Novel Highly Efficient Initiators For Two-Photon Induced Photopolimerization

In the last decade TPIP has attracted much attention of researchers and has been intensely studied for diverse three-dimensional (3D) applications such as different mechanical, electronic and optical micro devices, high-density 3D optical data storage, photonic crystals1,2, polymer-based optical waveguides on integrated circuit boards3, high-density 3D optical data storage4,5, and the like. Furthermore, the high resolution and the versatility in the building process offer new routes for medical applications, such as tailored replacement materials. In this real 3D rapid prototyping technique, a resin that contains mainly a multifunctional acrylate-based monomer and a photoinitiator (PI) is cured in the focal point of a femtosecond pulsed near infrared laser beam. With this unique process it is possible to produce complex 3D structures with resolutions (~120 nm) below the diffraction limit of the wavelength used6.

At the beginning of TPIP, research groups used typical commercially available radical PIs such as Irgacure 369, which suffer from having low TPA cross sections (σTPA).2 Relatively high concentrations, laser intensities, and long exposure times are needed when structuring with these one-photon initiators, often resulting in damage to the polymeric structures.

An efficient TPIP process requires active two-photon absorption photoinitiators (TPA PIs), which ensure high writing speeds, a low polymerization threshold and therefore high quality structures. In the last decades, plenty of TPA chromophores with large σTPA were synthesized but only a limited number of them can be used as efficient TPA PIs.7-9 The main reason is that the rate of free radical photopolymerization is mainly determined not only by the TPA of the PIs, but also the quantum yield of radical generation and the initiating efficiency of the formed radicals. Thus, a large σTPA of a PI is not necessarily associated with a high photoinitiation efficiency.10

TPA PIs usually comprise dipolar or quadrupolar TPA chromophores containing planar π systems with long conjugation length and strong donors and/or acceptors.11 Unlike classical UV-vis PIs the TPA PIs are still undergoing slow development and are actually being limited in their use to academic research. One important reason is that the formation of the desired

planar π systems up to now required multi-step synthesis and expensive catalysts. For example, double bonds are commonly introduced into the π systems as bridges to extend

the conjugation length. The wanted formation was mostly realized by classical Wittig12 or Horner-Wadsworth-Emmons (HWE) reaction13 of non-commercial Wittig or HWE salts with

corresponding aldehydes under strong alkaline conditions. A subsequent isomerization reaction with traces of iodine was generally required to convert the generated isomer to the

desired product.14 Therefore it is crucial to develop simple and economical synthesis routines to prepare efficient TPA PIs.

Recently, Wu’s group reported the synthesis of a series of benzylidene cyclopentanone dyes and successfully applied them as TPA PIs in TPIP.15,16 The D-π-A-π-D core structures of

benzylideneketone dyes can be formed in one step via a classical aldol condensation reaction. Although a few studies on the structure-property relationship of benzylidene ketone

dyes have been carried out by changing the terminal donor groups15,17 and extending the conjugation length18, the effects of the central ketone on their TPA behavior have not been

systematically studied up to now. In this article three benzylidene ketones with different central moieties were prepared and their photochemical behaviors in TPIP were investigated. For comparison, the highly efficient TPA PI R1 which is well-known from literature8 was also tested. Investigation on the photophysical properties of the initiators was conducted via UV-vis absorption and emission as well as z-scan measurements. Quantum chemical calculations were carried out to study the structure-property relationship and the calculated results were compared with the experimental ones. Finally, TPIP structuring tests at different laser intensities and feed rates were performed to evaluate the TPA initiation efficiency of each initiator.