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.

Vanadium(IV) magnetic centers are prime candidates as molecular quantum units. One longstanding question is how to obtain a scaffold that connects multiple centers and allows two communication modalities: magnetic and electronic. We have synthesized and studied a selection of vanadyl porphyrin dimers, as models of the most synthetically accessible linear porphyrin arrays. We show that a strongly π-conjugated backbone places the magnetic system in the strong coupling regime and protects the quantum coherence against electron pair flip-flop processes at low temperatures (<10 K). This result is a fundamental step towards the design of molecular materials for single-molecule devices controlled by microwaves with electrical readout.