Bis(2‐amino‐5‐thienyl)Ketone as Oxygen Tolerant Sensitizer for Conventional Radical Photopolymerization

TL;DR

This paper introduces a new sensitizer for light-based polymerization that works well in the presence of oxygen, making it useful for practical applications.

Contribution

The study presents a new oxygen-tolerant sensitizer for photopolymerization using bis(2-amino-5-thienyl)ketones.

Findings

TKs function effectively as sensitizers in photopolymerization under air using 395 nm UV-LED.

The use of TKs leads to higher network density in crosslinked materials due to efficient PET with iodonium salts.

TKs show good oxygen tolerance and potential for biocompatible applications due to low cytotoxicity.

Abstract

Bis(2‐amino‐5‐thienyl)ketones (TKs) operated as effective sensitizer in a multi‐component photoinitiating system for conventional radical photopolymerization. The distinct amino pattern (morpholino 1 versus diaryl moiety 2) followed an oxidative photoinduced electron transfer (PET) mechanism with the iodonium salt C1 and the sulfonium cation C2. 2 performed well under air upon UV‐LED exposure at 395 nm. The TKs mostly reacted from the triplet state (T1) with either C1 or C2 resulting in a cation radical with certain instability. This changes considering the radical anion studied by spectroelectrochemistry confirmed by reversibility. Electrochemical measurements and transient absorption experiments disfavor from a thermodynamic point of view a reductive mechanism of the T1 with C3. Long‐lived charge separated states discuss a new paradigm as a further alternative to form reactive intermediates even with C3. 1O2 generation by 3
TK* operated as additional pathway to form the initiating ∼N─CH2• radical of C3. The higher efficiency of photoinduced electron transfer (PET) with C1 explained the higher network density of the crosslinked material. The subsequent [4+2] cycloaddition of 1O2 to the thiophene moiety led to a mass increase of 32 mz
of either 1 or 2, which led to depletion of inhibiting 3O2. This additionally explains the unexpected, good oxygen tolerance. Furthermore, TK 2a showed no significant cytotoxic response explaining its future potential for biocompatible applications.

Bis(2‐amino‐5‐thienyl)ketones operate as sensitizer in multi‐component photoinitiating systems using either diaryl iodonium cation or amino benzoates as coinitiators to initiate conventional radical polymerization following either oxidative or reduction photoinduced electron transfer protocol, respectively. These systems result in formation of crosslinked polymers even under aerobic conditions. A 395 nm emitting LED operated as exposure source.

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