Design and nonlinear optical properties of benzotrithiophene-based covalent organic frameworks

Abstract

Covalent organic frameworks (COFs) possess infinitely extended large π-conjugated structures, which provide tremendous potential for enhancing the nonlinear optical (NLO) properties of materials. Herein, we have successfully constructed a series of benzotrithiophene-based covalent organic framework powders (BT-COFs) via Schiff-base condensation reaction. Furthermore, high-quality BT-COF thin films were fabricated by the in-situ growth method for the investigation of third-order NLO properties. The nonlinear optical performance parameters of BT-COF thin films were obtained through Z-scan measurements. Among them, BT-PT exhibits the highest nonlinear response, with a maximum nonlinear absorption coefficient (β) of −3.44 × 10⁻⁶ m W⁻¹, a maximum nonlinear refractive index (n₂) of 3.59 × 10⁻¹³ m² W⁻¹, and a maximum third-order nonlinear susceptibility (|χ⁽³⁾|) of 10.11 × 10⁻⁷ esu. This study regulates the electronic structure and conjugation degree of COFs by introducing heteroatoms, reveals the effective role of heteroatom doping in enhancing the third-order NLO properties of COFs, and provides a new idea for designing high-performance COFs NLO materials.