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Updated: Feb 27, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
Published on: November 29, 2018
Forbidden Chemistry: Two-Photon Pathway in [2+2] Cycloaddition of Maleimides
Mikhail V Tsurkan1, Christiane Jungnickel1,2, Michael Schlierf2
1Max Bergmann Center of Biomaterials, Leibniz Institute of Polymer Research , Hohe Strasse 6, 01069 Dresden, Germany.
Abstract:
Two-photon excitation provides high spatial resolution in three dimensions of the corresponding chemical or physical processes, allowing submicrometer structuring in stereolithography and three-dimensional (3D) microfabrication. While studying two-photon structuring applications, we observed an undescribed phenomenon in photochemistry that dictates reactivity of maleimide groups in two-photon mode. A low-absorbance transition formerly ignored in classical photochemistry has been found for maleimides. This transition was assigned to symmetry-breaking donor-acceptor complex formation, which revealed a formally forbidden pathway in [2+2] cycloaddition reactions of maleimide moieties. This synthetic pathway allowed for the creation of hydrogel materials under physiological conditions at low laser excitation energy (0.1 J/cm2 at 800 nm) without the use of photoinitiators, which makes it truly two-photon click chemistry.
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