Efficient room-temperature phosphorescence of covalent organic frameworks through covalent halogen doping

Abstract

Room-temperature phosphorescence in organic solids is attractive for practical applications but remains rare. Now, highly phosphorescent boroxine-linked covalent organic frameworks have been prepared by covalent doping with halogen atoms through the use of halogenated precursors. The resulting porous COFs exhibited oxygen-sensing capabilities with millisecond response time over a wide range of partial oxygen pressures. Organic room-temperature phosphorescence, a spin-forbidden radiative process, has emerged as an interesting but rare phenomenon with multiple potential applications in optoelectronic devices, biosensing and anticounterfeiting. Covalent organic frameworks (COFs) with accessible nanoscale porosity and precisely engineered topology can offer unique benefits in the design of phosphorescent materials, but these are presently unexplored. Here, we report an approach of covalent doping, whereby a COF is synthesized by copolymerization of halogenated and unsubstituted phenyldiboronic acids, allowing for random distribution of functionalized units at varying ratios, yielding highly phosphorescent COFs. Such controlled halogen doping enhances the intersystem crossing while minimizing triplet–triplet annihilation by diluting the phosphors. The rigidity of the COF suppresses vibrational relaxation and allows a high phosphorescence quantum yield ( Φ _Phos ≤ 29%) at room temperature. The permanent porosity of the COFs and the combination of the singlet and triplet emitting channels enable a highly efficient COF-based oxygen sensor, with an ultra-wide dynamic detection range (~10^3–10^−5 torr of partial oxygen pressure).