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

• Published in: Journal of the American Chemical Society Vol. 136; no. 42; pp. 14912 - 14923
• Main Authors: Lindquist, Rebecca J, Lefler, Kelly M, Brown, Kristen E, Dyar, Scott M, Margulies, Eric A, Young, Ryan M, Wasielewski, Michael R
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 22.10.2014; Amer Chemical Soc
• Subjects: absorption; biphenyl; Chemistry; Chemistry, Multidisciplinary; energy flow; fluorescence emission spectroscopy; Physical Sciences; Science & Technology; solar energy; structure-activity relationships; SINGLET EXCITON FISSION; MOLECULE LEVEL; PERYLENE-MONOIMIDE; TIME-RESOLVED EPR; PHOTOINDUCED ELECTRON-TRANSFER; EXCIMER FORMATION; TRIPLET-STATE FORMATION; PHOTOINITIATED CHARGE-TRANSPORT; CONFORMATIONAL DYNAMICS; SELF-ORGANIZATION
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja507653p

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.