π-Conjugated Structure Enhances the UV Absorption Performance of Carbon Dots and Application in the Design of Light-Colored Sunglasses

Traditional UV absorbers encounter challenges stemming from incomplete UV absorption and potential toxicity concerns. Carbon dots are a promising ecofriendly UV absorber. However, they still interfere with visible light transmission and have limited UV absorption efficiency. Furthermore, research on the stability of UV-absorbing carbon dots is limited. This study synthesized carbon dots (CDs) using citric acid (CA) and ethylenediamine (EDA) as precursors. The amide group is the cause of the ability of CDs to absorb UVA (long-wave ultraviolet light). To broaden the absorption region, o-phenylenediamine (o-PD) was employed as a modifier to synthesize efficient three-component full-band UV absorption carbon dots (NCDs). The introduction of o-PD constructed a large conjugated domain on the surface of NCDs, which enhances the absorption capacity of NCDs for UVB through π–π* transitions without affecting visible light. Furthermore, the salt resistance, acid–alkali resistance, thermal stability, and light-bleaching resistance of NCDs were investigated. These properties are linked to the hydrophilicity, structural stability, and electronic transition activity of NCDs. Additionally, NCDs were used in combination with polyurethane (PU) to design light-colored sunglasses that provide excellent UV protection and light-bleaching resistance. This work is significant for utilizing the electronic transition characteristics of conjugated structures to expand the UV absorption range of carbon dots.