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

Abstract

Luminescent covalent organic frameworks (COFs) can be used as metal-free phosphors to replace rare earth-based phosphors for LEDs. Here, a fully π-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 (Td) is ca. 495 °C, and its relative emission intensity at 90 ℃ is 80.0 % of that at 30 °C, at 150 ℃ is 58.9 % of that at 30 °C. Together with GaN-based blue-emitting chips (λem,max ≈ 460 nm, around 25.0 lm·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·W−1.