Modulating Linker Conjugation and Flexibility of Aggregation-Induced Emission Covalent Organic Framework for Enhanced Luminescence

Abstract

In order to inhibit non-radiative decay pathways of covalent-organic frameworks (COFs), a strategy is proposed to block intralayer conjugation and interlayer π–π stacking by using flexible aggregation-induced emission (AIE) building blocks (4′,4′′′,4′′′′′,4′′′′′′′-(1,2-ethenediylidene) tetrakis[1,1′-biphenyl]-4-carbaldehyde (TFBE)) connected by weakly conjugated flexible linker. By using the flexible TFBE as building blocks and changing the conjugation and flexibility of linker, TFBE-COFs with different luminescence properties are obtained. Experimental and theoretical results show that these TFBE-COFs have high crystallinity and large layer spacing, among which the photoluminescence quantum yield of hydrazone (Hz)-COF_TFBE-ODH (oxalyl dihydrazide (ODH)) in solid state reaches 26.28%, which is superior to most of the COFs reported so far. The excellent luminescence is attributed to the non-planar geometry and flexibility of TFBE, which inhibits aggregation-induced quenching. Moreover, the π-electron delocalization-induced non-radiative leaps are suppressed by weakly conjugated flexible linker, which further enhances the photoluminescence of TFBE-COFs. The Hz-COF_TFBE-ODH exhibits excellent sensing performance for trace tetracycline, the detection limit is 0.15 µm. In addition, a white light-emitting diodes coated with Hz-COF_TFBE-ODH is manufactured to achieve high-quality white light emission. This study provides a new strategy for the design and application of high-emission COFs.