Solid-State Self-Assembly: Exclusive Formation and Dynamic Interconversion of Discrete Cyclic Assemblies Based on Molecular Tweezers.

In contrast to self-assembly in solution systems, the construction of well-defined assemblies in the solid state has long been identified as a challenging task. Herein, we report the formation of tweezers-shaped molecules into various assemblies through a solid-state self-assembly strategy. The relatively flexible molecular tweezers undergo exclusive and quantitative assembly into either cyclic hexamers or a porous network through classical recrystallization or the exposure of powders to solvent vapor, despite the fact that they form only dimers in solution. The cyclic hexamers have high thermal stability and exhibit moderate solid-state fluorescence. The formation of heterologous assemblies consisting of different tweezers allows for tuning these solid-state properties of the cyclic hexamer. Furthermore, (trimethylsilyl)ethynyl-substituted tweezers demonstrate solvent-vapor-induced dynamic interconversion between the cyclic hexamer and a pseudocyclic dimer in the solid state. This assembly behavior, which has been studied extensively in solution-based supramolecular chemistry, had not been accomplished in the solid state so far.

Unusually Short H⋠⋠⋠H Contacts in Intramolecularly Cyclized Helically Fused Anthracenes.

The intramolecular coupling of dichloro-substituted helically fused anthracenes using the Yamamoto coupling yielded cyclized products with sterically congested molecular structures. The X-ray analysis and DFT calculations showed that the aromatic framework adopted a nonplanar structure with a twisted conformation about the newly formed single bond, which acts as a chiral axis. Interestingly, the X-ray structure obtained through the Hirshfeld atom refinement revealed short interatomic distances between the inner hydrogen atoms (1.648-1.692 Å), much shorter than the sum of their van der Waals radii. Owing to these unusually short contacts, the 1H NMR spectrum exhibited a significant deshielding (12.5 ppm) and a large nuclear Overhauser effect (44 %). Additionally, the IR spectrum displayed a high-frequency shift of the C-H stretching vibration. These observations, along with the noncovalent interaction plot indicative of a characteristic steric environment, strongly support the presence of steric hindrance. Moreover, dynamic NMR measurement of the mesityl-substituted derivative yielded a barrier to helical inversion of 84 kJ mol-1. The optical properties and crystal packing of the cyclized products are also reported.

Structure and Chiroptical Properties of Anthra[1,2-a]anthracene-1-yl Dimers as New Biaryls.

Dimers of anthra[1,2-a]anthracene-1-yl units and its mesityl derivative were synthesized by Ni(0)-mediated coupling of the corresponding chloro derivatives as new biaryls. The X-ray analysis and DFT calculations revealed that two polycyclic aromatic units with nonplanar deformations took a twisted conformation about the single bond as a chiral axis. Enantiomers of the nonsubstituted compound were resolved by chiral HPLC, and the enantiopure samples showed intense Cotton effects at 321 nm in the circular dichroism (CD) spectra and emission bands at 449 nm in the circularly polarized luminescence (CPL) spectra with dissymmetry factor of |glum| 3.6×10-3. The absolute stereochemistry of this biaryl was determined by the theoretical calculation of CD spectrum by the time-dependent DFT method. The barrier to enantiomerization was determined to be 108 kJ mol-1 at 298 K. The dynamic process proceeded via a stepwise mechanism involving the helical inversion of each aromatic unit and the rotation about the biaryl axis as analyzed by the DFT calculations.

Fluorescent Solvatochromic Probes for Long-Term Imaging of Lipid Order in Living Cells.

High-resolution spatio-temporal monitoring of the cell membrane lipid order provides visual insights into the complex and sophisticated systems that control cellular physiological functions. Solvatochromic fluorescent probes are highly promising noninvasive visualization tools for identifying the ordering of the microenvironment of plasma membrane microdomains. However, conventional probes, although capable of structural analysis, lack the necessary long-term photostability required for live imaging at the cellular level. Here, an ultra-high-light-resistant solvatochromic fluorescence probe, 2-N,N-diethylamino-7-(4-methoxycarbonylphenyl)-9,9-dimethylfluorene (FπCM) is reported, which enables live lipid order imaging of cell division. This probe and its derivatives exhibit sufficient fluorescence wavelengths, brightness, polarity responsiveness, low phototoxicity, and remarkable photostability under physiological conditions compared to conventional solvatochromic probes. Therefore, these probes have the potential to overcome the limitations of fluorescence microscopy, particularly those associated with photobleaching. FπCM probes can serve as valuable tools for elucidating mechanisms of cellular processes at the bio-membrane level.

