Covalent Organic Frameworks as Emerging Nonlinear Optical Materials

Abstract

The vastness of organic synthetic strategies and knowledge of reticular chemistry have made covalent organic frameworks (COFs) one of the most chemically and structurally diverse class of materials with potential applications ranging from gas storage, molecular separation, and catalysis to energy storage and magnetism. Recently, this class of porous materials has garnered increasing interest as potential nonlinear optical (NLO) materials. Traditionally, inorganic crystals, small-molecule organic chromophores, and oligomers have been studied for their NLO response. Nevertheless, COFs offer significant advantages over existing NLO materials in terms of higher mechanical strength, thermochemical stability, and extended conjugation. Herein, we discuss crucial aspects, terminology, and measurement techniques related to NLO, followed by a critical analysis of the design principles for COFs with NLO response. Furthermore, we touch on selected potential applications of these NLO materials. Finally, future prospects and challenges of COFs as NLO materials are discussed.

Cyclotriphosphazene-based organic frameworks as third-order nonlinear optical materials

“…9,10 Our recent minireview provides a comprehensive overview of the progress made in developing organic frameworks as NLO materials and highlighting the associated challenges. 11 Given that this research area is in its early stages, it is highly imperative to explore new NLOphores as potential building units and systematic structure–property analysis is crucial for advancing this evolving research field.…”

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms

“…These aspects can be duly addressed by fabricating COFMs with the robust and high in-plane π-conjugated (CC) skeletons associated with unique physicochemical properties. , However, this has proven to be quite a challenging task because of the low reversibility of the CC bond formation reaction and the sluggish kinetics involved in it. − Hence, developing a facile strategy to fabricate centimeter-scale, self-standing vinylene-linked COFMs would be an interesting study. On the other hand, integrating NLOphores (chromophores with NLO response), like porphyrins, phthalocyanine, keto-enamine, phenanthroline, etc., into a rigid polymeric framework has already been accomplished. − Notably, a higher NLO absorption coefficient (β) has been observed for porphyrin-based COFs, particularly ZnCu-Por-COF, attributed to their efficient π-conjugation (18 π-electrons) and charge-transfer transitions facilitated by this core. Nonetheless, efforts are underway to investigate new NLOphores in order to gain insights into structure–property relationships and optimize material efficiency.…”