Suppressing Excimers in H-Aggregates of Perylene Bisimide Folda-Dimer: Role of Dimer Conformation and Competing Assembly Pathways

Abstract

Long-lived excitons in H-aggregates hold great promise for efficient transport of excitation energy, provided they are not scavenged by structurallly relaxed excimers. We report solution self-assembly of a perylene bisimide (PBI) folda-dimer that exhibits two distinct kinetic stages: an initial fast assembly leads to metastable aggregates with large excimer contribution that is followed by a slower growth of stable, extended H-aggregates free of excimers. Mechanistic investigations reveal an interplay of two competing aggregation pathways, where suppression of excimers is directly linked to the crossover from an isodesmic to cooperative aggregation. How the comeptition between two self-assembly pathways is influenced by the conformational flexibility of the folda-dimer is also discussed.

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

“…Steadystate fluorescence spectra of ITNF4 and ITzNF4 films (Figure S18, Supporting Information) reveal broad, featureless emission profiles that are redshifted from the solution profiles (166 nm for ITNF4, 112 nm for ITzNF4), and consistent with excimer emission. [64,65] The presence of these features in the blends sug gests a highly crystalline acceptor morphology that is retained even in the presence of PBDBTF, consistent with the afore mentioned blend film GIWAXS results. Excimer states have been observed in other NFAs [37,64,66] and act as lowenergy trap states that inhibit charge separation of bound electron-hole pairs at the donor-acceptor interface, reducing mobilities and PCEs.…”

Safeguarding long-lived excitons from excimer traps in H-aggregated dye-assemblies

“…Recently, we reported the suppression of excimer formation in H-aggregates of a flexible PBI dimer through an interplay of competing aggregation pathways. 16 However, we found no direct evidence of the long-lived exciton. In this work, we present the first ever report of an ambient-stable, steady-state PL from long-lived excitons of a highly ordered H-aggregated PBI.…”

Cooperative Self-Assembly Driven by Multiple Noncovalent Interactions: Investigating Molecular Origin and Reassessing Characterization

“… 1 , 2 By virtue of reversibility and relaxed geometric restrictions, noncovalent interactions can drive a molecular self-assembly through multiple pathways and lead to a variety of assembled structures. 3 − 6 However, in order to maximize performance, it is critical to achieve the desired assembly with high structural order and narrow polydispersity, while also maintaining a high degree of aggregation. In recent times, programmable supramolecular polymerization strategies have fulfilled this objective using a two-pronged approach: by steering the self-assembly toward a preferred pathway 7 − 12 and ensuring that the assembled structure grows in a controlled manner.…”

“…While empirical observations suggest a correlation between multiple noncovalent interactions and cooperativity, 8 − 15 , 20 − 25 there are notable exceptions too. 6 , 23 , 26 This underscores the need for a molecular-level characterization of noncovalent interactions that could elucidate how different interactions influence each other nonadditively and could lead to the development of a rational design principle to achieve cooperativity. Crystal structure can provide valuable information about the nature of molecular packing and the interactions involved, but its applicability is severely limited in most supramolecular assemblies that are inherently inhomogeneous, and/or lack long-range structural ordering.…”