Topology Effects in Molecular Organic Electronic Materials: Pyrene and Azupyrene**

Abstract

Pyrene derivatives play a prominent role in organic electronic devices, including field effect transistors, light emitting diodes, and solar cells. The flexibility in the desired properties has previously been achieved by variation of substituents at the periphery of the pyrene backbone. In contrast, the influence of the topology of the central -electron system on the relevant properties such as the band gap or the fluorescence behavior has not yet been addressed. In this work, pyrene is compared with its structural isomer azupyrene, which has a -electron system with nonalternant topology. Using photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy, and other methods, it is shown that the electronic band gap of azupyrene is by 0.72 eV smaller than that of pyrene. The difference of the optical band gaps is even larger with 1.09 eV, as determined by ultraviolet-visible absorption spectroscopy. The nonalternant nature of azupyrene is also associated with a more localized charge distribution, as can be seen in 1 H and 13 C nuclear magnetic resonance shifts, as well as the C1s core-level shifts. Further insight is provided by density functional theory (DFT) calculations of the molecular properties and ab initio coupled cluster calculations of the optical transitions. The concept of aromaticity is used to interpret DFT-based structures and for the theoretical assignment of the vibrational modes of the infrared spectra, where major topology-related differences are apparent.

Topological Stone–Wales Defects Enhance Bonding and Electronic Coupling at the Graphene/Metal Interface

“…The multilayer spectra, which probe molecules not in direct contact with the metal surface, are similar for both molecules. The minor differences in the peak shapes with the slight asymmetry of the azupyrene spectrum have been explained by the different molecular topologies . The monolayer spectra of the two molecules show much larger differences, indicating their different interaction with the metal surface.…”

“…The S–W defect was chosen because it is a prototypical and ubiquitous intrinsic point defect of graphene and carbon nanotubes. − As a reference representing the topology of regular graphene, we use the benzenoid isomer of azupyrene, pyrene, which consists only of 6-membered rings (6-6-6-6 topology). In contrast, the 5-7-5-7 topology of azupyrene is classified as nonalternant and is known to strongly influence its electronic structure. , In related previous work, we compared the two-ring nonbenzenoid aromatic hydrocarbon azulene (5-7 topology) to its benzenoid isomer naphthalene (6-6 topology) regarding their interaction with Cu(111), Ag(111), and Pt(111) surfaces and found that the former binds stronger to all three surfaces. − Considering the structural analogy, we therefore proposed that nonbenzenoid defect sites in graphene may induce enhanced graphene/metal interactions. However, these previous studies were lacking in two essential points: First, the azulene motif rarely occurs as isolated defect, and thus azulene is not a representative model system.…”

Synthesis of Defective Nanographenes Containing Joined Pentagons and Heptagons

“…The precursors containing ketone structures (38 and 44) were obtained in a reaction sequence that included several main steps, such as nucleophilic addition, reduction, and cyclization. The treatment of precursors with reducing agents and then aromatization under high temperature afforded the final fully conjugated, red, platelike molecules (39) and black needle-like molecules (44). Interestingly, compound 39 exhibited a clear blueshift in the absorption spectrum compared to that of compound 5, which had a smaller conjugated network.…”

“…Azupyrene exhibits distinctive electronic properties compared to those of its alternant isomer pyrene owing to its nonhexagonal rings, such as smaller energy gap, more localized charge distribution, and a different conjugation pathway. [ 39 ] Furthermore, a modified synthetic procedure was developed by Anderson [ 40 , 41 ] and Jutz, [ 42 ] affording the product in moderate yield, and detailed studies involving theoretical calculations [ 43 , 44 , 45 ] and photochemical spectroscopies [ 39 ] elucidated the influence of the topological structure on the electronic configuration and molecular properties.…”

Self-interaction error induces spurious charge transfer artefacts in core-level simulations of x-ray photoemission and absorption spectroscopy of metal-organic interfaces

“…3 First principles simulation of core level spectroscopy has proved to be an effective tool to disentangle complex overlapping XP signatures of hybrid organic-inorganic interfaces for both monolayer 4 and multilayer thin films. 5 Recent example studies where core-level simulation was vital for the interpretation of experimental spectra include oxygen species on Pt(111), 6 and Porphin on Ag(111) and Cu(111) 7 . Klein et al have shown in a series of recent joint experimental and computational studies how XPS and Near-Edge X-ray Absorption Spectroscopy (NEXAFS) provide evidence for the differences in surface chemical bonding between Naphthalene (Nt) and its topological isomer Azulene (Az) adsorbed on Ag(111), Cu(111) and Pt(111) surfaces.…”