Synthesis and Properties of Rubicene-Based Aromatic π-Conjugated Compounds as Five-Membered Ring Embedded Planar Nanographenes.

Polycyclic aromatic hydrocarbons consisting of two or three rubicene substructures were designed as π-conjugated compounds embedding five-membered rings. The target compounds with t-butyl groups were synthesized by the Scholl reaction of precursors consisting of 9,10-diphenylanthracene units, even though a partially precyclized precursor was required for the synthesis of the trimer. These compounds were isolated as stable and dark blue solids. Single-crystal X-ray analysis and DFT calculations revealed the planar aromatic framework of these compounds. In the electronic spectra, the absorption and emission bands were considerably red-shifted compared with those of the reference rubicene compound. In particular, the emission band of the trimer extended to the near-IR region while retaining the emissive property. The narrowed HOMO-LUMO gap with the extension of the π-conjugation was confirmed by cyclic voltammetry and DFT calculations.

Structures and Supramolecular Properties of Inclusion Complexes of Anthracene-Triptycene Nanocages with Fullerene Guests and Their Dynamic Motion as Molecular Gyroscopes.

Three derivatives of macrocyclic cage compounds consisting of diarylanthracene and triptycene units were synthesized. These nanocages formed host-guest complexes with C60 and other fullerene guests as confirmed by 1 H NMR and fluorescence spectroscopy. The association constant of the mesityl and 2,4,6-tributoxyphenyl derivatives with C60 was determined to be 2.2 × 104  L mol-1 , which was larger than that of the pentafluorophenyl derivative. Direct experimental evidence of the complexation was obtained by X-ray diffraction analysis: the guest C60 molecule was included in the cavity via multipoint CH⋅⋅⋅π interactions. Dynamic disorders of the included C60 molecule in variable-temperature X-ray analysis indicated uniaxial motion, such as gyroscopic motion. The unique dynamic behavior of the spherical C60 rotor anchored by the cage stator via CH⋅⋅⋅π interactions in the crystal, as well as substituent effects on the association properties, are discussed with the aid of DFT calculations.

Synthesis, Structures, and Complexation with Phenolic Guests of Acridone-Incorporated Arylene-Ethynylene Macrocyclic Compounds.

Acridone units were incorporated into the arylene-ethynylene structure as polar arene units. Cyclic trimers consisting of three acridone-2,7-diyl units and three 1,3-phenylene units were synthesized by Sonogashira couplings via stepwise or direct route. X-ray analysis revealed that the trimer had a nearly planar macrocyclic framework with a cavity surrounded by three carbonyl groups. In contrast, the corresponding tetramer had a nonplanar macrocyclic framework. 1 H NMR measurements showed that the trimer formed a 1 : 1 complex as a macrocyclic host with dihydric phenol guests, and the association constants were determined to be ca. 1.0×103  L mol-1 for hydroquinone or resorcinol guests in CDCl3 at 298 K. The calculated structures of these complexes by the DFT method supported the presence of two sets of OH⋅⋅⋅O=C hydrogen bonds between the host and guest molecules. The spectroscopic data of the cyclic trimers and tetramers are compared with those of reference acridone compounds.

A pressure-induced ratiometric signalling chemosensor: a case of helical anthracenes.

One of the helical anthracenes, [4]HA, in which two fused anthracene ends are spatially arranged top and bottom, exhibits a ratiometric fluorescence response due to the hydrostatic pressure-dependent intramolecular [4+4] photocyclodimerization. This ratiometric signalling comes from the formation of an intramolecular stacked species and its subsequent photoreaction upon hydrostatic pressurization. The ratiometric indexes as a function of hydrostatic pressure may enable us to quantify an unknown pressure in solutions.

Synthesis, Structures, and Properties of Helically Fused Anthraquinones with Unusually Close Carbonyl-Carbonyl Contacts.

Electron-deficient aromatic ketones consisting of three fused anthraquinone units were synthesized by oxidation of the corresponding fused anthracenes. X-ray analysis revealed that these compounds had nonplanar helical structures with unusual contacts, C=O⋅⋅⋅C=O 2.467 Å, between the inner carbonyl groups. The role of n⋅⋅⋅π* interactions in the short contacts was evaluated using a noncovalent interaction plot and natural bond orbital analysis. The dynamic process involving helical inversion was observed by the variable temperature 1 H NMR measurement of a derivative with 2,4,6-trimethylphenyl groups, and the barrier was estimated to be 77 kJ mol-1 . DFT calculations indicated that the helical inversion proceeded via a multistep mechanism. The characteristic spectroscopic and electrochemical data due to the electron-deficient anthraquinone units and the sterically congested carbonyl groups are discussed with the aid of DFT calculations.

