Synthesis of Nonsymmetric NBN-Embedded [6]- and [7]Helicenes with Amplified Activities.

Two C1-symmetric heterohelicenes were constructed by nonsymmetrically extending the ortho-fused structures of a C2v-symmetric NBN-embedded phenalene derivative and featured intense luminescence, large Stokes shifts, and successive reversible redox behaviors. Increasing one fused phenyl unit in such a helical structure led to a 10-fold-enhanced dissymmetry factor. Their strong double hydrogen-bond-donating capability makes them distinctly red-shifted in absorption, emission, and CD and CPL spectra upon the addition of fluoride anion.

Divergent Synthesis of Double Heterohelicenes as Strong Chiral Double Hydrogen-Bonding Donors.

We developed a very efficient and expandable divergent approach initiated by a direct electrophilic borylation at phenyl rings to synthesize a series of double heterohelicenes. Their π-extended structures with pristine zigzag nitrogen (N)-boron (B)-nitrogen (N) edges offer them substantial physical properties and strong double hydrogen-bond donating capability. The isolated (P,P) and (M,M) enantiomers exhibit circularly polarized luminescence in response to hydrogen-bonding interactions.

2,4,6-Trimethylpyridine-Derived Vinylene-Linked Covalent Organic Frameworks for Confined Catalytic Esterification.

Knoevenagel condensation is a powerful tool for the construction of vinylene-linked covalent organic frameworks. Herein, we established a concise approach to vinylene-linked COFs by Knoevenagel condensation at the multi-methyl groups of a pyridine ring through in situ formation of an N-acyl pyridinium cation in the presence of various acylating reagents. Following this strategy, two vinylene-linked COFs were constructed using 2,4,6-trimethylpyridine and multi-aldehyde-substituted aromatic derivatives as monomers. The resultant COFs are highly crystalline and assembled into hexagonal lattices with specific surface areas as large as 1915 m2 g-1 (vs. 1972 m2 g-1 of the theoretical value). The stable and abundant pyridine-decorated regular nanochannels within the COFs allow for catalyzing the esterification of several pharmaceutical intermediates with distinct spatially confined selectivity and recyclability, representing an environmentally friendly catalytic organic transformation.

Calcium Based All-Organic Dual-Ion Batteries with Stable Low Temperature Operability.

In response to the application requirements of secondary batteries at low temperature, an all-organic dual-ion battery with calcium perchlorate contained acetonitrile as the electrolyte (CAN-ODIB) is fabricated in this work. The electrochemical energy is stored in CAN-ODIB via the association and disassociation of calcium and perchlorate ions in perylene diimide-ethylene diamine/carbon black composite based anode and polytriphenylamine based cathode with highly reversible redox states. Benefiting from the energy storage mechanism, CAN-ODIB exhibits excellent electrochemical performances in tests with the temperature ranging from 25 to -50 °C. Especially, CAN-ODIB at -50 °C reserves ≈61% of the capacity at 25 °C (83.4 mA h g-1 ) with the current density of 0.2 A g-1 . CAN-ODIB also shows excellent cycling stability at low temperature by retaining 90.3% of the initial capacity at 1.0 A g-1 after 450 charge-discharge cycles at -30 °C. The impedance analysis of CAN-ODIB at different temperatures indicates that the low temperature performance of CAN-ODIB depends more on the electrode materials than the electrolyte, which provides the important guidance for the further design of secondary batteries operable at low temperatures.

Synthesis of Ionic Vinylene-Linked Covalent Organic Frameworks through Quaternization-Activated Knoevenagel Condensation.

We developed a simple approach to synthesizing ionic vinylene-linked two-dimensional covalent organic frameworks (COFs) through a quaternization-promoted Knoevenagel condensation at three aromatic methyl carbon atoms of N-ethyl-2,4,6-trimethylpyridinium halide with multitopic aromatic aldehyde derivatives. The resultant COFs exhibited a honeycomb-like structure with high crystallinity and surface areas as large as 1343 m2 g-1 . The regular shape-persistent nanochannels and the positively charged polymeric frameworks allowed the COFs to be uniformly composited with linear polyethylene oxide and lithium salt, displaying ionic conductivity as high as 2.72×10-3  S cm-1 .

