Substituted Thioxanthone-Based Photoinitiators for Efficient Two-Photon Direct Laser Writing Polymerization with Two-Color Resolution

Abstract

Nanolithographic printing by direct laser writing (DLW) photopolymerization has attracted increased attention in recent years, as the speed of this printing has increased, while the feature sizes that have been realized have decreased well into the nanoscale regime. Specifically, isopropyl thioxanthone (ITX) has been utilized as one of the common photoinitiators in DLW polymerization processes because of its high-efficiency photoinitiating abilities and its ability to have its initiation properties inhibited through the application of a second wavelength of light. However, improved photoinitiating materials that are built from this successful archetype are required, by both academic and industrial circles, if advanced highthroughput nanomanufacturing techniques are to be implemented. Here, nextgeneration thioxanthone-based photoinitiators with tailored optical and charge transfer properties were computationally designed and subsequently synthesized. Particularly, branches with specifically modulated electron donor and electron acceptor qualities, relative to the ITX core, were coupled to the initial thioxanthone substrate. After having their molecular and optical properties characterized in full, it was evident that these initiators possessed a clear advancement in terms of photopolymerization initiation relative to ITX. As such, a champion photoinitiator chemistry was brought forward to demonstrate enhanced two-photon polymerization DLW such that superresolution properties were exhibited. In this way, we introduce a clear means by which to systematically design future photoinitiators for enhanced two-photon polymerization DLW nanoprinting processes.

Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone

“…It was therefore proposed that the inhibition mechanism in this photoresist involves depletion of the lowest triplet state ( T 1 ) of ITX when the deactivation beam is resonant with transition between T 1 and a higher triplet state. Although triplet absorption followed by RISC from a highly excited triplet state to a high singlet state has been suggested to be the deactivation mechanism ( Chi et al., 2019 , 2021 ; Harke et al., 2013 ), no direct evidence has been provided for this process.…”

“…With unfiltered emission, for powers up to 50 mW, the deactivation is limited to a few percent. A 435 nm bandpass filter with a full width at half maximum bandwidth of 48 nm restricts transmission to the wavelength region of the ITX fluorescence spectrum ( Figure S12 ), although some phosphorescence should fall within this window as well ( Chi et al., 2021 ). When this filter is used, virtually no deactivation is observed ( Figure 3 B).…”

“…Different mechanisms have been demonstrated for deactivation, and we will denote this general class of techniques as two-color lithography (2CL) ( Fourkas and Petersen, 2014 ). A second reason for our particular interest is that ITX has been shown to be a useful initiator for 2CL ( Chi et al., 2019 , 2021 ; Fischer and Wegener, 2011 ; Harke et al., 2012 , 2013 ; Mueller et al., 2014 ). Even though ITX is a Type II photoinitiator, this compound has been demonstrated to be able to initiate crosslinking of acrylates upon multiphoton excitation with pulsed 800 nm light in the absence of a coinitiator ( Harke et al., 2012 ).…”

“…It has been established that the mechanism of photodeactivation in ITX at these wavelengths is not stimulated emission ( Wolf et al., 2011 ). To date, there has been no clear evidence regarding the details of the deactivation process, although it has been proposed that triplet states are involved ( Chi et al., 2019 , 2021 ; Harke et al., 2013 ).…”

Rapid, continuous projection multi-photon 3D printing enabled by spatiotemporal focusing of femtosecond pulses

“…Another limiting factor for achieving high printing rates is the sensitivity of the photoresist. Much work has been done to develop two-photon photoinitiators with improved photoinitiating capabilities [48][49][50][51] .…”