Leveraging Multivalent Assembly towards High‐Temperature Liquid‐Phase Phosphorescence

High‐temperature phosphorescence (HTP) materials have attracted considerable attention owing to their expanded application prospects, whereas they still suffer from severe deactivation in polar media, limiting their reliability and utility. Here, we present an efficient multivalent assembly strategy to achieve high‐temperature liquid‐phase phosphorescence (HTLP). The supramolecular assembly of multivalent modules leads to extremely robust hydrogen‐bonding networks, which firmly immobilize the organic phosphors and protect triplet excitons from annihilation in high‐temperature polar media, resulting in excellent HTLP emission. Moreover, the photophysical properties of HTLP are significantly enhanced by boosting multivalent interactions using multitopic phosphors, demonstrating a visible afterglow of 5.0 s in boiling water, more than 2 s in dimethylsulfoxide at 460 K (187 °C), and a long lifetime of 70.3 ms in N‐methylpyrrolidone at 476 K (203 °C). Based on their fluidity and robust HTLP emission, in‐situ microcracks detection of high‐temperature operating instruments and spatial‐time‐temperature‐resolved anticounterfeiting are demonstrated.