Synthesis And Study Of Properties Of New Two-photon Polymerization Initiators: Anthraquinone And Anthracene Derivatives

Two-photon absorption (TPA) is a third-order nonlinear optical process and the region of TPA can be confined in a tiny volume around the focal point. Any subsequent process of TPA, such as two-photon induced polymerization (TPIP), is also localized to this small volume. The wavelength of laser source used in TPIP is infrared laser which can penetrate a large depth with a little loss. Therefore, resin including photoinitiator, monomer and crosslinker can be used to fabricate complex three-dimensional structures by utilizing TPIP technique. The crucial factor in TPIP is photoinitiator system which plays an important role in decreasing laser energy and improving fabrication speed. However, conventional ultra-violet (UV) active initiators have low two-photon sensitivity because of their low TPA cross section (δ_TPA). Thus, high excitation power and long exposure times are necessary to cure the resin, which often result in damage to the polymeric structure. For TPIP to become a commonly applied technique, photoinitiators with high-sensitivity are desired to make the process more reliable and allow structures to be patterned rapidly. Plenty of TPIP initiators with largeδTPA were reported and showed much higher sensitivity than conventional UV-active initiators. However, most of reported initiators still can't meet the requirement for improving fabrication speed and decreasing laser energy. I_n this work, we focused on synthesizing novel TPI_P initiators, studying their one- and two-photon optical properties, investigating their TPI_P initiating properties, studying the relationship between initiating efficiency and lateral spatial resolution (LSR) and investigating the relationship between different monomers and TPI_P properties.The whole paper consists of seven parts and the main point of each part is listed as follows:Chapter 1: This chapter covers literatures review. As an introduction, this chapter discussed photopolymerization and microfabrication, two-photon absorption process and its applications, research progress about TPI_P. The key problems faced for TPI_P initiators reported were pointed out. Finally, we addressed the purpose, significance and contents in this paper.Chapter 2: Research progress about TPI_P initiators was reviewed and the design for TPI_P initiators in this work was illustrated. We designed anthraquinone derivatives of D-Ï€-A-Ï€-D (I₁-I₄) and A-Ï€-A-Ï€-A (I₅-I₇) types and anthracene derivatives of D-Ï€-D-Ï€-D (II₁-II₃) and A-Ï€-D-Ï€-A (II₄-II₅) with C2v symmetrical molecular structure.Chapter 3: C2v symmetrical anthraquinone derivatives (I₁, I₂, I₃, I₄, I₅, I₆ and I₇) were synthesized by Wittig reaction. Their one-photon absorption and fluorescence, as well as two-photon absorption properties, were investigated. These compounds showed low fluorescence quantum yield. I₁ showed a larger two-photon absorption cross section of 1635 GM than that of 995 GM for I₂ at the wavelength of 800 nm. Two-photon absorption cross section of I₃, I₄, I₆ and I₇ are less than 100 GM at exctitation wavelengths from 700 to 850 nm.Chapter 4: C2v symmetrical anthracene derivatives (II₁, II₂, II₃, II₄ and II₅), were synthesized by Wittig reaction. Their one-photon absorption, fluorescence and two-photon absorption properties, as well as two-photon polymerization initiating property, were investigated. Compound II₁ shows lower one-photon fluorescence quantum yield and much larger two-photon absorption cross section than those of II₂, II₃, II₄ and II₅ at excitation wavelengths from 700 to 850 nm. Two-photon absorption cross section maximum of II₁, 466 GM, localizes at 750 nm. Chapter5: This chapter includes four parts. Firstly, TPI_P initiating properties of anthraquinone derivatives were investigated. I₁, I₂ and I₆ had higher sensitivity comparing with I₃, I₄, I₅, I₇ and Benzil. Threshold energys of resins with I₁, I₂ and I₆ as photoinitiator were 2.02 mW, 2.53 mW and 2.53 mW at the concentration of 0.03 mol%, respectively. The resin with I₁ as photoinitiator possessed much higher sensitivity with a threshold energy 2.02 mW at the concentration of 0.05 mol% compared to that of 12.72 mW for benzil. Secondly, TPI_P initiating properties of anthracene derivatives were investigated. II₁ had much higher sensitivity comparing with II₂, II₃, II₄ and II₅ and Benzil. Threshold energys of resins with II₁, II₂ and benzil as photoinitiator were 0.64, 1.60 and 6.37 mW at the concentration of 0.18 mol%, respectively. The mechanism related to this phenomenon was presented. II₁ has higher sensitivity compared to I₁. The resin with 0.18 mol% II₁ can polymerize as continous line at 2.53 mW with 1000μm/s linear scan speed. II₁ has higher sensitivity compared with I₁. Thirdly, relationship between initiating efficiency and lateral spatial resolution (LSR) were carefully studied. High initiating efficiency benefits to improvement of LSR. LSR was improved to 80 nm by using an II₁ as a highly sensitive and efficient photoinitiator. Theoretical calculations showed that LSR can be increased by reducing the laser power, indicating that LSR could be improved using more sensitive initiators in the future. I_n the end, the relationship between different monomers and TPI_P properties were investigated. TPI_P threshold and LSR were changed with different monomers. LSR can be improved by using two-photon copolymerization method.Chapter6: The conclusions were drawn in this chapter and the future researchs were discussed.Chapter7: Experimental part including measurement and synthesizing.