Structure and Photophysical Properties of 1,1,2,2-Tetra(1-anthryl)ethane: A C(sp3 )-C(sp3 ) Bond Substituted with Four Anthracene Units.

The title aromatic compound comprising four anthracene units was synthesized by the McMurry coupling of di(1-anthryl) ketone as a hydrogenated product in 65 % yield. The molecule forms a C2 symmetric structure with the ap conformation about the C(sp3 )-C(sp3 ) single bond, as revealed by X-ray analysis and DFT calculations. The UV/vis and fluorescence spectra of this compound were compared with those of anthracene, di(1-anthryl)methane, and 1,2-di(1-anthryl)ethane. The fluorescence spectrum showed a broad emission band at 450 nm having a long lifetime at 21 ns, which was assignable to an excimer-type emission, in contrast to the other reference compounds. The characteristic photophysical property is discussed in terms of the molecular structure with the aid of the noncovalent interaction plots and the conformational analysis.

Complexation of an Anthracene-Triptycene Nanocage Host with Fullerene Guests through CH⋠⋠⋠π Contacts.

A bicyclic anthracene macrocycle containing two triptycene units at the bridgehead positions was synthesized by Ni-mediated coupling of the corresponding precursor as a cage-shaped aromatic hydrocarbon host. This cage host formed an inclusion complex with C60 or C70 guest in 1 : 1 ratio in solution. The association constants (Ka ) determined by the fluorescence titration method were 1.3×104 and 3.3×105  L mol -1 for the C60 and C70 complexes, respectively, at 298 K in toluene. DFT calculations revealed that the guest molecules were included in the middle of the cavity with several CH⋅⋅⋅π contacts. The strong affinity of the cage host for the fullerene guests and the high selectivity toward C70 are discussed on the basis of spectroscopic and structural data.

Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes.

Invited for the cover of this issue is Shinji Toyota and co-workers at Tokyo Institute of Technology and Okayama University of Science. The image depicts a spirally rising dragon to represent the helical molecular structures in the manuscript. Read the full text of the article at 10.1002/chem.202004720.

Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes.

Polycyclic aromatic compounds consisting of four or five fused anthracene units were synthesized by PtCl2 -catalyzed cycloisomerization as novel long expanded helicenes. These compounds have helical structures with significant stacking of the terminal anthracene moieties at 0.33 nm interlayer distance. In the UV-vis and fluorescence spectra, the absorption and emission bands were red-shifted as the number of fused anthracene units was increased. The characteristic broad and long-lived emission bands of the long analogues are explained by the excimer-like stabilization of the excited state. These photophysical data as well as their cyclic voltammetric data are discussed on the basis of the π-conjugation and interlayer π⋅⋅⋅π interactions in the molecular structures and the molecular orbitals. The barrier and mechanism of helical inversion are also reported.

Multiple Fused Anthracenes as Helical Polycyclic Aromatic Hydrocarbon Motif for Chiroptical Performance Enhancement.

Polycyclic aromatic hydrocarbons consisting of three fused anthracene units were designed as new π-conjugated compounds having helical structures. These expanded helicenes were synthesized by Pt-catalyzed cycloisomerization of the corresponding ethynyl-substituted precursors. The nonplanar and helical structure was confirmed by X-ray analysis and DFT calculations, and the barrier to helical inversion was estimated to be 34 kJ mol-1 . The enantiomers of the diphenyl derivative were successfully resolved by chiral HPLC. Enantiopure samples showed good chiroptical performance in the CD (|Δϵ| 1380 L mol-1 cm-1 ) and CPL (|glum | 0.013) spectra, and these values were considerably large for simple organic molecules. The unique chiroptical properties are discussed on the basis of the molecular structure and the electronic state with the aid of time-dependent DFT calculations.

Exploration of Nano-Saturns: A Spectacular Sphere-Ring Supramolecular System.

