Two-Photon Excited Fluorescent 3,3'-Diamino-5,5'-Diboryl-2,2'-Bithienyls Featuring a Quadrupolar Structure.

The quest for fluorophores exhibiting large two-photon absorption cross sections and high fluorescence efficiency is an important topic. Two 2,2'-bithienyl derivatives are disclosed which contain two N,N-disubstituted amino and two dimesitylboryl groups at 3,3'- and 5,5'-positions, respectively. Despite the great steric effect of amino groups, the bithienyl skeleton still adopts a coplanar geometry. Herein, they are characterized by a quadrupolar structure and display good fluorescence efficiency and large two-photon absorption cross sections up to 473 GM.

Chiral Triarylborane-Based Small Organic Molecules for Circularly Polarized Luminescence.

Circularly polarized luminescence (CPL) has shown promising application potentials in 3D display, optical data storage, smart sensors/probers, CPL lasers, and light source for asymmetric photosynthesis. In the last decade, the CPL-active small organic molecules (CPL-SOMs) have attracted rapidly increasing research interest owing to the great advantages of SOMs, such as high luminescence efficiency, facile modification of chemical structure, fine emission wavelength tuning, precise relationships between structure and properties, and as well as easy fabrication. Promoted by the unique effects of boryl group, such as strong electron-accepting ability, great steric effect, and Lewis acidity to bind with Lewis bases, we herein summarized our recent research results about the creation of CPL-SOMs by modification of chiral scaffolds, such as [2.2]paracyclophane, [5]/[7]helicene, and binaphthyl, with boryl group. The preliminary results have well demonstrated that the chiral triarylborane-based SOMs exhibit promising CPL properties, such as intense CPL in combination of high luminescence dissymmetry factor (|glum |) with high fluorescence efficiency, solvent-induced sign inversion, facile emission wavelength tuning, high fluorescence efficiency in the solid, and substituent-induced sign inversion.

Circularly Polarized Luminescence Switching in Small Organic Molecules.

The circularly polarized luminescence (CPL) switching is of significant interest for applications in security technologies and sensing devices. Small organic molecules (SOMs) show several advantages over metal complexes, supramolecular assemblies, and polymers. Therefore, the recent progress on the CPL switching in SOMs is here reviewed. The results are summarized based on the strategies used to tune factors that influence the emission properties, and thus, to realize CPL switching. The strategies that have been adopted include promoting the excimer formation of fluorescent units, changing the conformation of fluorophores, tuning the electronic structure of the π-skeleton/substituent, and modulating the intramolecular charge-transfer dynamics.

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.

RIP2 deficiency attenuates cardiac hypertrophy, inflammation and fibrosis in pressure overload induced mice.

Although the pathological cardiac hypertrophy presents a leading cause of morbidity and mortality worldwide, our knowledge of the molecular mechanisms underlying the disease is still poor. Here, we reported that receptor-interacting serine/threonine-protein kinase 2 (RIP2), promoting pro-inflammatory gene expression, enhanced the pathological cardiac hypertrophy in animals. The effects of RIP2 on the cardiac hypertrophy triggered by pathological stimuli have not been fully investigated. In our study, mice were subjected to aortic banding (AB) surgery to explore the pathological, echocardiographic and molecular mechanisms. RIP2 expressed highly in cardiomyocytes after AB operation in wild type (WT) mice. RIP2-knockout (KO) attenuated cardiac hypertrophy, inflammation and fibrosis in mice 4 weeks after AB-surgery. First, RIP2 knockout down-regulated hypertrophic markers of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and ß-myosin heavy chain (ß-MHC) in the heart of AB-operated mice.in addition, RIP2-deficiency reduced toll-like receptor 4/myeloid differentiation factor 88/nuclear factor kappa B (TLR4/MyD88/NF-κB) activation, mitogen-activated protein kinases (MAPKs) phosphorylation and transforming growth factor-ß1 (TGF-ß1)/SMADs expressions, contributing to the suppression of inflammatory response and fibrosis, as further evidenced by down-regulated pro-inflammatory cytokines, including Tumor necrosis factor-α (TNF-α), interleukin (IL)-1ß, IL-6 and IL-18, as well as fibrosis markers of Collagen I, Collagen III and α-smooth muscle actin (α-SMA). Taken together, our data indicated that RIP2-deficience ameliorated cardiac hypertrophy, inflammation and fibrosis through modulating multiple signaling pathways.

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.

Charge-Transfer Emission in Organoboron-Based Biphenyls: Effect of Substitution Position and Conformation.

