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 spin–orbit coupling (SOC), intersystem crossing (ISC), 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 the RTP of doped PVA to its oxygen barrier and rigid hydrogen-bonded network. However, our research shows that these alone are insufficient. We demonstrate that typically non-phosphorescent organic small molecules like biphenyl and fluorene exhibit ultralong blue phosphorescence (λ_em = 455 nm, quantum efficiency: 8.72%, lifetime: 4.20 s) only within a PVA matrix. This suggests an unrecognized intrinsic PVA property promoting efficient SOC and 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.