π-Conjugated Two-Dimensional Covalent Organic Framework Containing Donor–Acceptor Bipolar Structural Units for White Light-Emitting Diodes

Luminescent covalent organic frameworks (L-COFs) can be used as phosphors for LEDs and are promising replacements for the currently expensive and nonrenewable rare-earth-based phosphors; herein, such a two-dimensional COF is reported. To achieve good luminescence performance, full π-conjugation and donor–acceptor (D–A) bipolar structural units are designed for this COF. The donor is mainly composed of a triphenylamine (TPA) group, while the acceptor is mainly constituted by a 2,4,6-tri([1,1′-biphenyl]-4-yl)-1,3,5-triazine (TBPT) group and a cyano group. The COF (TPA-TBPT-COF) features hexagonal mesopores with AA stacking, exhibiting yellow emission (530–720 nm, peaking at 589 nm) and a high quantum yield of 20.5%. Moreover, its thermal decomposition temperature is up to 500 °C, demonstrating exceptional thermal stability. By integrating the COF powders into an epoxy resin and coating the mixture onto GaN-based blue light chips (460 nm), a series of white LEDs can be fabricated. The optimized device achieves a high color rendering index of 82.3 and a CIE coordinate of (0.32, 0.33), which is very close to the (0.33, 0.33) of pure white light. This work provides an efficient method for designing COFs with high luminescence performance and facilitates the potential application of COFs as rare-earth-free phosphors in white lighting.