Donor–Acceptor Covalent Organic Frameworks Films with Ultralow Band Gaps to Enhanced Third-Order Nonlinear Optical Properties

Covalent organic frameworks (COFs) have attracted much attention in porous materials because of their structural tunability and stability. However, the problems of low transmittance and large light scattering due to the difficulty of powder phase processing have severely limited their practical applications in third-order nonlinear optics (NLO). Herein, we report the first successful synthesis of a series of donor–acceptor (D–A) COFs films (thickness of ∼100 nm) with uniform transparency and narrow optical band gap by the solvothermal and solid–liquid interface methods for third-order NLO applications. Most importantly, the designed PT–PA film exhibits excellent NLO performance, including a normalized transmission value of ∼2.55 at the beam focus, a modulation depth (As) of 139%, and a high nonlinear absorption coefficient (β) of 1.83 × 106 cm GW–1 at the pulse energy of 1 μJ, which is superior to most recently reported third-order NLO materials. The excellent third-order NLO properties are mainly attributed to the homogeneous transparency of the COFs films and the extended-ordered conjugated structure and narrow optical band gap of the COFs films, which enhance the charge transfer in their structures and thus yield excellent third-order NLO behavior. This work provides an option for the development of high-performance NLO optical devices and opens a new path for the future development of COFs films in the optical fields.