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 efficient and frequently used 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 via 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.

中文翻译：

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3D 激光打印光聚合过程中的两光子和三光子过程


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导致光化学反应，形成自由基，从而引发自由基聚合 (FRP)。在这里，我们研究了 7-二乙氨基-3-噻吩酰香豆素 (DETC)，它属于 3D 激光打印中高效且常用的光引发剂，并解释了 DETC 光活化引发 FRP 的分子基础。根据共引发剂的存在，DETC 在双光子或三光子激发时引起自由基生成，但迄今为止这些过程的机制尚不清楚。在这里，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其激发到高度激发的三重态所致。它们允许从季戊四醇三丙烯酸酯 (PETA) 单体到 DETC 的氢原子转移反应，从而形成反应性 PETA 烷基自由基，从而引发 FRP。活性 DETC 自由基的形成被证明不太自发。相反，在光引发剂光敏化至最低三重激发态之后，在鎓盐共引发剂存在下的光引发通过分子间电子转移进行。我们的量子力学计算证明了 DETC 多光子激活时的光物理过程，并解释了 DETC 光激活时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。