Sigurd Höger

Highly Efficient Methods for the Preparation of Shape-Persistent Macrocyclics

It has been about 25 years since Staab prepared a hexameric phenyl–ethynyl macrocycle by the statistical cyclization of the copper salt of m-iodo-phenylacetylene in 4.6% yield. Since that time, different methodologies have been investigated that allow not only the preparation of selectively functionalized structures, but also their formation in high yields. The repetitive formation of precursors followed by an intramolecular cyclization is only one approach to these structures. Alternatives include the use of covalently or noncovalently bound templates, as well as cyclization under thermodynamic control.

Shape-persistent macrocycles with intraannular alkyl groups: some structural limits of discotic liquid crystals with an inverted structure

The synthesis and thermal properties of new shape-persistent macrocycles of different sizes decorated with intraannular alkyl chains are described. The alkyl chain length is in all cases sufficient to cross the rings and to fill their interior completely. The investigation of the thermal behavior has shown that the smaller cycles do not exhibit thermotropic mesophases. Single crystal x-ray analysis indicates that the anisotropy in these compounds is too small to describe them as plates rather than spheres. For the larger macrocycles it is shown that longer adaptable substituents decrease the phase transition temperatures compared to previously described structures.

2D Assembly of Metallacycles on HOPG by Shape-Persistent Macrocycle Templates

The synthesis and scanning tunneling microscopy (STM) investigations of shape-persistent arylene-ethynylene-butadiynylene macrocycles along with their codeposites with metallacycles are reported. 2D ordered arrays of macrocycles and macrocycle/metallacycle architectures (1:1) have been obtained on HOPG by self-assembly under ambient conditions. It is found that the ordered macrocycle array acts as a template for the deposition of the adlayer molecules. For each underlying macrocycle, one metallacycle has been detected. The unit-cell data of both, the macrocycles and their codeposites, show that the structural information of the macrocycle layer is perfectly transformed to the guest molecules. A rather unexpected observation is that the present compound could not be coadsorbed with C(60), indicating that only a minor change in the structure of the macrocycle has a dramatic effect on the ability of the monolayer to bind additional guest molecules.

Fluctuating exciton localization in giant π -conjugated spoked-wheel macrocycles

Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended π conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment.

Empty helical nanochannels with adjustable order from low-symmetry macrocycles.

Natural channel-forming structures are mandatory for connecting different compartments within a living organism. For instance, transmembrane proteins function as ion channels, transporters, or antibiotics. Biomacromolecules that are formed during evolution self-assemble into tubular structures with precisely defined positions of functional groups. The stimuli-responsive activity of these molecules has inspired the search for artificial channel-forming structures that can mimic the functionality of the natural systems. Artificial channel systems may even include new functionalities in advanced chemical applications. Several attempts, including templating methods, have been reported for the de novo design of pore-forming structures that are stable both in solution and in the bulk state. In particular, macrocycles have an attractive topology for the formation of supramolecular channels if they organize in a columnar mesophase with close packing of successive rings. If properly designed, a channel is created with tight walls that do not allow the penetration of small molecules. In contrast to macrocycles that are held together by strong intermolecular forces, such as hydrogen bonds in cyclopeptides or cyclosaccharides, the increased mobility in the liquid-crystalline (LC) phase allows for self-healing and orientation of the channels by external forces (shear, electromagnetic fields, surface properties, etc.). When the channels are appropriately functionalized, the inclusion and manipulation of nano-objects becomes feasible. Columnar mesophases have indeed been found in macrocyclic polyamines. However, because of their flexibility, the macrocyclic rings assume a folded conformation and stable phases with large open voids have not been reported to date. This problem might be overcome by using shapepersistent macrocycles, and columnar liquid-crystalline compounds based on cyclic phenylene and phenylene–ethynylene oligomers with inner diameters of up to 1 nm, as deduced from X-ray diffraction studies (XRD), have been reported (Figure 1a). Powder XRD cannot provide details about the packing of the macrocycles on the molecular level, 12c] whereas solid-state NMR spectroscopy can provide this information with the help of quantum-chemical calculations. In contrast to diffraction techniques, NMR spectroscopy does not require strict periodicity and is therefore particularly suited to probe the local structure in LC phases. Moreover, NMR spectroscopy can be used to reveal the presence of guest molecules inside the channels, including back-folded side chains. Herein, we describe two phenylene–ethynylene–butydiynylene macrocycles 1a and 1b (Figure 1b), each of which contains two benzo[1,2-b:4,3-b’]dithiophene units that include a nanoscale interior with a diameter as large as approximately 1.3 nm (Figure 1a). At the expense of symmetry, we have introduced groups with different electron affinities. Both macrocycles 1a and 1 b were obtained by the statistical oxidative Glaser coupling of the appropriate “half-rings” under Pd/Cu catalysis, and were obtained in yields of 33% (1a) and 50% (1 b) after purification by recycling gel-permeation chromatography (recGPC) using THF as eluent, and subsequent precipitation from methanol and drying under vacuum. The compounds were obtained as slightly yellow powders that do not contain residual solvents as shown by NMR spectroscopy of solutions in dichloromethane (see the Supporting Information). Upon heating above room temperature, the compounds become waxy materials that are birefringent under optical microscopy (crossed polarizers). Differential scanning calorimetry (DSC) investigations (2nd heating; 10 8C min ) showed reversible endothermic transitions for both compounds, thus indicating different LC phases over a broad temperature range (1a : 22 8C (138.9 kJ mol ), 109 8C (85.2 kJmol ), 151 8C (2.8 kJ mol ); 1b : 33 8C (116.6 kJmol ), 160 8C (2.6 kJ mol )). The type of LC phases formed and the lattice parameters were determined by XRD. Upon cooling from the isotropic [*] Dr. M. Fritzsche, Dr. G. Richardt, Prof. Dr. S. H ger Kekul -Institut f r Organische Chemie und Biochemie Rheinische Friedrich-Wilhelms-Universit t Bonn Gerhard-Domagk-Str. 1, 53121 Bonn (Germany) Fax: (+ 49)228-73 5662 E-mail: hoeger@uni-bonn.de

