Manufacturing Multicolor-Switchable Photochromic Polymers via 3D Printing Using Visible-Light-Sensitive Hexaarylbiimidazole Derivatives as Photoinitiators

The rapid, efficient, and precise fabrication of photochromic materials is of great importance for the development of advanced smart materials. In this study, four hexaarylbiimidazole compounds (HABIs) exhibiting a distinct yellow-green photochromism were synthesized. Leveraging both the enhanced photochromic properties of HABIs and the function of initiating photopolymerization of HABIs/N-phenylglycine (NPG) systems, photochromic polymers were successfully manufactured in one step via digital light processing three-dimensional (DLP-3D) printing equipped with a 405 nm source. Furthermore, by incorporating a spiropyran derivative (SP) as an additional photochromic molecule alongside HABIs into the printing formula, together with the utilization of their different photoresponsive wavelengths and color expression, we successfully fabricated objects capable of three-color transitions, manifested as yellow (orange)-green-purple changes. Focusing exclusively on the photoinitiator functionality, HABIs exhibiting visible light absorption are particularly compatible with blue and green light-emitting diodes (LEDs), aligning with the ongoing trend of replacing mercury lamps by LED in photopolymerization, as the former causes ozone pollution and emits harmful ultraviolet (UV) light. In contrast, LEDs offer advantages such as ozone-free and enhanced safety, making them compatible with green chemistry concept. This work not only expands the type of visible-light-sensitive photoinitiators available for photopolymerization but also provides a straightforward, precise, and efficient strategy for manufacturing multicolor-switchable photochromic polymers via 3D printing, offering promising prospects for the development of advanced smart materials.