Strategies for the design and construction of non-linear, 2D and 3D conjugated macromolecules are presented in this critical review. The materials, termed here as star-shaped structures, feature a core unit which may or may not provide conjugated links between arms that radiate like spokes from a central axle. The arms of the macromolecules consist of linear oligomers or irregular conjugated chains lacking a formal repeat unit. The cores range from simple atoms to single or fused aromatic units and can provide a high level of symmetry to the overall structure. The physical properties of the star-shaped materials can be markedly different to their simple, linear conjugated analogues. These differences are highlighted and we report on anomalies in absorption/emission characteristics, electronic energy levels, thermal properties and morphology of thin films. We provide numerous examples for the application of star-shaped conjugated macromolecules in organic semiconductor devices; a comparison of their device performance with those comprising analogous linear systems provides clear evidence that the star-shaped compounds are an important class of material in organic electronics. Moreover, these structures are monodisperse, well-defined, discrete molecules with 100% synthetic reproducibility, and possess high purity and excellent solubility in common organic solvents. They feature many of the attributes of plastic materials (good film-forming properties, thermal stability, flexibility) and are therefore extremely attractive alternatives to conjugated polymers (210 references).

Star-shaped π-conjugated oligomers and their applications in organic electronics and photonics

3. 1 f 3 N-Branched 6390 3.1. 1 f 3 N-Branched, Alkyl Connectivity 6390 4. 1 f 3 P-Branched 6392 4.1. 1 f 3 P-Branched, Alkyl Connectivity 6392 5. 1 f 3 Si-Branched 6394 5.1. 1 f 3 Si-Branched, C2 Connectivity 6394 5.2. 1 f 3 Si-Branched, Vinyl Connectivity 6398 5.3. 1 f 3 Si-Branched, C3 Connectivity 6399 5.4. 1f 3 Si-Branched, (CH2)2S(CH2)3 Connectivity 6405 5.5. 1 f 3 Si-Branched, 1,4-(C6H4) Connectivity 6405 5.6. 1 f 3 Si-Branched, Si Connectivity 6405 5.7. 1 f 3 Si-Branched, S/Se/Te Connectivity 6406 5.8. 1 f 3 Si(O)-Branched, Alkyl Connectivity 6406 5.9. 1 f 3 Si(O)-Branched, Si(O) Connectivity 6409 6. 1 f 3 B-Branched, S Connectivity 6410 7. 1 f 3 Ge-Branched 6410 8. 1 f 3 Sn-Branched 6411 9. 1 f 3 Aryl-Branched 6412 9.1. 1 f 3 (3,4,5-)Aryl-Branched, Ether-Connectivity Dendrons 6412

Dendrimers derived from 1 → 3 branching motifs.

Polyfluorene (PF)-based light-emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low-energy emission band (at ∼ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low-energy emission band originates from fluorenone defects which are introduced by photo-oxidization, thermal oxidation, or during device fabrication. X-ray photo-emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron-transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.

Stabilized Blue Emission from Polyfluorene-Based Light-Emitting Diodes: Elimination of Fluorenone Defects

Enhanced Solid‐State Luminescence and Low‐Threshold Lasing from Starburst Macromolecular Materials

Two 2D supramolecular structures of macrocycle 1 and 1/C60 have been obtained on HOPG by self-assembly under ambient conditions and investigated by high-resolution STM. The monolayers of 1 are characterized by structures with perfect ordering over relatively large areas. In the case of 1/C60, the size of the macrocycle 1 and the presence of two individual bithiophene units per ring lead in the final superstructure to a 1:2 stoichiometry. The fullerenes are not trapped at the graphite surface inside the macrocyclic holes but are located around the periphery of the bithiophene units. This clearly shows that the donor−acceptor interaction between C60 and the electron-rich units of the ring is the dominant factor for the structure formation.

