Structure design of naphthalimide derivatives: toward versatile photoinitiators for near-UV/Visible LEDs, 3D printing, and water-soluble photoinitiating systems

Seven naphthalimide derivatives (NDP1-NDP7) with different substituents have been designed as versatile photoinitiators (PIs), and some of them when combined with an iodonium salt (and optionally N-vinylcarbazole) or an amine (and optionally chlorotriazine) are expected to exhibit an enhanced efficiency to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources (i.e., the LED at 385, 395, 405, 455, or 470 nm or the polychromatic visible light from the halogen lamp). Remarkably, some studied naphthalimide derivative based photoinitiating systems (PIS) are even more efficient than the commercial type I photoinitiator bisacylphosphine oxide and the well-known camphorquinone-based systems for cationic or radical photopolymerization. A good efficiency upon a LED projector at 405 nm used in 3D printers is also found: a 3D object can be easily created through an additive process where the final object is constructed by coating down successive layers of material. As another example of their broad potential, a NDP compound enveloped in a cyclodextrin (CD) cavity, leads to a NDP-CD complex which appears as a very efficient water-soluble photoinitiator when combined with methyldiethanol amine to form a hydrogel. The high interest of the present photoinitiator (NDP2) is its very high reactivity, allowing synthesis in water upon LED irradiation as a green way for polymer synthesis.The structure/reactivity/efficiency relationships as well as the photochemical mechanisms associated with the generation of the active species (radicals or cations) are studied by different techniques including steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin-trapping methods.