Dual-Ring-Locking Strategy Enables Persistent Blue Room Temperature Phosphorescence in Benzo[<i>b</i>]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[<i>b</i>]phospholium configuration, exhibiting intense persistent room temperature phosphorescence (RTP) in poly(vinyl alcohol) (PVA). Among them, dual-ring-locked <b>[P1]⁺[Cl]^-</b> showed near-ultraviolet fluorescence maximized at 400 nm in dichloromethane and blue RTP emission at 453 nm (Φ_phos ≈ 12.4%, τ_phos > 1200 ms) in the PVA matrix. In contrast, <b>[P2]⁺[Cl]^-</b> possessed a single ring-locked nucleus that had red-shifted emission and weak phosphorescence (Φ_phos < 1.8%, τ_phos = 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.