Synthesis, structure and density functional theory calculations of a novel photoluminescent trisarylborane-bismuth(III) complex.

A novel trisarylborane-Bi(III) complex, tris(4-(dimesitylboryl)phenyl)bismuthine [Bi(PhBMes2 )3 ], in which (Ph = phenyl, and Mes = mesityl), was synthesized via the reaction of bismuth (III) chloride (BiCl3 ) with three equivalents of lithiated (4-bromophenyl)- dimesitylborane [BrPhBMes2 ]. The new trisarylbismuthine was characterized by elemental analysis, ultraviolet-visible (UV-vis) spectroscopy, and NMR (1 H and 13 C) spectroscopy. The molecular structure of Bi(PhBMes2 )3 in the solid state was determined using single-crystal X-ray diffraction analysis, which showed short intermolecular C-H···H-C contact. The complex is a fluorescent emitter (λmax  = 395 nm) at room temperature and a phosphorescent emitter (λmax  = 423 nm) at 77 K, which displayed a long lifetime of 495 ms. The UV-vis transitions were investigated using density function theory (DFT) and time-dependent (TD)-DFT calculations. Natural bond orbital analysis showed that the bismuth (III) center was mainly Lewis acidic in nature.

Tuning the Colors of the Dark Isomers of Photochromic Boron Compounds with Fluoride Ions: Four-State Color Switching.

Combining a three-coordinated boron (BMes2) moiety with a four-coordinated photochromic organoboron unit leads to a series of new diboron compounds that undergo four-state reversible color switching in response to stimuli of light, heat, and fluoride ions. Thus, these hybrid diboron systems allow both convenient color tuning/switching of such photochromic systems, as well as visual fluoride sensing by color or fluorescent emission color change.

Assembly of a M4L4 "folded-cube" using a T-shaped, right-angled ligand.

A new bipyridine ligand based on a T-shaped 2,4,7-trisubstituted benzimidazole framework was synthesized with a 90° angle between the two nitrogen donors. When combined with convergent [Pt(dppp)](2+) corners, eight right angles (four metal and four ligand) combine to form a M4L4 cluster with an unusual folded-cube structure.

Photo- and thermal-induced multistructural transformation of 2-phenylazolyl chelate boron compounds.

The new N,C-chelate boron compounds B(2-phenylazolyl)Mes2 [Mes = mesityl; azolyl = benzothiazolyl (1a), 4-methylthiazolyl (2a), benzoxazolyl (3a), benzimidazolyl (4a)] undergo an unprecedented multistructural transformation upon light irradiation or heating, sequentially producing isomers b, c, d, and e. The dark isomers b generated by photoisomerization of a undergo a rare thermal intramolecular H-atom transfer (HAT), reducing the azole ring and generating new isomers c, which are further transformed into isomers d. Remarkably, isomers d can be converted to their diastereomers e quantitatively by heating, and e can be converted back to d by irradiation at 300 nm. The structures of isomers 1d and 1e were established by X-ray diffraction. The unusual HAT reactivity can be attributed to the geometry of the highly energetic isomers b and the relatively low aromaticity of the azole rings. The boryl unit plays a key role in the reversible interconversion of d and e, as shown by mechanistic pathways established through DFT and TD-DFT calculations.

Sugar complexation to silicone boronic acids.

A new class of surface-active compounds based on the combination of silicones and boronic acids is described. The properties of the compounds can be tuned by manipulation of both the hydrophobic (silicone size and 3D structure) and hydrophilic components (by binding different saccharides to the boronic acid). Stabilization of the four-coordinate boron structure is provided by Tris buffer that also maintains neutral pH to suppress silicone hydrolysis.

Impact of a dithienyl unit on photostability of N,C-chelating boron compounds.

A dithienyl unit in a N,C-chelate monoboryl compound has been found to completely stabilize a N,C-chelate boryl chromophore toward photoisomerization. N,C-chelate diboryl compounds that contain a dithienyl unit display a similar high stability toward photoisomerization. This greatly enhanced photostability is attributed to the π → π* transition and luminescence involving the dithienyl unit that competes effectively with photoisomerization of the boryl chromophore.

Steric and electronic influence on photochromic switching of N,C-chelate four-coordinate organoboron compounds.

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.

Enhancing the photochemical stability of N,C-chelate boryl compounds: C-C bond formation versus C=C bond cis,trans-isomerization.

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.

Reversible intramolecular C-C bond formation/breaking and color switching mediated by a N,C-chelate in (2-ph-py)BMes2 and (5-BMes2-2-ph-py)BMes2.

A diboron compound with both 3-coordinate boron and 4-coordinate boron centers, (5-BMes2-2-ph-py)BMes2 (1) and its monoboron analogue, (2-ph-py)BMes2 (2) have been synthesized. Both compounds are luminescent but have a high sensitivity toward light. UV and ambient light cause both compounds to isomerize to 1a and 2a, respectively, via the formation of a C-C bond between a mesityl and the phenyl group, accompanied by a drastic color change from yellow or colorless to dark olive green or dark blue. The structures of 1a and 2a were established by 2D NMR experiments and geometry optimization by DFT calculations. Both 1a and 2a can thermally reverse back to 1 and 2 via the breaking of a C-C bond, with the activation barrier being 107 and 110 kJ/mol, respectively. The N,C-chelate ligands in 1 and 2 were found to play a key role in promoting this unusual and reversible photo-thermal isomerization process on a tetrahedral boron center. Reactions with oxygen molecules convert 1a and 2a to 5-BMes2-2-[(2-Mes)-ph]-pyridine (1b) and 2-(2-Mes)-ph-pyridine (2b), respectively.