Saturn-like systems consisting of nanoscale rings and spheres are fascinating motifs in supramolecular chemistry. Several ring molecules are known to include spherical molecules at the center of the cavity via noncovalent attractive interactions. In this Minireview, we generalize the molecular design, the structural features, and the supramolecular chemistry of such "nano-Saturns", which consist of monocyclic rings and fullerene spheres (mainly C60 ), on the basis of previous experimental and theoretical studies. Ring molecules are classified into three types (loop, belt, and disk) according to their shapes and possible interactions. Whereas typical belt-shaped rings tend to form tight complexes due to the wide contact area via π-π interactions, flat disk-shaped rings generally form weak complexes due to the narrow contact area mainly via CH-π interactions. In spite of the small association energies, disk-shaped rings are attractive because such rings can mimic the planet Saturn precisely as exemplified by an anthracene cyclic hexamer-C60 complex.

Nano-Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C60.

An anthracene cyclic hexamer was synthesized by the coupling reaction as a macrocyclic hydrocarbon host. This disk-shaped host included a C60 guest in 1:1 ratio to form a Saturn-type supramolecular complex in solution and in crystals. X-ray analysis unambiguously revealed that the guest molecule was accommodated in the middle of the host cavity with several CH⋅⋅⋅π contacts. The association constant Ka determined by NMR titration measurements was 2.3×103  L mol-1 at 298 K in toluene. The structural features and the role of CH⋅⋅⋅π interactions are discussed with the aid of DFT calculations.

Synthesis and Electronic Properties of Length-Defined 9,10-Anthrylene-Butadiynylene Oligomers.

Linear π-conjugated oligomers consisting of anthracene and diacetylene units were readily synthesized by a one-pot process that involved desilylation and oxidative coupling from appropriate building units. We were able to isolate length-defined oligomers ranging from 2-mer to 6-mer as stable and soluble solids. The bathochromic shifts in the UV-vis spectra suggested that the π-conjugation was extended with elongation of the linear chain. Cyclic voltammetric measurements showed characteristic reversible oxidation waves that were dependent on the number of anthracene units.

A Directly Fused Subporphyrin Dimer with a Wavelike Structure.

[Ni(cod)2 ]-mediated intramolecular reductive coupling of ß-ß' linked meso,meso'-dibromosubporphyrin dimer gave the anti-isomer of meso-meso', ß-ß' doubly linked subporphyrin dimer as the first example of a fused subporphyrin dimer. The fused dimer 3anti displays an wavelike coplanar structure, a perturbed and red-shifted absorption spectrum, reversible redox behaviors with a decreased electrochemical HOMO-LUMO band gap, and a short S1 -state lifetime owing to the delocalized π-electronic network.

Boron Arylations of Subporphyrins with Aryl Zinc Reagents.

Boron arylations of B-(methoxo)triphenylsubporphyrin have been developed with a combined use of ArZnI⋅LiCl and trimethylsilyl chloride. Aryl zinc reagents bearing bromo, cyano, amide, and ester groups can be employed for the B-arylation reaction to provide the corresponding B-arylated subporphyrins in moderate yields. Postmodifications of B-arylated subporphyrins have been demonstrated without loss of the B-C bond. These modifications include conversion of the cyano group into a benzoyl group with PhMgBr, hydrolysis of the ester group to give B-(4-carboxyphenyl)subporphyrin, and Pd-catalyzed Suzuki-Miyaura coupling of the 4-bromophenyl group to give a 1,4-phenylene-bridged subporphyrin-ZnII porphyrin hybrid that displays intramolecular excitation energy transfer from the subporphyrin to the porphyrin. The newly synthesized B-arylated subporphyrins have been fully characterized by NMR, UV/Vis absorption and fluorescence spectroscopies, mass spectrometry, electrochemical measurements, and X-ray diffraction analysis.

Stable Boron Peroxides with a Subporphyrinato Ligand.

Subporphyrin B-peroxides have been synthesized in good yields by acid-catalyzed exchange reactions of subporphyrin B-methoxide with the corresponding hydroperoxides. Thermal dimerization of the subporphyrin B-hydroperoxide provided the peroxo-bridged bis(subporphyrin) quantitatively. These subporphyrin B-peroxides are fairly stable under ambient conditions, which allowed their isolation and full characterization as the first examples of structurally authenticated boron hydroperoxides, acyclic boron organylperoxides, and neutral peroxo-bridged diboron species. The subporphyrin B-peroxides thus prepared were investigated through their crystal structures, IR spectra, and cyclic voltammograms as well as by DFT calculations. The subporphyrin B-hydroperoxide oxidizes triphenylphosphine quantitatively to triphenylphosphine oxide.