Nonlinear optical properties of porphyrin-based covalent organic frameworks determined by steric-orientation of conjugation

Abstract

The influences of conjugation on the third-order nonlinear optical (NLO) properties of covalent organic frameworks (COFs) provide guidelines to design the structures for desired applications. In this work, two two-dimensional (2D) porphyrin-based COFs and corresponding topological polymerized three-dimensional (3D) materials have been prepared. Theoretical calculations and experimental results revealed that the linkages between the porphyrins played critical roles in the degree and orientation of conjugation. The topological polymerization of diacetylenyl groups led to increased conjugation of the structures but disturbed the delocalization of the 2D π-conjugation planes. As a result, the 2D COFs with well extended π-conjugated planes exhibited more excellent third-order nonlinear optical properties and stronger excited-state absorption. This work would provide valuable inspiration for the subsequent investigation on the structure–activity relationships by adjusting the conjugation orientation of COFs.