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

Area of Science:

• Materials Science
• Organic Chemistry
• Photophysics

Background:

• Commercial phosphines and phosphoniums typically exhibit unstable triplet dissipation due to flexible C-P geometry, leading to weak or absent phosphorescence.
• Restricting molecular motion and modifying electronic structures are key to enhancing triplet populations and stability.

Purpose of the Study:

• To investigate the effectiveness of a dual-ring-locking strategy in rigidifying phosphonium structures.
• To improve intersystem crossing (ISC) and triplet radiation for intense, persistent room temperature phosphorescence (RTP).

Main Methods:

• Synthesis of dual-ring-locked benzo[b]phospholium configurations.
• Characterization of photophysical properties, including fluorescence and RTP, in solution and polymer matrices (PVA).
• Computational analysis using time-dependent density functional theory (TD-DFT) to understand electronic structure and spin-flipping mechanisms.

Main Results:

• The dual-ring-locked compound [P1] exhibited intense blue RTP (453 nm) with high phosphorescence quantum yield (Φ_phos ≈ 12.4%) and long lifetime (τ_phos > 1200 ms) in PVA.
• A single ring-locked analogue [P2] showed weaker phosphorescence (Φ_phos < 1.8%, τ_phos = 74.2 ms) and red-shifted emission.
• TD-DFT revealed that improved spin-flipping in [P1] is attributed to integrated π-π*/n-π* transitions, rational energy splitting, and rigid excited states.

Conclusions:

• The dual-ring-locking strategy effectively enhances triplet stability and RTP in phosphonium compounds.
• The resulting materials demonstrate potential for applications in optical encryption due to their long-lasting afterglow.
• These phosphonium derivatives are promising candidates for emitting layers in light-emitting diode (LED) devices.