A fully π-conjugated triazine-based 2D covalent organic framework used as metal-free yellow phosphors in white light-emitting diodes

Luminescent covalent organic frameworks (COFs) can be used as metal-free phosphors to replace rare earth-based phosphors for LEDs. Here, a fully pi-conjugated 2D COF containing triazine units is synthesized via Knoevenagel condensation by using terephthalaldehyde (TPA) and 2,2 ',2 ''-((1,3,5-triazine-2,4,6-triyl)tris ([1 '',1 '''-biphenyl]-4 ',4 ''-diyl))triacetonitrile (TTTA) as monomers. This yellow-emitting COF consists of a periodic hexagonal pore structure and is in an eclipsed (AA) stacking mode. Its solid-state photoluminescence quantum yield (PLQY) is 26.0 %, its thermal decomposition temperature (T-d) is ca. 495 degrees C, and its relative emission intensity at 90 degrees C is 80.0 % of that at 30 degrees C, at 150 degrees C is 58.9 % of that at 30 degrees C. Together with GaN-based blue-emitting chips (lambda(em,max) approximate to 460 nm, around 25.0 lm center dot W-1), a series of cold/neutral/warm white LEDs are fabricated by using the COF as yellow phosphors at different concentrations in epoxy resin. At 2.0 wt%, a cold white LED is obtained with the best performances, its correlated color temperature (CCT) is 5743 K, color rendering index (CRI) is 83.1, Commission Internationale de L'Eclairage (CIE) chromaticity coordinate is (0.33, 0.29), luminous efficacy (LE) is 18.18 lm center dot W-1.

Automated summary

Luminescent covalent organic frameworks (COFs) containing triazine units were synthesized and used as metal-free phosphors for LEDs. These COFs had a hexagonal pore structure and an eclipsed stacking mode. When combined with GaN-based blue-emitting chips, a series of white LEDs with different color temperatures and performances were fabricated using the COFs as yellow phosphors. The best performing cold white LED had a CCT of 5743 K, a CRI of 83.1, a CIE chromaticity coordinate of (0.33, 0.29), and an LE of 18.18 lm center dot W-1.