Ultra-Long Room Temperature Phosphorescence with the Efficiency Over 64% Induced by 1parts per thousand Impurity Doping

Ultra-long room temperature phosphorescence (ULRTP) materials show valuable applications in encryption, biological imaging, and many other fields. Amazingly, the concomitant impurities from raw materials that are normally ignored contribute dramatically to the ULRTP. In this study, CzPMB [9-(4-bromo-3-methylphenyl)-9H-carbazole] with phosphorescent quantum efficiency of 64% is prepared from commercial carbazole, but the phosphorescent efficiency is drastically reduced to < 2% once trace impurity (5-(4-bromo-3-methylphenyl)-5H-benzo[b]carbazole) is separated. HPLC studies demonstrated the separated impurity is a byproduct derived from trace benzocarbazole in commercial carbazole. Subsequently, the ULRTP for the CzPMB synthesized from lab-made carbazole is totally unobserved, strongly confirming the dramatic impact of impurity. A defect trapping mechanism in multicomponent system rather than heavy atom effect is proposed for highly efficient ULRTP after carefully analyzing the crystal packings and molecular energy levels. Inspired by this discovery, a series of effective ULRTP bi-component systems with the highest phosphorescence efficiency of 64.1% are reproduced by directed host-guest doping. This strategy paves a viable path for the design of organic materials with highly efficient ULRTP.