A Mortise and Tenon Structure Triggered by Anisaldehyde‐Based Additive Unlocking a New Path to Multifunctional Flame Retardant Epoxy Resins

6,6'-((pentane-1,5-diylbis(azanediyl))bis((4-ethoxyphenyl)methylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (PDP) was synthesized by a simple one-pot method using biomass anisaldehyde, 1,5-pentanediamine and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as raw materials. A multifunctional epoxy resin was synthesized by adding PDP into pure epoxy resin. The designed mortise and tenon structure endowed the epoxy thermosets with superior mechanical properties. When the PDP addition was 5% (5-PDP/EP), the tensile strength, flexural strength, and impact strength of 5-PDP/EP were simultaneously improved by 13.1%, 57.7%, and 105.4%, respectively, at a low PDP addition content. Compared to pure epoxy thermoset, 5-PDP/EP also exhibited excellent flame retardancy, achieving a vertical burning (UL-94) V-0 rating, a limiting oxygen index (LOI) value of 35.7%, and reductions in peak heat release rate (PHRR) and peak smoke production rate (PSPR) of 58.9% and 44.0%, respectively. Meanwhile, because the amorphous area of epoxy thermoset increased, 5-PDP/EP showed a high transparency with the visible light transmittance as high as 92%. Moreover, the hydrophobicity and dielectric properties of PDP/EP were improved due to the unique structure of PDP and its interaction with epoxy resin matrix. Achieving simultaneous enhancements in flame retardancy, mechanical toughness, optical transparency, and dielectric performance, this study presents a paradigm for designing next-generation sustainable epoxy resins with synergistic multifunctionality.