Field-induced near-unity spin polarization in a highly emissive organic diradical

Spin polarization essentially drives a mixed spin population toward a well-defined state and represents a critical step in the construction of qubits. The degree of spin polarization directly determines the readout contrast and the signal-to-noise ratio of spin-based detection. Open-shell luminescent diradicals constitute a highly promising platform for optically addressable qubits, in which efficient spin polarization can be achieved via external magnetic fields, temperature control, and optical pumping. However, experimentally viable routes and representative examples for achieving near-unity spin polarization remain scarce. Here, we demonstrate that near-unity spin polarization can be realized in a luminescent diradical by applying a high magnetic field at low temperature. We first estimate the required temperature and magnetic-field strength through theoretical simulations, and then develop, via molecular engineering, a highly emissive luminescent diradical molecule. At 5.5 K and magnetic fields up to 46.6 T, we observe nearly saturated magnetoluminescence (MagLum) with a giant value of 112%, indicating that the spin population of a single ground-state triplet sublevel (T0,-) approaches unity. This work provides a representative case study toward realizing near-unity spin polarization in luminescent diradicals and offers a clear physical picture of MagLum under ultrahigh magnetic fields. Achieving high spin polarization is an important step toward controlling molecular spin systems for quantum applications. Here, the authors demonstrate near‑unity spin polarization in a luminescent organic diradical through molecular engineering and high‑field measurements.