Two-photon absorption cross-sections of common photoinitiators

Recent interests in and applications of two-photon absorption (2PA) induced photopolymerization have afforded advanced opportunities to perform three-dimensionally resolved polymerization, resulting in intricate microfabrication and imaging. Many of the reported 2PA-induced polymerizations make use of commercially available photoinitiators, and a key parameter to consider is the two-photon absorption cross-section (δ) of the initiator. To date, there has been no comprehensive investigation of two-photon absorptivity of commercial photoinitiators, though a few studies presenting novel photoinitiators for two-photon polymerization have appeared. Herein, we report the 2PA properties of common, commercially available photoinitiators typically utilized in conventional radiation curing science and technologies, and often used in 2PA-based polymerizations. Z-scan and white-light continuum (WLC) pump-probe techniques were utilized to obtain two-photon absorption cross-sections (δ). The results for most compounds were found to yield good agreement between the two methods. Most of the photoinitiators studied possess low δ, except Irgacure OXE01, indicating a need for the development of new photoinitiators with improved properties optimized for 2PA applications. A compound prepared in our laboratories exhibits high 2PA and was useful as a two-photon free-radical photoinitiator.