The Solid-State Multi-Color Fluorescence Switching from a [2.2]Paracyclophane-Based Triarylborane.

The fluorophores, the fluorescence of which can be switched between multi bright colors in the solid state, show promising applications not only in the sophisticated multicolor display but also in the advanced encryption and anti-counterfeiting systems. However, it is very challenging to obtain such fluorophores. Herein, we disclose such an example, g-BPhANMe2-Cp, which contains an electron-donating dimethylamino (NMe2) and an electron-accepting [(2-dimesitylboryl)phenyl]acetyl at the pseudo-gem position of [2.2]paracyclophane skeleton. This molecule can display tricolor mechanochromic luminescence (MCL) due to the different responses of the mechanically ground amorphous state to heating and solvent-fuming. Owing to the absence of intermolecular π-π interactions in the solid state, the fluorescence efficiency is very high irrespective of its morphological state (ΦF=0.60-0.87). Moreover, this molecule also displays reversible acidochromic luminescence (ACL) by protonation and deprotonation of NMe2 with trifluoroacetic acid (TFA) and triethylamine (TEA), respectively. The protonated sample fluoresces (ΦF=0.31) at much shorter wavelength due to the interruption of intramolecular charge transfer process. Therefore, with the combination of tricolor MCL and ACL properties, the solid-state emission of g-BPhANMe2-Cp can be switched among four bright fluorescence colors of yellow, green, cyan and blue via treatment with appropriate stimulus.

Two-Photon-Excited Fluorescent Tetrabranched Triphenylborane Featuring the Cooperative Effect of Branching in Two-Photon Absorption.

We here disclose a new type of two-photon-excited fluorescent triarylborane, tetrabranched triphenylborane 1, which contains four electron-donating [4-(N,N-diphenylamino)phenyl]ethynyl branches at 2,6-positions of two phenyl rings. The cross section of 1 reaches 275 GM (1 GM = 10-50 cm4 s photon-1) in tetrahydrofuran. Compared with dibranched triphenylborane 2, the 2-fold increase in the number of electron-donating branches induces a 3.6-fold increase in the two-photon absorption cross section, suggesting the great cooperative effect of branching in the enhancement of two-photon absorption.

The effect of composition changes on the structure and electronic properties of jamesonite: a DFT study.

Models of jamesonite with different compositions were built by different amounts of Sb or Pb substitution at Fe sites, and their structures and electronic properties were studied using the DFT method. The structure and properties of jamesonite significantly changed after Sb or Pb substitution. The lengths of the Sb-S and Pb-S bonds are larger than those of the corresponding Fe-S bonds of pure jamesonite, and the polarization of iron atoms adjacent to substitute atoms is weakened. After one Sb atom substitution, the S atoms bonded to Sb (substitution atom) gain more charges than those before Sb substitution. The Sb atom has more positive charges than the corresponding Fe atom before Sb substitution. For one Pb substitution system, the electrons transfer from the substituted Pb to adjacent S atoms, and the larger negative charge of the S atoms causes a slightly stronger Pb-S bond. With increasing Sb or Pb content, the electronic structural changes of the adjacent atoms are similar to those of one Sb or Pb substitution. However, the increase of 4-coordination Sb or 4-coordination Pb with the decrease of Fe atom changes the electronic structure of jamesonite, which will change its flotation performance.

Influence of external electric field on electronic structure and optical properties of ß-Ga2O3: a DFT study.

The influence of different electric fields on the electronic structure and optical properties of ß-Ga2O3 was studied by GGA+U method. The results show that appropriate electric field intensity can regulate the band gap of ß-Ga2O3 more effectively to improve the photoelectric characteristics. The band gap value of intrinsic ß-Ga2O3 is 4.865 eV, and decreases from 4.732 to 2.757 eV with the increase of electric field intensity from 0.05 to 0.20 eV Å-1. The length of the O-Ga bond along the electric field increases the fastest with the electric field intensity, and the distance between O and Ga reaches 2.52 Å when the electric field intensity is 0.20 eV Å-1. A new peak appears in the real and imaginary parts of the dielectric function for ß-Ga2O3 in the low frequency region under the electric field, and the conductivity increases obviously. The optical absorption peaks induced by the electric field were observed in the wavelength range of 400-600 nm. The optical absorption of ß-Ga2O3 is enhanced with an increase of electric field intensity, exhibiting a maximum value with the electric field of 0.15 eV Å-1. The electric field above 0.15 eV Å-1 causes a decrease of optical absorption intensity.

