Dual-Ring-Locking Strategy Enables Persistent Blue Room Temperature Phosphorescence in Benzo[b]phospholiums

Commercial phosphines and phosphoniums were commonly reported to have unstable triplet dissipation because of the flexible C–P pyramidal geometry, resulting in extremely weak or no phosphorescence. To boost triplet populations and stability by restricting the molecular motion and rebuilding the electronic structures, we reported that the dual-ring-locking strategy could enable elevated intersystem crossing (ISC) and triplet radiation for the rigid benzo[b]phospholium configuration, exhibiting intense persistent room temperature phosphorescence (RTP) in poly(vinyl alcohol) (PVA). Among them, dual-ring-locked [P1]+[Cl]− showed near-ultraviolet fluorescence maximized at 400 nm in dichloromethane and blue RTP emission at 453 nm (Φphos ≈ 12.4%, τphos > 1200 ms) in the PVA matrix. In contrast, [P2]+[Cl]− possessed a single ring-locked nucleus that had red-shifted emission and weak phosphorescence (Φphos < 1.8%, τphos = 74.2 ms). Time-dependent density functional theory (TD-DFT) disclosed that the improved spin-flipping of phosphoniums benefited from the integrated π–π*/n−π* transition, rational split energy, and rigid excited states. The impressive OU-RTP duration could function as an afterglow pattern for optical encryption or as an emitting layer for light-emitting diode (LED) applications.