Spin-Correlated Luminescence Enabled by Bright-Dark Radical Pairing in a Diradical System.

Spin-optical modulation lies at the core of emerging technologies in spintronics, spin-based optoelectronics, and quantum materials. Open-shell luminescent diradicals, featuring two unpaired and synthetically tunable spins, offer a molecular platform to achieve such control. However, previous studies have been restricted to symmetric systems, where spin interactions occur between two identical, luminescent radicals. Here, we demonstrate that a non-luminescent (dark) radical can effectively modulate the spin and photophysical behavior of a luminescent (bright) radical within an asymmetric diradical framework. The resulting molecule exhibits a unique three-stage magnetoluminescence (ML) response at low temperatures, arising from hyperfine coupling (HFC) (B < 0.05 T), the Δg-induced spin mixing (0.05-0.8 T), and spin polarization (0.8-7 T). Notably, the system exhibits pronounced ML enhancement (>14%) under ultra-low magnetic fields (B < 0.05 T), a previously unreported phenomenon in molecular spin-optical systems. These findings establish asymmetric bright-dark diradicals as a powerful new motif for spin-photon interface design, providing fresh insights into the fundamental photophysics of open-shell systems.