Fluorene-Based Monomers and Resins with High-Refractive Index and Optical Transparency for Preparing Microlens Arrays

In order to fabricate lightweight, thin, and highly integrated optical devices, a wide range of high-refractive-index (HRI) optical resins have been extensively studied. However, achieving both high-refractive index and high optical transparency still remains a challenge. In this study, based on a fluorene core, four acrylate monomers (MFTEA, PFTEA, BPFTEA, NFTEA) were designed and synthesized by incorporating sulfur atom and aromatic groups at the 9-position of the fluorene ring. The synthesis process has the advantages of mild conditions, simple operation, and high yields (≥95%). MFTEA and NFTEA are colorless liquid, while the other two are white powdery solid. All these monomers exhibited high transmittance within the visible range (absorption cutoff wavelength ≤ 330 nm) and high-refractive index (1.608–1.660). Under irradiation of a 365 nm UV LED, the homopolymerization of these monomers was rapidly initiated by photoinitiator 1173 with double bond conversion rates ≥93.5%, resulting in high-performance optical resins P1–P4 (refractive index: 1.667–1.709, Abbe number: 19.7–24.6, transmittance: >96.5%). Using a bifunctional HRI monomer as a cross-linking agent, three copolymer resins P5–P7 (P5: MFTEA+PFTEA; P6: MFTEA+BPFTEA; P7: MFTEA+NFTEA) with good processability, thermal stability, and tensile property were achieved. Microlens arrays fabricated by P5–P7 via the UV imprinting method demonstrated their excellent beam-shaping and homogenization capability, indicating great application potentials of these monomers and resins.