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: stefan.kowarik@physik.hu-berlin.de
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.