Bay-Linked Perylenediimides are Two Molecules in One: Insights from Ultrafast Spectroscopy, Temperature Dependence, and Time-Dependent Density Functional Theory Calculations

Abstract

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.

Photophysical implications of ring fusion, linker length, and twisting angle in a series of perylenediimide–thienoacene dimers

“…36 There is also precedence for this type of dual emissive behavior in twisted A-D-A complexes. 37,54 Correspondingly, the uniform emission observed in the fused dimer is likely a consequence of the locked structure imparted by the ring fusion. It is also of note that the fused CT transition has a lifetime of more than a factor of three greater than that of the non-fused CT, despite the two being very similar in energy (2.06 eV vs. 1.95 eV).…”

“…Ring-fused complexes have been shown to exhibit superior efficiency to their non-fused counterparts when tested in OPV devices. [15][16][17][18]34,35 Great strides have been made in improving device efficiency through such synthetic routes, however there have been few studies [36][37][38][39] focused on electronic processes in these increasingly complex molecules. Photoexcitation is the rst in a series of ultrafast processes that lead to exciton generation, charge separation, and charge carrier transport in photovoltaic materials, so understanding the molecular excited state/excitonic properties can be crucial for controlling photophysical processes and informing molecular design.…”

“… 36 There is also precedence for this type of dual emissive behavior in twisted A–D–A complexes. 37 , 54 …”

Hybrid Tetrameric Perylene Diimide Assemblies

“…To gain insight into the optical properties, the UV/Vis absorption and emission spectra of PDI 4 e and PDI 4 i were measured and compared to each other and reference compounds PDI 1 and PDI 2 . It has been frequently observed that the absorption profile of PDI is highly sensitive to its environment, especially the presence of folding, [38,39] or aggregates with H-or J-coupling. [40,41] First, looking at the absorption profiles in chloroform (Figure 5A) all four spectra are similar and typical of welldissolved PDI-based chromophores.…”

“…We believe that the diffuse emission of the dimer is due to either a symmetry-breaking charge-transfer process or a folded geometry, both of which have previously been observed for rigid rylene bisimide systems. [19,39] PDI 4 e has an emission maximum at 596 nm with a high energy shoulder at 560 nm. PDI 4 i has only a broad emission centered at 604 nm.…”