Rhenium(I) N‑Heterocyclic Carbene Complexes in Photoinitiating Systems for Polymerization upon Visible Light: Development of Photosensitive Resins for 3D and 4D Applications

Two <i>fac</i>-rhenium <i>tris</i>-carbonyl complexes bearing a π-accepting pyridoannelated N-heterocyclic carbene (NHC) ligand with general formula Re­(CO)₃(pyipy)­X, where pyipy is 2-pyridil-imidazo­[1,5-<i>a</i>]­pyridine-3-ylidene and X = Cl (<b>Re1</b>) and Br (<b>Re2</b>), are investigated as photoredox catalytic systems in light-promoted polymerization reactions. Both rhenium­(I) complexes are able to efficiently generate radicals to initiate free radical polymerization. Remarkably, excellent radical polymerization profiles and final conversions are obtained under soft irradiations (LED lamp, λ_exc = 405 nm) by using the proposed complexes with iodonium salt and amine as photoinitiating systems (PISs) for polyethyleneglycol (PEG)-acrylate monomers. The involved mechanism is investigated by means of different spectroscopic techniques and further supported by electrochemical data. In addition, using the prepared three-component PISs, outstanding 3D patterns with spatial resolution could be obtained through direct laser write experiments. Remarkably, the resulting material displays reversible deformation that can be controlled with heating–hydration cycles due to stimuli-responsiveness of PEG chains of the polymeric network, thus opening the way for innovative smart and stimuli-responsive 4D-printed materials and soft actuators.