Concise Approach to T-Shaped NBN-Phenalene Cored Luminogens as Intensive Blue Light Emitters.

Small organic molecules with finely tunable physical properties are highly desired for the fabrication of low-cost and high-performance organic electronic devices. In this work, the syntheses of a series of T-shaped NBN-embedded dibenzophenalene derivatives through the formation of a key brominated intermediate in a stoichiometrically controlled reaction are presented. The geometric and electronic structures of these T-shaped molecules can be simply tailored by attaching substituents along the direction perpendicular to the molecular main backbones, resulting in desirable physical properties, such as high thermal stability with a decomposition temperature of more than 350 °C, and intensive blue luminescence with a quantum yield up to 0.62. Organic light-emitting diode devices fabricated with such molecules as the emitting layer release pure blue light with CIE (0.16, 0.12).

π-Extended C2-Symmetric Double NBN-Heterohelicenes with Exceptional Luminescent Properties.

Two novel B,N-embedded double heterohelicenes, Ph-NBNDH and Naph-NBNDH, with intensively twisted quasi-C2 symmetrical structures are synthesized for the first time via a highly regioselective intramolecular Scholl reaction. The π-extended skeletons render them with exceptional physical properties, such as the highest fluorescence quantum yields up to 0.83 among multiple helicenes. Moreover, the isolated (M,M)- and (P,P)-enantiomers of Naph-NBNDH deliver intense circularly polarized luminescence. As the emitting layer, Ph-NBNDH exhibits strong electroluminescence at high working voltage.

Successive Annulation to Fully Zigzag-Edged Polycyclic Heteroaromatic Hydrocarbons with Strong Blue-Green Electroluminescence.

A Brønsted-acid-promoted alkyne benzannulation approach was developed to synthesize the amino-substituted dibenze[ a, j]anthracence derivatives in excellent yields, which were directly converted to fully zigzag-edged polycyclic heteroaromatic hydrocarbons via a nitrogen-directed electrophilic borylation. As the dopant in a blue-green electroluminescent device, the resulted compound exhibited relatively high stability.

1,1'-Binaphthyl Consisting of Two Donor-π-Acceptor Subunits: A General Skeleton for Temperature-Dependent Dual Fluorescence.

Temperature-dependent dual fluorescence with the anti-Kasha's rule is of great interest, but is a very challenging property to achieve in small organic molecules. The highly sensitive temperature-dependent dual fluorescence of 2,2'-bis(dimethylamino)-6,6'-bis(dimesitylboryl)-1,1'-binaphthyl (BNMe2 -BNaph), which essentially consists of two donor-π-acceptor (D-π-A) subunits, inspired the exploration of the importance of its structural features and the general utility of this molecular design. The reference compound MBNMe2 -BNaph, which lacks one electron-accepting Mes2 B, is found to show less sensitive temperature-dependent dual fluorescence, suggesting that the structure of BNMe2 -Bnaph, consisting of two symmetrical D-π-A subunits, is very important for achieving highly sensitive temperature-dependent dual fluorescence. In addition, it is found that another two 1,1'-binaphthyls, CHONMe2 -BNaph and CNNMe2 -BNaph, which also consist of two D-π-A subunits with Mes2 B groups replaced by CHO and CN, respectively, also show temperature-dependent dual fluorescence, with the fluorescence changing in a similar manner to BNMe2 -BNaph, indicating the general utility of the current molecular design for temperature-dependent dual fluorescence. Furthermore, the temperature-dependent dual fluorescence behaviors, such as the relative intensities of the two emission bands, the separation of the two emissions bands, and the sensitivity of the fluorescence intensity ratio to temperature, are greatly influence by the electron acceptors.

2,2'-Diamino-6,6'-diboryl-1,1'-binaphthyl: A Versatile Building Block for Temperature-Dependent Dual Fluorescence and Switchable Circularly Polarized Luminescence.

