Remarkable increase in electrochemiluminescence of isomeric bipyridine-based covalent organic frameworks via regulating the direction of imine linkage for sensing application

Covalent organic frameworks (COFs) with highly ordered structures and predictable optoelectronic properties provide an ideal platform to investigate the electrochemiluminescence (ECL) performance based on organic materials by atomically varying the molecular construction. Herein, the effect of imine-bond orientation on the ECL performance of COFs is investigated. We report two COFs (NC-COF_Py-Bpy and CN-COF_Py-Bpy) with different orientations of imine bonds using pyrene donor units (D) and bipyridine acceptor motifs (A) monomers. The direction of the imine linkage (carbon of imine bonds as the D unit and nitrogen as the A unit) in NC-COF_Py-Bpy oppose to the direction of donor and acceptor charge transfer between the molecular motifs. In contrast, CN-COF_Py-Bpy is in the same direction (D-A type imine COFs). However, the NC-COF_Py-Bpy demonstrates a superior ECL performance with 22.4-fold enhancement in ECL intensity compared to CN-COF_Py-Bpy, due to the effective separation of the highest and lowest occupied molecular orbitals to facilitate intramolecular charge transfer (IRCT) for strong ECL. Moreover, the ECL intensity of NC-COF_Py-Bpy remains higher stability than CN-COF_Py-Bpy at low-excited positive potential with tri-n-propylamine (TPrA) as the coreactant. The experimental and modeling investigation indicate that the bimodal ECL patterns of NC-COF_Py-Bpy were derived from the competitive oxidation mechanism of IRCT regulated: the co-reactant-mediated oxidation at lower potential and the direct oxidation at higher potential, but the CN-COF_Py-Bpy only has a co-reactant pathway. Notably, the NC-COF_Py-Bpy can be used to construct an ECL sensor for sensitive detection of doxorubicin (DOX). This research provides a protocol for the molecular design of COFs as ECL emitters with atomically regulated charge transfer direction in D-A systems.