We introduce a gold nanorod (AuNR) driven methodology to induce free radical polymerization in water with near infrared light (800 nm). The process exploits photothermal conversion in AuNR and subsequent heat transfer to a radical initiator (here azobisisobutyronitrile) for primary radical generation. A broad range of reaction conditions were investigated, demonstrating control over molecular weight and reaction conversion of dimethylacrylamide polymers, using nuclear magnetic resonance spectroscopy. We underpin our experimental data with finite element simulation of the spatio‐temporal temperature profile surrounding the AuNR directly after femtosecond laser pulse excitation. Critically, we evidence that polymerization can be induced through biological tissues given the enhanced penetration depth of the near infrared light. We submit that the presented initiation mechanism in aqueous systems holds promise for radical polymerization in biological environments, including cells.
The performance of a photoinitiator is key to control efficiency and resolution in 3D laser nanoprinting. Upon light absorption, a cascade of competing excited states and photoreactions leads to the radical formation that initiates free radical polymerization. Here we investigate 7-diethylamino-3-thenoylcoumarin (DETC), one of the most efficient photoinitiators for two-photon polymerization (TPP). Depending on the presence of a co-initiator, DETC causes radical generation either with two-photon or with a unique three-photon excitation, but the mechanism for these processes is not well understood. Here we show that the unique three-photon based radical formation of DETC in the absence of a co-initiator results from the excitation of special highly excited triplet states followed by multiple bond scission possibilities generating radicals. In contrast, photoinitiation in the presence of a co-initiator proceeds via intermolecular electron transfer or hydrogen atom transfer after the photosensitization of the photoinitiator to the lowest triplet excited state. Our quantum mechanical calculations explain the different pathways for the multiphoton activation mechanism of DETC and its radical formation, which enables the rational design of efficient photoinitiators to increase the speed and sensitivity of 3D laser nanoprinting.
We introduce two-photon (2P) pulsed laser polymerization (PLP) at 800 nm, demonstrating its working principle even through biological tissue. We show that 2P PLP is reliable in determining propagation rate coefficients on the example of the free radical polymerization of methyl methacrylate (MMA) at frequencies ranging from 10 to 100 Hz.
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