Tailored thioxanthone-based photoinitiators for two-photon-controllable polymerization and nanolithographic printing

Printing of high-resolution three-dimensional nanostructures utilizing two-photon polymerization has gained significant attention recently. In particular, isopropyl thioxanthone (ITX) has been implemented as a photoinitiator due to its capability of initiating and depleting polymerization on demand, but new photoinitiating materials are still needed in order to reduce the power requirements for the high-throughput creation of 3D structures. To address this point, a suite of new thioxanthone-based photoinitiators were synthesized and characterized. Then two-photon polymerization was performed using the most promising photoinitiating molecule. Importantly, one of the initiators, 2,7-bis[(4-(dimethylamino)phenyl ethynyl)-9H-thioxanthen-9-one] (BDAPT), showed a fivefold improvement in the writing threshold over the commonly used ITX molecule. To elucidate the fundamental mechanism, the excitation and inhibition behavior of the BDAPT molecule were evaluated using density functional theory (DFT) calculations, low-temperature phosphorescence spectroscopy, ultra-fast transient absorption spectroscopy, and the two-photon Z-scan spectroscopic technique. The improved polymerization threshold of this new photoinitiator presents a clear pathway for the modification of photoinitiators in 3D nanoprinting. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019