From Light to Structure: Photo Initiators for Radical Two‐Photon Polymerization

Abstract

Two‐photon polymerization (2PP) represents a powerful technique for the fabrication of precise three‐dimensional structures on a micro‐ and nanometer scale for various applications. While many review articles are focusing on the used polymeric materials and their application in 2PP, in this review the class of two‐photon photo initiators (2PI) used for radical polymerization is discussed in detail. Because the demand for highly efficient 2PI has increased in the last decades, different approaches in designing new efficient 2PIs occurred. This review summarizes the 2PIs known in literature and discusses their absorption behavior under one‐ and two‐photon absorption (2PA) conditions, their two‐photon cross sections (σ _TPA) as well as their efficiency under 2PP conditions. Here, the photo initiators are grouped depending on their chromophore system (D‐π‐A‐π‐D, D‐π‐D, etc.). Their polymerization efficiencies are evaluated by fabrication windows (FW) depending on different laser intensities and writing speeds.

Abstract

Two‐photon polymerization has established itself as one of the most important techniques for the fabrication of 3D structures in the micro to nanometer range. To ensure precise and rapid fabrication, photo initiators (PI) are used that have been specially developed for this purpose. In this review article, the PIs known to the literature are listed and compared in particular according to their structural elements, fabrication behavior and two‐photon cross‐section.