Revisiting C4N4 Fluorophore: Theoretical Elucidation of the Origin of Fluorescent Properties of 2,5‐Diaminopyrimidines and Strategic Applications to Pd Detection

Compact organic fluorophores are attractive scaffolds for probe design, yet their emission is often compromised by efficient nonradiative decay. Here, we introduce 2,5-diaminopyrimidine-based C4N4 as an intrinsically emissive, exceptionally small platform, which remains bright without extended π-conjugation. Systematic substitution at 4,6-positions with alkyl, alkenyl, and alkynyl groups reveals robust emission for alkyl and alkynyl derivatives, and a striking bimodality for alkenyls. TDDFT combined with ES/SC-AFIR mapped the S₁→S₀ internal-conversion landscape, providing a rationale for substituent-dependent relaxation behavior. Manipulation at the 5-position allowed for independent trigger-responsive emission behavior. This orthogonal design, where the 4,6-substituted C4N4 core serves as the fluorophore and the 5-position operates as ON/OFF gate, enabled the development of turn-on probes for Pd²⁺.