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Self-assembly of multi-stranded perylene dye J-aggregates in columnar liquid-crystalline phases
(2018)
Many discoid dyes self-assemble into columnar liquid-crystalline (LC) phases with packing arrangements that are undesired for photonic applications due to H-type exciton coupling. Here, we report a series of crystalline and LC perylene bisimides (PBIs) self-assembling into single or multi-stranded (two, three, and four strands) aggregates with predominant J-type exciton coupling. These differences in the supramolecular packing and optical properties are achieved by molecular design variations of tetra-bay phenoxy-dendronized PBIs with two N–H groups at the imide positions. The self-assembly is driven by hydrogen bonding, slipped π–π stacking, nanosegregation, and steric requirements of the peripheral building blocks. We could determine the impact of the packing motifs on the spectroscopic properties and demonstrate different J- and H-type coupling contributions between the chromophores. Our findings on structure–property relationships and strong J-couplings in bulk LC materials open a new avenue in the molecular engineering of PBI J-aggregates with prospective applications in photonics.
AbstractWater oxidation catalysis is a key step for sustainable fuel production by water splitting into hydrogen and oxygen. The synthesis of a novel coordination oligomer based on four Ru(bda) (bda = 2,2′‐bipyridine‐6,6′‐dicarboxylate) centers, three 4,4′‐bipyridine (4,4′‐bpy) linkers, and two 4‐picoline (4‐pic) end caps is reported. The monodispersity of this tetranuclear compound is characterized by NMR techniques. Heterogeneous electrochemical water oxidation after immobilization on multi‐walled carbon nanotubes (MWCNTs) shows catalytic performance unprecedented for this compound class, with a turnover frequency (TOF) of 133 s\(^{−1}\) and a turnover number (TON) of 4.89 × 10\(^6\), at a current density of 43.8 mA cm\(^{−2}\) and a potential of 1.45 V versus normal hydrogen electrode (NHE).
Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one. Exemplarily, we monitor exciton diffusion by annihilation in a perylene bisimide-based J-aggregate. We determine quantitatively the exciton diffusion constant from exciton–exciton-interaction 2D spectra and reconstruct the annihilation-free dynamics for large pump powers. The latter enables for ultrafast spectroscopy at much higher intensities than conventionally possible and thus improves signal-to-noise ratios for multichromophore systems; the former recovers spatio–temporal dynamics for a broad range of phenomena in which exciton interactions are present.
Understanding relationships between microstructure and electrical transport is an important goal for the materials science of organic semiconductors. Combining high-resolution surface potential mapping by scanning Kelvin probe microscopy (SKPM) with systematic field effect transport measurements, we show that step edges can trap electrons on the surfaces of single crystal organic semiconductors. n-type organic semiconductor crystals exhibiting positive step edge surface potentials display threshold voltages that increase and carrier mobilities that decrease with increasing step density, characteristic of trapping, whereas crystals that do not have positive step edge surface potentials do not have strongly step density dependent transport. A device model and microelectrostatics calculations suggest that trapping can be intrinsic to step edges for crystals of molecules with polar substituents. The results provide a unique example of a specific microstructure–charge trapping relationship and highlight the utility of surface potential imaging in combination with transport measurements as a productive strategy for uncovering microscopic structure–property relationships in organic semiconductors.