Synthesis, Structures, and Photophysical Properties of Zigzag BNBNB-Embedded Anthracene-Fused Fluoranthene.

Three zigzag BNBNB-embedded anthracene-fused fluoranthenes are synthesized from 1,3,2-benzodiazaboroles through an indole-type N-directed C-H borylation reaction. Single-crystal X-ray diffraction analyses confirm the double bond character of all four alternating B-N bonds and reveal the five-center four-π-electron nature of the BNBNB group. Experimental spectra and density functional theory calculations indicate that borylation remarkably enhances the planarity, extends π-conjugation, and leads to a bathochromic shift in the absorption and emission bands, with remarkable fluorescence quantum yields in solution (92%).

Two-Dimensional ZnS/SnS2 Heterojunction as a Direct Z-Scheme Photocatalyst for Overall Water Splitting: A DFT Study.

Direct Z-scheme photocatalysts have attracted extensive attention due to their strong redox ability and efficient separation of photogenerated electron-hole pairs. In this study, we constructed two types of ZnS/SnS2 heterojunctions with different stacking models of ZnS and SnS2 layers, and investigated their structures, stabilities, and electronic and optical properties. Both types of heterojunctions are stable and are direct Z-scheme photocatalysts with band gaps of 1.87 eV and 1.79 eV, respectively. Furthermore, their oxidation and reduction potentials straddle the redox potentials of water, which makes them suitable as photocatalysts for water splitting. The built-in electric field at the heterojunction interface improves the separation of photogenerated electron-hole pairs, thus enhancing their photocatalytic efficiency. In addition, ZnS/SnS2 heterojunctions have higher carrier mobilities and light absorption intensities than ZnS and SnS2 monolayers. Therefore, the ZnS/SnS2 heterojunction has a broad application prospect as a direct Z-scheme visible-light-driven photocatalyst for overall water splitting.

DFT study on the electronic structure and optical properties of an Au-deposited α-Fe2O3 (001) surface.

The electronic structure and optical properties of gold clusters deposited on an α-Fe2O3 surface were studied by using density functional theory (DFT), with a special emphasis on the influence of Au cluster sizes. There is a strong interaction between Au clusters and the α-Fe2O3 surface, and the binding energy increases with an increase of Au cluster size. The Au atoms of the gold cluster are bonded to the iron atoms of the α-Fe2O3 surface for the Au/α-Fe2O3 system, and the electrons transfer from the Au cluster to the α-Fe2O3 surface with the largest number of electrons transferred for 4Au/α-Fe2O3. The peaks of the refractive index, extinction coefficient and dielectric function induced by Au clusters appear in the visible range, which results in the enhanced optical absorption for the Au/α-Fe2O3 system. The optical absorption intensifies with increasing Au cluster size in the visible range, showing a maximum value for 4Au/α-Fe2O3. Further increasing the Au cluster size above 4Au results in a decrease in absorption intensity. The results are in good agreement with those of the refractive index, extinction coefficient and dielectric function.

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.

The mechanism of semiconductor to metal transition in the hydrogenation of VO2: a density functional theory study.

