Mauri Anna, Kiefer Pascal, Neidinger Philipp, Messer Tobias, Bojanowski N Maximilian, Yang Liang, Walden Sarah, Unterreiner Andreas-Neil, Barner-Kowollik Christopher, Wegener Martin, Wenzel Wolfgang, Kozlowska Mariana
Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT) Kaiserstraße 12 76131 Karlsruhe Germany
Institute of Applied Physics (APH), Karlsruhe Institute of Technology (KIT) Kaiserstraße 12 76131 Karlsruhe Germany.
Chem Sci. 2024 Jul 15;15(32):12695-709. doi: 10.1039/d4sc03527e.
The performance of a photoinitiator is key to control efficiency and resolution in 3D laser nanoprinting. Upon light absorption, a cascade of competing photophysical processes leads to photochemical reactions toward radical formation that initiates free radical polymerization (FRP). Here, we investigate 7-diethylamino-3-thenoylcoumarin (DETC), belonging to an efficient and frequently used class of photoinitiators in 3D laser printing, and explain the molecular bases of FRP initiation upon DETC photoactivation. Depending on the presence of a co-initiator, DETC causes radical generation either upon two-photon- or three-photon excitation, but the mechanism for these processes is not well understood so far. Here, we show that the unique three-photon based radical formation by DETC, in the absence of a co-initiator, results from its excitation to highly excited triplet states. They allow a hydrogen-atom transfer reaction from the pentaerythritol triacrylate (PETA) monomer to DETC, enabling the formation of the reactive PETA alkyl radical, which initiates FRP. The formation of active DETC radicals is demonstrated to be less spontaneous. In contrast, photoinitiation in the presence of an onium salt co-initiator proceeds intermolecular electron transfer after the photosensitization of the photoinitiator to the lowest triplet excited state. Our quantum mechanical calculations demonstrate photophysical processes upon the multiphoton activation of DETC and explain different reactions for the radical formation upon DETC photoactivation. This investigation for the first time describes possible pathways of FRP initiation in 3D laser nanoprinting and permits further rational design of efficient photoinitiators to increase the speed and sensitivity of 3D laser nanoprinting.
光引发剂的性能是控制3D激光纳米打印效率和分辨率的关键。光吸收后，一系列相互竞争的光物理过程会引发光化学反应，生成自由基，从而引发自由基聚合反应（FRP）。在此，我们研究了7-二乙氨基-3-噻吩甲酰基香豆素（DETC），它属于3D激光打印中一类高效且常用的光引发剂，并解释了DETC光活化后引发FRP的分子基础。根据共引发剂的存在情况，DETC在双光子或三光子激发时都会产生自由基，但目前对这些过程的机制尚不清楚。在此，我们表明，在没有共引发剂的情况下，DETC独特的基于三光子的自由基形成是由于其被激发到高激发三重态。它们允许季戊四醇三丙烯酸酯（PETA）单体向DETC发生氢原子转移反应，从而形成活性PETA烷基自由基，引发FRP。已证明活性DETC自由基的形成不太自发。相比之下，在存在鎓盐共引发剂的情况下，光引发剂在敏化到最低三重激发态后会发生分子间电子转移。我们的量子力学计算证明了DETC多光子活化后的光物理过程，并解释了DETC光活化后自由基形成的不同反应。这项研究首次描述了3D激光纳米打印中FRP引发的可能途径，并允许进一步合理设计高效光引发剂，以提高3D激光纳米打印的速度和灵敏度。
