Unraveling Ultralong Phosphorescence in Ar₂PO(H): n_(O) → σ*_(P-C) Transitions in Ar₂PO(H) Stabilize Triplet States Better than n_(P) → σ*_(P-C) in Ar₃P

Herein, we report the crystallization-induced persistent room-temperature phosphorescence (CIpRTP) characteristics of simple diarylphosphine oxides 1, 2, and 3. Under ambient conditions, crystalline solids of 1, 2, and 3 show photoluminescence (PL) lifetime of more than 100 ms. At 77 K, they exhibit a PL lifetime of nearly a second. Replacing one of the aryl moieties in Ar3P═O with hydrogen creates more space for intermolecular interactions in Ar2P═O (H); consequently, the latter shows better PL quantum yield (PLQY) than the former. The PLQYs of these compounds are tuned systematically by varying the steric perturbation around the phosphorus center. Detailed analysis of the crystal structure of these compounds revealed that the intermolecular interactions play a crucial role in stabilizing the triplet state in the solid state and bestowing them with CIpRTP. Thin films of these compounds do not show pRTP, further affirming that the intermolecular interactions indeed play a crucial role in stabilizing the triplet state. In the solution state, these compounds show ultraviolet fluorescence (λem ≈ 305 nm) with a small Stokes shift of ∼3200 cm–1 and narrow bandwidth (FWHM: 37, 35, and 33 nm for 1, 2, and 3, respectively). The calculated SOC (⟨T1|Ĥ|S0⟩) matrix element for 1, 2, and 3 is significantly lower than the value estimated for triarylphosphines; thus, the former show ultralong phosphorescence compared to the latter. Furthermore, the n(O) → σ*(P-C) transition plays a crucial role in controlling the optical features in these compounds.