Ultralong Phosphorescence From a Simple Push–Pull Naphthalonitrile

Achieving persistent room‐temperature phosphorescence (RTP) in purely organic, heavy atom‐free molecules is challenging due to intrinsically weak spin–orbit coupling and the occurrence of non‐radiative decay. Here, we investigate a donor–acceptor naphthalonitrile derivative ( NMe 2 ) that combines a pronounced charge–transfer (CT) character with a small singlet‐triplet energy gap (Δ E ST ), enabling efficient intersystem crossing (ISC), reverse intersystem crossing (RISC), and dual TADF/RTP emission. Comprehensive photophysical measurements reveal strong solvatochromism in solution and the emergence of temperature‐dependent delayed emission in rigid matrices. In glassy 2Me‐THF, polymethyl methacrylate (PMMA) films, and polymeric 3D‐printed objects, NMe 2 exhibits long‐lived phosphorescence with lifetimes in the millisecond‐to‐second range, facilitated by restricted molecular motion and reduced dioxygen quenching. Time‐gated and transient photoluminescence studies suggest the coexistence of TADF and phosphorescence in PMMA, with phosphorescence dominating at low temperature. TDDFT calculations support the experimental observations, showing CT‐dominated excited states, small Δ E ST values, and mixed singlet‐triplet character that promotes efficient ISC. These results highlight the potential of simple donor–acceptor scaffolds as versatile emitters for the development of heavy‐atom‐free luminescent materials for advanced photonic and optoelectronic applications.