Energy Flow Dynamics within Cofacial and Slip-Stacked Perylene-3,4-dicarboximide Dimer Models of π‑Aggregates

Robust perylene-3,4-dicarboximide
(PMI) π-aggregates provide
important light-harvesting and electron–hole pair generation
advantages in organic photovoltaics and related applications, but
relatively few studies have focused on the electronic interactions
between PMI chromophores. In contrast, structure–function relationships
based on π–π stacking in the related perylene-3,4:9,10-bis­(dicarboximides)
(PDIs) have been widely investigated. The performance of both PMI
and PDI derivatives in organic devices may be limited by the formation
of low-energy excimer trap states in morphologies where interchromophore
coupling is strong. Here, five covalently bound PMI dimers with varying
degrees of electronic interaction were studied to probe the relative
chromophore orientations that lead to excimer energy trap states.
Femtosecond near-infrared transient absorption spectroscopy was used
to observe the growth of a low-energy transition at ∼1450–1520
nm characteristic of the excimer state in these covalent dimers. The
excimer-state absorption appears in ∼1 ps, followed by conformational
relaxation over 8–17 ps. The excimer state then decays in 6.9–12.8
ns, as measured by time-resolved fluorescence spectroscopy. The excimer
lifetimes reach a maximum for a slip-stacked geometry in which the
two PMI molecules are displaced along their long axes by one phenyl
group (∼4.3 Å). Additional displacement of the PMIs by
a biphenyl spacer along the long axis prevents excimer formation.
Symmetry-breaking charge transfer is not observed in any of the PMI
dimers, and only a small triplet yield (<5%) is observed for the
cofacial PMI dimers. These data provide structural insights for minimizing
excimer trap states in organic devices based on PMI derivatives.