No abstract

A four-coordinate organoboron compound B(ppy)Mes(2) (1, ppy=2-phenylpyridyl, Mes=mesityl) was previously found to undergo reversible photochromic switching through the formation/breaking of a C-C bond, accompanied by a dramatic color change from colorless to dark blue. To understand this unusual phenomenon, a series of new four-coordinate boron compounds based on the ppy-chelate ligand and its derivatives have been synthesized. In addition, new N,C-chelate ligands based on benzo[b]thiophenylpyridine and indolylpyridine have also been synthesized and their boron compounds were investigated. The crystal structures of most of the new compounds were determined by X-ray diffraction analysis. UV/Vis, NMR, and electrochemical methods were used to monitor the photoisomerization process. DFT calculations were performed for all compounds to understand the photophysical and electronic properties of this class of molecules. The results of our study showed that the bulky mesityl group is necessary for photochromic switching. Electron-donating and electron-withdrawing groups on the ppy chelate have a distinct impact on the photoisomerization rate and the photochemical stability of the molecule. Furthermore, we have found that the non-ppy-based N,C-chelate ligands such as benzo[b]thiophenepyridyl can also promote photoisomerization of the boron chromophore in the same manner as the ppy chelate, but the product is thermally unstable.

An anti-solvent for graphene oxide (GO), hexane, is introduced to increase the surface area and the pore volume of the non-stacked GO/reduced GO 3D structure and allows the formation of a highly crumpled non-stacked GO powder, which clearly shows ideal supercapacitor behavior.

N,C-chelate boron compounds such as B(ppy)Mes(2) (ppy = 2-phenylpyridyl, Mes = mesityl) have been recently shown to undergo a facile and reversible C-C/C-B bond rearrangement upon irradiation with UV-light, quenching the emission of the sample and limiting their use in optoelectronic devices. To address this problem, four molecules have been synthesized in which the pi-conjugation is extended using either vinyl or acetylene linkers. These compounds, (ph-C[triple bond]C-ppy)BMes(2) (B1A), (ph-CH=CH-ppy)BMes(2) (B1), [p-bis(ppy-CH=CH)benzene](BMes(2))(2) (B2), and [1,3,5-tris(ppy-CH=CH)benzene](BMes(2))(3) (B3) have been fully characterized by NMR and single-crystal X-ray diffraction analyses. All four compounds are light yellow and emit blue or blue-green light upon UV irradiation. The acetylene compound B1A has been found to exhibit photochemical instability the same as that of the parent chromophore B(ppy)Mes(2). In contrast, all of the olefin-substituted compounds are photochemically stable, instead undergoing cis-trans isomerization exclusively upon exposure to UV light. Experimental and TD-DFT computational results establish that the presence of the olefinic bond in B1-B3 provides an alternate energy dissipation pathway for the B(ppy)Mes(2) chromophore, stabilizing the molecule toward photochromic switching via cis-trans isomerization. Furthermore, the incorporation of a cis-trans isomerization pathway may prove to be a useful strategy for the stabilization of photochemically unstable chromophores in other pi-systems as well.

No abstract

No abstract

The iron oxide nanoparticles were transformed to a matrix of iron-iron oxide on the graphene surface at an elevated temperature in a H(2)/Ar atmosphere. The resultant iron-iron oxide dispersed graphene was highly porous, robust and attractive for a variety of potential applications.

Many materials consisting of a p-conjugated organic backbone attached to either a tetrahedral or trigonal-planar boron center display enhanced fluorescence and charge-transport properties. This phenomenon can be attributed to either chelation-enhanced p-conjugation of the organic backbone by a tetrahedral boron center [1][2] or to the strong electronaccepting ability of a trigonal-planar boron center.[3] Hence, boron-containing molecules have found applications in OLEDs, organic transistors, and nonlinear optical materials. [1][2][3] We recently reported an unusual photochromic switch involving tetrahedral N,C-chelating boron chromophores such as B(ppy)Mes 2 (ppy = 2-phenylpyridine, Mes = mesityl).[4] These compounds undergo a thermally reversible photoisomerization from a colorless or light colored state to a dark colored state upon irradiation with 350-450 nm light (Scheme 1). These compounds are highly fluorescent in the light-colored state with tunable emission colors, whereas the dark-colored isomers are nonemissive.We have recently shown that this photoisomerization can be suppressed when the system is conjugated to an olefinic bond, which can dissipate the excitation energy through trans-cis isomerization.[4b] While this conjugation has the benefit of stabilizing the system towards photodegradation and preserving its luminescence properties, it also renders the system photochromically inert. To elucidate the impact of incorporating multiple photochromic boron centers into these materials, we prepared a series of new p-conjugated molecules as shown in Scheme 2. Our study has shown that isomerization of one chromophore prevents isomerization of the others, leading to amplified and reversible fluorescence quenching of the entire molecule. The details of our study are presented herein.The synthesis of the monoboryl B1 was reported previously.[4b] Diboryl B2 and B2', triboryl B3, and hexaboryl B6 were synthesized from a common intermediate 1 (see the Supporting Information). B2' was prepared by Hay coupling, [5] and B2, B3, and B6 were prepared by Sonogashira coupling [6] with the appropriate aryl halides. The crystal structures of B2, B2', and B6 (Figure 1) [7] reveal several important features of these materials. B2' and B2 have good coplanarity between the two boron chromophores and dihedral angles of 08 between the two ppy rings for B2 ' and 23.98 between the ppy and the central phenyl ring for B2, which is consistent with strong p conjugation and electronic communication between the boron centers in these molecules. In contrast, the electronic communication between the boron centers in B6 appears to be much weaker as a result of the high degree of steric congestion, as evidenced by the large dihedral angles of 22.38, 59.58, and 82.88 between the ppy chelates and adjacent phenyl rings.These trends in electronic communication are manifested in the reduction potentials and absorption spectra of these materials. The polyboryl compounds except B3 all show two well-resolved reduction peaks, and the first redu...