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

Abstract

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 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 π-extended aromatic excimers.

Striking the right balance of intermolecular coupling for high-efficiency singlet fission

“… 61 , 68 , 140 As already mentioned, the two-step dynamics we observe for the short-lived nascent triplet pairs is strikingly similar to the two-step excimer relaxation reported for several classical molecular ‘excimer’ forming materials that has also been associated with structural relaxation. 124 – 129 It is not unreasonable to conclude then that excimer relaxation, known to be detrimental to singlet fission, 61 , 68 , 90 is hindered by side chain sterics, which promote the formation of long-lived triplet pairs and thus high independent triplet yields (with a similar thesis drawn in a recent, related work 61 ).…”

“…123 The subsequent transfer of this energy to the surrounding environment can lead to a two-step formation process exactly analogous to that proposed for excimer excitons, 124 , 125 which has been attributed to intermolecular structural relaxation. 126 – 129 That is, the molecules comprising the exciton exhibit subtle changes in their relative intermolecular geometries so as to result in slight differences in their electronic structure and, in turn, transient absorption spectra. 130 , 131 …”

Charge‐Transfer Character in a Covalent Diketopyrrolopyrrole Dimer: Implications for Singlet Fission

“…We estimate that formation of the excimer is a minor pathway for TDPP2 that can form to the same degree in all solvents due to solution‐phase conformational flexibility, but has little impact on the average behavior of TDPP2 . This result is somewhat surprising, given the prevalence of excimer‐like emission in our previous TDPP thin film study . This indicates that the TDPP2 geometry favors another fluorescence‐quenching pathway, such as charge separation, in which the lifetime of the charge‐separated IP state depends on solvent polarity…”

Fluorinating π‐Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells

“…[ 64,65 ] The presence of these features in the blends suggests a highly crystalline acceptor morphology that is retained even in the presence of PBDB‐TF, consistent with the aforementioned blend film GIWAXS results. Excimer states have been observed in other NFAs [ 37,64,66 ] and act as low‐energy trap states that inhibit charge separation of bound electron–hole pairs at the donor–acceptor interface, reducing mobilities and PCEs. [ 36,67–70 ]…”