A series of organoboron-based biphenyls o,o'-NMe2, o,p'-NMe2, p,p'-NMe2, which contain an electron-donating NMe2 and an electron-accepting BMes2 groups at o,o'-, o,p'-, p,p'-positions of biphenyl skeleton, respectively, as well as o,o'-NBn2, which contains more bulky NBn2 rather than NMe2, were fully characterized to explore the effect of structural modification on the intramolecular charge-transfer emissions. In addition to significant effect of substitution position on photophysical properties, remarkable influence of conformation was also observed for o,o'-substituted compounds. The emission is substantially blue-shifted as conformation changes from the location of NMe2 and BMes2 at same side of biphenyl axis with a close B···N distance, and thus direct B···N electronic interaction in o,o'-NMe2, to the location of NBn2 and BMes2 on two opposite sides in o,o'-NBn2. And o,o'-NMe2 exhibits the longest emission wavelength, but the shortest absorption wavelength, and thus largest Stokes shift among these four organoboron-based biphenyls in both solution and solid state. The theoretical calculations demonstrated that the unique structure of o,o'-NMe2, in which boryl and amino located at the same side of biphenyl axis with close B···N distance and direct B···N electronic interaction, is helpful to stabilize the lowest singly occupied orbital in the exited state.

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.

Triarylboranes with a 2-dimesitylboryl-2'-(N,N-dimethylamino)biphenyl core unit: structure-property correlations and sensing abilities to discriminate between F(-) and CN(-) ions.

A series of triarylboranes, in which different substituents are introduced at the para position of the dimethylamino group of a 2-dimesitylboryl-2'-(N,N-dimethylamino)biphenyl core unit, have been comprehensively investigated to explore the effect of structural modification on photophysical properties. The introduction of electron-accepting substituents would facilitate the HOMO→LUMO charge transfer (CT) transition. In contrast, the intramolecular CT transition is significantly prohibited when electron-donating substituents are incorporated. Notably, the HOMO→LUMO CT transition mainly consists of the transition from the electron-donating amino group to an electron acceptor other than boryl when a strong electron acceptor such as the dicyanovinyl group is present. This dicyanovinyl-substituted compound displays sensing abilities to discriminate fluoride and cyanide ions. In solution in THF, the fluoride ions first bind to the boron center, then attack the α-carbon atom of the dicyanovinyl group, whereas the cyanide anion acts on the electron-accepting centers in the reverse sequence. As a result, the absorption and emission change in different manners upon addition of fluoride and cyanide ions.

Solid-state emissive B,S-bridged p-terphenyls: synthesis, properties, and utility as bifunctional fluorescent sensor for Hg2+ and F- ions.

The efficient synthesis has been disclosed to achieve a new class of ladder-type molecules, B,S-bridged p-terphenyls (BS-TPs). Their properties were fully characterized by UV-vis and fluorescence spectroscopy in both solution and solid state, time-resolved fluorescence spectroscopy, DFT theoretical calculations, and cyclic voltammetry. A detailed comparison between anti-BS-TP and its analogue B,N-bridged p-terphenyl (BN-TP) was made to elucidate the effect of displacement of bridging N with S atom on the properties. The introduction of S rather than N atom as bridging atom leads to increased fluorescence efficiency in both solution and solid state as well as enhanced reduction stability. And thus this new class of ladder-type molecules are highly emissive in both solution and solid state and display reversible reduction wave in cyclic voltammograms, denoting their promising potentials as electron-transporting solid-state emitters. In addition, this new class of molecules are capable of detecting F(-) and Hg(2+) with different fluorescence responses, owing to the high Lewis acidity of the B center to coordinate with F(-) anions and the great mercury-philicity of the S center to complex with Hg(2+) cations.

p-Quaterphenyls laterally substituted with a dimesitylboryl group: a promising class of solid-state blue emitters.

A new family of p-quaterphenyls 1-6 laterally substituted with a bulky electron-accepting dimesitylboryl group has been designed and synthesized. These compounds were characterized by X-ray crystallography, UV-vis and fluorescence spectroscopy, and DFT calculations as well as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and cyclic voltammetry (CV). X-ray single-crystal analysis revealed that the p-quaterphenyl main chain framework exhibits a twisted structure due to the steric effect of the lateral boryl group, and the intermolecular interactions are effectively suppressed in the solid state. Despite the significantly twisted main-chain structure, these molecules still display efficient intramolecular charge-transfer emissions with large Stokes shifts. An intriguing finding is that all these molecules show bright fluorescence with good to excellent quantum yields in the blue region in the solid state. In addition, the two representative p-quaterphenyls 3 and 4 containing both the electron-accepting boryl group and the electron-donating carbazolyl (3) or diphenylamino group (4) possess high thermal stability and good oxidation-reduction reversibility, which together with their excellent solid-state fluorescence efficiency make them promising bipolar transporting blue emitters.

