# Rigidifying Qubit Candidates in a Cu‐Porphyrin Nanohoop: Dipolar Coupling in Spin Pairs and Spin‐Polarized Ground State

## TL;DR

Researchers created a rigid nanohoop with copper centers that could be used as molecular qubits, showing unique spin interactions and light-induced polarization.

## Contribution

The study demonstrates a novel use of rigid nanohoops to achieve well-defined spin coupling and light-induced polarization in molecular qubit systems.

## Key findings

- The Cu[3]CPTA nanohoop exhibits well-defined dipolar coupling between spin centers.
- A rare light-induced polarization of a doublet ground state was observed in frozen solution.
- Nanohoop rigidity is shown to significantly influence spin interactions compared to more flexible structures.

## Abstract

Assembling molecular qubit candidates with precise control over the position and orientation of spin centers is an important contemporary challenge for synthesis. In this work, we show that the rigidity of highly strained macrocycles from the cycloparaphenylene family gives rise to distinct spin‐spin and light‐spin interactions that make such few‐qubit systems a promising testing ground for future quantum technologies. We synthesized conjugated nanohoop Cu[3]CPTA that comprises three Cu(II)porphyrin centers (S = ½) with a Cu–Cu distance of ca. 18 Å (by single‐crystal X‐ray diffraction). Continuous‐wave (cw) and pulse electron‐paramagnetic resonance (EPR) studies revealed that dipolar coupling in spin pairs is so well defined in this nanohoop that the Cu–Cu distance can be determined accurately via double electron–electron resonance (DEER). By transient cwEPR, we observed a rare case of a light‐induced polarization of a doublet ground state in frozen solution. The fact that ground state polarization was significantly less pronounced in a comparable, but more flexible macrocycle will inspire future efforts to better understand and harness this effect.

A conjugated nanohoop, Cu[3]CPTA, containing three Cu(II) porphyrin spin centers (S = ½) was synthesized. We show that the exceptionally high “nanohoop rigidity” results in well‐defined dipolar coupling (DEER) and a rare light‐induced spin polarization of the doublet ground state in frozen solution (transient cwEPR).