Metal‐Free OLED Triplet Emitters by Side‐Stepping Kasha’s Rule

With organic light-emitting diodes (OLEDs) emerging in ever more applications, such as smart phones, televisions, and lighting, it is easy to forget that the present technology is based on a brilliantly simple patch to an inherent problem of fluorescent hydrocarbons: three quarters of the electrically generated energy is dissipated as heat by triplet excitons. Radiative decay from the triplet state via phosphorescence is generally very weak, and has only been resolved in transient spectroscopy at low temperatures in select organic semiconductors. The solution to this problem has been to incorporate metal–organic emitters in OLEDs, which mix spin by enhancing intersystem crossing through spin-orbit coupling: the heavy-atom effect. As this approach relies on the longevity of triplet excitons and the associated diffusion lengths, it is highly effective: in a suitably homogeneous environment even ppm concentrations of covalently bound metal atoms are sufficient to activate electrophosphorescence. The second conceivable approach to harvesting energy from triplets is based on endothermic conversion to a fluorescent singlet by reverse intersystem crossing. This method necessitates control not only over spin–orbit coupling, requiring a heavy atom or a carefully engineered charge-transfer state, but also over the singlet–triplet exchange gap, which can be tuned by excitonic confinement. Although progress has been made recently, conceptually it parallels the former approach: all excitations are converted to either triplets or singlets, thereby losing information on the underlying spin correlations of charge carriers. Evidence is emerging, however, that spin correlations in excitonic electron–hole precursor pairs can be used for exquisitely sensitive measurements of magnetic fields and possibly even for quantum coherence phenomenology, with analogies to avian radical-pair photomagnetosensory processes. To quantify such spin correlations, it is desirable to develop materials without heavy-atom spin mixing that show both intrinsic fluorescence and phosphorescence. The third approach to triplet harvesting has not been explored previously: tuning spin–orbit coupling without heavy atoms such that non-radiative internal conversion from the triplet excited state to the singlet ground state is suppressed and phosphorescence is the only remaining relaxation mechanism. Even in low-atomic-order-number compounds such as hydrocarbons, the orbital component of the wavefunction can give rise to substantial magnetic moments, leading to non-negligible spin–orbit energy terms. The effect is well-studied in carbon nanotubes and graphene, where zero-field splitting correlates with nanoscale curvature. For molecular materials, orbital symmetry can induce unusual spin–orbital coupling effects, such as in pyrazine, which shows direct singlet–triplet absorption. Polycyclic aromatic hydrocarbons, such as triphenylenes or annulated compounds like phenazine, are classic metal-free materials that are known to exhibit substantial phosphorescence yields. Although triphenylenes have previously been explored both as emitters and as charge-transporting materials in OLEDs, there are no reports of direct electrophosphorescence, despite renewed interest in room-temperature organic phosphors. Herein, we demonstrate the feasibility of creating a triplet relaxation bottleneck to internal conversion that is so effective that radiative emission can even arise from higher-lying triplets in electroluminescence (EL), bypassing Kasha s rule of internal conversion. Figure 1a shows the two materials developed, a thiophene-decorated phenazine 1; and 1 linked to a triphenylene block in 2. Details of synthesis and characterization of the materials, and fabrication and performance of the OLEDs, are given in the Supporting Information. To construct OLEDs, we have to prevent concentration quenching, which is particularly strong for long-lived triplets. Therefore a dense film of the compounds cannot be used. The emitters are dispersed in a conducting matrix, poly(9-vinylcarbazole) (PVK). This material has a large optical gap, so that suitable electronand hole-transporting moieties have to be included: 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole (PBD) and N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzi[*] P. Klemm, S. Lautenschlager, A. Schmid, Dr. S. Bange, Prof. Dr. J. M. Lupton Institut f r Angewandte und Experimentelle Physik Universit t Regensburg Universit tsstrasse 31, 93053 Regensburg (Germany) E-mail: john.lupton@ur.de

