Ketocoumarin-Based Photoinitiators for High-Sensitivity Two-Photon Lithography

Abstract

Ketocoumarins are attractive and distinct photosensitizers due to their high molar extinction coefficients, high intersystem crossing coefficients, and high photochemical stability. As a classic commercial ketocoumarin-based two-photon initiator, 7-diethylamino-3-thenoylcoumarin (DETC) was widely used in two-photon lithography. However, the large fluorescence quantum yield and low twophoton absorption cross section value greatly limit its application in high-throughput nanofabrication. In this work, a series of DETC derivatives were developed by extending the length of the alkyl chain and integrating different donor and acceptor groups. These ketocoumarin-based initiators, namely, compounds 1−7, were designed, synthesized, and unambiguously characterized. Compared with DETC, these compounds exhibit higher molar extinction coefficient, lower fluorescence quantum yield, higher two-photon absorption cross section, and improved sensitivity in two-photon lithography. Among these molecules, compound 7 with expanded π-electron systems and structures with enhanced intramolecular charge transfer exhibits the best sensitivity in two-photon lithography. With compound 7-based photoresist, many kinds of complex threedimensional patterns can be fabricated using two-photon lithography at a writing speed of up to 60 mm s −1 . The high two-photon initiation sensitivity makes these compounds promising candidates for commercialization and provides a new design concept for the development of new initiators.

Light and matter co-confined multi-photon lithography

“…2a ): multiphoton absorption (MPA, S 0 → S 1 ), inter-system crossing (ISC, S 1 → T 1 ), and finally generating free radicals at T 1 state 34 . The T 1 -state DETC will form exciplex species through fast electron transfer, followed by proton transfer to produce a C-centered radical and an N-centered radical to initiate polymerization 35 . The generation of the above free radicals is confirmed by electron spin resonance (ESR) spectroscopy in Fig.…”

Push-Pull Carbazole-Based Dyes: Synthesis, Strong Ultrafast Nonlinear Optical Response, and Effective Photoinitiation for Multiphoton Lithography

“…This compound was selected among the others for MPL experiments, since it exhibited superior NLO absorptive response, which is crucial for achieving high-resolution and efficient fabrication. 70 As presented, the fabricated woodpile structure demonstrated low shrinkage and achieved a high resolution of ~280 nm. The laser intensity needed for fabricating high quality structures without presenting any deformations was ~1.14 TW/cm 2 , measured before the objective lens, and the writing speed was 50 μm/s.…”

Two-photon polymerization lithography for imaging optics

“…Consequently, precompensation is essential before initiating the fabrication process. The quest for resins boasting excellent optical quality, mechanical resilience, minimal shrinkage, ease of processing, and scalable size dimensions remains a formidable challenge [28,220,221]. Moreover, the development of photoresists that polymerize under continuous laser exposure holds great promise in expediting the fabrication process [222,223] and fostering innovation in fabrication methods, such as two-color, twostep, absorption-based, light-sheet 3D microprinting [224].…”