Environment-sensitive emission of anionic hydrogen-bonded urea-derivative-acetate-ion complexes and their aggregation-induced emission enhancement.

Anions often quench fluorescence (FL). However, strong ionic hydrogen bonding between fluorescent dyes and anion molecules has the potential to control the electronic state of FL dyes, creating new functions via non-covalent interactions. Here, we propose an approach, utilising ionic hydrogen bonding between urea groups and anions, to control the electronic states of fluorophores and develop an aggregation-induced emission enhancement (AIEE) system. The AIEE ionic hydrogen-bonded complex (IHBC) formed between 1,8-diphenylnaphthalene (p-2Urea), with aryl urea groups at the para-positions on the peri-phenyl rings, and acetate ions exhibits high environmental sensitivities in solution phases, and the FL quantum yield (QY) in ion-pair assemblies of the IHBC and tetrabutylammonium cations is more than five times higher than that of the IHBC in solution. Our versatile and simple approach for the design of AIEE dye facilitates the future development of environment-sensitive probes and solid-state emitting materials.

A fluorescent calix[4]arene with naphthalene units at the upper rim exhibits long fluorescence emission lifetime without fluorescence quenching.

We synthesised a new compound with four naphthyl groups in the upper rims of calix[4]arene (1). Compared to the monomer unit, compound 1 has redshifted absorption and fluorescence, together with high fluorescence quantum yield and long fluorescence lifetime, which is extremely rare because long fluorescence lifetime emission tends to reduce the quantum yield. Single-crystal X-ray analysis and quantum calculations in the S1 state revealed π-π through-space interactions between naphthalene rings.

A Pentacene-based Nanotube Displaying Enriched Electrochemical and Photochemical Activities.

Unlike previously well-studied, acyclic pentacene oligomers, the first synthesis of a cyclic pentacene trimer with a fixed tubular conformation is reported. A short-step synthesis starting from common pentacenequinone yielded the target molecule with a 1.5 nanometer length and a subnanometer pore. Steady-state spectroscopic analyses revealed that the close proximity of the non-conjugated, three pentacene chromophores allows the nanotube to display stepwise electrochemical/chemical oxidation characteristics. Furthermore, time-resolved transient absorption measurements elucidated the generation of an excited triplet state of the nanotube, with high quantum yield reaching about 180 % through intramolecular singlet fission and a very long triplet lifetime.

Polyaromatic molecular tubes: from strategic synthesis to host functions.

Ring- and tube-shaped molecules like crown ethers and cyclodextrins play a fundamental role in supramolecular chemistry since their initial discovery. To date, numerous intriguing properties and reactivities have been reported based on their unique inner microenvironments. While inner spaces encircled by aliphatic and/or small aromatic frameworks have been heavily investigated, tubular structures that feature polyaromatic frameworks remained largely unexplored until 2010, despite their undisputable potential. Polyaromatic rings provide appealing photophysical and electrochemical properties and thus allow for the construction of new functional cylindrical nanospaces. This feature article describes the recent progress in the synthesis and application of short tubular molecules bearing multiple (≥3) polyaromatic rings (e.g., anthracene, pyrene, chrysene, and HBC). The polyaromatic tubes reported herein display characteristic properties such as strong fluorescent emission, a selective molecular binding ability, efficient host-guest energy transfer and open-closed structural transformations.

Alkylation-, Heating-, and Doping-Induced Emission Enhancement of a Polyaromatic Tube in the Solid State.

A polyaromatic tube with a subnanometer-sized cavity was efficiently prepared on a gram-scale through the stereo-controlled cyclotrimerization of a diphenylanthracene derivative as a key step. The facile exterior alkylation of the polyaromatic framework leads to a moderately fluorescent tube (R=-OC10 H21 ; ΦF =20 %) in the solid state. The emission intensity of the solid-state alkyl-substituted tube is remarkably enhanced upon heating (up to 1.6 times, ΦF =31 %) as well as doping with fluorescent dyes (up to 4.2 times, ΦF =83 %) through efficient energy transfer.

Polyaromatic molecular peanuts.

Mimicking biological structures such as fruits and seeds using molecules and molecular assemblies is a great synthetic challenge. Here we report peanut-shaped nanostructures comprising two fullerene molecules fully surrounded by a dumbbell-like polyaromatic shell. The shell derives from a molecular double capsule composed of four W-shaped polyaromatic ligands and three metal ions. Mixing the double capsule with various fullerenes (that is, C60, C70 and Sc3N@C80) gives rise to the artificial peanuts with lengths of ∼3 nm in quantitative yields through the release of the single metal ion. The rational use of both metal-ligand coordination bonds and aromatic-aromatic π-stacking interactions as orthogonal chemical glue is essential for the facile preparation of the multicomponent, biomimetic nanoarchitectures.

