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

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 ³TK* 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 m_z 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.