Enhanced Coherence by Coupling Spins through a Delocalized π-System: Vanadyl Porphyrin Dimers

Abstract

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.

A Heterometallic Porphyrin Dimer as a Potential Quantum Gate: Magneto‐Structural Correlations and Spin Coherence Properties

“…T m values of 8 are instead slightly faster than those of 6 and 7 and also lower than those reported for dimeric V IV O bimetallic complexes, including the homometallic vanadyl porphyrin dimer. [11,22,49,50] The accurate determination of the Spin-Hamiltonian parameters of 8 allows us to envision the microwave operation of the dimeric unit as a C-NOT quantum gate. This is fundamental for realizing a universal quantum computer.…”

“…In addition, surface-induced reactivity has been exploited to produce flat triply-linked porphyrin nano tapes. [16][17][18] Studies on the exchange-coupling in porphyrin metal dimers evidenced moderate antiferromagnetic interactions promoted by the conjugation in doubly-and triply-linked porphyrins, [19][20][21][22][23] while meso-meso singly-linked homometallic Cu II dimers exhibit negligible exchange interactions. [19] We recently demonstrated that the vanadyl analog shows a measurable exchange interaction of the order of 10 À 2 cm À 1 .…”

“…In addition, the good coherence properties of the monomeric units are retained in the dimer, a finding that has been recently generalized to vanadyl dimers coupled through a delocalized π-system. [22] The increased exchange coupling in the vanadyl complex was justified by the fact that the vanadyl magnetic orbital lies slightly above the porphyrin plane. The saddlelike distortion of the porphyrin ring, [24] when coordinated to the V IV O unit, further induces the mixing of the in-plane porphyrin orbitals with the out-of-plane ones.…”

“…Similar to the singly-linked case, the sign of the coupling is consistent with Kahn's model. Its magnitude is intermediate between triply-fused Cu [19] and VO homometallic dimers, [22] the former being the largest.…”