p-π Conjugation-Promoted Electrochemiluminescence of Halogenated Covalent Organic Framework Nanoemitters

Covalent organic frameworks (COFs) are a kind of promising electrochemiluminescence (ECL) crystalline nanoemitters due to their rapid intrareticular charge transfer and predesigned structures. However, the high polarization of heteroatom-containing linkages impedes the charge transfer within reticular structures. In this study, we construct a series of halogenated COFs nanoemitters composed of terephthalaldehyde ortho-substitutions and 1,3,6,8-tetrakis(4-aminophenyl)pyrene ligands via a covalent halogenation predesign strategy. The incorporation of covalently bonded halogen atoms promotes a dense electron population in carbon-carbon antibonding molecular orbitals, thereby facilitating efficient p-π conjugation within the COF structure. Based on the improved intrareticular charge transfer along carbon skeleton, as evidenced by the Hall effect and terahertz spectroscopy, the brominated COF demonstrates a 49-fold enhancement in ECL intensity compared to nonhalogenated COF. Furthermore, the performance of four partially brominated COFs establishes a positive correlation between the degree of Br doping and ECL intensity. Beyond the corresponding model compounds, the conjugated frameworks of COFs significantly amplify the halogenation-induced enhancement effect. This halogenation-promoted p-π conjugation in reticular skeleton provides a universal strategy to sensitize crystalline nanoemitters for decoding ECL enhancement mechanism.