The Visible Light Challenge in Contemporary Photochemistry.

Catalyst-free and bond forming light induced reactions have seen an unprecedented renaissance in the realm of soft matter materials science due to their efficiency, spatio-temporal controllability and, sometimes, photoreversible nature. However, many of these reactions rely on the application of high energy UV light that can cause photo-degradation and is inapplicable in biological environments. If up-conversion systems or two photon processes are to be avoided, strategies for red-shifting catalyst-free ligation technology are critically required. This concept article introduces the reader to recent methods that lead to efficient, catalyst-free visible light induced ligation chemistry based on polyaromatic substituted photoreactive compounds - pyrene and anthracene - and furthermore emphasizes the broad and facile applicability of these molecules in polymeric material design. Concomitantly, we highlight that a careful action plot analysis of photochemical reactivity can provide deep insights into reactivity patterns, far beyond those suggested by the absorption spectrum. Indeed, we suggest that an action plot analysis is necessary for the evaluation any photochemical system and its response to structural chemical changes.