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.

Experimental General consideration: All reactions were performed under an inert atmosphere of dry N 2 with standard Schlenk techniques unless otherwise noted. All starting materials were purchased from Aldrich Chemical Co. and used without further purification. THF, Et 2 O, and CH 2 Cl 2 were purified using the solvent purification system (Innovation Technologies Co.). Deuterated solvents as chloroform-d 1 (D, 99.8%), methanol-d 4 (D, 99.8%), benzene-d 6 (D, 99.5%) and methylene chloride-d 2 (D, 99.9%) (Cambridge Isotopes) were used as received without further drying. NMR spectra were recorded on a Bruker Avance 400 spectrometer (400.13 MHz for 1 H, 100.62 MHz for 13 C, 376.50 MHz for 19 F), chemical shifts are referenced to the residual solvent peaks and have been reported in parts per million (ppm) relative to TMS (1 H and 13 C) and CFCl 3 (19 F). UV-Vis spectra were recorded on a Varian Cary-3 UV-Visible spectrophotometer. Cyclic voltammetry was performed using a BAS CV-50W analyzer with a scan rate of 500 mV/s to 4 V/s and a typical concentration of 5 mg of the compounds in 3 mL DMF. The electrolytic cell used was a conventional three-compartment cell, in which a Pt working electrode, a Pt auxiliary electrode, and a Ag/AgCl reference electrode were employed. The CV measurements were performed at room temperature using 0.10 M tetrabutylammonium hexafluorophosphate (TBAP) as the supporting electrolyte and DMF as the solvent. The ferrocenium/ferrocene couple was used as the internal standard (E 0 = 0.55 V). Elemental analyses were

A triarylboron compound Si-BNPA (1) containing a BMes(2) acceptor and an N-(2'-pyridyl)-7-azaindolyl (NPA) donor linked by a tetrahedral silane group has been synthesised. This molecule displays unusual white singlet-triplet dual emission at 77 K, with an exceptionally long phosphorescent decay time (2.2 s). Fluoride titration experiments established that the singlet and triplet emission peaks are due to acceptor-based Mes-->B charge transfer and donor-based 3pi-->pi* transitions, respectively. This dual emission was found to be persistent and observable at ambient temperature in its Pt(II) complex [Pt(N,N-Si-BNPA)Ph2] (2a). Furthermore, 2a was found to undergo intramolecular "roll-over" C-H activation to produce the N,C-chelate complex [Pt(N,C-Si-BNPA)(SMe2)Ph] (2b). This compound also displays ambient temperature singlet-triplet dual emission, but with a much greater phosphorescent efficiency than 2a due to the formation of a more stable chelate ring. Addition of fluoride was found to have little impact on the phosphorescent emission of 2a, but resulted in a large enhancement of the phosphorescent emission intensity of 2b. To establish the impact of donor-acceptor geometry on this singlet-triplet dual emission, the properties of linearly conjugated donor-acceptor complexes [Pt(N,N-BNPA)Ph2] (3a) and [Pt(N,C-BNPA)Ph2] (3b) were also examined. Consistent with 2a and 2b, the N,C-chelate complex 3b has a much higher phosphorescent efficiency than the N,N-chelate 3a. Although 3a and 3b show bright and fluoride-switchable phosphorescence at ambient temperature, they are not dual emissive and show only metal-to-ligand chage-transfer-based phosphorescence. The non-conjugated donor-acceptor geometry and the overlap of the donor and acceptor singlet and triplet excitation bands in Si-BNPA and its Pt(II) complexes may thus be the key for achieving singlet-triplet dual emission on two separated chromophores in a single molecule.

A Cu(I) complex, 1, and a Pt(II) complex, 2a, of a triarylboron ligand, Bnpa, with bright ambient-temperature phosphorescence have been obtained. The phosphorescence of these complexes is highly sensitive toward molecular oxygen and has a distinct response to fluoride ions. For 1, the fluoride ion causes phosphorescent quenching and Bnpa dissociation, and for 2a, it switches phosphorescent color from yellow to green. The Cu(I) complex has an exceptionally high emission quantum yield (0.88) in the solid state.

To investigate the impact of the linker on the electronic and photophysical properties of diboryl compounds, three new diboryl compounds that contain two BMes2 groups (Mes = mesityl) have been synthesized, including a planar 1,6-(BMes2)2pyrene (1), a V-shaped bis(p-BMes2phenyl)diphenylsilane (4), and a U-shaped 1,8-bis(p-BMes2phenyl)naphthalene (5). For comparison, two previously known compounds, p-(BMes2)2benzene (3) and 1,8-bis(p-BMes2-biphenyl)naphthalene (6), were also investigated. The aromatic linkers in these molecules have been found to have a dramatic impact on the electron-accepting ability and Lewis acidity of the diboryl compounds through their distinct steric and electronic properties. Compound 1 has the most positive reduction potential (E 1/2 red1 = −1.81 V, relative to FeCp2 0/+), while 5 has the most negative reduction potential (E 1/2 red1 = −2.34 V). All compounds are blue emitters with considerable variation of emission energy and efficiencies (e.g., λem = 446, 402, 395 nm, Φ = ∼1.0, 0.17, ∼1.0 for 1, 4, and 5, respectively), and each displays a distinct and selective response toward fluoride ions. Upon addition of fluoride ions, compound 1 displays an unusual red shift and an on−off response in both absorption and fluorescent spectra. By comparing the behavior of 1 to that of the monoboryl compound 1-BMes2pyrene (2) and 3, and with TD-DFT computations on 1 and its fluoride adducts 1F and 1F 2 , it has been found that the peculiar response of 1 toward fluoride ions is caused by the dominance of pyrene π orbitals at the HOMO level of 1F and the relatively low-energy charge transfer from the pyrene ring to the three-coordinate boron center in 1F. The crystal structures of 2, 4, 1F 2 , and 5F 2 were determined by X-ray diffraction analyses. The potential use of compound 1 as either a blue emitter or a bifunctional emitter in OLEDs has been demonstrated by the successful fabrication of double- and triple-layer electroluminescent devices.