Bay-linked diperylenediimide (di-PDI)
molecules are finding increasing
use in organic electronics because of their steric hindrance that
“twists” the two monomer units relative to one another,
decreasing molecular aggregation. In this paper, we explore the electronic
spectroscopy and ultrafast dynamics of the singly linked β-β-S-di-PDI (2,9′-di(undecan-5-yl)-2′,9-di(undecan-6-yl)-[5,5′-bianthra[2,1,9-def:6,5,10-d′e′f′]diisoquinolin]-1,1′,3,3′,8,8′,10,10′(2H,2′H,9H,9′H)-octaone). Excitation–emission spectroscopy reveals
two distinct emitting species, which are further characterized by
time-dependent density functional theory (TD-DFT), demonstrating that
the bay-linked PDI dimers exist in two geometrical conformations.
These conformations are an “open” geometry, where the
two monomer subunits are oriented nearly at right angles, giving them
more J-like coupling, and a “closed” geometry, in which
the two monomer subunits are nearly π-stacked, resulting in
a more H-like coupling. Given the extent of through-space and through-bond
coupling, however, neither di-PDI conformer can be well described
simply in terms of independently coupled monomers; instead, a full
quantum chemistry description is required to understand the electronic
structure of this molecule. Temperature-dependent experiments and
the TD-DFT calculations indicate that the “closed” conformer
is ∼70 meV more stable than the “open” conformer,
so that both conformers are important to the behavior of the molecule
at room temperature and above. We use a combination of steady-state
and femtosecond transient absorption and emission spectroscopies to
globally fit the multiple electronic transitions underlying the spectra
of both the “closed” and “open” conformers,
which agree well with the TD-DFT calculations. The fact that di-PDI
molecules are molecular species that adopt two distinct quasi-independent
chemical identities has important ramifications for charge trapping
and mobility in the organic electronic devices employing these materials.
