Ketocoumarin-Based Photoinitiators for High-Sensitivity Two-Photon Lithography
Jin Tang, Xiaoyi Xu, Xiaoming Shen, Cuifang Kuang, Hongzheng Chen, Minmin Shi, Ning Huang
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 two-photon 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 three-dimensional 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.