Solid-state emissive triarylborane-based BODIPY dyes: photophysical properties and fluorescent sensing for fluoride and cyanide ions.

We disclose two novel BODIPY dyes, which contain the bulky substituent, [(4-dimesitylboryl)phenyl]ethynyl at 2- and 2,6-positions. The steric bulkiness of the boryl group is effective to suppress the intermolecular interaction in the solid state and thus these two compounds display intense fluorescence not only in solution but also in the solid state. In addition, the BODIPY dyes display sensitive fluorescence responses to fluoride and cyanide anions through the complexation with the boron center of the boryl group and the subsequent decomposition of the BODIPY core, illustrating their potential uses for the fluorescence sensing of fluoride and cyanide ions.

2-(Dimesitylboryl)phenyl-Substituted [2.2]Paracyclophanes Featuring Intense and Sign-Invertible Circularly Polarized Luminescence.

We have disclosed a new type of [2.2]paracyclophanes that contain a 2-(dimesitylboryl)phenyl and a N,N-disubstituted amino groups at two different phenyl rings. They show intense circularly polarized luminescence combining high fluorescence efficiency (ΦF) and luminescence dissymmetry factor (|glum|), which are up to 0.93 and 1.73 × 10-2, respectively. In addition, the pseudo-meta derivatives display solvent-induced CPL sign inversion owing to the solvent-dependent excited-state dynamics.

Propeller Configuration Flipping of the Trivalent Boron-Inducing Substituent Dependence of the Circularly Polarized Luminescence Sign in Triarylborane-Based [7]Helicenes.

We here disclose two triarylborane-based [7]helicenes, which contain a dimesitylboryl or a 2-(dimesitylboryl)phenyl at position 9 of the [7]helicene skeleton. The change in the peripheral substituent from dimesitylboryl to 2-(dimesitylboryl)phenyl induced doubling of |glum| and sign inversion of the circularly polarized luminescence (CPL). The substituent dependence of the CPL sign is reasonably explained by the propeller configuration flipping of boron, which has a significant influence on the chiroptical properties.

Highly emissive diborylphenylene-containing bis(phenylethynyl)benzenes: structure-photophysical property correlations and fluoride ion sensing.

A series of 2,5-bis(dimesitylboryl)-1,4-bis(arylethynyl)benzenes 1-6 that contain various p-substituents on the terminal benzene rings, including NPh(2) (1), OMe (2), Me (3), H (4), CF(3) (5), and CN (6) groups, were synthesized, and the effects of the p-substituents on the absorption and fluorescence properties were investigated both in solution and in the solid state. Linear relationships were obtained not only between the Hammett sigma(p)(+) constants of the p-substituents and the absorption and fluorescence maxima, quantum yields, and excited-state dynamics parameters in solution, but also between the sigma(p)(+) constants and the fluorescence quantum yields in the solid state. An important finding extracted from these results is that the suppressed fluorescence quenching in the solid state is a common feature for the present laterally boryl-substituted pi-conjugated skeletons. Hence, the diborylphenylene can serve as a useful core unit to develop highly emissive organic solids. In fact, most of the derivatives showed more intense emission in the solid state than in solution. In addition to these studies, the titration experiment of 1 by the addition of nBu(4)NF was conducted, which showed the stepwise bindings of two fluoride ions with high association constants as well as a drastic change in the fluorescence spectra, while constantly maintaining high quantum yields (0.61-0.76), irrespective of the binding modes. This result also demonstrated the potential utility of the present molecules as an efficient fluorescent fluoride ion sensor.

Triarylborane-based [5]Helicenes with Full-Color Circularly Polarized Luminescence.

The efficient synthetic route was disclosed to prepare optically active triarylborane-based [5]helicenes, 7B-PhHC and 7B5N-PhHC. Their emission wavelengths are tunable by both the chemical structure modification and the tuning of excited state charge transfer dynamics via selection of appropriate solvents or addition of external F-, enabling the full-color circularly polarized luminescence with moderate to good quantum yields (0.07-0.51) and high luminescence dissymmetry factors (glum > 5 × 10-3).

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.