Reticular Electrochemiluminescence Nanoemitters: Structural Design and Enhancement Mechanism

ConspectusElectrochemiluminescence (ECL) is a powerful transduction technique, which depends critically on the formation of the excited emitter through the charge transfer between the electrochemical reaction intermediates of the emitter and the co-reactant/emitter. The exploration of ECL mechanisms for conventional nanoemitters is limited due to the uncontrollable charge transfer process. With the development of molecular nanocrystals, reticular structures such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been utilized as atomically precise semiconducting materials. The long-range order in crystalline frameworks and the tunable coupling among building blocks promote the quick development of electrically conductive frameworks. Especially, the reticular charge transfer can be regulated by both interlayer electron coupling and intralayer topology-templated conjugation. By modulating intramolecular or intermolecular charge mobility, reticular structures could serve as promising candidates for enhancing ECL. Thus, reticular crystalline nanoemitters with different topologies provide a confined platform to understand ECL fundamentals for designing next-generation ECL devices.Aiming at exploring the mechanism of ECL emission, our group has developed a series of ECL nanoemitters as well as enhancement strategies of ECL emission in the past 20 years. A series of water-soluble ligand-capped quantum dots were introduced as ECL nanoemitters to create sensitive analytical methods for detecting and tracing biomarkers. The functionalized polymer dots were also designed as ECL nanoemitters for imaging of membrane proteins with signal transduction strategies of dual resonance energy transfer and dual intramolecular electron transfer. To decode the ECL fundamental and enhancement mechanisms, an electroactive MOF with accurate molecular structure was first constructed with two redox ligands as a highly crystallized ECL nanoemitter in aqueous medium. Through the mixed-ligand approach, luminophores and co-reactants were integrated into one MOF structure for self-enhanced ECL. Furthermore, several donor-acceptor COFs were developed as efficient ECL nanoemitters with tunable intrareticular charge transfer. The atomically precise structure of conductive frameworks established clear correlations between the structure and charge transport in these materials. Therefore, reticular materials as crystalline ECL nanoemitters have demonstrated both proof of concept and mechanistic innovation.In this Account, taking advantage of reticular materials with accurate molecular structure, we survey the design of the electroactive reticular materials including MOFs and COFs as crystalline ECL nanoemitters at the molecular level. The enhancement mechanisms of ECL emission of various topology frameworks are discussed via the regulation of reticular energy transfer and charge transfer and the accumulation of anion/cation radicals. Our perspective on the reticular ECL nanoemitters is also discussed. This Account provides a new avenue for designing molecular crystalline ECL nanoemitters and decoding the fundamentals of ECL detection methods.