Protonation of Covalent Organic Framework with Robust Built‐In Electric Field for Dual Electrochemiluminescence Enhancement

Covalent organic frameworks (COFs) have emerged as promising electrochemiluminescence (ECL) emitters. To achieve the application, herein a dual ECL enhancement strategy is proposed for imine-linked COF by incorporating bipyridine units and implementing a post-modification method. After duplex protonation of bipyridine and imine nitrogen facilitated with ascorbic acid as a sacrificial agent, the HCOF_Py-Bpy exhibits a robust built-in electric field, reduced bandgap, and improved hydrophilicity, resulting in 9.63- or 4.11-fold ECL enhancement with tripropylamine as coreactant compared to COFs without protonation (COF_Py-Bpy) or with protonated imine nitrogen (HCOF_Py-PB). The ECL efficiency of 20.85% is 5.27- and 2.35-fold higher than those of COF_Py-Bpy and HCOF_Py-PB, respectively. Density functional theory calculations confirm that the protonated nitrogen in HCOF_Py-Bpy enhances the electron-withdrawing effect and enables a large dipole moment, which alters charge distribution to strengthen the D-A effect and thus enhances ECL performance. Utilizing HCOF_Py-Bpy as the emitter, an efficient ECL sensing platform is developed for ultrasensitive detection of iodide (I^‒) and iodixanol, which demonstrates a rational post-synthetic approach to design highly efficient ECL COF emitters, offering broad sensing applications.