Temperature-dependent dual fluorescence and switchable circularly polarized luminescence (CPL) are two highly pursued but challenging properties for small organic molecules (SOMs). We herein disclose a triarylborane π-system based on a 2,2'-diamino-6,6'-diboryl-1,1'-binaphthyl scaffold that can serve as a versatile building block for achieving these two properties by simply choosing different amino groups. BNMe2 -BNaph with less bulky dimethylamino groups displays temperature-dependent dual fluorescence, and can thus be used as a highly sensitive ratiometric fluorescence thermometer. On the other hand, BNPh2 -BNaph with bulky diphenylamino groups exhibits intense fluorescence in both solution and in the solid state. A change of solvent from nonpolar cyclohexane to highly polar MeCN not only shifts the CPL position to much longer wavelength but also inverts the CPL sign. In addition, the complexation of BNPh2 -BNaph with fluoride greatly enhances the CPL intensity.

Impact of the 2,2'-Bithienyl Framework on the Charge-Transfer Emission of Triarylborane-Based o,o'-Substituted Biaryls.

Two new triarylborane-based o,o'-substituted 2,2'-bithienyls, BT-BNMe 2 and BT-BNBn 2 , which contain BMes2 and NMe2/NBn2 groups at the 3,3'-positions, have been synthesized. Similar to the o,o'-substituted biphenyl analogues, BP-BNMe 2 and BP-BNBn 2 , which contain BMes2 and NMe2/NBn2 groups at the 2,2'-positions, the steric effect of the amino group has significant influence on the conformation of the 2,2'-bithienyl skeleton. The boryl and amino groups are located at the same side of 2,2'-bithienyls axis with a short B···N distance (3.63 Å) for the NMe2-substituted BT-BNMe 2 . On the contrary, the two substituents are arranged on the two different sides of the 2,2'-bithienyls axis for BT-BNBn 2 , which is modified with bulky NBn2. Despite the remarkable differences in the steric structure, the two 2,2'-bithienyls display fluorescence at close wavelengths, which is in sharp contrast to the much red-shifted fluorescence of BP-BNMe 2 than BP-BNBn 2 . The theoretical calculations demonstrated that the two 2,2'-bithienyls have close highest occupied molecular orbital-lowest unoccupied molecular orbital gaps in the excited state, which firmly support the experimental results. Thus, the parent main chain framework can exhibit great impact on the charge-transfer emission of o,o'-substituted biaryls.

A triarylborane-based biphenyl exhibiting abrupt fluorescence enhancement at a specific high temperature.

A triarylborane-based biphenyl, 4,4'-dibromo-2-dimesitylboryl-2'-(N,N-dimethylamino)biphenyl (Br2-BN-BPh), exhibits very unique temperature-dependent fluorescence behavior. Although Br2-BN-BPh is weakly emissive at room temperature, its fluorescence increases abruptly at a specific high temperature (68 °C in MOE, 88 °C in toluene). In addition, the fluorescence still remains very strong even after cooling to room temperature, providing the possibility of the fluorescence detection of a specific high temperature.

Charge-Transfer Emitting Triarylborane π-Electron Systems.

Triarylboranes have attracted significantly increasing research interest as a remarkable class of photoelectronic π-electron materials. Because of the presence of vacant p orbital on the B center, the boryl group is a very unique electron acceptor that exhibits not only electron-accepting ability through p-π* conjugation but also high Lewis acidity to coordinate with Lewis bases and steric bulk arising from the aryl substituent on the B center to get enough kinetic stability. Thus, the incorporation of a trivalent B element into π-conjugated systems is an efficient strategy to tune the electronic and stereo structures and thus the photoelectronic properties of π-electron systems. When an electron-donating group, such as amino, is present, triarylboranes would likely display intramolecular charge-transfer transitions. These kinds of molecules are often highly emissive. In addition, the geometry of the molecules has a great impact on the emission properties. In this Forum Article, we herein describe our recent progress on the charge-transfer emitting triarylborane π-electron systems with novel geometries, which include the lateral boryl-substituted π-system with amino groups at the terminal positions, the o,o'-substituted biaryl π-system with boryl and amino groups at the o,o'-positions, a triarylborane-based BODIPY system, and a B,N/S-bridged ladder-type π-system. We mainly put the emphasis on the molecular design concept, structure-property relationships, intriguing emission properties and great applications of the corresponding triarylborane π-systems.

