Chirality-induced luminescent ultra-thin covalent organic framework nanosheets

Compared with traditional materials with circularly polarized luminescence (CPL) activity, covalent organic frameworks (COF), a new type of crystalline porous material, have high potential due to their structural design capability, versatility, functional adaptability and chemical stability.

Currently, COFs with CPL activity and good crystallinity have not been created. 2D COFs with ultra-thin nanosheet structures can effectively prevent fluorescence quenching caused by π–π stacking. Introducing chiral and luminescent functions into 2D COFs is an efficient way to obtain CPL materials, but it remains a challenge.

In a study published in the Journal of the American Chemical SocietyProf. Zhang Jian, Prof. Gu Zhigang and their colleagues from the Fujian Research Institute of Structure of Matter, Chinese Academy of Sciences developed chirality-induced ultra-thin chiral COF nanosheets and rendered account of their application in the field of circularly polarized luminescence.

The researchers first prepared CPL materials based on ultrathin COF nanosheets (NS) using a chiral induced synthesis strategy. Chiral amines served as chiral inducers to give COF TpBpy with chirality and participated in the modification of TpBpy, inhibiting the fluorescence quenching caused by π–π stacking to form ultrathin luminescent chiral COFs (chirCOFs) NS. The resulting R-/S-TpBpy NS chirCOFs had strong chirality and an intense red CPL property with a |glum| of about 0.02.

They then post-modified the carboxyl containing green and blue fluorescent dye molecules on the NS chirCOFs (chirCOFs/Dyes) to obtain a color-tunable CPL.

Due to the chirality and energy transfer between chirCOF and dye groups, the obtained chirCOF/dye showed strong chirality and enhanced and tunable photoluminescence, exhibiting excellent CPL, tunable and amplified performance with maximum |glum| of about 0.1, which was about five times stronger than that of the prepared NS chirCOFs.

Additionally, researchers dispersed the corresponding NS chirCOFs in a polydimethylsiloxane (PDMS) matrix to form wafer-sized, highly transparent and flexible COFs/PDMS films for convenient CPL application.

This study opens a new strategy to prepare ultra-thin NS chirCOFs with strong and tunable CPL by chiral induction and provides a new approach for the preparation of transparent, large and flexible COFs composite films in chiral optical applications.