Printing of high-resolution 3D nanostructures using two-photon polymerization has gained significant attention because of the diverse materials that can be utilized in this technique; moreover, it provides an effective way to build from traditional 2D lithography to create 3D nanostructures rapidly. In particular, isopropyl thioxanthone (ITX) has been implemented as a photoinitiator for two-photon polymerization in 3D nanoprinting due to its strong capability of initiating polymerization and photoactivated inhibiting polymerization. Despite this, new photoinitiating materials are needed in order to reduce the power requirements for the high-throughput creation of 3D printed structures using two-photon polymerization processes. To address this point, we have synthesized and characterized a suite of new thioxanthone-based photoinitiators. Two-photon polymerization was performed using the most promising photoinitiator. Importantly, one of the initiators 2,7-bis((4-(dimethylamino) phenyl) ethynyl)-9H-thioxanthen-9-one) (BDAPT) demonstrated a 5-fold improvement in the writing threshold over the commonly-used ITX. In order to elucidate the mechanism behind this, the excitation and inhibition mechanisms associated with the BDAPT molecule were elucidated using density functional theory (DFT) calculations, ultra-fast transient absorption spectroscopy and the two-photon Z-scan spectroscopic technique. BDAPT has a larger two-photon absorption cross-section, which results in a more efficient absorption, and BDAPT also has a much shorter excited state lifetime relative to ITX. The performance difference between BDAPT and ITX was evaluated, and the target goal of reducing the excitation laser power while maintaining a polymerization depletion capability for BDAPT was successfully realized. The improved polymerization threshold of this new photoinitiator presents a clear design pathway for the strategic syntheses of next-generation photoinitiators for scalable 3D nanoprinting processes.

Synthesis and characterization of thioxanthone-based photoinitiators for two-photon controllable nanolithographic printing

2019 Fall National Meeting

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