Sustainable and Lignin-Assisted Polyester with Exceptional Optical Filtering via Highly Atom-Efficient In-Situ Polymerization Strategy

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. PDES performs a dual function: depolymerizing lignin by cleaving its ether bonds and simultaneously forming a crosslinked network of biopolyester through esterification. Lignin can form J-type structures due to its inherent π-π stacking while interacting with PDES through hydrogen bonding. Therefore, the biopolyester 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 possesses excellent mechanical properties, with a tensile strength of 10.12 MPa and a work of fracture of 6.29 MJ/m3. 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.