Modulating room temperature phosphorescence through intermolecular halogen bonding

Purely organic room temperature phosphorescence (RTP) luminogens have drawn much attention owing to their potential application in anti-counterfeiting, biological imaging and various sensors, etc. In this work, a series of halogenated derivatives (MPh-R, R = F, Cl, Br, I) based on 4-phenylmorpholine are designed and synthesized. The effect of halogen atoms on the crystal packing modes and RTP performance is systematically investigated. The experimental results, theoretical calculations and ESP analysis indicate that halogen bonding is successfully formed in MPh-Cl, MPh-Br, and MPh-I, which significantly accelerates the intersystem crossing (ISC) and enhances the solid-state phosphorescence quantum yields (QYs) by inhibiting the vibration and rotation of molecules. For MPh-F, only single emission of fluorescence is detected due to the absence of the halogen bonding. The phosphorescent QY is increased from 0.6% for MPh to 6.8% for MPh-Cl. The relatively long afterglow feature of MPh, MPh-Cl, and MPh-Br also realizes time-dependent anti-counterfeiting encryption. This work provides a new perspective for designing single-component organic RTP materials.