conjugated carbon structures can support amplified circulating currents that generate local magnetic fields. Within tight-binding and density functional theory (DFT) frameworks, we first study cycloparaphenylene (CPP) junctions where both electrodes are attached to the same phenylene unit on the nanohoop. We observe an energy-dependent ring current component that traverses the whole macrocycle by mapping the local current density. Importantly, we find that destructive interference near degenerate resonances can reverse the ring current direction and amplify it strongly relative to the source–drain current. We show that this interference-driven design principle is general, and also carries over to C60 junctions. In fullerene, lower-lying degenerate resonances are more easily accessible through electrostatic gating, reaching a magnetic field of 14.2 mT under a 100 mV source–drain bias. This work thus provides new insights into ring currents in radially π-conjugated carbon structures and highlights their potential as design platforms for single-molecule electromagnets.