Salicylideneaniline-Based Covalent Organic Frameworks: A New Family of Multistate Second-Order Nonlinear Optical Switches

This work demonstrates that covalent organic frameworks (COFs) can exhibit large second-order nonlinear (NLO) responses and that these NLO responses can be modulated as a function of successive enol-imine/keto-enamine tautomerisms, leading to efficient solid-state second-order NLO switches. The proof of concept is given by evidencing, by means of periodic boundary condition (time-dependent) density functional theory calculations, the large amplitudes of the second-order NLO susceptibility, χ(2), of two-dimensional COFs built from the assembly of tris(N-salicylideneaniline) units as well as their variations when switching between keto and enol forms. Calculations further demonstrate the key role of symmetry, that is, the distribution of enol and keto functions in the unit cell, on the χ(2) values as well as on their dipolar/octupolar character.