Watching molecular scale processes: From tailoring self-assembly with light to cooperative molecular switches Stefan Kowarik Institut für Physik, Humboldt-Universität zu Berlin Newtonstr. 15, 12489 Berlin E-mail: [email protected] X-ray measurements and optical spectroscopy are well suited to follow molecular processes, because they can be used for in situ remote observation on an atomic scale. Here we give two examples of in situ real-time studies from our work on (i) tailoring thin film growth and on (ii) light-driven molecular switches. i) Growth of molecular thin films is a complicated interplay of molecular processes and for quantifying individual growth processes real-time observation is very helpful. Importantly, from the real-time data we can infer ways to improve the growth. In one scheme we do not grow at a constant temperature but quickly modulate the temperature during deposition, so that nucleation and coalescence processes are optimised separately. In a second scheme, laser heating of certain molecular orientations in the film can also be used as a novel control parameter. We show that light can texture the crystal structure along the light polarisation even on amorphous substrates; further, light irradiation can also increase the phase purity. (ii) Molecular devices and machines demand the integration of molecular switches into hierarchical assemblies to amplify the response of the system from the molecular level to the meso- or macro-scale. Here, we demonstrate that multi-azobenzene oligomers can assemble to form robust molecular zippers and supramolecular nanofibers in which they can be switched repeatedly between the E- and Zconfiguration. We show that the intermolecular interactions can lead to cooperative switching, thereby increasing the quantum yield.