VO2 is a glamorous material with specific metal-semiconductor-transition (MST). The hydrogenation of VO2 could make it a promising material for application in an ambient environment. In this work, we reveal the hydrogenation of VO2 by modulating the hydrogen content and monitoring interaction models, adsorption energies, the density of states, electron density, and charge transfer between hydrogen and the VO2 surface. The monoclinic VO2(020) surface shows a distinct electronic polarization, and the majority spin band gap is larger than the minority spin band gap. The energy gap of the monoclinic VO2 surface is highly dependent on the majority spin band. The interaction between hydrogen and oxygen in the first layer of the VO2 surface is stronger than those in other layers. The energy gap on the surface of VO2 decreases gradually with increasing hydrogen content, and when twelve hydrogen atoms are adsorbed on the surface, an energy gap of 0 eV eventually appears, suggesting that monoclinic VO2 turns into a metallic conductor from a semiconductor. In the process of VO2 hydrogenation, the electron transfer only occurs between hydrogen and its connected oxygen atoms on the VO2 surface, and vanadium atoms just play an intermediary role.

Synthesis, Structure, and Photophysical Properties of BN-Embedded Analogue of Coronene.

Two BN-embedded benzo[ghi]perylene (Bzp) and coronene derivatives (BN-Bzp and BN-Cor) have been successfully synthesized from binaphthyl precursors by new efficient one-pot-multibond routes, and their single crystal structures were analyzed. Both experimental spectra and DFT theoretical calculations indicated that the absorption and emission of these BN-embedded polycyclic aromatic hydrocarbons are significantly enhanced comparing with those of their all carbon analogues. Especially, the fluorescence quantum yield of BN-Cor is nearly 20 times higher than that of ordinary coronene.

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.

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.

Enhanced Piezocatalytic Activity of Sr0.5Ba0.5Nb2O6 Nanostructures by Engineering Surface Oxygen Vacancies and Self-Generated Heterojunctions.

Piezocatalysis provides a promising strategy for directly converting weak mechanical energy into chemical energy. In this work, we report a simple one-step hydrogen reduction route for the simultaneous generation of surface defects and heterojunctions in Sr0.5Ba0.5Nb2O6 nanorods fabricated by a molten salt synthesis method. The as-fabricated Sr0.5Ba0.5Nb2O6/Sr2Nb2O7 nanocomposites with controllable oxygen vacancies exhibited excellent piezocatalytic activity under ultrasonic vibration, with an about 7 times enhancement of the rate constant (k = 0.0395 min-1) for rhodamine B degradation and an about 10 times enhancement of the water-splitting efficiency for hydrogen generation (109.4 µmol g-1 h-1) for the optimized sample (H2 annealed at 500 °C) compared to pristine Sr0.5Ba0.5Nb2O6 nanorods. This work demonstrates the essential role of a well-modulated oxygen vacancy concentration in the piezocatalytic activity and provides an inspiring guide for designing self-generated heterojunction piezocatalysts.

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.

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).

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.

Modification of [5]Helicene with Dimesitylboryl: One Way To Enhance the Fluorescence Efficiency.

The efficient synthetic route was disclosed to prepare structurally asymmetric [5]helicenes, which are substituted with either BMes2 (7B-HC) or both BMes2 and NMe2 (8B5NMe2-HC, 7B5NMe2-HC). Compared with the parent [5]helicene, these compounds show greatly enhanced fluorescence. In addition, they still retain fairly strong fluorescence in the solid state. Moreover, the complexation of 8B5NMe2-HC and 7B5NMe2-HC with fluoride can induce significant blue shift in fluorescence and the formed complexes are also highly fluorescent.

Solid-State Emissive Triarylborane-Based [2.2]Paracyclophanes Displaying Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence.

New types of [2.2]paracyclophane derivatives, g-BNMe2-Cp and m-BNMe2-Cp, in which electron-donating NMe2 and the electron-accepting BMes2 are introduced at the pseudo -gem and pseudo -meta positions, were designed and synthesized. The efficient through-space charge transfer enables the intense fluorescence with thermally activated delayed fluorescence characteristics. The quantum yields are up to 0.72 and 0.39 in cyclohexane. In addition, no significant fluorescence quenching was observed in the solid state with fluorescence quantum yields of powder up to 0.53 and 0.33. Moreover, the enantiomerically pure forms of g-BNMe2-Cp exhibit strong CPL signals with glum up to 4.24 × 10-3.