Sustainable and lignin-assisted polyesters with exceptional optical filtering via a highly atom-efficient in situ polymerization strategy

Abstract

Sustainable optical filters with visibly opaque but near-infrared transparent (VONIRT) capabilities show great promise in optical detection and information security. However, optical filter production often relies on fossil-based materials and involves complex, environmentally harmful processes. Here, we present a simple, highly atom-efficient, in situ polymerization strategy that confines industrial lignin into an esterification-polymerizable solvent to produce a fully bio-based VONIRT optical filter. In this process, lignin is mixed with a polymeric deep eutectic solvent (PDES) consisting of citric acid and 1,6-hexanediol. The PDES plays a dual role: depolymerizing lignin by cleaving its ether bonds and simultaneously forming a crosslinked network of polyesters through esterification. Lignin can form J-type structures due to its inherent π–π stacking while interacting with the PDES through hydrogen bonding. Therefore, the polyester optical filter with a dense structure exhibits extreme ultraviolet–visible shielding (transmittance of ∼0% at 400 nm and 600 nm) and high NIR transmittance (∼80% at 1400 nm). In addition, it exhibits excellent mechanical properties, with a tensile strength of 10.6 MPa and a work of fracture of 5.2 MJ m⁻³. The optical filter has demonstrated promising applications in NIR photography and information security. This simple, catalyst-free, highly atom-efficient approach is promising for producing a sustainable, high-performance bio-based optical filter.