Chirality Relay in 2,2'-Substituted 1,1'-Binaphthyl: Access to Propeller Chirality of the Tricoordinate Boron Center.

It is a challenging issue to achieve propeller chirality for triarylboranes owing to the low transition barrier between the P and M forms of the boron center. Herein, we report a new strategy to achieve propeller chirality of triarylboranes. It was found that the chirality relay from axially chiral 1,1'-binaphthyl to propeller chirality of the trivalent boron center can be realized when a Me2 N and a Mes2 B group (Mes=mesityl) are introduced at the 2,2'-positions of the 1,1'-binaphthyl skeleton (BN-BNaph) owing to the strong π-π interaction between the Me2 N-bonded naphthyl ring and the phenyl ring of one adjacent Mes group, which not only exerts great steric hindrance on the rotation of the two Mes groups but also gives unequal stability to the two configurations of the boron center for a given configuration of the binaphthyl moiety. The stereostructures of the boron center were fully characterized through 1 H NMR spectroscopy, X-ray crystal analyses, and theoretical calculations. Detailed comparisons with the analog BN-Ph-BNaph, in which the Mes2 B group is separated from 1,1'-binaphthyl by a para-phenylene spacer, confirmed the essential role of π-π interaction for the successful chirality relay in BN-BNaph.

Synthesis and Properties of Benzothieno[b]-Fused BODIPY Dyes.

Two benzothieno[b]-fused BODIPYs, BT-BODIPY and BBT-BODIPY, in which one parent BODIPY core is fused with one and two benzothieno rings, respectively, were synthesized from BODIPYs substituted with 2-(methylsulfinyl)phenyl at the ß-position. The first H2SO4-induced cyclization and deborylation afforded benzothieno[b]-fused dipyrrin derivatives, which can easily complex with BF3·OEt2 to form the desired benzothieno[b]-fused BODIPYs. It was revealed that the fusion of the benzothieno ring is more effective at extending conjugation than simple attachment of the 2-(methylthio)phenyl substituent, which presumably results from conformational restriction. Compared with the corresponding unstrained SPh-BODIPY and BSPh-BODIPY, which contain one and two 2-(methylthio)phenyl groups at the ß-position, BT-BODIPY and BBT-BODIPY display red shifted absorption, increased absorptivity, and fluorescence efficiency. Furthermore, the ring fusion is also helpful to increase stability of the formed cation in BBT-BODIPY. Thus, BBT-BODIPY exhibits very intriguing properties, such as intense absorption and emission in the red region, very sharp emission spectra, and reversible oxidation and reduction potentials.

A highly selective ratiometric bifunctional fluorescence probe for Hg(2+) and F(-) ions.

A triarylborane derivative BN-S, which contains a Hg(2+)-responsive dithioacetal group and a F(-)-responsive boryl group, has been designed and synthesized via the functionalization of 2-dimesitylboryl-2'-(N,N-dimethylamino)biphenyl core skeleton with a dithioacetal substituent. This compound displays intense intramolecular charge transfer fluorescence, even for its nano-aggregates in water. The Hg(2+)-promoted deprotection of the dithioacetal group and complexation of F(-) with the tri-coordinate boron center cause hypochromism of fluorescence to different extents. And thus BN-S behaves as a promising ratiometric bifunctional fluorescence probe to detect Hg(2+) and F(-) simultaneously. In addition, the detection of Hg(2+) is performable in aqueous medium using its nano-aggregates.

Synthesis and properties of B,N-bridged p-terphenyls.

The efficient synthesis has been described to achieve a new class of ladder π-conjugated molecules, B,N-bridged p-terphenyls. The bridging B atom exhibits a more significant effect than the bridging N atom on photophysical properties.