Important processes carried out by Nature’s machinery rely on proper regulation mechanisms. To achieve such control over various functions in man-made materials and devices light offers a superior advantage as an external stimulus and beyond as energy source. Photoswitchable entities provide an ideal platform to interface light with matter and therefore the proposed research program “Light 4 Function” aims at designing and developing new functional photochromic systems.Building on the PI’s previous achievements and expertise, this broad program will be developing photochromic systems in four project areas for:i) light-gated ligation, i.e. photocontrolling reversible attachment to complementary molecules, (bio)scaffolds, and surfaces to construct “smart” tags;ii) light-controlled catalysis, i.e. phototuning the activity and selectivity in living polymerization processes to obtain remote-controlled catalysts;iii) light-gated charge transport, i.e. photocontrolling current flow in single molecular junctions as well as in thin film organic transistors to create photoadressable organic devices;iv) light-driven molecular motion, i.e. photoswitching macromolecules to generate maximum geometry changes in order to create sensitive optomechanic materials and devices.The four project areas will be supported by efforts to further develop and optimize the utilized azobenzene and diarylethene photochromic components.The photochromic systems will be designed such that they allow for optimal control over the chosen physico-chemical processes by light. Therefore, photons will be exploited to stimulate and drive molecular “action” at specific locations and/or at defined times. This superior control will enable complicated molecular processes to be orchestrated with the “flip of a light switch” and should lead to the development of light-responsive “smart” tags, catalysts, materials, and devices.