2D Supramolecular Structures of a Shape-Persistent Macrocycle and Co-deposition with Fullerene on HOPG

Oligofluorenes (a trimer, pentamer, and heptamer) with one fluorenone unit in the center (OFnK: n=3, 5, or 7) were synthesized and used as models to understand the origin of the low-energy emission band in the photoluminescence and electroluminescence spectra of some polyfluorenes. All compounds form glasses with T(g) at 30 degrees C (OF3 K), 50 degrees C (OF5 K) and 57 degrees C (OF7 K). Oligomers OF5 K and OF7 K exhibit smectic liquid crystal phases that undergo transition to isotropic melts at 107 and 205 degrees C, respectively. Oligomer OF5 K could be obtained in form of single crystals. The X-ray structure analysis revealed the helical nature of the molecule and a helix reversal defect located at the central fluorenone unit. The packing pattern precludes formation of excimers. Electrochemical properties were investigated by cyclic voltammetry. The ionization potential (I(p)) and electron affinity (E(a)) were calculated from these data. Studies of the photophysical properties of OFnK in solution and thin film by steady-state and time-resolved fluorescence spectroscopic measurements suggest efficient funneling of excitation energy from the photoexcited fluorene segments to the low-energy fluorenone sites by both intra- and intermolecular hopping events whereby they give rise to green emission. Intermolecular energy transfer was investigated by using a model system composed of a highly defect free polyfluorene PF2/6 doped by OFnK. Forster-type energy transfer takes place from PF2/6 to OFnK. The energy-transfer efficiency increases predictably with increasing concentration of OFnK.

Monodisperse Oligofluorenes with Keto Defect as Models to Investigate the Origin of Green Emission From Polyfluorenes: Synthesis, Self-Assembly, and Photophysical Properties

A group of fluorene-based polymers, PF-1SOR and PF-2SOR, were synthesized and characterized as blue light-emitting materials. PF-1SOR and PF-2SOR displayed nematic liquid crystalline mesophase in films cast from solution. Compared with conventional polyfluorene, PF-1SOR and PF-2SOR display blue-shifted UV absorption and structureless blue fluorescence. The photoluminescence spectra of PF-1SOR and PF-2SOR were found insensitive against thermal treatment in air up to 200 degrees C and the blue electroluminescence in their light-emitting devices was independent of the driving voltage. Compared to the conventional polyfluorenes, the improved spectral stability of these polymers is attributed to the anti-oxidization effect of (3,5-di(tert-butyl)phenoxy)sulfonyl side groups attached to the backbone.

Substituent effect to prevent autoxidation and improve spectral stability in blue light-emitting polyfluorenes.

The synthesis, properties, and conformation of shape-persistent macrocycles with extraannular oligo-alkyl groups and an intraannular bis(hydroxymethyl)biphenylene diester are investigated. The key step in the preparation is the template-supported Glaser coupling of rigid bisacetylenes which allows obtaining the cyclic product in 81% yield. Contrary to the initial expectations, the compounds do not exhibit liquid crystalline phases. This could be explained by a nonplanar conformation of the molecules because the biphenylene bridge is longer than the diameter of the rings. On the other hand, scanning tunneling microscopy investigations indicate that the intraannular biphenylene bridge can be located in the same plane as the aromatic backbone of the macrocycles on graphite. Modeling involving molecular mechanics and molecular dynamics simulations unraveled this apparent inconsistency by showing that the molecules adopt a nonplanar conformation in the absence of the solid support whereas planarization takes p...

Covalent template approach toward functionalized oligo-alkyl-substituted shape-persistent macrocycles: Synthesis and properties of rings with a loop

Self-assembled monolayers of shape-persistent macrocycles have been adsorbed on highly oriented pyrolytic graphite and Au(111) substrates. Their structure and binding characteristics have been investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy experiments. Special functionalization of the macrocycles for the first time has enabled their self-assembling and binding on a metal substrate and opened a new route for new functional nanostructures.

Andreas Ziegler

Papers

Monodisperse Oligofluorenes with Keto Defect as Models to Investigate the Origin of Green Emission From Polyfluorenes: Synthesis, Self-Assembly, and Photophysical Properties

Substituent effect to prevent autoxidation and improve spectral stability in blue light-emitting polyfluorenes.

Covalent template approach toward functionalized oligo-alkyl-substituted shape-persistent macrocycles: Synthesis and properties of rings with a loop

SAMs of Shape-Persistent Macrocycles: Structure and Binding on HOPG and Au(111)