Charge-Transfer-Complex Photoinitiation for Visible-Light Additive Manufacturing

Charge-transfer complexes, also termed electron donor-acceptor complexes in this context, can form by ground-state association of complementary donor and acceptor molecules and generate an absorption response not present in the separated components. Visible-light excitation can drive electron transfer and create initiating species for radical or cationic photopolymerisation, while some systems also show thermal initiation. A focused 2019-2023 literature set demonstrates additive-manufacturing relevance: phosphine/iodonium CTCs [1], indole/iodonium CTCs [2], a water-compatible dye/triethanolamine CTC used for hydrogel printing [3], and bromoacetophenone/amine CTCs [4] were all taken into 405 nm patterning or 3D-printing demonstrations. The broader CTC review by Garra et al. provides the mechanism boundary between ground-state CTCs and excited-state complexes such as exciplexes [5]. These studies do not support a universal donor/acceptor recipe, induction time, conversion, cure depth, safety or storage-stability claim. ER-98 therefore requires direct evidence of complex formation or a validated mechanistic proxy, the emergent absorption/action window, donor-only and acceptor-only controls where mechanistically relevant, formulation-specific conversion/kinetics, printing evidence under defined optical conditions, and explicit separation of photochemical from thermal initiation. The phrase “photoinitiator-free” is treated as potentially misleading unless it is qualified to mean absence of a conventional single-molecule photoinitiator; the CTC donor/acceptor pair is still the initiating system.