Synthesis, Structures, and Properties of Highly Strained Cyclophenylene–Ethynylenes with Axial and Helical Chirality

Polycationic Open-Shell Cyclophanes: Synthesis of Electron-Rich Chiral Macrocycles, and Redox-Dependent Electronic States

π-Conjugated chiral nanorings with intriguing electronic structures and chiroptical properties have attracted considerable interests in synthetic chemistry and materials science. We present the design principles to access new chiral macrocycles (1 and 2) that are essentially built on the key components of main-group electron-donating carbazolyl moieties or the π-expanded aza[7]helicenes. Both macrocycles show the unique molecular conformations with a (quasi) figure-of-eight topology as a result of the conjugation patterns of 2,2',7,7'-spirobifluorenyl in 1 and triarylamine-coupled aza[7]helicene-based building blocks in 2. This electronic nature of redox-active, carbazole-rich backbones enabled these macrocycles to be readily oxidized chemically and electrochemically, leading to the sequential production of a series of positively charged polycationic open-shell cyclophanes. Their redox-dependent electronic states of the resulting multispin polyradicals have been characterized by VT-ESR, UV/Vis-NIR absorption and spectroelectrochemical measurements. The singlet (ΔES-T=-1.29 kcal mol-1) and a nearly degenerate singlet-triplet ground state (ΔES-T(calcd)=-0.15 kcal mol-1 and ΔES-T(exp)=0.01 kcal mol-1) were proved for diradical dications 12+2⋅ and 22+2⋅, respectively. Our work provides an experimental proof for the construction of electron-donating new chiral nanorings, and more importantly for highly charged polyradicals with potential applications in chirospintronics and organic conductors.

Di-2,7-pyrenidecaphyrin(1.1.0.0.0.1.1.0.0.0) and Its Bis-Organopalladium Complexes: Synthesis and Chiroptical Properties

A non-aromatic expanded carbaporphyrinoid, incorporating two built-in 2,7-pyrenylene moieties was synthesized. The intrinsically labile structure was demonstrated by proton-triggered conformational changes between the figure-of-eight and quasi-Möbius conformers. Upon treatment with Pd(OAc)₂ , the reaction produces two bis-Pd^II complexes with distinct coordination modes. Metal coordination serves to fix the macrocyclic frameworks with the net result that both bis-Pd^II complexes could be resolved by high performance liquid chromatography (HPLC) on a chiral stationary phase. The isolated enantiomers showed persistent chiroptical properties as evidenced by the intense response in the circular dichroism (CD) spectra and the record high absorption dissymmetry factors (g_abs of up to 0.038) seen in the near-infrared spectral region. Moreover, the mutual interconversion of these two Pd^II complexes was found to be stereospecific and to favor the more stable isomers under weakly acidic conditions.

Circularly Polarized Luminescence in a Möbius Helicene Carbon Nanohoop**

We present the first helicene carbon nanoohop that integrates a [6]helicene into [7]cycloparaphenylene. The [6]helicene endows the helicene carbon nanohoop with chiroptical properties and configurational stability typical for higher helicenes, while the radially conjugated seven para‐phenylenes largely determine the optoelectronic properties. The structure of the helicene carbon nanoohop was unambiguously characterized by NMR, MS and X‐ray analysis that revealed that it possesses a topology of a Möbius strip in the solid state and in solution. The chirality transfers from the [6]helicene to the para‐phenylenes and leads to a pronounced circular dichroism and bright circularly polarized luminescence, which is affected by the structural topology of the nanohoop.

Helical Oligophenylene Linked with [2.2]Paracyclophane: Stereogenic π-Conjugated Dye for Highly Emissive Chiroptical Properties

A stereogenic π-system based on dimer (2) and trimer (3) of [2.2]paracyclophane (PC) and biphenyl was prepared and its structural, photophysical, and chiroptical properties were investigated. X-ray analysis revealed that the quaterphenyl moieties in 2 adopt a double helical structure anchoring [2.2]PC from both sides. Furthermore, 3 forms a isosceles triangle structure with a large chiral cavity. A homodesmotic reaction using DFT calculations revealed that 2 has a larger strain energy than 3 owing to its highly twisted phenylene linkers. Electronic and circular dichroic (CD) spectra were recorded in CH₂ Cl₂ solution. The spectra of both 2 and 3 are similar, and their longest absorption band accompanying a remarkable Cotton effect is attributed to the transition from HOMO to LUMO, which is delocalized to the quaterphenyl moiety. These compounds exhibit fairly high fluorescence quantum yields (ϕ=0.70-0.83) and moderate dissymmetry factor (|g_CPL |=1.6×10^-3 ) in circularly polarized luminescence (CPL).

Small Figure-Eight Luminophores: Double-Twisted Tethered Cyclic Binaphthyls Boost Circularly Polarized Luminescence

Double-twisted cyclic binaphthyls, in which two naphthalenes are tethered by -O(CH₂ )_n O- linkage (n=1-3), have been synthesized. X-ray analyses and DFT calculations revealed a tightly constrained stereogenic figure-eight geometry. Tethering of two naphthalenes by short linkage forces a small dihedral angle, and the cyclic binaphthyls with short tether (n=1, 2) exhibit remarkable boosting of the g_lum value (1.0-1.6×10^-2 ) in circularly polarized luminescence (CPL) and unusual g_lum /g_abs ratios (0.93-1.3). These experimental high |g_lum | values are in accord with the results of excited state TD-DFT calculations, which show transannular interactions and that consequent extensive delocalization occurs throughout the figure-eight π-core. As a result, the present figure-eight luminophore promote the elongation of the magnetic transition dipole moment that results in significant increases in g_lum values.

Circularly Polarized Luminescence of a Stereogenic Curved Paraphenylene Anchoring a Chiral Binaphthyl in Solution and Solid State

A curved stereogenic [6]paraphenylene ([6]PP), anchoring a chiral binaphthyl scaffold at 7,7'-positions, was prepared and investigated for its properties as a solid-state circularly polarized luminescence (CPL) dye. X-ray analysis revealed a helically twisted structure of PP units induced by axial chirality of binaphthyl framework. The curved [6]PP exhibits fluorescence in powder and polymethyl methacrylate (PMMA) film as well as solution. A significant increase in quantum yield was observed for a non-fluid PMMA film owing the suppression of the molecular motion. The g_CPL values of the dye in solution and as PMMA film were almost the same (4.3-4.4×10^-3 ) and lager than that in powder. TD-DFT calculations in the excited state suggest that the exciton can be delocalized into a twisted PP unit to produce a larger magnetic transition dipole moment.