Harnessing Ultralong‐Lived Triplet Excitons in Color‐Tunable Carbon Dots for Multilayer Security Platform

ABSTRACT Achieving multicolor ultralong room‐temperature phosphorescence (URTP) from a metal‐free single benzenic framework represents a fundamental challenge owing to inefficient intersystem crossing. Herein, we demonstrate a precursor engineering strategy that enables excited‐state modulation in carbon dots (CDs) derived from a single benzenic core. Controlled co‐pyrolysis of urea with benzoic acid and methoxy‐substituted derivatives systematically tunes the electron‐donating character of the carbonizing precursor, thereby regulating electronic structure and triplet energy levels. During thermal condensation, the CDs are generated within a urea matrix and subsequently got confined in a cyanuric acid‐rich, hydrogen‐bonded rigid framework, which suppresses nonradiative pathways and stabilizes triplet excitons. This synergistic electronic modulation and structural rigidification afford color‐tunable URTPs spanning from blue to yellow with a maximum lifetime of 1.3 s and a photoluminescence quantum yield of 18.1%. These findings establish a general design principle for single‐core multicolor URTP systems and open avenues for advanced anti‐counterfeiting and optical information encryption technologies.