A Switchable Open/closed Polyaromatic Macrocycle that Shows Reversible Binding of Long Hydrophilic Molecules.

In spite of wide-ranging previous studies on synthetic macrocycles, the installation of open-close functions into the frameworks remains a challenge. We present a new polyaromatic macrocycle capable of switching between open and closed forms in response to external stimuli, namely, base and acid. The macrocycle, which is prepared in three steps, has a well-defined hydrophobic cavity with a length of around 1 nm, surrounded by four pH-responsive acridinium panels. The open and closed structures were confirmed by single-crystal X-ray analysis. The cylindrical cavity can bind long hydrophilic molecules up to 2.7 nm in length in neutral water and then release the bound guests through a reversible open-to-closed structural change upon simple addition of base.

A Truncated Molecular Star.

A pentanuclear coordination complex assembled from any palladium(II) component and non-chelating ligands is hitherto unreported. The pentanuclear complex [Pd5 (L1)5 (L2)5 ](BF4 )10 , 1 reported here was prepared by the spontaneous complexation of [Pd(DMSO)4 ](BF4 )2 with the non-chelating bidentate ligands 1,4-phenylenebis(methylene) diisonicotinate, L1 and 4,4'-bipyridine, L2 in a one-pot method at room temperature. The planar polycyclic complex 1 with outer diameters of ≈3 nm is termed as a "molecular star" owing to its resemblance with a pentagram shape. Interim paths leading to the star were also probed to decipher related dynamics of the system.

An M2 L4 Molecular Capsule with a Redox Switchable Polyradical Shell.

Preparation of molecular nanostructures with polyradical frameworks remains a significant challenge because of the limited synthetic accessibility which is entirely different from that of neutral and ionic ones. Herein we report the quantitative formation of a new M2 L4 molecular capsule from metal ions and dihydrophenazine-based ligands. The capsule has a spherical nanocavity (ca. 1 nm in diameter) enclosed by eight redox-active, dihydrophenazine panels. Electrochemical oxidation of the capsule leads to the generation of multiple radical cations on the shell framework. Moreover, a stable tetra(radical cation) capsule can be reversibly obtained by chemical as well as electrochemical oxidation.

Polycationic-Shelled Capsular and Tubular Nanostructures and Their Anionic-Guest Binding Properties.

For the development of novel nanospace with unique electrostatic character, we prepared new capsular and tubular nanostructures by the quantitative assembly of metal ions and bent bisacridinium ligands. The capsule and tube have closed spherical and open cylindrical cavities, respectively, with diameters of around 1 nm surrounded by cationic polyaromatic panels. Thanks to the facile synthetic protocol (three steps), another polycationic capsule with an elliptical nanocavity was also prepared by using an elongated ligand. In spite of the absence of pendant hydrophilic groups, the spherical polyaromatic capsule shows sufficient water solubility due to the polycationic shell. Moreover, the highly cationic cavity (12+) can selectively encapsulate anionic organic compounds in water.

A polyaromatic molecular tube that binds long hydrocarbons with high selectivity.

Long hydrocarbon chains are essential components of biomolecules used for structure and function in living organisms. The selective recognition and effective binding of hydrocarbons within synthetic host compounds are problematic owing to their conformational flexibility and the lack of specific binding sites. Here we report a molecular tube with polyaromatic frameworks prepared by the Zincke cross-coupling reaction. The tube has a well-defined cylindrical cavity with a diameter and length of ~1 nm encircled by multiple anthracene panels and thereby binds long hydrocarbons containing branched methyl groups and/or unsaturated carbon-carbon double bonds (for example, heptamethylnonane, nervonic acid ester and squalene) with high selectivity in aqueous solutions.

A bowl-shaped organic host using bispyridine ligands: selective encapsulation of carbonyl guests in water.

A bowl-shaped organic host was prepared by linking two anthracene-embedded bispyridine ligands with two methylene spacers. The water-soluble host has a hemispherical hydrophobic cavity (∼1 nm in diameter) with two cationic methylenebispyridinyl (Lewis acidic) moieties and shows the selective recognition and encapsulation of aromatic guest molecules containing carbonyl groups in water.