Quasi-Heavy Atom Effect for Room-Temperature Phosphorescence

Abstract

Organic room temperature phosphorescence (RTP) materials are vital for applications from bioimaging to anti-counterfeiting, offering advantages over inorganic materials. A key challenge is enhancing intersystem crossing (ISC), spin-orbit coupling (SOC), and stabilizing triplet excitons, which are prone to environmental quenching. Although molecular and material design strategies have been explored, the RTP mechanism in small molecules doped into polymer matrices, particularly poly(vinyl alcohol) (PVA), remains incompletely elucidated. Conventional explanations attribute PVA dopeds RTP to its oxygen barrier and rigid hydrogen-bonded network. However, our research shows these alone are insufficient. We demonstrate that typically non-phosphorescent organic small molecules like biphenyl and fluorene exhibit ultralong blue phosphorescence (λem = 455 nm, quantum efficiency: 8.72%, lifetime: 4.20 s) only within a PVA matrix. This suggests an unrecognized intrinsic PVA property promoting efficient ISC. We propose a novel "Quasi-Heavy Atom Effect," where PVA's unique characteristics facilitate SOC similar to heavy atoms, but without their toxicity or cost. This understanding is critical for designing novel RTP materials.