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

Herein,we report the crystallization-induced persistent room-temperaturephosphorescence (CIpRTP) characteristics of simple diarylphosphineoxides 1, 2, and 3. Under ambientconditions, crystalline solids of 1, 2,and 3 show photoluminescence (PL) lifetime of more than100 ms. At 77 K, they exhibit a PL lifetime of nearly a second. Replacingone of the aryl moieties in Ar3P=O with hydrogencreates more space for intermolecular interactions in Ar2P=O (H); consequently, the latter shows better PL quantumyield (PLQY) than the former. The PLQYs of these compounds are tunedsystematically by varying the steric perturbation around the phosphoruscenter. Detailed analysis of the crystal structure of these compoundsrevealed that the intermolecular interactions play a crucial rolein stabilizing the triplet state in the solid state and bestowingthem with CIpRTP. Thin films of these compounds do not show pRTP,further affirming that the intermolecular interactions indeed playa crucial role in stabilizing the triplet state. In the solution state,these compounds show ultraviolet fluorescence (lambda(em) approximate to 305 nm) with a small Stokes shift of similar to 3200 cm(-1) and narrow bandwidth (FWHM: 37, 35, and 33 nm for 1, 2, and 3, respectively). Thecalculated SOC (< T-1 vertical bar(H) over cap vertical bar S-0 >)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)) -> sigma*((P-C)) transition plays a crucial role in controlling the optical featuresin these compounds.