For generations, chemists have been taught that photochemical reactions will be most efficient when irradiated at the absorption maximum of the chromophore. But in 2017, a team led by Christopher Barner-Kowollik and Georg Gescheidt discovered that photopolymerizations occurred with much greater efficiency when irradiated at the red edge of the photoinitiator absorption spectrum. Until now, the reason for this mismatch between theory and experiment has remained unclear.

Discussing the initial results with the research community was as exciting as challenging: “The response we received when we presented the data at conferences was predictable: This cannot be correct, check your data and methods!” recalls Barner-Kowollik. Over the years the concept of action plots became widely accepted but a mechanistic explanation remained elusive. Several theories were put forward but were not generalizable.