Characterization of Excimer Relaxation via Femtosecond Shortwave- and Mid-Infrared Spectroscopy

Excimer formation plays a significant role in trapping excitons within organic molecular solids. Covalent dimers of perylene-3,4:9,10-bis-(dicarboximide) (PDI) are useful model systems for studying these processes as their intermolecular geometries can be precisely tuned. Using femtosecond visible-pump infrared-probe (fsIR) spectroscopy in the shortwave- and mid infrared regions, we characterize two PDI dimers with a cofacial and a slip-stacked geometry that are coupled through a triptycene bridge. In the mid-infrared region, fsIR spectra for the strongly coupled dimers are highly blue-shifted compared to spectra for monomeric (l)*PDI. The perylene core stretching modes provide a directly observable probe of excimer relaxation, as they are particularly sensitive to this process, which is associated with a small blue shift of these modes in both dimers. The broad Frenkel-to-CT state electronic transition of the excimer, the edge of which has previously been detected in the NIR region, is now fully resolved to be much broader and to extend well into the shortwave infrared region for both dimers and is likely a generic feature of x-extended aromatic excimers.