Nitrogen-rich covalent organic frameworks: a promising class of sensory materials

Covalent organic frameworks (COFs) have emerged as highly crystalline porous organic materials and proved to be the potential candidates for various interesting applications, such as gas adsorption and separation, catalysis, energy storage, and sensing, etc. Fluorescence-based sensing has gained great attention due to its high selectivity, rapid response time, and real-time monitoring. Lately, luminescent COFs have come to the fore as sensing probes to detect the various target molecules. In this review, we have targeted a comprehensive account of the chronological development of the field of COFs starting from 2005 till date. We have described the design principle with a focus on various topologies reported, classification of their structural diversity on the basis of thirty-two linkages and dimensionality, methods of synthesis with examples, and the use of various characterization techniques. Interestingly, almost one-half of the COFs contains linkages that are nitrogen-rich. With a background in different processes involved for fluorescence sensing, we have illustrated the potential applicability of nitrogen-rich COFs as sensors for numerousanalytes, such as explosives, volatile organic molecules, pH, temperature, toxic anions, metal ions and biomolecules from different media utilizing the striking features of COFs with predesigned structures and diverse functionality.