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

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 = 12) with a Cu–Cu distance of ca. 18Å(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.