Shape-persistent rings and wheels

Shape-persistent macrocycles based on the arylene-ethynylene backbone are synthetically challenging targets that can be obtained in good to high yields either by statistical or by template-supported oxidative Glaser coupling of the corresponding bisacetylenes. The macrocycles can be adsorbed on highly oriented pyrolytic graphite (HOPG) to form well-ordered monolayers as visualized by scanning tunneling microscopy (STM). Moreover, bithiophene-containing macrocycles are able to bind fullerenes at the electron-rich sites of the rings. One-dimensional tubular structures based on shape-persistent macrocycles can also be obtained by oxidative acetylene coupling. These contain intraannularly bound conjugated polymers and represent bichromophoric systems that allow an accumulation of excitation energy on the conjugated core. In the field of defined 2D objects we describe molecular spoked wheels with a lateral expansion of more than 5 nm. These compounds are highly rigid. In order to provide sufficient solubility, they contain peripheral substituents and additional substituents orthogonal to the plane of the molecules.

Large All-Hydrocarbon Spoked Wheels of High Symmetry: Modular Synthesis, Photophysical Properties, and Surface Assembly

In a convergent modular synthesis, a very efficient pathway to shape-persistent molecular spoked wheels has been developed and applied according to the covalent-template concept. The structurally defined two-dimensional (2D) oligo(phenylene-ethynylene-butadiynylene)s (OPEBs) presented here are about 8 nm sized hydrocarbons of high symmetry. 48 alkyl chains attached to the molecular plane (hexyl and hexadecyl, respectively) guarantee a high solubility of the compounds. The structure and uniformity of these defined, stable, D(6h) symmetrical compounds is verified by MALDI-MS, GPC analysis, and high-temperature (HT) (1)H and (13)C NMR. Detailed photophysical measurements of nonaggregated molecules in solution (as confirmed by dynamic light scattering (DLS)) focus on the identification of chromophores by comparison with suitable model compounds. Moreover, time-resolved measurements including fluorescence lifetime and depolarization support the chromophore assignment and reveal the occurrence of intramolecular energy transfer. Scanning tunneling microscope (STM) characterization at the solid/liquid interface demonstrates the efficient self-assembly of the OPEBs into hexagonal 2D crystalline layers with a periodicity determined by both the size of the OPEB backbone and the length of peripheral side chains. Atomic force microscope (AFM) studies show a very different assembly behavior of the two spoked wheel molecules, on both graphite and mica. While the hexyl-substituted wheel can form stacked superstructures, hexadecyl groups prevent any ordering in the film aside from the monolayer directly in contact with the surface.

Synthesis and Properties of Shape-Persistent Macrocyclic Amphiphiles with Switchable Amphiphilic Portions

Amphiphilic cyclic structures like 1, isomers of 1, and rings with enlarged amphiphilic units are available by the intermolecular Glaser coupling of the corresponding bisacetylenes, prepared by a flexible repetitive synthesis.

TEMPLATE-DIRECTED SYNTHESIS OF SHAPE-PERSISTENT MACROCYCLIC AMPHIPHILES WITH CONVERGENTLY ARRANGED FUNCTIONALITIES

Macrocyclic structures such as 1 and rings with different polar substituents pointing inwards can be prepared by the intramolecular Glaser coupling of tetraacetylenes held covalently in position around a temporary template.