In dye-sensitized solar cells (DSSCs) and in photo-catalytic devices designed for hydrogen production of CO₂ reduction, light triggers ultrafast molecular processes, such as electron, energy transfer or singlet fission. Since these processes are at the heart of the function of the devices and of their efficiencies, the design of new molecular photo-sensitizers or catalysts can be optimized rationally if these processes are monitored by ultrafast spectroscopy. [more]