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 threedimensional 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 twophoton 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.

Recent advances on benzylidene ketones as photoinitiators of polymerization

“…[291] When irradiated, some molecules are capable to absorb simultaneously two photons, giving rise to what is named a two-photon absorption (TPA). [292] As specificities, the probability of absorption of the electronic transition is proportional to the square of the incident intensity. As a result of this quadratic dependency, the absorption process is limited to the immediate vicinity of the focal point.…”

Lamp vs. Laser: A Visible Light Photoinitiator that Promotes Radical Polymerization at Low Intensities and Cationic Polymerization at High Intensities

“…There are numerous examples of photoinitiators that are activated through bi-and multi-photon excitation. 16,[32][33][34][35] However, the majority of these species are activated through non-resonant two-photon absorption (nr-TPA) wherein two photons, which individually are insufficiently energetic to populate an excited state, are absorbed simultaneously. Such systems would not be suitable for the current purposes as they would be transparent, and thus unreactive, under low intensity conditions.…”

Mechanical nanolattices printed using nanocluster-based photoresists

“…High-performance two-photon initiators can greatly enhance printing speed and resolution ( 11 , 12 ). Currently, almost all two-photon initiators are organic molecules that do not enhance the properties and structural complexity of the final printed products ( 13 , 14 ). In addition, each organic photoinitiator is typically efficient only for a single type of monomer.…”

“…This is different from organic two-photon initiators in which the S 1 state is short lived (ps-ns). In organic two-photon initiators, intersystem crossing to triplet states is generally necessary for the generation of reactive species ( 13 , 14 ), which may reduce overall photoinitiation efficiency (Fig. 1C, right).…”

“…Our nanocluster-PETA photoresists have a FOM ≥ 10 2 at velocities up to 100 mm/s. The minimum required concentration of the nanocluster in resin is 0.47 to 2.75 μmol/g (details in SI), which is among the lowest compared with hundreds of organic two-photon initiators ( 13 , 14 ). In addition, the nanocluster photoresists can be applied to more than one class of monomer, which provides an elegant strategy for printing high-quality nanocomposites.…”