Ultra-long room temperature phosphorescence, intrinsic mechanisms and application based on host-guest doping systems

Abstract

To construct efficient room-temperature phosphorescence (RTP) doping systems by simple doping methods, isomers 2-(2-(9H-carbazol-9-yl)benzyl)malononitrile (o-CzCN), 2-(3-(9H-carbazol-9-yl)benzyl)malononitrile (m-CzCN) and 2-(4-(9H-carbazol-9-yl)benzyl)malononitrile (p-CzCN) were designed and synthesized by choosing commercial carbazole. Based on the structure-function relationships of three isomers and excellent compatibility between carbazole and benzocarbazole, 2-(3-(9H-carbazol-9-yl)benzyl)malononitrile (Lm-CzCN) and 2-(3-(5H-benzo[b]carbazol-5-yl)benzyl)malononitrile (m-BCzCN) were prepared by self-made carbazole and 2-naphthylamine. Then, Lm-CzCN/m-BCzCN was constructed and optimized by dissolution and rapid evaporation, as well as tuning the mass ratios between Lm-CzCN and m-BCzCN. Lm-CzCN shows excitation dependent RTP and afterglow lifetimes, as well as concentration dependent RTP emission in poly(vinyl alcohol) (PVA) films, while 1% m-BCzCN@PVA film emits bright green afterglow, with RTP and afterglow lifetimes of 2.303 and 17 s in turn, as well as RTP quantum yield of 0.22. More importantly, Lm-CzCN/m-BCzCN presents ultra-long room temperature phosphorescence, with RTP and afterglow lifetimes of 597.58 ms and 8 s, respectively. Moreover, crystals m-CzCN and p-CzCN, as well as Lm-CzCN/m-BCzCN can be excited by visible light of 440 nm, showing yellow afterglow of 1–4 s. Noteworthy, polymorphism o-CzCN_Y and o-CzCN_B were found, whose different emission was investigated by molecular conformation, intermolecular arrangement and stacking patterns. Finally, multiple encryptions were successfully constructed based on the different luminescent properties.