Daniel L. Crossley

Facile Arylation of Four-Coordinate Boron Halides by Borenium Cation Mediated Boro-desilylation and -destannylation

The addition of AlCl3 to four-coordinate boranes of the general formula (C–N-chelate)BCl2 results in halide abstraction and formation of three-coordinate borenium cations of the general formula [(C–N-chelate)BCl]+. The latter react with both arylstannanes and arylsilanes by boro-destannylation and -desilylation, respectively, to form arylated boranes. Catalytic quantities of AlCl3 were sufficient to effect high-yielding arylation of (C–N-chelate)BCl2. Boro-destannylation is more rapid than boro-desilylation and leads to double arylation at the boron center, whereas in reactions with arylsilanes either single or double arylation occurs dependent on the nucleophilicity of the arylsilane and on the electrophilicity of the borenium cation. The electrophilicity of the borenium cation derived from 2-phenylpyridine was greater than that of the benzothiadiazole analogues, enabling the boro-desilyation of less nucleophilic silanes and the direct electrophilic borylation of 2-methylthiophene.

Highly Emissive Far Red/Near-IR Fluorophores Based on Borylated Fluorene–Benzothiadiazole Donor–Acceptor Materials

Abstract Stille, Suzuki–Miyaura and Negishi cross‐coupling reactions of bromine‐functionalised borylated precursors enable the facile, high yielding, synthesis of borylated donor–acceptor materials that contain electron‐rich aromatic units and/or extended effective conjugation lengths. These materials have large Stokes shifts, low LUMO energies, small band‐gaps and significant fluorescence emission >700 nm in solution and when dispersed in a host polymer.

Post-polymerization C–H Borylation of Donor–Acceptor Materials Gives Highly Efficient Solid State Near-Infrared Emitters for Near-IR-OLEDs and Effective Biological Imaging

Post-polymerization modification of the donor-acceptor polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole), PF8-BT, by electrophilic C-H borylation is a simple method to introduce controllable quantities of near-infrared (near-IR) emitting chromophore units into the backbone of a conjugated polymer. The highly stable borylated unit possesses a significantly lower LUMO energy than the pristine polymer resulting in a reduction in the band gap of the polymer by up to 0.63 eV and a red shift in emission of more than 150 nm. Extensively borylated polymers absorb strongly in the deep red/near-IR and are highly emissive in the near-IR region of the spectrum in solution and solid state. Photoluminescence quantum yield (PLQY) values are extremely high in the solid state for materials with emission maxima ≥ 700 nm with PLQY values of 44% at 700 nm and 11% at 757 nm for PF8-BT with different borylation levels. This high brightness enables efficient solution processed near-IR emitting OLEDs to be fabricated and highly emissive borylated polymer loaded conjugated polymer nanoparticles (CPNPs) to be prepared. The latter are bright, photostable, low toxicity bioimaging agents that in phantom mouse studies show higher signal to background ratios for emission at 820 nm than the ubiquitous near-IR emissive bioimaging agent indocyanine green. This methodology represents a general approach for the post-polymerization functionalization of donor-acceptor polymers to reduce the band gap as confirmed by the C-H borylation of poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2c,2cc-diyl) (PF8TBT) resulting in a red shift in emission of >150 nm, thereby shifting the emission maximum to 810 nm.

Inter- and intra-molecular C–H borylation for the formation of PAHs containing triarylborane and indole units

Inter-/intra-molecular electrophilic C-H borylation of C4-substituted indoles enables the formation of fused polycyclic aromatic structures containing triarylborane and N-heterocyclic units. These compounds are B-(C)n-N isosteres of carbocyclic PAHs that do not contain B-N bonds and comparison of one pair of BN/CC isosteres reveals that different resonance structures dominate. These compounds are highly sensitive to protodeboronation, of both the chloroborane intermediates and the mesityl protected products, which results in low isolated yields of the latter. Protodeboronation can be utilised productively for a C-H directed, C-H electrophilic borylation to make a previously unknown pinacol boronate ester by selective protodeboronation of the chloroborane intermediate. Intermolecular and double intramolecular electrophilic C-H borylation of a C4-substituted indole leads to a more highly fused structure containing two boracycles which represents a B-(C)n-N analogue of the unknown carbon isostere indeno[1,7ab]perylene.

CCDC 1836436: Experimental Crystal Structure Determination

Related Article: Daniel L. Crossley, Pakapol Kulapichitr, James E. Radcliffe, Jay J. Dunsford, Inigo Vitorica‐Yrezabal, Rachel J. Kahan, Adam W. Woodward, Michael L. Turner, Joseph J. W. McDouall, Michael J. Ingleson|2018|Chem.-Eur.J.|24|10521|doi:10.1002/chem.201801799

CCDC 1836437: Experimental Crystal Structure Determination

Related Article: Daniel L. Crossley, Pakapol Kulapichitr, James E. Radcliffe, Jay J. Dunsford, Inigo Vitorica‐Yrezabal, Rachel J. Kahan, Adam W. Woodward, Michael L. Turner, Joseph J. W. McDouall, Michael J. Ingleson|2018|Chem.-Eur.J.|24|10521|doi:10.1002/chem.201801799

CCDC 1836439: Experimental Crystal Structure Determination

Related Article: Daniel L. Crossley, Pakapol Kulapichitr, James E. Radcliffe, Jay J. Dunsford, Inigo Vitorica‐Yrezabal, Rachel J. Kahan, Adam W. Woodward, Michael L. Turner, Joseph J. W. McDouall, Michael J. Ingleson|2018|Chem.-Eur.J.|24|10521|doi:10.1002/chem.201801799

C-H Borylation / Cross Coupling Forms Twisted Donor-Acceptor Compounds Exhibiting Donor Dependent Delayed Emission.

Abstract Benzothiadiazole (BT) directed C−H borylation using BCl3, followed by B−Cl hydrolysis and Suzuki–Miyaura cross‐coupling enables facile access to twisted donor–acceptor compounds. A subsequent second C−H borylation step provides, on arylation of boron, access to borylated highly twisted D−A compounds with a reduced bandgap, or on B−Cl hydrolysis/cross‐coupling to twisted D‐A‐D compounds. Photophysical studies revealed that in this series there is long lifetime emission only when the donor is triphenylamine. Computational studies indicated that the key factor in observing the donor dependent long lifetime emission is the energy gap between the S1/T2 excited states, which are predominantly intramolecular charge‐transfer states, and the T1 excited state, which is predominantly a local excited state on the BT acceptor moiety.

Synthesis, Characterization, and Functionalization of 1‐Boraphenalenes

Abstract 1‐Boraphenalenes have been synthesized by reaction of BBr3 with 1‐(aryl‐ethynyl)naphthalenes, 1‐ethynylnaphthalene, and 1‐(pent‐1‐yn‐1‐yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench‐stable products. All of these 1‐boraphenalenes have LUMOs localized on the planar C12B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C5B ring in 1‐boraphenalenes is dramatically lower than the C6 rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1‐boraphenalenes.