This article describes bent π-conjugated systems composed of alternating o-phenylene and Z-vinylene units. all-Z-[n]Benzo[4n]annulenes are higher homologues of dibenzocyclooctatetraene (DBCOT) with a concave π system, and attempts were made to convert [20]- and [24]annulenes having partial belt structures of fullerenes and carbon nanotubes into [10]- and [12]phenacenes. A bent π-conjugated system composed of two DBCOT units showed dynamic syn–anti equilibrium in solution and behaved as dynamic molecular tweezers (DMTs). The syn isomers of the DMTs formed blue charge-transfer complexes with DDQ in solution, and this complexation and color change were applied to thermochromism. Furthermore, DMTs having two CN groups on each COT ring exhibited crystalline-state emission in the presence of solvent molecules, although almost no emission was observed in solution or the solid state. Based on this crystalline-state emission, a vapochromic system was established. The multifunctional properties of DMTs caused by the flexibility of the nonplanar π system are summarized.

We have investigated the photophysical properties of star-shaped oligothiophenes with three terthiophene arms (meta to each other, S3) or six terthiophene arms (ortho-, meta-, and para-arranged, S6) connected to an ethynylbenzene core to elucidate the relationship between their molecular structure and electronic properties by using a combination of ensemble and single-molecule spectroscopic techniques. We postulate two different conformations for molecules S3 and S6 on the basis of the X-ray structure of hexakis(5-hexyl-2-thienlyethynyl)benzene and suggest the coexistence of these conformers by using spectroscopic methods. From the steady-state spectroscopic data of compound S6, we show that the exciton is delocalized over the core structure, but that the meta-linkage in compound S3 prevents the electronic communication between the arms. However, in single-molecule spectroscopic measurements, we observed that some molecules of compound S3 showed long fluorescence lifetimes (about 1.4 ns) in the fluorescence-intensity trajectories, which indicated that π electrons were delocalized along the meta linker. Based on these observations, we suggest that the delocalized exciton is intensely sensitive towards the dihedral angle between the core and the adjacent thiophene ring, as well as to the substituted position of the terthiophene arms. Our results highlight that the fluorescence lifetimes of compounds S3 and S6 are strongly correlated with the spatial location of their excitons, which is mainly affected by their conformation, that is, whether the innermost thiophene rings are facing each other or not. More interestingly, we observed that the difference between the degrees of ring-torsional flexibility of compounds S3 and S6 results in their sharply contrasting fluorescence properties, such as a change in fluorescence intensity as a function of temperature.

One of the most important objectives in materials, chemical, and physical sciences is the creation of large conjugated macrocycles with well-defined shapes, since such molecules are not only theoretically and experimentally interesting but also have potential applications in nanotechnology. Fully unsaturated macrocycles are regarded as models for infinitely conjugated π systems with inner cavities, and exhibit unusual optical and magnetic behavior. Macrocycles have interior and exterior sites, and site-specific substitution at both or either site can afford attractive structures, such as 1D, 2D, and 3D supramolecular nanostructures. These nanostructures could be controlled through the use of π-extended large macrocycles by a bottom-up strategy. Numerous shape-persistent π-conjugated macrocycles have been synthesized, but only a few are on the nanoscale. This Review focuses on nanosized π-conjugated macrocycles (>1 nm diameter) and giant macrocycles (>2 nm diameter), and summarizes their syntheses and properties.

Eines der wichtigsten Ziele in den Materialwissenschaften, der Chemie und der Physik ist die Herstellung von großen konjugierten Makrocyclen genau definierter Form, da solche Moleküle nicht nur theoretisch und experimentell interessant sind, sondern auch potenzielle Anwendungen in der Nanotechnologie haben. Vollständig ungesättigte Makrocyclen werden als Modelle für vollständig konjugierte π-Systeme mit Hohlräumen herangezogen. Solche konjugierten π-Systeme zeigen aufgrund ihrer cyclischen Konjugation ein ungewöhnliches optisches und magnetisches Verhalten. Makrocyclen haben nach innen und nach außen gerichtete Bindungsstellen, und eine positionsspezifische Substitution an beiden oder nur einer der beiden Bindungsstellen kann attraktive Strukturen zugänglich machen. Von besonderer Bedeutung sind ein-, zwei- und dreidimensionale supramolekulare Nanostrukturen. Um die Bildung dieser Strukturen kontrollieren zu können, bietet sich die Bottom-Up-Synthese ausgehend von π-expandierten Makrocyclen an. Zahlreiche formstabile π-konjugierte Makrocyclen wurden synthetisiert, davon aber nur eine begrenzte Zahl an nanoskaligen Vertretern. In diesem Aufsatz konzentrieren wir uns auf nanometergroße π-konjugierte Makrocyclen (>1 nm Durchmesser) sowie Riesenmakrocyclen (>2 nm Durchmesser) und fassen deren Synthesen und Eigenschaften zusammen.

Novel dynamic molecular tweezers (DMTs) 3 a, 3 b, 4 a, 4 b, and 5 b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units, were designed and synthesized. The cyclooctatetraene (COT) rings of these DMTs readily invert in solution, and the molecular structure shows rigid syn and anti forms in an equilibrium mixture in solution. The syn and anti conformers can be observed by NMR. The isomerization barriers of 3 a, 3 b, 4 a, 4 b, and 5 b are in the range of 16.5–21.3 kcal mol⁻¹, depending on steric repulsion between substituents of the COT rings and protons of the central benzene ring. These DMTs form complexes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 1,2,4,5-tetracyano-benzene (TCNB) in solution and in the solid state. The binding abilities of these DMTs increase with electron-donating substituents on COT, which increase the electron densities of the cavity of the syn form, as supported by theoretical calculations. In addition, elongation of the terminal alkoxy chains of the DMTs was found to cause the enhancement of van der Waals contact with guest molecules. Therefore, 5 b, which has CH₂OMe groups on the COT rings and longer ethoxy groups on the terminal benzene rings, showed the highest electron density of the cavity and hence the highest binding ability with the electron-deficient guest molecules. Interestingly, solutions of 3 b, 4 b, and 5 b show thermochromism in the presence of DDQ. A solution of 3 b or 4 b with DDQ in CHCl₃ is green due to charge-transfer interaction at room temperature and the color changes from green to yellow upon heating to 60 °C and from green to blue upon cooling to −40 °C, whereas the high complexation ability of 5 b with DDQ only shows a change in the shade of blue.

Tris(tetrathiafulvaleno)dodecadehydro[18]annulene-hexaesters have a multi-functionality that is very sensitive to small differences in the ester side-chain. Self-aggregation of the [18]annulenes in amphiphilic media such as THF–H₂O (v/v, 1:1) either produce a fibrous structure or result in temperature hysteresis of the color and ¹H NMR signals. This temperature hysteresis in solution is due to both strong self-aggregation behavior and unique cluster formation in a binary solution of THF and water.