Synthesis of Functionalizable Boron-Containing π-Electron Materials that Incorporate Formally Aromatic Fused Borepin Rings

Synthesis and Structure-Property Relationships in Regioisomeric Alternating Borane-Terthiophene Polymers

Main-chain boron-containing π-conjugated polymers are attractive for organic electronic, sensing, and imaging applications. Alternating terthiophene-borane polymers were prepared and the effects of regioisomeric attachment of the conjugated linker and variations in the electronic effect of the pendent aryl groups (2,4,6-tri-tert-butylphenyl, Mes*; 2,4,6-tris(trifluoromethyl)phenyl, FMes) examined. Pd₂ dba₃ /P(t-Bu)₃ -catalyzed Stille polymerization of arylbis(2-thienyl)borane and arylbis(3-thienylborane) with 2,5-bis(trimethylstannyl)thiophene at 120 °C gave polymers with appreciable molecular weight but MALDI-TOF MS analyses showed evidence of unusually prominent homocoupling. These defects could be suppressed by using brominated rather than iodinated monomers, more hindered 2,5-bis(tri-n-butylstannyl)thiophene as comonomer, and Pd₂ dba₃ /P(o-tol)₃ as the catalyst at 100 °C. Under these conditions, macrocyclic species with n=3-10 repeating units formed preferentially according to MALDI-TOF MS analyses. Photophysical studies revealed a prominent effect of the regiochemistry and the nature of the pendent aryl groups on the absorption and emission, giving rise to orange, yellow-green, blue-green, and blue emissive materials respectively. The electronic effects were rationalized through DFT calculations on bis(terthiophene) model systems.

Isolation of Stable Borepin Radicals and Anions

Borepin, a 7-membered boron-containing heterocycle, has become an emerging molecular platform for the development of new materials and optoelectronics. While electron-deficient borepins are well-established, reduced electron-rich species have remained elusive. Herein we report the first isolable, crystalline borepin radical (2 a, 2 b) and anion (3 a, 3 b) complexes, which have been synthesized by potassium graphite (KC8 ) reduction of cyclic(alkyl)(amino) carbene-dibenzo[b,d]borepin precursors. Borepin radicals and anions have been characterized by EPR or NMR, elemental analysis, X-ray crystallography, and cyclic voltammetry. In addition, the bonding features have been investigated computationally using density functional theory.

Electron-Deficient Heteroacenes that Contain Two Boron Atoms: Near-Infrared Fluorescence Based on a Push-Pull Effect*

Electron-deficient heteroacenes that contain two tricoordinate boron atoms in their acene skeletons and planarized phenyl ether moieties at their periphery were synthesized via the borylation of silicon-bridged precursors. X-ray crystallographic analysis revealed quinoidal structures, which give rise to two-step reversible redox processes for both the reduction and oxidation. These compounds exhibit intense absorption and sharp fluorescence bands with vibronic structures in the near-infrared (NIR) region. These properties originate from the push-pull effect along the long axis of the molecule derived from the electron-donating ether moieties and the electron-accepting boron moieties. Of particular note is the NIR emission of the thienothiophene-centered heteroacene, which has a maximum at 952 nm with a narrow band width of 309 cm^-1 in cyclohexane. A Franck-Condon analysis revealed the crucial role of the sterically less-hindered thienothiophene spacer in achieving this sharp emission band.

The Thermal Rearrangement of an NHC-Ligated 3-Benzoborepin to an NHC-Boranorcaradiene

An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 °C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.

Stable Borepinium and Borafluorenium Heterocycles: A Reversible Thermochromic "Switch" Based on Boron-Oxygen Interactions

The first examples of N-heterocyclic carbene (NHC) and cyclic(alkyl)(amino) carbene (CAAC) stabilized borepinium and borafluorenium heterocycles are reported herein. The optical properties of the heterocyclic borenium cations were tuned by varying the Lewis base and by changing the number of atoms in the ring. More importantly, functionalizing the cationic boron ring system in the NHC-borafluorenium cation affords a temperature-sensitive molecule with reversible colorimetric "turn off/turn on" properties in solution. Notably, this is the first report of thermochromism in these cationic species. This property, which is mediated by an intermolecular boron-oxygen bond equilibrium, was examined in detail by X-ray crystallography, variable temperature-UV/Vis absorption spectroscopy (VT-UV/Vis), and density functional theory (DFT).

Photochemical synthesis of a ladder diborole: a new boron-containing conjugate material

Climbing the ladder: Reductive cyclization of alkynyl haloboranes lead to the bis-benzocycloborabutylidene rather than the expected ladder diborole, despite the former being much less thermodynamically favored. Photochemical conversion to the ladder diborole was, however, quite facile upon irradiation at 254 nm.