Few-cycle measurements of squaraine molecular crystals to investigate ultrafast population transfers

The influence of strong intermolecular coupling on the optical properties of organic molecular crystals is key in understanding light-matter interaction in organic semiconductors. Excited state dynamics, higher excited states beyond the reach of CW spectroscopy methods and the influence of optically dark states will be investigated using the ultrafast transient absorption setup at ELI Beamlines. Our system of interest, a class of semiconducting organic dyes called squaraines, is
particularly favorable as a model system as it has a very strong absorption in the visible to near IR range and grows many different polymorphs where the intermolecular alignment has major influence on the absorption spectrum. Many of the resulting aggregates have been thoroughly characterized by absorption and reflection microscopy, and recently the excited state dynamics of squaraines have started to gain attraction in the scientific community. In a project funded by the FWF, we are in the process of investigating the transient behavior of squaraines using transient absorption and provide a mechanistic understanding by connecting the experimental results with modern excited state simulations and suitable semi-empirical approaches. Measurements at the TA setup of ELI Beamlines will help us to shine light on fast relaxation processes, which are beyond the reach of the transient absorption setup in Graz, and allow us to connect the excited state absorption spectrum with theoretical calculations.